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Forced convection heat transfer from a cylinder in supercritical carbon dioxide Green, John Richard 1970

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FORCED CONVECTION HEAT TRANSFER FROM A CYLINDER SUPERCRITICAL CARBON DIOXIDE BY JOHN RICHARD GREEN B . A . S c , University of Bri t i sh Columbia, 1966 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in the Department of Mechanical Engineering We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH February, 1970 COLUMBIA In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Depart-ment or by h i s r e p r e s e n t a t i v e s . I t i s understood th a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. JOHN RICHARD GREEN Department of Mechanical Engineering The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date jQ-jOr", I )°)lO  ABSTRACT Heat transfer rates have been measured for forced flow of supercrit ical carbon dioxide normal to a horizontal heated cylinder. The 0.006 inch diameter cylinder was held at various constant temperatures by a feed-back bridge c i r c u i t . Free convection results are also included. The effects of bulk' f lu id temperature, bulk f lu id pressure, and surface temperature were studied for a range of bulk f lu id temperature and pressure of from 0.8 to 1.4 times the c r i t i c a l temperature and pressure for.several free stream velocit ies from zero to three feet per second. The temperature difference between the heated cylinder and the bulk f lu id was varied from 1 deg F to 320 deg F. Flow fields of a l l data runs were observed. S t i l l photographs and high speed movies have been taken at operating conditions of interest. In a supercrit ical f lu id the heat transfer rate increases smoothly and monotonically with increasing temperature difference, increasing velocity, and increasing pressure. In f lu id with the bulk temperature below the pseudo-critical temperature the..heat transfer coefficient shows large peaks when the cylinder temperature is near the pseudocritical temperature. Peaks are largest when the bulk f lu id pressure is near the c r i t i c a l pressure. The heat transfer coefficient decreases with increasing tempera-ture difference when the bulk f luid temperature is above the pseudo-critical temperature. The heat transfer rate noteably increases with increasing pressure only when vapor-l ike f lu id is in contact with the heated cylinder. Supercrit ical forced flow has been compared to forced flow boi l ing . The supercrit ical case does not exhibit the characteristic strong maxima in heat transfer rate shown in forced flow nucleate boi l ing . Heat transfer rates at larger temperature differences are very similar in forced flow film boil ing and supercrit ical forced flow heat transfer. With this horizontal, constant temperature cylinder, no "bubble-like" or "boiling-like" mechanisms of heat transfer were observed in supercrit ical free or forced convection. The flow f i e ld and heat transfer rate in free convection were found to be very unstable and sensitive to small temper-ature disturbances in the bulk f l u i d . TABLE OF CONTENTS Chapter Page 1 INTRODUCTION 1 1.1 Preliminary Remarks 1 1.2 Review of C r i t i c a l Region Terminology . . . 2 1.3 C r i t i c a l Region Heat Transfer -General 5 1.4 Free Convection Studies 10 1.5 Forced Convection Studies 23 1.6 Scope of Present Investigation 26 2 EXPERIMENTAL APPARATUS 28 2.1 General Concept of the Experimental Apparatus 2 8 2.2 Details of Apparatus Components 31 2.2.1 Main test section 31 2.2.2 Flow loop tubing and valves . . . . 33 2.2.3 Flow conditioning and transit ion section 34 2.2.4 Circulating pump 36 2.2.5 Heat exchangers 37 2.2.6 Transducer glands 40 2.2.7 Velocity measurement . . . . . . . . 40 2.2.8 Temperature measurement 42 2.2.9 Pressure measurement 43 2.2.10 Hot wire anemometer power supply 44 V Chapter Page 2.2.11 Probes and heated cylinders . . . 46 2.2.12 Photographic equipment and Schlieren system 48 3 EXPERIMENTAL PROCEDURE . . . 50 3.1 I n i t i a l Preparation and Calibration of the Apparatus . 5 0 3.2 Procedure in Free Convection Measurements 55 3.3 Procedure in Forced Convection Measurements 59 4 EXPERIMENTAL RESULTS 62 4.1 Effect of Free-stream Velocity 62 4.2 Effect of Bulk Temperature 69 4.3 Effect of Bulk Pressure 72 4.4 Interpretation of Photographs 74 5 DISCUSSION OF RESULTS . . 77 5.1 Free Convection Results 7 7 5.2 Forced Convection Results 81 6 CONCLUSIONS 87 6.1 General . 87 6.2 Free Convection Heat Transfer 87 6.3 Forced Convection Heat Transfer 88 6.3.1 Forced convection - general . . . 88 6.3.2 Effects of velocity 89 6.3.3 Effects of bulk temperature . . . 89 6.3.4 Effects of bulk pressure 90 6.3.5 Heat Transfer Mechanisms 90 v i Chapter Page REFERENCES 135 APPENDIX I - Properties of near-cr i t i ca l carbon dioxide 140 APPENDIX II - List ing and summary of Photographs taken 145 APPENDIX III - Calculations and error analysis .•. . . 149 LIST OF FIGURES Figure Page 1 Summary of Near-Crit ical Region Terminology . . 92 2 Typical Property Variations in Near-Crit ical Carbon Dioxide 93 3 Property Variations which would occur in Supercrit ical CO2 for the Free Convection Temperature Distribution Developed in Air . . . 94 4 Bubble-Like Heat Transfer Mechanism Previously Observed by Knapp 95 5 Summary of Previous Work with Heated Cylinders in Supercrit ical Fluids 96 6 Schematic of Near-Crit ical Carbon Dioxide Forced Flow Loop 97 7 Cutaway View of Test Section Arrangement . . . 98 8 Section View.of-Test Section Block' 99 9 General Layout of Experimental Equipment . . . 100 10 Thermocouple Measuring Circui t . . . . . . . . 101 11 Reproducibility of Data 102 12 Dimensions and Surface Condition of Heated Probe 103 13 Flow Field Around Probe and Supports in Free Convection 104 14 Arrangement of Schlieren System 105 15 Effect of Velocity on Heat Transfer Rate in C 0 2 at 80 deg F and 1100 psia . . . . . . . 106 16 Effect of Velocity on Heat Transfer Coefficient in CO„ at 8 0 deg F and 1100 psia . . . 107 v i i i Figure Page 17 Effect of Velocity on Heat Transfer Rate in C0 2 at 87 deg F and 1100 psia 108 18 Effect of Velocity on Heat Transfer Coefficient in CCu at 87 deg F and 1100 psia : . . . 109 19 Effect of Velocity on Heat Transfer Rate in C0 2 at 91 deg F and 1100 psia 110 20 Effect of Velocity on Heat Transfer Coefficient in CO„ at 91 deg F and 1100 psia I l l 21 Effect of Velocity on Heat Transfer Rate in C0 2 at 80 deg F and 1300 psia 112 22 Effect of Velocity on Heat Transfer Coefficient in CO„ at 80 deg F and 1300 psia 113 23 Effect of Velocity on Heat Transfer Rate in C0 2 at 86 deg F and 1300 psia . . . . . . . . 114 24 Effect of Velocity on Heat Transfer Coefficient in CC) at 86 deg F and 1300 psia 115 25 Effect of Velocity on Heat Transfer Rate in C0 2 at 80 deg F and 1500 psia . . 116 26 Effect of Velocity on Heat Transfer Coefficient in CO- at 80 deg F and 1500 psia . 117 27 Effect of Bulk Temperature on Heat Transfer Rate at 1100 psia (Re*=300) 118 28 Effect of Bulk Temperature on Heat Transfer Coefficient at 1000 psia (Re* = 300) 119 29 Effect of Bulk Temperature on Heat Transfer Rate at 1100 psia (Re*=600) 120 30 Effect of Bulk Temperature on Heat Transfer Coefficient at 1100 psia (Re*=600) 121 31 Effect of Bulk Temperature on Heat Transfer Rate at 1300 psia (Re* = 600) 122 ix Figure Page 32 Effect of Bulk Temperature on Heat Transfer Coefficient at 1300 psia (Re*=600) 123 33 Effect of Bulk Pressure on Heat Transfer Rate at 80 deg F (Re* = 600) 124 34 Effect of Bulk Pressure on Heat Transfer Coefficient at 80 deg F (Re*=600) ] 25 35 Flow Fie ld Variation with Velocity at 80 deg F and 1100 psia 126 36 Flow Fie ld Variation with Velocity at 80 deg F and 1300, 1500 psia 127 37 Flow Fie ld Variation with Circulation and Pump Vibration - Free Convection 128 38 Flow Fie ld Variation with Cylinder Temper-ature Increase in Subcrit ical and Super-c r i t i c a l Free Convection 129 39 Flow Fie ld Variation with Velocity in Subcrit ical Film Boiling 130 40 Flow Fie ld Variation with Pressure and Cylinder Temperature Variation in Forced Convection 131 41A Comparison, of Experimental Free Convection Data with Other Workers 132 41B Comparison of Experimental Free Convection Data with Other Workers 133 42 E l e c t r i c a l Resistance Change of Nichrome V with Temperature (Minimum Curve for Annealed Nichrome Wire From [40] . . . . . . . 134 NOMENCLATURE A surface area of the probe Ap flow channel area at the probe location A ^ area of the venturi meter throat Gr Grashof number I current dissipated by the heated cylinder (amperes) L effective length of the heated cylinder Nu Nusselt number P power dissipated by the heated cylinder (watts) P^ f luid bulk pressure (psia) Pr Prantl number 2 Q heat transfer rate (BTU/hr-ft ) Ra Rayleigh number Rc probe resistance at the bulk f lu id temperature (ohm) Re Reynolds number (~ Re* Reynolds number based on mean properties (—) Rop probe resistance at temperature (ohm) T^ f lu id bulk temperature (deg F) T ^ cylinder temperature (deg F) AT temperature difference between heated probe and bulk f lu id (deg F) f lu id velocity past probe V.