Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Mosadegh, Sepehr
Doctor of Philosophy - PhD
University of British Columbia
The global energy supply will rely on liquid fuels in the foreseeable future, with diesel-like variants witnessing the highest demand across various industries. Recent initiatives have focused on renewable biofuels such as biodiesel and ethanol, which are emerging as environmentally friendly alternatives to conventional diesel. Concurrently, incorporating nanomaterials, specifically Graphene Oxide (GO), into liquid fuels has demonstrated substantial enhancements in combustion power and emission characteristics. In the context of blended and doped fuels, atomization processes involving nucleation, expulsion, and droplet ejection play important roles, potentially influencing combustion characteristics. The focus of the present dissertation is quantifying the effects of atomization on burning rate, dynamics, and flame spread for single and multi-droplets. Suspensions of GO with doping concentrations of 0–0.1% in diesel, biodiesel, and ethanol, were prepared, and characterized. Results of experiments concerning ethanol doped with GO show consistent characteristics of ejected baby droplets for several test conditions. The results suggest the possible burning of the baby droplets within the flame envelope. A conservation-of-mass framework is used and it is shown that after droplets atomize, burning rate increases, but the rate of evaporation decreases. For ethanol droplets, GO addition and increased doping concentration improve the overall burning rate. In GO-doped diesel fuels, atomization induces a low-frequency intermittent coupling between the droplet size and flame chemiluminescence intensity, with amplified oscillations observed at higher GO concentrations. Such low-frequency oscillations are also observed for single and multi-droplet combustion of biodiesel blended with ethanol and doped with GO. For multi-droplet combustion, despite minimal impact on flame dynamics and spread rate among neighboring droplets, the addition of GO introduces a new mode of flame spread. Compared to doping with GO, blending of biodiesel with ethanol leads to significant increase of the flame spread rate due to intensified atomizations. The results presented in this thesis provides valuable insights into the intricate dynamics of atomization, combustion, and flame spread between atomizing droplets.
INFLUENCE OF ATOMIZATIONON SINGLE ANDMULTI-DROPLET COMBUSTIONOF DIESEL, BIODIESEL, ANDETHANOL DOPED WITHGRAPHENE OXIDEbySepehr MosadeghA dissertation submitted in partial fulfillment ofthe requirements for the degree ofDOCTOR OF PHILOSOPHYinThe College of Graduate Studies(Mechanical Engineering)THE UNIVERSITY OF BRITISH COLUMBIA(Okanagan)April 2024© Sepehr Mosadegh, 2024The following individuals certify that they have read, and recommend to the Collegeof Graduate Studies for acceptance, a dissertation entitled: INFLUENCE OF ATOMIZA-TION ON SINGLE AND MULTI-DROPLET COMBUSTION OF DIESEL, BIODIESEL,AND ETHANOL DOPED WITH GRAPHENE OXIDEsubmitted by Sepehr Mosadegh in partial fulfillment of the requirements of the degreeof Doctor of PhilosophyDr. Sina Kheirkhah, School of EngineeringSupervisorDr. Mohammad Arjmand, School of EngineeringSupervisory Committee MemberDr. Ian Foulds, School of EngineeringSupervisory Committee MemberDr. Cigdem Eskicioglu, School of EngineeringUniversity ExaminerDr. Swetaprovo Chaudhuri, University of TorontoExternal ExamineriiAbstractThe global energy supply will rely on liquid fuels in the foreseeable future, with diesel-like variants witnessing the highest demand across various industries. Recent initiativeshave focused on renewable biofuels such as biodiesel and ethanol, which are emerging asenvironmentally friendly alternatives to conventional diesel. Concurrently, incorporatingnanomaterials, specifically Graphene Oxide (GO), into liquid fuels has demonstrated sub-stantial enhancements in combustion power and emission characteristics. In the context ofblended and doped fuels, atomization processes involving nucleation, expulsion, and dropletejection play important roles, potentially influencing combustion characteristics. The fo-cus of the present dissertation is quantifying the effects of atomization on burning rate,dynamics, and flame spread for single and multi-droplets. Suspensions of GO with dopingconcentrations of 0–0.1% in diesel, biodiesel, and ethanol, were prepared, and characterized.Results of experiments concerning ethanol doped with GO show consistent character-istics of ejected baby droplets for several test conditions. The results suggest the possibleburning of the baby droplets within the flame envelope. A conservation-of-mass frameworkis used and it is shown that after droplets atomize, burning rate increases, but the rateof evaporation decreases. For ethanol droplets, GO addition and increased doping concen-tration improve the overall burning rate. In GO-doped diesel fuels, atomization inducesa low-frequency intermittent coupling between the droplet size and flame chemilumines-cence intensity, with amplified oscillations observed at higher GO concentrations. Such low-frequency oscillations are also observed for single and multi-droplet combustion of biodieseliiiblended with ethanol and doped with GO. For multi-droplet combustion, despite minimalimpact on flame dynamics and spread rate among neighboring droplets, the addition of GOintroduces a new mode of flame spread. Compared to doping with GO, blending of biodieselwith ethanol leads to significant increase of the flame spread rate due to intensified atom-izations. The results presented in this thesis provides valuable insights into the intricatedynamics of atomization, combustion, and flame spread between atomizing droplets.ivLay SummaryTraditional liquid fuels, such as diesel, are anticipated to provide for the world’s globalenergy in the foreseeable future and as the primary resource. Despite their widespreadapplication, these fuels are produced from limited natural resources, and poor combustionof these fuels can pollute the environment. Recent studies suggest that biofuels dopedwith nanomaterials may potentially help addressing the above challenges. However, severalfundamental questions remain to be addressed. This thesis aims to develop understandingregarding the characteristics and dynamics of both single and multi-droplet combustion ofsuch fuels. The findings presented in this thesis will facilitate designing engines capable ofsafely and efficiently burning biofuel blends doped with nanomaterials, paving the way fordesigning combustion engines that are more environmentally friendly.vPrefaceThe work presented in this dissertation was conducted under the supervision of ProfessorDr. Sina Kheirkhah from the School of Engineering at the University of British Columbia.The experiments were performed at the Combustion for Propulsion and Power Laboratoryat the University of British Columbia, Okanagan campus, Syilx Okanagan Nation Territory.The results in this thesis have led to journal articles and conference presentations, whichare listed in the following.Journal publications1. Mosadegh, S. and Kheirkhah, S. (2024) “Influence of the atomization on combustiondynamics and flame spread in multi-droplet configuration” Submitted.2. Mosadegh, S. and Kheirkhah, S. (2022) “The role of atomization in the couplingbetween doped droplets dynamics and their flames”, Physics of Fluids, 34, 087109 [1].3. Mosadegh, S., Ghaffarkhah, A., van der Kuur, C., Arjmand, M. and Kheirkhah, S.(2022) “Graphene oxide doped ethanol droplet combustion: Ignition delay and contri-bution of atomization to burning rate”, Combustion and Flame, Vol. 238, 111748 [2].Conference publications and presentations1. Mosadegh, S. and Kheirkhah, S. (2023) “Lateral flame propagation in an array ofatomizing fuel droplets”, Canadian Section of the Combustion Institute: 2023 SpringviTechnical Meeting, May 15–18, Edmonton, Canada.2. Mosadegh, S. and Kheirkhah, S., (2022) “The role of atomization in coupling betweenthe doped droplets dynamics and their flames”, Poster Presentation in 39th Interna-tional Symposium on Combustion, July 24–29, Vancouver, Canada.3. Mosadegh, S., Aboonasr Shiraz, M., van der Kuur, C., Arjmand, M., and Kheirkhah,S. (2021) “Burning dynamics of graphene oxide doped diesel droplets”, APS Divisionof Fluid Dynamics Meeting Abstracts, pp. M22-009 [3] (help online).viiTable of ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiLay Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viiiList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiiNomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xxivAcknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xxxiDedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xxxiiiChapter 1: Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.2.1 Droplet atomization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.2.2 Ignition delay and burning rate of the doped droplets . . . . . . . . . 91.2.3 Combustion dynamics of atomizing droplets . . . . . . . . . . . . . . . 131.2.4 Flame spread in an array of droplets . . . . . . . . . . . . . . . . . . . 151.2.5 Knowledge gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17viiiTABLE OF CONTENTS1.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181.4 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Chapter 2: Experimental methodology . . . . . . . . . . . . . . . . . . . . . . 222.1 Fuel preparation and characterization . . . . . . . . . . . . . . . . . . . . . . 222.1.1 GO doped ethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.1.2 GO doped diesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.1.3 GO doped and ethanol-blended biodiesel . . . . . . . . . . . . . . . . . 342.2 Apparatus and diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382.2.1 Experimental arrangement for the burning rate measurement . . . . . 392.2.2 Experimental arrangement for combustion dynamics measurement . . 422.2.3 Experimental arrangement for multi droplet combustion dynamicsand flame spread measurements . . . . . . . . . . . . . . . . . . . . . . 44Chapter 3: Droplet and flame detection . . . . . . . . . . . . . . . . . . . . . 473.1 Droplet detection and shadowgraphy data reduction . . . . . . . . . . . . . . 473.1.1 Droplets on the borosilicate tube . . . . . . . . . . . . . . . . . . . . . 473.1.2 Droplets on the Silicon carbide fibers . . . . . . . . . . . . . . . . . . . 493.1.3 Baby droplets ejected from the main droplet . . . . . . . . . . . . . . 493.2 Flame detection and OH* chemiluminescence data reduction . . . . . . . . . 523.2.1 Estimation of the ignition delay . . . . . . . . . . . . . . . . . . . . . . 533.2.2 Estimation of flame boundaries . . . . . . . . . . . . . . . . . . . . . . 56Chapter 4: Contribution of atomization to burning rate . . . . . . . . . . . 594.1 Ignition delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.2 Atomization characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 624.3 Contribution of atomization to doped ethanol burning rate . . . . . . . . . . 67ixTABLE OF CONTENTSChapter 5: Role of atomization in droplet and flame dynamics . . . . . . . 785.1 Characteristics of the droplets, flames, and atomization events . . . . . . . . 785.2 The coupling between the droplets, flames, and atomization events . . . . . . 87Chapter 6: Influence of atomization on flame spread and combustion dy-namics of multi droplets . . . . . . . . . . . . . . . . . . . . . . . 936.1 Effect of atomization on multi-droplet combustion dynamics . . . . . . . . . . 936.1.1 Effect of atomization on single droplet combustion dynamics . . . . . 946.1.2 Effect of atomization on multi-droplet combustion dynamics . . . . . . 1026.2 Effect of atomization on flame spread rate in an array of droplets . . . . . . . 106Chapter 7: Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1177.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1177.2 Suggestions for future directions of research . . . . . . . . . . . . . . . . . . . 124Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150Appendix A: Synthesis of highly oxidized GO . . . . . . . . . . . . . . . . . . . . . 150Appendix B: Spatial resolution of shadowgraphy technique . . . . . . . . . . . . . 152Appendix C: Selection of time period required for estimation of burning rate . . . 154xList of TablesTable 2.1 The liquid fuels used in the present thesis and their properties. . . . . 23Table 2.2 The test conditions for GO doped ethanol single droplet combustion. . 24Table 2.3 The test conditions for single droplet combustion of GO doped diesel.ODAGO: Graphene oxide functionalized with octadecyl amine groups. 32Table 2.4 The GO doped and ethanol blended with biodiesel test conditions.ODAGO: Graphene oxide functionalized with octadecyl amine groups. 35Table 4.1 Ethanol droplet ignition delay and those reported in [4]. . . . . . . . . 60xiList of FiguresFigure 1.1 Past, current, and future projection of the global energy consumptionby fuel resource. The data in the figure is adapted from [5]. The datarelated to nuclear, renewables, natural gas, coal, as well as liquid fuelsare highlighted by circular, triangular, square, cross, and diamondshape symbols, respectively. . . . . . . . . . . . . . . . . . . . . . . . 2Figure 1.2 Schematics of the regimes for spray combustion, with the spray num-ber density decreasing from (a) to (d). (a–d) represent externalsheath combustion, external group combustion, internal group com-bustion, and collection of single droplet combustion. The schematicsin the figures are adapted from [6] and are modified. . . . . . . . . . . 6Figure 1.3 Schematic presenting the stages of droplet atomization. (a) presentsthe formation of bubbles due to homogeneous and/or heterogeneousnucleation. (b) shows merging of the bubbles as well as their growthand reach to the surface. (c) presents the rupture of the motherdroplet surface, which is followed by the ejection of fuel vapor as wellas ligaments formation and their break-up forming baby droplets. (d)shows the ejected fuel vapor and baby droplets reach the flame area,get ignited, and burn. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Figure 2.1 Photographs of the doped ethanol. (a–f) correspond to Er0.01, Ep0.01,Eh0.01, Er0.1, Ep0.1, and Eh0.1, respectively. . . . . . . . . . . . . . 25xiiLIST OF FIGURESFigure 2.2 Raman spectrum of the synthesized hGO. . . . . . . . . . . . . . . . . 26Figure 2.3 High-resolution SEM images of (a) rGO, (b) pGO, and (c) hGO,respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Figure 2.4 The probability density functions of the utilized nanomaterials char-acteristic length. The PDFs are calculated from the coarse-resolutionSEM imaging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Figure 2.5 Absorption intensity versus wave-number of the GO that was usedfor doping ethanol. The results are from the FT-IR spectroscopy. . . 30Figure 2.6 The thermogravimetric analysis of the utilized nanomaterials for dop-ing ethanol. (a) W/W0 versus temperature and (b) |d(W/W0)/dt|versus temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Figure 2.7 Photographs of the doped fuels (a) immediately after preparation,and (b) 5 months after preparation. . . . . . . . . . . . . . . . . . . 32Figure 2.8 Scanning electron microscopy of the stacked GO used to dope diesel. 33Figure 2.9 The Probability Density Function of the GO hydrodynamic diameterfor the test condition of D0.02. . . . . . . . . . . . . . . . . . . . . . 34Figure 2.10 (a), (b), and (c) are the pictures of the fuels immediately, 40 min,and 60 min after preparation of the fuels. The first to sixth columnspertain to B, B0.01,B0.1, BE, BE0.01, and BE0.1, respectively. . . . . 36Figure 2.11 SEM of the ODAGO; (a) small platelets, (b) stacked two-dimensionallayers, and (c) the side view of the folded layers. . . . . . . . . . . . . 36Figure 2.12 Raman spectrum of the utilized octadecyl amine graphene oxide. . . . 37Figure 2.13 The XRD signal versus twice the incident angle for ODAGO. . . . . . 38xiiiLIST OF FIGURESFigure 2.14 Technical drawing of the experimental apparatus and diagnostics usedfor GO-doped ethanol droplet combustion. (a) is the OH∗ chemilu-minescence setup, (b) is the shadowgraphy setup, and (c) is a closeview of the ignitor and the droplet. The numbered items are: (1)setup holder box, (2) tube holder, (3) ignitor module, (4) dropletsuspension mechanism, (5) ignitor power source, (6) solenoid actua-tor, (7) actuator holder, (8) relay module, (9) data acquisition unit(10) high-speed camera, (11) image intensifier, and (12) back-lightLED. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40Figure 2.15 (a) The relative timing of the solenoid actuation, ignitor operation,and data acquisition. (b) The relative timing of the camera andintensifier for OH∗ chemiluminescence imaging. (c) The timing ofthe camera for shadowgraphy imaging. . . . . . . . . . . . . . . . . . 41Figure 2.16 The experimental apparatus and diagnostics used for studying thediesel droplets combustion. (a) Close view of the cross-fiber config-uration and the ignitor, (b) the shadowgraphy setup, and (c) theOH∗ chemiluminescence setup. The numbered items are: (1) cross-fiber, (2) ignitor, (3) solenoid actuator, (4) high-speed camera, (5)back-light LED, (6) image intensifier, and (7) data acquisition unit. . 43Figure 2.17 (a) The experimental setup and diagnostics arrangement used tostudy GO doped biodiesel and GO doped ethanol blended with biodieseldroplets. (b) Close-view of the multi-droplet combustion configuration. 46Figure 3.1 Shadowgraphy image obtained for (a) the suspension mechanism and(b) the droplet suspended on the suspension mechanism. . . . . . . . 48xivLIST OF FIGURESFigure 3.2 An ellipse with major and minor diameters of a and b fit to a shad-owgraphy image of a droplet. . . . . . . . . . . . . . . . . . . . . . . . 49Figure 3.3 (a) A representative raw shadowgraphy image and (b) the binarizedimage using the adaptive thresholding technique. The baby dropletsare circled in both (a) and (b). . . . . . . . . . . . . . . . . . . . . . . 50Figure 3.4 Effects of the threshold and window size on the number of detectedand ejected baby droplets. The results pertain to the test conditionof Eh0.01 in Table 2.2. . . . . . . . . . . . . . . . . . . . . . . . . . . 51Figure 3.5 (a) Raw flame chemiluminescence image. (b) is the results in (a) af-ter background subtraction, white field correction, and median-basedfiltering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Figure 3.6 Variation of A/Amax versus time for the test conditions of (a) Ep0.01and (b) Eh0.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54Figure 3.7 (a) Normalized flame area versus time, (b–f) correspond to the flamechemiluminescence images acquired at t = 47.5, 50.5, 53.5, 56.5, and59.5 ms, respectively. (g) presents the location of the flame base(green solid curve) and the top of the flame (dashed pink curve)versus time. The results pertain to the pure ethanol test condition. . 55Figure 3.8 The “hot” color bar is the flame chemiluminescence normalized by itsmaximum. The contours are 5, 10, 15, 20, and 25% of the maximumflame chemiluminescence. The black circles are the droplets obtainedfrom the shadowgraphy image simultaneously acquired with the flamechemiluminescence. The results correspond to a representative testcondition of B0.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Figure 3.9 Flame spread time versus IN for representative test conditions ofB0.01 and BE0.01. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57xvLIST OF FIGURESFigure 4.1 Effect of graphene oxide type and concentration on the ignition delayof ethanol droplets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61Figure 4.2 (a) Variation of the normalized flame area versus time and (b-d) arethe OH∗ chemiluminescence images pertaining to t = 2.47, 2.63, and2.87 s, respectively. The results pertain to the test condition of Ep0.01. 62Figure 4.3 Shadowgraphy images of baby droplets formation and ejection. Theresults pertain to the test condition of Eh0.01. . . . . . . . . . . . . . 64Figure 4.4 Baby droplet ejection projected trajectories. The results pertain tothe test condition of Er0.01. . . . . . . . . . . . . . . . . . . . . . . . 64Figure 4.5 The probability density functions of (a) the x − y projected initialvelocity of the ejections, (b) the initial diameter of the ejected babydroplets, and (c) the x− y projected trajectory length of the ejections. 66Figure 4.6 JPDF of (a) the initial projected velocity of the baby droplets, (b)the initial diameter of the ejected baby droplets, and (c) the lengthof the ejected baby droplets projected trajectories versus time. (d),(e), and (f) are the JPDFs of V0 versus d0, s versus d0, and s versusV0, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Figure 4.7 Regression of the droplet diameter squared versus time (a) withoutatomization and (b) with atomization. The results in (a) and (b)pertain to Er0.1 and Ep0.01 test conditions, respectively. Both thevertical and horizontal axes values are normalized by the droplet ini-tial diameter squared (D20). . . . . . . . . . . . . . . . . . . . . . . . . 68Figure 4.8 Shadowgraphy images of ethanol droplet doped with 0.01% of rGOduring combustion. Images in (a–d) are separated by 75 ms andpertain to t = 2200, 2275, 2350, and 2425 ms. The red dashed circleshighlight the location of doping agent aggregate. . . . . . . . . . . . . 71xviLIST OF FIGURESFigure 4.9 Variation of (a) Term I versus K2, (b) Term I versus K1, and (c)Term II versus K1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73Figure 4.10 K ′2 versus K1 for the cases that atomization is present. The figureis color-coded based on the total mass of the ejected baby droplets.The dashed line is y = x. . . . . . . . . . . . . . . . . . . . . . . . . . 74Figure 4.11 Effect of graphene oxide type and concentration on the burning rateof ethanol droplets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75Figure 4.12 Variation of K2 versus the area under the FT-IR curves (see Fig. 2.5)for ethanol doped with rGO, pGO, and hGO. . . . . . . . . . . . . . 77Figure 5.1 (a) and (b) are D2 and D2′versus t. (c–e) present the droplet imagesduring an atomization event. (f) is the spectrogram of D2′. . . . . . 79Figure 5.2 (a) and (b) are If and If′ versus t. (c–e) are the flame chemilumi-nescence during an atomization event. (f) is the spectrogram of If′.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80Figure 5.3 (a) and (b) are the phase-space trajectories of the droplet and flamechemiluminescence oscillations. (c) and (d) are the correspondingPoincare´ maps. The results in (a and c) and (b and d) pertain toD0.01 and D0.02 test conditions, respectively. . . . . . . . . . . . . . 82Figure 5.4 (a) Time dependent PDF of the atomization intensity, Ia. (b–e) arethe representative shadowgraphy images during atomization eventswith Ia = 1.05, 1.2, 1.4, and 1.8, respectively. . . . . . . . . . . . . . 84xviiLIST OF FIGURESFigure 5.5 (a) The RMS of theD2′fluctuations versus the number of atomizationevents and the mean atomization intensity for all test conditions andtheir repeats. (b) and (c) are the 2D presentation of the data in (a),with the colored and enlarged data points presenting the averagedvalues. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85Figure 5.6 (a–c) present the shadowgrpahy images of the droplet with grpaheneoxide featuring the Brownian motion. (d–g) present the motion of agraphene oxide aggregate during a circulatory flow inside the droplet.(a–f) correspond to t = 1.125, 1.150, 1.175, 1.738, 1.775, 1.825, and1.840 s, respectively. The results pertain to the test condition of D001. 86Figure 5.7 The relation between the inverse of the time at which the first atom-ization occurs and the droplet diameter squared RMS. . . . . . . . . . 87Figure 5.8 (a) Variation of D2 versus time. The results correspond to D0.01test condition. (b) and (c) are the probability density functions ofthe inverse of the time between to consecutive atomization eventswith 1.05 ≤ Ia < 1.4 and Ia ≥ 1.4, respectively. (d) and (e) are thePSDs of the droplet diameter squared oscillations corresponding tot(Ia = 1.05) ≤ t < t(Ia = 1.4) and t ≥ t(Ia = 1.4), respectively. . . . . 88Figure 5.9 (a–e) are representative oscillations of the droplet center along thevertical axis versus time for test conditions of D, D0.001, D0.005,D0.01, and D0.02, respectively. (f) presents the averaged power spec-trum densities of the these oscillations. . . . . . . . . . . . . . . . . . 90Figure 5.10 The most probable value of 1/∆ta for atomization events with Ia ≥I∗a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91xviiiLIST OF FIGURESFigure 5.11 Spectra of the droplet diameter squared (black) and flame chemilu-minescence (red) oscillations as well as the PDF of the inverse of thetime difference between two consecutive atomization events (blue).The gray curves are the spectra of the droplet vibrations along thevertical axis. (a–e) correspond to test conditions D, D0.001, D0.005,D0.01, and D0.1, respectively. . . . . . . . . . . . . . . . . . . . . . . 92Figure 5.12 Variations of the dominant frequencies of the droplet oscillationsalong the vertical axis (fn), flame chemiluminescence oscillations (ff),and the droplet diameter squared oscillations after the occurrence ofthe first intense atomization event (fd). fa is the most probable fre-quency of intense atomization events. . . . . . . . . . . . . . . . . . . 92Figure 6.1 (a) Variations of the droplet diameter squared (black) and normalizedflame chemiluminescence (red) versus time. (b) Horizontal positionof the droplet (black) and flame chemiluminescence (red) centroidssubtracted by their mean values versus time. (c–j) are the flamechemiluminescence and droplet images corresponding to the instantsmarked in (a) and (b). Specifically, the results in (c–j) correspond tot = 0, 0.54, 0.69, 0.74, 0.78, 1.47, 1.50, and 1.51 s, respectively. Theresults are for the representative test condition of B0.01. . . . . . . . 95Figure 6.2 The spectrogram of (a–c) spatially integrated flame chemilumines-cence, (d–f) horizontal location of the flame chemiluminescence cen-troid, and (g–i) horizontal location of the droplet center. The resultsin the first, second, and third columns are for biodiesel, biodieseldoped with 0.01% of GO, and biodiesel doped with 0.1% of GO, re-spectively. The results correspond to single droplet combustion. . . . 97xixLIST OF FIGURESFigure 6.3 (a) Variations of the droplet diameter squared (black) and normalizedflame chemiluminescence (red) versus time. (b) Horizontal positionof the droplet (black) and flame chemiluminescence (red) centroidssubtracted by their mean values versus time. (c–j) are the flamechemiluminescence and droplet images corresponding to the time in-stants marked in (a) and (b). Specifically, (c–j) correspond to t = 0,0.08, 0.13, 0.14, 0.22, 0.46, 0.65, and 0.80 s, respectively. The resultsare for the representative test condition of BE0.01. . . . . . . . . . . . 99Figure 6.4 The spectrograms of single droplet combustion. (a–c) are for spatiallyintegrated flame chemiluminescence, (d–f) are for the horizontal lo-cation of the flame chemiluminescence centroid, and (g–i) are for thehorizontal location of the droplet center. The results in the first,second, and third columns pertain to ethanol blended with biodiesel,ethanol blended with biodiesel and doped with 0.01%, and ethanolblended with biodiesel and doped with 0.1% by mass of GO, respec-tively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100xxLIST OF FIGURESFigure 6.5 (a) and (b) are the rms of the flame chemiluminescence and the rmsof the flame chemiluminescence centroid horizontal position, respec-tively. In the figure, B-NA and B-A correspond to data related tobiodiesel test conditions before and after the atomizations, respec-tively. BE-A1 and BE-A2 correspond to the ethanol blended withbiodiesel during homogeneous nucleation and atomization as well asheterogeneous nucleation and atomizations, respectively. Th solid,dashed, and dotted-dashed ellipses in (a) correspond to test condi-tions with no atomizations, heterogeneous nucleation and atomiza-tions, as well as homogeneous nucleation and atomizations, respec-tively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102Figure 6.6 The spectrogram of the flame chemiluminescence for multi-dropletcombustion with the center droplet lit. The results in (a–c) cor-respond to biodiesel and (d–f) correspond to ethanol blended withbiodiesel. The results in the first, second, and third columns pertainto neat fuel, fuel doped with 0.01% by mass of GO, and 0.1% by massof GO, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104Figure 6.7 The spectrogram of the flame chemiluminescence for multi-dropletcombustion with the side droplet lit. The results in (a–c) correspondto biodiesel and (d–f) correspond to ethanol blended with biodiesel.The results in the first, second, and third columns pertain to neatfuel, fuel doped with 0.01% by mass of GO, and 0.1% by mass ofGO, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104xxiLIST OF FIGURESFigure 6.8 The dominant frequency of the flame chemiluminescence oscillations.The results in (a), (b), and (c) correspond to single droplet combus-tion, multi-droplet combustion with the center droplet ignited, andmulti-droplet combustion with the side droplet ignited. The triangu-lar, square, and circular data point pertain to diesel, biodiesel, andethanol blended with biodiesel, respectively. . . . . . . . . . . . . . . 106Figure 6.9 Modes of flame spread for (a–c) GO doped biodiesel and (d) biodieselblended with ethanol droplets. The results presented in (a–c) and (d)correspond to B0.1 and BE0.1, respectively. . . . . . . . . . . . . . . . 108Figure 6.10 Ignition time, propagation time, and flame spread rate versus thenormalized inter-droplet spacing. (a–c) and (d–f) correspond to thebiodiesel and ethanol blended with biodiesel droplets combustion,respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111Figure 6.11 The flame spread rate versus the rms of the flame chemiluminescencecentroid lateral movements during the spread time. (a and c) cor-respond to Modes 1, 2, and A (B). (b and d) correspond to ModeA (BE). Data in (a and b) are color-coded based on the inter-dropletspacing. Data in (c and d) are color coded based on GO dopingconcentration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115Figure 6.12 The flame spread rate versus the normalized inter-droplet spacing.Overlaid on the figure are the results of Park et al. [7, 8] and Wanget al. [9]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116Figure 7.1 Image of the USAF1951 target acquired with settings identical tothose used in the shadowgraphy experiments. . . . . . . . . . . . . . . 152xxiiLIST OF FIGURESFigure 7.2 (a) Variation of contrast (C) versus the number of line pairs permillimeter for the present study and that of [10]. The vertical axis onthe right hand side is the maximum threshold required to distinguishthe white rectangles. (b–d) are the variations of IN across the whiterectangles versus x/L for L = 111.4, 55.4, and 44.2 µm, respectively. . 153Figure 7.3 Variations of R2 and β obtained from Eqs.(7.3a) and (7.3b) versus∆t/τ . The results are presented for the representative tested condi-tion of Ep0.01. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154xxiiiNomenclatureSymbolsA AreaAmax Maximun areaAp Area of particles in SEM imagesa Major diameter of ellipseb Minor diameter of ellipseC ContrastCOO Carboxylate functional groupCO Carbonyl functional groupCO2 Carbon dioxidecl Droplet diameter squared obtained from a linear fitD Droplet diameterD2 Mean of the measured droplet diameter squaredD′ Droplet equivalent diameter in the absence of atomizationsD2′Fluctuations of the droplet diameter squaredDa Droplet diameter at the instant that first atomization is observedDe Droplet diameter at the end of an atomizationDi Droplet diameter at the begining of an atomizationD2′rms Root mean square of droplet diameter squaredD2′ rms Mean of the rms of the fluctuations of the droplet diameter squaredxxivNOMENCLATURED0 Initial droplet diameterdh Hydrodynamic diameter of the GO plateletsdl Inter-layer distnce of GOd0 Projected initial diameter of the baby dropletsd0(i) Initial diameter of the ith detected baby dropletf Frequencyfa The most probable value of intense atomization events frequencyfd Frequency of the droplet diameter squared oscillationsff Frequency of the flame chemiluminescence intensity oscillationsfIf Frequency of the flame chemiluminescence oscillations for a group ofdropletsfn Natural frequency of the droplet oscillationsfXd Frequency of the horizontal location of the droplet center oscillationsfXf Frequency of the horizontal location of the centroid of the flame chemi-luminescence oscillations[GO] Graphene Oxide doping concentrationHCl Hydrocloric acidH2O WaterH2O2 Hydrogen peroxideH2SO4 Sulfuric acidI IntensityIa Inensity of atomizationI∗a Threshold for atomization intensityIa Mean inensity of atomization averaged for each test conditionIa Mean inensity of atomizationIF Intensity of FFT-IR spectrumxxvNOMENCLATUREIf Flame chemiluminescence intensityIf′ Fluctuations of flame chemiluminescence intensityI f Means of flame chemiluminescence intensityIf,rms Root mean square of the flame chemiluminescence intensityIf(x, y) Local flame chemiluminescence intensityImax Maximum intensityImin Minimum intensityIN Flame chemiluminescence intensity normalized by the corresponding max-imumIN Spatially averaged flame intensity normalized by the corresponding max-imumIR Intensity of Raman scattering spectrumIX Intensity of XRD spectrumi ith detected baby dropletKCl Pottasium chlorideKMnO4 Potassium PermanganateKT Total droplet burning rateK1 Droplet burning rate during the non-atomizing periodK2 Droplet burning rate during the atomizing periodK ′2 Droplet equivalent burning rate during the atomizing period, providedthat the baby droplet ejections are absentL Width of the slots on USAF1951 target platel Particles characteristic lengthm˙EV Time rate of change of the main droplet mass due to evaporationMn2O7 Dimanganese heptoxideMw Mean intensity in a windowxxviNOMENCLATUREm0(i) Initial mass of the ith detected baby dropletN Number of detected baby dropletsNa Number of atomization eventsNa Number of atomization events averaged for each test conditionNd Number of data pointsOH Hydroxyl functional groupOH∗ Excited hydroxyl radicalR2 Coefficient of determinationr RadiusS Inter-droplet spacingS Mean inter-droplet spacingS⃗0 Projected initial discplacement of the baby dropletsSiC Silicon Carbides Length of the rojected trajectory of the baby dropletsT Temperaturet Timeti Time instant at which the side droplet is ignitedtp Time instant at which the flame has just completely propagated aroundthe side dropletth Threshold valuethmax Maximum threshold valueV VolumeVs Volume of the suspension mechnismV0 The magnitude of the initial x−y projected velocity of the baby dropletsW Nanomaterials WeightW0 Nanomaterials Weight at room temperaturexxviiNOMENCLATUREw Window sizeX Horizontal distance from the originXd Horizontal location of the droplet centerXd Mean of the horizontal location of the droplet centerXf The horizontal location of the flame centroidXf Mean of the horizontal location of the flame centroidXf,rms Root mean square of the horizontal location of the flame chemilumines-cence centroidx Horizontal distancexmax Maximum horizontal location of acquired flame imagexmin Minimum horizontal location of acquired flame imagey Vertical distancey′ The droplet center vertical position oscillationsyd Vertical distance of the droplet center from the originymax Maximum vertical location of acquired flame imageymin Minimum vertical location of acquired flame imageGreek symbolsβ Linear regression coefficient∆mEJ Change of droplet mass due to the baby droplet ejection∆mEV Change of droplet mass due to evaporation∆mT Total droplet mass loss∆ta Time difference between two consecutive atomizationsθ Angle of incidence of the X-rays to a given crystal lattice planeκ Wave numberλ Wavelength of X-ray radiationxxviiiNOMENCLATUREτ Droplet lifetimeτa Time instant that the first atomization is observedτi Side droplet ignition time periodτid Ignition delayτp Side droplet propagation time periodτs Flame spread timeρl Liquid densityAbbreviationsB BiodieselBE Ethanol-blended biodieselBE-A1 Biodiesel blended with ethanol combustion characteristics during homo-geneous nucleationsBE-A2 Biodiesel blended with ethanol combustion characteristics after homoge-neous nucleationsB-A Biodiesel combustion characteristics after atomizationB-NA Biodiesel combustion characteristics before atomizationD DieselDAQ Data Acquisition BoardDLS Dynamic Light ScatteringDMF 5-dimethylfuranE EthanolEDX Energy-Dispersive X-ray SpectroscopyFFT Fast Fourier TransformFT-IR Fourier-Transform InfraredGO Graphene OxidexxixNOMENCLATUREhGO Highly oxidized Graphene OxideJO Jatropha oilJPDF Joint Probability Density FunctionODAGO Octadecyl amine functionalized Graphene OxidePDF Probability Density FunctionPSD Power Spectrum DensitypGO Partially oxidized Graphene OxiderGO Reduced oxidized Graphene OxideRHS Right-Hand-SideRMS Root Mean SquareSEM Scanning Electron MicroscopyTGA Thermogravimetric AnalysisUV Ultra VioletXRD X-Ray Diffraction SpectroscopyxxxAcknowledgmentsI am immensely grateful for the guidance and support provided by my supervisor, Dr.Sina Kheirkhah, throughout the entire journey of my doctoral studies. He has been aninvaluable mentor, offering not only expert academic advice but also fostering a nurtur-ing environment that allowed me to flourish as a researcher. Dr. Kheirkhah consistentlydemonstrated a profound commitment to my academic and personal development, provid-ing constructive feedback, encouragement, and insights that greatly enhanced the quality ofmy work. His unwavering support and dedication have played an instrumental role in thesuccessful completion of this dissertation, and I am truly fortunate to have had him as mymentor.I extend my sincere appreciation to the members of my supervisory committee, Dr.Mohammad Arjmand and Dr. Ian Foulds, for their valuable input and constructive feedback.Their insightful comments and thoughtful suggestions have significantly contributed to therefinement of this dissertation. I am grateful for the time and expertise they generouslyshared, which undoubtedly strengthened the overall quality of my research.I am grateful for the financial support from the Natural Sciences and Engineering Re-search Council (NSERC) of Canada as well as the company Zentek.I would also like to thank Dr. Colin van der Kuur, Dr. Ehsan Ebrahimnia-Bajestan,Dr. Gethin Owen, Dr. Sudip Shrestha, Dr. Seyyed Arash Haddadi, Dr. Ahmadreza Ghaf-farkhah, Dr. Robert Godin, Dr. Matthew Brown, Negin Jalili, Majed Amini, and SutriptoKhasnabis, for their assistance with the nanomaterials preparation, and characterization.xxxiThroughout my doctoral studies, I undertook experimental setup development, a taskmade possible by the invaluable support of Raymond Seida, Tim Giesbrecht, Praveen Rajan,Durwin Bossy, Ryan Mandau, Jiyee Yoon, Chris Seib, Emily Zhang, and Alec Smith. Iam deeply grateful for their assistance and appreciate their contributions to the successfulexecution of these endeavors.I extend my gratitude to numerous friends at the University of British Columbia for theinsightful discussions that greatly contributed to the development of my research. Specialthanks to Dr. Sajjad Mohammadnejad Daryani, Leslie Saca, Deepanshi Sisodiya, BehroozKhatir, Ali Rostami, Omid Niksan, Vladimir Shevliakov, Milad Goodarzi, and HamounNabilou for their valuable contributions and support.Finally, I would like to thank my family that helped and supported me during myPh.D. studies. I am particularly grateful for the unwavering love and support of my wife,Ladan Tazik, my mother, Narges Torkjazi, and my father, Hamidreza Mosadegh. Theirunconditional encouragement has been my anchor, empowering me to confront and overcomethe challenges and hardships that I encountered throughout these years.xxxiiDedicationThis dissertation is a heartfelt dedication to my wife, Ladan, whose continuing support,boundless encouragement, and enduring love have infused profound meaning into everystep of my academic journey. Additionally, I extend this dedication to my parents, Nargesand Hamidreza, whose nurturing love and unwavering belief in my potential have been theemotional bedrock, shaping the very essence of my educational pursuits.xxxiiiChapter 1IntroductionThis chapter is organized in four sections. The motivation of the thesis is presented inthe first section. In the second section, the background concerning the single and multi-droplet combustion is presented. This background is then used to highlight the gap in theliterature towards the end of this section. The thesis objectives are presented in the thirdsection. Finally, the outline of the thesis is presented in the fourth section.1.1 MotivationThe resources which provided for the world’s energy consumption over the past fewdecades as well as the contributions of these resources to our future energy consumption by2050 were studied and presented in a report by the International Energy Agency [5]. Theinformation adapted from [5] is presented in Fig. 1.1. As can be seen, liquid fuels, namelypetroleum and biofuels, have been serving and will continue to serve as the primary resourcesof the world’s energy in the foreseeable future. Specifically, the consumption of energy rele-vant to liquid fuels is increasing from about 170 × 1015 Btu in 2020 to about 250 × 1015 Btuin 2050. Such significant increase for liquid fuels consumption is due to the existing andwell-developed infrastructure in many sectors including transportation, power generation,petrochemical, as well as residential and commercial [11]. The transportation sector, alone,consumed about 29% of the worldwide energy, accounting for approximately 50% of theenergy produced globally from liquid fuels [12] in 2019. The combustion engines used in thetransportation sector feature a relatively long lifespan, e.g., 25–30 years for aviation and11.1. MOTIVATIONmarine engines, and nearly 15 years for road vehicles [13]. This thesis is concerned with theliquid fuels used in some of these engines, and given the above discussion, the liquid fuelsdiscussed later will serve as our primary resource for many types of transportation and forthe foreseeable future. Despite the widespread application of liquid fuels, their combustionin relevant engines suffer from two major drawbacks. First, the traditional liquid fuels arelimited natural resources; and, second burning the fuels can pollute the environment [14].Therefore, while it appears that we will continue to burn liquid fuels to transport, improve-ments should be made to reduce the fuel consumption and the emission of pollutants fromthe relevant engines [14].2010 2020 2030 2040 2050050100150200250Fig. 1.1. Past, current, and future projection of the global energy consumption by fuelresource. The data in the figure is adapted from [5]. The data related to nuclear, renewables,natural gas, coal, as well as liquid fuels are highlighted by circular, triangular, square, cross,and diamond shape symbols, respectively.Of relevance to the present thesis is the diesel fuel which features a relatively high de-mand among all petroleum products [11]. Diesel is used in a multitude of sectors includingtransportation (road, rail, marine, and aviation), power generation (in remote areas and21.1. MOTIVATIONduring emergencies), construction activities (heavy machinery and mining trucks), agricul-ture (powering tractors, combines, and irrigation pumps), oil and gas industry (for drilling,pumps, and compressors), heating needs (across residential, commercial, and industrialdomains), as well as military and defense applications, including tanks and armored vehi-cles [15, 16]. Aiming to mitigate the first drawback of liquid fuel consumption discussedabove, i.e. these fuels being limited natural resources, renewable biofuels such as ethanoland biodiesel are becoming popular for blending with traditional diesel [14]. The blending ofbiodiesel and ethanol with diesel yields a more environmentally friendly product by reducingthe overall greenhouse gas emissions such as carbon dioxide, nitrogen oxides, and unburnedhydrocarbons, as well as other pollutants including carbon monoxide, sulfur dioxide, andparticulate matter [14, 17]. Such blending is especially desirable since the above renewablefuels can be integrated with current combustion technologies with relatively minimal mod-ifications [5]. Ethanol and biodiesel are also shown to form stable suspensions when mixedwith diesel [18–25], suggesting the above blends can be used as “drop-in” fuels.In addition to blending, doping of (renewable) liquid fuels with nanomaterials has provencapabilities for improving combustion characteristics, such as generated power and emission[17, 26–30]. The improvement primarily arises from the significant specific surface area ofthe nanomaterials, leading to their enhanced thermal properties, reactivity, and catalyticbehavior [31]. Besides combustion performance and emissions, nanomaterials doped liquidfuels exhibit enhanced suspension stability compared to fuels with larger doping agents. Thischaracteristic renders nanomaterials doped liquid fuels well suited for integration as drop-in fuels. Compared to nanomaterials, such as those that contain metal based compounds,carbonaceous nanomaterials feature accessibility, relatively large energy density, and absenceof toxic metal-oxide emissions. This is advantageous for the carbonaceous nanomaterialscompared to the metal-based counterparts [17, 31, 32].Various carbonaceous nanomaterials are considered as fuel additives, including graphite-31.1. MOTIVATIONbased, graphene-based, carbon nanotube, and carbon black [17]. Among these, GrapheneOxide (GO) stands out as a promising choice due to its remarkable thermo-physical andoptical properties, significant energy content [32], and the flexibility to incorporate variouschemical functional groups [17]. This not only enables the suspension of GO in a wide rangeof fuels, but also provides an opportunity to further customize the GO reactivity. Whilefacilitating the mentioned improvements, laboratory-grade GO can be relatively inexpensiveto synthesize and is accessible [17, 27, 33, 34]. Nevertheless, the cost of production may befurther reduced provided such additives are used at large-scale.In addition to the mentioned enhancements, doping of liquid fuels with nanomaterials(such as GO) and blending them with volatile biofuels (e.g. with ethanol), can lead tothe atomization of fuel droplets [26, 35]. This phenomenon arises due to the buildup ofinternal pressure within the droplets, caused by the confinement and superheating of morevolatile species (such as ethanol in comparison to diesel and biodiesel) or nucleation aroundnanomaterials within the droplets [26, 35]. The atomization of the droplets leads to the for-mation of smaller “baby” droplets which evaporate more rapidly (compared to the “mother”droplet), achieving faster air mixing, and burning more efficiently [26, 35] which can implythe followings. First, an equivalent mass of fuel can undergo combustion at an acceleratedrate, thereby (potentially) requiring less space for complete burning. This, in turn, resultsin an increased ratio of power generated by the engine to its size. Second, the mismatch involatilities or boiling points, which drives the atomization process for blended fuels, facili-tates generating high temperature environment which can accelerate the combustion of lessvolatile fuels, making their utilization more feasible without considerable concerns abouttheir vaporization rates. Third, the atomization of droplets enables a reduction in the en-ergy required to spray the fuel while maintaining a specific droplet size distribution. Thismeans that larger droplets formed during fuel injection can eventually undergo atomizationsleading to the formation of smaller droplets. This is generally attractive and required for41.1. MOTIVATIONimproved spray combustion [26].The modus operandi of several engineering application that burn liquid fuels is spraycombustion. To improve and customize the combustion characteristics of conventional liquidfuels that are doped with nanomaterials and blended with volatile compounds, developnumerical models that are capable of simulating the spray combustion of the above fuels,and successfully design novel combustors that burn the sprays, it is imperative to understandthe effects of the atomizations on spray combustion. The regimes of spray combustion areidentified based on the droplet number density (i.e. the number of droplets per volume).Following the discussions presented in [6], four regimes of spray combustion can be identified,with their schematic presented in Figs. 1.2(a–d). In the figure, moving from (a) to (d), thespray number density decreases. In the figure, the black solid circles represent the dropletsand the red circles show the flame. The region occupied between the dashed black circlesin Fig. 1.2(a) and that inside the dashed circle in Fig. 1.2(b) highlight where the dropletand significant fuel vapor coexist together. As shown in Fig. 1.2(a–d), reducing the dropletnumber density leads to the regime of spray combustion changing from a global flame aroundvaporizing group of droplets to groups of droplets combustion and eventually single dropletcombustion. Thus, understanding spray combustion requires development of knowledgeat a more fundamental level concerning single or multi-droplet combustion configurations.Such configurations allow for (ideally) removing the effect of surroundings (e.g. turbulence)on combustion and focusing on how nanomaterials or volatile addition impact the liquidfuel combustion. Despite the relatively large body of research available in the literature,our understanding related to the combustion characteristics of doped with nanomaterialsor blended with volatile compounds single or multi-droplet combustion is limited and thisrequires further investigations.51.2. BACKGROUNDFig. 1.2. Schematics of the regimes for spray combustion, with the spray number densitydecreasing from (a) to (d). (a–d) represent external sheath combustion, external groupcombustion, internal group combustion, and collection of single droplet combustion. Theschematics in the figures are adapted from [6] and are modified.1.2 BackgroundOf relevance to the present thesis are the atomization characteristics, how fast a singledroplet is ignited and burnt, the combustion dynamics of droplets, and how fast flamespreads in a multi-droplet configuration, with the relevant background discussed in thefollowing. Finally, the presented discussions are used to highlight the gap in the literature.1.2.1 Droplet atomizationIn this thesis, similar to the definition provided in [26], atomization is referred to theprocess that leads to the generation of baby droplets from a mother droplet. It is importantto note that, in some engineering applications that concern sprays, a column/jet of liquidis injected in a cross-/co-flow, and the column/jet is broken up into droplets. Such processis also (and traditionally) referred to as the atomization in the combustion/fluid mechanicsliterature, see for example [36, 37]. This is not of interest in the present thesis. In thecontext of droplet combustion, it is shown that doping and blending of liquid fuels with61.2. BACKGROUNDnanomaterials and biofuels may lead to the occurrence of the atomization see for example [24,26, 38–45]. The atomization phenomenon in burning droplets is comprised of several stages,namely, nucleation and bubble growth, droplet surface rupture, ligament formation and itsbreakup, and the ejection and ignition of baby droplets and fuel vapor [26]. These stagesare schematically shown in Fig. 1.3 and described below.(a)Bubble(s) formation(b)Bubble growth and surface rupture(c)Baby droplet(d)FlameFig. 1.3. Schematic presenting the stages of droplet atomization. (a) presents the formationof bubbles due to homogeneous and/or heterogeneous nucleation. (b) shows merging of thebubbles as well as their growth and reach to the surface. (c) presents the rupture of themother droplet surface, which is followed by the ejection of fuel vapor as well as ligamentsformation and their break-up forming baby droplets. (d) shows the ejected fuel vapor andbaby droplets reach the flame area, get ignited, and burn.Two types of nucleation are identified for droplet combustion and are elaborated in theliterature. The first type, referred to as the homogeneous nucleation, occurs at the liquid-liquid interface of multi-component fuels which feature significant differences in the volatilityof the constituent components [43, 44]. In the droplet combustion of blended liquid fuels(which usually feature a mismatch between the volatilities of the constituent components), asthe droplet burns, the concentration of the more volatile component at the droplet surfacedecreases due to the larger rate of evaporation for the volatile component compared tothat of the less volatile component. Thus, the concentration of less volatile component atthe droplet surface increases during the droplet combustion compared to that of the morevolatile component. Subsequently, the less volatile component at the droplet surface isheated up to its boiling temperature, which is higher than the boiling temperature of themore volatile component. As a result of this, the more volatile component in the dropletcore is superheated by the heat transfer from the less volatile component around it. Such71.2. BACKGROUNDsuperheating of the more volatile liquid in the droplet leads to the nucleation and bubbleformation at the interface of the liquid fuel components, which is referred to as homogeneousnucleation [35]. Compared to this, the second type of nucleation occurs at the liquid-solidinterface and is referred to as the heterogeneous nucleation. The solid interface is eitherthe droplet suspending mechanism, the doping nanomaterials, or both [26, 35, 38]. For theheterogeneous nucleation, the flame radiative heat is absorbed at the solid surface, leadingto the heating of the liquid around it. This heating, in turn, results in the formation ofbubbles. Pandey et al. [46] studied the nucleation of dodecane droplets doped with ceriumoxide nanomaterials. Performing timescale analysis, they [46] observed that the internalboiling of the droplet only occurs provided the droplet combustion lifetime is larger thanthe sum of the time required for nanomaterial aggregates to form and the time required forbubbles around them to grow. It was discussed in [46] that, for the aggregation to happen,the particles must move and collide with each other. Inside the droplet, the particles movedue to particle diffusion and internal flow patterns [46]. Comparing the particle diffusionand internal flow timescales, they [46] showed that the aggregation time is dominated bythe internal flow patterns. Then, comparing the bubble formation and growth timescaleswith the droplet lifetime, Pandey et al. [46] showed that a minimum initial droplet sizeexists for boiling to occur. This minimum droplet size decreases with increasing the dopingconcentration since the presence of more nanomaterials facilitates the aggregation and as aresult boiling [46].The growth and coalescence of bubbles inside the droplet are influenced by the dropletinternal circulatory flow patterns, transfer of heat to the bubbles, and the surface regressionof the main droplet that lead to formation of relatively large bubbles [26, 30, 38]. Onceinternal boiling is triggered and large bubbles are formed, they follow the droplet internalflow pattern and reach the droplet surface. This leads to the droplet surface rupture,followed by ligaments formation, see the schematic in Fig. 1.3(b). Finally, the ligaments81.2. BACKGROUNDmight breakup, and as a result, the baby droplets and fuel vapor eject from the droplet,which might burn later in the flame [39–41, 47–50], see Figs. 1.3(c and d). Miglani et al. [49]showed that the breakup and subsequent ejections are strongly influenced by the ratio ofbubbles to droplet volume. The above described processes (from nucleation to ejections)may repeat several times during the main droplet lifetime.The intensity of the atomization events depends on the type of nucleation. For dropletsatomizing due to homogeneous nucleation, as the volatility difference between the con-stituent components increases, the amount of superheating increases, which results in ahigher nucleation rate and more intense atomizations [35]. Also, for fuel blends (with dif-ferent volatilities of the constituents), as the concentrations of the blending componentsbecome similar, the atomizations become stronger [35]. For heterogeneous nucleation oc-curring at the nanomaterials aggregate surfaces and for moderate doping concentrations,the nanomaterials form a shell at the droplet surface that is continuously ruptured duringthe atomization events [40]. However, for excessively large doping concentrations of thenanomaterials, the shell is relatively strong, which traps the bubbles, and suppresses theatomization.1.2.2 Ignition delay and burning rate of the doped dropletsPrior to the burning and once the droplet is introduced to an ignition source, it takessome time for the droplet to heat up and ignite. This time period is referred to as theignition delay [4, 51–53]. The ignition delay depends on many parameters such as thethermal power of the ignition source, the size of the droplet, the proximity of the ignitionsource to the droplet, etc. As such, comparisons of ignition delays between different studiesare challenging at best. Possibly influenced by the above parameters, past investigations[4, 51–53] show some controversy in the reported results related to the effect of nanomaterialsaddition on liquid fuel ignition delay. On one hand, Ooi et al. [51, 52] reported that the91.2. BACKGROUNDignition delay of diesel and biodiesel decreases 47% by addition of 0.01–0.1% by weightgraphite oxide. On the other hand, Bennewitz et al. [4] reported adding 1% by weightof graphene nanomaterials increases the ignition delay of rocket-grade kerosene by 20%.Similar to Bennewitz et al. [4], Singh et al. [53] reported that the ignition delay increasesabout 3% and 10% by adding 3% by weight of acetylene black nanomaterials to petrodieseland biodiesel, respectively.The droplet burning rate is defined as the negative of the derivative of the main dropletdiameter squared (D2) with respect to time (t) [35], with t = 0 marking the time at which aflame is formed around the droplet. Doping of liquid fuels with nanomaterials influences theburning rate of the droplets through two mechanism [26–28, 38–41, 47–52, 54–58]. First,doping with nanomaterials influences the heat transfer to the liquid fuel. This can potentiallyincrease the evaporation rate and as a result the droplet burning rate [26–28, 38, 51, 52, 54–57]. Second, the presence of the nanomaterials leads to the occurrence of the atomizationevents and ejection of droplet mass in the form of baby droplets [46] which could impactthe droplet burning rate. While the first mechanism and its effect on the burning rateis studied thoroughly, the second mechanism is not studied extensively. Specifically, paststudies reported the amounts of improvement in burning rate as a result of doping bynanomaterials, but did not elaborate how such improvements are achieved. Acknowledgingthis gap, in the following, the effect of nanomaterials doping on the droplet heat transferand evaporation is reviewed first. This is then followed by reviewing the literature thatreported the burning rate alteration as a result of doping by nanomaterials.Addition of nanomaterials to liquid fuels can change conductive, convective, and ra-diative heat transfer to the droplet, which, in turn, influence the droplet burning rate[26–28, 51, 52, 54–57]. The thermal conductivity is highly influenced by the type of theadded nanomaterials and their concentration, temperature of the fuel and nanomaterialssuspension, and the size of the aggregates. Generally, increasing the doping concentra-101.2. BACKGROUNDtion [59–61], increasing the suspension temperature [61, 62], and decreasing the size of thenanomaterial aggregates [61, 62] are expected to increase the thermal conductivity. Forexample, Agarwal et al. [63] and Aboalhamayie et al. [55] reported that adding 0.2 and3% by weight of carbon nanotubes and graphene nanoplatelets to jet fuel and kerosenemay increase the thermal conductivity by 23% and 29%, respectively. The increase of thethermal conductivity by the addition of nanomaterials is due to enhancement of the Brow-nian motion [64, 65] and generation of solid-liquid interface thermal bridge [66–68] insidethe droplet. However, excessive doping of the liquid fuels with nanomaterials may leadto formation of aggregates, which may or may not increase the fuel thermal conductivity.For example, studies of [67, 69, 70] reported that nanomaterials aggregation increases thethermal conductivity, while studies of [38, 62] suggest nanomaterials aggregation decreasesthe thermal conductivity.Compared to the conductive heat transfer, the convective heat transfer inside the nano-materials doped liquid fuel droplets is improved as a result of circulatory flow inside thedroplets which influences/is influenced by aggregation of the nanomaterials and their mor-phology [32]. Moreover, the Brownian motion of the particles induce micro-convection inthe droplet that enhances the convective heat transfer [71].Radiation heat transfer can also be improved by the addition of nanomaterials [28, 51, 52,54, 56, 57]. Majority of liquid fuels are relatively transparent to the radiation wavelengthsemitted from the flame. However, the addition of carbonaceous nanomaterials decreasesthe droplet optical transmittance, which increases the radiation absorption [28, 54]. Theamount of increase depends on the emission wavelength and the droplet size, as shown in,for example [28].Addition of nanomaterials to liquid fuels alters the heat transfer to/from the droplet,which influences the droplet evaporation rate, see for example [30, 55, 56, 72–74]. Resultsof Tanvir et al. [56] suggest that the circulatory flow inside the droplet accumulates the111.2. BACKGROUNDnanomaterials at the droplet surface, creating a porous shell. This shell formation mayeither positively or negatively impact the evaporation rate. On one hand, the shell formationdecreases the convective heat transfer from the surrounding to the liquid, which limits theenhancement of the evaporation rate by the addition of the nanomaterials [26]. Also, oncethe porous shell is formed at the droplet surface, the liquid fuel permeates from the dropletcore through the porous shell and to the droplet surface. This phenomenon may negativelyinfluence the evaporation rate. On the other hand, the addition of the nanomaterials canlead to the droplet temperature rise [26], which enhances permeation of the liquid fuelthrough the porous shell [41] and potentially increasing the evaporation rate. Both theformation of the porous shell as well as the droplet temperature rise may lead to competingeffects on the evaporation rate, and the resultant effect depends on the type of the utilizednanomaterials and the amount of the nanomaterials doping. For example, Gan and Qiao [54]used a mercury lamp and investigated the effect of radiation absorption on the evaporationrate of their droplets. Their results suggest that, for a fixed loading concentration of 0.1% byweight and at a radiation intensity of 175 W, the addition of multi-walled carbon nanotubesto ethanol increased the evaporation rate by 26%. However, this increase was 6% for ethanoldoped with carbon nanoparticles for an identical lamp radiation intensity.The combined effects of heat transfer to the droplet, evaporation, and atomization (withthe details discussed above) dictate how fast the droplets burn. Many past studies investi-gated the burning rate of doped fuels, see for example [27–29, 38, 51–54, 57, 75–80]. Resultsof Ghamari and Ratner [38] suggest that there exists an optimum doping concentrationat which the burning rate is maximized. Specifically, up to about 10% enhancement ofjet fuel burning rate was reported by adding 1.5, 0.25, and 0.1% by weight of activatedcarbon nanoparticles, multi-walled carbon nanotubes, and graphene nanomaterials, respec-tively. Similarly, Ooi et al. [51, 52], Singh et al. [53], and Yadav et al. [75] added 0.01–0.1%graphite oxide, 2% acetylene black nanomaterials, and 0.2% by weight of graphene nano-121.2. BACKGROUNDmaterials into diesel, biodiesel, and rocket-grade kerosene; and, they reported about 29, 13,and 14%, increase in the burning rate, respectively. Decreasing the nanomaterial size canincrease the burning rate of the liquid fuels. For example, Tanvir et al. [28] showed thatdecreasing the graphite nanomaterials size from 100 to 50 nm can increase the burning rateof ethanol doped droplets by about 64%. Compared to the effect of decreasing the nano-materials size on the burning rate, excessive doping and poor suspension stability may leadto a decrease of the burning rate [38].1.2.3 Combustion dynamics of atomizing dropletsThe influence of atomization on combustion dynamics of a droplet has been studiedin the past, see for example [41, 48, 49]. For relatively dilute doping concentrations ofnanomaterials, Miglani et al. [49] showed that moderate atomizations occurred for theirdroplets which led to small-scale droplet deformation. However, for larger doping concen-trations, they showed that relatively intense ejections occurred which led to large amplitudedroplet surface deformations and droplet oscillations. Regardless of the doping concentra-tion, Miglani et al. [48] showed that the ratio of the bubbles volume to that of the dropletsignificantly influences the droplet breakup, the atomizations, and as a result the dropletsurface deformation and oscillations. In a later study by Pandey et al. [41], the relationbetween the droplet and its flame chemiluminescence was investigated. They [41] showedthat, for ethanol mixed with water droplets, the droplet shape and the flame chemilumines-cence featured oscillations at about 32 and 43 Hz, respectively. It was shown [41] that theaddition of nanoceria particles to these droplets decreases both of the above frequencies toabout 13 Hz. Their [41] results suggest that the droplet shape deformation and the flamechemiluminescence oscillations are coupled through two mechanisms. First, as discussed in[48], the atomization events induce the droplet shape deformation. This changes the fuelvapor flux into the flame which alters the flame chemiluminescence. Second, the ejected131.2. BACKGROUNDbaby droplets are ignited as they reach the flame, and this induces alteration of the flamechemiluminescence. The above reported mechanisms were further expanded to include theeffect of acoustics by Sim et al. [81]. They [81] studied the coupling between acoustics andthe burning of nanoaluminum doped ethanol droplets. They [81] showed that the burningrate enhances as a result of acoustically forcing the droplets. Such forcing at two frequenciesof 898 and 332 Hz was performed in [81], and larger enhancement of burning was reportedat 898 Hz compared to that at 332 Hz.Though many