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Wind tunnel investigation of jet fan aerodynamics Mutama, Kuda Ronald 1995

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WIND TUNNEL INVESTIGATION OF JET FAN AERODYNAMICSbyKUDA RONALD MUTAMAB.Sc.(Hons.), University of Leeds (England, U.K.)M.Sc., University of Manchester (U.M.I.S.T.), (England, U.K.)A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIES(Department of Mining and Mineral Process Engineering)We accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIAApril 1995© Kuda R. Mutama 1995In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature)_________________________________Department of MuJ tJ& -I- I’kL1\J4L JJ&The University of British ColumbiaVancouver, CanadaDate_____________DE-6 (2/88)11ABSTRACTThis study has investigated the performance aerodynamics of jet fans in order to identifyand understand the fundamental parameters in their use in mine and tunnel ventilation.Despite their advantages over other ventilation methods, jet fans have not been often usedin mining due to an inability to predict performance accurately. They have been used inlongitudinal ventilation of vehicle tunnels and other installations with encouraging resultsdespite the fact that such systems have been designed with limited data.The current studies used a wind tunnel to study jet fan ventilation. The fan was simulatedby aluminum pipes of different diameters connected to a centrifugal blower. The aluminumpipes were inserted at the entrance section of the wind tunnel and jet outlet velocitiesranging between 20 and 40 m/s were used to produce the flow field. In order to studypassage wall effects on the flow the jet fan was traversed from a near-wall position to thewind tunnel axis in equal successive steps. The axial pressure development for all positionswere determined together with a detailed velocity distribution and the overall entrainmentcharacteristics. Both the magnitudes of the pressure drop and rise depended on the jet fanposition from the walls. Near-wall jet fan positions tended to have initially larger pressuredrops and lower pressure rises than positions farther from the wall which had lowerpressure drops and higher pressure rises. The consequence of this pressure variation wasthat generally at near wall positions the jet fan entrained more air into the tunnel than atpositions farther from the wall. The smaller diameter jet fan produced higher frictionlosses (as much as 15 % at the wall position) than the larger diameter fan with loweroutlet velocity which had about 8 %. The flow field was found to develop rapidly withaxial distance. The jet axis velocity developed faster than that of a free jet of the sameinitial velocity and revealed that jet fans can move air over distances greater than 70 jet fandischarge diameters and still maintain a minimum air velocity of at least 0.5 in/s. For fan111positioning at F < 0.4, a region of backflow was identified. The backflow fraction was0.72 and 0.55 for the smaller and larger diameter fan respectively. The performanceparameter E of the jet fan determined from pressure and flow (eturainment) ratioconsiderations QT / Q (P — F ) / (P — P) was found to decrease as the jet fan wasmoved away from the tunnel wall despite higher friction losses at near-wall positions. Thejet fan performance parameter is generally below 12 % as verified by mathematicalderivations. The larger diameter (lower velocity, Uj = 21.4 mIs) jet fan hadperformance values almost twice that of the smaller diameter jet fan (Uj = 40 mIs). Thevalue ranged between 4.5 to 6 % for the larger diameter fan. High entrainment ratiosachieved at near wall positions generally improve jet fan performance.Theoretical equations based on momentum and energy considerations were formulated.These derivations identified a range of flow ratios (n = 0.1 to 0.9) which can be used todesign an effective jet fan ventilation system. For each flow ratio (n) there is an optimumarea ratio (ce) for maximum induction of secondary flow.The present studies have established a procedure for jet fan performance analysis usingwind tunnel investigations and have provided useful information for jet fan ventilationdesign.ivTABLE OF CONTENTSABSTRACT iiLIST OF FIGURES viiLIST OF SYMBOLS xACKNOWLEDGMENTS xiiCHAPTER ONE 1INTRODUCTION 11.1 Importance of Ventilation 11.2 Mine Ventilation Fans 21.2.lMainfans 21.2.2 Auxiliary Fans Ducted auxiliary fans Ductless Fans (Jet fans) 51.3 Research Objectives 81.4 Rationale 91.6 Remaining Chapters 11CHAPTER TWO 22LITERATURE SURVEY 222.1 Theoretical Considerations of Incompressible Turbulent Jets 222.1.2 Jets in Coflow 282.1.3 Round Jets with Swirl 292.1.5 Summary of Previous Work on Confined Jets 312.2 Jet Fan Measurements in Mine Ventilation 322.3 Jet Fan Investigations in Vehicle Tunnel Ventilation 39CHAPTER THREE 53EXPERIMENTAL PROGRAM 533.1 Design and Construction of Experimental Apparatus 533.1.1 Wind Tunnel 533.1.2 Construction of the Wind tunnel 563.1.3 Wind Tunnel Instrumentation 573.1.4 Wind Tunnel Testing 583.1.4.1 Test Results 583.2 Jet Fan Simulation and Arrangement 603.3 Experimental Description of Jet Fan Perfoimance Measurements 623.3.1 Jet Fan Velocity Field Measurements 623.3.2 Jet Fan Pressure Measurements 63VCHAPTER FOUR .78DATA REDUCTION AND ANALYSIS 784.1 Analysis of Pressure Results 784.2 Analysis of Velocity Readings 794.2.1 Jet Axis Velocity Decay Profile 804.3 Backflow Analysis and Jet Expansion Angle Determination 804.3.1 Jet Expansion Angle Determination 814.4 Entrainment Results 814.5 Jet Fan Performance 824.7 Longitudinal Velocity Fluctuations 824.8 Uncertainty Analysis in Measured and Derived Quantities 83CHAPTER FIVE 85DISCUSSION OF PRESSURE FIELD RESULTS 855.1 Effect of Jet Fan Position FP on Axial Pressure Development 855.1.1 Pressure Results for DR=0. 11 Without Tunnel Coflow 855.1.2 Pressure Results for DR = 0.17 and Uj=21.4 (Re=21 165) 875.2 Comparison of Pressure Variation for Two Differing Jet FanDischarge Velocities 885.3 Comparison of Pressure Variation for Two Jet Fan to TunnelDiameter Ratios 895.4 Axial Static Pressure Variation in the Presence of a Strong TunnelCoflow 905.5 Pressure Ratio as a Function of Jet Fan Position (Fe) Inside theTunnel 91CHAPTER SIX 103DISCUSSION OF THE JET FAN VELOCITY FIELD DEVELOPMENT 1036.1 Velocity Distribution for Jet Fan to Tunnel Diameter Ratio DR=O. 11and Uj=40m/s 1036.2 Velocity Distribution for Jet Fan to Tunnel Diameter ratio DR=O. 17and Uj=21.4 rn/s 1056.3 Jet Axis Velocity Decay Inside Tunnel for a Jet Fan at VariousPositions 1066.4 Jet Expansion Angle and Reverse Flow Phenomena 1076.4.1 Jet Expansion Half Angles 1076.4.2 Description of Reverse Flow 1096.4.3 The Quantity of Reverse Flow as Fraction of Jet Dischargeand Total Tunnel Flow 112CHAPTER SEVEN 134DISCUSSION OF JET FAN PERFORMANCE ANALYSIS 1347.1 Tunnel Axis Longitudinal Turbulence Levels 1347.2 Entrainment Rate as Function of Jet Fan Position 1357.3 Jet Fan Performance Assessment 137viCHAPTER EIGHT.143THEORETICAL TREATMENT OF JET FAN PERFORMANCE USING 143MOMENTUM AND ENERGY CONSIDERATIONS 1438.1 Jet Fan Nozzle Energy Equation 1458,2 Momentum Balance in the Tunnel 1468.4 Jet Fan Performance Efficiency 1488.5 Theoretical Estimation of the Backflow Fraction 1498.6 Jet Fan Analysis from Energy Considerations 1508.7 Analysis of the Performance i 155CHAPTER NINE 163PRACTICAL APPLICATIONS OF JET FAN WIND TUNNEL STUDIES 1639.1 Jet Fan Application Case Study 1 1639.2 Jet Fan Application Case Study 2 165CHAPTER TEN 170CONCLUSIONS 170CHAPTER ELEVEN 174RECOMMENDATIONS 174CHAPTER TWELVE 176CLAIMS TO ORIGINAL RESEARCH 176REFERENCES 177APPENDIX 181viiLIST OF FIGURESFigure Page1.1 Schematic layout of a jet fan 121.2 Construction of a centrifugal fan also showing blade types 131.3 Construction of an axial flow fan 141.4 Two booster fan system in an airway 151.5 Typical representation of flow losses (Linsell, 1953) 161.6 A simplified diagram of a mine ventilation system 171.7 Ducted Auxiliary ventilation fan operating in the exhaust mode 181.8 Ducted auxiliary ventilation fan operating in the forcing mode 191.9 Jet fan ventilation in a mine heading 201.10 illustration of a tunnel ventilated by a jet fan 212.1 A jet issuing into a fluid reservoir 452.2 Submerged turbulent jet (not to scale) 462.3 Submerged free jet in a coflowing stream 472.4 Ducted jet showing regions of development (not to scale) 482.5 Mine heading test site (Matta et al. study) 492.6 Mine test area with an inadquate source of fresh air 502.7 Mine test area ventilated by a jet fan (Matta et al. studies) 512.8 illustration of McElroy’s different phases of velocity decay of a 52freely expanding turbulent jet 643.1 Minimum length for contractions, without separation 653.2 View of wind tunnel showing support structure 663.3 Wind tunnel north wall showing static pressure holes 673.4 Wind tunnel south wall showing hot wire access holes 683.5 View of wind tunnel layout 693.6 View of wind tunnel from the axial fan discharge end 703.7 Arrangement of instrumentation 713.8 Axial static pressure variation for two flow settings for theworking section of the wind tunnel 723.9 Wind tunnel inlet velocity variation with time 723.10 Velocity profiles at Re = 219560 733.11 Velocity profiles at Re = 427253 743.12 Pressure drop-Reynolds number plot for wind tunnel contractionpiece 753.13 Schematic of jet fan simulation mechanism 763.14 Photograph of jet fan simulation arrangement 775.1 Axial pressure variation (F1, = 0.06 to 0.17) 945.2 Axial pressure variation (F1, = 0.22 to 0.5) 94viii5.3 Axial static pressure variation on tunnel side walls (F1, = 0.06 and0.94) 955.4 Axial static pressure variation on tunnel side wall (F1, 0.11 and0.89) 955.5 Axial static pressure variation on tunnel side wall (F1, 0.17 and0.83) 965.6 Axial static pressure variation on tunnel side wall (Fr, = 0.22 and0.78) 965.7 Axial static pressure variation on tunnel side wall (F0 = 0.44 and0.56) 975.8 Axial static pressure variation on both sides of the tunnel (F0 = 0.5) 975.9 Axial static pressure variation vs axial distance for jet fan DR=O.l7 985.10 Static pressure variation of jet fan at two different Reynoldsnumbers (F1, = 0.17) 995.11 Static pressure variation of jet fan at two different Reynoldsnumbers (F0 = 0.33) 995.12 Comparison of pressure variation for two jet fan diameter ratios 1005.13 Pressure variation for jet fan with tunnel coflow 1005.14 Pressure variation with tunnel coflow (U= 3 m/s) 1015.15 Comparison of pressure variation with and without tunnel coflow 1015.16 Plot of various pressure ratios vs jet fan position (DR = 0.11) 1025.17 Plot of various pressure ratios vs jet fan position (DR = 0.17) 1026.1 Velocity profiles of jet fan at various positions (DR = 0.11) 1166.2 Velocity profiles for jet fan inside tunnel (DR = 0.17) 1176.3 Plot of U/Umax for different fan positions (DR = 0.11) 1186.4 Plot of U/Umax for different fan positions (DR = 0.17) 1196.5 Tunnel velocity profiles at X/D1 = 45.3 (jet fan DR = 0.11) 1206.6 Tunnel velocity profiles at X/D1 = 30.2 (jet fan DR = 0.17) 1216.7 Jet axis velocity decay (jet fan DR = 0.17) 1226.8 Jet axis velocity decay (jet fan DR = 0.11) 1226.9 Jet axis velocity decay for two jet fan sizes 1236.10 Plot of jet expansion angle vs jet fan position (DR = 0.11) 1246.11 Plot of jet expansion angle vs jet fan position (DR = 0.17) 1246.12 Tunnel cross - sectional backflow velocity profile for jet fan at(X/D1 = 18.2) 1256.13 Plot of backflow velocity profile vs distance across tunnel 1266.14 Width of backflow at various jet fan positions (DR = 0.11) 1276.15 Width of backflow at various jet fan positions (DR = 0.17) 1276.16 Backflow width for jet fan with tunnel coflow velocity of 0.5 m/s 1286.17(a) Flow visualization photographs (DR = 0.11) 1296.17(b) Flow visualization photographs (DR = 0.17) 1306.18 Extent of backflow length vs jet fan position (DR = 0.11) 131ix6.19 Extent of backflow length vs jet fan position (DR = 0.17) 1316.20 QR/QT vs X/D1 for jet fan diameter ratios DR = 0.11 and 0.17 1326.21 Plot of backflow fraction vs jet fan position 1336.22 Backflow fraction QR/Q1 and QR/QT vs jet fan position 1337.1 Tunnel centreline longitudinal turbulence levels 1407.2 Flow ratio vs jet fan position inside wind tunnel (DR = 0.11) 1417.3 Flow ratio vs jet fan position inside wind tunnel ((DR = 0.17) 1417.4 Jet fan performance vs position (DR = 0.11) 1427.5 Jet fan performance vs position (DR = 0.17) 1428.1 Schematic description of jet fan - tunnel system 1588.2 Velocity decay of an axisynimetric free jet showing mixing concept 1598.3 Figure 8.3 Plot of theoretical performance vs flow ratio n 1608.4 Figure 8.4 Plot of theoretical performance vs flow ratio (n) forvarious jet fan positions (friction loss factors) 1608.5 Optimum flow ratio and performance vs optimum pressure ratio 1618.6 Figure 8.6 Friction loss ç vs jet fan position F 1618.7 Figure 8.7 Performance vs area ratio for various flow ratio n 1628.8 Figure 8.8 Flow ratio n vs optimum area ratio 1629.1 Figure 9.1 Example of jet fan used in through flow to increaseairflow in other mine ventilation districts 1679.2 Figure 9.2 Jet fan used in a closed heading with curtain to reducereturn air entrainment 168Figure 9.3 illustration of flow ratio vs total flow for a 5 and 10 169m3/s jet fanFigure 9.4 Jet fan fitted with an entrainment tube as in an ejector 169xLIST OF SYMBOLSA3, A Cross sectional area of jet fan and tunnel respectivelyA, Ae Area through which secondaiy flow enters the tunnel (= As-A)A0b Area occupied by obstructions in the tunnelD, D Diameter of jet fan and tunnel respectivelyDR Diameter ratio (= D/D)Em Energy input from jet fanE0 Energy in the tunnel outlet flowE tunnel mixing loss energyEfi Energy due to friction lossJet loss energy at discharge or in the nozzleEn Recirculation or reverse flow energy lossF Dimensionless jet fan position (= Y/D)Y Distance across the tunnelF Dimensionless fan position (= Y/D)Lr, LR Backflow length in the tunnelm, me Entrained mass flown Jet fan discharge mass flowm Total tunnel mass flow (= mi-I-me)n Flow ratio (QJQJ = me/mi)Entrance pressure for entrained flowP Jet fan total discharge flowTunnel maximum static pressure achieved over test sectionr0 radius of jet nozzleS Swirl number of jetT Angular momentum of jetW Axial momentum of jetQe, Q, Entrained or secondary volume flowQ Jet fan discharge volume flowQT Total tunnel volume flowQR Backflow volume flowU VelocityU Tunnel centreline velocityUe Entrainment velocityU Jet fan discharge average velocityU Average tunnel velocityii’ Velocity fluctuationRms or turbulence level of the fluctuating velocity at the tunnel axisWR Width of back flow at any axial distance (metres)WRIt), Dimensionless backflow widthX Axial distance from jet fan nozzle (metres)x0 Distance from jet where secondary and jet flow start to mixxix1 Distance at which jet flow and secondary flow are fully mixedX / D Dimensionless distance from jet fan nozzleInduction efficiency defined by equation 3Tunnel to jet velocity ratio UJUJ ratioe Thring and Newby similarity parameter (= ( + Qj(D, / 2)I(QDj)C, 0 Craya -Curtet parameter for ducted confmed jetsa Area ratio of jet fan to secondary stream inlet area (= A/A3)Area ratio of jet fan to tunnel (= A/As)p Air density (kg/rn3)ç Loss coefficients of nozzle, secondary flow and tunnel friction coefficient.cr Jet recirculation or backflow fraction of total tunnel flowJet fan performance parameter described in Chapter 7xiiACKNOWLEDGMENTSI am most thankful to my supervisor Dr. Allan E. Hall for the conception of the idea of jetfans in mine ventilation at a time when very little was known about the subject. Throughhis knowledge and wisdom he was able to direct me; leading to the successful completionof this thesis. I also benefited greatly from the many informal discussions during the courseof this thesis.My sincere gratitude are due to Professor Ian S. Gartshore of the Mechanical EngineeringDepartment, University of British Columbia for his constructive advise during the courseof this research. His contribution is greatly appreciated.I would also like to thank Mr. Frank Schmidiger for the total commitment he gave that ledto the successful completion of this project. The help given by the conmiittee members Dr.Richard Poulin, Dr. Rimas Pakalnis and Dr. Ray Meadowcroft is appreciated. My specialthanks are also to Dr. J. A. Meech.I would also like to thank my friends Dr. Nathaniel Makoni, Dr. David Mchaina, CharlesMasala, Allan and Ndeninsia Issangya, Peter and Hora Mtui, Joseph and Edna Mmbaga,Sunil Kumar, Elena Alonso and Dr. Marcello Veiga.This thesis is dedicated to my wife Hope Nwaobilo, and my three children Kudakwashe,Imogen and Munashe. They showed great strength and understanding during the yearsthey stayed in England without me. I also wish to mention my mother Stella, fatherNevison and my late stepmother Eleanor and grandmother Mbuya Adeke “Deke iro!”1CHAPTER ONE1. INTRODUCTIONJet fans are free-standing, unducted axial flow fans used in mine and vehicular tunnellongitudinal ventilation, and are often fitted with a nozzle as shown in Figure 1.1. Theirapplicationin mining operations includes pressure boosters and ventilation of developmentends, underground workshops, battery charging bays, pump and machine chambers. Inindustrial applications, they are used for road tunnel ventilation, cooling of furnaces andkilns, degassing of tanks and ship hulls; and ventilation of service tunnels during repairs.Although jet fans have been used successfully in longitudinal ventilation of road tunnelstheir use in mining is not common because their operation is not well understood. In orderto appreciate the reasons for this, it is necessary to review the basics of typical mineventilation systems.1.1 Imnortance of VentilationFresh air is required in subsurface environments such as mining and tunneling because (i)sufficient air is needed for personnel to breathe. (ii) Noxious fumes or gases producedduring mining must be diluted to a safe level of concentration, to prevent adverse effectson people exposed to the conditions (iii) The greatest amount of comfort possible shouldbe provided at a reasonable and economic price and (iv) the ventilation provided should becost effective.Failure to provide adequate ventilation which meets the legislated standards can result inthe closure or suspension of the production operation. The resulting adverse health effectswhich may appear long term incur significant compensation claims, and thus2environmental control measures are an essential and integral part of the mine productioncycle.Ventilation in underground mining and tunneling is achieved using fans which induceairflow in mine openings. Fans offer the most convenient means of supplying ventilationalthough compressed air injectors are used in limited circumstances by some mines.Natural ventilation effects may assist or work against the fan system. These effects can besignificant in deep mines.1.2 Mine Ventilation Fans1.2.1 Main fansMain fans provide the whole system with air and can either be centrifugal or the axial flowtype. Large fan ratings are required particularly in large and deep workings. Each personworking underground requires approximately 0.1 m3/s but this is seldom a limiting value inN. America. The extensive use of diesel equipment underground requires the provision of0.06-0.09 m3/s of ventilation air per kilowatt of engine power. Ventilation circuit headlosses are often considerable in mining. Main mine fan installations can be required toovercome pressures beyond 6000 Pascals delivering up to 1000 m3/s into the very largeand deep mines. Power consumption for this duty would be 8.57 MW for a fan mechanicalefficiency of 70 % and cost over $300 000 per month at a cost of 5 cents/kWh.Centrifugal fans can have radial, forward or backward curved impeller blades. Figure 1.2shows a centrifugal fan. In centrifugal fans, air is drawn into a rotating impeller anddischarged radially into an expanding scroll casing. The tangential velocity of the air3entering and leaving the impeller increases the centrifugal (static) energy and isproportional to the work done.In axial flow fans, pressure is produced by imparting a tangential acceleration as it passesthrough the impeller of the fan. As air leaves the impeller, the energy of rotation isconverted into a linear flow of energy. It is usual to locate guide vanes in the diffusercasing following the impeller and these are most effective in converting the rotativeenergy. The blades of the impeller can either be fixed or have a variable pitch. Most largeaxial-flow fans are of the vane axial type. Figure 1.3 shows the construction of an axial-flow fan.The application of main fans to mine circuits is subject to significant uncertainty.According to the advance of the workings, new shafts, roadways, and stopes are added tothe circuits so that the air volume as well as the necessary fan pressure is alteringconstantly. Final dimensions of airways often differ from designed figures because ofscaling and overbreak in blasting. Airways are irregularly obstructed by mobile equipmentand rock falls. The design of the fan system should allow for these developments andaccommodate them adequately. Another complication is that in the mine between theshafts and working faces, leakage may occur through old workings or through strata. Theleakage is significant and on average about 45 % of short-circuiting can occur. Thesefactors complicate the design specification for the fan.Main ventilation systems usually consist of independent circuits of differing resistance.High resistance mining circuits may require the provision of a booster fan to overcome thehead loss. Provision of a booster fan is preferable to increasing the pressure of the mainfan and regulating the other circuits because it requires less energy to be supplied. Atypical mining booster fan is shown in Figure 1.4. Because of the large pressure difference4across the fan, partitions are used to prevent uncontrolled recirculation and to ensuremaximum development. Walls (1983), gave some useful features of large fan installations.An example of the magnitude of mine ventilation flow losses is shown in Figure 1.5(Linsell, 1953). In Figure 1.6, air is introduced through the mine shaft A and is exhaustedto the atmosphere through shaft B. Airways ventilated by the main and booster fans aredesignated as being in a condition called through-flow. Closed end portions and newadvancing workings of the mine circuit cannot be ventilated by through ventilation andrequire the provision of auxiliary ventilation systems.1.2.2 Auxiliary FansAn auxiliary fan can either be used attached to suitable ducting commonly known asventilation tubing or as a “stand alone” fan i.e. ductless. In the latter case when used in thismode it is called a jet fan. Most auxiliary fans including jet fans are designed as eithersingle stage or multistage axial flow fans. The auxiliary fans used in mining have diametersranging between 300 and 1400 mm. Axial flow type auxiliary fans vary in size and duty.They can range from a rating of 4.5 kW, 1 m3/s units to 11 m3/s or more. These are usedin the ventilation of dead-end workings, where mining is taking place and is most neededand by doing so they move the air to where it is needed. Auxiliary fans are used in almostall underground mines for both development work and exploitation. Ducted auxiliary fansIn metal mines; drifts, raises, shafts, winzes and stopes with one entrance require auxiliaryventilation. In coal mines auxiliary ventilation is required in all entries beyond the last(connecting) crosscut and the auxiliary fans fitted with tubing ventilate the dead-endworkings which are often partitioned using line brattice. The auxiliary ventilation system5must be able to purge the area of harmful gases and dust and maintain adequate ventilationstandards required by legislation. Figures 1.7 and 1.8 illustrate auxilliary ventilationsystems commonly used in mines. Hartman et al.