^ f lu id velocity through venturi meter throat c, venturi meter discharge coefficient s p e c i f i c heat a t constant p r e s s u r e diameter of the probe 2 heat t r a n s f e r c o e f f i c i e n t (BTU/hr-ft -deg F) thermal c o n d u c t i v i t y of the bulk f l u i d d i f f e r e n t i a l p r e s s u r e across v e n t u r i meter e x p e r i m e n t a l l y measured c o e f f i c i e n t of r e s i s t a n c e change with temperature ^ (deg F) r a t i o of v e n t u r i t h r e a t diameter to upstream tube diameter change i n ( ) 3 f l u i d v i s c o s i t y ( l b / f t ) dynamic v i s c o s i t y ( l b / f t - h r ) 2 kinematic v i s c o s i t y ( f t /sec) ACKNOWLEDGEMENTS The author wishes to express his deep gratitude to his thesis supervisor, Dr. E .G. Hauptmann, for the amount and quality of guidance throughout the project. Additional thanks are due to Dr. Z. Rotem and Dr. I .S. Gartshore for invaluable comments on phases of the research. The author would also l ike to thank Mr. D. Workman and the members of the technical staff of the Mechanical Engineering Department for their assistance in the actual construction of the apparatus. Support for this research from the National Research Council of Canada is gratefully acknowledged. Com-puting f a c i l i t i e s were provided by the University Computing Centre. 1. INTRODUCTION 1.1 Preliminary Remarks Heat transfer to fluids in the region of the thermodynamic c r i t i c a l point has become of great importance i n the last 15 years due to the very high heat transfer rates possible in this region. Current applications involving heat transfer to fluids above their c r i t i c a l pressure include supercrit ical water in power station boi lers , supercrit ical water as the working f lu id in thermal power stations, and supercrit ical hydrogen as the fuel and coolant for the com-bustion chamber in large regeneratively-cooled rocket engines. Proposed applications include: supercrit ical methane as the fuel in the SST as well as an aerodynamic and engine heat sink: near-cr i t i ca l helium to cool the coi ls of superconduct-ing electromagnets: and limited applications in the l i q u i f i -cation of hydrocarbons from natural gases. An entirely supercrit ical turbine power cycle using supercrit ical carbon dioxide has been devised. Such a turbine is now feasible and would offer many advantages for such applications as e lectr ic power generation for terres t ia l or space applications and shaft power for surface or sub-surface marine propulsion. An entirely supercrit ical turbine would have high thermal efficiency, low volume to power rat io , no blade erosion in the turbine, no cavitation i n the pump, and use a single stage 2 t u r b i n e and pump. The d e v e l o p m e n t o f t h i s t u r b i n e and o t h e r e n g i n e e r i n g a p p l i c a t i o n s o f f l u i d s above t h e c r i t i c a l p r e s s u r e has b e e n hampered by l a c k o f b a s i c i n f o r m a t i o n a b o u t f l u i d b e h a v i o u r i n t h e c r i t i c a l r e g i o n . Of more f u n d a m e n t a l i n t e r e s t , h e a t t r a n s f e r i n a n e a r - c r i t i c a l f l u i d i s an e x t r e m e example o f h e a t t r a n s f e r i n a s i n g l e - p h a s e f l u i d w i t h v e r y l a r g e p r o p e r t y v a r i a t i o n s . P r o p e r t y v a r i a t i o n s i n f l u i d n e a r t h e c r i t i c a l p o i n t c a n be so s e v e r e t h a t u n u s u a l s e c o n d a r y mechanisms, s i m i l a r t o s u b -c r i t i c a l b o i l i n g , have been p r o p o s e d f o r t h e s u p e r c r i t i c a l s i n g l e - p h a s e f l u i d . To d a t e most work i n s u p e r c r i t i c a l f l u i d h e a t t r a n s -f e r has b een c o n c e r n e d w i t h f l o w s i t u a t i o n s i n s i d e h e a t e d t u b e s . T h i s g e o m e t r y i s o f i m m e d i a t e e n g i n e e r i n g i m p o r t a n c e and d e s i g n o f h e a t e x c h a n g e r s and r e l a t e d e q u i p m e n t r e q u i r e d methods o f p r e d i c t i n g h e a t t r a n s f e r r a t e s . E n g i n e e r i n g c o r r e l a t i o n s o f e x p e r i m e n t a l d a t a have been o b t a i n e d w h i c h a r e v a l i d o v e r l i m i t e d r a n g e s , and a v e r y few methods o f p r e d i c t i n g h e a t t r a n s f e r r a t e s have been p r o -p o s e d . To d a t e p r e d i c t i o n s have o n l y b een v a l i d i n c e r t a i n s e c t i o n s o f t h e n e a r - c r i t i c a l r e g i o n and e v e n t h e b o u n d a r i e s o f t h e s e r e g i o n s o f p r e d i c t a b l e h e a t t r a n s f e r have n o t b een f i x e d . 1.2 Review o f C r i t i c a l P o i n t D e f i n i t i o n s P r o b l e m s i n v o l v i n g h e a t t r a n s f e r i n t h e n e a r - c r i t i c a l r e g i o n a r e o f more t h a n a c a d e m i c i n t e r e s t . D e s i g n f o r g r e a t e r 3 efficiency in boilers and regeneratively-cooled rocket com-bustion chambers has forced operating temperatures and pressures into the c r i t i c a l region. The c r i t i c a l region is considered to include those thermodynamic states within 0.8 to 1.5 times the c r i t i c a l temperature and pressure. The c r i t i c a l pressure is defined as that pressure above which the f luid can not exist as two dist inct phases, while the c r i t i c a l temperature is the highest temperature at which the f lu id can exist as two dist inct phases. For engineering purposes these l imiting values define a c r i t i c a l point. At the c r i t i c a l point the physical properties of the l iquid and vapor phases are identical and the characteristic surface tension between the l iquid and vapor phases has disappeared. Fluid above the c r i t i c a l pressure is therefore a continuous single-phase medium. Properties vary extremely rapidly with both tempera-ture and pressure in.the c r i t i c a l region, and change continu-ously from those of a l iquid to those of a vapor with bulk temperature increase. Fluid above the c r i t i c a l pressure is termed supercri t ica l , while f lu id below the c r i t i c a l pressure is termed subcr i t ica l . For a given supercrit ical pressure the temperature at which the maximum specific heat occurs is known as the pseudo-critical or transposed c r i t i c a l tempera-ture for that pressure. Both terms are in common usage in the l i terature . The pseudo-critical temperature may equiya-lently be defined as the temperature at which the maximum rate of change of enthalpy with temperature at that constant 4 p r e s s u r e o c c u r s . The l o c u s o f t h e p s e u d o - c r i t i c a l t e m p e r a t u r e v a r i a t i o n w i t h p r e s s u r e may be c o n s i d e r e d as an e x t e n s i o n o f t h e s u b c r i t i c a l s a t u r a t i o n c u r v e . F i g u r e (1) shows a g r a p h i c a l summary o f t h e s e d e f i n i t i o n s . S u p e r c r i t i c a l f l u i d b e l o w t h e p s e u d o - c r i t i c a l t e m p e r a t u r e i s t e r m e d l i q u i d - l i k e . The f l u i d h as p r o p e r t i e s v e r y much l i k e a l i q u i d , t h e t h e r m a l c o n d u c t i v i t y and t h e dynamic v i s c o s i t y d e c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e as i n o r d i n a r y l i q u i d s . The s u p e r c r i t i c a l f l u i d above t h e p s e u d o -c r i t i c a l t e m p e r a t u r e has p r o p e r t i e s somewhat l i k e a n o r m a l g a s . The t h e r m a l c o n d u c t i v i t y and dynamic v i s c o s i t y a r e n o t g r e a t l y a f f e c t e d by i n c r e a s i n g t e m p e r a t u r e . I n i t i a l l y t h e v i s c o s i t y and t h e r m a l c o n d u c t i v i t y d e c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e by a s m a l l amount and t h e n t h e s e p r o p e r t i e s b e g i n t o i n c r e a s e ' w i t h i n c r e a s i n g t e m p e r a t u r e . S u p e r c r i t i c a l f l u i d above t h e -p s e u d o - c r i t i c a l t e m p e r a t u r e i s known as v a p o r - l i k e f l u i d . F i g u r e (2) shows t h e d e n s i t y , dynamic v i s c o s i t y , and P r a n d t l -number v a r i a t i o n w i t h t e m p e r a t u r e f o r s u p e r c r i t i c a l c a r b o n d i o x i d e a t 1100 p s i a . F u r t h e r f i g u r e s s h o w i n g t h e e f f e c t o f p r e s s u r e on d e n s i t y , t h e r m a l c o n d u c t i v i t y , d y n a m i c v i s c o s i t y and P r a n d t l number a r e r e p r o d u c e d i n A p p e n d i x ( I ). Note t h a t t h e p r o p e r t i e s c i t e d show c o n t i n u o u s v a r i a t i o n f r o m t h e l i q u i d -l i k e t o t h e v a p o r - l i k e f l u i d and t h a t t h e maximum r a t e o f change w i t h r e s p e c t t o p r e s s u r e o c c u r s r o u g h l y a t t h e p s e u d o -c r i t i c a l t e m p e r a t u r e f o r t h a t p r e s s u r e . F i g u r e (3) shows t h e f r e e c o n v e c t i o n t e m p e r a t u r e f i e l d a r o u n d a h e a t e d h o r i z o n t a l c y l i n d e r ; i n a i r . .The c y l i n d e r 5 t e m p e r a t u r e i s 130 deg F and t h e a i r i s a t a b u l k t e m p e r a t u r e o f 80 d e g • F . P l o t t e d t o t h e same s c a l e a r e r e p r e s e n t a t i v e p r o p e r t y v a r i a t i o n s w h i c h w o u l d o c c u r a c r o s s t h e b o u n d a r y l a y e r i n s u p e r c r i t i c a l c a r b o n d i o x i d e a t 1100 p s i a and 80 deg F b u l k t e m p e r a t u r e f o r t h e same t e m p e r a t u r e f i e l d . The a c t u a l f l o w f i e l d i n c a r b o n d i o x i d e w o u l d o f c o u r s e be v a s t l y d i f f e r -e n t f r o m t h a t i n a i r b u t t h e model f l o w does show t h e tremendous p r o p e r t y v a r i a t i o n s w h i c h must o c c u r i n a s u p e r c r i t i c a l f l u i d when t h e w a l l t e m p e r a t u r e and b u l k f l u i d t e m p e r a t u r e span t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . 1.3 C r i t i c a l R e g i o n Heat T r a n s f e r - G e n e r a l The e a r l i e s t work i n n e a r - c r i t i c a l h e a t t r a n s f e r was r e p o r t e d by S c h m i d t [1] i n t h e e a r l y 1930"s. S c h m i d t r e a s o n e d t h a t as t h e N u s s e l t number i n n o r m a l f l u i d s i s a f u n c t i o n o f t h e G r a s h o f and P r a n d t l numbers, t h e h i g h v a l u e s o f t h e G r a s h o f and P r a n d t l numbers e x p e c t e d due t o t h e l a r g e p r o p e r t y v a r i a t i o n s s h o u l d p r o d u c e v e r y e f f e c t i v e h e a t t r a n s f e r i n t h e n e a r - c r i t i c a l r e g i o n . E x p e r i m e n t a l r e s u l t s were p r e s e n t e d i n terms o f an a p p a r e n t t h e r m a l c o n d u c t i v i t y (the t h e r m a l c o n d u c t i v i t y r e q u i r e d o f a s o l i d b a r o f t h e same d i m e n s i o n s as t h e t e s t chamber t o t r a n s f e r t h e e x p e r -i m e n t a l l y o b s e r v e d amount o f h e a t ) . F o r s u p e r c r i t i c a l animonia S c h m i d t l a t e r [15] f o u n d an a p p a r e n t t h e r m a l c o n d u c -t i v i t y as l a r g e as 4000 t i m e s t h a t o f p u r e c o p p e r b u t n o t e d t h a t t h i s l a r g e v a l u e e x t e n d e d o n l y o v e r a v e r y n a r r o w r a n g e o f t e m p e r a t u r e s . 6 L i t t l e further investigation into supercrit ical heat transfer was done unt i l the mid-1950's when studies were begun to provide engineering design data for super-c r i t i c a l boilers and regeneratively-cooled rocket engines. Notable is the work of Powell [2] in supercrit ical hydrogen, oxygen, and nitrogen, the work of Goldmann [3] in super-c r i t i c a l water, and the work of Deissler [4] in supercrit ical water. These studies were primarily interested in obtaining experimental pressure drop and heat transfer characteristics for supercrit ical and near-cr i t i ca l fluids in heated tubes. Controversy about the heat transfer rates, the f lu id behaviour, and even the fundamental mechanisms of near-cr i t i ca l heat transfer immediately arose. Boi l ing- l ike mechanisms were proposed by some [3, 5, 6] but other workers [4, 1, 8] main- ' tained that property variations alone would explain the heat transfer rates. In attempts to resolve the controversy, a large number of experimental studies were done in a variety of fluids with extremely varied results. Some investigators observed large peaks in surface temperature along the heated tubes, others observed minimums in surface temperature along the heated tube, while s t i l l others observed conventional temperature distributions. Some workers noted large pressure drops, pressure osci l lat ions as high as 0.3 of test pressure, singing noises in the heated tubes, and even destruction of the test section. Others observed only some or none of these unusual heat transfer characteristics. 7 Workers i n t h e f i e l d o f s u p e r c r i t i c a l h e a t t r a n s f e r have b e e n a b l e t o c o r r e l a t e t h e i r own d a t a e q u a l l y w e l l assum-i n g o r n o t a s s u m i n g a b o i l i n g - l i k e mechanism. I n no c a s e has a c o r r e l a t i o n b e e n o b t a i n e d w h i c h i s v a l i d e x c e p t i n s e l e c t e d s e c t i o n s o f t h e n e a r - c r i t i c a l r e g i o n . V a r i o u s r e s e a r c h e r s have p o i n t e d t o t h e same e x p e r i m e n t a l d a t a and c o n c l u d e d t h a t a b o i l i n g - l i k e phenomena does o r does n o t e x i s t . I n s h o r t , h e a t t r a n s f e r i n t h e c r i t i c a l r e g i o n i s so complex t h a t t h e most s o p h i s t i c a t e d e n g i n e e r i n g p r e d i c t i o n s c a n d i f f e r f r o m e x p e r i m e n t a l r e s u l t s by ± 100% and r e s e a r c h e r s a r e o n l y now b e g i n n i n g t o be a b l e t o p r e d i c t t h e a r e a s i n w h i c h u n u s u a l h e a t t r a n s f e r r e s u l t s a r e l i k e l y . The o b s e r v e d d i f f e r e n c e s f r o m h e a t t r a n s f e r s i t u a t i o n s i n v o l v i n g f l u i d s w i t h m o d e r a t e p r o p e r t y v a r i a t i o n s a r e so s e v e r e t h a t a p h y s i c a l model o f h e a t t r a n s f e r i n t h e c r i t i c a l r e g i o n has n o t y e t been a c c e p t e d . A r e v i e w o f t h e h e a t t r a n s f e r l i t e r a t u r e t o 1965 i s " c o n t a i n e d i n t h e work o f Hauptmann [ 9 ] . A s u r v e y o f t h e p r o g r e s s i n i n t e r n a l f l o w h e a t t r a n s f e r s i t u a t i o n s t o 1968 has been g i v e n by P e t u k o v [ 1 0 ] . P e t u k o v i n c l u d e s some l i t t l e known R u s s i a n work b u t s t i l l c o n c l u d e s t h a t some a t t e n t i o n s h o u l d now be g i v e n t o t h e a c t u a l mechanisms o f h e a t t r a n s f e r i n t h e s i n g l e - p h a s e c r i t i c a l r e g i o n . As r e c e n t l y as mid-1969 H e n d r i c k s and Simoneau [11] c o m p i l e d a v e r y c o m p l e t e s u r v e y o f h e a t t r a n s f e r t o n e a r - c r i t i c a l f l u i d s . 196 p u b l i c a t i o n s a r e c r i t i c a l l y r e v i e w e d and summaries o f p r o g r e s s t o d a t e on a l l f a c e t s o f n e a r - c r i t i c a l h e a t t r a n s f e r a r e i n c l u d e d . 8 O n l y p a p e r s o f s p e c i a l i n t e r e s t o r d i r e c t l y r e l e -v e n t t o t h i s s t u d y w i l l be r e v i e w e d h e r e . As n o t e d above most work i n t h e n e a r - c r i t i c a l r e g i o n has b een done i n t h e g e o m e t r y o f i m m e d i a t e i n t e r e s t — t h e h e a t e d tube; f i l l e d w i t h s u p e r c r i t i c a l f l u i d , and w i t h t h e f l u i d o f i m m e d i a t e i n t e r e s t — w a t e r , h y d r o g e n , n i t r o g e n , and o x y g e n . V e r y l i t t l e work has been done on t h e d e t e r m i n a t i o n o f t h e h e a t t r a n s f e r mechanism by d i r e c t o b s e r v a t i o n o r i n h e a t t r a n s f e r w i t h e x t e r n a l s u p e r c r i t i c a l f l o w s i t u a t i o n s . The s u p e r c r i t i c a l f l u i d s now u s e d i n e n g i n e e r i n g a p p l i c a t i o n s i n v o l v e e i t h e r v e r y h i g h t e m p e r a t u r e s and p r e s -s u r e s (water w i t h T c r = 705 deg F and P e r = 3206 p s i a ) o r m o d e r a t e p r e s s u r e s and low t e m p e r a t u r e s ( h y d r o g e n w i t h T c r = -400 deg F and P e r = 188 p s i a and o x y g e n w i t h T c r = -182 deg F and P e r = 730 p s i a ) o r a r e e x p l o s i v e (methane). T h e s e p r o p e r -t i e s s e v e r e l y r e s t r i c t e x p e r i m e n t a l e q u i p m e n t t o be v e r y s o p h i s t i c a t e d and c o s t l y . Measurements o f f u n d a m e n t a l v a r i a b l e s become d i f f i c u l t and e x p e r i m e n t a l e r r o r s c a n be l a r g e . The l a r g e p r e s s u r e o s c i l l a t i o n s p r e v i o u s l y o b s e r v e d e x p e r i m e n t a l l y c o u l d a l s o r e q u i r e v e r y c o n s e r v a t i v e e q u i p m e n t d e s i g n f o r s a f e t y c o n s i d e r a t i o n s . Most w o r k e r s c o n c e r n e d w i t h t h e f u n d a m e n t a l mechanisms and v a r i a b l e s o f h e a t t r a n s f e r i n t h e c r i t i c a l r e g i o n have a g r e e d t h a t a l t h o u g h measurements made i n one f l u i d c a n n o t be d i r e c t l y a p p l i e d t o a n o t h e r f l u i d , t h e b a s i c mechanism s h o u l d n o t be d e p e n d e n t on t h e i n d i v i d u a l t e s t f l u i d . 