investigations have been performed to understand the effect of atomiza-tion on the dynamics of a single droplet combustion, only some [44, 82–84] reported theoccurrence of the atomization events in a multi-droplet combustion configuration and afew [44, 82] discussed the influence of the atomizations on the multi-droplet combustiondynamics. Meng et al. [44, 82] studied the atomization events in heated multi-dropletcombustion of ethanol blended with biodiesel. They showed that decreasing the differencebetween the concentration of biodiesel and ethanol leads to the occurrence of intense at-omizations [44]. They also reported that increasing the environment temperature increasedthe atomization intensity and as a result this decreased the lifetime of the droplets [44]. Fora given background gas temperature, they showed that decreasing the size of the dropletsincreases the probability of intense atomizations [82].Antonov et al. [83] studied the atomizations for two droplets of rapseed oil and wateremulsions placed in a high temperature air stream. They showed that the atomizationbegins sooner for the lead droplet compared to the downstream droplet, and as the inter-droplet spacing increases, the time difference between the first atomization events of thedroplets decreases. This was attributed to the effect of leading droplet on the heat transferto the downstream droplet [83]. Antonov et al. [84] extended their studies to a three-droplet configuration and confirmed that similar to a two-droplet configuration, as the airtemperature increased, the atomizations began sooner.141.2. BACKGROUND1.2.4 Flame spread in an array of dropletsThe spread of flame in an array of droplets has been investigated in several past studies,see for example [9, 85–92]. Review of past work suggests that the spacing between thedroplets (referred to as the inter-droplet spacing), the background gas temperature and itscomposition, as well as the size of the droplets play a major role in the characteristics ofthe flame spread.The inter-droplet spacing allowed to identify the modes of flame spread [85, 86]. For arelatively small inter-droplet spacing normalized by the initial droplet diameter, while thediffusion flame of the main droplet grows (due to its evaporation), the side droplet becomesswallowed into the main droplet flame [85, 86]. The heat from the main droplet flameevaporates the side droplet, leading to its ignition. This is referred to as Mode 1 of flamespread between two droplets in Mikami et al. [85, 86]. For relatively larger inter-dropletspacing, the leading edge of the diffusion flame reaches the flammable mixture layer thatis formed around the side droplet and the flame propagates in the fuel and air mixturelayer to form the diffusion flame around the side droplet. This is referred to as Mode 2of flame spread [85, 86]. For significantly larger inter-droplet spacing, the side droplet isauto-ignited through heating by the main droplet diffusion flame, whose leading edge doesnot reach the flammable mixture layer around the side droplet. This mode of flame spreadis referred to as Mode 3 in [85, 86]. For all of the above discussed modes, the rate of flamespread is quantified using SD0/τs, with S, D0, and τs being the inter-droplet pacing, theinitial diameter of the droplets, and the spread time respectively. As discussed in [85, 86],the spread time is the time it takes for the droplet to which the flame is propagating to(referred to as the side droplet) to be enveloped by the propagating flame.The effects of background gas temperature and composition on the flame spread ratewere studied in Mikami et al. [85] and Nomura et al. [87]. They showed that increasingthe background gas temperature increases the rate of droplet evaporation, decreasing the151.2. BACKGROUNDspread time, and as a result increasing the rate of flame spread [85]. Nomura et al. [87]showed that increasing the fuel-air equivalence ratio of the background gas increased therate of flame spread between the droplets. They [87] showed the influence of the backgroundgas equivalence ratio on the flame spread rate became more pronounced as the inter-dropletspacing increased. The effect of the droplets diameter on the flame spread rate was studiedin Mikami et al. [88]. Their results showed that the flame spread rate decreased withincreasing D0. This is because the time it takes for the side droplet to heat up due toconduction increased with the side droplet diameter times a thermal conduction time, withthe latter also positively relating to D0.Although the above studies are of significant importance as they provided insight intothe modes of flame spread in an array of droplets as well as the effects of governing pa-rameters on the flame spread rate, the investigations are performed for fuels/test conditionsthat do not yield the droplet atomization. Understanding the effect of atomization on theflame spread rate in an array of droplets is important, as majority of the fuels used in en-gineering applications are blends of constituents with different volatilities, which promoteatomizations. Despite the above importance, a few recent studies [89–91] only observed andreported the atomization events during the flame spread in a multi-droplet configuration.For example, Wang et al. [89] argued that the molecules produced by pyrolysis of fattyacids in jatropha oil (which is their utilized fuel) feature small boiling point, and initiatetheir observed atomizations in a multi-droplet configuration. Chen et al. [90] showed anarray of falling dodecane and water emulsion droplets undergo atomization during combus-tion. They [90] showed that, once the atomization occurs, the main flame expands and mayspread to downstream droplets. Zheng et al. [91] studied the combustion of Kerosene in atwo-droplet configuration. They [91] showed that the maximum inter-droplet spacing forwhich successful ignition of side droplet occurs increases due to the occurrence of atomiza-tions. To the author’s best knowledge, only the studies of Yamada et al. [92] and Wang et161.2. BACKGROUNDal. [9] aimed to relate the flame spread in an array of droplets to atomizations. Specifically,Yamada et al. [92] investigated the flame spread rate in an array of n-dodecane and wateremulsions. They showed that the flame spread rate decreased with increasing the waterconcentration. They also showed that the flame spread rate of the oil in water emulsionwere more than those of the water in oil emulsions. Recently, Wang et al. [9] measured theflame spread rate in a one dimensional array of Jatropha oil blended with dimethylfuran.They [9] showed that, for a given inter-droplet spacing, the addition of dimethylfuran (morevolatile component) to Jatropha oil increased the flame spread rate.1.2.5 Knowledge gapsThe above review of literature shows that, even though numerous studies are performedin the past few decades to understand the combustion of liquid fuels blended with biofuelsor doped with nanomaterials in a single or multi-droplet configuration, several gaps ofknowledge exist and many questions remain to be addressed. The identified knowledge gapsare listed and elaborated in the following.1. Even though several studies reported the occurrence of atomization in nanomaterialdoped fuels, see for example [38, 51, 53, 57], our quantitative understanding relatedto the characteristics of this phenomenon is limited and this requires further investi-gations. For example, the trajectory and velocity of the ejected baby droplets as aresult of atomization, as well as how such ejections influence the droplet burning rateare not quantified in the literature.2. Only a few studies, see for example [41, 81], have investigated the droplet and flameoscillations for atomizing droplets. Though Pandey et al. [41] elaborated the cou-pling mechanism between the droplet and its flame; and, they [41] quantified thefrequency/frequencies of the oscillations, it is yet unclear why and how much the dop-ing with nanomaterials influence the reported frequencies. Especially, it is unclear,171.3. OBJECTIVEScompared to the nanomaterials used in [41, 81], what the frequency of oscillations isfor liquid fuels doped with carbonaceous nanomaterials such as graphene oxide.3. Though Pandey et al. [41] reported and quantified the dynamics of single fuel dropletscombustion during heterogeneous nucleation, it is unclear if such dynamics also existsduring homogeneous nucleation and how they differ with the heterogeneous counter-part. It also remains to be investigated if the dynamics that are present during thesingle droplet combustion also exists for a burning multi-droplet configuration.4. Though the studies of [9, 89–92] reported the presence of atomizations during thespread of flame in a multi-droplet configuration and observed that they influence theflame spread, the effect of either homogeneous or heterogeneous atomizations on theflame spread rate was not quantified.1.3 ObjectivesGiven the above knowledge gap, the overarching goal of the present thesis is to improveour understanding of atomization events and how they influence the single/multi-dropletcombustion of liquid fuels. Specifically, the four identified gap of knowledge led to definingfour objectives for this thesis, which are presented below.1. This thesis aims to quantify the contribution of atomization to the burning rate ofnanomaterial doped liquid fuels. To address this objective, first, the atomizationevents are characterized by quantifying the ejected baby droplets size, trajectory ofmotion, initial velocity of ejection, and length of travel. Then, the effects of suchatomizations on the evaporation mass loss and the overall droplet burning rate arestudied.2. The second objective of the present thesis is to investigate if the droplet and flamecoupling observed in Pandey et al. [41] for nanoceria doped ethanol also exists for181.4. OUTLINEGO doped fuels. GO doped diesel droplets were considered to study the root-causeof such coupling, and to quantify the effect of GO doping concentration on the abovecoupling.3. Several studies [17, 93–96] suggested that the addition of GO into diesel-biodiesel-alcohol blends improve the overall performance and emissions form diesel engines. Theaddition of GO or blending with alcohols promotes heterogeneous and homogeneousatomizations. The third objective of this thesis is to investigate the effect of suchatomization on the dynamics of both single and multi-droplet combustion.4. The last objective of the thesis is to quantify the effect of the homogeneous andheterogeneous atomizations on the flame spread rate in a multi-droplet configuration.1.4 OutlineThe outline of the thesis is provided below.Chapter 1: IntroductionIn this chapter, the motivation of the dissertation was discussed first. Then, the relevantbackground related to droplet atomization, ignition delay and burning rate, combustiondynamics of atomizing droplets, and the flame spread in an array of droplets were reviewed.Finally, the knowledge gap, the objectives of the thesis, and this outline are presented.Chapter 2: Experimental methodologyIn this chapter, first the liquid fuels and the nanomaterials doping agents studied in thepresent thesis are introduced. Then, the procedures for the preparation and the character-ization of the tested fuels are elaborated. Finally, the developed apparatuses and utilizeddiagnostics are discussed.191.4. OUTLINEChapter 3: Droplet and flame detectionThis chapter provides the detailed post processing steps performed to analyze the col-lected images. First, the methods of droplet(s) detection for different droplet(s) suspensionmechanisms are presented. Then, the necessary data reduction for the detection of the babydroplets are discussed. Next, the procedure for reducing the flame images is explained. Fi-nally, details for estimation of the ignition delay as well as obtaining the location of theflame kernel are discussed.Chapter 4: Contribution of atomization to burning rateIn this chapter, first the results pertaining the ignition delay of the GO doped ethanoldroplet are discussed. Then, the effect of atomizations on the flame behavior is presented.Next, the atomization events and the kinematics of the ejected baby droplets are character-ized. Then, the burning rate of the droplet during the non-atomizing and atomizing periodsare discussed. Finally, using a conservation of mass framework, the effect of atomization onthe droplet mass loss is quantified.Chapter 5: Role of atomization in droplet and flame dynamicsIn this chapter, first, the characteristics of a single atomizing droplet and its flamedynamics are discussed. Next, the atomization intensity is quantified. Then, the relationbetween the above intensity and flame characteristics are investigated. Finally, the droplet,flame, and atomization frequencies for all test conditions are quantified and compared.Chapter 6: Influence of atomization on combustion dynamics and flamespread in multi-dropletsThe dynamics of atomization for single and multi-droplet configurations during bothheterogeneous and homogeneous nucleation is investigated first. Then, the effect of related201.4. OUTLINEatomizations on the flame spread rate are studied.Chapter 7: Conclusions and future workFirst, a summary of Chapters 2–6 are presented. Then, future investigations that couldbe performed with the already developed and available hardware/software as well as thosethat require long-term investments are discussed.21Chapter 2Experimental methodologyIn this chapter, the procedure for preparing the doped/blended fuels, the characteristicsof the doping agents, and the test conditions are presented in the first section. Then, thedeveloped experimental apparatuses and the utilized diagnostics are discussed in the secondsection.2.1 Fuel preparation and characterizationGraphene Oxide (GO) and its variations are used as the doping agent in this thesis.The rationale behind this selection is that, first, the resource (graphite) for synthesizing GOfeatures notable accessibility. For example, the industrial partner of this project ownsa graphite mine in Thunder Bay (Ontario, Canada). This mine carries approximately24 megatonnes of 99% pure graphite, referred to as Albany graphite [97]. Past studies,see for example [98] suggest 1 gr of graphite can be converted to 0.4 gr of graphene ox-ide, i.e. 40% conversion efficiency. This suggests about 0.4 × 24 × 109 = 9.6 × 109 kgof graphene oxide can be synthesized from the industrial partner’s mine. The largestdoping concentration tested in the present study is 0.1% by weight. This means that1 kg of liquid fuel0.001 kg of GO× 9.6 × 109 kg of GO = 9.6 × 1012 kg of liquid fuel can be doped withthe GO produced from the mine of the industrial partner. Using the lower heating value ofdiesel, and that GO heat of combustion is negligible compared to the diesel [99], it is esti-mated that 9.6× 1012kg of diesel ×42.7MJ/kg of diesel = 4.09× 1014MJ can be generatedthrough combustion of the doped diesel. Second, GO has a relatively large energy density,222.1. FUEL PREPARATION AND CHARACTERIZATIONand it features exceptional thermo-physical and optical characteristics (compared to othersolid additives) [17, 27, 32–34]. Third, the chemical structure of GO can be engineered withdifferent functional groups, which allows for improving the stability of the doped fuel sus-pension. These functional groups influence the GO chemical and physical properties, as wellas the combustion characteristics [27, 34, 51, 77, 100–102]. Thus, extensive characterizationof GO is important and was performed, with the details reported in this section.Graphene oxide and its derivatives were suspended in three base fuels: diesel, biodiesel,and ethanol. Biodiesel, ethanol, and their blends emerge as prime candidates for dieselengines owing to their propensity in enhancing combustion emissions [14, 103]. Desirably,these fuels can be mixed with GO, forming stable suspensions [18–25]. The density at roomtemperature, flash point, and boiling point for the above liquid fuels are extracted from thecorresponding safety data sheets and are provided in Table 2.1.Table 2.1. The liquid fuels used in the present thesis and their properties.Base fuel Density (kg/m3) Flash point (oC) Boiling point (oC)Ethanol [104] 789 13 78Diesel [105] 844 65–87 130–365Biodiesel [106] 881 130 > 200In the following, the preparation procedure and characteristics of the GO nanomaterials,as well as the test conditions are presented. Since the fuel preparation and characterizationdepend on the base fuel, the rest of the discussions are grouped into those relating to GOdoped ethanol, GO doped diesel, and GO doped ethanol blended with biodiesel, with detailspresented in subsections 2.1.1, 2.1.2, and 2.1.3 respectively.2.1.1 GO doped ethanolThree types of synthesized GO were suspended in ethanol. Highly oxidized grapheneoxide (hGO) was used, which was synthesized from a high purity (more than 99%) graphitepowder, referred to as Albany graphite [97]. Then, hGO was thermally treated using a vac-232.1. FUEL PREPARATION AND CHARACTERIZATIONuum assisted oven at 120 oC with separate short and long treatment time periods, generatingpartially oxidized graphene oxide (pGO) and reduced oxidized graphene oxide (rGO), re-spectively. Details related to the synthesis procedure of hGO is provided in the Appendix ??.In total, three doping concentrations of 0, 0.01 and 0.1% (by mass) were considered. Alltest conditions for GO doped ethanol are tabulated in Table 2.2. In the table, the first rowpertains to a test condition without the addition of nanomaterials, which is pure ethanol.The first column presents the labels of the test conditions. In this column, ErX, EpX, andEhX refer to rGO, pGO, and hGO added to ethanol with a doping concentration of X% (bymass). The second column presents the type of the base fuel, i.e., ethanol. The third col-umn presents the type of the added nanomaterial to ethanol. The last column is the testeddoping concentration by mass, denoted by [GO]% (by mass). For the purposes of estimatingthe uncertainties of the reported results, each test condition in the table was repeated 12times. In total, 7 test conditions × 12 experiments per imaging technique (discussed later)and per test condition × 2 separate imaging techniques = 168 experiments were conductedfor GO doped ethanol fuel.Table 2.2. The test conditions for GO doped ethanol single droplet combustion.Tested Condition Base fuel Nanomaterials [GO]% (by mass)E Ethanol — 0Er0.01 Ethanol rGO 0.01Er0.1 Ethanol rGO 0.1Ep0.01 Ethanol pGO 0.01Ep0.1 Ethanol pGO 0.1Eh0.01 Ethanol hGO 0.01Eh0.1 Ethanol hGO 0.1To avoid interference with the droplet combustion, no surfactant was used for stabi-lization during the suspension procedure. In fact, it has been shown that relatively stablesuspensions of carbonaceous nanomaterials (for example graphite [28, 56] and functional-ized graphene sheets [27]) in ethanol can be prepared without using surfactants. In order to242.1. FUEL PREPARATION AND CHARACTERIZATIONprepare the suspensions used in the present study, each GO sample was first added to 20 mLof ethanol, which was poured inside a vial. Then, water was added to an ultrasound bath(Elma Ultrasonic Cleaner, EP100H), the vial was placed in the bath, and the suspensionwas sonicated for 60 min. To avoid the fuel temperature rise, the water inside the bathwas replaced by room temperature water every 5 min. The combustion experiments wereperformed immediately after the suspension was prepared.To assess the stability of the suspensions, their photographs were acquired every 5 min.The photographs corresponding to rGO, pGO, and hGO are presented in the first, second,and third rows of Fig. 2.1, respectively. The results in Figs. 2.1(a–c) and (d–f) pertainto doping concentrations of 0.01 and 0.1%, respectively. The results suggest that, for thesmaller doping concentration, the fuels are stable for about 10–15 min; however, for thelarger tested concentration, the fuels are stable for at least one hour. This means that theduration for which our suspensions are stable is at least 10 min × 60 s/min = 600 s. Thisis 120 times longer than the duration of our droplet experiments, which is less than 5 s(discussed later in the chapter 4).Fig. 2.1. Photographs of the doped ethanol. (a–f) correspond to Er0.01, Ep0.01, Eh0.01,Er0.1, Ep0.1, and Eh0.1, respectively.Raman spectroscopy, similar to [107], was performed using the First guard Raman spec-252.1. FUEL PREPARATION AND CHARACTERIZATIONtrometer (from Rigaku) to assess the characteristics of the hGO. The Raman shift was variedfrom 100 to 2300 cm−1 with a resolution of 0.65 cm−1. Figure 2.2 shows the variation ofthe Raman scattering intensity normalized by the corresponding maximum (IR) versus theRaman shift wave-number. Two peaks, highlighted by D and G are detected, which corre-spond to the wave-numbers of 1336 and 1601 cm−1, respectively. The peak occurring at Dpertains to the defects in the structure of GO, likely due to the presence of hydroxyl andepoxide groups [108, 109]. The peak highlighted by G pertains to the planar vibration modeof the carbon atoms that are arranged in hexagonal structures [109]. The Raman spectrumis similar to that previously reported in past investigations related to GO, see for example[98, 109–111] and confirm that the hGO material, indeed, features the molecular structureof the GO.500 1000 1500 20000.40.60.81Fig. 2.2. Raman spectrum of the synthesized hGO.A thermal reduction method [112] was used to reduce the oxygen content of hGO,generating pGO and rGO. Specifically, an Across International vacuum oven (AT09-UL) wasfirst preheated to 120 oC. Then, hGO was heated in the oven for 2 hours generating pGO.Similarly, rGO was generated by heating hGO using the oven, but for 6 hours. ScanningElectron Microscopy (SEM) images were collected to characterize the size of rGO, pGO,262.1. FUEL PREPARATION AND CHARACTERIZATIONand hGO. SEM with high-resolution (9.7 nm/pixel with the field-of-view of 14.9 µm ×9.9 µm) and coarse-resolution (0.5 µm/pixel with the field-of-view of 512.0 µm× 512.0 µm)were performed. For the high-resolution SEM, FEI Helios 650 nanolab was used, with theaccelerating voltage and current being set to 5 kV and 25 pA, respectively. Such relativelysmall accelerating voltage facilitated [113] capturing a relatively detailed structure of theGO nanomaterials. Figures 2.3(a), (b), and (c) show exemplary high-resolution SEM imagesof rGO, pGO, and hGO, respectively. As evident in Fig. 2.3, all tested nanomaterials featurea layered structure similar to that presented in past studies, see for example [114–116]. Theinsets of the images presented in Fig. 2.3 show that the thickness of the stack of 2D layersis smaller than about 100 nm for all tested nanomaterials.Fig. 2.3. High-resolution SEM images of (a) rGO, (b) pGO, and (c) hGO, respectively.Although the SEM results presented in Fig. 2.3 facilitate visualizing the layered structureof the utilized GO owing to the high resolution of the images, they do not allow for measuringa characteristic length of the utilized particles. In order to obtain this, the large field-of-272.1. FUEL PREPARATION AND CHARACTERIZATIONview/coarse-resolution SEM imaging was used. For the coarse-resolution SEM imaging,Tescan Mira 3 XMU was utilized, with the accelerating voltage and current set to 20 kVand 15 pA, respectively. The coarse-resolution SEM images were binarized using MATLAB,and a characteristic length (l) was estimated for each detected particle using l =√4Ap/π,with Ap being the area of particles in a binarized image. For instance, for a circular disk,l equates the diameter of the disk. The Probability Density Function (PDF) of l wasestimated for all types of utilized nanomaterials, and the results are presented in Fig. 2.4.The bin size of the PDF distributions was set to 2.3 µm, and this was selected following atrial and error process, leading to the best presentation of the PDFs. All PDFs at l = 0were set to zero. The results in the figure suggest that the PDF distribution is similarfor all tested nanomaterials, with the most probable characteristic length of about 2 µm.Generally, the thermal behavior of the doped fuel is influenced by the nanomaterial type,the doping concentration, and the size distribution of the added nanomaterials. Since thePDF of the particle size is rather similar for all types of the utilized nanomaterials (seeFig. 2.4), the investigated combustion characteristics are expected to be influenced by onlythe nanomaterial type and the doping concentration for ethanol doped fuels.0 5 10 15 20 2500.10.20.30.4Fig. 2.4. The probability density functions of the utilized nanomaterials characteristiclength. The PDFs are calculated from the coarse-resolution SEM imaging.282.1. FUEL PREPARATION AND CHARACTERIZATIONAlthough the SEM images show that the synthesized nanomaterials feature a relativelysimilar size distribution (for l ≳ 2 µm), further insight related to the chemical characteris-tics of the tested nanomaterials can be obtained using Fourier-Transform Infrared (FT-IR)spectroscopy [117, 118]. The FT-IR experiments were performed using Thermo ScientificNicolet iS20 Spectrometer, with the wave-number (κ) scanning range of 600 to 4000 cm−1and a resolution of 0.48 cm−1. Figure 2.5 presents the FT-IR absorption intensity (IF) forrGO, pGO, and hGO, which are shown by the blue, green, and red colors, respectively.Absorption intensity is defined as the radiant power absorbed by the sample normalized bythe radiant power of the incident light. Several dominant peaks are detected that are relatedto the molecular structure of the tested GO nanomaterials. The results in the figure showthat all tested GOs feature peaks at 1050, 1110, 1240, and 3400 cm−1, which correspond toCO-O-C [119], C-O and C-C [120, 121], N-H [122], and O-H [123] groups, respectively. Thepeaks at 875, 1580, and 1720 cm−1 correspond to C=O [124–126]. The peak at 2360 cm−1is related to a background carbon dioxide gas and is not related to the molecular structureof the tested materials. The results in the figure show a significant presence of carbon andoxygen bonds in the molecular structure of hGO compared to that of pGO and rGO. Thus,it is concluded that the hGO with absorption intensity variation highlighted by the red colorcorrespond to GO with the highest level of oxidization. However, those shown by the blueand green colors feature similar spectra.The results presented in Fig. 2.5 highlighted several relatively dominant peaks (relatedto oxygen-containing groups). However, the variations pertaining to rGO and pGO arerather similar. In order to investigate the differences, the Thermogravimetric Analysis(TGA) was performed using TGA Q500 V20.13, which operated with nitrogen gas. Separate5 mg batches of GO samples were used for the TGA. The temperature was varied fromambient to 800 oC, with an increased rate of 10 oC/min. Variations of the nanomaterialsweight (W ) normalized by that at room temperature (W0) as well as |d(W/W0)/dt| versus292.1. FUEL PREPARATION AND CHARACTERIZATION1000 2000 3000 4000481216Fig. 2.5. Absorption intensity versus wave-number of the GO that was used for dopingethanol. The results are from the FT-IR spectroscopy.temperature were obtained for all tested nanomaterials and are presented in Figs. 2.6(a andb), respectively. W/W0 and |d(W/W0)/dt| related to rGO, pGO, and hGO are shown by theblue, green, and red curves, respectively. Three regions associated with a significant decreasein the normalized materials weight can be identified in the figure. The region highlightedby the blue text pertains to the temperature ranging from the initial temperature of TGAsamples to ∼190 oC and corresponds to the evaporation of the adsorbed water [127–129].Studies of [98, 127, 128] suggest that the weight loss pertaining to the temperature range of∼190 and ∼470 oC is due to the removal of unstable oxygen-containing functional groups,such as CO, COO, and OH [130]. This region is highlighted by the green color text inFigs. 2.6(a and b). The inset of Fig. 2.6(b) shows that, for T ≈ 190 oC, the rate ofweight loss pertaining to the red curve is larger than that of the green curve. Similarly,this parameter associated with the green curve is larger than that of the blue curve. Thissuggests that hGO features the largest oxidation level, followed by pGO, and rGO. Theregion pertaining to T ≳ 470 oC and highlighted by the orange text in Figs. 2.6(a and b)corresponds to the decomposition of carbon from the skeleton of the nanomaterial [131]. In302.1. FUEL PREPARATION AND CHARACTERIZATIONessence, the results presented in Fig. 2.6 suggest that the thermal reduction of hGO, indeed,leads to the preparation of GO with significant and relatively moderate loss of oxygen, whichwere referred to as rGO and pGO, respectively.0 200 400 600 8000204060801000 200 400 600 80013579Fig. 2.6. The thermogravimetric analysis of the utilized nanomaterials for doping ethanol.(a) W/W0 versus temperature and (b) |d(W/W0)/dt| versus temperature.2.1.2 GO doped dieselGraphene oxide functionalized with octadecyl amine groups (ODAGO) was used asthe doping agent in ultra-low sulfur diesel fuel. The presence of such functional groupsenhances the suspension stability of GO in non-polar solvents like diesel. In total, fivedoping concentrations of 0, 0.001, 0.005, 0.01, and 0.02% (by mass) were examined. All testconditions corresponding to diesel as a base fuel are presented in Table 2.3. The first rowpertains to a test condition without the addition of nanomaterials, which is also referred toas the pure diesel. The first column presents the labels of the test conditions. In this column,DX refers to GO added to diesel with a doping concentration of X% (by mass). The secondcolumn shows the base liquid fuel, which is diesel. The third column pertains to the typeof the added nanomaterial to diesel. The last column is the tested doping concentration,denoted by [GO]% (by mass). For the purposes of estimating the uncertainties of the312.1. FUEL PREPARATION AND CHARACTERIZATIONreported results, each test condition was repeated 12 times. In total, 5 test conditions ×12 experiments per imaging technique (discussed later) and per test condition × 2 separateimaging techniques = 120 experiments were conducted.Table 2.3. The test conditions for single droplet combustion of GO doped diesel. ODAGO:Graphene oxide functionalized with octadecyl amine groups.Tested Condition Base fuel Nanomaterials [GO]% (by mass)D Diesel — 0D0.001 Diesel ODAGO 0.001D0.005 Diesel ODAGO 0.005D0.01 Diesel ODAGO 0.01D0.02 Diesel ODAGO 0.02The GO nanomaterials functionalized with octadecyl amine groups were suspended indiesel and the suspensions were sonicated for 60 min prior to each combustion experiment.To assess the stability of the suspensions, their photographs were acquired. The photographscorresponding to D, D0.001, D0.005, D0.01, and D0.02 are presented in the first to fifthcolumns of Fig. 2.7, respectively. The images presented in Figs. 2.7(a and b) correspondto those collected immediately after the preparation of the fuel as well as 5 months later.As can be seen, the doped fuels were stable and no sedimentation was observed during theabove period.Fig. 2.7. Photographs of the doped fuels (a) immediately after preparation, and (b) 5months after preparation.An FEI Helios 650 nanolab was used to study the chemical structure and morphology ofthe GO, performing Energy-Dispersive X-ray Spectroscopy (EDX) and SEM, respectively.322.1. FUEL PREPARATION AND CHARACTERIZATIONThe EDX results showed that the tested GO platelets were mainly comprised of about 80%and 20% by weight of carbon and oxygen, respectively. The SEM imaging was performedwith the accelerating voltage and magnification of 1 kV and 12000, respectively. Figure 2.8shows an SEM image of the stacked GO aggregates after letting the diesel in the dopedfuel to completely evaporate. The layered structure of GO nanomaterials and their stackedplatelets are observed in Fig. 2.8. Despite that the size of the GO platelets suspended inliquid fuel cannot be measured from such SEM (due to aggregation of the platelets), theGO sheet thickness can be obtained and was about 100 nm, see the inset of Fig. 2.8.Fig. 2.8. Scanning electron microscopy of the stacked GO used to dope diesel.To estimate the hydrodynamic diameter (dh) of the GO platelets suspended in dieselthe Dynamic Light Scattering (DLS) experiments were conducted for a representative testcondition of D0.02, using a NanoLab 3D. This device utilizes a 638 nm light source. Thetemperature of GO and diesel suspension and the scattering angle of the DLS measurementswere 25oC and 150o, respectively. The DLS experiments were conducted for a run durationof 45 s, and each run was repeated 10 times. The GO platelets were assumed to have aspherical shape, and the hydrodynamic diameter of the GO platelets was obtained usingthe method of cumulants discussed in Hassan et al. [132]. The PDF of the hydrodynamic332.1. FUEL PREPARATION AND CHARACTERIZATIONdiameter was obtained and is presented in Fig. 2.9. The results showed that the GO plateletssuspended in diesel feature a most probable hydrodynamic diameter of about 300 nm, seeFig. 