(1982) covers the subject of mineventilation adequately and explains the various ventilation schemes practiced in mining.Tubing and line curtain used in auxiliary ventilation do not have as high a capital cost asthe fan. The problem of these attachments is that solid ducting is difficult to transport andstore in mines. Flexible ducting is easier to handle but is readily torn or damaged. Thisresults in excessive leakage and significant sections of tubing are frequently replacedbecause of damage. This increases the cost of the system and labour required formaintenance. In addition the face conditions are unsatisfactory because of the leakage andthe requirement to shut down the system to replace the defective ducting.Ducted systems and line curtains provide an obstruction in the airway cross-section andthe airway is frequently driven over-size to its final required dimension to accommodate aducted ventilation system during construction. Inadequate installation and clearances frommoving equipment can cause damage to the tubing during mining operations. It is notuncommon for operators to tie off parts of the ducting to clear mobile equipment. Thismay reduce the available flow area by up to a third and throttle the airflow resulting inshock and friction losses. It is therefore attractive to mines to consider the use ofventilation systems without ducting. Ductless Fans (.Tet fans)A jet fan operates by discharging an incompressible turbulent air jet in the area to beventilated. The jet exchanges its momentum with the surrounding secondary air in exactlythe same manner as a jet pump; this causes air pressure to fall below ambient and6consequently surrounding air is entrained by the jet and a continuous airflow is established.In underground mine auxiliary ventilation, jet fans are used to boost air pressure and todirect airflow where mining operations are in progress. The presence of a duct as shown inFigures 1.7 and 1.8 may interfere with mining operations and there are usually restrictionson its size. High velocities in long ducts of small area result in large energy costs. Becauseof leakages due to damage, ducts have to be replaced frequently at an additional cost.Fig. 1.9 shows a jet fan situated at the upstream corner of the last connecting airway (orcrosscut) and discharging air in the fom of a turbulent jet along the wall into the opening.The jet expands with increasing distance from the fan until, ideally, air is flowing in halfthe width of the opening and it exhausts in the other half back to the main airway. Thevelocity of the flow reduces downstream because of the increasing mass of air beingentrained and by this process the jet fan delivers a volume much greater than its own inletvolume. Some of the entrained air is recirculated at the end wall of the opening.Recirculation can also occur at the inlet of the opening and can be as much as 20 to 40 %.Recirculation is not a major problem in longitudinal ventilation of vehicle tunnels becausetunnels are open ended. The presence of sharp curves may cause pockets of localrecirculation to be established. In tunnel ventilation it is necessary to locate jet fans in aposition that allows enough airflow to go through. Fans are inserted in a short circularduct which is streamlined and sound insulated to reduce noise levels. They are placed atregular distances along the tunnel. In some mining operations jet fans are placed in anairway in order to boost the pressure of the flow as shown in Figure 1.10. In this case adesired pressure rise is achieved.7Jet fan ventilation is very effective because it uses the opening itself as a duct and onaverage air velocities in the opening, typically are higher between 1 and 2 rn/s. This type ofventilation can be operated remotely with the aid of computer control.Jet fan (or induction) ventilation is simple in principle but the mechanisms by which the jetinteracts with the induced flow in confined places is still not well understood. Investigationof the subject is still incomplete and there are no established guidelines for the use of jetfan ventilation systems. Jet fan ventilation design requires a rigorous knowledge of theprinciples of fluid flow and momentum exchange between the primary air from the fan andthat of the secondary stream. Only a well developed theory supported by experimentalinvestigations gives insight to designing induction systems. Any empirical rules may leadeither to an oversized system or to a faulty ventilation system.The studies described in this thesis recognize that the key to the design of jet fanventilation systems rests in the characterization of the complex nature of the aerodynamicsof the discharged turbulent jet from the fan. Objectives of this study have been formulatedin order to provide knowledge which have either been unavailable previously or providefull attention to the subject of jet fan performance in mine ventilation particularly overallflow and pressure characteristics of jet fans. The data obtained will therefore contributeconsiderable information on the subject particularly in mine ventilation where jet fanapplication has very been limited. In vehicle tunnel ventilation previous work has notprovided an adequate data base for effective future jet fan ventilation designs. Futureventilation systems should be inexpensive, effective and easy to commission.81.3 Research ObjectivesThe objectives of the present study have been formulated after a comprehensive review ofthe current literature on the subject in both road tunnel and mine ventilation.Consideration of mine situations shows that there are large numbers of configurations ofairways which can be ventilated by jet fans. Ventilation problems may be dilution of stratagases which require a certain air velocity to be provided at the point of emission or dust orfume mitigation which requires a larger quantity of air to be provided at a particularworking face area. It is clear that there are an infinite number of individual problems whichmay occur in mines. Previous research had examined some specific problems and theresults had limited application to general principles of jet fan performance and positioning.There are two major ways in which a jet fan can be used in mine ventilation. (i) It can beused to ventilate a dead end working as shown in Figure 1.9 (i.e. closed end case). or (ii)It can be used in an open ended situation (Figure 1.10). When used in the open endedsituation it can be used to increase the pressure and also achieve the desired flow volumein the airway. Most previous studies have examined jet fan performance in existinginstallations where it was difficult to obtain general data which could be used in othersituations. In this study, a wind tunnel and jet fan test facility for modelling jet fans inunderground mine environment and tunnel conditions is set up. The test conducted is foran open-ended situation but it is considered that the results obtained can also be used inthe closed-end mine development situation. The objectives of this research can be stated asfollows.(1) To set up a wind tunnel- jet fan facility(2) To establish pressure and flow conditions of jet fans applied to geometry appropriateto mine openings with respect to wall interactions.9(3) To formulate a mathematical representation of the jet fan performance based onmomentum and energy principles.(4) To show how the observed results can be applied to a mine ventilation problem.1.4 RationaleThe wind tunnel to be used in this study had to be designed, constructed and testedbecause no other suitable apparatus for this type of study existed at University of BritishColumbia. No one has ever used a wind tunnel for jet fan studies in mine ventilation.Setting up an experimental test facility for jet fans and using a wind tunnel to create similarconditions as in mine and tunnel ventilation is the most effective way to perform theinvestigations because several geometric and dynamic factors can be varied with ease. Awind tunnel enables flow and pressure parameters to be measured more accurately andcreates a convenient atmosphere for the study to be carried out while maintaining goodsimulation of a real situation. Computer simulations need experimental data for validation.Reliable data is not available at present.Parameters such as positioning of the jet fan in cross section are known to affect theaerodynamics of the jet fan but there is no data to quantify the effects. Walls affect thebehaviour of many flows of engineering importance in any application and ventilation isnot an exception. It is important to study the influence of the tunnel walls on the flow andpressure field.The ratio of the jet fan diameter to tunnel diameter is important because it specifiesgeometric ratios for an optimum design. It is thus important to study how this parameter10influences performance of the ventilation system. A knowledge of the effects of theinsertion of the jet fan inside the tunnel is important in evaluating performance.The parameters defining the flow or velocity ratios of the jet fan to that prevailing in thetunnel are important for assessment of the system efficiency. Entrainment data can bedetermined from the flow field and its effects on the axial static pressure variation or viceversa can also be assessed. From these measurements, it is then possible for the overalllosses of the jet fan ventilation system to be determined from momentum or energyconsiderations.One question which needs to be addressed is to what extent the axial static pressuregradient influences the aerodynamics of the flow. Since jet fans discharge turbulent air jetsin the ventilated area; it is interesting to determine how free and confined turbulent jettheory can be applied to explain observed results of this study. Development of atheoretical framework can generate tools for use in solving mine ventilation problems.These tools can be developed in the future by perfonning additional testwork with theapparatus constructed in order to verify the applicability of mathematical equationsderived in this work under all conditions which might be met in mine ventilation.It is important to apply the results obtained to a real mining case of jet fan ventilation inorder to show how the results can be beneficial. The results and analysis of this study canprovide design data for jet fan ventilation in underground mines and tunnels.In this chapter an introduction to the present research has been given in which theobjectives are clearly stated. The next chapter deals with previous work on the subject andany information pertinent to the current research.111.6 Remaining ChaptersIn chapter two a general description of turbulent jets is presented together with some ofthe most outstanding previous work related to jet fan ventilation in vehicular tunnels andmines. Chapter three gives a full description of the experimental program that wasfollowed in this study. In chapter four the data reduction and analysis is presented in orderto make the results of this study easier to follow. In chapters five and six the discussion ofthe jet fan pressure and velocity field developments respectively are presented. Chapterseven discusses the jet fan jet flow entrainment process. Chapter eight gives a theoreticaltreatment of the jet fan flow based on momentum and energy considerations. Chapter ninegives two examples of jet fan application in an open end and closed passage. In Chapterten a declaration of the achievements of this work is stated. The conclusion andrecommendations of this work are given in Chapter 11 and 12 respectively.-I,CD CD Cl)C)ZrCD 3 C) 0 0 0 0 CD -4,0 D-13Scroll casingmpellerBladeInletShaft04DischargeForward curved backward curved radialFig.1 .2 Construction of a centrifugal fan also showing blade types14Inlet cone_______BladeShaft ( — Dc hargeL... DiffuserHubGuide vaneHousingBladeI...-’WidthFig. 1.3 Construction of an axial flow fan15rS.airlock doorsI I0High pressure airI-AirwayPartitionFig. 1.4 A two booster tan sytem in an airway16Input power =475 kW = 100%mine fan0//0for modern fan)Sundry flows66.5 kW = 140’/086.2 kW= 18.1%Throttling in sundry flows16.6 kW=3.5%Throttling in ventilation section35.2 kW = 7.4 %Air power= 252 kW=53 %Ventilation short circuits47.5 kW =10 %Net air powerFigure 1.5 Typical representation of flow losses (Linsell, 1953)17exhaust air outmainfan air inB A Intake entryregulatordoorsleakage flow D Dthrough ventilationBooter fanadvancing headingFig. 1.6 A schematic diagram of a mine ventilation system0‘I004-’C)-D18fan ventilation tubingairflow into headingmine headingFigure 1 .7 Ducted auxiliary ventilation fan operating in the exhaust mode19L mine headingairflow out ofeadingN fan ventilation tubingFigure 1 .8 Ducted auxiliary ventilation fan operating in the forcing mode20jet expansionLreturn air— — —— —— — — —pjetfan______D —C)____Cl) . ø.Cl) Co 0I.entraitied airmining headingoC>3)D0IFigure 1 9 Jet fan ventilation in a mine heading21-— AtUj Airflowjetfan- / +tunnelLPpressure dropFig. 1 .10 Illustration of a tunnel ventilated by a jet fan22CHAPTER TWOLITERATURE SURVEYThis chapter discusses the behaviour of turbulent jets as they apply to jet fan ventilation.The major part of the chapter presents a review of the previous work known to have beencarried out on the subject in both mine and tunnel ventilation.2.1 Theoretical Considerations of Incompressible Turbulent .TetsThe behaviour of the mixing of a jet in a confined flow is of great engineering importance,e.g. in ejector design, combustors and design of any device involving the transfer ofmomentum and energy.Since their flow is confined, jet fans can be characterized approximately by confmedturbulent jet phenomenon. Due to the complex rough geometry of mine airways andtunnels, where jet fans are used it is necessary to carry out more studies which can provideadequate information for ventilation design purposes. Jet fans fall in the category ofinduction systems because they move air by entrainment and momentum transfer.Approximate theoretical equations have been developed for inducted systems involving jetfans. The theoretical treatment of the subject and results of previous work are discussed inthis section.The subject of jets is well covered by Abramovich (1963), Rajaratnam (1976) and Blevins(1984), among others. A simplified summary of the most salient features on jets is givenfor brevity. Figure 2.1 shows a submerged free jet. Mass, momentum and energy enter the23control volume by means of a nozzle which transports fluid of velocity U0 through area A0and concentration of species C0 above or below the ambient level of the reservoir. Theconcentration of pollutants or temperature of fluid is represented by C0. The equations ofconservation of mass, momentum and species can be applied to the non deformablecontrol surface of Figure 2.1, and are represented by equations 2.1, 2.2 and 2.3respectively. The entrainment velocity is represented by Ue; and the flow passes throughthe lateral sides of the control surface into the jet with this velocity. Equations 2.1 and 2.2give conservation of mass and momentum respectively;p(U0A+UeA3)=PfUdA (2.1)pU2A =pfU2dA (2.2)00 AdA is an element of the area of the right hand side of the control surface through which thejet exits, A is the area of one lateral side of the control surface and p is fluid density. Acomparison of equations 2.1 and 2.2 dictates the existence of entrainment to satisfy thebalance of momentum and mass conservation. The fact that momentum has to beconserved in free jets is very important in their analysis. Equation 2.2 states that the flux ofaxial momentum M0 (i.e. momentum passing through a plane per unit time) of a jet isconserved even as the jet disperses. Once past the nozzle jets develop free of externallyapplied constraints and at some downstream point many jets become “self preserving” i.e.the flows at various axial stations are dynamically similar when non dimensionalised bylocal length and time scales. To obtain the properties of the fully developed jet, thesescales and their evolution with the flow must be determined.24Equation 2.3 describes the behaviour of self preserving jets where r is the distance fromthe origin of the jet and x is the axial distance. Urn is the centreline axial velocity of the jetwhere r=0, and is maximum i.e. f(0) = 1.0.(2.3)Equation 2.3 can be rewritten in a form that incorporates equation 2.2:IU2dA =U1j dA = UAQ = Const. (2.4)U02A is the initial momentum flux of the jet. The element of the control surface is anannular ring for an axisymmetric (round )jet and dA = 2itrdr. Equation 2.3 becomes:UX2j27_d(!LJ=UAo = C (2.5)o Urn X XTherefore UX2 = const.The centreline velocity of an axisymmetric jet must be inversely proportional to the axialdistance X:U7/cAo (2.6)25Using the above expression for centreline velocity the volume flow of an axisymmetric jetIs= IA= JU(2Itr)drUrnX221J—_Z-d(T_j (2.7)From the assumption that U / Urn = f (r / x) which is based on equation 2.4 the integral onthe right hand side of equation 2.7 is a constant independent of X or r. Q must beproportional to the quantity UrnX2. Substituting equation 2.6 for Um, the volume flow inan axisymmetric jet increases linearly with axial distance:Q Const.X 2 8A(.)A similar argument can be used to demonstrate that the rate of decrease of the speciesconcentration C along the centreline of an axisymmetric jet is:tXC A2= Const.—-- (2.9)ACm XThe equations generally hold well for both laminar and turbulent flow for X greater thanabout 5-8 nozzle diameters.Submerged turbulent jets are jets of a turbulent fluid flow into a reservoir of similar fluid.A submerged turbulent jet is shown in Figure 2.2 as a sum of three regions: an initialregion, a transition region, and the fully developed jet. The initial region has a length x,26and it consists of the core flow and the surrounding shear layer. The velocity in the coreflow is equal to the nozzle exit velocity Uo for a uniform exit velocity. The core flow isfree of shear and the term potential core is often used. The core flow is surrounded by aturbulent shear layer, which forms the boundary between the core flow and the reservoirfluid. The flows in the core, the initial region and the transition region bear the imprint ofthe nozzle details. At some point in the transition region, the turbulent eddies in the shearlayer will efface the details of the nozzle core flow.The boundary surface between the jet and the reservoir fluid is called the intermittencysurface. The reservoir fluid, on the outside of the intermittency surface is non turbulentand is irrotational. The jet fluid within the intennittency surface, is both turbulent androtational. Approximate solutions for turbulent jets have been developed by a number ofworkers and are based on the boundary layer time-averaged forms of the Navier-Stokesequations. For axisymmetric flow at constant pressure, these equations areau au ia( au —-U—+V——=——I vr——ruv (2.10)ax ar rar ar )The axial and transverse components of the flow velocities are the sum of the timeaveraged component and time-dependent deviation, The time averaged values u’ and v? arezero but their averaged product u’v’ is not. The kinematic viscosity is denoted by V.Turbulent shear stress u’v’ models have been developed to solve jet flow problems and thesimplest of these models applicable to the axisymmetric jet is the eddy viscosity modelaUuv =—E—ayFor this model the quantity Um — = Um equals the centreline mean velocity.27Using u’v’ = — , substituting this equation in (2.10) and neglecting kinematic viscosityv in comparison with eddy viscosity of turbulencee0result in the following equationa au aI (2.11))x ar rar\ ar)The species profile in the jet is a simple function of the axial flow velocity profile:=[_J (2.12)Thus if the axial velocity is known for a plane or axisymmetric jet, then the species profilecan be calculated from equation 2.12, and agree with experimental data for anaxisymmetnc jet e / 87- = 1/1.4 and / e- 0.5 for a plane jet. The centreline velocity andprofile of velocity is given by:Plane jet: U (x, y) = 12()Uoe (2.13)2Axisymmetric jet: U(x,r) = 12(JUoeJ (2.14)Equations 2.13 and 2.14 are for submerged turbulent jets.Entrainment velocities are defined as follows:Plane jet: u = (2.15)e 2bdx28ldQAxisymmetnc Jet: u = (2.16)2itbdxThe entrainment velocities are the component of flow velocity across the intermittencysurface toward the axis of the jet. Q is the volume flow rate of the jet, i.e.Q(x)=IU(x,y)dA(y) (2.17)(volume flow rate per unit length of slot for the plane jet) which increases with the axialdistance X. The cross sectional plane area A, corresponds to X = constant; b* = 2.5b is anestimate of the distance from the centreline of the axisymmetric jet to the edge of theintermittency surface. The required radius is b, for the axial flow velocity U to fall to onehalf its value along the axis. Generally turbulent jets entrain more air than laminar jets.2.1.2 .Tets in CoflowA jet in a coflow discharges into a fluid flowing in the same direction as the jet; as shownin Figure 2.3. A coflowing jet is also temied a compound jet or in the case of anaxisymmetric jet, a coaxial jet. The ratio of the nozzle exit velocity to the velocity of thecoflow characterizes a jet in coflow. As the velocity of the surrounding fluid is decreasedto zero, the coflowing jet becomes a submerged jet issuing into still fluid. The jetdisappears if the velocity of the surrounding fluid becomes equal to its own and if itexceeds the coflowing jet it becomes a wake.29Conservation of momentum in the axial direction for this case of a moving control surfacein a uniform pressure field in the absence of any mechanically applied forces for a plane jetin coflow is given by the equation:u0(u—u1)2b= —u1)dy (2.18)where b = jet width and b = 0.1 X for a submerged jet. b = C1X for UJU1>> 1.0. Forweak coflow jetsbzzcixCl (2.19)2.1.3 Round Jets with SwirlSwirl is the circumferential component of velocity which causes a round jet to rotate aboutits axis which may at the discharge end of jet fans unless they are fitted with flowstraighteners. A form of ductless fans called vortex fans used in mine ventilation are usedto provide a strong swirling jet and in most cases have high entrainment capabilities.The dimensionless swirl number S is defmed as the ratio of the angular momentum of thejetT = 24r2uwdr (2.20)to the total axial momentum times the nozzle radius r030Wr02ici f(p —p + pu2)rdr = 2ltroJ°”[u2 —j_]rdr (2.21)S = T / (Wr0) (2.22)P and P are the static pressures of the jet and the reservoir into which the swirling jetissues respectively and P < P, in a swirling jet. The velocities u and w are time averagedcomponents of axial and circumferential velocity, respectively; r is the radial distance fromthe centreline and r0 is the nozzle radius. Both axial momentum and angular momentumare conserved in a jet with swirl. The swirl number S is constant for a given nozzle flow.In addition to swirl from S=0, the following changes take place in the jet: (i) the centrelinevelocity decreases more rapidly with axial distance (ii) the jet spreads more rapidly withaxial distance (iii) The static pressure along the jet centreline decreases below reservoirpressure.The presence of swirl increases the mixing in a round jet by increasing the jet width whiledecreasing the jet axial velocity. Proffles of axial velocity, radial velocity, circumferentialvelocity, and static pressure are axisymmetric about the jet axis.Wbile the maximum axial velocity and minimum static pressure fall on the jet centreline,the maximum circumferential velocity is located at approximately r / x = 0.12 where r isthe radial distance from the nozzle. In summary swirl increases the spread of the jet. Theswirl number S can vary from zero to a very strong value of 1.4.312.1.5 Summary of Previous Work on Confined JetsFigure 2.4 shows a ducted jet defming four flow regions. in Region I the inner and outerjet flows are separated by a shear layer. In Region lithe shear layer has extended to theduct walls and the fluid is entrained from the surrounding stream rapidly enough to reducethe velocity of that stream. Region III is a region of possible eddy formation andrecirculation. If the pressure gradient is large enough and the coflow velocity is smallenough, the central jet will laterally entrain all the fluid in the coflow before the jet hasspread to the wall. Thus, a recirculating eddy will be established. Region IV begins afterthe point of reattachment and marks the beginning of conventional duct flow. Jet fan flowscan be considered as ducted flows and therefore can be approximately described bygeneral jet flow relationships with some modifications.Hill (1965) using the differential-integral technique was able to predict the mean flow fieldfor jet mixing in the presence of fixed and varying area cross sections. Hill evaluated thevelocity shear distribution and similarity profile in integration terms from free jet data.Confined jet mixing differs from free jets in that momentum is not conserved. Some resultsand techniques of the free jet investigations are readily extendible to confined flows. Thepresence of the confining walls causes a pressure gradient which modifies the rate ofspread of the jet, rate of growth of the boundary layer, and velocity profile shape. Theflow is more complicated than the boundary layer flow with an imposed adverse pressuregradient. The pressure gradient, while determined primarily by the mixing of the twostreams is related to the boundary layer growth. Therefore, the flow in the early mixingregion is much like a conical diffuser flow.Curtet and Ricou (1964) conducted an experimental investigation of the axisymmetricconfined jet to check the theoretical approach of Curtet (1958). Measured velocity profiles32were not strictly similar but the terms neglected by the assumption of similarity were foundto be small. Razinsky and Brighton (1971) have also investigated the mixing of an air jetwith a lower velocity air stream in a constant diameter pipe. The flow was investigatedfrom the inlet where the jet and secondary velocities were uniform (but different to alocation downstream where the flow is fully developed). Their measurements includedwall static pressures, mean velocities, turbulence levels and Reynolds stresses.2.2 .Iet Fan Measurements in Mine VentilationResearch in mine ventilation into jet fans has been very limited and the available previouswork has been conducted by various workers with the U.S. Bureau of mines(USBM).Only a few articles have been published which are presented below.Matta et al. (1978) evaluated jet fan effectiveness using SF6 tracer gas analysis in theventilation of dead headings. Results of their study demonstrated the ability of jet fans toventilate dead headings beyond maximum penetration distance indicated by smoke-tubedetection of air movement and to redistribute available fresh air through larger workingareas. They further commented that there must be sufficient fresh air for the jet fan tooperate with in order to avoid the recirculation of contaminated air in the heading. Oftenthe mine has sufficient fresh air flowing through it but directing the air where it is actuallyneeded is always a problem. In