9 I t i s n o t e x p e c t e d t h a t d i r e c t c o m p a r i s o n s o f h e a t t r a n s f e r and f l u i d f l o w b e h a v i o u r w i l l be p o s s i b l e f r o m one f l u i d t o a n o t h e r i n t h e c r i t i c a l r e g i o n . Thermodynamic p r o p -e r t i e s f o r a p u r e s u b s t a n c e c a n sometimes be c o n s i d e r e d t o c o l l a p s e t o a s i n g l e r e l a t i o n s h i p f o r many f l u i d s when t h e p r o p e r t i e s a r e n o r m a l i z e d by t h e c r i t i c a l c o n s t a n t s . N e a r t h e c r i t i c a l r e g i o n h o w e v e r , t h i s c o l l a p s e does n o t o c c u r ; t h e t h e r m o d y n a m i c p r o p e r t i e s , w h e n g e n e r a l i z e d , may d i f f e r by 25% o r more. T h i s g e n e r a l i z e d p r o p e r t y v a r i a t i o n o r law o f c o r r e s -p o n d i n g s t a t e s has n o t b een f o u n d t o a p p l y t o t h e t r a n s p o r t p r o p e r t i e s o f a f l u i d and t h e r a t e s o f change o f t h e t r a n s p o r t p r o p e r t i e s show ev e n more d e v i a t i o n f r o m f l u i d t o f l u i d . The r a t e s o f change w i t h t e m p e r a t u r e and p r e s s u r e o f s u c h t r a n s -p o r t p r o p e r t i e s as t h e r m a l c o n d u c t i v i t y , d e n s i t y , and v i s c o s i t y a r e o f p r i m a r y i m p o r t a n c e i n a n e a r - c r i t i c a l h e a t - t r a n s f e r f l u i d - f l o w s i t u a t i o n . A l t h o u g h t h e r e a r e s t i l l f o r m i d a b l e p r o b l e m s a s s o c i -a t e d w i t h d e t e r m i n i n g h e a t t r a n s f e r r a t e s and mechanisms i n any f l u i d n e a r t h e c r i t i c a l r e g i o n t h e s e p r o b l e m s a r e minim-i z e d by w o r k i n g w i t h a f l u i d w i t h c r i t i c a l v a l u e s e a s i l y o b t a i n e d i n t h e l a b o r a t o r y . Most w o r k e r s s t u d y i n g t h e f u n d a -m e n t a l s o f s u p e r c r i t i c a l h e a t t r a n s f e r and f l u i d f l o w have c h o s e n e i t h e r c a r b o n d i o x i d e o r one o f t h e F r e o n s as t h e w o r k i n g f l u i d . The c r i t i c a l c o n s t a n t s o f c a r b o n d i o x i d e a r e F r e o n s - t h e s e r i e s o f f l u o r o - c h l o r o h y d r o c a r b o n s h a v i n g t h e g e n e r a l s t r u c t u r e C,H,C1 F (k,l,m,n v a r i a b l e ) . 10 T c r = 89 deg F and Pc ~ 1071 psia while those, of a typical Freon, Freon 114A and Tcr = 2 94 deg F and Per = 4 73 psia. 1. 4 Free Convection Studies Perhaps the earl iest work directed at isolating the mechanism in supercrit ical heat transfer was that of G r i f f i t h and Sabersky [12]. Heat transfer rates were measured from a 0.010 inch diameter horizontal wire in Freon 114A. Free convection was studied at both subcrit ical and super-c r i t i c a l pressures. Heat transfer rates were observed to be smooth functions of wire temperature but bubble-like formations were photographed in the supercrit ical f l u i d . The super-c r i t i c a l photographs show great s imilarity to photographs presented of subcrit ical nucleate boil ing but the heat trans-fer rate in the supercrit ical f lu id does not show the char es-ter is t i c subcrit ical departure from nucleate boil ing peak. G r i f f i t h and Sabersky concluded that an unusual boi l ing- l ike heat transfer mechanismr as proposed by Goldmann [ 3 ], was present in the single phase f lu id above the c r i t i c a l pressure. Doughty and Drake [13] also used a 0.010 inch diameter horizontal heated cylinder to study free convection heat transfer in supercrit ical Freon 12. Heat transfer rates and heat transfer coefficients were presented as functions of wire temperature but no flow visualization was attempted. The data presented were reasonably correlated by the McAdams [14] formulation that the Nusselt number was a function of the f luid Grashop and Prandtl numbers but the peaks in heat transfer coefficient were not explained. It was noted that the heat transfer rates obtained in the supercrit ical f lu id are up to 10 times those observed in the subcri t ical vapor at the same bulk temperature. An elusive phenomenon which increased the heat transfer coefficient by 100% was also noted occasionally at some bulk conditions. Doughty and Drake concluded that the heat transfer behaviour observed could be accounted for by the f lu id property variations and that the agreement with the correlation proposed by McAdams would be improved i f more accurate property data were available for supercrit ical Freon 12. An interesting series of experiments directed at c lar i fy ing the heat transfer mechanism from a horizontal s tr ip in near-cr i t i ca l hydrogen was performed by Graham et a l . [16]. Heat transfer rates were measured as a function of temperature difference between heater surface and bulk f lu id temperature for subcri t ical and supercrit ical hydrogen (Tcr = -400 deg F and Per = 188 psia) for a variety of accelerations from 1 to,10 Earth gravit ies . The induced gravity effect was produced in a centrifuge arrangement. The hydrogen-filled tank was mounted on a trunnion arrangement which automatically oriented the heater s tr ip so that the resolved acceleration vector was perpendicular to the heater surface. The heated str ip was 0.062 inches wide and mounted flush on the face of a large bakelite block. The heater geometry could be considered as a horizontal heated patch under various gravity conditions. 12 Graham noted that in both the subcri t ical and supercrit ical f lu id the heat flux variation with temperature difference was smooth and was similar to that obtained by previous workers in different near-cr i t i ca l fluids and with other heater geometries. The established film boi l ing region in a subcri t ical f lu id and the supercrit ical heat transfer process were sensi-tive to an enhancement due to the induced multi-gravity effect. The established nucleate boil ing region in subcri t ical hydrogen was not influenced by an induced gravity effect of 7 g. The nucleate boil ing region and the transition zone from nucleate to film boi l ing were influenced by the past history of the test f lu id but this effect was absent in established film boi l ing and in supercrit ical heat transfer. Graham suggested that the observations of heat transfer rates, the effect of multi-gravity, and the high speed movies taken, a l l indicate that supercrit ical heat transfer resembles established film boil ing in a subcrit ical f l u i d . Graham concluded that while suitable selection of the constants can make the existing boil ing correlations f i t the experimental data, no accurate predictions of heat transfer rate can be made. An early experimental study of free convection from a horizontal cylinder in supercrit ical water was done by Holt [17]. Holt's study was an extension of a subcrit ical boil ing investigation to supercrit ical pressures. Both a 0.010 inch diameter horizontal cylinder and a short (1/8 th inch high) vert ical wall were manufactured of platinum and mounted i n a h i g h p r e s s u r e t e s t s e c t i o n . H o l t m e a s u r e d h e a t t r a n s f e r r a t e as a f u n c t i o n o f t e m p e r a t u r e d i f f e r e n c e f o r v a r i o u s p r e s s u r e s , b o t h s u b c r i t i c a l and s u p e r c r i t i c a l , and f o u n d t h e h e a t t r a n s f e r r a t e a t s u p e r c r i t i c a l p r e s s u r e s t o be a smooth i n c r e a s i n g f u n c t i o n o f t e m p e r a t u r e d i f f e r e n c e . A t s u p e r c r i t i c a l p r e s s u r e s t h e peak n u c l e a t e h e a t f l u x p o r t i o n , o f t h e h e a t f l u x v s t e m p e r a t u r e d i f f e r e n c e c u r v e had d i s a p p e a r e d and no d e t e r i o r a t e d r e g i o n o f h e a t t r a n s f e r r a t e s u c h as o b s e r v e d d u r i n g t r a n s i e n t f i l m b o i l i n g e x i s t e d . T h i s e f f e c t . ^ o c c u r r e d b o t h f o r t h e h o r i z o n t a l c y l i n d e r and t h e v e r t i c a l w a l l . The h e a t t r a n s f e r r a t e f r o m t h e v e r t i c a l w a l l was a p p r o x i m a t e l y t h e same as f r o m t h e h o r i z o n t a l c y l i n d e r and t h e h e a t f l u x v s t e m p e r a t u r e - d i f f e r e n c e c u r v e s have t h e same shape f o r e a c h g e o m e t r y . H o l t n o t e d t h a t t h e f l o w v i s u a l i z a t i o n was o b s c u r r e d i n t h e n e a r - c r i t i c a l f l u i d due t o g r o s s c i r c u l a t i o n o f s h e e t s o f f l u i d t h r o u g h t h e t e s t chamber and no s e c o n d a r y b u b b l e - l i k e mechanism was v i s i b l e . H o l t ' s t e s t s a t s u p e r c r i t i c a l p r e s s u r e were done a t t h e b u l k t e m per-a t u r e c o r r e s p o n d i n g t o t h e maximum s p e c i f i c h e a t and t h e i n f l u e n c e o f b u l k t e m p e r a t u r e on h e a t t r a n s f e r r a t e was assumed s m a l l . H o l t c o n c l u d e d t h a t f r e e c o n v e c t i o n above t h e c r i t i c a l p r e s s u r e r e s e m b l e d s i n g l e - p h a s e f r e e c o n v e c t i o n t o a c o n s t a n t p r o p e r t y f l u i d and n o t e d t h a t t h e d e t e r i o r a t e d r e g i o n a s s o c i a t e d w i t h t r a n s i t i o n f i l m b o i l i n g v a n i s h e d w h i l e t h e h e a t t r a n s f e r r a t e r e m a i n e d v e r y c l o s e t o t h a t o b s e r v e d i n s t a b l e f i l m b o i l i n g . 