2.9. Given the hydrodynamic diameter of about 0.3 µm from DLS experiments andplatelet thickness of about 0.1 µm from SEM experiments, it is expected that most of the GOplatelets dispersed in the diesel feature an area of about π/6×0.3 µm3/0.1 µm = 0.15 µm2.Although the SEM image shown in Fig. 2.8 was acquired for agglomerated GO, it containsGO platelets with areas comparable to the estimation above.Fig. 2.9. The Probability Density Function of the GO hydrodynamic diameter for the testcondition of D0.02.2.1.3 GO doped and ethanol-blended biodieselThe GO nanomaterials functionalized with octadecyl amine groups were used to dopetwo liquid fuels: biodiesel as well as 60% (by mass) biodiesel blended with 40% (by mass)ethanol. In total, three GO doping concentrations of 0, 0.01, and 0.1% (by mass) wereexamined. All test conditions corresponding to biodiesel as the base fuel are tabulated inTable 2.4. The first row is the test condition for which biodiesel was used; and, the secondrow is the condition for which ethanol blended with biodiesel was used. No doping agentwas used for the test conditions presented in the first and second rows of the table. The342.1. FUEL PREPARATION AND CHARACTERIZATIONfirst column presents the acronyms of the test conditions. In this column, BX refers to GOadded to biodiesel with a doping concentration of X% (by mass), and BEX refers to X%(by mass) of GO added to ethanol blended with biodiesel. The second and third columnsshow the mass percentage of liquid fuels in the blend. The fourth column is the type of theadded nanomaterial. The last column presents the tested doping concentration, denoted by[GO]% (by mass). For the purposes of estimating the uncertainties of the reported results,each test condition was repeated 12 times. In total, 6 test conditions × 12 experiments perimaging technique (discussed later) and per test condition × 1 simultaneous run of the twoimaging techniques = 72 experiments were conducted.Table 2.4. The GO doped and ethanol blended with biodiesel test conditions. ODAGO:Graphene oxide functionalized with octadecyl amine groups.Tested Condition [Biodiesel]% (by mass) [Ethanol]% (by mass) Nanomaterials [GO]% (by mass)B 100 0 — 0BE 60 40 — 0B0.01 100 0 ODAGO 0.01BE0.01 60 40 ODAGO 0.01B0.1 100 0 ODAGO 0.1BE0.1 60 40 ODAGO 0.1The suspensions of GO and the base fuel were sonicated for 60 min prior to each combus-tion experiment. Figures 2.10 (a), (b), and (c) present the pictures of the vials containingthe fuels immediately, 40 min, and 60 min after their preparation, respectively. The firstto sixth columns pertain to B, B0.01, B0.1, BE, BE0.01, and BE0.1, respectively. As canbe seen, except BE0.1, all doped fuels were stable at least one hour after the preparation.BE0.1 was stable at least for 40 min, which is significantly longer than the duration of thecombustion experiments.Similar to the SEM images presented in Fig. 2.8 for the stacked ODAGO (after dieselwas evaporated), the SEM images of ODAGO after the biodiesel was evaporated was alsocollected and are presented in Fig. 2.11. The results in Fig. 2.11(a) show that small ODAGOplatelets exists among the aggregates. The layered structure of GO can be observed in352.1. FUEL PREPARATION AND CHARACTERIZATIONFig. 2.10. (a), (b), and (c) are the pictures of the fuels immediately, 40 min, and 60 minafter preparation of the fuels. The first to sixth columns pertain to B, B0.01,B0.1, BE,BE0.01, and BE0.1, respectively.Figs. 2.11(b and c). In Fig. 2.11(b), the layers are stacked on each other, while in Fig. 2.11(c)the stacked layers are folded as can be seen by the side view.Fig. 2.11. SEM of the ODAGO; (a) small platelets, (b) stacked two-dimensional layers, and(c) the side view of the folded layers.Raman spectroscopy was performed using the First guard Raman spectrometer to assesscharacteristics of the synthesized graphene oxide. The Raman shift was varied from 100 to2300 cm−1 with a resolution of 0.5 cm−1. Figure 2.12 shows the variation of the Ramanscattering intensity normalized by the corresponding maximum versus the Raman shiftwave-number. Similar to the discussion presented in Section 2.1.1 for hGO, the normalized362.1. FUEL PREPARATION AND CHARACTERIZATIONRaman scattering intensity (IR) of ODAGO features two peaks at the wave-numbers of1343 and 1603 cm−1, which correspond to to the defects in the structure of ODAGO andthe planar vibration mode of the carbon atoms, respectively. The Raman spectrum is similarto that previously reported in past investigations related to GO, see for example [98, 109–111] and confirms the material used, indeed, features primarily the molecular structure ofgraphene oxide.Fig. 2.12. Raman spectrum of the utilized octadecyl amine graphene oxide.X-Ray Diffraction (XRD) experiments were performed to examine the defects and crys-tallinity of the ODAGO. Measurements were performed using a D8 Advance DaVinci, withan Eiger2R 500K detector, using a copper anode. For the measurements, the incident slit,the step size angle, and step time were 0.681 mm, 0.03o, and 8 s, respectively. Twice theincident light angle (2θ) was varied between 5o and 35o. The variation of the XRD signalversus twice the incident angle for the ODAGO is presented in Fig. 2.13. Depending on 2θ,four groups of peaks are identified in the figure, which are indicated by the red ellipses. Forgroup A, the peaks near 5.99o and 7.10o correspond to the intercalation of the octadecylchain as suggested in [133–137]. Group B corresponds to the peaks around 9.98o and 11.87o372.2. APPARATUS AND DIAGNOSTICSwhich appear due to synthesizing GO based materials from graphite, see [133–138]. Group Cinclude peaks at 19.85o, 21.08o, and 23.24o, which are due to the removal of C–O–C elementsfrom the basal plane, see [134–136]. Finally, the peak at 26.48o in Fig. 2.13 correspondsto the existence of a well-structured and layered arrangement defined by the unimpairedgraphitic exterior [133, 134, 136, 138]. The results presented in Fig. 2.13 show that thedominant peaks occur at 2θ = 19.85 o and 21.08o. The inter-layer distance (dl) associatedwith these peaks was calculated using dl = λ/2 sin(θ), with λ being the wavelength of theX-ray employed in the experiments, which was 1.54060 A˚. Specifically, the dominant peaksobserved at 2θ = 19.85o and 21.08o correspond to the ODAGO inter-layer spacing of 0.45 nmand 0.42 nm, respectively.Fig. 2.13. The XRD signal versus twice the incident angle for ODAGO.2.2 Apparatus and diagnosticsDepending on the objective of the experiments, different experimental arrangementswere employed. Overall, three arrangements of experimental apparatuses and diagnosticswere employed. The details of these three experimental arrangements are provided in the382.2. APPARATUS AND DIAGNOSTICSfollowing subsections.2.2.1 Experimental arrangement for the burning rate measurementTo quantify the contribution of atomizations to the burning rate of GO doped ethanoldroplets, high-speed OH∗ chemiluminescence and high-speed shadowgraphy techniques wereseparately employed, which facilitated the detection of the flame and droplet, respectively.Specifically, The experimental arrangements related to the chemiluminescence and shadowg-raphy measurements of GO doped ethanol are illustrated in Figs. 2.14(a and b), respectively.For clarity purposes, an inset of Figs. 2.14(a and b) is enlarged and shown in Fig. 2.14(c).The experimental setup used for the chemiluminescence and shadowgraphy experiments arecomposed of 12 components/modules, which are highlighted by the corresponding numbersin Fig. 2.14. A 20×20×15 cm3 aluminum box (item 1) that supports a tube holder atthe top (item 2) and an ignitor module at the bottom (item 3) were manufactured. Threegrooves on the side walls of the box were machined. The grooves facilitated the motion of atranslation stage, which carried a calibration plate (not shown in the figure). Item 2 is analuminum tube, which was screwed to the box, carried 4 nylon set screws, and held a dropletsuspension tube (item 4). The tube was made of borosilicate, was 150 mm tall, had an outerdiameter of 1 mm, and was the first suspension mechanism in the present study. A propanetorch was used to form a nearly symmetrical bead with a diameter of 1.16±0.01 mm at thebottom of the tube, increasing the surface tension between the bottom of the tube and theliquid, and as a result, improving the droplet suspension. The droplet was deposited on thebead using a 100 µL, model 1710, Gastight syringe from Harvard Apparatus (not shown inthe image). The volume of the GO doped ethanol droplets was fixed, and the correspondingdroplet diameter was 2.14 ± 0.01 mm. Smaller diameter of suspension mechanism did notallow for proper suspension of the droplets. The ignitor utilized a plasma-assisted ignitionstrategy. Such ignition strategy is desirable for droplet combustion research mainly due392.2. APPARATUS AND DIAGNOSTICSto the ability to control the duration of plasma generation [4], the relatively small powerof the ignitor (∼ 20 mW), and the improved repeatability of the ignition process. Theignitor module was composed of a four-prong plasma arc generator (see Fig. 2.14(c)), anignitor control unit, a power source (item 5), a linear solenoid actuator (item 6), as wellas an actuator holder (item 7). Eight set screws are installed on the holder, securing theactuator. Care was taken to position the ignitor module components with respect to thedroplet such that the ignitor arc was positioned about 1 mm below the droplet when theactuator reached its full range of motion. The duration of the plasma arc for igniting GOdoped ethanol droplets at a distance of 1 mm from the droplets was approximately 50 ms.Items 5 and 6 were controlled using an electric relay (item 8) and an NI PCIe 6361 dataacquisition board (item 9).1223145 96781011Fig. 2.14. Technical drawing of the experimental apparatus and diagnostics used for GO-doped ethanol droplet combustion. (a) is the OH∗ chemiluminescence setup, (b) is theshadowgraphy setup, and (c) is a close view of the ignitor and the droplet. The numbereditems are: (1) setup holder box, (2) tube holder, (3) ignitor module, (4) droplet suspensionmechanism, (5) ignitor power source, (6) solenoid actuator, (7) actuator holder, (8) relaymodule, (9) data acquisition unit (10) high-speed camera, (11) image intensifier, and (12)back-light LED.LabVIEW was used to control the board and the diagnostics. The relative timingof the solenoid actuation, ignitor operation, and data acquisition signals are presented inFig. 2.15(a). As shown in the figure, the data acquisition (dotted black curve) begins 60 msprior to the return of the actuator (see the descend in the red dotted dashed curve); and,402.2. APPARATUS AND DIAGNOSTICSthe ignitor operated 10 ms after the start of the data acquisition (see the blue dashed curve)for 50 ms. For both chemiluminescence and shadowgraphy techniques, a high-speed camera(Photron Fastcam Nova S12), shown as item 10 in Fig. 2.14, was used. The imaging acqui-sition frequency for each technique is 2000 Hz. For OH∗ chemiluminescence measurements,a Nikkor 50 mm (f/# = 1.4) lens was used to connect an Invisible Vision UVi 1850B in-tensifier (item 11) to the camera. The intensifier was equipped with a UV Nikon Rayfact105 mm (f/# = 1.4) lens. A 310± 10 nm band-pass filter was installed on the UV lens tocollect the flame chemiluminescence near wavelength of the OH∗ emission. As illustrated inFig. 2.15(b), the camera exposure time was set to 100 µs. The intensifier gate was set to90 µs, and it was centered with respect to the camera exposure time. The intensifier gainwas set to 45%. The pixel resolution of the chemiluminescence measurements (defined as theratio of the field-of-view side length to the corresponding number of pixels) is 44 µm/pixel.Fig. 2.15. (a) The relative timing of the solenoid actuation, ignitor operation, and dataacquisition. (b) The relative timing of the camera and intensifier for OH∗ chemiluminescenceimaging. (c) The timing of the camera for shadowgraphy imaging.For the shadowgraphy experiments, the camera was equipped with a Zeiss Makro-Planar100 mm (f/#=8) lens. As schematically illustrated in Fig. 2.15(c), the camera exposuretime was 1/14000 s ≈ 71 µs. A FotodioX C-300RS FlapJack LED (item 12) was used forbackground illumination. The LED brightness (which was fixed for all experiments) was412.2. APPARATUS AND DIAGNOSTICSobtained using a trial and error process, maximizing the quality of the acquired images.The pixel resolution (with similar definition provided for the chemiluminescence imaging)of the shadowgraphy technique was 38 µm/pixel. Spatial resolution of the shadowgraphymeasurements is estimated using the USAF1951 resolution target, similar to [10], and wasapproximately 44 µm. The details of spatial resolution estimation are provided in theAppendix ??.2.2.2 Experimental arrangement for combustion dynamics measurementAlthough the previously discussed apparatus allowed for measuring the effect of atomiza-tions on the droplet burning rate, it was not suitable for studying the combustion dynamicsof the GO doped diesel droplets. Specifically, the borosilicate tube used as the suspensionmechanism is relatively thick and damps out the droplet oscillations, influencing its dy-namics. Therefore, the suspension mechanism was modified and thin fibers were utilizedto allow for the droplet oscillations. Specifically, silicon carbide (SiC) fibers are preferredcandidates among thin fibers due to their non-combustible nature and feature low thermalconductivity [39], and as a result are extensively used for droplet combustion experiments,see for example [32, 39, 53, 77, 86, 139, 140].The experimental arrangement shown in Fig. 2.14 was refined and the modified setupis shown in Fig. 2.16. Three 142 µm diameter SiC fibers (SCS-6, from Specialty Materials)were utilized as the suspension mechanism. As shown in Fig. 2.16(a), the fibers (item 1) wereplaced inside a horizontal plane. Each fiber formed an angle of 15o with the neighboringfiber. For this suspension mechanism, three factors can potentially impact the droplets,their atomization, and their flame dynamics. First, the presence of the fiber may promotethe occurrence of atomization events through nucleation at the solid-liquid interface. Thefiber size and configuration were identical for all test conditions. Thus, it is anticipated thatthe observed trends which are discussed later in Chapters 5 and 6 are due to the addition of422.2. APPARATUS AND DIAGNOSTICSthe GO nanomaterials. Second, the vibrations of the fiber(s) may interfere with the dropletatomization and their flame dynamics. Although this is a potential influencing factor, ourshadowgraphy imaging confirmed that the fibers are fixed and do not vibrate during theexperiments. Third, the heat transfer to the fiber can potentially influence the combustiondynamics of the droplets. At the beginning of the droplet lifetime, this influence is deemedto be small, however, as the droplet size decreases (due to combustion, atomization, andevaporation) the heat transfer effect becomes more pronounced. Nonetheless, this occurs forcombustion experiments in which the droplet is suspended from a suspension mechanism,and as a result, such effect cannot be eliminated for these experiments.Fig. 2.16. The experimental apparatus and diagnostics used for studying the diesel dropletscombustion. (a) Close view of the cross-fiber configuration and the ignitor, (b) the shad-owgraphy setup, and (c) the OH∗ chemiluminescence setup. The numbered items are: (1)cross-fiber, (2) ignitor, (3) solenoid actuator, (4) high-speed camera, (5) back-light LED,(6) image intensifier, and (7) data acquisition unit.A gastight syringe was used to suspend the diesel droplet at the fibers point of inter-section using a syringe pump (11 Elite pump from Harvard Apparatus). The initial dropletdiameter was 2.00± 0.02 mm. The plasma arc (item 2) was positioned about 1 mm belowthe droplet, and operated for approximately 500 ms for ignition purpose. After this periodends, the plasma-assisted ignitor is retracted using the linear solenoid, item 3. Separatehigh-speed shadowgraphy and OH∗ chemiluminescence techniques were employed to studythe GO doped diesel droplet and flame dynamics, respectively. These diagnostics are shownin Figs. 2.16(b and c), respectively. The acquisition frequency for both techniques was set432.2. APPARATUS AND DIAGNOSTICSto 4000 Hz. For the shadowgraphy experiments, the camera (item 4) was equipped with aSIGMA Macro 105 mm (f/# = 2.8) lens. Similar to the studies of Bennewitz et al. [4, 141],a Semrock FF01–433/530 dual bandpass filter was installed on the camera lens to improvethe visibility of the droplet in the presence of the diesel sooty flame. Though this was notnecessary for the ethanol droplet combustion, without this filter, shadowgraphy imaging forthe diesel droplet was not possible due to presence of soot. An LED light (item 5) wasused to illuminate the region of interest and help capturing the shadow of the droplets. Thecamera exposure time for the shadowgraphy technique was 100 µs. The imaging resolutionwas obtained using the USAF1951 target plate following the procedure similar to that dis-cussed in Appendix ??, and the shadowgraphy resolution was 22 µm/pixel. For the OH∗chemiluminescence measurements, the intensifier (item 6) gate was set to 90 µs, and thepixel resolution of the chemiluminescence measurements was 23 µm/pixel. All diagnosticswere controlled and synchronized using an NI PCIe 6361 data acquisition board (item 7).2.2.3 Experimental arrangement for multi droplet combustion dynamicsand flame spread measurementsIn order to study the combustion dynamics and flame spread in a multi droplet envi-ronment, the experimental apparatus presented in Fig. 2.16 was modified to accommodatesuspending more than one droplet of GO doped and ethanol blended biodiesel. Further-more, to investigate the flame spread in a multi-droplet configuration, it is desirable toobtain synchronized data regarding the flame and the droplets location. Thus, simulta-neous OH∗ chemiluminescence and shadowgraphy imaging techniques were preferred. Theconfiguration of the apparatus and diagnostics used in the previous subsection were modifiedand is presented in Fig. 2.17(a).Simultaneous shadowgraphy (with acquisition frequency of 80 Hz and pixel resolution of13 µm/pixel) as well as OH∗ chemiluminescence (with acquisition frequency of 4000 Hz and442.2. APPARATUS AND DIAGNOSTICSpixel resolution of 43 µm/pixel) were performed, see Fig. 2.17(a). The OH∗ chemilumines-cence hardware were similar to those presented in the previous subsection. A 310±10 nmbandpass filter was installed on the UV lens of the chemiluminescence camera to facilitatecollecting flame chemiluminescence near the wavelength of the OH∗ emission at 308 nm. Forshadowgraphy, Andor’s Zyla 5.5 sCMOS camera was used. The Semrock FF01–433/530 200dual bandpass filter was installed on the shadowgraphy camera lens to improve the visibil-ity of the droplet in the presence of sooty flames and similar to the experiments presentedin subsection 2.2.2. Following [29], a beam splitter with 45o orientation was used to reflectthe light with wavelength smaller than 350 nm for OH∗ chemiluminescence and to transmitlight wavelength larger than 350 nm for shadowgraphy imaging.For each test condition, three 1.9±0.02 mm diameter droplets were deposited at theintersection of three 140 µm SiC fibers that form an angle of 15o with one another, seeFig. 2.17(b). The initial inter-droplet spacing was set to 4 mm. Immediately after thedeposition of the droplets, the 20 mW plasma-arc ignitor (which was mounted on top of alinear actuator) was positioned about 1 mm below the center or a side droplet and operatedfor 400 ms. After the plasma arc operation was complete, the actuator retracted the ignitorto minimize its interaction with the flame. Then, the flame envelops the ignited droplet. Thisdroplet is referred to as the main droplet in discussions concerning multi-droplet combustionin the thesis. The main droplet flame ignites the side droplets. Once the ignition is complete,the main and side droplets may slightly move along the horizontal axis and the inter-dropletspacing may change due to various reasons, including the effect of the recoiling force fromthe atomizations of the droplet.452.2. APPARATUS AND DIAGNOSTICSFig. 2.17. (a) The experimental setup and diagnostics arrangement used to study GOdoped biodiesel and GO doped ethanol blended with biodiesel droplets. (b) Close-view ofthe multi-droplet combustion configuration.46Chapter 3Droplet and flame detectionTo detect the droplet and flame, it is necessary to post-process the images collected viashadowgraphy and OH∗ chemiluminescence techniques, respectively. This chapter presentsthe procedures for reducing the images obtained from the aforementioned imaging tech-niques.3.1 Droplet detection and shadowgraphy data reductionThe shadowgraphy technique was utilized to study the location and size of the main, side,and baby droplets. The methods used for the estimation of the droplet diameter dependon the utilized suspension mechanism discussed in Section 2. In the following, first, thedroplet measurements for those deposited on a borosilicate tube are discussed. Then, thedroplet measurement for those deposited at the intersection of SiC cross fibers are presented.Finally, the employed data reduction techniques for detection and characterization of theejected baby droplets are elaborated.3.1.1 Droplets on the borosilicate tubeTo estimate the size of the droplets deposited on the borosilicate tube, first, one image ofthe utilized droplet suspension mechanism was acquired. This image is shown in Fig. 3.1(a).Then, using an edge finding algorithm in MATLAB, the exterior of the suspension mecha-nism was obtained, see the yellow curve shown in Fig. 3.1(a), and overlaid on Fig. 3.1(b).The same algorithm was also used to estimate the edge of the droplet, which is shown by473.1. DROPLET DETECTION AND SHADOWGRAPHY DATA REDUCTIONthe blue curve in Fig. 3.1(b). In several past investigations, see for example [38, 142, 143],the droplet features an ellipsoid shape, and the droplet equivalent diameter is estimated byequating the volume of the ellipsoid to that of a sphere, whose diameter is the droplet equiv-alent diameter. However, as shown in Fig. 3.1, the droplets deposited on the borosilicatetube do not resemble an ellipsoid. As a result, the volume occupied between the droplet’souter surface and the suspension mechanism’s outer surface was estimated and equated tothe volume of a sphere, whose diameter is the droplet’s equivalent diameter (D).-1 0 1-1012-1 0 1Fig. 3.1. Shadowgraphy image obtained for (a) the suspension mechanism and (b) thedroplet suspended on the suspension mechanism.The following procedure was implemented to estimate the droplet equivalent diameter.First, the combined droplet and suspension mechanism volume was assumed to be made ofseveral disks, stacked along the vertical axis. The volume of each disk is πr(y)2∆y, where∆y = 38µm is the pixel height and r(y) is the radius of the disk at a given height, y.Then, the total droplet and suspension mechanism volume (V ) was calculated by summingup the volumes of the disks. A similar approach was used to calculate the volume of thesuspension mechanism (Vs) as well. The droplet equivalent diameter D was then obtained,using D = [6(V − Vs)/π]1/3.483.1. DROPLET DETECTION AND SHADOWGRAPHY DATA REDUCTION3.1.2 Droplets on the Silicon carbide fibersUnlike the borosilicate tube, the droplets deposited on the SiC cross-fibers feature anellipsoid shape, as shown in Fig. 3.2. To estimate the location and diameter of these droplets,a procedure similar to that in Hicks et al. [142], Dietrich et al. [143], and Ghamari andRatner [38] was followed. First, the droplet edges were obtained using the shadowgraphyimages. For a representative droplet, these edges are shown by the red color data pointsin Fig. 3.2. Then, an ellipse with the major and minor diameters of a and b, and with thecenter location (with respect to the origin) of Xd and Yd was fit to the droplet edges, seethe yellow ellipse in Fig. 3.2. Finally, the droplet equivalent diameter was calculated usingD = (ab2)1/3.Fig. 3.2. An ellipse with major and minor diameters of a and b fit to a shadowgraphy imageof a droplet.For multi-droplet configurations, the mean inter-droplet spacing (S) of each droplet pairwas determined by calculating the time-average of the distance between the centers of thecorresponding ellipses.3.1.3 Baby droplets ejected from the main dropletThe projected trajectory of the baby droplets and their mass were obtained using theshadowgraphy images. This is rather challenging due to the shadowgraphy images beingcontaminated by noise and that the ejected droplets can move out of the camera’s focusingvolume. Binarization with a spatially constant threshold removes some of the baby droplets493.1. DROPLET DETECTION AND SHADOWGRAPHY DATA REDUCTIONfrom the images, which is undesirable. To address this, an adaptive threshold [144] techniquewas utilized. To demonstrate the performance of this technique, a raw shadowgraphy imageis shown in Fig. 3.3(a), and the detected baby droplets after binarization of the raw imagewith the adaptive thresholding method is presented in Fig. 3.3(b). Care was taken toensure no secondary droplet is removed as a result of the applied technique, and that thebaby droplets were distinguished from the background noise.-1 0 1-2-101-1 0 1Fig. 3.3. (a) A representative raw shadowgraphy image and (b) the binarized image usingthe adaptive thresholding technique. The baby droplets are circled in both (a) and (b).The discussions presented in Appendix ??, and specifically Fig. 7.2(a) suggest that babydroplets as small as nearly one pixel may be detected in our measurements. However,such ability was substantially influenced by the post-processing of the collected images.Specifically, the threshold value used for the binarization of the images as well as the sizeof the filter influenced the ability to detect the baby droplets. The technique used here forthe detection of the baby droplets is similar to the method presented in [144]. First, analgorithm is developed in MATLAB to detect candidate baby droplets. Then, the center ofeach candidate baby droplet was obtained, a w × w pixel2 window was considered aroundthe baby droplet center, and the mean intensity inside the window (Mw) was calculated.Finally, pixels whose intensities were larger thanMw(1−th) were disregarded, with th beingthe selected threshold. This procedure was applied for the signal detection in Appendix ??,and the maximum threshold (thmax) that allowed for detecting the signals was obtained and503.1. DROPLET DETECTION AND SHADOWGRAPHY DATA REDUCTIONpresented against the number of line pairs per millimeter in Fig. 7.2(a) using the orangetriangular data points. It was obtained that at the resolution of 44 µm, the maximumthreshold that allows for detecting the signal is about 23%. On one hand, values of th largerthan 23% removed the signal from the post-processed images and are not recommended tobe used. On the other hand, too small threshold values did not allow for distinguishingbetween the background noise and the signal. Inspecting the sequence of images capturedfrom the shadowgraphy technique, it was obtained that th ≲ 15 % can lead to the detectionof noise as a baby droplet. Thus, the selected threshold should vary between 15% and 23%.For the above range of threshold (i.e. 15% ≲ th ≲ 23%), the effects of both th and w onthe number of ejected baby droplets (N) were assessed. The window size (w×w) was variedfrom 7× 7 to 21× 21 (pixel2). After careful examination of the raw images, it was obtainedthat N ≈ 29, which corresponds to the yellow region in Fig. 3.4. Thus, it is expected thatselecting several combinations of th and w values (which correspond to the yellow region inFig. 3.4) can lead to the correct estimation of the number of ejected baby droplets. In thepresent study, th = 19% and w = 17 pixels were used.15 16 17 18 19 20 21 22 2379111315171921010203040Fig. 3.4. Effects of the threshold and window size on the number of detected and ejectedbaby droplets. The results pertain to the test condition of Eh0.01 in Table 2.2.513.2. FLAME DETECTION AND OH* CHEMILUMINESCENCE DATA REDUCTION3.2 Flame detection and OH* chemiluminescence datareductionThe flame chemiluminescence measurements were used to study the ignition of droplets,flame dynamics, and flame spread. In the following, first, the preliminary pre-processingapplied to the OH∗ chemiluminescence images is discussed. Then, further data reductiontechniques for estimating the ignition delay and obtaining the flame edges are presented.The basic chemiluminescence data reduction is similar to that discussed in several pastinvestigations, see for example [145–147]. Figure 3.5(a) presents a raw chemiluminescenceimage. All acquired chemiluminescence images were first subtracted by a background. Thebackground was obtained by collecting and averaging 1000 images with the camera lenscapped. Then, using intensifier settings identical to that used for the chemiluminescencemeasurements, 1000 white field images were collected and averaged. For this, the LEDlight shown in Fig. 2.14 was in place and turned on. As discussed in past investigations[145–147], the position of the LED did not influence the white field images. For all collectedwhite field images, the LED brightness was set to 30%. Then, the background-subtractedchemiluminescence images were divided by the white field to normalize for the spatiallydependent sensitivity of the imaging equipment. In order to minimize electronics noise, a7×7 pixels2 window median-based filter was applied to the white field-corrected chemilumi-nescence images. The resultant filtered image is shown in Fig. 3.5(b). It was confirmed thatreducing the size of the utilized filter to 3×3 pixels2 windows did not influence the reportedresults significantly.The discussed data reduction allowed for studying the flame oscillations. In the following,further data reduction for estimation of ignition delay and flame boundaries are discussed.523.2. FLAME DETECTION AND OH* CHEMILUMINESCENCE DATA REDUCTION-3 0 3-30360418-3 0 3-30360maxFig. 3.5. (a) Raw flame chemiluminescence image. (b) is the results in (a) after backgroundsubtraction, white field correction, and median-based filtering.3.2.1 Estimation of the ignition delayIt can be inferred from the study of Aggarwal [148] that the ignition delay (τid) is thetime difference between the instant that the droplet is introduced to the ignition sourceand the instant that the flame size reaches its maximum value. Similarly, Bennewitz et al.[4, 141] calculated the ignition delay using the flame area measured by infrared imaging. Asimilar approach is used in the present dissertation, except that compared to the infraredimages, the flame chemiluminescence images are used. Here, the summation of the area ofthe pixels that feature flame chemiluminescence larger than zero was obtained and is referredto as A. Variation of A/Amax for two representative conditions of Ep0.01 and Er0.01 areshown in Figs. 3.6(a) and (b), respectively. It was verified that using larger threshold valuesdoes not significantly change the variation of A normalized by the corresponding maximumvalue (Amax) versus time. In Figs. 3.6(a) and (b), t = 0 corresponds to the instant that theignitor spark is formed. This was facilitated by starting the data acquisition 10 ms prior tothe ignitor operation, as shown by the dotted black and dashed blue curves in Fig. 2.15(a).As schematically demonstrated in Fig. 2.15(a), the ignitor spark operates for about 50 ms.During the plasma arc operation 0 ≲ t ≲ 50 ms, the chemiluminescence signal is con-taminated by the light emission from the ignitor spark, and as a result, the correspondingdata is presented for t ≳ 50 ms, see the inset of Fig. 3.6(a). After t ≈ 50 ms, the solenoid533.2. FLAME DETECTION AND OH* CHEMILUMINESCENCE DATA REDUCTION0 1 2 3 400.20.40.60.810 0.02 0.04 0.060.50.550 1 2 3 40 0.02 0.04 0.060.480.5Fig. 3.6. Variation of A/Amax versus time for the test conditions of (a) Ep0.01 and (b)Eh0.1.is retracted during 50 ≲ t ≲ 55 ms. This was confirmed through high-speed imaging ofthe ignitor retraction process, which is not presented here for brevity. During the solenoidretraction time period (50 ≲ t ≲ 55 ms), the flame size decreases, causing the decreasingvalues of A/Amax, see the insets of Fig. 3.6(a and b). Such behavior is independent of thetest conditions, and is speculated to be linked to the motion of the ignitor. To investigatethe validity of such speculation, variation of A/Amax pertaining to the test condition ofpure ethanol is shown Fig. 3.7(a). The flame chemiluminescence pertaining to t = 47.5,50.5, 53.5, 56.5, and 59.5 ms are presented in Figs. 3.7(b–d), respectively. During the timeperiod shown in Fig. 3.7(a), A/Amax is influenced by the retracting motion of the igni-tor. In order to demonstrate this, the global maximum of flame chemiluminescence during47.5 ms ≲ t ≲ 59.5 ms was identified, and the flame chemiluminescence contour correspond-ing to 10% of its maximum was obtained. Then, the vertical locations of the contour atx = 0 were identified and used to highlight the flame base and top, which are shown by thegreen and pink data points in Figs. 3.7(b–f), respectively. These locations are also shown bythe green solid and pink dashed curves in Fig. 3.7(g). As can be seen, for about 5 ms, theflame base moves upward faster than the flame top, which decreases the flame size, hencethe decrease of A/Amax shown in Fig. 3.7(a) during 47.5 ≲ t ≲ 53.5 ms. Such decreasingtrend (which is induced by the ignitor retracting motion) pertains to a relatively short time543.2. FLAME DETECTION AND OH* CHEMILUMINESCENCE DATA REDUCTIONperiod (5 ms), is independent of the tested condition, and is relatively negligible comparedto the ignition delay.45 50 55 600.560.580.60.620.6445 50 55 60 65 70 75-4.7-4.4-4.1-3.84.34.75.15.55.9-4 -2 0 2 4-4-20246-4 -2 0 2 4 -4 -2 0 2 4 -4 -2 0 2 4 -4 -2 0 2 40maxFig. 