working areas where mining is in progress, large quantitiesare necessary to dilute and remove contaminated gases.Auxiliary fans redistribute this air and jet fans are in fact auxiliary fans that do not usebulkhead or tubing. Krause (1973) has shown that the penetration distance of a free jet isdoubled by placing the fan outlet along a mine rib. The penetration was measured as thedistance from the fan to where the velocity dropped to 0.33 ni/s. The air is only entrained33from one side of the jet and as a result a “ half jet “is formed. Computer studies and mineventilation surveys show that jet ventilation systems can considerably improve thedistribution of air currents in mines with large cross-sectional areas.The Matta et al. (1978) study of the US.B.M. was reasonably successful in using sulfurhexafluoride (SF6) tracer gas to determine the effectiveness of a jet fan in providing freshair. Studies by Drivas et al. (1972) using SF6 tracer gas in the evaluation of ventilationsystems in buildings have enabled the effective purging of various rooms and provided thedecay curve of the tracer gas. The Drivas method was used by the Matta et al. study(1978) in dead-end headings to evaluate the effectiveness of different fan sizes, effect oftheir positioning and the ability of the jet fans to redistribute fresh air. The effectiveness ofjet fans in purging a fixed volume V, for SF6 released uniformly throughout the volumewith concentration C, at time, t, can be determined in terms of a concentration decay.C =0exp(—Q/V)t (2.23)Q is the amount of fresh air purging the volume, and C0 is the initial SF6 concentration. Asemi-log plot of the concentration versus time yields a straight line with a slope equal to -Q/V. The slope is given by the equation-Q/V= ln(C1 IC2)(2.24)tl —where C1 and C2 are any two concentrations along the straight line at time t1 and t2respectively. The amount of fresh air Q can be determined for a heading of volume V.34Figure 2.5 shows the dead-heading test site used by Matta and coworkers for theirinvestigations. Fan sizes of the following diameters: 635, 737, and 762 mm were used toventilate the headings and to show the resulting SF6 decay curves for each size of fan.Figure 2.6 shows a test area with an inadequate source of fresh air, and Figure 2.7 shows atest area ventilated by a jet fan. SF6 measurements were used to detennine theeffectiveness of jet fan ventilation in the test site.Typical SF6 decay curves for the 635 and 762 mm fan were determined in order to assessthe rate of potential pollutant clearance by the jet fans. The 635 mm diameter fan hadpenetration distances of up to 27 m into the heading. The Matta study demonstrates theeffectiveness of SF6 in evaluating ventilation studies of this kind.A more detailed study was carried out by Thimons et al. (1986) in which they tried severalstrategies of face ventilation for oil shale mining to dilute and remove pollutants from theworking face area. They considered all possible sources of air pollutants expected in oilshale mining and in particular the problem of methane and diesel pollutants was welladdressed. For each of the pollutants including respirable dust, Thimons et al. quantifiedthe amount of fresh air required to dilute these pollutants below the Threshold LimitValues (TLVs).Thimons et al. favoured a jet fan system for non gassy oil shale mining and reversible fanswith rigid ducts for gassy mines. Each system had a common design basis including (i)154.8 m3/s capacity (ii) low power consumption (iii) components that must be handledwith a minimum of special equipment (iv) two speed operation to conserve powerconsumption when full flow was not required.35In the characterization of the turbulent jet from the jet fan Thimons et al used a simplifiedmethod developed by McElroy (1945) even though theoretical analysis performed byAbramovich (1963) existed. McElroy?s work was limited because it was solely based onemprical relationships. McElroy developed a group of equations to describe the decay incentreline velocity with increasing distance from the fan outlet. These equations arecorrelated with four phases of behaviour, as shown in Figure 2.8. For a round jet in phase1 and 2 the centreline velocity U, at a distance X from the fan outlet is characterised byU, (2.25)U0 = outlet discharge velocity , a = constant ( between 1.0 and 1.2)Equation 2.25 holds fairly well up to 5 outlet diameters. The constant a, decreases withdecreasing velocity. In phase 3u = KU0D (2.26)xwhere K = constant (between 3 and 10), D is outlet diameter, X is distance from outlet.In the transition zone the cenireline velocity U, decays rapidly to the range that ispredicted using the flowrate and one half the area of the opening. After this zone U, isgiven bykUDu = (2.27)lOGx36kf is a constant related to the ratio of the outlet diameter to opening dimension and G =O.026kfk 122DSa (2.28)wSa = aspect ratio of the fan outlet (= 1.0 for a round jet). W is the large dimension of theopening. These equations developed by McEfroy are for freely expanding jets. Wallsrestrict the growth of the jet and increase the distance in which phase 3 behaviour isobserved.The equations developed by McEfroy are similar to the work reported by Krause (1972)and were tested by fitting the relationships to experimental data from Lewtas (1980).Values of K (in equation 2.26) varied between 5.0 and 11.3 for fans located within threefan diameters from the wall. An approximate linear relationship was observed between thedistance from the fan outlet to the transition zone and the outlet velocity divided by thedischarge diameter.Thimons et al. (1986) showed that the zone heading that is of critical importance in miningapplications is the transition zone and beyond. The entrainment action increases rapidlycausing a rapid decrease in flow velocity. In the design process it is important to ensurethat the jet can force air to the face with sufficient velocity to provide good air quality anda complete face sweep. The design of the jet fan system in Thimons et al. work was basedon the following criteria: (i) Flow capacity based upon expected rates of pollutantemissions at the face (ii) Fan diameter selected such that the jet reaches the face with 0.5rn/s velocity. Using K = 5.0 in equation (2.26) a flow rate of 47 m3/s, and openingdimensions of 17 m wide by 9 m high, fans with diameters of between 1219 and 1524 mm37were predicted to project air in the range of 91 m with a minimum velocity of 0.5 mIs. A1400 mm diameter fan was chosen for this system.The jet fan tests by Thimons et al. showed superior performance in clearing out dieselexhaust and methane from muckpile tests. The duct system was more effective in clearingout blast gases and methane layering. The jet fan air recirculation value was 28.4 % andthat for the ducted system was 23.8 %.For a given room volume V, Thimons et al. (1986) gives the following equation tocalculate the time to reach a TLV of a pollutant in a dead heading.T=—---(lnC0--1nTLV) (2.29)Qejwhere T is the time in seconds to reach a TLV, Qe is the effective flowrate of the fanequal to EdQFan (m3Is). The dilution efficiency Ed is equal to volume of fresh air delivereddivided by volume flowrate of the fan. The peak pollutant concentration in ppm, isdescribed by C0.Thimons et al. concluded that the jet fan was more efficient at a flowrate of 28.32 m3/sthan at 41.72m3/s and delivered similar dilution rates suggesting some interaction betweenthe turbulent jet and room dimensions that is still not well understood.Dunn et al. (1983 ) carried out some extensive studies on testing of jet fans in metal andnon metal mines with large cross-sectional airways. Their work tested various sizes of jetfans in open airways, dead headings and face areas, and they were able to develop some38basic guidelines on the positioning and sizing of jet fans. Fan tests were conducted in threemines with cross-sectional areas ranging from 88 to about 186 m2 and mainly in room andpillar mining. It was concluded that for open airways large jet fans are less efficient insmaller airways but effective in larger open airways in achieving both uniformity andgreater air entrainment when inclined about 100 or elevated. Jet fans must be placedagainst the rib of a heading closest to the incoming airstream from the crosscut to reducerecirculation. This work also used tracer gas analysis to determine airflow recirculationand distribution.The ventilation current propagated by a jet fan along a longwall face has been investigatedby Radchenko et al (1965). Stochinsky and Komarov (1969) reported the use of jet fansfor increasing airflow in mine workings and provided some empirical formulas to computejet fan air quantities.Goodman et al. (1990) conclude that jet fans wifi be widely accepted in future for deepadvance mining and as a general auxiliary method of ventilation compatible with computerassisted mining. Research along these lines is being carried out by the U.S.B.M.Meets and Meyer (1993) conducted some ventilation tests in room and pillar headings inSouth African coal mines using two types of ductless fans. One was called a jet fanbecause it had a fitted nozzle and the other a vortex fan because its fan blade design wasdeveloped from vertical take-off aircraft. The latter fan produced a vortex or swirling jetand there was no nozzle to increase the outlet velocity. The purpose of these studies wasto develop better methods of ductless ventilation and to replace the traditional ventilationsystem with its disadvantages. This study provided airflow patterns for the ductless fanssituated at various positions with obstructions in the heading and determined the39percentages of recirculated air. This work was a significant contribution to the subject butit did not go far enough in providing fundamental ductless ventilation design data.2.3 Jet Fan Investigations in Vehicle Tunnel VentilationEck (1973) gives the following expression for excess pressure developed by the fan in atunnel ventilated by momentum drive:IXP = pU(A I A1 +(lI(1— A3 / 4 ))(U1 /U3 — A /A1)2—(U IU)) (2.30)where U is the ti.mnel air velocity and U is the fan outlet velocity. At tunnel cross-sectional area and A is the fan outlet area. This equation applies to any tunnel situationwhere induction ventilation system is used but it does not account for wall friction andtunnel length. Induction ventilation systems for tunnels have been the subject of majorresearch efforts (eg Kempf, 1965). The experience gained in tunnel ventilation has neverbeen extended to auxiliary ventilation in underground mines and it is the author’s beliefthat theories developed by early studies in tunnel ventilation can find some ground in mineventilation.Baumann (1973) evaluated friction coefficients and mean wall roughness of Swiss tunnelsfrom pressure and volume flow measurements. Baumann gives the pressure drop in atunnel ventilated by a jet fan to betP1 ± AP = (ç + ce + f p4-+4-c ..(N+(U —U1)2—N(U3+U1)2 (2.31)and the pressure rise due to the jet fans to be given by40AP =-A_pQj(uj —ui) (2.32)where A is the pressure generated by the jet fans, Af is the sum of the external forces(barometric, thennostatic and wind forces), f is the friction coefficient (.i 2AP / (pU2),ç is the pressure drop coefficient for entry into tunnel portal,ce is the pressure dropcoefficient for tunnel exit, p is the air density, n is the number of jet fans in the tunnel, K,is the reduction factor for jet fan thrust, Cc is vehicle drag coefficient, L is tunnel length,D is the diameter of the tunnel, Uis the jet fan outlet velocity and U is the tunnelvelocity. Q stands for volume flow of the jet fan, A is the tunnel cross sectional area, Ais the cross sectional area occupied by vehicles, N, N is the number of vehicles in thedirection of flow and against flow respectively. The friction coefficient can be estimatedfrom measurements of velocity and pressure fields.Rohne (1964) assumes the fan thrust to beF=pQ(U—U)=APA (2.33)from which the pressure head developed by a single fan, AP results.PUL[1_J (2.34)and in tenns of velocity and area ratiosAP = pU2(l—?)=pu1c(l—)/2 (2.35)41Meidinger (1964) applied continuity and mechanical energy equations to obtain=—2I —2Q— )2c2] (2.36)where ‘I is velocity ratio, and 2 is area ratio A/AL. Reale (1968) obtainedtsp = pu i(1_)2 = pu(l_dI)J (2.37)by applying momentum and continuity equations.Reale (1973) defines an induction efficiency as the ratio of the ventilation power outputto the power transmitted to the fluid by the fan.= pQ(u-u) =(X) (2.38)and total efficiency of the ventilation system is given by= lljTIf (2.39)where is the overall fan efficiency. Reale (1973) promotes the use of experimentaltests on ventilation models because a theoretical analysis alone is insufficient in givingdesign data useful for modem systems.42In tunnel ventilation, the losses due to vehicle motion and the effect of winds must beconsidered when evaluating the performance of jet fans. Possibilities for the reduction ofenergy consumption in ventilation systems using jet fans are considered by Pinter (1982)particularly for tunnels with lengths of up to 2 km. Pinter divides the possibilities of savingenergy in jet fan ventilation systems into three categories (i) reduction of the tunnelresistance (ii) increase of efficiency of the jet fan ventilation and (iii) reduction ofoperating hours a year. Tunnel resistance is comprised of entrance, exit and friction losses.Entrance loss coefficients for well designed tunnel portals are around ç1 = 0.3 and can beas low as 0.1. Traffic signs and the piston effect of moving vehicles inside the tunnelincrease its resistance. In the mining situation mine vehicles and other obstructionscontribute to resistance.Fan efficiencies of up to 70 % are possible with axial-flow fans. A number of factors affectthe efficiency of the fan arrangement and include the reduction of thrust due to wallfriction and uneven diffusion of the jet if the fan is installed near the wall. Factors have tobe incorporated in the equations to account for losses. Pinter suggests fitting conicallyextended transition pieces to the impeller stage of the fan, to improve efficiency of the jetand to better utilise the free space in the tunnel cross section. This results in lower jetspeeds and higher jet efficiencies and with significant lower energy consumption of up to55 %. The reduced jet speeds result in lower thrust and require more jet fans for a givenlength of tunnel, which increases capital costs.Noon and Smith (1990) compared the performance and sound levels of jet fans inlaboratory and road tunnel tests. The main parameters measured were volume flow, axialthrust, input power and sound level using the BS848: Part 1:1980 Type Chamber A testmethod. Noon and Smith conclude that more work needs to be carried out in order to find43methods of improving jet fan system efficiency, and in particular to establish a reliablemethod of measuring the pressure rise in a tunnel due to a fan.Mizuno and Araie (1989) carried out some measurements of pressure rise performance ofa jet fan in a tunnel by the use of a 1/35 scale model experiment. Their model tunnel was8m long and used a 230 mm diameter acrylic circular tube. The size of the jet pipe was 46mm in diameter. They used U / U values of 0.2, 0.4, and 0.66 and traversed the jet fromthe centre of the duct to the wall. In this case U / U is the ratio of mean velocity of theduct to the jet velocity Uj. In order to predict the pressure rise coefficient CPth they usedthe expressionCplh =2(1_)[1+..(1+)] (2.40)which is close to Meidinger’s (1964) equationCPh 22(1—)++2ç3—22cI—2(2.41)Equations 2.40 and 2.41 reduce toCpjh = 2(1—(2.42)because the second terms can be ignored since they are negligible. In the above equation2 is the area A/As and 1 is the velocity ratio of the tunnel and jet discharge. Mizuno and44Araie observed that 15 % of the momentum is lost by friction when the jet is in contactwith the wall.Hayward (1973) used model road tunnels to evaluate a longitudinal tunnel ventilationsystem. Smith (1982) investigated the design aspects of high reaction fans from both anaerodynamic and mechanical design perspective in order to increase jet fan performance.Other jet fan studies in road tunnel ventilation were performed by Baba and Ishida (1985)to determine economic considerations. Like Pinter they favoured low jet fan outletvelocity as opposed to high velocities of up to 30 mIs. They showed that the input energyof a 30 mIs velocity jet fan is about 3.4 times that of a 20 m/s velocity jet fan so that for a20 year period, the total cost of a low velocity fan application wifi be lower than that of ahigh velocity fan application. Fudger and Lowndes (1985), and Ohashi et al. (1976) alsoprovided some useful information on vehicle tunnels ventilated by jet fans.Previous studies have addressed various aspects of jet fan ventilation but they have notanswered all questions pertaining to system effects of jet fan aerodynamics. More researchis needed in this area. The nature and extent of work proposed in the present study willhave far reaching results. The present research investigates the most important andfundamental aspects ofjet fan ventilation.45Fig. 2.1 A jet issuing into a fluid reservoir(Ue is the entrainment velocity)yuer — Control surface 1Uo’‘4 V“0+bb4-ue462bo[bo=ro and y=r for axisymmetric jet]Fig. 2.2 Submerged turbulent jet (not to scale)Um/2virtualorigin surfaceinitial ‘ransitionlengthFully developed jet472bofor axisymmetric jet, r replaces yFig. 2.3 Submerged free jet in a cotlowing stream48Dj DtII III IVFig. 2.4 Ducted jet showing regions of development(not to scale).8.5m.0,Samplingpoint45.7mfromcrosscutFarthestpenetrationofanyfanasdeterminedbyDragertubeDEcoN.U)0Samplingpoint18.3mfromcrosscut(‘3a)IU)4-.0).1c,)CAirflow_________(Ua)f26.9nIsiiI.ft.LJLcICc’jIa).1Lt50Test area = 59465 mSeam thickness, 6.7 mGob pilec7I/IL/Fig. 2.6 Mine test area with an inadquate source of fresh air51scale mSampling SF6 releaseç7\)1//I0Figure 2.7 Mine test area ventilated by a jet tan (Matta et al. 1978 study)52Fig. 2.8 Illustration of McElroy’s different phases of velocity decayxFanPhase 1 and 2(0 - 5 fan diam)___________Phase 3 flow(5 to 75 fan diam)Gausian airvelocityistributionTransition zonePhase 4 flow—(bevoIdfhase 3)—of a freely expanding turbulent jet53CHAPTER THREEEXPERIMENTAL PROGRAM3.1 Design and Construction of Experimental Annaratus3.1.1 Wind TunnelIn order to investigate jet fan aerodynamics in mine ventilation a wind tunnel was used tosimulate a mine environment or road tunnel arrangement. It was the philosophy of thiswork that jet fans can be simulated by carefully using laboratory test facilities. The use ofmodels in experimental work provides many advantages in both cost and convenience. Inorder to predict the performance of a full scale industrial jet fan from the small scalemodel, complete similarity of the flow pattern is required.This requires both kinematic and dynamic similarity as well as geometric similarity.Kinematic similarity requires that velocities and velocity gradients be exactly proportionaland dynamic similarity requires that various force ratios be equal in each case. It is notalways possible to obtain complete similarity. Generally it is important to maintain similarflow conditions by keeping the same Reynolds number. Dynamic dissimilarity arisesprincipally in the boundary layer flow due to frictional and shear forces and shock lossesCommon design methods for sizing ducts may be listed as (a) constant velocity, (b)velocity reduction , (c) equal friction and (d) static regain. The theory of the design oflow speed wind tunnels is given by Bradshaw and Pankhurst (1964), and Pope andHarper( 1966) and many others.54The studies to be carried out in this research demanded a unique wind tunnel to be solelyavailable all the time. Wind tunnel facilities in the Mechanical Engineering Department(UBC) could not be used due to the fact that they were being used all the time for otherstudies and that they were not quite suitable for mine ventilation measurements. There aretwo main factors which influenced the present wind tunnel design (i) a 7.5 kW variablepitch axial flow fan of diameter 630 mm was already available for use in the wind tunneldesign. The fan was manufactured by Woods of Coichester (England) and can handle anairflow of more than 10 m3/s if required. (ii) A rectangular shaped wind tunnel wasrequired to simulate the general geometry of mine drifts and vehicular tunnels. The crosssectional dimensions had to be large enough for the insertion of objects and allow easyaccess to personnel working in the tunnel.The space available for the layout of the tunnel also limited the maximum length that couldbe achieved. The current design is unique because the size of the wind tunnel that wasrequired in relation to the area of the room in which it was to be situated, made the task achallenging one. There are many standing structures at the walls of the room and near theend, the roof is abruptly lower to accommodate a concrete explosion duct. The mostimportant objective, despite the unfavourable conditions of the room was to achieve aninlet airflow to the wind tunnel that was as smooth as possible. Budgetary constraints alsolimited the size and material that could be used for the construction of the wind tunnel.After consideration of all the factors it was decided that the wind tunnel cross sectionaldimensions should be 900 by 900 mm. The length of the wind tunnel depended on the flowconditions to be achieved in the working section. The flow conditions should be as smoothas possible over the test section especially if accurate aerodynamic studies were to beconducted to reduce air fluctuations.55Most flows that exist in mine ventilation are turbulent. For a fully developed turbulentflow the length L, to diameter D, ratio is given by= 14.2log0Re— 46 (3.1)for Re> 10000. L is the inlet development length of the flow, and D is the diameter of theduct. For a tunnel diameter of 0.9 metres the above equation would require a length of atleast 10 m for a Re=10000. The entrance length L, cannot really be treated mathematicallysince there is a downstream reduction of the “potential core” because of the boundarylayer growth profile. The wind tunnel is a multi-purpose one and the entire length is usedfor measurements especially for jet fan aerodynamic studies. The main body of the tunnelof 8 metres was deemed a sufficient test length for the various studies to be carried outand when required honeycombs and screens can be fitted at the inlet to straighten andreduce the turbulence of the flow.A contraction piece was designed to join the square cross section wind tunnel to the 630mm diameter axial flow fan. The contraction piece design was critical for uniform smoothflow from the wind tunnel to the axial flow fan. A good design was essential in order topromote good fan performance. Poorly designed contraction pieces lead to very highpressure losses. The contraction piece is square at the tunnel side and circular at the fanside. The contraction area ratio is 2.61:1 and according to Figure 3.1 (Chmielewski, 1974)gives a length to diameter ratio l / D1 of 0.7. The contraction length is l and diameter ofthe wind tunnel D is equal to 900mm. This l / D ratio avoids flow separation whichshould be prevented in the contraction section because this would bring flow irregularitiesto the fan and eventually cause it to stall. The length of the contraction piece worked outto be 0.63 metres.56In order to produce smooth flow conditions in the tunnel an inlet section was designedwhich ensured maximum aerodynamic performance. A beilmouth entrance piece gives thelowest flow loss coefficient. The minimisation of entry losses to any wind tunnel is criticalto obtaining good measurements. The pressure loss coefficient K, for bell mouth inletpieces is dependent on the ratio r/D where r, is the radius of curvature of the bell mouthpiece and D, is the diameter of the tunnel. For values of K approaching zero, the ratio r/Dgradually approaches unity. The radius of curvature r, for the wind tunnel bell mouth inletpiece was chosen to be 900 mm to give an r/d ratio of 1. It was determined that the bellmouth piece should be 540 mm long and the inlet dimensions are 1264 by 1264 mm. It hasthe same dimensions as the wind tunnel where the two are joined by flanges.3.1.2 Construction of the Wind tunnelThe bell mouth entrance piece is made out of aluminum and it has a smooth profile toreduce inlet turbulence. A plywood transition piece of 900 by 900 mm cross section and800 mm long is connected to the inlet piece and the main body of the wind tunnel. Themain body of the wind tunnel is 7314 mm long and is constructed in three equal parts. Theroof and the floor of the wind tunnel are made from wood board whose surface wasspecially treated with Danish oil fmish an oil resin sealer and varathane diamond finish aninterior penetrating polymer coating with twice the abrasion resistance of polyurethane togive it a smooth texture when it was cured. The wind tunnel sides are made fromplexiglass 9.5 mm thick. The advantage of plexiglass is that, it enables flow visualizationtests to be made since it is transparent. The plexiglass pieces are inserted into 9.5 mm widegrooves of the same depth along the entire length of the roof and floor of the tunnel.Special gaskets cushion the joints of the plexiglass and the wood making the wholestructure completely air tight. At every joint, wooden ribs of 50 by 75 mm cross sectionare fastened to the structure one each side of the tunnel