14 I n an e x p e r i m e n t a l s t u d y s p e c i f i c a l l y d e s i g n e d t o i n v e s t i g a t e t h e b u b b l e - l i k e s u p e r c r i t i c a l h e a t t r a n s f e r mechanism o b s e r v e d by G r i f f i t h and S a b e r s k y [ 1 2 ] , Knapp [18] s t u d i e d f r e e c o n v e c t i o n h e a t t r a n s f e r f r o m a h o r i z o n t a l c y l i n d e r i n s u p e r c r i t i c a l c a r b o n d i o x i d e . Knapp u s e d a c o n s t a n t c u r r e n t power s u p p l y t o h e a t a 0.010 i n c h d i a m e t e r n i c h r o m e c y l i n d e r w h i c h i s a l s o c o n n e c t e d as one l e g o f a Whea t s t o n e b r i d g e . A c o n s t a n t c u r r e n t power s u p p l y f i x e s t h e h e a t f l u x w h i c h must be d i s s i p a t e d by t h e h e a t e d c y l i n d e r and t h e c y l i n d e r t e m p e r a t u r e must v a r y t o r e a c h t h e r m a l e q u i l i b r i u m . Knapp m e a s u r e d t h e c y l i n d e r r e s i s t a n c e and c a l c u l a t e d t h e w i r e t e m p e r a t u r e . A t low t e m p e r a t u r e d i f f e r e n c e ( l e s s t h a n 100 deg F) a l a m i n a r t y p e o f f l o w r i s i n g t o a t u r b u l e n t plume e x i s t e d o v e r t h e e n t i r e h e a t e d c y l i n d e r . A t much h i g h e r t e m p e r a t u r e d i f f e r e n c e s ( g r e a t e r t h a n 300 deg F) a b u b b l e - l i k e f l o w , v e r y s i m i l a r t o t h a t s e e n by G r i f f i t h and S a b e r s k y , e x i s t e d u n d e r t h e h e a t e d c y l i n d e r . Knapp o b s e r v e d and p h o t o g r a p h e d an o s c i l l a t i n g f l o w r e g i o n a t i n t e r m e d i a t e t e m p e r a t u r e d i f f e r e n c e s ( f rom 150 t o 250 deg F) i n w h i c h t h e f l o w c h anged r e p e a t e d l y f r o m t h e l o w - t e m p e r a t u r e - d i f f e r e n c e l a m i n a r f r e e c o n v e c t i o n t o t h e h i g h - t e m p e r a t u r e - d i f f e r e n c e b u b b l e - l i k e mechanism. The o s c i l l a t i n g r e g i o n was marked by l a r g e t e m p e r a t u r e - d i f f e r e n c e and h e a t - f l u x o s c i l l a t i o n s and by a s h a r p i n c r e a s e i n t h e a v e r a g e d h e a t t r a n s f e r r a t e . P h o t o g r a p h s o f t h e s e two modes t a k e n f r o m a h i g h s p e e d movie p r o d u c e d by Knapp a r e r e p r o d u c e d as F i g u r e (4 ) . The o n s e t o f t h e o s c i l l a t i n g r e g i o n was n o t e d t o be b u l k t e m p e r a t u r e and p r e s s u r e d e p e n d e n t . Knapp c o n c l u d e d t h a t t h e h e a t t r a n s f e r r a t e was g r e a t l y i n c r e a s e d by t h e b u b b l e - l i k e mechanism and t h a t t h i s i n c r e a s e c o u l d t r i g g e r t h e w i r e t e m p e r a t u r e o s c i l l a t i o n by s u d d e n l y c o o l i n g t h e h e a t e d w i r e . Knapp a l s o s t u d i e d a s h o r t (0.125 i n c h h i g h ) v e r t i c a l w a l l and a n a r r o w (0.125 i n c h wide) h o r i z o n t a l s t r i p i n t h e same e x p e r i m e n t a l a p p a r a t u s b u t o b s e r v e d o n l y a l a m i n a r f r e e c o n v e c t i o n f l o w e v e n a t h i g h t e m p e r a t u r e d i f f e r e n c e s . The h e a t t r a n s f e r r a t e f o r t h e v e r t i c a l w a l l o r h o r i z o n t a l s t r i p was a smooth f u n c t i o n o f t e m p e r a t u r e d i f f e r e n c e . The h e a t t r a n s f e r r a t e f r o m t h e h e a t e d c y l i n d e r w i t h t h e b u b b l e - l i k e mechanism was o b s e r v e d t o be as much as f i v e t i m e s t h a t f r o m e i t h e r o f t h e h e a t e d s t r i p s a t t h e same t e m p e r a t u r e d i f f e r e n c e and b u l k f l u i d c o n d i t i o n s . Knapp c o n c l u d e d t h a t u n d e r c e r t a i n c o n d i t i o n s a b u b b l e - l i k e mechan i s m , a l t h o u g h n o t as g e n e r a l a phenomena as s u b c r i t i c a l b o i l i n g , c a n e x i s t i n t h e s i n g l e - p h a s e s u p e r c r i t i c a l f l u i d , and t h a t t h i s b u b b l e - l i k e mechanism can have a s t r o n g e f f e c t on h e a t t r a n s f e r r a t e . W h i l e t h e work o f Knapp was i n p r o g r e s s , G o l d s t e i n and Aung [20] a l s o s t u d i e d f r e e c o n v e c t i o n h e a t t r a n s f e r f r o m a h o r i z o n t a l c y l i n d e r i n s u p e r c r i t i c a l c a r b o n d i o x i d e . G o l d s t e i n a l s o u s e d a c o n s t a n t c u r r e n t power s u p p l y b u t t h e h e a t e d c y l i n d e r was a 0.015 i n c h d i a m e t e r p l a t i n u m w i r e . G o l d s t e i n i n d e p e n d e n t l y s t u d i e d v e r y n e a r l y t h e same f l u i d b u l k c o n d i t i o n s as Knapp and o b s e r v e d a b u b b l e - l i k e f l o w a t 16 h i g h w i r e t e m p e r a t u r e s ( g r e a t e r t h a n 500 d e g F) when t h e b u l k t e m p e r a t u r e was b e l o w t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . G o l d s t e i n a l s o s t u d i e d w i r e t e m p e r a t u r e s up t o 1500 deg F and f o u n d t h e h e a t t r a n s f e r r a t e t o be a smooth f u n c t i o n o f t e m p e r a t u r e d i f f e r e n c e f o r t h e e n t i r e r a n g e i n v e s t i g a t e d . The o c c u r r e n c e o f t h e b u b b l e - l i k e f l o w was n o t marked by t h e s h a r p i n c r e a s e i n h e a t t r a n s f e r r a t e o b s e r v e d by Knapp. G o l d s t e i n n o t e d t h a t t h e b u b b l e - l i k e f l o w he o b s e r v e d o c c u r r e d a t r o u g h l y t h e same h e a t f l u x as t h e b u b b l e - l i k e f l o w f o u n d by.Knapp b u t s t r e s s e d t h a t t h e w i r e t e m p e r a t u r e he m e a s u r e d (400 t o 500 deg F) was v e r y much l a r g e r t h a n t h a t o b s e r v e d by Knapp (150 t o 200 deg F) f o r t h e same b u l k c o n d i t i o n s . G o l d s t e i n c o n c l u d e d t h a t h i s r e s u l t s i n f r e e c o n v e c t i o n f r o m a c y l i n d e r c a n be e x p l a i n e d i n t e r m s o f s u i t a b l y s e l e c t e d f l u i d p r o p e r t i e s . G o l d s t e i n p r o p o s e d t h a t t h e N u s s e l t (Nu) number was a f u n c t i o n o f t h e G r a s h o f (Gr) and P r a n d t l (Pr) numbers as g i v e n by Mcadams [14] b u t t h a t t h e Gr and P r numbers s h o u l d be e v a l u a t e d a t t h e a r i t h m e t i c mean temper-a t u r e . G o l d s t e i n c o n c l u d e d t h a t a b u b b l e - l i k e mechanism c o u l d e x i s t i n s u p e r c r i t i c a l c a r b o n d i o x i d e b u t n o t e d t h a t t h e a b s e n c e o f any s t r o n g e f f e c t on t h e h e a t t r a n s f e r r a t e due t o t h e b u b b l e - l i k e f l o w m i g h t be e x p l a i n e d by t h e a b r u p t manner i n w h i c h h e a t f l u x was v a r i e d . The i d e a t h a t t h e r a t e o f change o f h e a t t r a n s f e r r a t e g r e a t l y i n f l u e n c e s t h e h e a t t r a n s f e r mechanism and h e a t t r a n s f e r r a t e o r i g i n a t e d w i t h Yamagata e t a l . [ 2 1 ] . 17 Yamagata a l s o s t u d i e d f r e e c o n v e c t i o n f r o m a h o r i -z o n t a l c y l i n d e r i n s u p e r c r i t i c a l c a r b o n d i o x i d e . The e x p e r i -m e n t a l d a t a t a k e n by Yamagata a r e r e p r o d u c e d and c o r r e l a t e d i n a r e p o r t by N i s h i k a w a and M i y a b e [ 2 2 ] , Yamagata u s e d a 0.008 i n c h d i a m e t e r p l a t i n u m c y l i n d e r , a l s o l o c a t e d i n a w h e a t s t o n e b r i d g e and h e a t e d by c o n s t a n t c u r r e n t . Yamagata a l s o s t u d i e d v e r y n e a r l y t h e same b u l k c o n d i t i o n s as Knapp [19] and G o l d s t e i n [ 2 0 ] . A l t h o u g h Yamagata o b s e r v e d a h y s t e r -i s i s e f f e c t i n w h i c h t h e h e a t t r a n s f e r r a t e - seemed t o be a f u n c t i o n o f t h e r a t e o f change w i t h t i m e o f t h e h e a t t r a n s f e r r a t e i t s e l f a t low t e m p e r a t u r e d i f f e r e n c e s ( l e s s t h a n 20 deg F) t h e h e a t t r a n s f e r r a t e a t h i g h e r t e m p e r a t u r e d i f f e r e n c e s (up t o 400 deg F) was a smooth f u n c t i o n o f t e m p e r a t u r e d i f f e r e n c e . Yamagata d i d n o t o b s e r v e t h e b u b b l e - l i k e mechanism r e p o r t e d by Knapp and G o l d s t e i n b u t t h e e x p e r i m e n t a l t e m p e r a t u r e d i f f e r e n c e s were s l i g h t l y l e s s t h a n G o l d s t e i n o b s e r v e d n e c e s s a r y t o i n i t i a t e t h e b u b b l e - l i k e f l o w . The e x p e r i m e n t a l t e m p e r a t u r e d i f f e r e n c e s were however g r e a t e r t h a n n o t e d by Knapp f o r i n i t i a t i o n o f t h e b u b b l e - l i k e f l o w and t h e o n s e t o f t h e h e a t f l u x - t e m p e r a t u r e o s c i l l a t i o n . N i s h i k a w a and M i y a b e c o r r e l a t e p a r t o f Yamagata"s d a t a u s i n g t h e c o n s t a n t p r o p e r t y r e l a t i o n s a s s u m i n g t h e p h y s i c a l p r o p e r t i e s a t t h e mean t e m p e r a t u r e between t h e h e a t e d c y l i n d e r and t h e b u l k f l u i d . The d a t a t a k e n i n t h e r e g i o n d e s c r i b e d as p s e u d o -b o i l i n g by Yamagata a r e c o r r e l a t e d by ah e x t e n s i o n o f a n u c l e a t e and f i l m b o i l i n g model p r o p o s e d e a r l i e r by N i s h i k a w a [ 2 3 ] , A g r e e m e n t o f t h e e x p e r i m e n t a l d a t a w i t h t h e p r o p o s e d 18 e x p r e s s i o n i s r e a s o n a b l e b u t a g a i n t h e c o n s t a n t s i n t h e e x p r e s s i o n s have been c h o s e n t o f i t t h e p a r t i c u l a r e x p e r i -m e n t a l d a t a . N i s h i k a w a c o n c l u d e s t h a t c e r t a i n r e g i o n s o f h e a t t r a n s f e r i n a s u p e r c r i t i c a l f l u i d c a n n o t be c o r r e l a t e d r e a d i l y h owever,and t h a t f u r t h e r i n v e s t i g a t i o n i s r e q u i r e d . O t h e r w o r k e r s have a l s o s t u d i e d f r e e c o n v e c t i o n h e a t t r a n s f e r f r o m a h e a t e d c y l i n d e r i n s u p e r c r i t i c a l c a r b o n d i o x i d e . D u b