3.7. (a) Normalized flame area versus time, (b–f) correspond to the flame chemilu-minescence images acquired at t = 47.5, 50.5, 53.5, 56.5, and 59.5 ms, respectively. (g)presents the location of the flame base (green solid curve) and the top of the flame (dashedpink curve) versus time. The results pertain to the pure ethanol test condition.For t ≳ 55 ms, A/Amax increases and features a global maximum, see Figs. 3.6(a andb). Ignition delay (τid) is defined as the time difference between the instant that the sparkis formed (i.e. t = 0) and the instant that the normalized flame area reaches 95% of itsglobal maximum, see this duration highlighted in Figs. 3.6(a and b). Analysis of the resultspresented in Fig. 3.6 suggests that A/Amax features relatively large fluctuations near theglobal maximum. This leads to large uncertainty in the estimation of the time at whichA/Amax equals unity. In order to reduce this uncertainty, 95% of A/Amax was used forestimating the ignition delay.553.2. FLAME DETECTION AND OH* CHEMILUMINESCENCE DATA REDUCTION3.2.2 Estimation of flame boundariesAs discussed in Chapter 1, the flame spread time (τs) is defined as the time differencebetween the instants that the main and side droplets are enveloped by their correspondingflames [85]. Thus, to estimate τs, it is key to find the moments at which the main and sidedroplets are enveloped by the flame. For this, the flame chemiluminescence contours shouldbe used [85, 86, 88]. These contours can be obtained for different fractions of the globalmaximum flame chemiluminescence. To examine how varying the above fraction impacts theflame contour and its position with respect to the droplets, simultaneous shadowgraphy andflame chemiluminescence images are required. A representative flame chemiluminescencesuperimposed by the droplets are presented in the Fig. 3.8. The results correspond tothe test condition of B0.1. In the figure, the size and location of the main (right) andside (left) droplets were determined using the droplet detection process described earlier,with the droplets shown by the black circles in Fig. 3.8. Also overlaid on the figure arethe contours of flame chemiluminescence normalized by its maximum (IN). Specifically,contours corresponding to IN = 5, 10, 15, 20, and 25% are presented. As can be seen, thecontours for IN = 5% and 10% show that the side droplet is enveloped by the flame, whilecontours relating to IN > 15% suggest otherwise. The results in Fig. 3.8 show that theflame chemiluminescence can be influenced by the presence of the wires, which is mostlypronounced for IN > 15% (see the arrow on the right-hand-side of the figure). Thus, tominimize the above influence, the flame contours corresponding to IN ≤ 15% should beused. It was confirmed that the above conclusion is not influenced by the test condition.As will be discussed in Chapter 6, the cusps on the left-hand-side of the contours (see thearrows near x ≈ y ≈ 0 and on the left-hand-side of the figure) are mostly influenced by theflame propagation at the top and bottom of the side droplet.For several values of IN, τs was calculated for all test conditions. It was concluded thatthe flame chemiluminescence and, as a result, τs are substantially influenced by blending563.2. FLAME DETECTION AND OH* CHEMILUMINESCENCE DATA REDUCTIONFig. 3.8. The “hot” color bar is the flame chemiluminescence normalized by its maximum.The contours are 5, 10, 15, 20, and 25% of the maximum flame chemiluminescence. Theblack circles are the droplets obtained from the shadowgraphy image simultaneously ac-quired with the flame chemiluminescence. The results correspond to a representative testcondition of B0.1.Fig. 3.9. Flame spread time versus IN for representative test conditions of B0.01 and BE0.01.biodiesel with ethanol, with representative variation of τs versus IN for test conditionsB0.01 and BE0.01 presented in Fig. 3.9 using solid black and red dotted-dashed curves,respectively. The results in the figure show that, for IN ≥ 9%, the flame spread timeis not influenced by the value of the selected IN. Given the above discussions, values of573.2. FLAME DETECTION AND OH* CHEMILUMINESCENCE DATA REDUCTION9% ≤ IN ≤ 15% yields accurate prediction of the flame spread time. In the present thesis,IN = 15% was used.58Chapter 4Contribution of atomization toburning rateThe contribution of atomization to burning rate of single droplets for ethanol (E) dopedwith graphene oxide (GO) are investigated in this chapter. Prior to this, first, the resultsrelated to the ignition delay are presented in section 4.1. The, the characteristics of atom-ization and contribution of atomization to the burning rate are presented in sections 4.2and 4.3, respectively.4.1 Ignition delayFollowing the procedure discussed in section 3.2, the ignition delay (τid) was estimated.τid pertaining to pure ethanol is shown in Table 4.1. Also, presented in the table are theignition delays for pure ethanol droplets extracted from the study of [4]. As can be seen,the ignition delay varies from 71 to 158 ms. Several factors could alter the ignition delay.The ignition method/ignitor power, the location of the ignitor with respect to the droplet,the droplet suspension mechanism, and the droplet size could cause the differences betweenthe ignition delays reported in Table 4.1 for pure ethanol. Correcting for the effects ofthe ignitor and suspension mechanism on the ignition delay is not possible. However, τidcan be normalized for the effect of the droplet size on the ignition delay, using the dropletdiameter. Since heat transfer by convection and buoyancy are, respectively, related to the594.1. IGNITION DELAYTable 4.1. Ethanol droplet ignition delay and those reported in [4].Study D0 (mm) τid (ms) τid/D20 (ms/mm2) τid/D30 (ms/mm3)Present investigation 2.14 158.5 34.6 16.2Bennewitz et al. [4] 1.35 71 38.9 28.5droplet surface area and volume, τid was also normalized by D20 and D30. As the resultsin Table 4.1 suggest, such normalization does not allow for the collapse of the data, andthe scatter of the ignition delay results for ethanol is expected to be due to the differencesbetween the utilized ignitor, suspension mechanism, and droplet size in the present studyand those of [4].Nevertheless, since the ignitor, the suspension mechanisms, and the droplet diameterare unchanged in the present investigation, comparisons for the effect of the nanomaterialaddition on the droplet ignition delay can be made. The ignition delay results are presentedin Fig. 4.1. In the figure, each error bar pertains to twice the standard deviation of themeasured ignition delay (corresponding to 12 repeats of each experimental condition). Theresults in Fig. 4.1 show that, for the ethanol droplets doped with rGO and pGO, the ignitiondelay increases with increasing the doping concentration. However, for hGO, the effect ofthe doping concentration on the ignition delay is different. That is, for hGO, the maximumignition delay pertains to the doping concentration of 0.01%. The results in the figureshow that for the doping concentration of 0.01%, a maximum increase of 18.3, 9.2, and6.8% occurs for the ignition delay of ethanol droplets doped with hGO, pGO, and rGO,respectively. The maximum increases of the ignition delay for the doping concentration of0.1% are 20.9, 16.8, and 16.6% pertaining to ethanol droplets doped with pGO, rGO, andhGO, respectively. The increase in the ignition delay of the liquid fuels doped with GOnanomaterials is consistent with the observations reported for carbonaceous doping agentsin [4, 53, 57]. It is speculated that, generally, two mechanisms may influence the ignitiondelay increase as a result of the addition of the nanomaterials. First, doping with GO leads604.1. IGNITION DELAYto increase of the radiation absorption by this material without an immediate increase ofthe droplet temperature. Second, presence of the GO nanomaterials at the droplet surfaceand/or formation of a shell [56] decrease the amount of the droplet surface area availablefor evaporation. These two mechanisms decrease the vapor pressure at the droplet surface,increasing the ignition delay. Nonetheless, the effect of GO addition on ethanol ignitiondelay is presented in this thesis for the first time.0 0.01 0.1150170190210Fig. 4.1. Effect of graphene oxide type and concentration on the ignition delay of ethanoldroplets.Although the chemiluminescence measurements allow for characterizing the ignition de-lay, further insight regarding the droplet burning dynamics can be obtained using suchmeasurements. Results presented in Fig. 3.6(a) show that, after the ignition delay period,A/Amax decreases with time. Our results show that, for some test conditions, the reductionof the droplet surface area is followed by a significant increase in the flame surface area, seethe second peak in Fig. 3.6(b). Variation of A/Amax versus time pertaining to the secondpeak and for the test condition of Ep0.01 is shown in Fig. 4.2(a). The flame chemilumines-cence images pertaining to t = 2.47, 2.63, and 2.87 s, are shown in Figs. 4.2(b), (c), and(d), respectively. As can be seen, at t = 2.63, the flame surface area is maximized, which ismainly due to the increase of the flame size along the vertical axis. Similar observations arereported in the literature, see for example [48]. Past studies, such as Basu et al. [26], argue614.2. ATOMIZATION CHARACTERIZATIONthat the reason for such behavior can be linked to the droplet atomization. Specifically,during the atomization period, ejection of the baby droplets can increase the flame size, asdemonstrated in Fig. 4.2(c). After the atomization, the flame surface area decreases untilthe flame extinction takes place. The ejected baby droplets cannot be visualized by the OH∗chemiluminescence; however, further details related to the atomization characteristics arestudied using the shadowgraphy technique and are discussed in the following subsections.2.4 2.6 2.8 30.70.80.91-3 0 3-50510-3 0 3 -3 0 30maxFig. 4.2. (a) Variation of the normalized flame area versus time and (b-d) are the OH∗chemiluminescence images pertaining to t = 2.47, 2.63, and 2.87 s, respectively. The resultspertain to the test condition of Ep0.01.4.2 Atomization characterizationFigures 4.3(a–h) show a series of shadowgraphy images of a burning droplet during the at-omization period. The images are separated by 1 ms corresponding to 2591 ms ≤ t ≤ 2598 ms,and pertain to test condition of Eh0.01. For the test condition presented in Figs. 4.3(a–h),the atomization period started at t = 2521 ms and finished at t = 3075 ms. Thus, only ex-emplary results are presented in the figure. As can be seen, four baby droplets (highlightedby #1–4) are formed and ejected from the main droplet. The baby droplets are color-coded624.2. ATOMIZATION CHARACTERIZATIONbased on the first instant that they are observed. For example, baby droplet #1 is observedat t = 2591 ms and exists for about 4 ms, as shown in figures 4.3(a–e). Analysis of allshadowgraphy images suggests that atomization may occur for all test conditions of dopedethanol droplets. However, pure ethanol droplets do not feature atomization. Past studies,see for example [26, 47, 48, 58], suggest that the difference between the boiling points ofmolecules inside a multi-component fuel with one another and/or with water (due to po-tential water vapor condensation for alcohols [149]), the existence of a droplet suspensionmechanism, and/or presence of solid particle doping agents can lead to the formation ofbubble(s) inside the droplet. Since the doping agents are not present for our pure ethanolexperiments, and that atomization is not observed for these experiments, it is not expectedthat the presence of the suspension mechanism and possibly water condensation cause theatomization. Thus, it is speculated that the presence of GO nanomaterials is the potentialreason for the bubble formation, and as a result, the atomization. Specifically, the studiesof [26, 30, 38, 48, 73] show that bubbles formed at the aggregated nanomaterials sites mergeand grow in size initially. Then, the merged bubbles reach the droplet surface and rupturethe surface. This leads to formation of ligaments, which undergo break-up and cause theejection of the baby droplets [39–41, 47–50].For all test conditions, the baby droplets were identified following the procedure dis-cussed in section 3.1. Figure 4.4 presents the projected trajectory of all baby dropletscorresponding to the test condition of Er0.01. The suspension mechanism is marked by theblack solid curve in the figure. The trajectories are color-coded based on the time at whichthe corresponding baby droplet appears first. In the figure, t is normalized by the totaldroplet lifetime (τ). As can be seen, all the ejection events take place during t/τ ≥ 0.7.It can also be observed that the location at which a baby droplet is initially formed andejected is rather random. For example, the dark red trajectories (occurring towards the endof the droplet lifetime) may appear on both sides of the droplet.634.2. ATOMIZATION CHARACTERIZATION-2 -1 0 1-2-101-2 -1 0 1 -2 -1 0 1 -2 -1 0 1-2 -1 0 1-2-101-2 -1 0 1 -2 -1 0 1 -2 -1 0 1334 43123111 12 3Fig. 4.3. Shadowgraphy images of baby droplets formation and ejection. The results pertainto the test condition of Eh0.01.-6 -4 -2 0 2 4 6-6-4-20240.70.9Fig. 4.4. Baby droplet ejection projected trajectories. The results pertain to the testcondition of Er0.01.For all test conditions, the magnitude of the initial x− y projected velocity of the babydroplets (V0) were estimated using the corresponding trajectory and the finite differencingscheme [150]. Specifically, the magnitude of the projected baby droplet initial displacement(S⃗0) was obtained and normalized by the corresponding time period (0.5 ms). This isschematically shown in the inset of Fig. 4.4. The PDF of V0, the initial x−y plane projectedbaby droplet diameter (d0), and the total length of an x− y plane projected trajectory (s)644.2. ATOMIZATION CHARACTERIZATIONwere obtained for all test conditions. The PDFs of V0, d0, and s are presented in Figs. 4.5(a),(b), and (c), respectively. In the figure, open and solid symbols correspond to 0.01 and0.1% by weight concentrations of the doping agent, respectively. Blue, green, and red colorspertain to rGO, pGO, and hGO, respectively. The resolution of the PDFs depends onthe number of the data points available due to the baby droplet ejections and the spatio-temporal resolution of the measurements. The bin size for the PDFs was selected using atrial and error process, which leads to the best presentation of the results. All PDFs at zerovalues of V0, d0, and s were set to zero. The results show that the most probable value ofV0 is 0.5±0.2 m/s, which is comparable to the ethanol and air laminar flame speed at thestoichiometric condition. The results in Fig. 4.5(b) show that the most probable value ofthe ejected baby droplet initial diameter corresponds to 40 µm ≤ d0 ≤ 60 µm, with themajority of the test conditions featuring a most probable d0 value of about 50 µm. This isabout 55 times smaller than the main droplet initial diameter (D0). The values of V0 andd0 estimated in the present study are similar to those reported in [42]. Specifically, Yao etal. [42] showed that, for a 2 mm diameter ethanol droplet undergoing atomizations, the babydroplets with the diameter of ∼ 50 µm feature a mean projected velocity of ∼ 0.5 m/s. Themost probable value of s is about 1 mm, as shown in Fig. 4.5(c), which is comparable tothe main droplet diameter. The results in Fig. 4.5(c) show that the possibility of relativelylong projected trajectories occurrence is non-zero, similar to those shown in Fig. 4.4.Although the results presented in Fig. 4.5 allow for understanding the most probablevalues of V0, d0, and s, they do not demonstrate (potential) correlations between the mostprobable values. The Joint Probability Density Function (JPDF) of the above parameterswas calculated to address this. Our analysis shows, generally, the time duration (comparedto the droplet lifetime) and the number of occurrences of the baby droplet ejections arerelatively small for each test condition, which does not facilitate a proper presentation ofthe JPDFs. However, the results presented in Fig. 4.5 showed that the PDFs of the baby654.2. ATOMIZATION CHARACTERIZATION0 1 2 300.20.40.60.810 50 100 150 200 1 3 5 7 9 11Fig. 4.5. The probability density functions of (a) the x− y projected initial velocity of theejections, (b) the initial diameter of the ejected baby droplets, and (c) the x− y projectedtrajectory length of the ejections.droplet ejections are not significantly influenced by the amount of doping as well as thetype of the added nanomaterials. Thus, data related to all test conditions pertaining tothe atomization were combined, facilitating the calculation of the JPDFs with improvedresolution of the presentation. The JPDF of V0, d0, and s versus time are presented inFigs. 4.6(a–c), respectively. The results in the figure show that the most probable valueof the above parameters occurs around 0.55 ≲ t/τ ≲ 0.95, which is in agreement withthe results presented in Fig. 4.4. Figures 4.6(d–f) present the JPDFs of V0, d0, and sversus one another. The results in Fig. 4.6(d) show that relatively large baby droplets maybe ejected with relatively large initial velocities. Also, relatively large baby droplets maytravel longer trajectories, see the white dashed arrow in Fig. 4.6(e). However, the results inFigs. 4.5 and 4.6(d–f) show that majority of the ejected baby droplets are relatively small,feature projected velocity that is comparable to the flame speed, and travel relatively shortdistances compared to the droplet equivalent diameter. This suggests the majority of theejected baby droplets feature a relatively short lifetime and they may burn prior to leavingthe flame. However, there exist relatively large baby droplets with large projected trajectorylength, which may escape the flame envelope. The GO nanomaterials that are carried bythe baby droplets may reside in the flame. Also, the dissociation temperature of these664.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATEnanomaterials ranges between about 500 and 800 oC (see Fig. 2.6), which is comparable tothe adiabatic flame temperature. Thus, it is likely that the GO particles may burn inside theflame, influencing the ethanol combustion chemistry. However, investigating the combustionchemistry was not the focus of the present thesis.0.55 0.75 0.950.51.52.5024680.55 0.75 0.955010015020002.557.5100.55 0.75 0.951357902581150 100 150 2000.51.52.50511172350 100 150 200135790102030400.5 1.5 2.51357906121824Fig. 4.6. JPDF of (a) the initial projected velocity of the baby droplets, (b) the initialdiameter of the ejected baby droplets, and (c) the length of the ejected baby dropletsprojected trajectories versus time. (d), (e), and (f) are the JPDFs of V0 versus d0, s versusd0, and s versus V0, respectively.4.3 Contribution of atomization to doped ethanol burningrateAlthough the above discussion allows for characterizing the baby droplets ejections, theeffect of such ejections on the burning rate is unknown and is investigated in this subsection.Variation of the droplet diameter squared versus time for two test conditions with andwithout atomizations are presented in Figs. 4.7(a) and (b), respectively. The results arenormalized by the droplet initial diameter squared (D20). In the figures, t = 0 correspondsto the instant that the ignitor spark is initially formed. The results in Fig. 4.7 show that,674.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATEfor about 0.05 (s/mm2) × 2.142 (mm2) ≈ 0.2 s, the droplet diameter does not changesignificantly. During this period, highlighted as Phase 0 in Figs. 4.7(a and b), the dropletsize is influenced by both gasification (during which the combustible mixture is formed) andthe droplet thermal expansion. The former reduces the droplet diameter; however, the latterincreases the diameter. As can be seen, the effects of these two competing mechanisms onthe droplet equivalent diameter cancel out, leading to a nearly constant value of the dropletequivalent diameter during Phase 0. This observation is consistent with the results reportedin the literature for pure ethanol [32], α-methylnaphthalene [151], and Bakken crude oil [57].0 0.2 0.4 0.6 0.80.20.610 0.2 0.4 0.6 0.80.20.61Fig. 4.7. Regression of the droplet diameter squared versus time (a) without atomizationand (b) with atomization. The results in (a) and (b) pertain to Er0.1 and Ep0.01 testconditions, respectively. Both the vertical and horizontal axes values are normalized by thedroplet initial diameter squared (D20).Analysis of all test conditions with and without atomization suggests that about 5%of the droplet lifetime corresponds to Phase 0. This was also confirmed by calculating thechange in the slope of the D2/D20 variation versus time. After the heat-up period (i.e.after Phase 0), for the test conditions without atomization, the droplet equivalent diametersquared decreases linearly with a nearly constant slope. Following the regression analysis684.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATEpresented in Appendix ??, it was obtained that about 90% of the total droplet lifetime leadsto the most accurate estimation of the slope. This period is highlighted in Figs. 4.7(a andb) as ∆t/τ = 90%. The rest of the droplet lifetime (0.95 ≲ t/τ ≲ 1) pertains to the flameextinction period, and is referred to as Phase 3.Compared to the results presented in Fig. 4.7(a), those shown Fig. 4.7(b) suggest that thediameter squared regression curve of the droplet features a noticeable change in the slope.In Fig. 4.7(b), the time instant at which the first baby droplet is ejected is highlighted by thedashed line at t/τ = 0.63. As can be seen, this time instant corresponds to when the slopeof the curve changes. Similar observations were made for the rest of the test conditions thatfeature atomization. For the test condition presented in Fig. 4.7(b), the period between theend of Phase 0 and the instant that the first baby droplet is ejected is referred to as Phase 1.The atomization period is highlighted by Phase 2. For the test conditions without the babydroplet ejections, Phase 1 is limited between the end of Phase 0 and beginning of Phase 3.For test conditions with no atomization, Phase 2 does not exist.Past studies [26, 29, 141, 152] suggest that the non-linear behavior of the D2 regressioncurves may occur due to several reasons, which are non-spherical shape of the droplets [152],the utilized liquid fuels being made of multi-components [141], atomization or micro-explosions[26, 29, 141], as well as the presence of the suspension equipment [141]. In the present in-vestigation, during Phase 1 and for all test conditions, the droplets are non-spherical andthe suspension mechanism exists. Yet, the droplet equivalent diameter squared features alinear variation with a constant slope during this phase. For this reason and similar to paststudies [4, 29, 38, 41], a line was fit to the surface regression curves pertaining to Phase 1for all test conditions. The absolute value of the slope of the line pertaining to Phase 1is referred to as K1. In the present study, the slope of the regression curve changes onceatomization takes place, but the slope remains nearly constant after the occurrence of theatomization and the slope absolute value is referred to as K2.694.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATEPast studies, see for example [41], suggest two distinct mechanisms lead to mass loss fromthe main droplet during Phase 1 (when atomization is not present). These are gasificationof liquid fuel at the droplet surface (which is present for both pure and doped ethanol)and permeation of the liquid fuel through a porous shell of nanomaterials formed near thedroplet surface. Gan et al. [39] suggest that the fluid motion, the droplet surface regression,the droplet expansion as a result of bubble formation, and the internal flow circulation maylead to the formation of the porous shell. Although our measurements do not allow forimaging the shell formation, evidence of flow circulation inside the droplet is present in ourstudy. Figure 4.8 shows a series of shadowgraphy images that underline the movement ofaggregated particles inside the droplet due to a possible internal flow circulation. The resultsin the figure pertain to Er0.01 test condition. Figures 4.8(a–d) correspond to t = 2200, 2275,2350 and 2425 ms, respectively. As can be seen, a nanomaterial aggregate highlighted bythe red dashed circle in the figure undergoes a clockwise motion, possibly due to an internalcirculatory flow inside the droplet. During Phase 2, in addition to gasification and possiblepermeation of fuel through the nanomaterial porous shell, mass loss due to atomization isalso present. Thus, the larger absolute value of the slope of D2 versus time was observedcompared to that of Phase 1.It is of interest to quantify the effect of atomization on the burning rate of the dopedethanol droplets. Assuming that the total mass loss during the droplet lifetime is due tobaby droplets ejection and evaporation mechanisms, conservation of mass suggests∆mT = ∆mEJ +∆mEV, (4.1)where ∆mT, ∆mEJ, and ∆mEV are the total mass loss, the mass loss due to the babydroplet ejections, as well as the mass loss due to evaporation. It is important to highlightthat ∆mEJ only pertains to Phase 2; however, ∆mEV is present during the entire droplet704.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATE-1 0 1-101-1 0 1-1 0 1-101-1 0 1Fig. 4.8. Shadowgraphy images of ethanol droplet doped with 0.01% of rGO during com-bustion. Images in (a–d) are separated by 75 ms and pertain to t = 2200, 2275, 2350, and2425 ms. The red dashed circles highlight the location of doping agent aggregate.lifetime. The total mass loss depends on the droplet density and the main droplet initialdiameter, given by ∆mT = −ρlπD30/6, with ρl being pure ethanol density. In fact, resultsof [26, 33, 51, 60, 153–155] suggest that the droplet density does not change substantiallyfor the doping concentrations studied in the present investigation. The first term on theright-hand-side (RHS) of Eq. (4.1) is given by∆mEJ = −N∑i=1ρlπ6d30(i), (4.2)where d0(i) is the initial diameter of the ith detected baby droplet, and N is the totalnumber of the detected baby droplets. In Eq. (4.2), it is inherently assumed that theconcentration of the doping agent inside the baby droplets remains small (similar to themain droplet). Thus, the density of the baby droplets equals the ethanol density. Assumingthat the droplet burning rates during the heat-up (0 ≤ t ≤ 0.05τ) and flame extinction(0.95τ ≤ t ≤ τ) periods equate to those in Phases 1 and 2, respectively, ∆mEV is given by:714.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATE∆mEV ≈∫ τ0m˙EV(t)dt, (4.3)where m˙EV(t) is the time rate of change of the main droplet mass due to evaporation.Assuming that the liquid density remains constant during the droplet evaporation, it canbe shown thatm˙EV(t) =ρlπ4D′(t)ïdD′2(t)dtò, (4.4)where D′(t) is the main droplet equivalent diameter provided that the baby droplet ejectionsare absent. Only during Phase 1, D′(t) = D(t) since the baby droplet ejections do not occurduring this phase; however, D′(t) ̸= D(t) during Phase 2. It is important to highlight thatD′(t) pertaining to Phase 2 cannot be measured experimentally, since the atomization ispresent during Phase 2. Following the above discussion, Eqs. (4.3) and (4.4) were combinedand the integral in Eq. (4.3) was expanded and is given by∆mEV ≈ ρlπ4∫ τa0D(t)ÅdD2(t)dtã︸ ︷︷ ︸−K1dt+∫ ττaD′(t)ÅdD′2(t)dtã︸ ︷︷ ︸−K′2dt , (4.5)with τa being the time instant at which the first baby droplet appears, referred to as theatomization time delay, see Fig. 4.7(b). During Phase 1, baby droplet ejections are absentand dD2(t)/dt = −K1. In Eq. (4.5), dD′2(t)/dt pertains to only the droplet evaporation,does not equate to −K2 shown in Fig. 4.7, and is referred to as −K ′2. In other words, thetime rate of change of D′2 is the contribution of evaporation to droplet burning rate onlydue to evaporation during Phase 2. Using the above discussions, it is straightforward toshow that D(t) (pertaining to Phase 1) and D′(t) (pertaining to Phase 2) can be obtainedfromD(t) ≈»D20 −K1t, 0 ≤ t < τa, (4.6a)724.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATED′(t) ≈√D2a −K ′2(t− τa), τa ≤ t ≤ τ. (4.6b)Combining Eqs. (4.1), (4.2), (4.5), (4.6a), and (4.6b), it can be shown that K ′2 is given byK ′2 ≈ÅD2aτ − τaã︸ ︷︷ ︸Term I+Ç−∑Ni=1 d2(i)τ − τaå︸ ︷︷ ︸Term II. (4.7)In Eq. (4.7), Term I is similar to K2, except this term pertains to τa ≲ t ≲ τ , but K2 isobtained using a linear fit to the droplet diameter squared for τa ≲ t ≲ 0.95τ . Variation ofTerm I versusK2 is shown in Fig. 4.9(a). In the figure, open and solid symbols pertain to GOconcentration of 0.01 and 0.1 %, respectively. As can be seen, the results nearly follow theline of y = x, confirming the assumption made earlier that the droplet burning rate duringPhase 2 and during 0.95τ ≲ t ≲ τ are similar. Variations of Term I and Term II versus K1are presented in Figs. 4.9(b) and (c), respectively. The results in Fig. 4.9(b) suggest thatTerm I is always larger than K1, which is expected and is due to the combined effects ofatomization and evaporation. Term II, however, is negative and can become significantlysmall (about -1 mm2/s), see Fig. 4.9(c).1 1.2 1.4 1.611.21.41.60.9 1 1.111.21.41.60.9 1 1.1-1-0.500.5Fig. 4.9. Variation of (a) Term I versus K2, (b) Term I versus K1, and (c) Term II versusK1.It is of interest to investigate the effect of atomization on the rate of evaporation duringPhase 2. Thus, the variation of K ′2 versus K1 is presented in Fig. 4.10, and the results arecolor-coded based on the total mass loss due to baby droplets ejections. As can be seen,734.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATEfor a relatively small amount of baby droplet ejections (shown with dark blue symbols), therate of evaporation during Phase 2 is close to that of Phase 1. However, as the total massof the ejected baby droplets increases, K1 decreases to values as small as 0.3 mm2/s, seethe red data symbols in Fig. 4.10. This means that, for significant amounts of the babydroplet ejections (about 2.5% of initial droplet mass), the evaporation rate is suppressed.Nonetheless, the combined effect of mass loss due to evaporation and atomization is suchthat K2(or Term I) ≳ K1.0.9 0.95 1 1.05 1.10.30.50.70.91.11.30.1255075100Fig. 4.10. K ′2 versus K1 for the cases that atomization is present. The figure is color-codedbased on the total mass of the ejected baby droplets. The dashed line is y = x.The averaged values of K1 and K2 for all test conditions are presented in Fig. 4.11 usingblue and green bars, respectively. The values of pure ethanol burning rate were extractedfrom the studies of [4, 29, 38, 141, 156] and are presented in the figure by the blue barsas well. In order to facilitate comparisons, as well as to assess the overall burning rate ofthe doped droplets, a line was fit to the diameter squared regression curves, see the reddashed lines in Fig. 4.7(b). The absolute value of the slope of the line is referred to asKT, and the values are presented in Fig. 4.11 using red bars. The results in the figureshow that the reported burning rate of ethanol in the present study is about 35% largerthan the average value of the results from the literature. This is due to the difference744.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATE0.60.811.21.41.61.8Fig. 4.11. Effect of graphene oxide type and concentration on the burning rate of ethanoldroplets.between the droplet suspension mechanism used in the present study and those used in[4, 29, 38, 141, 156] as well as the ratio of the droplet to the suspension mechanism diameter.It is speculated that the relatively large diameter of the suspension mechanism used in thepresent study enhances heat transfer from the suspension mechanism to the droplet. Infact, past investigations [157–161] reported that increasing the diameter of the suspensionmechanism increases the burning rate. For example, Farouk and Dryer [161] showed thatdecreasing the ratio of n-decane droplet diameter to the suspension mechanism diameterfrom about 170 to 7 increases the enhancement of the burning rate due to existence of thesuspension mechanism from about 0.5% to 20%.Although the droplet suspension mechanism can influence the burning rate, since thesuspension mechanism geometry is fixed in this study, comparisons can be made betweenthe conditions tested here. The results show that increasing the doping concentration andreducing the oxygen content of graphene oxide both increase K1. That is doping pureethanol with 0.1% of reduced graphene oxide leads to the maximum enhancement of about8% in K1. It can be inferred from the results presented in [34, 59–61] that increasing the754.