as shown in Figure 3.2. All legs57are made of wood and are distributed evenly in order to anchor the structure properly. Acontraction piece joins the wind tunnel to the fan through flanges and rubber gaskets. Thecontraction piece is made from aluminum.Figure 3.3 shows static pressure holes driiied on one side of the tunnel and spaced at 180mm apart in each of the six sections of the tunnel. On the other side of the tunnel six setsof 5 holes are located in each section for the insertion of velocity measurement instrumentsas shown in Figure 3.4. Complete velocity traverses can be made at each station coveringthe entire cross section and length of the wind tunnel. Figures 3.5 and 3.6 show the layoutof the wind tunnel as viewed from the bell mouth inlet piece and the axial fan dischargeoutlet respectively. The layout of the wind tunnel in relation to the room can also be seenclearly in these figures. Mutama and Hall (1993) give a detailed description of design,construction and testing of the wind tunnel.3.1.3 Wind Tunnel InstrumentationMeasurement techniques for airflow are described by Ower and Pankhurst (1977) amongothers. The instrumentation available included an array of low pressure transducers whichwere connected to the static pressure holes to give the axial pressure disthbution in thetunnel. The pressure transducers were connected to an analog-digital data acquisitionboard system linked to a computer. These pressure measurements were necessary whenassessing jet fan aerodynamics.The instrumentation available for the measurement of velocity comprised an AirflowDevelopments MED 500 digital micromanometer which worked in conjunction with apitot static tube and was capable of displaying velocity, pressure and volumetric flowreadings in both S.I. and English units. This instrument had extra features which gave the58user reliability in assessing the various aspects of airflow. The instrument could also beconnected to an analog data logger so that the readings could be recorded for furtheranalysis on a spreadsheet. Various sizes of pitot static tubes and a hot wire anemometerwere available for the measurement of velocity and these were adapted so that they couldbe used with the electronic traversing ruler. This ruler could determine the position of avelocity traverse to within 1/100 of a millimeter. An Omega Thermo-electronic vaneanemometer with analog outputs was used mainly to measure the inlet velocity profiles ofthe wind tunnel.3.1.4 Wind Tunnel TestingThe test program for the wind tunnel was aimed at assessing flow distributions anduniformity at two different flow settings. Flow patterns at the inlet bell mouth piece weredetermined. Axial pressure variations were measured for both a high and low Reynoldsnumber regime. Pressure drops across the contraction piece between the fan and the windtunnel were determined. The above mentioned instruments were used to measure the staticpressure and velocity. The instrumentation was arranged as shown in Figure 3.7. Resultsof the test program are presented and discussed in the following section. Test ResultsIn Figure 3.8 axial static pressure variation is shown for the two flow settings investigatedin the test program. The wind tunnel was tested at two Reynolds numbers of 219560 and427253, both of them being high turbulent airfiows. For the Re=219560 the static pressurecan be said to be constant throughout the main body of the wind tunnel i.e from the end ofthe inlet piece to the start of the contraction section which joins the fan. For the high flowsetting the pressure changes relatively little throughout the tunnel but fluctuations are59noticeable because of the higher turbulence that is present. In fact for this type of windtunnel the static pressure is not expected to change by any significant percentagethroughout the constant area portion of the tunnel.Figure 3.9 shows the centreline entrance velocity time plots of the wind tunnel for the twoflow settings investigated. The fluctuations shown in Figure 3.9 for the Reynolds numberof 219560 are not as large as those for Re = 427253. The inlet flow disturbances increasesignificantly as the volume flow is increased. A large proportion of this fluctuation resultsfrom the wind tunnel room. The structure occupies a third of the space available in thelaboratory and at high Reynolds numbers air currents exist throughout the whole room. Infact turbulence around the wind tunnel room was deliberately increased in order toestablish its effect on the inlet velocity and Figure 3.9 shows this effect. The nature of theaerodynamic tests performed inside this tunnel are not affected by the existence ofbackground turbulence in the room as long as circulating air currents at the inlet arereduced by a significant proportion. Turbulence could be reduced by eliminating some ofthe return currents from the fan discharge to the wind tunnel inlet.Results of the velocity traverses can be seen in Figures 3.10 and 3.11 for three stations inthe tunnel. In the Figures the length of the tunnel is presented in non-dimensionalized formas X/Dt where X is the axial distance from the inlet and Dt is the wind tunnel diameter.Z/Dt represents non-dimensionalized distance from the wind tunnel roof to the floor. Thefirst station was at X/Dt = 2.21 (1.99 m), the second at X/Dt = 4.93 (4.44 m) and finallythe third was established at X/Dt = 7.65 (6.89 m) from the entrance of the tunnel. Thesethree stations were carefully chosen from the six traversing stations available in the tunnel.As can be seen from the graphs five traverses were performed at each station at positionsof ZJDt of 1/6, 1/3, 1/2, 2/3 and 5/6. The traverses were performed from one side to the60other side wall of the wind tunnel. The velocity profiles are of a turbulent nature and thedistributions are quite flat. In Figure 3.10(a) to 3.10(b) it can be seen that the velocities ofthe five traverses of each station are all within a close magnitude. The profiles show aneven distribution of the flow with very thin turbulent boundary layers.At the last measuring station at the lower flow setting the velocities exhibit a very smoothprofile showing good flow symmetry although in Figure 3.11(b) unlike 3.10(b) themaximum velocities were obtained at the bottom of the tunnel. One would expect thevelocities at Z/Dt of 2/3 and 5/6 to be lower than those at Z/Dt = 1/2 since this is thecentre of the tunnel. In an ideal situation the profile at Z/Dt of 1/6 and 5/6 are similar andthose at Z/Dt of 1/3 and at 2/3 are also expected to be the same since they are taken at thesame distances relative the top and bottom walls respectively.One other aspect of the wind tunnel testing was to establish the pressure drop variationacross the contraction piece which joins the tunnel to the fan as a function of Reynoldsnumber or flowrate. The result of this test was quite satisfactory as shown in Figure 3.12and it conforms to the square law relationship i.e. the pressure drop is proportional to thesquare of velocity.3.2 Jet Fan Simulation and ArrangementThe sizes of the fans available on the market that could be fitted in the wind tunnel for jetfan tests were too large to make an effective simulation. The smaller fans were not able toproduce a jet outlet velocity sufficient to enable effective jet fan assessment unless theywere fitted with extension pipes of smaller diameter than the fan in order to increase thefan discharge velocities.61A compressed air jet was a good option for jet fan simulation and it offered the advantagethat the primary flow can be varied and measured easily but it could not be installed in thewind tunnel. A centrifugal fan was selected and adapted for the jet production. This wasachieved by redesigning its discharge side. A plenum air chamber measuring 295 by 262mm and 300 mm long is fitted to the discharge side of the fan. A conical contractionaccelerates the flow to 100 mm diameter aluminum pipe. The aluminum pipe is 200 mmlong and can be fitted with an orifice plate between its flange and that of another 100 mmdiameter aluminum pipe. The second aluminum pipe extends into the tunnel for a distanceof 1400 mm. The end of the aluminum pipe can be fitted with nozzles of differentdiameters ranging from 50 to 100 mm. The nozzles can be either straight, or converging.Air jet velocities ranging from 1 to 20 rn/s can be achieved with this centrifugal fan whichhas a 1/3 hp motor on a 100 mm diameter pipe without the orifice plate. When the orificemeter is installed it causes so much resistance such that there is hardly any flow.A much more powerful centrifugal blower with a 1 hp motor capable of discharging 0.378m3/s (800 cfm) was purchased in order to provide higher outlet jet velocities. The orificemeter was used to determine the air jet mass flow rate. A detailed arrangement of the jetmechanism in relation to the wind tunnel is shown in Figures 3.13 and 3.14.A range of jet outlet velocities could be produced by adjusting the suction area of the fan..The jet producing centrifugal fan is mounted on traversing rails or grooves such that thedischarge pipe or jet is situated on the centreplane of the tunnel i.e. 450 mm from the floorof the tunnel. Therefore the jet can be traversed from wall to wall of the tunnel. Thisenabled jet wall proximity to be investigated. The secondary flow in the wind tunnel couldbe varied by adjusting the pitch of the axial flow fan blades if required.623.3 Experimental Descrintion of Jet Fan Performance Measurements3.3.1 .Tet Fan Velocity Field MeasurementsThe instrumentation used to assess the jet fan performance is as described in section 3.1.3and shown diagrammatically in Figure 3.7. Velocity measurement was carried out using ahot wire anemometer which can determine one velocity component and its fluctuation. AnAirflow Developments MED500 digital micromanometer working in conjunction with apitot static tube was also used as a backup for some of the measurements. The velocityreadings were recorded on a data logger or an IBM PC. An electronic vane anemometerof 25 mm diameter specially adapted for the present study was used to determine themagnitude and direction of the flow whenever required and could be traversed across thewind tunnel. All the velocity and pressure traversing probes were coupled to a digitaltraversing ruler described earlier.Velocity traverses were performed at a height ofO.5DL from the tunnel floor although atfirst they were performed at each of the five vertical holes of the six stations in order toassess the complete velocity distribution of the flow at a particular cross section. After theinitial measurements were completed it was sufficient to determine velocity profiles at theheight of O.5D from the tunnel floor for all the six stations and at each fan position. At the5th and 6th measuring stations complete velocity grids were determined in order to obtainthe total volumetric flow rate. The amount of air entrained by the fan could be calculatedby subtracting the initial fan discharge volume flow from the total flow out of the windtunnel. The purpose of velocity measurements were twofold; (i) to determine the jetdiffusion process from the fan and (ii) to determine the entrainment characteristics of theconfined jet fan, The axial velocity fluctuation pattern could also be determined giving agood indication of the turbulence in the tunnel.633.3.2 Jet Fan Pressure MeasurementsPressure measurements were carried out using an array of transducers which wereconnected to the static pressure holes to give the axial pressure distribution of the tunnel.The pressure transducers were connected to an analog-digital data acquisition boardsystem coupled to a computer. In addition a digital micromanometer was used quitesuccessfully to record the axial variation of static pressure. It proved to be a reliablesource of pressure measurements. The purpose of the pressure measurements was todetermine how the pressure field varied axially and obtain pertinent data from it. This wasa useful exercise because very interesting results were obtained.The fan was set to deliver jets with 20.8 and 40 m/s outlet velocities from a jet fandiameter of 100 mm i.e. DR (D/Dt) of 0.11. A second jet fan to tunnel diameter ratio of DR= 0.17 was also used with a jet outlet velocity of 2mi/s. The jet fan was fitted withstraight nozzles in each case. The range of velocities of between 20 and 40 in/s is the usualone used in jet fan ventilation. In this study it was necessary to use the two velocityextremes in order make a good comparison. The two chosen jet fan to tunnel diametersDR of 0.11 and 0.17 were a good starting point for the study of jet fan-tunnel geometrywhich is an important parameter.The jet fan was traversed at positions F,, (Y/D1) of 0.06, 0.11, 0.17, 0.22, 0.28, 0.33,0.39, 0.44 and 0.5. from one tunnel wall. These positions corresponded to Y/D steps of0.055 from one wall to the centre of the tunnel. The reason for doing this was to studythe effect of the tunnel walls on jet fan perfonnance. The first task was to record axialstatic pressure distributions on both sides of the wind tunnel. The first measurementswere taken with the jet fan in contact with the wall and subsequent measurements weretaken at varying Y/D positions until the jet fan was at the centre of the tunnel64corresponding to YfD = 0.5. For each jet position 36 axial wall static pressure readingswere taken. It was necessary to record a large sample of readings for each static pressureposition so that a representative average could be determined.65210__________ __________ __________ __________0 5 10 15 20Contraction ratioFigure 3.1 Minimum length for contractions, without separationNo sei aration:ration66Figure 3.2 View of wind tunnel showing support structure67top_________ ________________ ________________000000 000000 000000000000 000000000000___6 12 18 2’ 3 36Ietfan fanbottom Dt=900mmF’ressure riole positions from jet nozzleN X N X N X N x0. mm 0. mm 0. mm 0. mm1 150 10 1870 19 3760 28 55502 330 11 2050 20 3940 29 57303 510 12 2230 21 4120 30 59104 690 13 2580 22 4300 31 61905 870 14 2760 23 4480 32 63706 1050 15 2940 24 4660 33 65507 1330 16 3120 25 5010 34 67308 1510 17 3300 26 5190 35 69109 1690 18 3480 27 5370 36 7090Fig. 3.3 Wind Tunnel South Wall showing static pressure holes68Fig. 3.4 Wind Tunnel north wall showing hot wire probe access holes‘.Stations 1—— —jet fan2 3 400--.-0050000600-Q00400-0---0000—o—0000--0-00635mm 1820mm 2080mm3270mm4530mmholes spaced 150mm apart vertically5720mm from jet fa69Figure 3.5 Wind tunnel layoutFigure3.6ViewofWindtunnelfromtheaxialfandischargeend71airflow directionarray of pressure transducers9 5 5 5wind tunnel0)Co1cD>0)o N.:::zC) 0) (1)t1. •I ——G) .E0C)interfacing computerwith dataacquisition cardpitot static tubeor hot wire anemometerdigitalm icromanometerFig. 3.7 Arrangement of instrumentation72Cl)00G)>C)4-,0Eci-5-10-150Cl)cts --25-30ci)-35-40-45Reynolds No. = 219560Reynolds No. = 427253:0 1 2 3 4 5 6 7 8Axial Distance (metres)Fig. 3.8 Axial static pressure variation for two flow settingsfor the working section of the wind tunnel65432100 50 100 150Time (Seconds)Fig. 3.9 Wind tunnel inlet velocity variation with time200735X/Dt = 2.214A A 4 A A A- ‘i a • a •0 0 • • 1 1. I •_A A>.‘00ci)>10 100 200 300 400 500 600 700 800 900(a)Distance across tunnel (mm)5X/Dt = 7.654 000000>4-C.)02ci)>1•00 100 200 300 400 500 600 700 800 900(b) Distance across tunnel (mm)Z/Dt = 1/6 Z/Dt = 1/3 A Z/Dt = 1/2 0 ZJDt = 2/3• ZJDt=5/6Fig. 3.10 Velocity profiles at Re = 2195607498 *XIDt=2.21•0 • * t A ** • * * 0o 00DDE0>21100 200 300 400 500 600 700 800 900(a) Distance across tunnel (mm)12XIDt = 7.651O . . . . I • • • •I •I0000000000(°8 .0000000E a A A A A A AA A A A A A A A Ae . g • • t 8 O .D0>200 100 200 300 400 500 600 700 800 900(b) Distance across tunnel (mm)o Z/Dt = 1/6 * Z/Dt = 1/3 A Z/Dt = 1/2 0 Z/Dt = 2/3Z/Dt = 5/6Fig. 3.11 Velocity profiles at Re = 427 253752502000U)0iv0 0100 0DU)00000 100 200 300 400 500Reynolds Number/i 000Fig. 3.12 Pressure drop-Reynolds number plot for wind tunnel contraction piece76&ndtunneI.I.xSide waIl 2Jet fan traverseIJet fanwall positionSide waIl 1IL =900 mmSECTION AA SHOWING JET FAN TRAVERSEZveilicalfX axial1/Fp= Y/Dt fan position parameter1 .centrifugal fan with plenum chamber2 graduated traversing railsFig. 3.13 Jet fan simulation mechanismDt=diameter of tunnelAbelimouth piecealuminum nozzle Y2ACl)iiCICIII__,Fp=0.577Figure 3.14 Photograph showing jet fan simulation arangement78CHAPTER FOURDATA REDUCTION AND ANALYSISAn explanation of the analysis and processing of experimental results is given in thischapter. The primary data is essentially pressure and velocity measurements which areused to determine secondary parameters of interest in the analysis of the jet fan jet flowfield.4.1 Analysis of Pressure ResultsAxial static pressure measurements have been plotted as a dimensionless pressure or innormalised form in order to make comparison easier from one set of measurements toanother. The axial static pressures are presented as (P — P ) / (0. 5pU) and are plottedagainst distance in jet nozzle diameters X/D3.e is the pressure of the secondary stream atentry in the tunnel. The pattern of static pressure variation is compared for both tunnelwalls in order to assess differences. A few static pressure results are also presented wherethe tunnel had a coflow velocity which was obtained by running the main axial flow fan atthe discharge end. The jet fan then discharged into an already moving tunnel air stream.The purpose of this analysis was to determine the pattern of pressure variation in a strongtunnel air stream which is not initially entrained by the jet fan from outside. Therefore theeffect of different jet fan to tunnel Reynolds number ratio can be examined.The data presented is for two sizes of jet fan to tunnel diameter ratio DR and three jetdischarge Reynolds numbers or three jet outlet velocities U. The jet position from onetunnel wall is characterised by a parameter F, which can be defined as the distance from79the tunnel wall divided by the tunnel diameter. This parameter is very important indetermining the performance of the jet fan in this study and has been plotted against thepressure ratio (P — f ) / (O.5pU). This pressure ratio is used to define a performanceparameter also plotted against F. The measured pressure rise — F) was plotted as aratio of the theoretical pressure rise (P1 — P )th * This was necessary in order to assess themomentum losses of the system.4.2 Analysis of Velocity ReadingsThe distribution of radial velocity of the jet fan - tunnel system at various axial locationswas necessary in order to determine the development of the flow throughout the entiretunnel. For each jet fan diameter ratio DR the velocity U/Us was plotted against thedistance across the tunnel Y/D i.e. the velocity was normalised using the jet dischargevelocity and the distance across the tunnel Y was norrnalised using the tunnel diameter Dfor all the profiles at various axial locations. Several distribution plots were made for thejet position parameter F.Since the flow is confined the jet fan flow field falls into the category of confined jetswhere self similarity of the flow is not usually achieved. It was necessary to check this byplotting the velocity U/Umax against the distance across the tunnel Y/D for selected values.When the profiles are self similar they fall under the same curve when plotted this way.A complete set of velocity profiles at X/D=45.3for DR=O. 11 and X/D=3O.2 for theDR=O. 17 is presented in order to (i) assess the flow symmetry of the tunnel and (ii)determine the tunnel mean velocity which can then be used to determine the volume flow80rate. The velocity profiles are normalised using the tunnel bulk velocity UB and are plottedagainst YfD at various tunnel heights Z/D.4.2.1 .Jet Axis Velocity Decay ProfileThese velocities are taken at the jet axis and plotted as Urn / Uj against dimensionless axialdistance XID. These velocities show how the jet discharge velocity declines from its initialhigh value to approximately the average tunnel mean velocity at some point at the end ofmixing between the primary and secondary stream. The jet axis velocity decline iscompared for the two jet fan diameters. The theoretical jet axis decay curveU D. (4.1)U Xfor an axisymmetric jet is also plotted on the same graph as the measured values in orderto compare the two data sets.4.3 Backflow Analysis and Jet Expansion Angle DeterminationIt was necessary to plot the backflow velocity distribution independently for the various jetposition parameters F and diameter ratios DR. Complete cross sectional profiles of thebackflow velocity distribution have been obtained in order to provide a full assessmenttunnel height velocity distribution. The backflow UR is normalised by the backflow bulkvelocity UBr.The backflow quantity QR (m3/s) has been determined from the velocity measurements foreach cross section and at all the jet fan position parameters F. The quantity QR/QT hasbeen plotted against axial distance X/D. The average backflow quantity QR normalised by81QT and Q for each jet fan position has been plotted against the parameter F,. It was veryuseful to be able to compare the amount of air recirculated as a fraction of the total tunnelflow or jet discharge for the assessment of system energy losses.For each jet fan - tunnel arrangement the backflow field has been characterisedcompletely. One variable of interest was the width YJD of the backflow at each axiallocation of the tunnel which has also been presented graphically as a function of XfD. Thebackflow length LR/DJ has been plotted against the jet fan position parameter F.4.3.1 Jet Expansion Angle DeterminationThe jet expansion angle of the jet fan has been estimated from the measured results andprimarily from knowledge of the backflow length LR. An assumption is made that the jetfrom the jet fan expands like a free jet until it reaches the walls of the tunnel. At this pointsome of the air stream is recirculated back to the entry region of the tunnel. By estimatingthis length from the jet nozzle discharge the jet expansion angle can be determined with areasonable accuracy. A complete account of how the jet expansion angles have beendetermined in this study is detailed in Chapter Seven dealing with theoreticalconsiderations. The half jet expansion angle is plotted against the jet fan positionparameter F, for both diameter ratios DR=O. 11 and Entrainment ResultsThe entrainment results are derived from velocity measurements across the entire tunnelcross section where backflow was totally absent. The total tunnel airflow quantity QT(m3/s) is determined from knowledge of the mean velocity. QT/Q has been plotted againstthe jet fan position parameter F, in order to compare the entrainment characteristics of82the jet fan at various positions from the wall. The amount entrained flow Q has beenplotted as a fraction of the jet discharge quantity Q. This parameter is generally regardedas the flow ratio n in this study since it expresses the ratio between the secondary and theprimary flow.4.5 Jet Fan PerformanceThe jet fan performance has been plotted as a performance parameter q derived frompressure results by multiplying the pressure ratio (P-Pe)/(P-P by the flow ratio n. TheTiparameter is plotted against the jet fan position parameter F. Another performanceparameter termed the induction efficiencyC 24 Vi—i11=1 ii I (4.2)1l—2Al+)has also been plotted for comparison purposes. The symbols and 2 are the velocity andarea ratios U/U and A/As respectively.4.7 Longitudinal Velocity FluctuationsThe velocity fluctuations measured at the tunnel axis are plotted and normalised by thetunnel axis velocity or local centreline velocity. The tunnel axis is not necessarily the jetaxis in this case. The purpose of these measurements was to give an indication of thelongitudinal turbulence in the tunnel with the aim of gaining a deeper understanding of theflow structure and its mechanisms.834.8 Uncertainty analysis in measured and derived uuantitiesDetailed guidelines to the determination of experimental design and uncertainty aredescribed by Coleman and Steele (1989), Holman (1984), Moffat (1988) and are alsogiven in the ANSI/ASME PTC.The methods recommended are that the precision and the bias limit be determined in orderto determine the overall uncertainty of a result. The bias error is the fixed, systematic orconstant entity of the total error. When the true bias error is defined as I, the quantity B isthe experiment’s 95 percent confidence estimate such that B. The precision error ±eis the random component of the total error and is the lack of repeatability of the result.The ±e interval about a result (single or averaged) is the experiment’s 95 percentconfidence estimate of the band within which the mean of many such results would fall, ifthe experiment was repeated many times under the same conditions and using the sameequipment.The ±a interval about the result is the band within which the experimental result is at 95percent confidence level of the true value. The 95 percent confidence uncertaintya iscalculated from the root-sum square (RSS)a = [B2 + e2j°5 (4.3)Bias limits from M elemental error sources can be combined using RSS asM 0.5B =[(BJ)k2] (4.4)For the data reduction equationr=r(X1,X2...,Xj) (4.5)84the bias limit for the experimental result is found from the uncertainty analysis expression0.5(4.5)(4.6)Similar expressions can be written for the precision error e and the overall estimate of theuncertain 0 can be calculated from the uncertainty equation which combines both the biasand precision limit for each result.03-2 / 2iar 1 iar 1 Ira, = L%0”J +-axJ+ +axj (4.7)Tabulated experimental uncertainties based on the above analysis have been given in theappendix for a representative portion of the data.85CHAPTER FIVEDISCUSSION OF PRESSURE FIELD RESULTSThis chapter and the next present a detailed discussion of the results obtained in thisinvestigation. Jet fan ventilation results are described by the pressure and flow fieldmeasurements and their subsequent analysis. The importance of the pressure results istwofold. (i) The pressure field results help in determining the entrainment rates of the jetflow field of the fan. (ii) From the pressure rise characteristic of the jet fan, powercalculations can be performed and the effectiveness of the system determined. The mainvariables are (i) the diameter of the jet fan nozzle in relation to the tunnel diameter (Dj/Dtor DR), (ii) the jet discharge velocity (Ui) and (iii) the influence of the jet fan positioningfrom the walls (Y/Dt or F) on the measured quantities. It will be shown that thesevariables play a significant role in determining jet fan performance.5.1 Effect of .Tet Fan Position F on Axial Pressure Develonment5.1.1 Pressure results for R=°•11Without Tunnel CoflowThe axial static pressure results have been normalised by the dynamic pressure of the jetfan in most cases. Two values of jet fan to tunnel diameter ratio DR were used: 0.11 and0.17 with jet fan discharge velocities of 40 and 21.4 m/s respectively. In this section theaxial static pressure results are discussed for the diameter ratio of 0.11 (U1 = 40m/s) andthese are presented in Figures 5.1 to 5.8.Figure 5.1 shows the axial static variation for three fan positions (Fe) of 0.06, 0.11 and0.17. The position F=0.06 is closest to the wall. The axial static pressure varies in the86same way at these three jet fan positions. At the point of the jet discharge the staticpressure is zero i.e. it is at the atmospheric or ambient level and at this stage the walls ofthe tunnel do not affect the jet pressure field. Initially the pressure falls slightly and thenrises within eight to ten jet fan nozzle diameters downstream, due to the influence of thetunnel wall, when the jet flow is expanding. When the jet reaches the wall the pressureonce again falls due to inward deflection of the jet flow away from the wall. Minimumpressure which is below atmospheric is attained at just over 25 nozzle diametersdownstream in all three cases. The respective minimum pressures are -0.005, -0.007 and -0.009 of the jet discharge dynamic pressure for the three positions F=0.06, 0.11 and 0.17.After the minimum is reached the jet flow pressure rises monotonically until a maximum ofabout 0.016 jet dynamic pressure is achieved, after which it remains unchanged over theentire test section. At this point the jet is thought to have reached both tunnel wails andthe flow develops in the same way as ordinary pipe flow.Figure 5.2 shows pressure results for the jet fan position parameter F=tL22, 0.28, 0.33,0.39, 0.44 and 0.5. These are presented together because they follow similar trends whichdiffer from the positions close to the wall shown in Figure 5.1. In these positions thepressure continues to drop from the jet discharge until a minimum of about —0. 