r o v i n a and S k r i p o v [24] u s e d a 0.0012 i n c h d i a m e t e r w i r e t o i n v e s t i g a t e t h e h e a t t r a n s f e r mechanism a t v e r y low (0.9 deg F) t e m p e r a t u r e d i f f e r e n c e s a l o n g i s o -therms i n t h e c r i t i c a l r e g i o n . D u b r o v i n a and S k r i p o v n o t e d t h a t t h e peak h e a t t r a n s f e r c o e f f i c i e n t a t a p a r t i c u l a r t e m p e r a t u r e d i f f e r e n c e i s s t r o n g l y a f f e c t e d by v e r y s m a l l c h a n g e s i n b u l k t e m p e r a t u r e o r p r e s s u r e . The peak h e a t t r a n s f e r c o e f f i c i e n t o c c u r s when t h e b u l k t e m p e r a t u r e i s s l i g h t l y above t h e p s e u d o - c r i t i c a l t e m p e r a t u r e and t h e peak v a l u e d e c r e a s e s and s h i f t s w i t h i n c r e a s i n g b u l k t e m p e r a t u r e . D u b r o v i n a and S k r i p o v a l s o showed t h e e f f e c t o f p r e s s u r e on h e a t t r a n s f e r r a t e and h e a t t r a n s f e r c o e f f i c i e n t f o r l a r g e r (up t o 20 deg F) t e m p e r a t u r e d i f f e r e n c e s f o r f l u i d a t one b u l k t e m p e r a t u r e . The o b s e r v e d h e a t t r a n s f e r r a t e was a smooth f u n c t i o n o f t e m p e r a t u r e d i f f e r e n c e e v e n t h o u g h t h e h e a t t r a n s f e r c o e f f i c i e n t showed l a r g e p e a k s as t h e w a l l t e m p e r a t u r e e x c e e d e d t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . I n c r e a s i n g p r e s s u r e c a u s e d an i n c r e a s e i n h e a t t r a n s f e r r a t e e x c e p t a t t e m p e r a t u r e d i f f e r e n c e s l e s s t h a n 3 deg F. D u b r o v i n a and S k r i p o v a l s o s t u d i e d a v e r t i c a l w i r e i n t h e same e x p e r i m e n t a l a p p a r a t u s and c o n c l u d e d t h a t t h e r a t i o o f h e a t t r a n s f e r c o e f f i c i e n t o f t h e h o r i z o n t a l w i r e t o t h e h e a t t r a n s f e r c o e f f i c i e n t f o r t h e v e r t i c a l w i r e v a r i e d f r o m 1.2 t o 1.5, d e c r e a s i n g as t h e p r e s s u r e became f u r t h e r f r o m t h e c r i t i c a l p r e s s u r e . F o r b o t h g e o m e t r i e s t h e h e a t t r a n s f e r r a t e was a smooth f u n c t i o n o f t e m p e r a t u r e d i f f e r -e n c e and no u n u s u a l o r b u b b l e - l i k e h e a t t r a n s f e r mechanisms were o b s e r v e d . D u b r o v i n a and S k r i p o v c o n c l u d e d t h a t f o r f r e e c o n v e c t i o n i n a s u p e r c r i t i c a l f l u i d t h e Nu = f ( G r , P r ) r e l a t i o n s h i p c a n be u s e d t o c o r r e l a t e t h e e x p e r i m e n t a l r e s u l t s b u t t h a t t h e f o r m o f f ( G r , P r ) must be d e t e r m i n e d by e x p e r i m e n t . D a n i e l s and B r a m a l l [25] a l s o i n v e s t i g a t e d f r e e c o n v e c t i o n f r o m a s m a l l h o r i z o n t a l c y l i n d e r i n s u p e r c r i t i c a l c a r b o n d i o x i d e . B o t h h e a t t r a n s f e r r a t e and h e a t t r a n s f e r c o e f f i c i e n t were p r e s e n t e d as f u n c t i o n s o f t e m p e r a t u r e d i f f e r e n c e f o r b o t h s u b g r i t i c a l and s u p e r c r i t i c a l p r e s s u r e s . The e f f e c t s o f b u l k f l u i d t e m p e r a t u r e were assumed s m a l l and D a n i e l s c o n c l u d e d t h a t t h e h e a t t r a n s f e r r a t e was a l m o s t c o n s t a n t w i t h i n c r e a s i n g p r e s s u r e a t p r e s s u r e s g r e a t e r t h a n 1100 p s i a . D a n i e l s d i d n o t n o t e t h e s h a r p i n c r e a s e i n h e a t f l u x o b s e r v e d by Knapp a t r o u g h l y t h e same b u l k c o n d i t i o n s . No c o r r e l a t i o n o f t h e d a t a i s p r e s e n t e d b u t D a n i e l s c o n c l u d t h a t t h e s u p e r c r i t i c a l h e a t t r a n s f e r mechanism was n o r m a l 20 f r e e c o n v e c t i o n and t h i s i s augmented by a s t i r r i n g a c t i o n c a u s e d by t h e movement o f c l u s t e r s o f d i f f e r i n g - d e n s i t y i f l u i d . R e c e n t l y K a t o e t a l . [26] s t u d i e d f r e e c o n v e c t i o n f r o m a 0.080 i n c h d i a m e t e r c y l i n d e r i n s u p e r c r i t i c a l c a r b o n d i o x i d e . O n l y low t e m p e r a t u r e d i f f e r e n c e s ( l e s s t h a n 90 deg F) were a t t a i n e d b u t t h e h e a t t r a n s f e r r a t e and h e a t t r a n s f e r c o e f f i c i e n t d i d n o t show t h e h y s t e r i s i s e f f e c t s o b s e r v e d by Yamagata f o r t h e same b u l k c o n d i t i o n s . The b u b b l e - l i k e mechanism p h o t o g r a p h e d by Knapp [19] and G o l d s t e i n [20] was n o t o b s e r v e d and t h e h e a t t r a n s f e r r a t e was a smooth f u n c t i o n o f t e m p e r a t u r e d i f f e r e n c e . The h e a t e d c y l i n d e r u s e d by K a t o e t a l . was much l a r g e r t h a n t h o s e u s e d by o t h e r w o r k e r s and t h e t e m p e r a t u r e d i f f e r e n c e s were n o t h i g h enough ; t o e x p e c t b u b b l e - l i k e f l o w . Heat t r a n s f e r r a t e s were s i m i l a r t o t h o s e o b s e r v e d by Yamagata and G o l d s t e i n b u t l o w e r , as e x p e c t e d b e c a u s e o f t h e l a r g e r d i a m e t e r c y l i n d e r . K a t o a l s o s t u d i e d f r e e c o n v e c t i o n f r o m a s h o r t (0.090 i n c h h i g h ) v e r t i c a l w a l l and o b s e r v e d t h a t t h e h e a t t r a n s f e r r a t e f o r t h e w a l l was j u s t s l i g h t l y l o w e r t h a n f o r t h e h e a t e d c y l i n d e r a t t h e same b u l k c o n d i t i o n s . K a t o a l s o p e r f o r m e d a l i m i t e d s e r i e s o f e x p e r i m e n t s w i t h f o r c e d f l o w p a s t t h e h o r i z o n t a l c y l i n d e r and a g a i n o b t a i n e d h e a t t r a n s f e r r a t e s w h i c h were smooth f u n c t i o n s o f t e m p e r a t u r e d i f f e r e n c e . K a t o c o n c l u d e d t h a t t h e f a m i l i a r Nu = f ( G r , P r ) e x p r e s s i o n f o r f r e e c o n v e c t i o n and t h e e x p r e s s i o n Nu = F ( R e , P r ) f o r f o r c e d c o n v e c t i o n c o u l d be u s e d t o c o r r e l a t e t h e h e a t t r a n s f e r c o e f f i c i e n t s i f i n t e g r a t e d mean f l u i d p r o p e r t i e s a r e u s e d t o e v a l u a t e t h e d i m e n s i o n l e s s g r o u p s . K a t o was u n a b l e t o n o t e any c o r r e l a t i o n between t h e o b s e r v e d g e n e r a t i o n o f t h r e a d - l i k e columns i n t h e s u p e r c r i t i c a l f l u i d and t h e h e a t t r a n s f e r c o e f f i c i e n t . G r i g u l l and A b a d z i c [27] a l s o s t u d i e d h e a t t r a n s f e r f r o m a h e a t e d h o r i z o n t a l w i r e i n n e a r - c r i t i c a l c a r b o n d i o x i d e and F r e o n 13. Heat t r a n s f e r r a t e s f r o m a 0.0004 i n c h d i a m e t e r w i r e were s t u d i e d a t p r e s s u r e s f r o m 0.75 t o 1.0 o f t h e c r i t i c a l p r e s s u r e . The b u l k t e m p e r a t u r e o f t h e l i q u i d a t t h e v a r i o u s p r e s s u r e s was m a i n t a i n e d v e r y n e a r t h e s a t u r a t i o n t e m p e r a t u r e and r e s u l t s a r e p r e s e n t e d as h e a t t r a n s f e r r a t e v s t e m p e r a t u r e d i f f e r e n c e between t h e h e a t e d w i r e and t h e s a t u r a t i o n t e m p e r a t u r e . The r e g i o n s o f f r e e c o n v e c t i o n , n u c l e a t e b o i l i n g and f i l m b o i l i n g were o b s e r v e d and p h o t o g r a p h e d . G r i g u l l and A b a d z i c c o n c l u d e d t h a t t h e h e a t t r a n s f e r r e s u l t s were s i m i l a r i n t h e two f l u i d s and t h a t o n l y l a c k o f p r o p e r t y i n f o r m a t i o n f o r n e a r - c r i t i c a l F r e o n 13 p r e v e n t e d a c o r r e l a t i o n s i m i l a r t o t h a t p r e s e n t e d as Nu = 0. 