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATEconcentration of the doping agent as well as reducing the oxygen content of the grapheneoxide increase the thermal conductivity of the fuel. It is speculated that the enhancement ofK1 with addition of graphene oxide is due to a potential increase of the thermal conductivity,which is similar to the conclusions reported in [27, 53, 55, 57].The results presented in Fig. 4.11 show that the values of K2 are larger than K1 forall test conditions. As can be seen, for the doping concentration of 0.01%, the largestvalue of K2 pertains to pGO, followed by hGO and rGO. As discussed in [40], adding thedoping agent increases the number of sites prone to absorption of radiation heat, whichleads to the formation of bubbles, and as a result the atomization. As discussed in [162],most of the flame thermal radiation may correspond to the infrared wavelength rangingfrom about 1 to 24 µm, which relates to the wave-number varying from 400 to 10000 cm−1.This is mostly due to radiation from CO2 and H2O [162]. The FT-IR results related tothe radiation absorption of the tested nanomaterials allowed for characterizing the emissionabsorption versus the wave-number ranging from 600 to 4000 cm−1, see Fig. 2.5. Variationof K2 versus the area under the absorption curves in Fig. 2.5 is presented in Fig. 4.12. Theblue, green, and red data symbols in the figure correspond to ethanol droplets doped withrGO, pGO, and hGO, respectively. In the figure, open and solid symbols present 0.01 and0.1% by weight concentrations of the doping agent, respectively. As can be seen, increasingthe area under the absorption curves is accompanied by an increase in the values of K2for the smallest test concentration; however, this increase is not significant for the largesttest concentration. The results in Fig. 4.12 suggest that the burning rate in Phase 2 canbe significantly increased for ethanol doped with GO by increasing this material radiationabsorption. However, this improvement depends on the doping concentration. In fact,Basu and Miglani [26] argued that the increase in the doping concentration might lead tothe formation of a stronger aggregation shell which could resist the pressure build-up (asa result of bubble formation) and could potentially suppress the ejection events. This is764.3. CONTRIBUTION OF ATOMIZATION TO DOPED ETHANOL BURNING RATE270 290 310 3301.21.41.61.8Fig. 4.12. Variation of K2 versus the area under the FT-IR curves (see Fig. 2.5) for ethanoldoped with rGO, pGO, and hGO.consistent with the results shown in Figs. 4.11 and 4.12 for the doping concentration of 0.1%compared to 0.01%.The results show that adding the carbonaceous doping material increases the overallburning rate of ethanol. This is of significant importance for the industrial application ofsuch nanofuels. Specifically, the results in Fig. 4.11 suggest that, for the doping concentra-tion of 0.01%, the addition of rGO, pGO, and hGO to ethanol leads to an enhancementof the overall burning rate by 6.2, 4.8, and 4.1%, respectively. Similarly, for the dopingconcentration of 0.1%, adding rGO, pGO, and hGO results in an enhancement of 8.4, 6.9,and 6.4% of the overall burning rate, respectively. Thus, it is concluded that the maximumenhancement of about 8% can be achieved by adding 0.1% of rGO to ethanol droplets.77Chapter 5Role of atomization in droplet andflame dynamicsThe role of atomization in single droplet combustion dynamics for diesel (D) dopedwith octadecyl amine functionalized graphene oxide (GO) droplets are investigated in thischapter. In the first section, the characteristics of the droplet, flame, and atomizationdynamics are investigated. In the second section, the coupling between the droplet andflame, as well as the root cause for such coupling is presented.5.1 Characteristics of the droplets, flames, and atomizationeventsVariations of the droplet diameter squared (D2) versus time is presented in Fig. 5.1(a) fora representative test condition of D0.01. In addition to D2, the present study is concernedwith fluctuations of the droplet diameter squared (D2′). Due to the inherently transientnature of the droplet combustion, a time-dependent mean diameter squared was obtainedand subtracted from D2 to estimate D2′. That is D2′= D2 − D2. The size of the timewindow for averaging the droplet diameter squared was selected to be large enough such thatthe droplet dynamics is resolved. The droplet diameter squared fluctuations correspondingto the results presented in Fig. 5.1(a) are shown in Fig. 5.1(b). As can be seen, D2′featuresrelatively large amplitude oscillations. Similar results are reported in several past studies, see785.1. CHARACTERISTICS OF THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSfor example Miglani et al. [48], Pandey et al. [163], Han et al. [164], and Li et al. [165]. Threerepresentative time instances corresponding to a large amplitude oscillation were selectedand marked in Figs. 5.1(a and b), with the relevant droplet images shown in Figs. 5.1(c–e).As can be seen in Fig. 5.1(c), a bubble is present near the droplet surface immediately beforeits rupture. At this time instant, the bubble has reached its maximum size and the D2′timeseries features a peak. After the main droplet surface is ruptured, a baby droplet is ejected,causing the main droplet surface corrugations as shown in Fig. 5.1(d). After about 10 msthat D2′featured a peak, this parameter is minimized and the next bubble appears at thedroplet surface, see Fig. 5.1(e). The bubble detected in Fig. 5.1(e) leads to generation ofthe next baby droplet, and this process repeats.Fig. 5.1. (a) and (b) are D2 and D2′versus t. (c–e) present the droplet images during anatomization event. (f) is the spectrogram of D2′.The time-varying Fast Fourier Transform (FFT) of D2′(denoted by |FFT |2) was cal-795.1. CHARACTERISTICS OF THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSculated and is shown in Fig. 5.1(f). The size of the time window utilized for calculationof the spectrogram in Fig. 5.1(f) is 0.1 s. This was selected using a trial and error processthat led to best presentation of the results. As can be seen in Fig. 5.1(f), the droplet di-ameter squared oscillations feature a dominant frequency of fd ≲ 15 Hz. This frequencycorresponds to the large amplitude fluctuations in Fig. 5.1(b).Fig. 5.2. (a) and (b) are If and If′ versus t. (c–e) are the flame chemiluminescence duringan atomization event. (f) is the spectrogram of If′.Variation of the flame chemiluminescence normalized by the corresponding maximumduring the droplet lifetime (If) versus time for representative results pertaining to the testcondition of D0.02 is presented in Fig. 5.2(a). During the ignitor operation period (0 s ≤t ≲ 0.5 s), the chemiluminescence signal is contaminated by the ignitor arc emission. Thus,the chemiluminescence data for t ≳ 0.5 s is presented and used for analysis. The normalizedflame chemiluminescence intensity fluctuations signal (If′ = If − I f) is shown in Fig. 5.2(b).A sliding window size identical to that used for obtaining the droplet diameter squared805.1. CHARACTERISTICS OF THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSfluctuations was used for obtaining I ′f . Three time instants corresponding to a relativelylarge variation of I ′f were selected and highlighted in Figs. 5.2(a and b). The correspondingflame chemiluminescence intensity of these three time instants are presented in Figs. 5.2(c–e). The relatively large value of If′ corresponding to Fig. 5.2(c) is due to the ignitionof ejected baby droplets and/or the entrainment of the fuel vapor into the flame area, assuggested by Pandey et al. [41]. Our flame chemiluminescence imaging confirmed the former.Indeed, analysis of the flame chemiluminescence videos show that once the baby dropletsare ejected, they are ignited, and as a result, If increases to relatively large values. Oncethe baby droplets are burnt, the value of If decreases substantially, see Figs. 5.2(d and e).Figure 5.2(f) presents the spectrogram of I ′f (denoted by |FFT |2). The window size used forthe results in Fig. 5.2(f) is 0.1 s and is set following a trial and error process that led to bestpresentation of the results. As can be seen, similar to the results presented in Fig. 5.1(f),small frequency oscillations (ff ≲ 15 Hz) are present towards the end of the droplet lifetime.The results presented in Figs. 5.1(b) and 5.2(b) suggest that the droplet diameter squaredand flame chemiluminescence may feature intermittent oscillations. In order to asses this,the phase-space trajectories [166] and the Poincare´ maps [167] of D2′and I ′f were obtainedand presented in Fig. 5.3. Specifically, Figs. 5.3(a) and (b) present variations of D2′andI ′f versus their corresponding first derivative with respect to time. The central differenc-ing scheme [150] was used to estimate the derivatives. The results in the figures are colorcoded based on time. As can be seen, the phase-space trajectories feature a large numberof data points at their centers and some data points around their centers for both D2′andI ′f . The second derivatives of D2′ and I ′f were estimated and the data points at which thefirst derivatives of D2′and I ′f change sign were obtained. The collection of these data pointsis referred to as the Poincare´ map [168]. These maps for the droplet diameter squared andflame chemiluminescence oscillations are presented in Figs. 5.3(c) and (d), respectively. Inthe figures, the blue circular (red cross) data symbols correspond to those for which the815.1. CHARACTERISTICS OF THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSfirst derivative of both D2′and I ′f change sign from negative (positive) to positive (nega-tive). Agreeing with the phase-space trajectories, the Poincare´ maps show that the datapoints are both clustered at the origin and scattered around the origin. The above analysissuggests that D2′and I ′f feature intermittent oscillations. This is similar to the character-istics of pressure and flame chemiluminescence oscillations presented in for example Nair etal. [169], Kheirkhah et al. [170], Guan et al. [171], and Ma et al. [172]. Although the resultscorresponding to the test conditions of D0.01 and D0.02 are presented here, indeed, similarintermittent behavior was also observed for the rest of the test conditions. The intermittentoscillations of D2′and I ′f for atomizing droplets are observed, characterized, and reportedfor the first time in here.Fig. 5.3. (a) and (b) are the phase-space trajectories of the droplet and flame chemilumi-nescence oscillations. (c) and (d) are the corresponding Poincare´ maps. The results in (aand c) and (b and d) pertain to D0.01 and D0.02 test conditions, respectively.It is hypothesized that the similarity between the characteristics of the droplet diameter825.1. CHARACTERISTICS OF THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSsquared and the flame chemiluminescence oscillations (presented in Figs. 5.1–5.3) is not co-incidental, and it is due to the occurrence of the atomization events. In order to characterizethese events as well as to assess the above hypothesis, the D2 time series, similar to thatshown in Fig. 5.1(a), are used. Analyses of the D2 data as well as the shadowgraphy imagesshow that the beginning and the end of an atomization event correspond to a relatively largevariation in the D2 values, as evident in Figs. 5.1(c–e). Here, the atomization intensity (Ia)is defined as Ia = D2i /D2e , with D2i and D2e being the droplet diameter squared immedi-ately at the beginning and at the end of an atomization event, respectively. The dropletdiameter squared primarily changes due to the atomization and evaporation. Analysis ofD2 data suggests that the smallest value of D2 that can be detected is 1 mm2 (see thered dashed line in Fig. 5.1(a)) due to the presence of the droplet suspension mechanism;the maximum duration of an atomization event is ∼30 ms; and that |dD2/dt| features amaximum of 1.5 mm2/s in the absence of atomization. This means that the percentilechange in the droplet diameter squared due to evaporation features a maximum of about(1.5 mm2/s × 0.03 s)/1 mm2 ≈ 5%. As a result, events with Ia ≳ 1.05 during timeperiods smaller than 30 ms are expected to correspond to the atomization. Indeed, usingthe shadowgraphy images, this was confirmed that D2i is larger than D2e by at least 5%during an atomization event. The values of Ia were obtained for the 12 repeats of all testconditions, the Probability Density Function (PDF ) of this parameter normalized by thecorresponding maximum was obtained, and the results are presented in Fig. 5.4(a). In thefigure, the PDF was calculated for several temporal bins, aiming to highlight how the in-tensity of atomization change with time. As can be seen, Ia can become as large as about3; and, the atomization events mostly occur towards the end of the droplet lifetime. Thisagrees with the results presented in Figs. 5.1(f) and 5.2(f) and suggests there may exist acausal relation between the atomization events and the droplet diameter squared as well asthe flame chemiluminescence oscillations. For demonstration purposes, representative shad-835.1. CHARACTERISTICS OF THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSowgraphy images corresponding to representative events with Ia = 1.05, 1.2, 1.4, and 1.8are shown in the series of images in Figs. 5.4(b–e), respectively. As can be seen, all imagesshow the presence of bubbles prior to rupturing of the droplet surface (see the blue circlesand arrows). This confirms that, indeed, Ia ≥ 1.05 allows for detection of the atomizationevents. While this criterion can change for other doped fuels and shadowgraphy settings, itallows for detection of the atomization events in the present study.Fig. 5.4. (a) Time dependent PDF of the atomization intensity, Ia. (b–e) are the repre-sentative shadowgraphy images during atomization events with Ia = 1.05, 1.2, 1.4, and 1.8,respectively.The relation between the atomization events and the root mean square of the dropletdiameter squared fluctuations was investigated statistically. Figure 5.5(a) presents variationof D2′rms versus both the mean atomization intensity (Ia) as well as the number of the atom-ization events (Na) for all test conditions and their repeats. For clarity purposes, variationof D2′rms averaged for all repeats of each test condition (denoted by D2′rms) versus the meanatomization intensity also averaged for the repeats of each test condition (Ia) is presentedin Fig. 5.5(b). Variation of D2′ rms versus the number of the atomization events averagedfor the repeats of each test condition (Na) is presented in Fig. 5.5(c). The lengths of the845.1. CHARACTERISTICS OF THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSerror bars in Figs. 5.5(b and c) present the standard deviations of the relevant parameters.The data shown by the light gray color in Figs. 5.5(b and c) represent the variability dueto the repeats of the test conditions. As can be seen, generally, increasing the number andintensity of the atomization events increase the root mean square of the droplet diametersquared oscillations. The results in the figure show that increasing the doping concentrationfrom 0.001 to 0.02% increases D2′ rms by about 2 folds.Fig. 5.5. (a) The RMS of the D2′fluctuations versus the number of atomization events andthe mean atomization intensity for all test conditions and their repeats. (b) and (c) are the2D presentation of the data in (a), with the colored and enlarged data points presenting theaveraged values.In order to assess the underlying reason for the scatter of the results presented in Fig. 5.5,a representative sequence of shadowgraphy images related to test condition D0.001 is pre-sented in Fig. 5.6. Our analyses show that, at the beginning of the droplet lifetime, thedroplet internal motion is random-like, as shown in Figs. 5.6(a–c). Indeed, past investiga-855.1. CHARACTERISTICS OF THE DROPLETS, FLAMES, AND ATOMIZATION EVENTStions show such motion is Brownian [61, 173]. The Brownian motion facilitates the formationof nucleation sites via generation of nanomaterials aggregates. The dynamics of such Brow-nian motion is influenced by the size, density, and concentration of the nanomaterials, aswell as the viscosity of the base fuel [61, 173]. Compared to the early stage of the dropletcombustion, as the nucleation sites are generated and bubbles grow inside the droplet, acirculatory flow is established, which can be visualized by the motion of the GO aggregateshighlighted by the red dashed circles in Figs. 5.6(d–g). During this period, the droplet masstransport and as a result the atomization dynamics is governed by this circulatory flow,which leads to the occurrence of the first atomization event. The time at which the firstatomization event happens is denoted by τa. It is hypothesized that stronger circulationslead to sooner occurrence of atomization, which prolongs the time-periods with relativelylarge RMS droplet diameter squared fluctuations. Variation of D2rms versus 1/τa for all re-peats of the test conditions are presented in Fig. 5.7. The results in the figure suggest thata positive relation between 1/τa and D2rms exists; however, the scatter of the results (whichwas originally present in Fig. 5.5) is not removed using the above analysis. Such scattermay be removed by quantifying the 3D circulatory motion inside droplet, which was notfeasible using the shadowgraphy technique used in the present study.Fig. 5.6. (a–c) present the shadowgrpahy images of the droplet with grpahene oxide featuringthe Brownian motion. (d–g) present the motion of a graphene oxide aggregate during acirculatory flow inside the droplet. (a–f) correspond to t = 1.125, 1.150, 1.175, 1.738, 1.775,1.825, and 1.840 s, respectively. The results pertain to the test condition of D001.865.2. THE COUPLING BETWEEN THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSFig. 5.7. The relation between the inverse of the time at which the first atomization occursand the droplet diameter squared RMS.5.2 The coupling between the droplets, flames, andatomization eventsAlthough the results in Figs. 5.5 and 5.7 show that the atomization characteristics areinfluenced by the doping concentration, it is unclear if the droplet and flame dynamics areinfluenced by the atomization events at all. Such potential coupling is discussed in thissubsection. For a representative test condition of D0.01, the variation of D2 versus timeis presented in Fig. 5.8(a). Overlaid on the figure are the solid circular data points thatcorrespond to the atomization events, which are highlighted for Ia ≥ 1.05. These datapoints are also color-coded based on the value of Ia, i.e. dark blue corresponds to intenseatomization events. As can be seen, generally, large intensity atomization events occurtowards the end of the droplet lifetime, which confirms the results presented in Fig. 5.4(a).It is of interest to investigate if there exists a relation between the intensity of theatomization events and the time separation between two consecutive atomization event,referred to as ∆ta. In order to study this, the atomization events were categorized using athreshold value of Ia = 1.4, and the PDF of 1/∆ta was calculated. For the representativetest condition of D0.01, the PDFs of 1/∆ta for atomization events with 1.05 ≤ Ia < 1.4875.2. THE COUPLING BETWEEN THE DROPLETS, FLAMES, AND ATOMIZATION EVENTS1.051.351.651.95Fig. 5.8. (a) Variation ofD2 versus time. The results correspond to D0.01 test condition. (b)and (c) are the probability density functions of the inverse of the time between to consecutiveatomization events with 1.05 ≤ Ia < 1.4 and Ia ≥ 1.4, respectively. (d) and (e) are the PSDsof the droplet diameter squared oscillations corresponding to t(Ia = 1.05) ≤ t < t(Ia = 1.4)and t ≥ t(Ia = 1.4), respectively.and Ia ≥ 1.4 are presented in Figs. 5.8(b) and (c), respectively. The results in Fig. 5.8(c)suggest that, for Ia ≥ 1.4, the atomization events become relatively scarce such that a bestpresentation of the PDF is not possible. The results show that the most probable values of1/∆ta occur at about 25 and 5 Hz for 1.05 ≤ Ia < 1.4 and Ia ≥ 1.4, respectively. Thesedominant frequencies were calculated using the area-weighted average of the probabilitydensity function for PDF > 0.5 × max{PDF}. The results presented in Figs. 5.8(a–c)suggest that, for this test condition, atomization event with large intensities occur lessfrequently and towards the end of the droplet lifetime.The results in Fig. 5.8(a) as well as the atomization intensity were used to identify themoments corresponding to the first time at which Ia = 1.05 and 1.4. These are referred toas t(Ia = 1.05) and t(Ia = 1.4). Results in Figs. 5.8(d) and (e) present the power spectrumdensities (PSD) of the droplet diameter squared oscillations for t(Ia = 1.05) ≤ t < t(Ia =885.2. THE COUPLING BETWEEN THE DROPLETS, FLAMES, AND ATOMIZATION EVENTS1.4) and t ≥ t(Ia = 1.4), respectively. As can be seen, during the the time period that theatomization events are not intense, the PSD of the droplet diameter squared oscillationsfeature a dominant frequency near 25 Hz, which is similar to that reported in Fig. 5.8(b).However, for the rest of the droplet lifetime (corresponding to intense atomization events)the droplet diameter squared oscillations feature a dominant frequency near 5 Hz, which issimilar to that reported in Fig. 5.8(c). The above argument suggests a causality relationbetween the atomization events and the droplet diameter squared oscillations.It is noted that the amplitude of the PSD presented in Fig. 5.8(e) and at f ≈ 5 Hzis about one order of magnitude larger than that for the PSD presented in Fig. 5.8(d)at f ≈ 25 Hz. This is discussed using the droplet dynamics. The shadowgraphy imagingshowed that the droplets could oscillate primarily along the y–axis. It was observed that theretracting motion of the ignitor, which occurs at t ≈ 0.5 s, induced the droplet oscillations.These oscillations are present prior to the first atomization event and are quantified usingthe vertical position of the droplet center. It is claimed that the droplet oscillations withrespect to the wires are not the root cause of a potential coupling between the dropletdiameter squared and the flame chemiluminescence oscillations. Representative variationsof the droplet center vertical position oscillations versus time for test conditions of D, D0.001,D0.005, D0.01, and D0.02 are presented in Figs. 5.9(a–e), respectively. The power spectrumdensities of the oscillations of the droplet along the vertical axis were calculated for all12 repeats of each test conditions. The power spectrum densities were then averaged andpresented in Fig. 5.9(f). The PSDs in the figure were stepped for clarity purposes. As canbe seen, for all test conditions, the droplets vibrate along the vertical axis at a frequency ofabout 25 Hz.Following the above discussion, the oscillations at 25 Hz are present prior to the oc-currence of the first atomization event, are due to the droplet vibrations along the verticalaxis, and are originally induced as a result of the retracting motion of the ignitor. It is895.2. THE COUPLING BETWEEN THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSFig. 5.9. (a–e) are representative oscillations of the droplet center along the vertical axisversus time for test conditions of D, D0.001, D0.005, D0.01, and D0.02, respectively. (f)presents the averaged power spectrum densities of the these oscillations.of interest to confirm if the observations presented in Fig. 5.8 hold for all test conditions.Following the above discussions, the PDF of 1/∆ta was estimated for Ia ≥ I∗a , with I∗a beinga threshold atomization intensity value. For all test conditions, the most probable valueof 1/∆ta for several values of 1.05 ≤ I∗a ≤ 1.4 were obtained, averaged for 12 repeats ofeach test condition, and presented in Fig. 5.10. The results in the figure confirm that thefrequency of occurrence of the intense atomization events decrease as I∗a increases, and thisis consistent for all test conditions.Although the above discussions elaborate the connection between the atomization eventsand the droplet diameter squared fluctuations, it is unclear how these relate to the flamechemiluminescence oscillations, specifically for intense atomization events. In order to in-vestigate this, the time instant at which the first atomization event with Ia ≥ 1.4 (i.e.the intense atomization) occurs was obtained. The power spectrum density of the dropletdiameter squared fluctuations for the time period immediately after the occurrence of the905.2. THE COUPLING BETWEEN THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSFig. 5.10. The most probable value of 1/∆ta for atomization events with Ia ≥ I∗a .first intense atomization event is presented in Fig. 5.11, see the black curves. The resultsin Figs. 5.11(a–e) pertain to the test conditions of D, D0.001, D0.005, D0.01, and D0.02,respectively. The dominant frequencies for the droplet diameter squared oscillations wereextracted and presented in Fig. 5.12, see the black data symbol in the figure. The errorbars in the figure are the Full Width at Half Maximum of the PSDs. The PSDs of the flamechemiluminescence oscillations were also calculated for all test conditions and are presentedby the solid red curve in Fig. 5.11. The dominant frequency of the flame chemiluminescenceoscillations was extracted from the results in Fig. 5.11 and are presented in Fig. 5.12 usingthe red data points. For comparison purposes, the PDF of the inverse of the time differencebetween the occurrences of two consecutive atomization events (for Ia ≥ 1.4) as well as thePSD of the oscillation of the droplet center along the vertical axis (which is also presentedin Fig. 5.9) are shown by the blue and gray curves in Fig. 5.11, respectively. The results inFig. 5.11 are stepped for clarity purposes. The dominant frequency of the natural vibrationsspectrum (fn) and the most probable value of intense atomization events frequency (fa) areshown by the gray and blue data points in Fig. 5.12. As can be seen, for all test condi-tions, the dominant frequencies of the droplet diameter squared oscillations spectrum, themost probable frequency of intense atomizations, and the dominant frequency of the flame915.2. THE COUPLING BETWEEN THE DROPLETS, FLAMES, AND ATOMIZATION EVENTSchemiluminescence oscillations spectrum match. The results show that fn is appreciablylarger than fd, fa, and ff . It is also observed that, although addition of GO can change theatomization intensity (as shown earlier in Fig. 5.5), fn, fd, fa, and ff are not influenced byGO addition.Fig. 5.11. Spectra of the droplet diameter squared (black) and flame chemiluminescence(red) oscillations as well as the PDF of the inverse of the time difference between two con-secutive atomization events (blue). The gray curves are the spectra of the droplet vibrationsalong the vertical axis. (a–e) correspond to test conditions D, D0.001, D0.005, D0.01, andD0.1, respectively.Fig. 5.12. Variations of the dominant frequencies of the droplet oscillations along the ver-tical axis (fn), flame chemiluminescence oscillations (ff), and the droplet diameter squaredoscillations after the occurrence of the first intense atomization event (fd). fa is the mostprobable frequency of intense atomization events.92Chapter 6Influence of atomization on flamespread and combustion dynamicsof multi dropletsThe influences of atomization on multi-droplet combustion dynamics and flame spreadrate for biodiesel (B) doped with octadecyl amine functionalized graphene oxide (GO)droplets as well as that for droplets of biodiesel blended with ethanol (BE) and dopedwith GO are investigated in this chapter. In the first section, the spectral characteristicsof (multi-)droplet combustion is discussed for single and multi droplets of B and BE fuelsdoped with GO. In the second section, the modes of flame spread for the above fuels and fora one-dimensional array of droplets are first presented. Then, the effect of atomization onthe flame spread rate is quantified and the results are compared with those in the literature.6.1 Effect of atomization on multi-droplet combustiondynamicsIn the first subsection, the results related to the GO-doped biodiesel and GO-dopedbiodiesel blended with ethanol single droplets are presented. Then, in the second subsection,the results for multi-droplet combustion are presented and compared with those for a singledroplet.936.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICS6.1.1 Effect of atomization on single droplet combustion dynamicsFigure 6.1(a) presents the variations of the biodiesel droplet diameter squared, D2,versus time using the black curve. Also, the local flame chemiluminescence, If(x, y), wasspatially integrated, and the variation of this parameter normalized by its maximum (IN)versus time is shown in Fig. 6.1(a) using the red curve. The shadowgraphy images showthat the droplet size becomes comparable to the size of suspension fibers towards the end ofthe droplet lifetime, and as a result, the droplets cannot be discerned from the fibers. Theminimum detectable droplet size from the shadowgraphy images is shown by the dashedline in Fig. 6.1(a). In addition to D2 and IN, the horizontal locations of the droplet andthe flame centroid are of interest. The variation of the former subtracted by its mean value(Xd − Xd) versus time is presented in Fig. 6.1(b) using the black curve. The horizontallocation of the flame chemiluminescence centroid, Xf was calculated usingXf =∫ xmaxxmin∫ ymaxyminxIf(x, y)dxdy∫ xmaxxmin∫ ymaxymindxdy, (6.1)with xmin, xmax, ymin, and ymax being the horizontal and vertical extents of the acquiredflame chemiluminescence data. The variation ofXf subtracted by its mean value is presentedin Fig. 6.1(b) using the red curve. The flame chemiluminescence images show, once theignition mechanism is turned off, a small flame kernel is formed above the droplet; and,this kernel immediately propagates around the droplet. Here, t = 0 corresponds to themoment that the flame kernel has just enveloped the droplet. The flame chemiluminescenceacquired at t = 0, 0.54, 0.69, 0.74, 0.78, 1.47, 1.50, and 1.51 s are presented in the top panelsof Figs. 6.1(c–j), respectively. Also presented in the bottom panel of Figs. 6.1(c–j) are thesimultaneously acquired shadow of the droplets. To aid the discussions, circles with diameterof D (which represent the droplet) are superimposed on Fig. 6.1(c–g). The center of thesphere is located at the horizontal and vertical droplet center (Xd, Yd). After the droplet is946.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICSignited, initially, the area of significant flame chemiluminescence increases, which is shownto be due to the vaporization of the droplet [41], compare the flame chemiluminescence inFigs. 6.1 (c) and (d).Fig. 6.1. (a) Variations of the droplet diameter squared (black) and normalized flamechemiluminescence (red) versus time. (b) Horizontal position of the droplet (black) andflame chemiluminescence (red) centroids subtracted by their mean values versus time. (c–j)are the flame chemiluminescence and droplet images corresponding to the instants markedin (a) and (b). Specifically, the results in (c–j) correspond to t = 0, 0.54, 0.69, 0.74, 0.78,1.47, 1.50, and 1.51 s, respectively. The results are for the representative test condition ofB0.01.As the droplet combustion proceeds, bubbles are generated at either at the suspendingfiber or at the GO aggregates. This bubble formation at liquid-solid interface is referred toas the heterogeneous nucleation [26]. Such bubbles are shown in Figs. 6.1 (d–f) using thedashed blue circles. When these bubbles reach the droplet’s surface, it ruptures, leadingto atomizations during 0.5 s ≲ t ≲ 0.8 s. The results in Fig. 6.1(a) show that, for suchatomizations, the droplet diameter and spatially integrated flame chemiluminescence donot feature significant fluctuations. Compared to these, the results presented in Fig. 6.1 (b)show that, during the above atomization, both centroids of the droplet and flame movelaterally back and forth, together, and without delay. The back and forth movement ofthe droplet is due to the recoiling force [41] induced as a result of atomizations. This956.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICSmechanistic relation between the droplet and flame chemiluminescence centroids confirmsthe results presented in Chapter 5. Compared to the results presented in Chapter 5, whichwere obtained based on separate measurements, those presented in Fig. 6.1 (b) are obtainedbased on simultaneous measurements of the droplet and its flame. As shown in Figs. 6.1 (aand b), towards the end of the droplet lifetime, a few intense atomizations are observed.Such events impact the flame topology via formation of baby droplets and ejection of fuelvapor into the flame. The relevant atomization events are shown in Figs. 6.1(h–j) where thechanges in flame topology are highlighted by the dashed blue ellipses.Aiming to understand the dynamics of the above described oscillations during the atom-izations, the spectrograms of If , Xf , and Xd are obtained and presented in Figs. 6.2(a–c),(d–f), and (g–i), respectively. The results in the first, second, and third columns of the figurecorrespond to test conditions for which the fuel was biodiesel, biodiesel doped with 0.01%by mass of GO, and biodiesel doped with 0.1% by mass of GO, respectively. To generatethe spectrograms, the corresponding data was, first, subtracted by the data averaged overa sliding window with a size of 0.2 s. For the Fast Fourier Transform (FFT) analysis of thespectrograms, the number of FFT points was set to 8192. The size of the window used forsubtraction of the sliding average and the number of FFT points were selected to ensurebest quality of presentation for the spectrograms. The horizontal axis of the spectrogramspresent time normalized by the droplet lifetime (t/τ). The spectrograms were calculatedfor all repeats of each test condition, the results were averaged over the repeats, and werenormalized by the maximum. Overlaid on the spectrograms are the straight solid lines inmagenta color, which is the mean of the normalized time at which the atomizations start.This time is obtained from the shadowgraphy images. The standard deviation of the abovetime was calculated based on the 12 repeats of the experiments. The mean normalized timeat which the atomizations start plus and minus half of the corresponding standard deviationare also presented using dashed lines in magenta color.966.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICSFig. 