003k pUis reached in all cases at twenty jet nozzle diameters. After the minimum is reached thepressure rises monotonically until a maximum is reached as in the previous cases. It isinteresting that in the first twenty nozzle diameters the pressure variation for all thesepositions is very close both qualitatively and quantitatively.Considering the pressure variation in both Figures 5.1 and 5.2 it is observed that thepressure drops below ambient level before rising to a maximum value. The initial pressure87drop is responsible for inducing secondary flow into the tunnel. The secondary flow thenmixes with the jet primary stream and causes the static pressure to rise to a certain level.The closer the jet fan is to the wall the greater the initial pressure drop on one side of thetunnel.In Figures 5.3 to 5.8 the same pressure results are presented but this time showing acomparison of the variation from both sides of the tunnel. The main feature of thiscomparison is that although the variation pattern is very similar on both sides of thetunnel, the pressure rise is much steeper on the side of the tunnel for which F 0.5.Figures 5.3 to 5.8 also show that the static pressure is not symmetrical on both sides of thetunnel. In the first twenty diameters in Figures 5.3 and 5.5 for F 0.5 the pressure dropvariation does not strictly follow that for F 0.17. This is to be expected in this casebecause of the differences of the jet fan location from either wall. The fan positionparameter F is defmed from both walls in Figures 5.3 to 5.12. For example the positionF=0. 11 from one wail is F=0.89 from the other wall. When the jet fan is moved towardsthe axis of the tunnel the pressure magnitude is almost the same in the first twenty nozzlediameters downstream as can be seen in Figures 5.7 and Pressure Results for = 0.17 and U=21.4 (Re=21165The results for the jet fan to tunnel diameter ratio DR=0.17 are presented in Figure 5.9 forfour jet fan positions Fp of 0.083, 0.17, 0.33 and 0.5. The purpose of these results was toestablish the effect of having a larger fan discharge diameter with a lower discharge outletvelocity but keeping the outlet mass flow rate approximately the same. The axial staticpressure variation is similar to the variation for the DR=O. 11 (lJj=40m/s) case for similarvalues of jet fan position from the wall F. When the larger diameter jet fan is nearest tothe wail the pressure initially rises above ambient and declines to negative values before it88begins to rise but the pressure drops are much less than those reported in Figures 5.1 and5.2. The farther the jet fan is moved from the wall the faster the static pressure rises toreach a constant value over the measuring test section.Comparing Figures 5.1 and 5.9 it can be seen that the larger the jet fan diameter the morerapid the pressure field develops in the tunnel. For F 0.33 the pressure variation iseffectively the same, and it reaches a minimum in this case at about 10 jet fan nozzlediameters compared to 15 and 18 nozzle diameters for the positions F of 0.08 and 0.17respectively. An important feature of Figure 5.9 is that the normalised static pressureshave a higher magnitude than in Figure 5.1 where the jet fan diameter is smaller and theoutlet velocity is higher. It can be deduced from the two figures that for approximately thesame jet discharge mass flow rate, larger diameter jet fans have a higher jet pressureutilisation factor than smaller diameter fans.5.2 Comparison of Pressure Variation for Two Differing .Tet Fan DischargeVelocitiesFigures 5.10 and 5.11 compare axial pressure variation for a jet fan of the same diameterpositioned at the same distance from one tunnel wall with two outlet discharge velocities.In the first case the jet fan is positioned close to the wall F = 0.06 and in the other caseit is situated further away from the wall at F = 0.33. The most important point to make isthat the normalised pressure values stay almost the same up to the point where the jet witha discharge Reynolds number of 13714 reaches its maximum value. The fully developednormalised pressure for the Re=26374 case (Uj=40m/s) is about one and half times higherthan that for Re=13714 (U=20.8m/s). In Figure 5.10 results are also presented for jet fanposition F = 0.83 which compares the pressure variation from the other side of the89tunnel with that for the position F of 0.06 for the two jet discharge Reynolds numbers. Itcan be seen that the pressure variation is not symmetrical for both tunnel side walls.For any jet fan discharge mass flow rate the parameter which controls the manner in whichthe axial static pressure varies is the jet fan position relative to the confining tunnel wallsrather than the discharge velocity or Reynolds number. Figures 5.10 and 5.11 clearly showthis fact.5.3 Comparison of Pressure Variation for Two Jet Fan to Tunnel Diameter RatiosIn Figure 5.12 pressure results for two jet fan to tunnel diameter ratios of DR=O. 11 and0.17 are presented. The average jet discharge flow is approximately the same in both casesbut the outlet velocities are 21.4 and 40 m/s for the larger and smaller diameter jet fanrespectively. Qualitatively the pressure varies in the same way. For the larger diameter jetfan the pressure develops rapidly axially and reaches a peak at shorter distances. Themagnitude of the normalised pressure is also higher in this case. The minimum normalisedpressures are lower than for the smaller diameter jet fan. For the smaller diameter jet fanthe magnitude of the pressure change from jet discharge to minimum and then tomaximum is more gradual; and lower peaks are reached at larger downstream distancesthan for the larger diameter jet fan. After the maximum values are reached in the tunnel thepressure gradually falls downstream to ambient level again. In both situations the pressuredevelops more rapidly when the jet fan is positioned at the tunnel axis (F=0.5) and thanwhen it located near the wall (F=0.O6 and 0.08)905.4 Axial Static Pressure Variation in the Presence of a Strong Tunnel CoflowIn a number of situations jet fans are used to boost the pressure of an existing flow in atunnel or mine airway. The pressure rise characteristic in this case is of interest in theventilation design of the jet fan and airway or tunnel system because in this case the fanderives its airflow from the airway and gives it a momentum boost.Figure 5.13 shows axial static pressure plots for a jet fan to tunnel diameter ratio DR=O. 11and initial velocity ratio U/U=O.O9.The tunnel mean velocity was 3.6 m/s before the jetflow was introduced. In this case the coflow velocity is considered to be very strong. Thepressure varies in a very interesting way for both jet fan positions F=0.06 and 0.5. In thesituation when the jet fan is located close to the wall (F=O.O6) the pressure rises and fallsin the first 10 jet nozzle diameters and then remains below ambient for most of the tunnel.At about 50 nozzle diameters it starts to rise slowly and at X/D=7Oit is still rising and thistime it is well above ambient with a value of about 0.3% of the jet discharge dynamicpressure. When the jet fan is located at the tunnel axis (F=0.5) the pressure starts fromwell below ambient with a value of -0.0075 of the jet discharge pressure. The pressuregradient is positive right from the start. The pressure rises above ambient at aboutX/D=3S and at X/D=70 the pressure is still rising with a value of 0.065% of the jetdischarge diameter. When Figures 5.1, 5.2 and 5.13 are compared it can be seen that theeffect of tunnel coflow is to stretch the pressure variation and therefore reduce thesteepness of the pressure gradients in all cases. In Figures 5.1 and 5.2 there is no initialtunnel flow i.e. zero coflow velocity and it is observed that the pressure rise is muchsteeper and the pressure reaches a maximum value much earlier than in Figure 5.13 wherethere is strong tunnel coflow. Therefore the effect of tunnel coflow is to retard thedevelopment of the jet flow which explains why the pressure develops more slowly.91In Figure 5.14 results are presented for a stronger coflow of velocity ratio UjlJ=O.l5 andfor the jet diameter ratio DR=O. 17 for four jet fan positions. In this case the pressure risesmore slowly than for the case in Figure 5.13. The tunnel pressure variation for an averagevelocity of 3 mIs, without a jet fan is also shown and this shows no change over the entiretest section. At some point the jet fan will have no marked effect on the secondary (tunnel)stream for a sufficiently high coflow velocity. Thus jet fans are useful where there is weakor no coflow in the confining space. In Figure 5.15 a comparison is made for two sets ofpressure results for ajet fan with and without coflow for a diameter ratio of DR=O.l7.Thisgraph shows that the rate of pressure rise is steeper with no coflow than when there is acoflow. The importance of these results is that they are a useful guideline in ventilationdesign for a jet fan used to boost airway pressure in a mine or tunnel.5.5 Pressure Ratio as a Function of Jet Fan position (Fe) Inside the TunnelFigures 5.16 and 5.17 show the measured to theoretical tunnel end pressure ratio(1’tn — ‘e )exp / (P — F) as a function of jet fan position from the tunnel wall. Thispressure is the maximum pressure rise achieved over the tunnel test section. Thetheoretical pressure (F — F )jh is determined from the following equation:P —F =cLpU(1—an) (5.1)Equation 5.1 is derived from momentum balance considerations of the jet flowsecondary or entrained flow meUe and total tunnel flow momentum mTUT and expressedas:92mU + meUe = mTUT + iXpA (5.2)The symbols U ,c,n and p represent the jet fan discharge velocity, jet fan outlet tosecondary flow entry area ratio, flow ratio and air density respectively. The last term inequation 5.2 ApA, is the momentum due to tunnel pressure forces. The assumption madein the derivation of the above equations is that there are no wall friction losses, thusequation 5.1 represent ideal conditions. From the ratio (P,2 — P )exp I (P Pe), the lossesdue to wall friction can be estimated. In Figure 5.16 it is very clear that walls have markedeffect on the pressure rise due to excessive friction. When the jet fan is situated close tothe wall it is observed that 15% of the momentum is lost to friction in this investigation.Mizuno and Araie (1990) also observed that as much as 15 % of the momentum is lostdue to friction when the jet flow is close to the wall. The percentage momentum loss whenthe jet fan is close to the wall can vary depending on the roughness of the walls and the jetflow Reynolds number. The results shown in Figure 5.16 are for the jet fan-tunneldiameter ratio DR=O. 11 and a jet fan discharge velocity of 40 m/s.As the jet fan is moved towards the tunnel centre the momentum loss due to friction loss isreduced. At the tunnel centre (F = 0.5) the difference between measured and theoreticalpressure rise is about 9 %. In Figure 5.17 for the larger jet fan-tunnel diameter ratioDR = 0.17 and lower jet discharge velocity U1 21.4 m/s the percentage loss due tofriction when the jet fan is close to the wall is about 9.5 %. As the jet fan is traversedtowards the tunnel centre, the pressure loss due to friction diminishes much faster than inthe previous case. At the jet fan location F = 0.25 the measured pressure rise is almostthe same at the theoretical one derived from equation 5.1. However at jet fan position= 0.33 the measured pressure falls sharply to only about 94 % of the theoretical one93i.e. the losses due to friction increase to 6 %. This value is reduced to about 1 % as the jetfan is moved closer to the tunnel centre position.From Figures 5.16 and 5.17 it can be said that pressure losses are less for larger diameterjet fans with lower outlet velocities. Thus the selection of an optimum position for aneffective system depends on the physical parameters of the jet fan such as outlet area andvelocity. The width and height of the opening or tunnel should be of a certain size whichreflect the jet fan flow and geometrical conditions.In Figures 5.16 and 5.17 a second pressure ratio is also plotted. This pressure ratio defmesthe pressure rise of the tunnel airsiream to the pressure drop by the jet flow(P1—e) / (P —P111). The symbols P, e and P are the tunnel end, secondary stream andjet fan discharge pressure. Jet fan positions closer to the wall have lower pressure ratiosmainly because of friction. In Figure 5.16 at the position closest to the wall (F = 0.06)the pressure rise is 1.7 % of the jet discharge pressure drop and when the jet fan is at thetunnel centre (F = 0.5) the pressure rise is 1.9 %. The variation follows that ofexperimental to theoretical pressure rise plotted on the same figure. In Figure 5.17 theratio of the pressure rise to jet total pressure drop is higher than that shown in Figure 5.16for DR=O. 11 and U=40m/s. In Figure 5.17 the diameter ratio DR=O. 17 and jet dischargevelocity is 21.4 rn/s. The deduction that larger diameter jet fans with a lower dischargevelocity have a higher pressure utiisation capability is a reasonable one.940.02cJ —0015• 0o___A 03. 0.01ci - 0-89:.. 0.005 - - -80— OOo00A oe -8_& 0 --0.005 0 -o-_00-0.010 10 20 30 40 50 60 70 80X/Dj- Fp=0.06 o Fp=0.11 A Fp=0.17 Uj=40m/s (DR=0.11)Fig. 5.1 Axial pressure variation (Fp=0.06 to 0.17)0.025’ II” •“0015 *1_- Uj=40m/s(Re=26374)0.00500 *a -0.005o 10 20 30 40 50 60 70 80X/Dj- Fp=0.22 0 Fp=0.28 Fp=0.33 * Fp=0.39o Fp=0.44 * Fp=0.5 (DR=0.1 1)Fig. 5.2 Axial pressure variation (Fp=0.22 to 0.5)95- Fp=0.11 0 Fp=0.89Fig. 5.4 Axial static pressure variation on tunnel side walls (Fp=0.1 1 and 0.89)(DR=O.1 1 Re=26374 Uj=40m/s)0.02cJDQ— 0.0150.019 0.0050-0.005-0.01- Fp=0.06 0 Fp=0.94Fig. 5.3 Axial static pressure variation on tunnel side walls (Fp=0.06 and 0.94)(DR=O.1 1 Re=26374 Uj=40m/s)0.02C”10 20 30 40 50 60 70 80X/Dja.— 0.0150.01a)0.0050-0.005___00 --0 -010 20 30 40 50 60 70 80X/Dj96I- Fp=O.17 ° Fp=O.83Fig. 5.5 Axial static pressure variation on the tunnel walls (Fp=O.17 and 0.83)0.02c’J___ ___ ___ ___ ______ ___ ___0.0150.01- Fp=0.22 o Fp=0.78 DR=0.11 Re=26374Uj=40m/scJ0.020.0150.010.0050-0 .005o0p6 -0_ -10 20 30 40 50 60 70 80X/Djo Q0op0.0050-0.0050.01o 10 20 30 40 50 60X/Dj70 80Fig. 5.6 Axial static pressure variation on the tunnel walls (Fp=0.22 and 0.78)970.0150.010.0050Fig. 5.7 Axial static pressure variation on the tunnel walls (Fp=O.44 and 0.56)c’J£ 0:15__ ____ ____Fp=0.5 - North waJi o South wall DR=0.1 1 Re=26374 Uj=40m/s0.02c%J-0.005o Fp=0.4410 20 30 40 50 60 70 80X/DjFp=0.56 DR=0.1 1 Re=26374 Uj=4OmIs0.02o .._- --c5- ---50.00500.005o 10 20 30 40 50 60X/Dj70 80Fig. 5.8 Axial static pressure variation on both sides of the tunnel980.05cJ****** ******0.04 AA0O.O30-90.02 *0000.01 A 000 0 -0001- -- .-0.0210 20 30 40 50X/Dj- Fp=0.083 0 Fp=0.167 Fp=0.33 * Fp=0.5Uj=21 .4m/s (Re=21 165)Fig. 5.9 Axial static pressure variation vs axial distance for jet fan DR=O.1 7990.02c15’ o2--0.0150... 0.01 ——AcL -0.005aA0-0.005o 10 20 30 40 50 60 70 80X/DjUj=20.8 rn/s - Uj=40 rn/sUj=40m/s Fp=0.83Fig. 5.10 Static pressure variation of jet fan for two jet Reynolds numbers(Fp=0. 17)0.02N5,0.015000000000000 000000000 _--_-_-_0.01 0 - -0-0.005 e- 00053o0.005o 10 20 30 40 50 60 70 80X/Dj- Uj=20.8 mIs 0 Uj=40 rn/s Fp=0.33Fig. 5.11 Static pressure variation of jet fan for two jet Reynolds numbers0.05C\J 0.04a— Fp=0.08 cIFp=O.17 * Fp=0.5 DR=O.17- Fp=O.06 OFpO.17 Fp=O.5 DR=0.11Fig. 5.12 Comparison of pressure variation for two jet fan diameter ratiosc’J0.01c’.Ja)aii0-0.00510 20 30 40 50 60Distance from nozzle X/DJ- Fp=0.06 0 Fp=0.5 Ut/Uj=0.09 Uj=40m/s DR=O.1 1Fig. 5.13 Pressure variation for jet fan with tunnel coflow70 8010 20 30 40 50 60 70 80000000OO000oO00 - -0 --0p0 -000- - ------ 0__- - -- -- - j_00 - -000000000101Dc’j0cL0.051:::0- 0.02a0.010-0.01-0.020.03o0.02* *** ****0.01 - **** *** -*** - QO0_-** --o-_000 Q-0.01-0.02 oTunnel at Ut=3m/s without jet fan-0.0310 20 30 40 50XIDj- Fp=0.08 0 Fp=0.17 Fp=0.33DR=0.17 Ut/Uj=0.15Fig. 5.14 Pressure variation with tunnel coflow Ut=3m/s10-Fp=O.O8 —•— Fp=O.17Fp=O.08 -e— Fp=O.17DR=0.17 UJ=21.4m/s20X/Dj30-A— Fp=O.5 (Ut/Uj=O.15)— Fp=O.5 (no coflow)40Fig. 5.15 Comparison of pressure variation with and without tunnel coflow102.c:a,00><a)a)0e4-..111.02-= 14-0.980.96><CI)0.940.920.9a)aa)C4-.0-.0.920.90.880.860.84(Ptn-Pe)/(Pj-Ptfl)0.01 950.01 850.0175_______0.01650.5 0.60 0.1 0.2 0.3 0.4Jet fan position FpDR=0.1 1 Uj=4OmfsFig. 5.16 Plot of various pressure ratios vs jet fan position Fp(Ptn-Pe)/(Pj-Ptfl)0.0460.0440.042C.100.0400.0380.60 0.1 0.2 0.3 0.4 0.5Jet fan position FpDR=0.17 Uj=21.4mJsFig. 5.17 Plot of various pressure ratios vs jet fan position Fp103CHAPTER SIXDISCUSSION OF THE JET FAN VELOCITY FIELD DEVELOPMENTVelocity results are presented in this chapter. The velocity field development is importantin evaluating the airflow or aerodynamic characteristics of the jet fan inside the tunnel.The velocity profiles were taken at half the distance from the floor of the wind tunnel tothe roof i.e. at half the tunnel diameter and measured from one side wall to the other. Thejet fan was traversed across the cross section of the tunnel and the results presented arefor four jet fan positions for two diameter ratios and jet discharge velocities.6.1 Velocity Distribution for Jet Fan to Tunnel Diameter Ratio DR=O.11 andUj=4OmJsFigure 6.1 shows the normalised velocity distribution (U /U) for a jet fan (DR=O. 11,U=40 mis) at four tunnel positions F 0.06, 0.17,0. 33 and 0.5. is the distance of thejet fan from the wall normalised by the tunnel diameter. The profiles presented start fromX I D = 6.35 up to 32.7. The velocity profile development is affected by the presence ofthe tunnel walls. At the position F=0.06 (Figure 6. la) the velocity profiles have acommon feature in that the velocities are positive for a part of the tunnel cross section andnegative on the other. The jet flow field is close to one tunnel wall and shows lack ofsymmetry. Its closeness to one tunnel wall limits the way it spreads with axial distance. Onthe other side of the tunnel the backflow covers a large area and by 45.3 jet fan outletdiameters the reverse flow has disappeared. When all the velocity profiles are comparedfrom Figure 6.la to 6.ld it is found that the jet flow spreads less rapidly when the jet fan isclose to the tunnel wall than away from it. As the jet fan is moved towards the tunnel axisthe size of the backflow decreases. A much more detailed analysis of the backflow will be104given in chapter eight. When the jet fan is at the tunnel axis the flow is symmetrical andthe flow field is observed to oscifiate with periods varying between two to four seconds.This takes places in wall regions up to 0.17 tunnel diameters from the wall. The flowchanges in direction in this region from positive to negative and this extends from a shortdistance downstream of the jet discharge to nearly 32.7 jet fan outlet diameters.In Figure 6.3 the same velocity profiles are nomalised using Umax and it is clear that theflow does not vary smoothly across the tunnel at most of the measurement stations mainlydue to the high turbulence intensity inside the tunnel. Even at X / D1 = 45.3 for the jet fanposition F = 0.06 the flow is still skewed on one side of the tunnel. It seems it wouldtake a longer distance from the jet fan for the flow to be uniform. The backflow is shownclearly in this Figure. It is apparent that the flow in this setting does not approach selfsimilarity which is not surprising for a confmed jet, unlike a free jet.Figure 6.5 shows a complete velocity distribution for the entire tunnel cross section atX ID = 45.3 for three jet fan positions F of 0.11, 0.33 and 0.5. The velocity profiles arenormalised by the local bulk velocity UB and are measured at fractional height of the windtunnel, Z/D of 0.17, 0.33, 0.5, 0.67 and 0.83. At this axial location there is no reverseflow. However the velocity distribution is not evenly distributed for the jet fan positionaway from the tunnel axis as shown in Figures 6.5a and 6.6b. The turbulence of the flowfield at this point contributes to the large scatter observed in Figures 6.5a and 6.5b. InFigure 6.5c the flow is effectively symmetrical.1056.2 Velocity Distribution for .Tet Fan to Tunnel Diameter Ratio DR=O.l7and Uj=21.4Velocity distribution (U IU1) for the larger jet fan shown in Figure 6.2 is very similar tothe previous case (Figure 6.1). When the jet fan is not located at the axis of the tunnel theflow develops on one side with a significant reverse flow covering a significant part of thetunnel. The reverse flow area is not as large as in the smaller diameter, higher velocity jetfan. The flow develops faster than in the smaller diameter jet fan and by X / D = 21.8 thejet flow has spread to cover the entire cross section of the tunnel. When the jet fan islocated at the axis of the tunnel the velocity distribution is symmetrical. The oscillationobserved for the smaller diameter, and higher velocity jet fan is not as strong in this case.Velocity profiles (U / Um) are presented in Figure 6.4. They are similar to the smallerdiameter jet fan velocity profiles and they show no self similarity. These results also showthe presence of backflow and the rapid development of the jet flow at positions closer tothe tunnel axis. The velocity distribution at X ID1 = 21.8 for the jet fan located at thetunnel axis (F,, = 0.5) shown in Figure 6.4d is completely evenly distributed suggestingthe jet flow is totally mixed with the secondary stream.Velocity profiles at X / D = 30.2 normalised by the local bulk velocity presented inFigure 6.6 are similar to the profiles shown in Figure 6.5 (for the smaller diameter jet fan).At jet fan positions closer to the wall the velocity distribution remains skewed atdownstream points. The profile in Figure 6.6c is very similar to that shown in Figure 6.5csince they are for the same jet fan position i.e. the tunnel axis location and they arereasonably symmetrical.1066.3 Jet Axis Velocity Decay Inside Tunnel for a Jet Fan at Various PositionsThe normalised centreline velocity Urn IU decay of a confined jet fan in the tunnel isshown in Figures 6.7 to 6.9. These results are for two jet fan to tunnel diameter ratios andfor various tunnel jet fan locations. Figure 6.7 shows that the centreline velocity of a jetfan located