1/8 (Gr*Pr) ' f o r c a r b o n d i o x i d e . G r i g u l l . and A b a d z i c a l s o c o n c l u d e t h a t t h e r e g i o n o f s t a b l e f i l m b o i l i n g o c c u r s a t l o w e r t e m p e r a t u r e d i f f e r e n c e s as t h e b u l k f l u i d p r e s s u r e i s r a i s e d n e a r t o the. c r i t i c a l p r e s s u r e . The v a r i o u s mechanisms o b s e r v e d i n t h e s u b c r i t i c a l f l u i d , s u c h as v a p o r columns c o n t a i n i n g v a p o r c l u s t e r s and s h e e t s o f v a p o r were c o n -22 e l u d e d n o t t o have a s t r o n g e f f e c t on t h e a c t u a l h e a t t r a n s f e r r a t e . F u r t h e r i n f o r m a t i o n on t h e mechanism of. h e a t t r a n s -f e r i n s u b c r i t i c a l f l u i d v e r y n e a r t o t h e c r i t i c a l p r e s s u r e i s c o n t a i n e d i n a r e c e n t p a p e r by B a u m e i s t e r and Simoneau [ 2 8 ] . B a u m e i s t e r and Simoneau s t u d i e d f i l m b o i l i n g f r o m a 0.001 i n c h d i a m e t e r w i r e i n s a t u r a t e d n i t r o g e n a t p r e s s u r e s f r o m 0.03 t o 0.99 o f t h e c r i t i c a l p r e s s u r e . The mechanism was o b s e r v e d t o change f r o m a x i a l f l o w a l o n g t h e w i r e r e s u l t i n g i n l a r g e v a p o r b u b b l e s t o c i r c u m f e r e n t i a l f l o w a b o u t t h e w i r e w h i c h r e s u l t e d i n v a p o r columns as t h e b u l k p r e s s u r e i s i n c r e a s e d . The d a t a were c o r r e l a t e d v e r y w e l l by t h e r e l a t i o n s h i p d e v e l o p e d e a r l i e r by B a u m i e s t e r f o r f i l m b o i l i n g a t low p r e s s u r e s by a s i m p l e i n c r e a s e i n t h e c o e f f i c i e n t t o f i t t h e e x p e r i m e n t a l d a t a . B a u m e i s t e r and Simoneau c o n -c l u d e d t h a t t h e h e a t t r a n s f e r r a t e i n a n e a r - c r i t i c a l , b u t s u b c r i t i c a l , f l u i d f o l l o w s t h e t r e n d d e v e l o p e d f o r f i l m ' b o i l i n g a t low p r e s s u r e s and t h a t t h e o b s e r v e d change f r o m a x i a l t o c i r c u m f e r e n t i a l f l o w on t h e w i r e i s i n a c c o r d a n c e w i t h t h e p r e v i o u s l y d e v e l o p e d t h e o r e t i c a l m o d e l s . To d a t e t h e p r o b l e m o f f r e e c o n v e c t i o n f r o m a h o r i z o n t a l h e a t e d c y l i n d e r i n a s u p e r c r i t i c a l f l u i d has n o t been s o l v e d t h e o r e t i c a l l y e x c e p t when t h e p r o p e r t y v a r i a t i o n s c a n be c o n s i d e r e d s m a l l and t h e r e f o r e a c o n s t a n t p r o p e r t y e x p r e s s i o n c a n be e m p i r i c a l l y m o d i f i e d t o f i t e x p e r i m e n t a l d a t a . L i m i t e d work has b een done on p r e d i c t i n g h e a t t r a n s f e r j r a t e s w i t h t h e s i m p l e r g e o m e t r y o f a v e r t i c a l p l a t e i n v' s u p e r c r i t i c a l f l u i d w i t h l a m i n a r f r e e c o n v e c t i o n by Hasegawa and Y o s h i o k a [ 2 9 ] . Hasegawa and Y o s h i o k a u s e d a p e r t u r b a t i o n a p p r o a c h t o a c c o u n t f o r p r o p e r t y v a r i a t i o n e v a l u a t e d i n terms o f an e n t h a l p y v a r i a t i o n . The a p p r o a c h i s r e s t r i c t e d t o low t e m p e r a t u r e d i f f e r e n c e s and does n o t c o n s i d e r any u n u s u a l h e a t t r a n s f e r mechanism. Hasegawa and Y o s h i o k a c o n c l u d e t h a t t h e p e r t a b a t i o n method u s i n g a r e f e r e n c e e n t h a l p y i s l i k e l y t o be s u c c e s s f u l b u t much f u r t h e r work i s r e q u i r e d . I n summary, f o r t h e s p e c i f i c c a s e o f f r e e c o n v e c -t i o n f r o m a h e a t e d h o r i z o n t a l c y l i n d e r i n a s u p e r c r i t i c a l f l u i d no s a t i s f a c t o r y method o f p r e d i c t i n g h e a t t r a n s f e r r a t e has y e t been d e v e l o p e d and t h e r e i s s t i l l no g e n e r a l a g r e e -ment a b o u t t h e e x p e r i m e n t a l h e a t t r a n s f e r r a t e s and mechanisms. 1.5 F o r c e d C o n v e c t i o n S t u d i e s E x p e r i m e n t a l o r t h e o r e t i c a l s t u d i e s o f e x t e r n a l f l o w f o r c e d c o n v e c t i o n a r e e x t r e m e l y l i m i t e d . Hauptmann [30, e x p e r i m e n t a l l y s t u d i e d f o r c e d f l o w o f s u p e r c r i t i c a l c a r b o n d i o x i d e o v e r a h e a t e d f l a t p l a t e . Hauptmann p r e s e n t e d r e s u l t s i l l u s t r a t i n g t h e e f f e c t s o f b u l k t e m p e r a t u r e , b u l k p r e s s u r e , and f r e e - s t r e a m v e l o c i t y on h e a t t r a n s f e r r a t e and h e a t t r a n s f e r c o e f f i c i e n t . V i s u a l s t u d i e s o f t h e h e a t t r a n s f e r mechanisms u s i n g a m o d i f i e d s e m i - f o c u s i n g s c h l i e r e n 24 c o l o r s y s t e m a r e p r e s e n t e d . Hauptmann c o n c l u d e d t h a t peak v a l u e s i n o b s e r v e d h e a t t r a n s f e r c o e f f i c i e n t were p r i m a r i l y due t o t h e l a r g e p r o p e r t y v a r i a t i o n s i n t h e n e a r - c r i t i c a l f l u i d . K a t o e t a l . 126] a l s o e x p e r i m e n t a l l y s t u d i e d f o r c e d c o n v e c t i o n i n an e x t e r n a l f l o w . K a t o p r e s e n t e d r e s u l t s a t one b u l k p r e s s u r e , f i v e b u l k t e m p e r a t u r e s , and a n a r r o w r a n g e o f f r e e - s t r e a m R e y n o l d s number (Re) f o r a h e a t e d c y l i n d e r i n c r o s s f l o w a t low ( l e s s t h a n 25 deg F) t e m p e r a t u r e d i f f e r e n c e s . K a t o c o n c l u d e d t h a t t h e h e a t t r a n s f e r r a t e c o u l d be c o r r e l a t e d 0 6 33 by an e x p r e s s i o n Nu = 0.27 Re ' P r * where a l l f l u i d p r o p e r t i e s a r e e v a l u a t e d a s i n t e g r a t e d mean p r o p e r t i e s . K a t o was a b l e t o c o r r e l a t e h i s d a t a t o ± 2 5 % b u t was u n a b l e t o comment on t h e h e a t t r a n s f e r mechanism. I t i s n o t a b l e t h a t t h e e x p e r i m e n t a l h e a t t r a n s f e r c o e f f i c i e n t was a l w a y s above t h e c a l c u l a t e d h e a t t r a n s f e r c o e f f i c i e n t and t h a t K a t o f o u n d t h e f o r c e d c o n v e c t i o n h e a t t r a n s f e r c o e f f i c i e n t was l e s s s e n s i t i v e t o c y l i n d e r t e m p e r a t u r e v a r i a t i o n t h a n t h e f r e e c o n v e c t i o n c o e f f i c i e n t . The g e n e r a l c a s e o f e x t e r n a l f l o w o f a s u p e r -c r i t i c a l f l u i d o v e r any g e o m e t r y has n o t been s o l v e d b u t two a n a l y t i c a l s t u d i e s i l l u s t r a t e t h e p r o g r e s s made t o d a t e . Simoneau and W i l l i a m s [31] have r e c e n t l y s o l v e d t h e c a s e o f l a m i n a r c o u e t t e f l o w i n a n e a r - c r i t i c a l f l u i d . T h i s s i m p l e c a s e i n v o l v e s o n l y t h e i n f l u e n c e o f t h e v i s c o s i t y and c o n -d u c t i v i t y v a r i a t i o n s w i t h t e m p e r a t u r e . Simoneau and W i l l i a m s c o n c l u d e d t h a t t h e v a r i a t i o n i n t r a n s p o r t p r o p e r t i e s g r e a t l y r e d u c e d t h e h e a t t r a n s f e r r a t e i n n e a r - c r i t i c a l l i q u i d - l i k e p a r a - h y d r o g e n b u t t h a t v a p o r - l i k e p a r a - h y d r o g e n c o u l d be a d e q u a t e l y t r e a t e d by a s s u m i n g c o n s t a n t p r o p e r t y r e l a t i o n -s h i p s . Simoneau and W i l l i a m s a l s o c o n c l u d e d t h a t t h e c o u e t t e f l o w c a s e may p r o v e u s e f u l b e c a u s e o f t h e s i m i l a r i t y w i t h b o u n d a r y l a y e r f l o w b u t s t r e s s e d t h a t no p r o p e r t y v a r i a t i o n s a l o n g t h e p l a t e s have been i n c l u d e d . V e r y r e c e n t l y Graham [32] has s u g g e s t e d a model o f t h e s u p e r c r i t i c a l h e a t t r a n s f e r mechanism i n v o l v i n g a t i m e - d e p e n d e n t p e n e t r a t i o n mechanism, somewhat s i m i l a r t o b o i l i n g . Graham c o n s i d e r e d t h e h e a t t r a n s f e r p r o c e s s t o be augmented by f l u i d p a c k e t s w h i c h s p r i n g d i r e c t l y f r o m t h e w a l l r e g i o n i n t o t h e f l u i d s t r e a m . He p r e s e n t e d t y p i c a l c o r r e l a t i o n s w h i c h e n a b l e d a p e n e t r a t i o n model t o e x p l a i n d a t a t a k e n w i t h n e a r - c r i t i c a l h y d r o g e n , c a r b o n d i o x i d e , and w a t e r w h i c h c o u l d n o t be p r e d i c t e d by a t u r b u l e n t p i p e - f l o w h e a t t r a n s f e r c o r r e l a t i o n . Graham c o n c l u d e d t h a t i n some c a s e s t h e p r e d i c t i o n s u s i n g a p e n e t r a t i o n m odel gave much c l o s e r v a l u e s than, c l a s s i c a l t e c h n i q u e s b u t t h a t much f u r t h e r work i n t o t h e i n f l u e n c e o f t h e s u b - l a y e r s t r u c t u r e i n n e a r -c r i t i c a l f o r c e d f l o w i s r e q u i r e d . I n summary, a s t u d y o f f o r c e d c o n v e c t i o n h e a t t r a n s f e r mechanisms and h e a t t r a n s f e r r a t e s i n e x t e r n a l f l o w 26 s i t u a t i o n s has n o t been done and i s r e q u i r e d f o r f l u i d s i n t h e n e a r - c r i t i c a l r e g i o n t o a i d i n t h e d e v e l o p m e n t o f h e a t t r a n s f e r c o r r e l a t i o n s f o r t e c h n i c a l a p p l i c a t i o n s . 