6.2. The spectrogram of (a–c) spatially integrated flame chemiluminescence, (d–f)horizontal location of the flame chemiluminescence centroid, and (g–i) horizontal locationof the droplet center. The results in the first, second, and third columns are for biodiesel,biodiesel doped with 0.01% of GO, and biodiesel doped with 0.1% of GO, respectively. Theresults correspond to single droplet combustion.It is important to note that, since the shadowgraphy images do not allow for visualizingthe droplets past about 70% of the droplet lifetime, and also, the frequency of the shad-owgraphy imaging is 80 Hz, comparisons between the spectral content of Xd with those ofIf and Xf can only be made for t/τ ≲ 0.7 and f ≲ 80/2 = 40 Hz. Given the above, severalconclusions can be drawn from the results in Fig. 6.2. First, for single biodiesel droplets(with and without the doping agent), the spectral content of If and Xf are similar, suggest-ing that the dynamics of the flame chemiluminescence is dominated by its movements alongthe horizontal axis, which confirms the results presented in Fig. 6.1(b). For t/τ ≲ 0.7 andf ≲ 80/2 = 40 Hz, the spectral content of Xd is similar to those of If and Xf . Specifically,after about 20% of the droplet lifetime (at which point the atomizations start), both theflame and the droplet dynamics feature low-frequency (less than 10 Hz) oscillations for alltest conditions. Towards the end of the droplet lifetime, t/τ ≳ 0.8, If and Xf feature largeamplitude oscillations for 10 ≲ f ≲ 100 Hz.Compared to the dynamics of biodiesel droplet combustion, drastically different dy-976.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICSnamics are observed for ethanol-blended biodiesel droplet combustion. To illustrate this,the test condition of BE0.01 is selected as a representative test condition. The variationsof the droplet diameter squared as well as the normalized flame chemiluminescence areshown by the black and red curves in Fig. 6.3(a) and for the above test condition. Thevariations of the droplet center and flame chemiluminescence centroids subtracted by thecorresponding mean values are shown in Fig. 6.3(b) by the black and red curves, respec-tively. Figures 6.3(c–j) present the flame chemiluminescence (top panel) and shadowgraphy(bottom panel) images simultaneously acquired at t = 0, 0.08, 0.13, 0.14, 0.22, 0.46, 0.65,and 0.80 s, respectively. Also overlaid on the flame chemiluminescence images is the dropletand the flame chemiluminescence centroid, which are shown by the blue circle and crossdata symbol, respectively. As evident by the shadowgraphy image in Fig. 6.3(c), almostimmediately after the droplet is lit, both boiling and atomizations occur. This is due to therelatively smaller boiling temperature of ethanol (about 78oC) compared to that of biodiesel(≳ 200oC, which is provided in the safety data sheet acquired from Consolidated BiofuelsLTD.). The results in Fig. 6.3(c) show that boiling initiates at the liquid-liquid interface,which is referred to as the homogeneous nucleation [26]. As shown in Fig. 6.3 (a), both thedroplet diameter squared and the normalized flame chemiluminescence exhibit pronouncedfluctuations, which are due to an early and intense atomization occurring immediately afterthe droplet is lit. During these intense atomizations, the flame structure undergoes signifi-cant and frequent changes due to the ‘puffing’ of the fuel vapor [174] and the baby dropletsejection as illustrated in Figs. 6.3(c-j). The influence of these early atomizations on theflame is such that it becomes locally extinct at several locations, but the flame does notextinguish. Comparison of the results in Figs. 6.3(b) and 6.1(b) show that the additionof ethanol mitigates the lateral coupling between the droplet and flame chemiluminescencethrough homogeneous nucleation and atomization. It is also observed that the lateral flamelocation features larger fluctuations compared to those in Fig. 6.1(b). After the above in-986.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICStense and early atomization period, towards the end of the droplet lifetime, it undergoesrelatively mild atomizations. This is due to the heterogeneous nucleation at the liquid-solid,which are the fuel-GO/fiber, interfaces. Such atomizations are similar to those reported forGO-doped biodiesel test condition, see Fig. 6.1(b). Similar results were obtained for othertested GO doping concentrations.Fig. 6.3. (a) Variations of the droplet diameter squared (black) and normalized flamechemiluminescence (red) versus time. (b) Horizontal position of the droplet (black) andflame chemiluminescence (red) centroids subtracted by their mean values versus time. (c–j) are the flame chemiluminescence and droplet images corresponding to the time instantsmarked in (a) and (b). Specifically, (c–j) correspond to t = 0, 0.08, 0.13, 0.14, 0.22, 0.46,0.65, and 0.80 s, respectively. The results are for the representative test condition of BE0.01.It is of interest to compare the spectral contents of the flame chemiluminescence, theflame chemiluminescence centroid, and the lateral position of the droplet centroid for BEfuels with those of B fuels. To this end, the spectrograms of If , Xf , and Xd were obtainedand are presented in Fig. 6.4(a–c), (d–f), (g–i), respectively. In the calculation of thespectrograms, the size of sliding window used for subtracting the mean and the number ofthe FFT points were identical to those used for the calculation of the spectrograms presentedin Fig. 6.2. The results in the first, second, and third columns of Fig. 6.4 pertain to ethanolblended with biodiesel droplets that are doped with 0, 0.01, and 0.1% by mass of GO dopingconcentration, respectively. As the results in Fig. 6.4 show, the addition of graphene oxideto the ethanol blended with biodiesel droplets does not substantially influence the dynamics996.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICSof If , Xf , and Xd. However, comparison of the results presented in Fig. 6.2 with those inFig. 6.4 shows differences. As can be seen, as soon as the ethanol doped droplets are lit,large amplitude oscillations for f ≲ 50 Hz appear. The inspection of the shadowgraphyimages show that the large amplitude oscillations that appear immediately after the dropletignition are due to the homogeneous nucleation followed by intense atomizations. The meannormalized droplet lifetime at which such intense atomizations are completed is shown bythe solid straight magenta line. The dashed lines present the the above normalized timeplus and minus half of the normalized time standard deviation, which is estimated basedon the 12 repeats of each test condition. The shadowgraphy images (not presented here)and the results in Fig. 6.4 show that, after about 50% of the droplet lifetime, the intenseatomizations are followed by heterogeneous nucleation and relatively mild atomizations,with a frequency of about 10 Hz.010101Fig. 6.4. The spectrograms of single droplet combustion. (a–c) are for spatially integratedflame chemiluminescence, (d–f) are for the horizontal location of the flame chemilumines-cence centroid, and (g–i) are for the horizontal location of the droplet center. The resultsin the first, second, and third columns pertain to ethanol blended with biodiesel, ethanolblended with biodiesel and doped with 0.01%, and ethanol blended with biodiesel and dopedwith 0.1% by mass of GO, respectively.The results presented in Figs. 6.1–6.4 suggest, though the addition of GO may influence1006.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICSthe flame dynamics, significant alterations to these dynamics can be observed as a resultof blending with ethanol. Aiming to quantify such alterations, the root-mean-square (rms)of If and Xf were calculated. Since Xd cannot be obtained towards the end of the dropletlifetime, the rms of this parameter was not calculated. In calculating the rms of If andXf , the corresponding data was partitioned into two segments, reflecting the periods ofintense or moderate atomizations. To this end, the time highlighted by the magenta colorsolid lines in Figs. 6.2 and 6.4 were used. Specifically, for biodiesel droplets, the rms of theabove data was calculated for time periods before (B-NA) and after (B-A) the atomizations.For ethanol blended with biodiesel droplets, the rms of the above data (If,rms and Xf,rms)was calculated for those associated with the initial intense atomizations (BE-A1) and thosecorresponding to after this period (BE-A2). The rms of If and Xf versus GO concentrationare presented in Figs. 6.5(a) and (b), respectively. The error bars in the figures are thestandard deviation of the corresponding data and are calculated based on the repeats ofeach test condition. For data points without an error bar, the standard deviation wassmaller than the size of the corresponding data symbol and the bars are not presented. Theresults in Fig. 6.5 show that, generally, the addition of graphene oxide does not substantiallyinfluence the rms of If and xf . For biodiesel droplets, If,rms and Xf,rms are relatively largefor the periods that the mild atomizations occur compared to those for no atomizationperiods (compare the black and red square data points). For ethanol blended with biodieseldroplets, If,rms and xf,rms during the homogeneous nucleation and atomization are at leasttwice those during the heterogeneous nucleation and atomizations (compare the red withblack triangular data points). Also, the results in Fig. 6.5 show that If,rms and Xf,rmsare similar during the heterogeneous nucleation and atomizations for both B and BE fuels(compare the red square with the black triangular data points). In summary, the datapoints that feature no atomizations, heterogeneous nucleation and atomizations, as well ashomogeneous nucleation and atomizations are grouped by the solid, dashed, and dotted-1016.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICSdashed ellipses in Fig. 6.5(a). Similar groups are also identified for the results in Fig. 6.5(b)but are not shown to avoid clutter in the presentation. Overall, the results show thathomogeneous nucleation and atomization lead to significantly large values of If,rms andXf,rms. This finding has implications for the flame spread in multi-droplet as well as spraycombustion. The former is discussed in the next subsection as well as the next section, butthe latter is a topic for future research.0 0.01 0.100.20.40.60.810 0.01 0.100.20.40.60.81B-NAB-ABE-A2BE-A1Fig. 6.5. (a) and (b) are the rms of the flame chemiluminescence and the rms of the flamechemiluminescence centroid horizontal position, respectively. In the figure, B-NA and B-Acorrespond to data related to biodiesel test conditions before and after the atomizations,respectively. BE-A1 and BE-A2 correspond to the ethanol blended with biodiesel duringhomogeneous nucleation and atomization as well as heterogeneous nucleation and atomiza-tions, respectively. Th solid, dashed, and dotted-dashed ellipses in (a) correspond to testconditions with no atomizations, heterogeneous nucleation and atomizations, as well as ho-mogeneous nucleation and atomizations, respectively.6.1.2 Effect of atomization on multi-droplet combustion dynamicsTwo configurations for the multi-droplet combustion are considered. After three dropletswere deposited on the suspending fibers, either the droplet at the center or that on the right-hand-side is lit. This droplet is referred to as the main droplet in the discussions. Sincethe characteristics of flame spread between droplets are discussed in the next section, thespectral content of the multi-droplet combustion is characterized and comparisons are made1026.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICSwith those of a single droplet combustion discussed earlier.For the spectral analyses, the horizontal position of the droplet centroid is not used,since more than one droplet is present and analysis similar to those presented for singledroplet combustion could not be performed. Also, the horizontal location of the flamechemiluminescence centroid did not allow for best presentation of the atomization eventsdynamics and was not used here. Instead, the spectrogram of the spatially integratedflame chemiluminescence was used to study the spectral characteristics of the group dropletcombustion. Such spectral characteristics depend on whether the droplet at the center orthat on the side is lit. Thus, the results corresponding to the former and the letter aregrouped accordingly and presented in Figs. 6.6 and 6.7, respectively. The results in the firstand second rows of the figures correspond to biodiesel and ethanol blended with biodiesel,respectively. Those in the first, second, and third columns of both figures correspond to theneat fuel, fuel doped with 0.01% by mass of GO, and fuel doped with 0.1% by mass of GO,respectively. Overlaid on both figures are the straight solid lines in magenta color whichpresent the instants at which the flame envelops the first side and second side dropletsnormalized by the three-droplet lifetime. The dashed lines in magenta color present theabove normalized times plus and minus half of a standard deviation, which is calculatedbased on the 12 repeats of the experiments for each test condition.Comparison of the results in Fig. 6.6(a–c) show that the addition of GO does not sub-stantially influence the dynamics of the spatially integrated flame chemiluminescence for thecenter-droplet-lit test conditions. However, for test conditions that the side droplet is lit,the time between the ignition of the center droplet and the far most droplet is larger thanthat for the test conditions that the center droplet is lit, compare the distance between themagenta solid lines in Fig. 6.6 with those in Fig. 6.7. This is because once the center dropletis lit, heat transfer to both side droplets start. However, for scenarios that the side dropletis lit first, the farthest droplet is lit after the center droplet is ignited, leading to long delay1036.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICS0101Fig. 6.6. The spectrogram of the flame chemiluminescence for multi-droplet combustion withthe center droplet lit. The results in (a–c) correspond to biodiesel and (d–f) correspond toethanol blended with biodiesel. The results in the first, second, and third columns pertainto neat fuel, fuel doped with 0.01% by mass of GO, and 0.1% by mass of GO, respectively.0101Fig. 6.7. The spectrogram of the flame chemiluminescence for multi-droplet combustionwith the side droplet lit. The results in (a–c) correspond to biodiesel and (d–f) correspondto ethanol blended with biodiesel. The results in the first, second, and third columns pertainto neat fuel, fuel doped with 0.01% by mass of GO, and 0.1% by mass of GO, respectively.for the ignition of the farthest droplet. For biodiesel test conditions with the center dropletlit, since the ignition of both side droplets occur at a relatively short time period from oneanother, heterogeneous combustion only occurs towards the end of the multi-droplet com-bustion lifetime. This leads to the large oscillations amplitude near ∼10 Hz. However, forscenarios that the side droplet is lit, heterogeneous atomizations occur at the end of the sidedroplet lifetime (see the large amplitude oscillations before t/τ = 0.5 in Fig. 6.7(a–c)) which1046.1. EFFECT OF ATOMIZATION ON MULTI-DROPLET COMBUSTION DYNAMICSis followed by the ignition of the farthest droplet. Towards the end of farthest droplet com-bustion lifetime, the heterogeneous atomizations occur, see the large oscillations amplitudein the second half of the multi-droplet lifetime in Fig. 6.7(a–c).The results presented in Fig. 6.3 for single droplet combustion of ethanol blended withbiodiesel as well as those in the second row of Figs. 6.6 and 6.7 show that the presence ofethanol leads to the homogeneous atomizations immediately after the ignition of the maindroplet which leads to an early ignition of the second droplet, compare the times associatedwith the first magenta solid lines in the first row of Figs. 6.6 and 6.7 with those in thesecond rows of the figures. It is also observed that, for either of the ignition scenarios, thelast droplet ignited sooner for the ethanol blended with biodiesel test conditions compared tobiodiesel test conditions, compare the time difference between the magenta solid lines in thefirst and second rows of either of Figs. 6.6 and 6.7 and for each test condition. The resultsin Figs. 6.6(d–f) and 6.7(d–f) also show that after the flame propagation is completed, theethanol blended with biodiesel droplets continue to feature atomizations and towards theend of the multi-droplet lifetime. The shadowgraphy videos showed that the large values ofthe FFT immediately after the second solid magenta lines are associated with heterogeneouscombustion.To compare the spectral contents of the dynamics presented in Figs. 6.6 and 6.7, theFFT of the flame chemiluminescence oscillations was calculated for all test conditions, thedominant frequency of oscillations was obtained, and the results are presented in Fig. 6.8.The lengths of the error bars are the full-width at half maximum of the FFTs near themost dominant frequency. The results in Fig. 6.8(a–c) correspond to a single droplet, multi-droplet combustion with center droplet lit, and multi-droplet combustion with the sidedroplet lit, respectively. For comparison purposes, the results from Chapter 5 (single dieseldroplet combustion) are also compiled and presented in Fig. 6.8(a). The results show that,overall, the low-frequency of oscillations observed for single droplet combustion also exists1056.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSfor multi-droplet combustion. This finding highlights that the low frequency dynamics thatexists for single droplet combustion also exists for an array of three droplets and is dueto the atomizations. Whether such low frequency oscillations induced by atomization alsoexist for an array of droplets with more than three droplets or for turbulent spray flames isnot studied in this thesis and is a topic for future investigations.DBBEFig. 6.8. The dominant frequency of the flame chemiluminescence oscillations. The resultsin (a), (b), and (c) correspond to single droplet combustion, multi-droplet combustion withthe center droplet ignited, and multi-droplet combustion with the side droplet ignited. Thetriangular, square, and circular data point pertain to diesel, biodiesel, and ethanol blendedwith biodiesel, respectively.6.2 Effect of atomization on flame spread rate in an arrayof dropletsAnalysis of the flame chemiluminescence and shadowgraphy images allowed for identi-fying the modes of the flame spread, with exemplary results presented in Fig. 6.9. For theresults in the figure, the flame spreads from right to left. In the first column of Fig. 6.9,the right (left) solid black curve presents the horizontal location of the main (side) dropletcenter versus time. In the figure, the dotted black curves present the horizontal locationsof the droplets edges. The time required for the heat from the main droplet’s flame toreach the side droplet, locally vaporize the side droplet, create a combustible mixture at itsvicinity, and establish a sustainable flame kernel is referred to as τi. The time required for1066.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSthe side droplet to absorb heat from the flame kernel, create the fuel vapor, and finally theflame kernel to propagate through the fuel-air mixture around the side droplet is referred toas τp. The flame chemiluminescence images show that the flame propagates both at the topand at the bottom of the main droplet towards the side droplet. In order to quantify τi andτp, the leading points of the flame both at the top and at the bottom were monitored. Thehorizontal locations of the flame’s leading point at the top (bottom) are shown using theblue (magenta) colors, respectively, in Figs. 6.9(a–d). The time instant at which the bottomflame leading point reaches the side droplet is referred to as ti, with the corresponding pointshown by a green cross data symbol on the x−t diagrams in Figs. 6.9(a–d). The time instantat which the flame leading points at the top and at the bottom reach together is referred toas tp. At this time, the side droplet is fully enveloped by the flame, and the correspondingpoint is highlighted by both tp and a green cross data symbol in Figs. 6.9(a–d). Given theabove time instants and that t = 0 is set to be the time at which a flame envelops the maindroplet, the ignition and propagation times can be calculated from τi = ti and τp = tp − ti.For either of the modes presented in the first column of Fig. 6.9, several flame chemilumi-nescence images are presented to the right, elaborating the flame propagation. Overlaid onthe figures are the contours of the flame chemiluminescence at the top (blue color) and thebottom (magenta color). The leading points of the above contours (the most left location onthe contours) are shown by the solid circular data points with matching colors in the figures.Also overlaid on the figures are the main and side droplets shown by the black circular datapoints. The size of the circular data points resemble the diameter of the relevant droplet.For the figures that the main droplet is not shown, its size is close to the suspending fibersand the droplet cannot be detected from the shadowgraphy images. It is observed that forall test conditions and their repeats, the flame propagates at the top before it propagatesat the bottom. Since the experiments were conducted under normal gravity conditions,the upward natural convection increases the heat transfer to the upper portion of the side1076.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSFig. 6.9. Modes of flame spread for (a–c) GO doped biodiesel and (d) biodiesel blendedwith ethanol droplets. The results presented in (a–c) and (d) correspond to B0.1 andBE0.1, respectively.droplet and this is speculated to increase the fuel vapor formed at the top of the side dropletand as a result the sooner propagation of the flame at the top. The results show that Mode1 occurs for GO-doped biodiesel droplets with an inter-droplet spacing (S) smaller thanhalf-width of the main droplet flame. This width is defined as half of the distance betweenthe most left and most right sides of the main droplet flame contour at t = 0. The averagingfor S is performed during the time period between t = 0 and the smaller between ti andthe latest time that the main droplet can be detected from the shadowgraphy images. ForMode 1, the flame spread is completed before the occurrence of the atomization events, andas a result, the flame spread is not affected by the atomizations.Mode 2 of flame spread occurs for the values of S that are large enough that the maindroplet flame does not reach the side droplet immediately after the main droplet flameis formed. The flame chemiluminescence images associated with Mode 2 of propagation1086.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSare shown in the second row of Fig. 6.9. For this mode, the main droplet is ignited andthe corresponding flame envelops this droplet. During about 1 s, the main droplet flameis established, the main droplet burns, and the side droplet receives heat from the maindroplet flame. During this time, an ignitable fuel vapor and air mixture is formed betweenthe side droplet and the main droplet flame, and as a result, the flame of the main dropletpropagates towards the side droplet, see the flame chemiluminescence images in the secondrow of Fig. 6.9. For Mode 2, τi is significantly larger than that for Mode 1.The third mode of flame spread, which is identified in the present thesis for the first time,is associated with atomizations and is referred to as Mode A. For this mode, after the maindroplet is ignited, the inter-droplet spacing is large enough that the flame does not propagatetowards the side droplet initially. However, towards the end of the main droplet lifetime,this droplet undergoes atomizations and due to the flame’s lateral movement (discussed inthe previous subsection), the main droplet flame eventually reaches the side droplet andpropagates around this droplet, see the flame chemiluminescence image corresponding tot = 1.64 s in the third row of Fig. 6.9.Depending on the occurrence of heterogeneous or homogeneous nucleation and atom-ization (which took pace for biodiesel and ethanol blended with biodiesel droplets), twovariations of Mode A, which are Mode A (B) and Mode A (BE), are identified. For ModeA (B), typically, the main droplet combustion is completed, and the kernel formed aroundthe side droplet slowly propagates to envelop the side droplet. This is shown in the flamechemiluminescence images presented in the third row of Fig. 6.9. For ethanol blended withbiodiesel droplets (which exhibit Mode A (BE)), atomization begins immediately after themain droplet flame is formed, and the side droplet is lit due to such atomizations. Comparedto the biodiesel droplet combustion for which the main droplet is completely burnt beforethe side droplet flame is formed, for ethanol blended biodiesel droplets, the main flame en-velops the side droplet immediately. The results presented in the fourth row of Fig. 6.9 show1096.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSthe flame propagation associated with Mode A (BE). Comparison of the results presentedabove with those in the literature show that Modes 1 and 2 are similar to those discussedin a few past investigations, see for example [85, 86]. However, the mechanistic descriptionof how atomizations can lead to flame spread in an array of burning droplets, which areModes A (B) and A (BE), are not previously discussed in the literature and are elaboratedhere for the first time to the author’s best knowledge.The discussions presented above suggested that both the inter-droplet spacing and blend-ing with ethanol can influence the flame spread between the droplets. Here, the quantifica-tion of the flame spread rate is of interest. To this end, the flame spread time (τs = τi+ τp)was first calculated. τs is the time that it takes for the flame to envelop the side droplet afterthe main droplet is lit. To quantify the flame spread rate, and similar to past investigations,see for example [85–87], SD0/τs was estimated, see [85]. For all test conditions, the sidedroplet ignition time, the flame propagation time around the side droplet, and the flamespread rate were obtained and are presented against the normalized spacing in the first,second, and third columns of Fig. 6.10, respectively. The results pertaining to biodiesel andethanol blended with biodiesel droplets are shown in Figs. 6.10(a–c) and Figs. 6.10(d–f),respectively. Data presenting Modes 1 and 2 are shown by the black circles and blue crosssymbols, respectively. Data relating to Mode A (B) is presented using the red square symbolfor biodiesel, and that for Mode A (BE) is shown using the red diamond symbol for ethanolblended with biodiesel droplets. The scatter in the data in each panel is due to the 12repeats of each test condition, two tested configurations for the multi-droplet combustion(which is lighting the side or center droplet), and the three tested concentrations of GO.Thus, 12 × 2 × 3 = 72 data points exist in each panel of Fig. 6.10. To study the impactof spacing on τi, τp, and SD0/τs as well as for later comparisons with the results in theliterature, the scattered data points in Fig. 6.10 were grouped into several bins. Then, thedata in the bins was averaged. Finally, for each Mode of the flame spread, the averaged1106.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSdata points were connected by solid lines, see Fig. 6.10. These lines facilitate understandingthe influences of inter-droplet spacing and Modes of flame spread on τi, τp, and SD0/τs. InFig. 6.10, the length of the error bars are equal to the standard deviation of the scattereddata point within each bin.Fig. 6.10. Ignition time, propagation time, and flame spread rate versus the normalizedinter-droplet spacing. (a–c) and (d–f) correspond to the biodiesel and ethanol blended withbiodiesel droplets combustion, respectively.The results in Figs. 6.10 (a–c) show that Mode 1 of flame propagation is present forS/D0 ≲ 2.5. The observation that Mode 1 of flame propagation correspond to relativelysmall inter-droplet spacing is consistent with that in Mikami et al. [85, 86]. For this mode ofpropagation, as soon as the main droplet is enveloped by its flame, it becomes in contact withthe side droplet and the main droplet flame vaporizes the side droplet rather immediately.This is accompanied by the formation of the initial flame kernel close to the side droplet.Provided sufficient heat is available, the time that is takes for the fuel vapor and air toignite relates to the combustion chemistry of the fuel and is generally very short (on theorder of milliseconds [175, 176]). Thus, it is expected that τi primarily depends on howsoon the heat generated by the flame of the main droplet produces a combustible mixture1116.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSnear the side droplet. As can be seen in Fig. 6.10(a), once the normalized spacing (S/D0)exceeds approximately 2.5, the main droplet flame does not reach the side droplet, andMode 2 prevails. Compared to Mode 1, for Mode 2, the transfer of heat from the flame ofthe main droplet to the side droplet is decelerated, and as a result, τi estimated for Mode2 is larger than that for Mode 1. The results in Fig. 6.10(a) show that, for S/D0 ≳ 3,both Mode 2 and Mode A (B) can exist. It is observed that, for a fixed value of the inter-droplet spacing, τi is larger for Mode A (B) than that for Mode 2. This suggests that therandom atomizations associated with Mode A (B) distribute the heat generated from themain droplet combustion in random directions leading to decreased heat transfer to the sidedroplet and as a result increasing τi compared to that for Mode 2. Nonetheless, for largevalues of S/D0, atomizations can lead to ignition of the side droplet (via Mode A (B) andin addition to Mode 2), which is desirable for practical engineering applications.Compared to τi that depends on how soon a flame kernel is formed at the vicinity ofthe side droplet, τp is influenced by the amount of the combustible mixture that is formedaround the side droplet. The results presented in Figs. 6.10(a and b) show that τi and τp areboth about 0.5 s for Mode 1. However, for S/D0 ≲ 3, increasing the inter-droplet spacingincreases (decreases) τi (τp). It is observed that, for relatively large normalized inter-dropletspacing (S/D0 ≳ 3) and for Mode A (B), increasing S/D0 plateaus τp. This is because, forsuch spacing and for Mode A (B), the ignition time plateaus with increasing the normalizedinter-droplet spacing. As a result, for S/D0 ≳ 3, the time that the side droplet is exposed tothe heat of combustion from the main droplet flame does not change, leading to formationof similar amount of vapor around the side droplet and plateau of τp. Comparison of theresults in Fig. 6.10(a) for Mode 2 and Mode A (B) shows that τi is longer for the latter thanthe former mode. That is, for S/D0 ≳ 3, even though the side droplet is exposed to themain droplet flame for a longer period of time and for Mode A (B) compared to that forMode 2, τp is similar for both modes and this parameter does not continue to decrease with1126.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSincreasing the inter-droplet spacing. This is due to the atomization of the main droplet andloss of combustion heat for Mode A (B).Compared to biodiesel droplets combustion, for ethanol blended with biodiesel dropletscombustion, the main droplet atomizations begins as soon as it is lit, which substantiallyinfluences how fast the flame spreads. Comparison of the results presented in Fig. 6.10(a)with those in Fig. 6.10(d) show that the ignition time for Mode A (BE) is several timessmaller than that for Mode A (B). This is because of two reasons. First, larger inter-dropletspacing leads to longer time to generate combustible mixture near the side droplet. Second,increasing the inter-droplet spacing requires larger number of baby droplets to be ejectedso they can reach to and ignite the side droplet, which leads to large values of τi. Althoughit was concluded that increasing the inter-droplet spacing increases τi, conclusions for theeffect of S/D0 on τp cannot be drawn. This is because, on one hand, increasing S/D0increases the time for the droplets evaporation which can increase the amount of vaporaround the droplet and, as a result, decreasing τp. However, on the other hand, increasingthe inter-droplet spacing leads to significant loss of combustion heat from the droplet priorto the flame propagation, which could reduce the amount of fuel vapor around the sidedroplet and potentially decreasing τp. Nonetheless, the results in Fig. 6.10(e) suggest thatmaximum propagation time occurs for 2.5 ≲ S/D0 ≲ 3.Several conclusions are obtained comparing the flame spread rate for Modes 1, 2, andA (B) with that for Mode A (BE). It can be seen that, for Modes 1 and 2, SD0/τs increasesfrom about 6 to 8 increasing the normalized inter-droplet spacing from about 2 to 3. Thereason for this increasing trend is due to the significant decrease of τp by increasing thenormalized spacing. Compared to those for Modes 1 and 2, relatively small values of SD0/τsis reported for Mode A (B), which is due to the prolonged ignition time caused by randomatomizations of this mode. Compared to the results in Fig. 6.10(c), those in Fig. 6.10(f)show significant scatter of the flame spread rate (6–8 versus 10–40). Nonetheless, owing to1136.