closer to the wall is slower in its decrease than when the jet fan is positioned atthe tunnel centre after distances X I D 5. Results shown in this figure are for thediameter ratio DR = 0.17 and U = 21.4 mIs. At the jet fan position Fp=0.08 the centerlinevelocity after 22 jet outlet diameters is greater than at the other positions shown on thefigure. At this distance the centreline velocity is less than 15 % of the initial jet fandischarge velocity for all the jet fan positions in the tunnel. The velocity decays rapidly inthe first 15 jet fan outlet diameters to less than 50 % of the initial jet discharge velocity.For the purpose of comparison the axial centreline velocity decay profile of anaxisymmetric free jet derived from the formulaU (6.1)U Xis plotted on the same figure. In the above equation D is the free jet outlet diameter equalto the jet fan diameter and X is the axial distance from the jet outlet. The centrelinevelocity decline for a free jet is lower and more gradual than that for a confined jet fan. Atabout X I D 22, the velocity for an axisymmetric jet is about six times higher than thatfor a jet fan located at the tunnel axis. The difference is caused by fluid friction, mixingand reverse flow losses in the main tunnel flow.107Figure 6.8 shows the centreline velocity decline for a lower diameter ratio DR = 0.11 andhigher jet fan discharge velocity U =40 ni/s. The same trends are observed as for thelarger diameter ratio jet fan but in this case the velocity decline is slower. Generally thedecline of centreline velocity is slower as the jet is moved closer to the wall. In both casesthe axisymmetric jet declines more gradually and smoothly than for a confined jet fan.Results for the two jet fan diameter ratios are plotted together in Figure 6.9 where it isclear that the centreline velocity decline is faster for the larger diameter jet fan. A steepdecline of the jet fan flow means that the flow is spreading and mixing very rapidly. Thiscauses the jet flow to slow down and at distances much further downstream the final flowvelocity is less than 2.5 % of the initial jet discharge velocity U3. Most of the energy lostby the jet fan to mixing is within the first 20 jet outlet diameters.6.4 Jet Expansion Angle and Reverse Flow Phenomena6.4.1 .Iet Expansion Half AnglesThe jet expansion angle of the jet fan flow is a useful property which enables the distanceof confining space covered by the jet during its expansion before it impinges on the wallto be determined. Information concerning reverse flow or recirculation of some of theflow stream can be estimated from this distance. The estimation of jet expansion angle hasbeen described in chapter four. The assumption made was that the jet from the fanexpands as in a free jet until it reaches the wall. In this study the distance it takes for thejet to reach the wall has been estimated by assuming that the axial static pressure at thatpoint reaches a maximum and therefore the distance can be determined by measurement.The other method used in this study was to assume that the point where reverse flowbegins downstream in the tunnel is where approximately the jet reaches the wall.Experience from this study has shown these to be reasonable assumptions.108Figure 6.10 shows jet expansion half angle for the jet fan with DR = 0.11 and U =40 m/sfor various positions in the tunnel. The average jet expansion half angle 0 determined forthis flow setting is 12.43°. The minimum half angle determined is 10.5° when the jet fan islocated at the tunnel axis. The maximum half angle obtained is about 14° when the jet fanis at position F = 0.33. For every flow condition i.e. jet fan Reynolds number or outletvelocity, jet fan outlet diameter and configuration, the jet expansion angle should beindependent of jet fan position inside the tunnel. Thus a jet fan expansion half angle of12. 43° ± 1.5° for this flow condition is a reasonable estimate. The differences in jetexpansion half angle shown in Figure 6.10 for the jet fan at various positions inside thetunnel are due to the uncertainty in determining the point where the jet reaches the wall.However the jet expansion angle estimation gives reasonable results because thedifferences at the various positions are not as large.Figure 6.11 also shows jet expansion half angles for a jet fan with discharge velocityU = 21.4 nb’s and diameter ratio DR = 0.17 for various tunnel positions. The average jetexpansion half angle was determined to be 12.90 with a maximum and minimum value ofabout 16.5° and 110 respectively. Although the average value of 12.9° in this case ishigher than that shown in Figure 6.10 of 12.43° for the smaller diameter jet fan with ahigher velocity ratio these values can be considered to be quite close. Large diameter jetfans with a low outlet velocity should have slightly larger jet expansion angles due to thefact that the development length of the resulting jet is shorter than in a high velocity case.The expansion angle of the jet is larger for divergent nozzles than for straight andconvergent nozzles.The results obtained in this study for jet expansion half angles are in reasonable agreementwith those reported in the literature. Abramovich (1963) reports a value of 0 = 13.5° for a109circular free jet. Wesely (1984) measured a jet propagation half angle of 14° in theuninhibited horizontal direction for a jet fan inside an arch-supported mining roadway.Thimons et al. (1986) obtained values between 120 and 140 for the jet expansion halfangle of a jet fan ventilating a heading 8.5 m wide by 50 m long. The values of these halfangles encountered in many practical situations will tend to vary between 9 and 160depending on jet fan configurations and conditions. The results from this study of 12.430and 12.90 fall within this range. When the jet fan is close to the wall the expansion of thejet is restricted on the side that is close to the wall.6.4.2 Description of Reverse FlowReverse or backflow is a feature of jet fan ventilation which needs special attention in mineventilation due to the potential hazards that exist in some underground situations wherethe level of pollutants might be very high. Recirculation of dangerous gases or dust is anundesirable feature in mine ventilation, therefore its onset can best be treated by carefulstudies.In this investigation reverse flow was observed when the jet fan was not located at thetunnel axis. In most situations auxiliary fans are placed near side walls of openingsparticularly for the ventilation of dead end headings. In a heading a well placed jet fan cancause air to flow all the space with an average velocity of 1 to 2 mIs. The jet flow reachesthe face of the heading and returns to the last open crosscut using the remainder of thespace. Possible recirculation takes place around the intake of the jet fan or at the face ifthe air supply to the fan intake is inadequate. In this investigation the flow of the jet fan isallowed to discharge to a straight open end of the tunnel. The reverse flow analysis isuseful particularly in some mining situations where jet fans are used as pressure boosters110and in headings to see the jet effect and the interaction of the jet flow with the openingwalls.Figure 6.12 shows typical reverse flow velocity profiles at the jet fan position of F = 0.11at 6.35 nozzles diameters from the jet fan for a jet fan/tunnel diameter ratio DR = 0.11.The profiles are at various heights from the tunnel floor. In Figure 6.12 the proffles areeffectively flat and uniform regardless of height from the tunnel floor signifying thinturbulent boundary layers even right from the edge of the interface between the axial flowand the reverse floor. Figure 6.12 is representative of all reverse flow profiles measured atall the axial distances from the jet fan in this study.Figure 6.13 shows normalised reverse flow velocity profiles UR / UBR at various axialdistances (X /D) at the normaiised height from the tunnel floor H of 0.5 for the jet fandiameter ratio DR = 0.17. UR and UBR are the reverse and bulk reverse flow velocityrespectively. The reverse flow covers about two thirds of the tunnel cross section at theposition F = 0.08 and 0.17 at X / D3 4.23. At larger axial distances from the jet fannozzle the cross sectional area occupied by the reverse flow diminishes. The reverse floworiginates at the point where the jet flow reaches the wall and extends to the jet fannozzle. Comparing Figures 6.13 (a) to (c) it can be seen that reverse flow diminishes asthe jet fan is moved towards the tunnel axis i.e. at larger values of F 0.5. Thus themagnitude of the reverse flow at = 0.33 is less than that at = 0.08 for both sizes ofthe jet fan diameters investigated.Results of backflow or reverse flow are also plotted in Figures 6.14 to 6.16 in the form ofthe width of the reverse flow (WR /D1) at selected stations for both jet fan sizesinvestigated at various positions from the tunnel walls. WR is the width of the backflow111and D is the diameter of the tunnel. It is easier to visualize the extent of the backflow byobserving Figures 6.14 to 6.16. For each jet fan position from the tunnel wall the backflowwidth is zero near the jet fan nozzle and reaches a maximum value at 6.35 diameters in thecase shown in Figure 6.14. After this maximum is reached the width decreases with axialdistance until it is zero at some downstream location. This data confirms that the size ofthe backflow decreases with jet fan distance from the wall. The trends are similar for thetwo jet fan sizes. The normalised data in Figure 6.15 for the larger diameter jet fanindicates that the backflow lengths are shorter. In Figure 6.16 data is presented forbackflow width obtained for a weak tunnel coflow velocity of 0.5 tn/s. Even with a initialtunnel velocity of 0.5 rn/s reverse flow is established by the jet flow. The initial tunnel flowhas to be strong enough to prevent reverse flow from being established. If this is the caseit would not be necessary to use a jet fan. Jet fans should only be used to move air wherethe surrounding flow is weak or stagnant.Flow visualization of the backflow was achieved by attaching strings of ribbons to acommon thread and this was fixed across the wind tunnel at half the tunnel diameter fromthe floor. The ribbon arrangement was moved from one axial location to another eachtime observing and photographing the flow. It worked very well and illustrated theexistence of reverse flow qualitatively. Results of this exercise can be seen in Figure 6.17for various jet fan positions from the wall. Figure 6.17 reinforces the data presented inFigures 6.13 to 6.16. During the testing the ribbons were lifted by the flow in bothdirections i.e. (i) by the forward moving jet flow and in the other by (ii) reverse flow. Thisflow phenomena was also recorded by a video camera and was an effective way ofanalysing the backflow. The flow visualization provided a very convincing way ofaddressing the backflow issue.112At each jet fan position the length of the reverse flow was estimated. This was plotted innormalised form LR/D against jet fan position and is shown in Figure 6.18 and 6.19 for thetwo jet fan sizes respectively. As has already been described the length of the reverse flowdecreases with the distance of the jet fan away from the tunnel wall. At the tunnel axisposition of the jet fan i.e F 0.5 the reverse length could not be determined but it couldbe assumed to be zero. The jet flow for the tunnel axis position for the smaller size jet fanwas found to produce both positive and negative velocities intermittently within 0.17tunnel diameters from both side walls in periods of 2 to 4 seconds. This could been causedby the jet failing to attach to the walls upon reaching them and combined with anoscillation of the jet flow field itself. The jet flow oscillation was observed by attachingcotton threads at the jet fan discharge nozzle circumference. In Figure 6.19 the backflowlength is largest for jet fan positions close to the wall and decreases sharply for positionsaway from the wall. It reaches a minimum at the position F,, = 0.33 and increases slightlyat the remaining jet fan position for which backflow was observed. The backflow lengthwas important in the calculation of jet expansion angles and it was also a necessary part ofthe tunnel flow analysis.6.4.3 The Ouantity of Reverse Flow as Fraction of Jet Discharge and Total TunnelFlowResults of the reverse flow as a fraction of the total tunnel flow for a few jet fan positionsare presented in Figures 6.20 to 6.22. The amount of recirculated fluid at some specifiedaxial locations is estimated for a few selected jet fan positions as shown in Figures 6.20(a)to (c). In Figure 6.20(a) the normalised reverse flow QR / QT is zero at X / D = 0 andthen rises to a maximum level of 1.5 for positions F,, = 0.08 and 0.17, after which it dropsto zero at some downstream point for all the jet fan positions Fp. QR and QT are therecirculated quantity and total tunnel flow (m3/s) respectively. Results in Figure 6.20(a)113are for a jet fan with 21.4 nils discharge velocity and diameter ratio DR = 0.17. In Figure6.20(b) the recirculated fluid fraction is presented for the smaller diameter jet fan with thehigher discharge velocity. The quantity QR / QT varies in the same way as that shown inFigure 6.20(a) with sharper peaks at the maximum level. At the peak of recirculation thereis almost twice the amount of tunnel flow in reverse flow. The tunnel jet fan positions ofF,, of 0.06, 0.08, 0.17 and 0.33 are representative of the axial variation of the recirculatedfluid for both jet fan sizes.Figure 6.20(c) compares the recirculated quantity QR I QT for the two jet fan sizes at twopositions from the wall, From Figure 6.20 it is clear that there is more recirculated fluidfor the smaller diameter jet fan than the larger jet fan for the same discharge mass flow.Figure 6.20 can be better understood with the aid of Figure 6.13 to 6.19 described earlier.The amount of recirculated fluid as a fraction of the total tunnel flow QR / Q is plottedagainst jet fan position F in Figure 6.21 for the two jet fan sizes. The quantity QR I Qvaries relatively little with jet fan position where reverse flow exists and is around 0.72 forthe smaller diameter jet fan DR = 0.11, and U1 = 40 nils. For the larger diameter ratio jetfan plotted on the same Figure the quantity QR / QT is around a value of 0.55. There is adrop in QR / QT from 0.58 to 0.53 from the position Fp of 0.08 to 0.17 before a constantvalue of 0.55 is reached for jet fan positions further from the tunnel wall.Figure 6.22 gives the recirculated quantity nomalised by the jet fan discharge flowQR I Q and plotted against jet fan position in the tunnel for both fans. The results fromFigure 6.21 are also plotted in Figure 6.22 for the purpose of comparison. The quantityQR I Q varies from 1.4 to about 1.1 for the fan positions F of 0.06 to 0.33 for thesmaller diameter setting (DR = 0.11 and U = 40 m/s). In the larger diameter fan114(DR = 0.17 and U, = 21.4 mIs) the quantity QR / Q1 varies from 0.85 down to 0.63 forthe jet fan positions Fp of 0.08 to 0.33. Again this fraction is lower than in the other case.Figure 6.22 shows that when the reverse flow QR is nomalised by the total tunnel flow QTit seems to vary relatively little regardless of jet fan position but when it is normalised byQ (the jet fan discharge volume flow) the change is quite noticeable. This observationresults from the amount of recirculated air which is higher when the jet fan is close to thewall than away from it. In a larger diameter jet fan with a lower discharge velocity theamount of recirculated air is less than in a smaller jet fan with a higher discharge velocity.When the total tunnel flow QT is used to normalise the reverse flow QR it is observed thatthe fraction of recirculated fluid QR/QT varies very little with jet fan position in the tunnelfor both jet fan sizes. The higher velocity jet fan has a higher proportion of recirculated airthan the lower outlet velocity fan for the same mass flow (and different diameters). Thereason for this is that the total tunnel flow is different for different jet fan positions. The jetfan entrains more air at some positions closer to the wall than when moved towards thetunnel axis. The amount of recirculated fluid also goes up in proportion to the total tunnelflow. Thus the more secondary air is entrained in the tunnel the more air is recirculated butthe ratio QR / QT remains almost invariant for a specified jet fan discharge condition. Theamount of recirculated air however is greater than the initial jet fan discharge flow and thisvaries for each jet fan position in the tunnel as shown in Figure 6.22. This recirculationphenomena obtained in this study has never been reported in previous work to the best ofthe author’s knowledge. No previous work has ever produced a flow field for a jet fantraversed from near walls to tunnel axis conditions for through flow situations. A fewstudies in the literature (e.g. Curtet, 1958) have described confined jet flow andrecirculation for centrally positioned ducted jets.115Curtet (1958) developed an approximate theory on confined jets and recirculationphenomena in an attempt to give a full account of the effects of furnace walls and thesurrounding environment on turbulent diffusion flames for a jet located on the duct axis(F = 0.5). Higher velocities were used (up to 140 mIs) and the amount of secondary flowwas controlled. Curtet (1958) observed backflow under these conditions on both walls ofthe confining duct. The resulting velocity profiles resemble those in the present studies forthe centrally positioned jet fan with U1 =40 rn/s and DR = 0.11 shown in Figure 6.1(d).The Craya-Curtet parameter C1 which is similar to Thring and Newby (1953) similitudeparameter 0 = (Q1 + Q )(D, /2) / (Q1D) defines the possibility of recirculation. Theterms Q., D and D1 are the jet discharge, secondary stream flow, jet diameter andtunnel diameter respectively. For values of C1 0.075 the extent of backflow is significantand decreasing with increase of C1. According to Barchilon and Curtet (1964) at C1 > 0.9there is no backflow. In this study values of C1 are less than 0.25 and recirculation isobserved at most jet fan positions F,, <0.44. The controlling factors for the onset ofbackflow are (i) the jet fan to tunnel diameter ratio (ii) jet fan position from the wall andthe tunnel to jet fan outlet velocity ratio U1 /U.1.21.21Fp=O.061-Fp=0.17-0.8DD0.60.600-0.400.400000Q00.200.20S_____________________________________________‘-a0I1I8IaA8.00.2o0. D060.4-0.40000000000000.20000000000-0.2o0000*-....S....7.S...5S.*5aSk_aSS.S..000--0.2o0.°18.220.8•32.7°45.30)Fig.6.1Velocityprofilesofjetfaninsidetunnelatvariouspositions1.51.2-X/Dj=4.230X/Dj=12.13Fig.6.2VelocityprofilesforjetfaninsidetunnelDR=O.17X!Dj=13.87*X/Dj=21.810.5 0D D D D1Fp=0.17FP-0o8-.D0.80.600000000000-0.4-o0..,.0.2•••-T7TTV-3ooaa;;2k-aoooooooo0oo00. Y/Dt0.805o 1.2 1Fp=0.330.80.6-0.4-00000-0000.200000‘‘o5;3s;;-0.2-0.4o0. D1.41.2Fp=0.510.80.600040000.0000_00020000.Oo••.•—-—X/Dj=18.2X/Dj=20.8-——X/Dj=32.7X/Dj=45.3Fig.6.3PlotofU/Umaxatvariousaxiallocationsatdifferentfanpositionsx a5 E D D x E D D(a)Y/DtDR=O.11(b)YIDtx E D D1.5xlE D —05D00. 10.5 0- ci E D E D1.51.5 10.5 0-0.51E D x ci E D D0.5(a)0-0.500. 10.5-0.50YIDt0Y/DtDR=0.171.2ciiE D0.60.40.2—————X/Dj=13.87XIDj=21.8Fig.6.4VelocityplotU/Umaxatvariousaxiallocations0.10.2(c) 2’>,- •*01.5 a -- n,•001DAI’0.5 a(a)00 0.2 0.4 0.6 0.8Distance Across Windtunnel Y/Dt2D Fp=0.33 a 00 0•0 00•a’e 0 a.2 0 -— • •0 •0e og• 4 0”0OioAAA4a4a!0•Oe A C0.5(b) 00 0.2 0.4 0.6 0.8Distance Across Windtunnel Y/D2Fp=0.5DD.2 n * a 8a00*aeA*wa a1 A e • 0So0.5(c)00 0.2 0.4 0.6 0.8Distance Across Windturinel X/Dt- Z/Dj=0.17 0 ZIDj=0.33 ZIDj=0.5 • ZIDj=0.67 a Z/Dj=0.83Fig. 6.5 Tunnel Velocity profiles at XIDJ = 45.3 (jet fan DR=O.1 1)DDC.)0a)>DDC)0a)>DDC.)0a)>21.51Fp=0.08121-0* ‘II •::0**(a)20.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Distance Across Windtunnel Y/Dt1.51Fp=0.33—0 •, •a*0. - - _00m;-t8: ; j .•e 2o0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Distance Across Windtunnel Y/Dt0.5(b) 02Fp=0.51.508*.2 a1 •0 00.500 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9(c) Distance Across Windtunnel Y/Dt- ZIDJ=0.17 ° Z/Dj=0.33 Z/Dj=0.5 ZIDj=0.67 OZ/Dj=0.83Fig. 6.6 Tunnel Velocity profiles at X/Dj = 30.2 (DR=0.17)1221.11O.9E08D• distance from jet fan X/DjFp=0.08— Fp=0.5Fp=0.17Axisymmetric jetFp=0.33Fig. 6.7 Jet axis velocity decay (DR = 0.17, Uj = 21.4 m/s)>% distance from jet fan X/DjFp=0.06-- Fp=0.5-.--Fp=0.17 --Fp=0.33-e- Axisymmetric jet data5 10 15 20 255 10 15 20 25 30 35Fig. 6.8 Jet axis velocity decay (DR = 0.11, Uj = 40m/s)1231.110.9E Fp=0.17 0 Fp=0.33 Fp=0.5 DR=0.17 Uj=21.4 rn/s0 10 20 30 40 50• Fp=0.17 • Fp=0.33X/DjA Fp=0.5DR=0.11 Uj=40m/sFig. 6.9 Jet axis velocity decay for two jet fan sizes12416/averageane=12.43degr0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45Distance of Jet Fan from Wall FpDR =0.11, Uj=40m/sFig. 6.10 Plot of jet expansion angle vs jet fan position18Coa)160)a)- 14ci0)asl2ctS0(I)c50><a)0.5DR = 0.17, Uj = 21.4 mIsFig. 6.11 Plot of jet expansion angle vs jet fan position0.2 0.3Jet fan position Fp0.412500ci>0C,0—1-2-3200 300 400 500 600 700 800Distance across the tunnel mm900Fig. 6.12 Tunnel cross-sectional backflow velocity profile for jet at X/Dj=18.2mDDDDctDDX/Dj=4.23- X/Dj=13.87X/Dj=1 2.13-—-—-—-—X/Dj=21.8 DR=O.17Uj=21.4m/s1260.3 0.4 0.5 0.6 0.7 0.8 0.9Distance across tunnel Y/Dt121.50.5021.50.50432100.40.3 0.4 0.5 0.6 0.7 0.8 0.9Distance across tunnel Y/Dt0.5 0.6 0.7 0.8Distance across tunnel0.9Y/DtFig. 6.13 Plot of backflow velocity profile vs distance across tunnel Y/Dt1270.8O.7O.60.110 20 30 40 50Axial distance XlDjetFp=0.06 --Fp=0.11 - Fp=0.22 -“-Fp=0.17 & Fp=0.0.28-- Fp=0.39Fig. 6.14 Width of backflow at various jet fan positions for DR=0.11, Uj= 40 rn/s0.8V0. 5 10 15 20X/Dj-.--Fp=0.083-.—Fp=0. 17 ——Fp=0.25 -— Fp=0.33 -e— Fp=0.42 Fp=0.44Fig. 6.15 Width of backflow at various jet positions for DR=0.1 7, Uj=21 .4 rn/s25Ua)ECa-oa)0()CoCaLZ-o0-*— Fp=O.06—*— Fp=O.22DR=O.1 1, Uj=40 rn/s-.— Fp=O.11-e- Fp=O.28-A•- Fp=O.17-•- Fp=O.391280. 6.35 18.2 20.8 32.7 43.5 52.7Distance from nozzle X/DjFig. 6.16 Backflow width for jet fan with tunnel coflow velocity of 0.5 rn/s129XIDj = 18.2X/Dj = 20.8Figure6.17(a) Flow visualization showing reverse flow (Uj = 40 ITI/S, DR 0.11)130II, j‘I ii;’Jj•j_Jstrong shearfiowXIDI = 12.13X/Dj = 13.87Figure 6.17(b) Flow visualisation showing reverse flow (Uj = 21.4 mIs, DR = 0.17)1315040 AA30 AC A AAAD 200100 0.1 0.2 0.3 0.4 0.5Jet fan position FpDR=O.11, Uj=40m/sFig. 6.18 Extent of backf low length vs jet fan position Fp26U240222 0(‘3D200ci180+— iaC 1k? 04-,xw14- 012- 0lOo 0.11 0.2 0.3 0:4 0.5Jet fan position FpDR=0.1 7, Uj=21 .4m/sFig. 6.19 Extent of backt low length vs jet tan position FpIa:000.5Ia:00132(a)o 5 10 15 20 25Backflow length Xr/DjDR=0.17, Uj=21.4 rn/s—FpO.08 — —Fp=0.17 -—-Fp=0.332.521.510.50(b)10 20 30 40Xr/DjDR=0.1 1, Uj=40 rn/s—Fp=0.06 — — Fp=0.17 -—“Fp=0.332.521.510.50000 10 20 30 40(c)______Xr/Dj• Fp=O.06 DRO.11 Fp=O.08 DR=O.17• Fp=O.33 DR=O.11 0 Fp=O.33 DR=O.17Fig. 6.20 Qr/QT vs X/Dj for jet fan diameter ratios DR=O.1 1 and 0.170.2 0.25Jet fan position FpFig. 