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSrelatively small values of both τi and τp, values of SD0/τs for Mode A (BE) are approximatelythree times those for Mode A (B).It is anticipated that the flame spread rate is influenced by either inter-droplet spacing,blending with ethanol, the presence of GO, the occurrence of atomizations, or a combinationof these. It is acknowledged that separating the effects of the above parameters on the flamespread rate is not possible from the results of the present investigation. However, conclusionsconcerning the significance of the effect of either parameter on the flame spread rate may bedrawn. Aiming to study this, the rms of the flame chemiluminescence centroid lateral move-ments during the spread time (Xf,rms) is calculated for all test conditions and its variationversus the flame spread rate is presented in Fig. 6.11. The results in Fig. 6.11 are symbolcoded based on the Modes of the flame spread. Additionally, those in Figs. 6.11 (a and b)are color coded based on the normalized inter-droplet spacing; and, those in Figs. 6.11 (cand d) are color coded based on the GO doping concentration. The results in Fig. 6.11 showthat the effects of the normalized droplet spacing and GO concentration on the flame spreadrate are not as pronounced as the effect of ethanol addition to biodiesel. While the datapoints related to the biodiesel test conditions (see the circle, cross, and square symbols) areclustered and correspond to small spread rate and small Xf,rms, for Mode A (BE), the flamespread rate and Xf,rms are positively related. That is, for non-atomizing droplets, whosecombustion mode is denoted by Modes 1 and 2 (see the square and circular data symbols),the flame’s normalized lateral displacement rms remains small (Xf,rms/D0 ⪅ 0.05) for alltested normalized spacing values and GO doping concentrations. Comparison of the resultsfor Modes 1 and 2 with those for Mode A (B) shows that, even though the atomizationincreases Xf,rms for biodiesel, the flame spread rate remains unchanged and about 6 mm2/s.However, for Mode A (BE), the atomizations lead to relatively large values of Xf,rms, whichis positively related to the flame spread rate. Specifically, for Mode A (BE), Xf,rms as largeas ∼2 mm and flame spread rates as large as about 40 mm2/s are reported.1146.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSFig. 6.11. The flame spread rate versus the rms of the flame chemiluminescence centroidlateral movements during the spread time. (a and c) correspond to Modes 1, 2, and A (B).(b and d) correspond to Mode A (BE). Data in (a and b) are color-coded based on theinter-droplet spacing. Data in (c and d) are color coded based on GO doping concentration.It is of interest to compare the flame spread rate estimated in the present study withthose of past investigations. For comparison purposes, the segmented lines in Figs. 6.10(cand f) versus the normalized spacing are shown in Fig. 6.12. Also overlaid on the figure arethe results of Park et al. [7, 8] and Wang et al. [9]. In the studies of Park et al. [7, 8], the flamespread rate was measured for pure decane and hexadecane array of droplets. Wang et al. [9]measured the flame spread rate in an array of 5-dimethylfuran (DMF) blended Jatrophaoil (JO) droplets. The boiling temperatures of decane, DMF, hexadecane, and Jatropha oilare 174 oC [177], 92 oC [9], 297 oC [7], and 355 oC [9], respectively. The results in Fig. 6.12may suggest that the small boiling temperature/large volatility of decane and DMF isaccompanied by about an order of magnitude larger flame spread rate compared to that ofJetropha oil and biodiesel, which are fuels with relatively large boiling temperature/smallvolatility. The results of Wang et al. [9] also show that blending fuels with different volatility1156.2. EFFECT OF ATOMIZATION ON FLAME SPREAD RATE IN AN ARRAY OF DROPLETSleads to mixture with flame spread rate values in between those of the original fuels. This issimilar to the observations reported in this thesis. That is the addition of ethanol increasesthe flame spread rate. The results of both the present study and those of Park et al. [7, 8] andWang et al. [9] show that, for a given fuel blend, the flame spread rate is nearly insensitiveto S/D0 and for the tested normalized spacing. However, for fuels with large volatility, theflame spread rate varies by changing the tested normalized spacing.Fig. 6.12. The flame spread rate versus the normalized inter-droplet spacing. Overlaid onthe figure are the results of Park et al. [7, 8] and Wang et al. [9].In essence, the findings of past investigations and those in this thesis show that, forlow volatility fuels (such as biodiesel), the flame spread rate is primarily influenced by theaddition of a higher volatility fuel (such as ethanol) and due to the homogeneous nucleationand atomization. It is concluded, for such low volatility fuels, the inter-droplet spacing andthe addition of GO have minimal impacts on the flame spread rate.116Chapter 7ConclusionsThe research conducted in this dissertation has yielded several key conclusions. Thischapter begins by examining these concluding remarks. Following that, recommendationsare presented to support future studies that can build upon the findings of the currentresearch.7.1 ConclusionsThe influences of atomization on the burning rate, combustion dynamics, and flamespread rate of droplets were studied in this thesis. The tested liquid fuels were ethanol,diesel, and biodiesel. The fuels were doped by several types of Graphene Oxide (GO). Theconcluding remarks are summarized in the following.Effect of atomization on the burning rate of ethanol droplets: The effects ofatomization on the burning rate of ethanol droplet doped with GO were investigated usingseparate high-speed OH∗ chemiluminescence and high-speed shadowgraphy imaging. Thedroplets were deposited on a borosilicate tube and were lit using a plasma-assisted igni-tor. Three different types of GO, categorized by their oxidation level were tested. Thesewere reduced, partially oxidized, and highly oxidized GO, which were referred to as rGO,pGO, and hGO, respectively. GO doped ethanol with doping concentrations (by mass)of 0, 0.01, and 0.1% were utilized. Characterization of such GO nanomaterials involvedRaman spectroscopy, high- and coarse-resolution Scanning Electron Microscopy (SEM),Fourier-Transform Infrared (FT-IT) spectroscopy, and thermogravimetric analysis (TGA).1177.1. CONCLUSIONSThe Raman spectroscopy results confirmed the distinctive GO characteristics of the usednanomaterials, namely the G and D bands were present. The results from high-resolutionSEM illustrated the layered structure of GO, indicating that the thickness of 2D layers inGO nanomaterials is less than approximately 100 nm. Coarse-resolution SEM was employedto estimate the Probability Density Function (PDF) of the characteristic length of the testedGO nanomaterials. The analysis suggested a similar PDF for all three types of grapheneoxide, with the most probable characteristic length being about 2 µm. Fourier-TransformInfrared spectroscopy of the three samples exhibited peaks at similar wave-numbers, sug-gesting the presence of matching oxygen-containing functional groups. The combined resultsfrom infrared spectroscopy and thermogravimetric analysis indicated that rGO, pGO, andhGO indeed correspond to GO with the least, moderate, and maximum amounts of oxida-tion, respectively. The suspensions were prepared, and it was confirmed that they are stablefor time periods longer than the combustion experiments.The results showed that the atomization phenomenon occurred during combustion of alltested doped fuels; however, it did not take place for pure ethanol droplets. It was concludedthat, for ethanol doped droplets, nucleation occurred at the liquid-solid (GO) interface(namely heterogeneous nucleation), which led to the atomizations. For all experimentsfeaturing the atomization phenomenon, the ejection of baby droplets only occurred duringthe second half of the droplet lifetime. The most probable values of the baby droplets initialprojected velocity, the baby droplets initial diameter, and the length of the baby dropletsprojected trajectory were approximately 0.5 m/s, 50 µm, and 1 mm, respectively. Thesevalues were independent of the GO type and the tested doping concentrations. The mostprobable values for the baby droplets initial projected velocity and length of the projectedtrajectory were comparable to the flame speed of ethanol and air mixture at stoichiometriccondition and the size of the main droplet, respectively. The Joint Probability DensityFunctions of these parameters were calculated. It was shown that majority of the baby1187.1. CONCLUSIONSdroplets initially feature a relatively small diameter and travel about 1 mm with an initialprojected velocity of about 0.5 m/s. This finding suggested that most baby droplets have arelatively short lifetime and may potentially burn before escaping the flame.The burning rate, defined as the absolute value of the slope of the droplet diametersquared versus time, significantly increased once atomizations started. The burning ratesduring periods with and without atomizations were denoted by K1 and K2, respectively.The burning rate analysis suggested that increasing the doping concentration and reducingthe oxidation level of GO led to an increase of K1. This enhancement was speculated tobe related to the improved thermal conductivity of the fuel due to the addition of GOnanomaterials. K2 was largest for pGO followed by those for rGO and hGO. The resultsof FT-IR spectroscopy indicated the above observation is linked to the amount of thermalradiation absorption for the tested GO. Specifically, such absorption integrated over theresolved wavelengths was positively related to K2, with the test condition of ethanol dopedwith 0.01% of pGO featuring the largest values of K2 and the above integral. Using amass-balance framework, the effect of atomization on the droplet mass loss was studied. Itwas found that in the presence of relatively intense atomizations (indicating a large totalmass of ejected baby droplets), the evaporation mass loss of the droplet is suppressed. Theresults suggested that adding GO to ethanol and increasing the doping concentration from0.01 to 0.1% improves the overall burning rate. The maximum improvement was 8.4% forethanol droplets doped with 0.1% of rGO.Effect of atomization on single droplet combustion dynamics: The influence ofatomization on the coupling between single diesel droplets doped with GO and their flameswas investigated using separate high-speed OH∗ chemiluminescence and high-speed shad-owgraphy imaging. The droplets were deposited at the intersection of three silicon carbidefibers and ignited using a plasma-assisted ignitor. Octadecyl amine functionalized GO wasused as the doping agent for diesel. GO doped diesel droplets, with doping concentrations1197.1. CONCLUSIONS(by mass) of 0, 0.001, 0.005, 0.01, and 0.02%, were used in the experiments. The SEMimages illustrated the 2D structure of the functionalized GO, with a thickness of approx-imately 100 nm. Suspensions of GO in diesel were prepared, and they were stable for atleast 5 months. Dynamic Light Scattering experiments were conducted, suggesting that themost probable hydrodynamic diameter of the stable GO platelets suspended in diesel wasapproximately 300 nm.The results showed that the atomization occurred for all test conditions, leading tooscillations in both the droplet diameter squared and flame chemiluminescence. Theseoscillations exhibited intermittent behavior, as evidenced by the phase-space trajectoriesand Poincare´ maps. The intensity of the atomization events was quantified by calculatingthe ratio of the droplet diameter squared at the beginning of each event to that at theend. The time-varying PDF of the atomization intensity indicated an increased likelihoodof large-intensity atomization events toward the end of the droplet lifetime. It was observedthat an increase in the mean number and intensity of the atomization events resulted inlarger root mean square (rms) values of the droplet diameter squared oscillations. Dopingconcentrations ranging from 0.005 to 0.02% showed approximately 100% larger rms valuescompared to those with smaller doping concentrations.The atomization intensity was utilized to condition the droplet diameter squared oscilla-tions. Before the occurrence of the intense atomization events, both the PDF of the inverseof the time separation between two consecutive atomization events and the Power SpectrumDensity (PSD) of the droplet diameter squared oscillations exhibited high probabilities andpowers at around 25 Hz. Following the occurrence of intense atomization events, this fre-quency decreased to approximately 5 Hz for both mentioned parameters. The PSD of thesquared droplet diameter oscillations at 5 Hz was about one order of magnitude larger thanthat at 25 Hz. It was demonstrated that atomizations led to large amplitude oscillations at5 Hz, while the retracting motion of the ignitor induced oscillations at 25 Hz. For all test1207.1. CONCLUSIONSconditions, the dominant frequencies of flame chemiluminescence, droplet diameter squared,the vertical position of the droplet center, and atomization events were determined. It wasobserved that, depending on the doping concentration, the frequency of droplet vertical os-cillations that are triggered by the retraction of the ignitor can vary from about 21 to 25 Hz,while the dominant frequencies of the others vary from about 5 to 13 Hz. In essence, forall doping concentrations, the frequencies of droplet diameter squared and flame chemilu-minescence oscillations locked onto that of intense atomization events, which are relativelysmall (less than 15 Hz). It was concluded that the atomizations induce a coupling betweenthe droplet and its flame.Effects of atomization on multi-droplet combustion dynamics and flame spreadrate: The impacts of atomization on the combustion dynamics and flame spread rate wereinvestigated for biodiesel and ethanol blended with biodiesel droplets. Octadecyl aminefunctionalized GO was used as the doping agent, with concentrations of 0, 0.01, and 0.1%by mass. The investigations were performed for both single and an array of three dropletspositioned along a horizontal line. The latter was referred to as multi-droplet configuration.Simultaneous and temporally resolved OH∗ chemiluminescence and shadowgraphy imagingwere performed. The droplet(s) were deposited at the intersections of silicon carbide fibers,and a plasma-assisted ignitor was used for ignition. For multi-droplet combustion experi-ments, either the droplet at the center or that on the right-hand-side was ignited. Ramanspectroscopy and X-Ray Diffraction were conducted to characterize the GO nanomaterials.The former showed the presence of G and D bands, which are characteristics of grapheneoxide nanomaterials. The results of the latter suggested four groups of peaks which wererelated to the presence of the octadecyl chain, synthesis of GO from graphite, removal ofC–O–C from the basal plane of graphene oxide, and the layered structure of GO. The SEMof graphene oxide was performed and the images confirmed the layered structure of theutilized nanomaterials. The suspensions were prepared, and it was confirmed that they1217.1. CONCLUSIONSwere stable for time periods significantly longer than the combustion experiments. In thefollowing, the conclusions are presented for the dynamics of both single and multi-dropletcombustion as well as the flame spread rate.For single (GO doped) biodiesel droplet combustion, after about 20% of the dropletlifetime, heterogeneous nucleation occurred at the liquid-GO/fiber interface, leading to at-omizations. This caused coupled, lateral movements of the droplet and its flame. Towardsthe end of the droplet lifetime, these atomizations intensified, significantly altering thedroplet and flame movements. Though adding GO did not substantially influence the abovedynamics, blending ethanol with biodiesel drastically influenced the dynamics. Specifically,immediately after the droplet was lit, nucleation at liquid-liquid interface (namely homo-geneous nucleation) occurred leading to intense atomizations. The rms of the spatiallyintegrated flame chemiluminescence and its lateral movements were calculated for all testedfuels, and for scenarios with no atomizations, heterogeneous atomizations, and homoge-neous atomizations. It was concluded that the rms of flame oscillations increases in thepresence of atomizations, with that during homogeneous nucleation and atomization beinglarger than that during heterogeneous nucleation and atomization. The spectral analysisof biodiesel droplet combustion dynamics suggested that the heterogeneous nucleation andatomization led to large amplitude oscillations of the flame chemiluminescence and its cen-troid lateral movements at about 10 Hz. Similar to biodiesel droplets, ethanol blended withbiodiesel droplets also featured large amplitude oscillations at about 10 Hz and towards theend of the droplet lifetime. However, ethanol blended with biodiesel droplets featured largeamplitude flame chemiluminescence and its centroid lateral oscillations for frequencies lessthan 50 Hz and immediately after the droplet was lit. Such dynamics of ethanol blendedwith biodiesel droplet combustion was not significantly influenced by the addition of GO.The low frequency dynamics observed for single droplet combustion was also present formulti-droplet combustion with both side and center droplet ignition configurations. Such1227.1. CONCLUSIONSdynamics was pronounced once each droplet was ignited and burnt. The Fast Fourier Trans-form analysis of the spatially integrated flame chemiluminescence was performed, and it wasshown that both single and multi-droplet combustion of biodiesel and ethanol blended withbiodiesel feature pronounced dynamics at about 10 Hz. This was not influenced by thedoping concentration of GO.The impact of atomization on the flame spread in a multi-droplet configuration wasstudied. Three modes of flame spread were identified. The first two modes were identical tothose reported in past studies and were driven by the diffusion of heat and mass. However,for the last mode, the flame spread was driven by the atomizations. The results showedthat, for both pure and GO-doped biodiesel and for relatively large inter-droplet spacing, thespread of flame from the main to the side droplet occurred due to heterogeneous nucleationand atomizations, referred to as Mode A (B). For ethanol blended with biodiesel, the flamewas spread due to homogeneous nucleation and atomization. That is, independent of the GOdoping concentration and the inter-droplet spacing, the flame spread occurred immediatelyafter the atomization for ethanol blended with biodiesel droplets. This was referred to asMode A (BE).For all of the above identified modes, the flame spread rate (which is the inter-dropletspacing multiplied by the droplet initial diameter divided by the flame spread time) wasestimated. The flame spread time was defined as the difference between the time that themain droplet (i.e. the ignited droplet) was enveloped by its flame and the time that a sidedroplet was enveloped by its flame. It was observed that for the first two modes of flamespread, increasing the droplet spacing slightly increases the flame spread rate from about 6to 8 mm2/s, which was argued to be linked to elongated heat up period of the side dropletand (potentially) an increased amount of fuel vapor around the side droplet. For ModeA during the heterogeneous nucleation and atomization, the flame spread rate was smallerthan other modes and for similar droplet spacing, which was linked to the random ejection1237.2. SUGGESTIONS FOR FUTURE DIRECTIONS OF RESEARCHof baby droplets and loss of combustion heat during the atomizations. Compared to these,for ethanol blended with biodiesel droplets, the flame spread rate was several times largerthan the others (10–40 mm2/s). It was shown that, compared to heterogeneous nucleationand atomization, the flame spread rate featured an increasing trend with the rms of theflame chemiluminescence lateral movement for homogeneous nucleation and atomization.Comparing the results of the present study with those of past investigations, it was concludedthat the impacts of inter-droplet spacing and GO addition on the flame spread rate areminimal for low volatility fuels (such as biodiesel). However, adding volatile components tosuch fuels can substantially increase the flame spread rate.This thesis presented observation and quantification of atomization events for severalliquid fuel droplets doped with GO or blended with ethanol. The impacts of doping liquidfuels with GO or blending were quantified in terms of burning rate, combustion dynamics,and flame spread rate. The findings suggested doping with GO or blending with a volatilefuel influences the above. This warrants future investigations to understand the impacts ofdoping or blending with ethanol on more industrially relevant flames such as spray flamesin terms of combustion dynamics and emissions. Detailed suggestions for future researchare presented below.7.2 Suggestions for future directions of researchWhile this thesis aimed to partially address ongoing research questions concerning thecombustion of atomizing droplets, future investigations are necessary to substantiate someof the findings presented in this dissertation. Additionally, there is a need to expand thesefindings to other studies and address remaining questions about the effects of atomizationon droplet combustion. This section provides details regarding the suggested future inves-tigations.1247.2. SUGGESTIONS FOR FUTURE DIRECTIONS OF RESEARCH1. In the present thesis, each droplet was deposited on either a 1.16 mm diameter borosil-icate tube or at the intersection of three 142 µm diameter silicon carbide fibers. Tofurther enhance the droplet suspension and minimize the impact of the suspendingmechanism on the droplet combustion, it is recommended to employ smaller diameterfibers [140]. For small diameter fibers, additional care is necessary to ensure the fibersare properly positioned. The smaller fiber diameter would decrease heat transfer be-tween the fibers and droplets, promote the formation of spherical droplets, and reducethe possibility of the occurrence of heterogeneous nucleation at the fuel-fiber interface.2. In this study, the stability of blended and doped fuels was qualitatively assessedthrough fuel photography. To ensure the development of blended and doped fuels withhigh stability until their combustion, it is recommended to conduct Zeta potential ex-periments to measure fuel stability accurately. This method enables the developmentof a fuel suspension with an optimal pH level, Zeta potential, and stability.3. Few studies, for example [95, 96] observed a reduction in unburnt hydrocarbons andsoot formation in diesel engines using doped and blended fuels relevant to this study.To clarify the mechanisms responsible for these improvements, it is recommended tocollect and characterize the residues after droplet burnout. For instance, comparingthe Raman spectrum of the residues with that of graphene oxide can elucidate theextent of burning of graphene oxide additives. Indeed, investigations into combustionresidues could offer valuable insights into the emission characteristics of the studiedfuels.4. It was suggested that the droplet internal circulation could potentially influence thenucleation process, consequently affecting atomization dynamics. However, the reso-lution constraints of the utilized shadowgraphy technique impeded a thorough exam-ination of internal circulation. To address this limitation, it is advised to employ a1257.2. SUGGESTIONS FOR FUTURE DIRECTIONS OF RESEARCHlong distance microscope and perform an in-depth investigation to study the relationbetween the internal circulation characteristics and the heterogeneous nucleation.5. The experiments demonstrated that doping the droplets with partially oxidized GO,which exhibited the maximum area under the curve in Fourier-Transform Infraredspectroscopy, resulted in the highest droplet burning rate during the atomizationperiod. To validate the impact of thermal radiation absorption on nucleation andatomization processes, conducting UV-Vis experiments across a broad spectrum, in-cluding the infrared region containing flame thermal radiation wavelengths, is recom-mended. Subsequently, utilizing a high-speed and high-resolution shadowgraphy tech-nique would enable studying how the thermal radiation absorption of doping agentsinfluences heterogeneous nucleation and the rate of bubble formation.6. To engineer and customize fuels with a specific atomization propensity, it is crucial tocomprehend the interplay between boiling and the surface tension of the droplets, andhow this interplay is influenced by the presence of the doping agents. It is suggested toinvestigate the impact of GO addition on both surface tension using a tensiometer andthe rate of nucleation using high-speed and high-resolution shadowgraphy techniques.These investigations should allow understanding the optimum concentration of GOthat leads to most pronounced atomization events.7. This thesis demonstrated that the atomization influences the spread of flame amonga one-dimensional array of droplets. It proved challenging to eliminate the impactof fuel vapor formation resulting from evaporation before combustion. To investigatethis effect, it is recommended to conduct thermogravimetric analysis on the dopeddroplets. Maintaining the droplets in an environment with a temperature below theboiling point of all components would aid understanding the rate of fuel evaporationbefore its ignition. This approach would offer an estimation of the amount of fuel1267.2. SUGGESTIONS FOR FUTURE DIRECTIONS OF RESEARCHvapor generated around the side droplet(s) during the flame spread.8. Finally, this dissertation provided insights into the combustion of atomizing droplets.However, several real-world engineering applications (such as aircraft engines) involvespray combustion, which is associated with interactions between droplets dynamics,combustion chemistry, and turbulence. The extent to which the findings of the presentthesis can be extrapolated to spray environments remains unanswered. It is recom-mended that future research to be conducted using 2D and 3D arrays with a largenumber of droplets. 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First, 10 g of graphite powder was soaked inside 300 mL of H2SO4 (with98% purity), and the suspension was stirred at 50oC with a rotation speed of 300 rpmfor 18 hours. Then, the suspension container was positioned inside an ice bath, reducingits temperature to 5oC. Next, 40 g of KMnO4 was gradually (about 0.7 g/min) added tothe suspension. Then, the suspension was stirred, increasing its temperature to 50oC. Atthis temperature, the suspension was stirred for 12 hours. It is assumed that dimanganeseheptoxide is formed through the following chemical reactions [178, 179]KMnO4 + 3H2SO4 −−→ K+ +MnO3+ +H3O+ + 3HSO4−, (7.1a)MnO3+ +MnO4− −−→ Mn2O7. (7.1b)The generated dimanganese heptoxide (which serves as the oxidant) enters in betweenthe graphite layers, intercalates the graphite, and increases the graphite layers spacing[178–180]. During the above step, manganese esters are formed as a by-product [178].Then, the suspension was positioned inside an ice bath, cooling the suspension. After this,600 mL of distilled water and 4 mL of diluted hydrogen peroxide (30% H2O2 and 70%by weight distilled water) was added to the suspension, resulting in the hydrolysis of the150manganese esters and the formation of manganese-oxo, which is soluble in water [178]. Inaddition to these species, cyclic sulfate groups could also be formed during the previoushydrolysis process [181]. In order to remove impurities, e.g., SO2+4 and K+ as well as tohydrolyze the cyclic sulfate [182], the resulting material (chemically intercalated graphite orgraphite oxide) was washed two times with water and two times with a 1 M HCl solution (1mole of HCl in 1 liter of HCl and water). HCl features a larger acid dissociation constant(compared to H2SO4), which reacts with K+ (formed as a result of reaction 7.1a) andgenerates KCl. The resultant materials are washed two times with ethanol (to removethe remaining impurities), using the Eppendorf-5804 centrifuge operating at a rotationspeed of 11000 rpm. The final sediments are dried using a temperature-controlled surface(SCILOGEX) at 60oC for 12 hr. Finally, the GO is synthesized through the delamination ofgraphite oxide via sonication [178, 179]. The product features the maximum oxygen content,is referred to as hGO, and is thermally reduced, which yielded to GO with less oxidization.151Appendix B: Spatial resolution of shadowgraphy techniqueSpatial resolution of the shadowgraphy measurements is estimated using the USAF1951resolution target, similar to [10]. An image of the target is shown in Fig. 7.1. Identicalcamera settings to those used for the shadowgraphy measurements were used for acquiringthe image. In the figure, the yellow-dashed rectangle highlights the lines whose thicknessis close to the pixel resolution, i.e. the side length of the field-of-view divided by thecorresponding number of pixels (39 mm/1024 pixels = 38 µ mm/pixel). The pink dashedbox highlights the white rectangles whose width is almost equal to the shadowgraphy imagespixel resolution.Fig. 7.1. Image of the USAF1951 target acquired with settings identical to those used inthe shadowgraphy experiments.In order to estimate the imaging resolution, first, a parameter referred to as contrast(C) was estimated using [10]C =Imax − IminImax + Imin, (7.2)where Imax and Imin are the maximum and minimum light intensity (I) acquired acrosseach group of three white rectangles shown in Fig. 7.1. Variations of the contrast versus152the number of line pairs (combination of one white and black rectangles) in one millimetercalculated in the present study and extracted from [10] are shown by the solid and openblue circular data symbols, respectively, in Fig. 7.2(a). As expected, increasing the numberof line pairs per one millimeter (i.e. selecting smaller thickness lines) decreases the abilityof the camera to distinguish the lines. The extent of the horizontal axis is the inverse of thetwice of the field-of-view resolution. Papageorge et al. [10] used C = 0.2 to estimate theirimaging resolution. The value of the imaging resolution at C = 0.2 is approximately 44 µmin the present study.0 1 2 3 4 5 6 7011 3 5 7 9 11 1300.20.40.60.810204060801000 1 2 3 4 5 6 7010 1 2 3 4 5 6 701Fig. 7.2. (a) Variation of contrast (C) versus the number of line pairs per millimeter for thepresent study and that of [10]. The vertical axis on the right hand side is the maximumthreshold required to distinguish the white rectangles. (b–d) are the variations of IN acrossthe white rectangles versus x/L for L = 111.4, 55.4, and 44.2 µm, respectively.Figures 7.2(b–d) show the variations of light intensity normalized by the correspondingmaximum intensity (IN = I/Imax) versus x normalized by the corresponding white rect-angle width (L). Results in Figs. 7.2 (b–d) correspond to L = 111.4, 55.4, and 44.2 µm,respectively. In Figs. 7.2(b–d), the purple-dashed lines are used to mark perfect signals withleast modulations. The results in Fig. 7.2(d) show that for the line pairs with L = 44.2,three distinct peaks related to three white rectangles are detected. This means that, for theresolution of about 44 µm (which is larger than but close to the pixel resolution), the truesignal can be detected.153Appendix C: Selection of time period required for estimationof burning rateIn section 4.3, it is argued that D2/D20 data corresponding to 90% of the droplet lifetime(i.e. ∆t = 0.9τ) was used for estimation of K1. For this estimation, the time instantcorresponding to the median of the data was set to t = 0.5τ . For all tested conditions,∆t = 0.9τ was used to avoid nonlinear variation of D2/D20 corresponding to the dropletheat-up and flame extinction periods. A smaller number of data points (corresponding tosmaller value of ∆t/τ) can also be used for estimation of K1. However, as it is argued inthe following, ∆t = 0.9τ leads to the most accurate estimation of K1.20 40 60 800.9930.9950.9970.9990.10.30.50.70.9Fig. 7.3. Variations of R2 and β obtained from Eqs.(7.3a) and (7.3b) versus ∆t/τ . Theresults are presented for the representative tested condition of Ep0.01.The quality of the linear fits was used to assess the value of ∆t required for the calculationof K1. Two criteria were used to assess this quality, similar to [141]. Specifically, thecoefficient of determination (R2) and the linear regression coefficient (β) were estimatedusing:R2(∆t) = 1−∑Nd(∆t)j=1 [D2j −D2j (∆t)]2∑Nd(∆t)j=1 (D2 − cl,j)2, (7.3a)β = R2∆tτ, (7.3b)154where j is the number of the utilized data points, Nd is the total number of data pointscorresponding to ∆t, D2j is the experimentally measured data (i.e., the droplet equivalentdiameter squared), D2 is the mean of D2j , and cl,j is the value of D2 obtained from a linearfit. For a given ∆t, this fit was obtained using the least square technique. Both values ofR2 and β being close to unity suggest that the selected ∆t allows for accurate estimation ofK1, and that the fit has an acceptable quality.Values of R2 and β were obtained for several ∆t/τ using Eqs. (7.3a) and (7.3b), andthe results are presented in Fig. 7.3 using dashed and solid black curves, respectively. Theresults correspond to Ep0.01 test condition. Similar results were obtained for the rest ofthe tested conditions with ethanol as the base fuel. As can be seen, the values of R2 arerelatively large and close to unity; however, β maximizes for ∆t = 0.9τ . Thus, 90% ofD2/D20 data were used for calculation of K1.155
Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Kheirkhah, Sina
Influence of atomization on single and multi-droplet combustion of diesel, biodiesel, and ethanol doped with graphene oxide
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