6.21 Plot of backflow fraction Vs jet fan position1.51.4Q 1.3o 1.2o 1110. for DR=0.167Jet fan position FpQr/Qj DR=0.1 1 Uj=40 rn/sQr/Qj DR=0.167 Uj=21 .4m/sQr/QT for DR=0.11133I0c0t0C)m0.80.750.70.650.60.550.5DR=0.11 Uj=40m/s67 Uj=21 .4m/s0.05 0.1 0.15 0.3 0.35Fig. 6.22 Backflow fraction Qr/Qj and Qr/QT Vs jet fan position134CHAPTER SEVENDISCUSSION OF JET FAN PERFORMANCE ANALYSIS7.1 Tunnel Axis Longitudinal Turbulence LevelsAxial turbulence levels / U were measured at the tunnel axis at various downstreamlocations, for different jet fan positions. The turbulence levels for the jet fan positionsother than on the tunnel axis (i.e. F = 0.5) do not coincide with the axis of the jet flow.These velocity fluctuations were normalised using the local tunnel axis or centrelinevelocity Uc and these are presented in Figure 7.1 for the jet fan of diameter ratioDR = 0.11 and outlet velocity U. =40 mIs. The importance of the longitudinal turbulencelevels was to establish their relationship with entrainment rates in the tunnel and to serveas an aid in the understanding of the structure of the tunnel flow field.The data presented in Figure 7.1 includes Curtet (1958) and free jet axis turbulence levels.These data are included for comparison purposes. When the jet fan is not positioned atthe tunnel axis the turbulence levels obtained at the tunnel axis are greater than 50 % atX / D = 6.35. The turbulence level then reduces to minimum levels between X /D1 of18.2 and 20.8, after which it rises to a peak and begin to decrease again. A peak of 55 %is reached at X / D = 32.7 for the jet fan positions of F of 0.22 and 0.33. At theposition F = 0.06 the turbulence levels continue to rise monotonically and atX /D = 57.2 the turbulence level is nearly 65%. High entrance values occur becauseclose to the tunnel entrance there are high velocity fluctuations observed in the entrainedflow which reduce downstream and then increase to a peak again.135The turbulence levels for the jet fan at the tunnel axis position are somewhat different fromthe other positions in the first twenty jet fan nozzle diameters mainly becausemeasurements are taken directly on the jet axis. From the jet nozzle up to the end of thepotential core of the jet flow at X / D <20, the turbulence levels resemble that of a freejet i.e. initially very low and rising gradually. The free jet turbulence levels gradually reacha constant level of around 25 % at greater distances away from the nozzle and those of thejet fan rise significantly at X / D = 18.2 and reach a peak at 43.5 nozzle diameters beforedropping to a value of about 30 %. This value is almost half that of the turbulence levelwhen the jet fan is located at near the wall. Curtet’s turbulence data for a centrallypositioned jet varies in the same manner in the first twenty jet nozzle outlet diameters as inthe present case when the jet fan is located at the tunnel axis. Further downstream theturbulence levels in Curtet’s case rise continuously up to a value of 80 % as shown inFigure 7.1. The presence of the confining walls causes a larger. increase in the turbulencelevel than would be obtained in a free jet.7.2 Entrainment Rate as Function of Jet Fan PositionThe pressure and flow results including the turbulence levels are needed to understand thejet fan entrainment data. The entrainment results are a direct measure of jet fanperformance. The results presented are for a wind tunnel without an initial flow when thejet fan flow was introduced. The total tunnel flow is the sum of the induced secondaryflow and primary jet flow for two conditions. (i) In Figure 7.2 the flow ratio Qe / Q andQT / Q are plotted against jet fan position F,, for the jet fan diameter to tunnel diameterratio of DR = 0.11 and U =40 rn/s. (ii) Figure 7.3 presents the same quantities but forthe diameter ratio DR = 0.17 and U = 21.4 mj’s. In both situations the same Reynolds136number is maintained. The quantities Q,, Q and QT (m3/s) are the jet fan discharge,entrained (secondary) and total tunnel flow (QT = Qe + Q).Figures 7.2 and 7.3 show that the amount of secondary air entrained is higher when the jetfan is closer to the tunnel wall. The entrained air reduces when the jet fan is moved awayfrom the tunnel walls and reaches a minimum level at F = 0.39 for the smaller diameterjet fan (Figure 7.2) and at F = 0.42 for the larger diameter jet fan. The amount ofentrained air increases significantly at the tunnel axis position but does not reach the samelevel as when the jet fan is located at the wall as shown in Figure 7.2 for the smallerdiameter jet fan. In the larger diameter jet fan (Figure 7.3) there is a small increase in theentrained air at the tunnel axis position after the minimum entrainment level is attained. InFigure 7.2 it is clear that the jet fan can entrain an amount of secondary flow equivalent toits own outlet discharge volume flow when situated near the tunnel wall. The tunnel totalflow thus varies from about twice to one and a half times the jet fan flow for the highervelocity jet fan (Figure 7.2). The larger diameter jet fan (with lower outlet velocity) doesnot entrain as much air as the higher velocity, lower diameter fan. At the peak value inFigure 7.3 the jet fan entrains secondary air equal to half its discharge volume flow (i.e.Qe / Q 0.5). The total tunnel flow varies between one and a half times and l.lQ.An explanation for the entrainment results presented in Figures 7.2 and 7.3 can beaccomplished with the aid of the axial pressure results described in Chapter 5. Because thejet fan flow is confined a pressure gradient is obtained as shown in Figures 5.1 to 5.11.The pressure initially is negative i.e. below ambient in the first twenty jet fan nozzlediameters from the outlet and this causes secondary air to be entrained into the tunnel as ina jet pump or ejector. When the jet fan is situated close to one tunnel wall it is observedthat the initial pressure drops are quite considerable compared to the other positions andthus suction of secondary air is enhanced even though strong reverse flow is present in this137situation. The slight recovery in entrained air seen at the tunnel axis position is probablydue to a combination of high turbulence mixing between the jet flow and secondary flowand the symmetrical positioning of the jet fan in the tunnel, which might favour theentrainment of secondary air.7.3 Jet Fan Performance AssessmentJet fan performance criteria can be derived from the pressure and flow measurement. Thisenables jet fans of different outlet diameters, discharge outlet velocity and Reynoldsnumbers to be compared directly.In Figures 7.4 and 7.5 two major performance parameters are plotted for two jet fans ofdifferent outlet velocities and diameter but with the same discharge mass flow or Reynoldsnumber. The first parameter defining jet fan performance E is obtained by the product ofthe pressure ratio (P — P )/ (P — P) and the flow ratio QT I Q as(7.1)The pressure ratio in equation (7.1) results from the pressure rise of the total tunnel flow(F — P) and the total pressure of the jet fan minus the total pressure in the tunnel(P—f). The above equation is an energy ratio of the tunnel flow to the jet fandischarge. In jet pumps it is common to use the flow ratio Qe I Q instead QT I Q used inequation (7.1). This is also presented in Figure 7.4 but yields lower performance values.Equation (7.1) agrees reasonably well with another performance parameter defined byReale (1973) as induction efficiency1and given by equation (7.2) as13821___(7.2)In the above equation 4is the velocity ratio of the tunnel to the jet outlet flow U / U and2 is the area ratio of the jet fan to the tunnel A / A. The induction efficiency can bedefmed as the ratio of the ventilation power output to the power transmitted to the fluidby the jet fan. The induction efficiency is plotted in Figures 7.4 and 7.5 for various jet fanpositions in the tunnel. The values of performance parameters defmed by both equations7.1 and 7.2 vary from 3.5 % to a minimum of about 3 %. It can be seen that the jet fanperformance is better for positions closer to the tunnel wall than away from it. When thejet fan is at the tunnel axis the performance also improves but is still lower than that at thewall positions. The fact that equations 7.1 and 7.2 yield similar results plotted in Figure7.4, is very encouraging, and gives confidence in the current measurements.Figure 7.5 also presents the performance parameter and the induction efficiencyi1 forthe jet fan with outlet velocity 21.4 rn/s and DR = 0.17. Both performance values decreasewhen the jet fan is moved away from the tunnel wall and again after a minimum value isreached there is some recovery at the tunnel axis position. The results of jet fanperformance presented in Figure 7.5 range from about 6 % down to 4.6 %. At jet fanpositions F,, <0.2 the induction efficiency values are slightly higher than the resultsderived from the measured pressure ratios in both Figures 7.4 and 7.5. At jet fan positionscloser to the tunnel axis the induction efficiency results are slightly lower.Although the results shown in Figures 7.4 and 7.5 show low performance parameters, jetfans are still considered a convenient way of ventilation in vehicle tunnels and someunderground applications. When multiple jet fans are used to create high ratios of flow,139areas and pressure, the efficiency of the system is improved significantly. Figures 7.4 and7.5 give a very good indication of the effect of position on the performance system. Theperformance variation follows that of flow ratio against jet fan position shown in Figures7.2 and 7.3. It is worth mentioning that in the test facility room the amount of space waslimited and this could reduce the amount of entrained air considerably. In a real facilitythere are fewer limitations than in a wind tunnel facility.The results presented in Figures 7.4 and 7.5 show that larger diameter jet fans with lowoutlet velocities give better performance than low diameter ones with higher dischargevelocities. The performance results presented in Figure 7.5 are almost double that shownin Figure 7.4 for the smaller diameter jet fan simulation even though in the latter case moresecondary air is entrained. Therefore most of the energy in the jet fan flow is lost througha number of ways. An overall energy balance of the system can be formulated as follows:=E01 +Efl +ER +E,rj +E17 (7.3)where Em is the initial amount of energy in the jet flow. The other energy terms E0,E7,ER, E,,. and E17 are respectively the work output of the secondary flow, energy loss dueto friction, reverse flow, mixing, and jet loss. The reverse flow, mixing and friction lossesaccount for a significant amount of the losses incurred by the system. A special treatmentof this topic is given in Chapter 8 where an attempt is made to give theoreticalformulations of jet fan tunnel system.140C.)D0.• Fp=0.0610 20 30 40X/Dj50A Fp=0.33Curtet (Ct=0.152)60p Fp=0.5Fig.7.1 Tunnel centreline longitudinal turbulence levels141o.5o 0.6Jet fan position FpDR=O.11 Uj=40m/sFig. 7.2 Flow ratio vs jet fan position inside wind tunnel201.50.:0.6Jet fan position FpDR=O.1 7 Uj=21 .4 rn/sFig. 7.3 Flow ratio vs jet fan position inside wind tunnel1420C.)LU43.532.521.510.50 0.1 0.2 0.3 0.4 0.5Jet fan position Fp-V1---”induction efficiency (.0.6Fig. 7.4 Jet fan performance vs position inside tunnel (DR = 0.11)04)0II-.Ui0•((Ptn-Pe)QT)/(Qj(pi..ptn))on ((PtnbasedPe)Qe)I(Qj(Pe)Qe)/(Qj(Piptn))\6.565.554.540 0.1 0.2 0.3 0.4 0.5 0.6Jet fan position Fpinduction efficiency 1- .A1÷based on ((Ptn-Pe)QT)/(Qj(Pi-Ptn))Fig. 7.5 Jet fan performance vs position inside tunnel (DR =0.17)143CHAPTER EIGHTTHEORETICAL TREATMENT OF .JET FAN PERFORMANCE USINGMOMENTUM AND ENERGY CONSIDERATIONSTheoretical considerations are necessary in analysing a flow system fully and theseconsiderations are required to understand the mechanisms of jet fan performance. Oneway of analysing jet fans is to perform a momentum and energy analysis by considering thelosses that are encountered when two fluid streams of dissimilar velocities are mixed. Jetpump performance is often analysed in this way e.g. McClintock and Hood (1946),Cunningham (1957), and Cunningham (1976). However the analysis for jet fans is muchmore complicated because of the three dimensional nature of the flow but only a simplifiedtheoretical approach will be developed for this study. It is reasonable to assume thatdensity differences between the primary jet flow and the secondary induced stream arenegligible.Figure 8.1 shows a schematic diagram of the jet fan-tunnel system. The area of the crosssection occupied by the fan in the tunnel can be defined as follows:and from continuity= m +144A. A and A are the areas of the jet fan outlet or nozzle, the area through which thesecondary flow is introduced to the tunnel and the tunnel cross sectional areasrespectively. The mass flows ma., m, and represent the jet flow, secondary stream andtotal tunnel mass flow respectively.A. A.Let = x, then (i) L____A5 A1 1+cm m m-÷m(ii) Let —f- = n (iii) then = = 1 + n (iv)inj m mThe velocity of the secondary stream u8, and tunnel flow u1 can be expressed in tems ofthe jet discharge velocity u3 as follows:= anu (v)c(1+n)Ut = u (vi)1+ccThe axial static pressure of the flow at the initial mixing of the primary jet and secondarystream and at the fmal mixed stage is given by the following equation if zero friction loss isassumed,(P1Pe)’4 =mu+m5u—m1 (8.1)By using equations (i) to (vi) the above equation reduces to145Pt Pepuj(1_2wz+çj2n2)Pt Pe =apu(1—n)2 (8.2)The above equation is valid only when there are no losses in the system which is not thecase in reality. A number of equations can be written for each part of the jet fan ventilationsystem.8.1 Jet Fan Nozzle Energy Eouationp2 p2 ppfl /p represents the nozzle energy loss. Let = p1 + pu/2 the total pressure of the jetU2and Pfl—cP--The jet fan nozzle equation is then1’j Pe =(1+ç).I- (8.3)Secondary or entrained flow energy equation146An equation similar to (8.3) can be written for the secondary stream21 PeP and Pe are the Suction pressure at entry and start of mixing point respectively. ç, is aSuction loss coefficient. The above equation can now be expressed as1 Pe = (1+ç)2npu (8.4)8.2 Momentum Balance in the TunnelMomentum balance in the tunnel is the form of equation (8.1) but all possible losses areconsidered. The momentum equation can be written as follows,mu+mu3—m1—cA=(p pe)Aj (8.5)If significant obstructions are present which might contribute to the overall momentumloss then equation (8.5) can be expressed as follows;mjuj +mu—m,u Pe)4 (8.6)147c1 is the friction loss due to tunnel or confining walls, c0b is drag or a resistancecoefficient due to the presence of obstructions and f0b is the fraction of tunnel areaoccupied by obstructing objects giving fobA1 to be the frontal area of these objects.The tunnel experiences a significant amount of recirculation in the mixing section butmomentum is not lost. This momentum is equivalent to mrur where m and U,. are theback or recirculating mass flow and average backflow velocity throughout the tunnelrespectively.By using equations (i) to (vi) equation (8.6) becomes2_______ ____________mu +n mju — mu—(c1+cQbf A2÷2(pt pe)4 (8.7)ob) I 2(1+cz)By making the substitution mu = Apu =pu in the above equation the pressuredrop in the tunnel is given by2 c2nA 2________a2(1+n)Pt — Pe =+ pu — ()2 A1pu—(ç +cObfOb)AI 2(1+)2 PUjJ(8.8)1+ 1+ 2 — (l +c0bf) 2(1+)2 ) (8.9)P1 — Pe = 2(1+n)Z 2(1+n)(i+)2x U1 2 cL(1+n)2 c(1+n)2 (8.10)P1 Pe—(1÷cL) —(c1 +cObfOb) 2(1+e))Pt Pe = ()2[a(1+)+2n2(1+x)_c2(l+n)2 ( +çfa2(1+n)28.11)148Pu2 IP Pe=22a(1_czn)_ 2(1+(ct+cObfOb)) (8.12)2(1+cL)The pressure drop of the jet flow is given by subtracting equation (12) from (3) andobtaining3U2 pu2 / 2 2P—p1 =(i+ç)—-— ‘ 2a(i—an) —a (1+n)2(ct+cobfob)) (8.13)2(1+x)and can be further simplified topu2 /P —Pt = 2( 2(1+a)1+ç)—2a(1—an) +a2(1+n)(c+c0bf)) (8.14)1+c)8.4 Jet Fan Performance EfficiencyA jet fan performance parameter can now be described as the ratio of energy output toenergy input‘. nmE0 (ptpe)(ptpe) (8.15)E Pt) (8.16)in p\jT=Eoui/Ejn=fl(PtPe)I(PjPt) (8.17)149— 2a(1—an)—a2(1+n)c+c0bf)2 2 2 2 (• )(1+a) (1+ç)—2a(1—wz) +a (1+n) (ç+c0bf)Equation (18) shows that the efficiency of the jet fan depends on the entrainment or themass flow ratio n i.e. the secondary stream to the primary jet fan mass flow ratioand the area ratio a which is the ratio of the area occupied by the jet fan at the tunnelentry to the area available for the secondary flow to enter the tunnel AS/AJ. The losscoefficients ç and c0b can be determined directly or indirectly. The friction coefficient ccan be obtained from wall shear stress measurements. The resistance caused byobstructions ;b can be estimated fairly accurately. If the pressure drop for a clear tunnelis known, the differences can be attributed to the presence of obstructions and thereforethe coefficient c0b can be found. If the pressure drop in the tunnel or opening and the flowratio n are known then the loss coefficients can be estimated by solving equation (14).Both n and the recirculation or backflow fractioncr are a function of the jet fan positionY/D, from the confining walls.8.5 Theoretical Estimation of the Backflow Fraction crA jet fan of diameter D is situated at a distance Y metres from the tunnel wall to its axis.The tunnel is of hydraulic diameter D. Since from experimental observation it is knownthat recirculation takes place on one side of the tunnel when the jet fan is situated atdistance * < 0.5 an estimation of the volume occupied by the body of the backflow canbe carried out by assuming that the jet develops like a free jet until it reaches the tunnelwalls. The jet wifi expands with angle 0 from the nozzle and the distance it takes for it toreach one side of the tunnel wall is Lr the recirculation length.150e D —Y—O.5D.tan1= L (8.19the backflow length is then given byL,. (8.20)The area occupied by the backilow eddy if assumed to be approximated by triangularshape can be expressed asAr =0.5(J) _Y_O.SDJ)(DI s)O.5(D _Y_0.5Dj)Lr (8.21)If an average backflow velocity Ur is assumed within this area then the recirculationO.5(D _Y_O.5Dj)UrLr O.5(D—Y—O.5D1)u,.fraction can be calculated ascr = LrUIDt = uSD1 (8.22)and in terms of the jet discharge velocity— o.s(1+)(D,—Y—o.5D).2cr— c(1+n)uD,(8. 3)8.6 .Tet Fan Analysis from Energy ConsiderationsThe jet fan performance can be analysed from energy considerations by carrying out anenergy balance in the tunnel. This is done by first assessing all possible losses and takinginto account the energy input and output. The losses are accounted as follows.1511. Friction losses(i) Secondary stream energy loss = cm4 (8.24)(ii) Jet fan nozzle loss= çm -j- (8.25)(iii) Tunnel friction loss=ç1m4- (8.26)2. Backflow (recirculation) energy 1OSSEr mr 4 crmt — (8.27)3. Jet loss which the jet experiences a loss during its discharge from the fan. This loss inenergy isEji=(pe—pa)=p=mj (8.28)(4) Mixing Energy LossesIn addition to the other losses mixing energy losses occur because two streams of differentvelocities are brought together. The mixing loss isE,,d=nl (8.29)In the above equation m1 is the tunnel mass flow and is the mixing velocity which willbe dealt with later.152An overall energy balance can be written as followsEIfl=W,+ETI+E,,d+Efl+EJI (8.30)W = E0, i.e. energy out of the tunnel.The total energy loss is therefore = En + Erni + Efi + E7Efi is the sum of all frictional energy losses and isEfi (8.31)Efi = t:c1m- (8.32)Efi = mU(2n3c+ cn +2ç J (8.33)The recirculation loss En is En = cm1 =ç2 m (8.34)Equation (29) describes a mixing energy loss. The mixing velocity has to be defmed anddetermined. The following reasoning can be applied. First it is necessary to assume thatthe jet once past the nozzle of the fan develops like an axisymmetric free jet until it reachesthe confining walls. The centreline velocity of axisymmetric jet is given byUrn = 12-u (8.35)153àXjThe mixing velocity is given by Um = (xi- j (u — u1 )dx (8.36)x0)0rnThe limits of integration x0 and x1 are the end of the jet potential core and point where jetcentreline velocity Urn = u1 respectively. At this point the body of the tunnel airflow isassumed to be fully mixed (see Figure 8.2).12r0u fXl()fXl 12r0(8.37)Uj;= (,—x0) , (x,—x0)The mixing energy loss is thenE,,.=q(12roujin(.-)—u)=(i+n)-(12r UIn(x1\2(X,-X) — (i+)X0)U I l2r ((÷)\2= (i+ n)m--() XO) (1+cL)) (8.38)The total energy loss can be determined by summing all the lossesU. 2 (1+n)3 U U2 (12?b (x1) a(1+n)2a2(1+n)3 UE10 =c 2n3mJ._+cmJ+cL (1+)2 cm—-+(1+n)m- x1_x0 0 (i÷c) ) +cr (i+a)2 mJT(1+n)3(1+ n)(12r0______ _________E103 = m (ça2n3+ +22 c + , 0 0 (i÷) ) + c (i+cz)2(8.39)154E1 =L(P —p1) (8.40)nmE (pipe)ptpe) (8.41)out p—= (8.42)+ 2 c + (i + n)( l2ro h() — (1+n)\2 2 (i+n)E103 = m(c5x2n3 +2 (i+n)3_____ _____ _______(i+) x1 —x0 X0 (i+)I + c (1÷)2 )u2 (i÷) +(i+ n)(_! _In(L.)_cL(1+n)2 2(1+n)3(i, —p1)—n(p—pj = p--(çan3+ç +a2 2 x (1+cL) j + c1- (1+)2(8.43)Efficiency can now be defined asi1— 11=1—mj2L(8.44)in,mj__2= —mj—- I(ca2n3 + ç + a2(1+n)3___ _____ __(1÷)2 ct + (i + n)( l2roa(1÷n)2 2(1+n)3x1 —x0 x0 (i÷) r (1÷a)2______I_____ ___ __1[ 2 \x1 —X0 X0 (1+c) cr (1+)2 )= (p_p)JPfl +÷a2 (1÷)2 c1 +(i+ n)(12r0 in()_(1+n))2+2(1(8.45)1558.7 Analysis of the Performance rThe dependency of the performance parameter 1 on flow ratio (n) and area ratio(c) frommomentum considerations can be evaluated by plotting equation (8.18) graphically.Equation (8.18) is a product of flow ratio (n) and the pressure ratio(P1 Pe ) / (P — ps). The numerator (PtPe) is the tunnel pressure rise due to the jet fanand (P-p) is the pressure drop of the jet fan. For ease of understanding, equation (8.18) isrepeated below as equation (8.46)=2a(1—an) —a2(1+)2(ct+c0bf) (8.46)(1+a)1+ç)—2a(1—xn)+cL2(1+n)2(ç1+cbf0b)To simplify the problem, it is assumed that there are no obstructions in the tunnel andtherefore the quantity çf =0. Equation 8.18 becomes22a(1—an)—ç(1+n)2 (8.47)(1+ct) (l+ç)—2a(1—an) +aç1(1+n)Equation (8.47) is depicted in Figure 8.3 as a plot of (‘n) vs. flow ratio n (= Q/Q) forconstant values of the friction loss factor ;. The area ratio (a) is the area that is notoccupied by the fan (and available for the secondary stream to enter the airway), to that ofthe airway. A value of a = 0.99 obtained from the experimental work is used. A frictionloss term ç = 0.009 is used in this example. This value is obtained by using the Blausiusfriction formula for smooth surfaces i.e. ç1 = 0.079Re°25 for a tunnel Reynolds numberof 6000 for the purpose of this example. A jet fan nozzle loss coefficient of ç of 0.01 isused. In Figure 8.3 the value of n discounting nozzle losses peaks at about 0.1 for a flow156ratio (n) of 0.28. For flow ratios at this set of conditions, when the entrainment flowexceeds 90 % of the jet flow (n > 0.9), the performance () becomes meaningless (i.e. itsvalue drops below zero).When the nozzle losses are included, one obeserves that the peak value of (11) is reducedto 0.084 and occurs at a lower flow ratio (n) of 0.22. As well the practical range ofmeaningful flow ratios declines to the upper limit of 0.61. Figure 8.3 shows that i = 0, atn = 0, and i will increase with n, until a maximum is reached at an optimum value of nbetween 0.2 and 0.3 for the two plots. The higher the nozzle loss factor and friction in thepassage the lower the performance value r. It is important to note that the jet fanperformance values will always be low (less than 0.2 in all cases).Figure 8.4 compares theoretical and experimental performance parameter (TI) values. Thisshows that each performance curve represents a frictional factor value whichcorresponds to an experimental value of (ri). Experimental data fit the theoretical curve atits lower end, at low performance (ri) and high flow ratio (n). To obtain optimumperformance values from measured data, high pressure ratio values (p - Pe)’(fP) arerequired. Figure 8.5 shows optimum pressure ratio at optimum performance (TI) and flowratio (n). On average the optimum pressure ratio is 0.27. The experimental values are afactor of 10 less than the optimum and at a large flow ratio (n) along the performancecurve.An increase in the frictional factor loss term results in a decrease of performance (TI)values and a narrowing of the flow ratio (n) range. Each curve in Figure 8.4 represents aparticular fan position parameter F and therefore a friction factor loss ; value whichcovers a particular range of flow ratio (n) on the performance curve. In Figure 8.6, the157calculated frictional loss term ç is plotted against fan position parameter F. The frictionloss factor is high for the small (n) value range which occur at positions farther from thewall. This small n range corresponds to a low tunnel Reynolds number at jet fan positionsfarther from the wall.In Figure 8.7 the performance parameter (TI) is plotted against area ratio (a) for a familyof flow ratio n values of 0.1, 0.3 and 0.7. At each flow ratio, there is an optimum arearatio ((x), The larger the flow ratio e.g. n = 0.7 the higher the performance parameter (ri =0.19) and the smaller the optimum area ratio (a 0.3). The optimum area ratios forsmaller n values are larger e.g. a = 0.48 for n = 0.3, a = 0.78 for n = 0.1. Figure 8.8shows the optimum area ratio plot for various flow ratios n. The value of optimum arearatio declines rapidly with increasing flow ratio. In most mining applications the optimumvalue of area ratio is not used because the sizes of the passages are determined primarilyby the mining method and not by ventilation needs. Fans are of limited range in diameterand therefore on average, area ratios encountered in mining are above 0.8. Only in a fewsituations is a smaller area ratio encountered and it would be normally greater than 0.5.The values of flow ratio plotted in Figure 8.4 specify a range of n which can be varied bycontrolling the amount of secondary air entrained. In the experiment the amount ofsecondary fluid entrained measured by the ratio n is not controlled and is found to dependon jet fan position F. The foregoing analysis has demonstrated that equation 8.46 can beused in jet fan performance assessment. For each set of conditions defined by the frictionloss term or jet fan position there is an optimum performance value and an optimum flowratio (n) for a given entrained flow area ratio a.Ae secondary flow inlet areaAjjetfan outlet areaAt tunnel cross sectional areaPt tunnel pressurePe sec. flow entry pressurePj jet total pressureAob obstruction areamj jet fan mass flowmt tunnel total mass flowY distance of jet fan from tunnel walle jet expansion angleLr backflow length158LrmsmjAj/-—øDtregionmt2mixing lion3developed regionFigure 8.1 Schematic description of jet fan - tunnel systemED>00a)>Cl)><ci)l=xl-xo (mixingDistance along tunnellength) Ut=tunnel velocity159xo xlFigure 8.2 Velocity decay of an axisymmetric jet and mixing velocity concept1600.120.1CCI)C.) 0.08CE0.060.040.0200o<099Figure 8.3 Plot of theoretical performance vs flow ratio n—7 N0036\./ / 0. 441/ -LII -/[/ N////iL1/ -, 0.62 N N0.5 0.6 0.Flow ratio n=QefQj0.110.1C.CI) 0.09C.)0.08€ 0.07a)°- 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Flow ratio n=QeIQj=o.o1 °(=0.99experiment DR=0. 11, Uj=40 mis • experiment DR=0. 17, Uj=21 .4 mIsFigure 8.4 Plot of theoretical performance vs flow ratio (n)for various jet fan positions (friction loss factors)161C—C)2i0.35 0.1- 0.09S 0.30C”0.250.072Di 0.2 0.0600. 0.020.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 0.31 0.32 0.33 0.34Optimum pressure ratio (pt-pe)/(Pj-pt)Figure 8.5 Optimum flow ratio and performance vs optimum pressure ratio0. 0.22 0.33Jetfan position FpFigure 86 Friction loss factorc vs jet fan position Fp0.44162Ca,020a) ratio 0<Figure 8.7 Performance 9 vs area ratio for various flow ratio n0.9 —\\NFlow ratio n0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 102200. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Figure 8.8 Flow ratio n vs optimum area ratio 0<163CHAPTER NINEPRACTICAL APPLICATIONS OF .IET FAN WIND TUNNEL STUDIESThe results of the present study were obtained using an open ended wind tunnelinvestigation and it is necessary to give a practical application of the results. There aremany potential applications of jet fans in mine and industrial ventilation. In mineventilation, the present results directly provide information for a jet fan used in an openended airway such as that shown in Figure Fan Application Case Study 1It is desired to supply airflow to airway B, D and E following a disruption of airflow dueto the collapse of airway C which used to supply airflow to D and E. The ventilationdistrict is arranged as in Figure 9.1. The airflow should all be supplied from airway Awhich has an airflow of 60 m3/s. We need to supply D and E with approximately 10 and20m3/s respectively and this requires an airflow quantity of 30 m3/s in airway B. This canbe done by installing a mobile jet fan in airway B of capacity 20m3/s, with a diameter of800 mm and about 40 rn/s discharge velocity. For operational reasons the jet fan must beplaced at least 12 meters from the intersection of airway A and B. For an effectiveventilation system airway B should be no more than 150 m in length in order to maintainthe pressure of the system. The cross sectional hydraulic diameter of airway B should beno more than ten times that of the jet fan. In order to determine the amount of air the jetfan entrains, Figure 7.2 is used. For a jet fan positioned at F 0.05 the entrainment ratioQIQJ = 0.9 (or a total flow of 1.9 times the original jet fan discharge volume flow) for adischarge velocity of = 40 rn/s. Therefore on average, about 38 m3/s will flow in164airway B and for a cross sectional area of 16 m2 the average velocity will be 2.4 rn/s. Ifother jet fan discharge velocities are used the entrainment or flow ratio will differ. Thepositioning of the fan from the walls will also affect the flow ratio.To determine the pressure rise in the airway caused by the jet fan, equation 8.12 is usedi.e.P1 Pe=2(2cc(1—ccn)—cc2(1+n)ç+c0bf)) (8.12)2(1 + cc)If it is assumed that there are no obstructions in airway B then the drag resistance ç(), iszero. The airway friction factor c can be estimated fairly reasonably. The area ratio cc isthe area not occupied by the fan to that of the total airway. Using n = 0.9, Uj = 40 m/s, cc= 0.969, p = 1.2 kg/rn3, the dimensionless friction factor of the airway ç = 0.00 1 (for 10% pressure drop). The pressure rise of the airway (ps- Pe) according to equation 8.12 willbe 7.01 N/m2 and this should be constant along the length of the airway to maintain theflowrate of 38 m3/s. The pressure Pe and Pt are the pressure upstream of the jet fan andmaximum pressure attained respectively.To calculate the induction efficiency of the system, equation 4.2 is used= 24(1—1) / ((1— )(1 + c1))Since the airway to jet fan velocity ratio ‘1 = 0.06, and the jet fan to airway area ratio 2 =0.0314, the above equation gives an induction efficiency of about 11 % i.e.= 2x0.06(1-0.06)/((1-0.0314)(1-t-0.06)) 0.10991659.2 .Tet Fan Application Case Study 2A closed end mining heading is to be ventilated by a jet fan. The velocity at the productionface is to be at least 0.5 m/s for the effective removal of pollutants. The heading is 30metres long and has cross sectional dimensions of 10 metres wide by 5 metres high. Thejet fan can be positioned as shown in Figure 9.2 with a curtain running along part of theheading to reduce re-entrainment of the return air from the working face and allowing thejet fan to entrain as much fresh air as possible from the main airway. In order to dilutepollutants below their TLVs at least 10 m3/s is required to reach the working face. A jetfan of 10m3/s capacity is chosen and with a discharge velocity of 30 m/s. The fan diameterD is therefore 0.65 metres. To calculate what the centreline velocity will be at the face 30metres away, the free jet centreline velocity decay equation is used as a guideline (i.e.equation 6.1). This yields a centreline velocity of 3.9 rn/s. From Figures 6.7 to 6.9, it isclear that the actual value lies between 1 and 3.9 rn/s since the jet is not totally confined asin the wind tunnel case. The total flow can be obtained using Figure 9.3 for a 10 m3/s jetfan if the entrainment ratio n is known or estimated. For any entrainment ratio n theamount of total flow will increase linearly. If an entrainment ratio n = (Q/Q) = 0.5 isassumed the total amount of fresh air reaching the working face area will be Qj+Qe and Qe= 0.5 x 10m3/s. Therefore the total amount of air will be 15 m3/s. This flow takes place inhalf of the heading area and the average velocity throughout the volume is 15m3/s/25 m20.6 rn/s. This is a good velocity for the removal of pollutants in a mining area.According to Thimons et al (1986) the recirculation of contaminated air can be up to 28 %which is not a problem considering the amount of excess fresh air entering the heading. Inthis case, a curtain of 5-10 metres in length can be used to separate the fresh air from thereturn air as shown in Figure 9.2. It is over this length that there is a possibility of returnair being entrained and therefore the curtain can reduce this likelihood.166The advantage of a jet fan in this application is (i) more air at a higher average velocity issupplied to the working face and (ii) the need for ventilation tubing has been removedtherefore cutting ventilation costs.Jet fans can be fitted with entrainment tubes as shown in Figure 9.4 to control the amountair entrained from the return air in a closed airway. The use of a tube in this applicationwould significantly reduce the recirculation fraction. There are other applications of jetfans in mine ventilation. Jet fans can be used for cooling purposes in hot working climatesby increasing local air velocities. They can also be used to dilute gas emissions in largeexcavations by directing flow to the required point of application. Ventilation engineershave been hesitant to use jet fans in mines because of the lack of knowledge regardingtheir effects. The data contained in this work can assist the ventilation engineer inunderstanding the best way to employ these fans.AC)E0Co167main ventilationdrift/ // / /B/Ec’JC/ //// / . / /E///38m3/s R___—etfann=O.9 Qj=20 m3/s Qe=1 8 m3/s QT=Qe+QT=38 m3Isclosed areaFigure 9.1 Example of jet fan used in through flow to increase airflowin other mine ventilation districts16804-.C30m —return air4-,0curtainfresh air-jet fanclosed headingn=O.5, Qj=1O m3Is, QT=(1+n)Qj=15 m3IsFigure 9.2 Jet fan used in a closed heading with curtain to reduce return air entrainmentU)000100a)EFigure 9.3 Illustration of flow ratio vs total flow for a 5 and 10 m3Is jet fan20169iOn 3/sjetf________— --5m1/sjetfij...__— .——00 0.1 0.2 0.3 0.4 0.5 0.6 0.7Entrainment ratio n0.8 0.9entrained flow______L______jet fanentrained flowentrainment tubeentrainment tube length L up to 5 x nozzle diameterFigure 9.4 Jet fan fitted with entrainment tube as in an ejector170CHAPTER TENCONCLUSIONSThis study has sought to provide data concerning jet fan ventilation in mines and tunnels.The studies have examined the pressure rise characteristics, the velocity development andthe overall air entrainment of a jet fan simulated in a wind tunnel. Two jet fan sizes wereused with different outlet velocities. By varying the jet fan position relative to the windtunnel walls the optimum conditions and overall aerodynamics have been investigated. Theconclusions drawn from the results of this investigation are presented in this chapter.(1) A unique wind tunnel - jet fan test facility was designed, constructed and tested at theUniversity of British Columbia, Department of Mining and Mineral Process Engineeringfor mine ventilation jet fan perfomiance analysis in an open end case. The wind tunnelfacility can be used for other mine ventilation studies in the future. This is a majorcontribution of this work.(2) The pressure and flow conditions of jet fans applied to geometry appropriate to mineopenings with respect to wall interactions was investigated.(a) For the same discharge velocity, a jet fan situated at near wall positions (F,, 0.17)produced larger pressure drops in the first thirty jet nozzle diameters downstream in therange of magnitudes between -0.005 and -0.006 jet discharge dynamic pressure. At theother positions (F,, 0.22) the corresponding pressure drop values were less than -0.003of the jet dynamic pressure; for the fan with a discharge velocity of 40 m/s and diameterratio DR=O. 11. The pressure recovery for jet fans at positions F> 0.22 is higher by asmuch as 7 % than that at the closest jet fan position to the wall (F1, = 0.06). The larger171diameter fan (DR=O.l7)with a discharge outlet velocity of 21.4 rn/s shows similar pressurevariations to those of the smaller fan for the same discharge Reynolds number. Howeverthe pressure recovery factors expressed as a ratio of the jet discharge dynamic pressure areon average 2.5 times larger than for the smaller fan. When the experimental pressurerecovery is compared to the theoretical one, it is found that there is a 15 % difference fornear wall jet fan position at a discharge velocity of 40 rn/s (DR=O. 11) and 9 % for thedischarge velocity of 21.4 rn/s (DR=O.l7). This indicates that the discharge velocity isimportant in determining the frictional pressure loss for a fan positioned close to the wallof an airway. This must be balanced by the requirement for the air to be moved. The wallfriction losses for the smaller jet fan situated at the tunnel axis (DR 0.11) is about 8 %and that for the larger fan (DR = 0.17) is less than 2 %. This shows that for a given airquantity, it is better to use a larger diameter fan and an optimum discharge velocity. Thepressure rise for the smaller diameter fan reaches a peak at about 70 jet fan nozzlediameters and that for larger fan (and lower discharge velocity) peaks at about 45 nozzlediameters.(b) Comparing the velocity proffles of the smaller and larger diameter fan, it can beconcluded that the flow develops in the same manner qualitatively at similar jet fanpositions from the wall. Generally for fan positions F < 0.4 there is a region of reverseflow on one side of the tunnel which extended for 20 and 40 jet nozzle diameters for thenear wall jet fan position for the larger and smaller diameter fan respectively. Reverse flowreduces as the fan is traversed towards the axis of the tunnel. On average the magnitude ofthe reverse flow expressed as a fraction of the total flow at the tunnel cross sections isfound to be about 0.72 and 0.55 for the smaller and larger fan respectively. Jet fan jetexpansion angles ranged between 24° and 26’ for both fan sizes and velocity discharges.(c) From the velocity profiles results, it can be concluded that the jet fan can provide aventilation airflow velocity greater than 0.5 m/s for distances up to 70 fan diameters from172discharge. The distances are slightly higher for fans positioned near walls. In a real mineapplication, the distances of ventilation can be even larger because fans of up to 50 m3/sdischarge volume can be used.(d) Entrainment ratios for the smaller jet fan (diameter ratio, DR = 0.11, Uj = 40 mIs)ranged from 0.92 when the fan was located near the wall F = 0.06 to about 0.6 when thefan is at position F = 0.39. The entrainment ratio n for the larger jet fan (DR = 0.17, Uj =20 mIs) ranged between 0.5 for the fan wall position to a minimum of 0.1 at position F =0.44.(e) The measured performance parameter,defined by equation 7.1 as the product ofthe flow ratio QT / Q1 and the pressure ratio (P — F ) / (P —P1,), agrees fairly well withthe calculated induction efficiency rt (equation 7.2) and is higher for near wall positionsthan at positions away from the wall. The values obtained with the smaller fan at the nearwall positions were = 0.034 and = 0.036. Values of = 0.058 and = 0.062 wereobtained for the same position with the larger fan at same Reynolds number. These valuesdecreased as the jet fan was moved away from the wall with some recovery for the tunnelaxis position. The performance of a jet fan can be evaluated using both entrainment andpressure considerations.(3) Theoretical formulations have shown that performance i is dependent on the flowratio n. For zero flow ratio, performance is also zero and it will rise to reach a maximumat a particular n value and drop as n is further increased. The useful range of flow ratio nis 0< n 1. This flow ratio range ensures a total flow in any ventilation passage greaterthan the initial jet fan discharge volume flow. The range of flow ratios n decreases as thevalue of the friction factor loss increases. The optimum performance parameter (TI)decreases from a value of about 0.1 to 0.024 as the friction loss factor ç is increasedfrom zero to 0.6. This friction loss factor corresponds to jet fan position in the tunnel.173Fan positions with a low range of flow ratio (n) therefore low tunnel Reynolds numberhave higher friction loss factors and lower performance values. When performance isplotted against area ratio a for each flow ratio value n there is an optimum a for each n(e.g. n = 0.1, aopt = 0.78, n 0.3, c = 0.48, n = 0.7, aq = 0.3). The values will changea little depending on the friction loss factors or coefficient terms used in the equations.Agreement between theoretical performance values and experimental ones was obtainedon the lower range of the performance curve corresponding to the upper limit of the flowratio (n) for each jet fan position. Apart from the theoretical analysis of Chapter 8 whichuses the flow ratio (n) the parameter and the induction efficiency i provide analternative way of analysing jet fan performance which is based on the tunnel total flow.(4) This study has demonstrated that jet fans can be used to solve ventilation problems inopen air passages and also in closed end situations by careful selection of the rightcapacity jet fan with a discharge velocity ranging between 20 and 40 mIs. Entrainmentratio 0< n 1 wifi provide excess mine ventilation air in production areas and thereforeincrease mine productivity. This makes it possible to mine out difficult areas which werenot accessible because of ventilation problems.174CHAPTER ELEVENRECOMMENDATIONSThe results described in this thesis identified major parameters in the assessment of jet fanperformance. These are size ratios, outlet velocities, flow ratios or entrainment andpressure rise characteristics.(1) Jet fans should be used where there is a plentiful supply of fresh air so that enough airfor the fan and entrainment can be available. In open passages jet fans positioned close tothe wall (F,, 0.17 ) with an adequate source of air can move a larger mass of air thantheir own initial discharge volume for distances greater than 50 jet nozzle diameters. Therange of discharge velocity to be used is 20 to 40 m/s. The optimum probably lies betweenthese values and should be researched further. In closed end headings, jet fans can be usedto increase airflow requirements to the working areas by selecting a fan of the rightcapacity Q and size and then positioning it close to the wall while extending the suctionwell into the fresh air. An entrainment ratio of 0.5 can be assumed to calculate the totalamount of air reaching the face i.e. (Q + 0.5Q = 1.5Q). To reduce any possibility ofentrainment of return air from the heading end a restricting curtain can be installed in thefirst 10 - 15 m of the heading. The use of jet fans therefore eliminate the need forventilation tubing. Jet fan ventilation is very effective in clearing out pollutants because ofgood mixing of the jet flow and contaminants.(2) Jet fans used in open tunnels should be used to boost the pressure of the flow in orderto overcome friction losses of the passage walls so that air can be moved over largedistances. Jet fans are also ideal for balancing flows in short interconnecting airways (e.g.175up to 150 metres) as shown in Chapter 9.1. They can be placed near a major airway with alarge airflow. The fan will move its own inlet volume of air plus secondary air moved byentrainment. The result is that a larger amount of air can be moved at a high pressure. Thisstudy shows that if the fan is close to the wall, the amount of air moved is much largerthan when the fan is closer to the airway axis. For a jet fan situated at any position in theairway, the present results show that these fans can be used without any partition or longventilation tubing. Jet fans always increase the pressure of the secondary stream bymomentum and energy exchange and also provide more air than ducted fans for any flow(entrainment) ratio n >0.176CHAPTER TWELVECLAIMS TO ORIGINAL RESEARCHI claim the following as original research from this work(1) The design and construction of a wind tunnel facility for studying jet fan applications intunnels and mines(2) The establishment of position dependency of jet fan performance (pressure and flowratio characteristics) in the ventilation of open passages i.e. wall interaction effects.(3) The establishment of mathematical formulations to predict performance and frictionlosses based on flow ratio of the entrained and jet stream.177REFERENCESAbramovich, G.N., 1963, The Theory of Turbulent Jets, MIT Press, Cambridge, MA;1963, pp. 444-475, pp.633-654.ANSIJASME PTC 19.1-1985 Part 1, 1986, Measurement UncertaintyBaba, T., and Ishida, M., 1985, “Possibilities for the Reduction of Total Costs ofVentilation Systems with Low Velocity Jet Fans,” 5th International Symposium on theAerodynamics and Ventilation of Vehicle Tunnels, BHRA Fluid Engineering, Lille,France.Baba, T., Hatttori, Y., and Seki, T., 1979, “Characteristics of Longitudinal VentilationSystem Using Noimal Size Jet Fans, “ Third International Symposium on theAerodynamics and Ventilation of Vehicle Tunnels, BHRA Fluid Engineering, Sheffield,England.Barchilon, M., and Curtet, R., 1964, “Some Details of the Structure of an AxisymmetricConfined Jet With Back Flow, “A.S.M.E. Journal ofBasic Engineering, pp. 777-787.Baumann, H., 1973, “ Evaluation of Friction Coefficients and Mean Wall RoughnessFrom Pressure and Volume Flow Measurements,” mt. Symposium on the Aerodynamicsand Ventilation of Vehicular Tunnels, BHRA Fluid Engineering, Kent, Cantebury,England.Blevins, R.D., 1984, Applied Fluid Dynamics Handbook, Van Nostrand Reinhold Co.Inc. pp. 38-123, pp. 229-278.Bradshaw, P., and Pankhurst, R. C., 1964, “ The Design of Low - Speed WindTunnels,” Progress in Aeronautical Sciences, Vol. 5, eds. D. Kuchemann and L. H. G.Sterne.Chmielewski, G. E., 1974, “Boundary Layer considerations in the Design ofAerodynamic Contractions,” J. Aircraft, Vol. 11, No.8. p.435-438.Coleman, H.W., and Steele Jr., G.W. 1989, Experimentation and Uncertainty Analysisfor Engineers, John Wiley and Sons.Cunningham, R. G., 1957, “Jet Pump Theory and Performance with Fluids of HighViscosity,” Trans. ASME, Vol. 79, pp. 1807-1820.178Cunningham, R.G., 1976, “Liquid Jet Pump Modelling: Effects of Axial Dimensions ofTheory - Experiment Agreement,” 2nd Symposium on Jet & Ejectors and Gas LftTechniques, BHRA Fluid Engineering, Bedford, England.Curtet, R., 1958, “Confined Jets and Recirculation Phenomena with Cold Air,”Combustion and Flame, Vol.2, pp. 383-411.Curtet, R., and Ricou, F.P., 1964, “On the Tendency of Self-Preservation inAxisymmetric Jets,” ASME, Journal of Basic Engineering, Dec, 1964, pp. 765-771Drivas, P.J., Simmonds, P.G. and Shair, F.H., 1972, “Experimental Characterization ofVentilation Systems in Buildings,” Enviro. Sci. Technol. Vol. 6, No. 7, pp. 609-614.Dunn, M.F., Kendorski, F.S., Rahim, M.O., and Mukherjee, A., 1983, “Testing JetFans In Metal/Non Metal Mines with Large Cross-Sectional Airways,” (ContractJ0318015, Eng. Tin.) Bureau of Mines OFR 106-84, L32p., NTIS PB 84-196393.Eck, L B., Fans, Translated by R. S. Azad and D. R. Scott, 1973, Pergamon Press pp.409-441.Fudger, G., and Lowndes, J. F. L., 1985, “Longitudinal Ventilation of Road Tunnels inthe United Kingdom,” 5th Tnt. Symposium on the Aerodynamics and Ventilation ofVehicle Tunnels, BHRA Fluid Engineering, 1985.Goodman, G.V.R., Taylor, C.D., Divers, E. F., 1990, “Ventilation Schemes for DeepAdvance Mining Systems,” Proceedings of the 5th U.S. Mine Ventilation Symposium,1990. ed. Wang, Y. J.Hartman, H. L., Mutmansky, J. M., Wang, Y. J., Mine Ventilation and AirConditioning, John Wiley and Sons, 1982 pp. 255-385.Hayward, A. J. T., 1973, “Ventilation Tests on a Model Road Tunnel,” Tnt. Symp. onthe Aerodynamics and Ventilation of Vehicle Tunnels, BHRA Fluid Engineering, Kent,Canterbury, England.Hill, P.G., 1965, “Turbulent Jets in Ducted Streams,” J. Fluid Mech. Vol. 22, part 1, pp.161-186.Holman, J.P., 1984, Experimental Methods for Engineers, 4th ed., McGraw-Hill, NewYork.Kempf, J., 1965, “Wall Effect on the Efficiency of Booster Fan,” (In German), SchweizerBauzeitung, 83 Jg, Heft 4, S47.Krause, D., 1972, “ Freistrahien bei der Sonderbewetterung (Free Jets in AuxiliaryVentilation),” Neue Bergbautechnik, V.2 No.1, pp. 44-52.179Lewtas, T. A., 1980, Assessment of Induction Fan Effectiveness, (Contract 30387223,Forster-Miller Associates, Inc.) BuMines OFR 8 1-82, 62pp, NTIS PB 82235987.Linseli, Mine Ventilation, VDI-Z, 1953, p.429.Matta, J.E., Thimmons, E.D., Kissell, F.N., 1978, “Jet Fan Effectiveness as Measuredwith SF6 Tracer Gas,” Report ofInvestigations 8310, U.S.B.M., l4p.McClintock, F. A., and Hood, H. L., 1946, “Aircraft Ejector Performance,” Journal ofthe Aeronautical Sciences, Vol. 13, No. 11, PP. 80-89.McElroy, G. E., 1945, “Role of Air Jets in Mine Ventilation,” Trans. AIME Vol. 163 pp.349-414.Meets, E. J., and Meyer, C. F., 1993, “Some Applications of Ductless Fans in Bord andPillar Headings in South African Coal Mines,” Proceedings of the 6th US MineVentilation Symposium, Chapter 71, pp. 475-48 1.Meidinger, U., 1964, “Longitudinal Ventilation of Vehicle Tunnels with Axial FlowFans,” Scheweizeriche Bauzeitung, No.28, pp. 498-501.Mizuno, A., and Araie, K., 1989, “Measurement of Pressure Rise Performance of a JetFan in a Tunnel by Model Experiment,” Nippon Kikai Gakkai Ronbunshu, B. Hen/Trans.of the Japan Society ofMechanical Engineers, Part B, no. 514, pp. 1613-1617.Moffat, R.J., 1988, “Describing the Uncertainties in Experimental Results,” ExperimentalThermal and Fluid Science, Vol 1, pp. 3-17.Mutama, K.R. and Hall, A.E., 1993, “Development of a Wind Tunnel for MineVentilation Aerodynamic Measurements,” Proceedings of the 6th US Mine VentilationSymposium, Chapter 61, pp.407-412.Noon, C. L. B., and Smith, T. W., 1990, “Comparison of the Performance and SoundLevels of Jet Fans Measured in a Laboratory and Two Road Tunnels,” Proc. ofLMech.E., European Conference, Installation Effects in Fan Systems, London, England,1990.Ohashi, H., Kawamura, T., Baba, T., 1976, “A Study on a Longitudinal VentilationSystem Using the Enlarged Jet Fans,” 2nd mt. Symp. on the Aerodynamics andVentilation of Vehicle Tunnels, BHRA Fluid Engineering, Cambridge, England. 1976.Ower, E., and Pankhurst, R.C. The Measurement of Airflow, 5th ed. Pergamon Press,1977.180Pinter, R., 1982, “Possibilities for the Reduction of Energy consumption on VentilationSystems with Jet Fans,” 4th mt. Symp. on the Aerodynamics and Ventilation of VehicleTunnels, BHRA Fluid Engineering, York, England, 1982.Pope, A., and Harper, J. J., 1966, Low-Speed Wind Tunnel Testing, John Wiley andSons Inc.Radchenko, G. A., et al., 1965, “Practical Investigations in a Ventilation CurrentPropagated Along the Long Wall Face, “Fiz. Tekhn., Probi. Razrabotki Polezn. Iskop.,No.3, May-June,pp.118-23.Rajaratnam, N., 1976, Turbulent Jets, Elsevier Scientific Publishing Co.Razinsky, E., and Brighton, J. A., 1971, “Confined Jet Mixing for Non SeparatingConditions,” Trans. ASME J. of Basic Engineering, 1971, pp. 333-347.Reale, F., 1968, “Induction Ventilation of Vehicle Tunnels by Means of Axial Fans,”Ingeneri. No. 51, pp. 45-52.Reale, F., 1973, “Fundamentals and Applications of Induction Ventilation Systems ofVehicle Tunnels,” In Symp. on the Aerodynamics and Ventilation of Vehicle Tunnels,BHRA Fluid Engineering, Kent, England. 1973.Rohne, E., 1964, “Longitudinal Ventilation of Vehicle Tunnels and Axial Fans,” (LaVentilation Longitudinale des Tunnels Routiers et Les Ventilateurs Helicodes),Schweizerische Bouzeitung, No. 48, pp. 3-7 (in French).Smith, A., 1982., “High Reaction Fans,” International Conference on Fan Design &Applications, Guildford, England.Stochinsky, A., and Komarov, V., 1969, Mine Ventilation., Mir Publishers, Moscow,580p.Thimons, E.D., Brechtel, C.E., Adam, M.E., Agapito, J.F.T., 1986, “Face Ventilationfor Oil Shale Mining, “ Infor. Circular 9118, U.S. Bureau of Mines, 15p.Thring, M. W., and Newby, M. P., 1953, Fourth Symposium (International) onCombustion, p.789. Williams and Wilkins: Baltimore.Walls, R. A., Axial Flow Fans and Ducts, 1983, John Wiley and Sons, Inc. pp. 1-136.Wesely, R., 1984, “Airflow at Heading Faces with Forcing Auxiliary Ventilation,” ThirdInternational Mine Ventilation Congress, Harrogate, (UK), pp.73-94181APPENDIXThe error or uncertainty analysis presented in Chapter four was used to estimate the errorsin the present study. These errors represent the worst possible cases that are present in themeasurements.Table Al Experimental uncertainty estimates of the dataQuantity Error estimate %2.67(P—1)/+(pu)U/Ui 2.75WRIDI 8.68.7R1 !jQR/QT10.676.05Iu2 /U(i -Pjexpi(f ‘e)th1.442.77(-),(p,-i)Qe / Q and QT / Q6.52(n)Qe/Qj(”j)7.08(r’ —lJQT /Q(P —j 7.08


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