1.6 Scope o f T h i s I n v e s t i g a t i o n i P r e v i o u s r e s u l t s [18, 21] i n n e a r - c r i t i c a l h e a t t r a n s f e r had i n d i c a t e d t h a t a s e c o n d a r y h e a t t r a n s f e r mechanism w h i c h g r e a t l y a f f e c t e d h e a t t r a n s f e r r a t e m i g h t be p o s s i b l e w i t h a h o r i z o n t a l h e a t e d c y l i n d e r . O t h e r r e s u l t s [13, 20] had i n d i c a t e d t h a t t h e h e a t t r a n s f e r c o e f f i c i e n t c o u l d show l a r g e peak v a l u e s i n t h e n e a r - c r i t i c a l r e g i o n and t h a t u n u s u a l e f f e c t s on h e a t t r a n s f e r r a t e m i g h t be p o s s i b l e . The e n h a n c e d mode o f h e a t t r a n s f e r p r e v i o u s l y o b s e r v e d had b e e n a c c o m p a n i e d by l a r g e c y l i n d e r t e m p e r a t u r e and c u r r e n t o s c i l l a t i o n s and t h e r e was some d o u b t as t o t h e a c t u a l r e g i o n i n w h i c h t h i s i m p r o v e d r e g i m e o f h e a t t r a n s f e r c o u l d be e x p e c t e d . F o r t h e p r e s e n t s t u d y i t was f e l t t h a t a h e a t e d c y l i n d e r h e l d a t c o n s t a n t t e m p e r a t u r e , as i s an anemometer p r o b e , w o u l d be a very, u s e f u l t o o l t o i n d i c a t e h e a t t r a n s f e r b e h a v i o u r u n d e r an o s c i l l a t i n g mechanism as h e a t t r a n s f e r r a t e o s c i l l a t i o n s w o u l d o n l y show up as c u r r e n t f l u c t u a t i o n s . I t was a l s o f e l t t h a t i f a r e g i o n o f v a s t l y 1 i m p r o v e d h e a t t r a n s f e r e x i s t e d i n f r e e c o n v e c t i o n a f r e e -s t r e a m v e l o c i t y m i g h t i m p r o v e t h e h e a t t r a n s f e r r a t e s t i l l 27 f u r t h e r and be v e r y u s e f u l i n modern a p p l i c a t i o n s . I t was a l s o f e l t t h a t i n v e s t i g a t i o n o f t h e h e a t t r a n s f e r mechanism f r o m t h e s i m p l e g e o m e t r y o f a h o r i z o n t a l h e a t e d c y l i n d e r m i g h t i n d i c a t e what f a c t o r s were c a u s i n g t h e l a r g e d i s c r e p -a n c i e s r e p o r t e d i n most n e a r - c r i t i c a l h e a t t r a n s f e r s t u d i e s . A c c o r d i n g l y t h e p r e s e n t e x p e r i m e n t a l s t u d y was d e s i g n e d w i t h t h e i d e a o f u s i n g a c o n s t a n t t e m p e r a t u r e h e a t e d c y l i n d e r t o f i r s t l y i n v e s t i g a t e f r e e c o n v e c t i o n e f f e c t s and i s o l a t e t h e f a c t o r s a f f e c t i n g t h e h e a t t r a n s f e r r a t e and s e c o n d l y t o i n v e s t i g a t e t h e e f f e c t o f a f r e e - s t r e a m v e l o c i t y on any i m p r o v e d r e g i o n s o f h e a t t r a n s f e r . P r e v i o u s s t u d i e s [18, 26, 20] had i n d i c a t e d t h a t u n u s u a l h e a t t r a n s f e r e f f e c t s were most l i k e l y n e a r t h e c r i t i c a l p r e s s u r e , and when t h e b u l k f l u i d was i n t h e l i q u i d - l i k e c o n d i t i o n , and i f t h e c y l i n d e r t e m p e r a t u r e was w e l l above t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . P r e s e n t o p e r a t i n g c o n d i t i o n s were c h o s e n t o i n v e s t i g a t e t h e e f f e c t s o f b u l k t e m p e r a t u r e , and b u l k p r e s s u r e w i t h b o t h l i q u i d - l i k e and v a p o r - l i k e f r e e - s t r e a m f l u i d . 28 2. EXPERIMENTAL APPARATUS 2.1 G e n e r a l C o n c e p t o f t h e E x p e r i m e n t a l A p p a r a t u s The p r e s e n t s t u d y i s c o n c e r n e d w i t h m e a s u r i n g h e a t t r a n s f e r r a t e s f r o m a h e a t e d c y l i n d e r mounted i n a u n i f o r m s t r e a m o f s u p e r c r i t i c a l c a r b o n d i o x i d e . I t was r e q u i r e d t h a t t h e f l u i d b u l k t e m p e r a t u r e , f l u i d b u l k p r e s s u r e and t h e f r e e - s t r e a m v e l o c i t y t h r o u g h t h e t e s t s e c t i o n be v a r i a b l e . D e t e r m i n a t i o n o f t h e mechanisms o f h e a t t r a n s f e r i n t h e c r i t i c a l r e g i o n was a p r i m e g o a l o f t h i s s t u d y so i t was n e c e s s a r y t h a t t h e f l o w f i e l d s p r o d u c e d c o u l d be o b s e r v e d and p h o t o g r a p h e d . C a r b o n d i o x i d e was c h o s e n as t h e w o r k i n g f l u i d b e c a u s e t h e c r i t i c a l s t a t e i s e a s i l y r e a c h e d i n t h e l a b o r a t o r y . C a r b o n d i o x i d e a l s o has t h e a d v a n t a g e s t h a t i t i s n o n - t o x i c , does n o t r e a c t o r decompose e a s i l y , i s r e a d i l y a v a i l a b l e , and t h e p h y s i c a l p r o p e r t i e s i n t h e c r i t i c a l r e g i o n have been e x t e n s i v e l y s t u d i e d and documented. . The h e a t e d c y l i n d e r was mounted i n a c l o s e d c i r c u i t , r e c i r c u l a t i n g f l o w l o o p . The t e s t f l u i d was c i r c u l a t e d by a c e n t r i f u g a l c o n s t a n t - r p m c a n n e d - r o t o r pump and f l o w v e l o c i t y t h r o u g h t h e t e s t s e c t i o n was c o n t r o l l e d by a b y p a s s a r r a n g e m e n t . The s y s t e m p r e s s u r e was m a i n t a i n e d by a s e t o f f r e e - p i s t o n h y d r a u l i c a c c u m u l a t o r s u s i n g c o m p r e s s e d a i r as t h e h i g h p r e s s u r e f l u i d . The s y s t e m b u l k f l u i d t e m p e r a t u r e was l o w e r e d 29 by c o l d w a t e r h e a t e x c h a n g e r s and r a i s e d by f r i c t i o n h e a t i n g f r o m t h e r e c i r c u l a t i n g pump. A s c h e m a t i c o f t h e f l o w l o o p i s shown i n F i g u r e ( 6 ) . The main t e s t s e c t i o n was a v e r t i c a l s t a i n l e s s s t e e l t u b e 2 i n c h e s i n d i a m e t e r and 3 f e e t l o n g w i t h a remov-a b l e t e s t s e c t i o n b l o c k l o c a t e d a t m i d - h e i g h t . The t e s t s e c t i o n b l o c k was f i t t e d w i t h t e m p e r e d p y r e x v i e w i n g p o r t s . The t e s t s e c t i o n b l o c k was a l s o a h i g h p r e s s u r e chamber t o house t h e t e s t s e c t i o n i n s e r t . The t e s t s e c t i o n i n s e r t s m o o t h l y c h a n g e d t h e f r e e - s t r e a m f r o m t h e r o u n d t u b e i n t o t h e p a r a l l e l - s i d e d v i e w i n g s e c t i o n , and was f o u n d t o be n e c e s s a r y t o e l i m i n a t e d i s t u r b a n c e s t o t h e f r e e - s t r e a m c a u s e d by t h e v i e w i n g p o r t s . A s e c t i o n i l l u s t r a t i n g t h e smooth t r a n s i t i o n s e c t i o n i s shown i n F i g u r e ( 7 ) . The f l o w c o n d i t i o n i n g s e c t i o n c o n s i s t e d o f a p a c k e d t e f l p n - w o o l m a t r i x , a honeycomb s e c t i o n , and a s e r i e s o f v e r y f i n e s c r e e n s and was l o c a t e d i n t h e main t e s t s e c t i o n u p s t r e a m o f t h e h e a t e d c y l i n d e r . The g e n e r a l a r r a n g e m e n t o f t h e h e a t e d c y l i n d e r and t h e t e s t s e c t i o n components i s shown i n t h e removed s e c t i o n o f F i g u r e ( 8 ) . The h e a t e d c y l i n d e r s u s e d i n t h i s s t u d y were e i t h e r f i n e n i c h r o m e w i r e s o r s p e c i a l l y m o d i f i e d h o t f i l m anemometer p r o b e s . Power was s u p p l i e d t o t h e h e a t e d c y l i n d e r by a h o t w i r e anemometer s e t , f a c t o r y m o d i f i e d t o p r o v i d e power l e v e l s fi 2 o f more t h a n 2 x 10 B T U / f t h r . The c y l i n d e r t e m p e r a t u r e * was d e t e r m i n e d by t h e o v e r h e a t r a t i o u s e d on t h e anemometer O v e r h e a t r a t i o P r o b e o p e r a t i n g r e s i s t a n c e . P r o b e r e s i s t a n c e a t b u l k f l u i d t e m p e r a t u r e s e t . Power d i s s i p a t e d by t h e h e a t e d c y l i n d e r was d e t e r m i n e d f r o m t h e c u r r e n t s u p p l i e d t o t h e c y l i n d e r and t h e c y l i n d e r o p e r a t i n g r e s i s t a n c e . The anemometer s e t c o n t a i n e d a f e e d -back l o o p w h i c h k e p t t h e c y l i n d e r r e s i s t a n c e and h e n c e t h e c y l i n d e r t e m p e r a t u r e a t t h e s e t v a l u e by v a r y i n g t h e c u r r e n t s u p p l i e d . The h e a t e d f l o w f i e l d p r o d u c e d i n t h e S c h l i e r e n s y s t e m was m o n i t o r e d d u r i n g a l l d a t a r u n s and s t i l l 35 mm p h o t o g r a p h s as w e l l as h i g h - s p e e d 16 mm m o t i o n p i c t u r e s a t 5000 pps were t a k e n a t a l l o p e r a t i n g c o n d i t i o n s o f i n t e r e s t . B u l k f l u i d t e m p e r a t u r e was m e a s u r e d by 2 s h e a t h e d t h e r m o c o u p l e s immersed d i r e c t l y i n t h e t e s t f l u i d and d i s -p l a y e d on a s p e c i a l l y c a l i b r a t e d b a l l i s t i c g a l v a n o m e t e r . T e m p e r a t u r e o f t h e b u l k f l u i d was m easured b o t h i n t h e t e s t s e c t i o n b l o c k and downstream o f t h e v e n t u r i m e t e r . System b u l k p r e s s u r e was m easured by a H e i s e b o u r d o n t y p e p r e s s u r e gauge r e a d i n g t o 1500 p s i a and f a c t o r y c a l i b r a t e d t o 0.1% a c c u r a c y . The b u l k p r e s s u r e t a p was l o c a t e d i n t h e s t a g n a n t f l u i d between t h e t e s t s e c t i o n i n s e r t and t h e t e s t s e c t i o n b l o c k . V e l o c i t y t h r o u g h t h e t e s t s e c t i o n was d e t e r m i n e d f r o m t h e d i f f e r e n t i a l p r e s s u r e d e v e l o p e d a c r o s s a v e n t u r i m e t e r l o c a t e d i n t h e main f l o w l o o p . D i f f e r e n t i a l p r e s s u r e was d i s p l a y e d on a B a r t o n d i f f e r e n t i a l p r e s s u r e m e t e r c a p a b l e o f m e a s u r i n g 0 t o 10 p s i a t a w o r k i n g p r e s s u r e up t o 5000 p s i a . A d d i t i o n a l s p e c i f i c d e t a i l s on t h e d e s i g n , c o n -s t r u c t i o n , and s p e c i a l f e a t u r e s o f t h e v a r i o u s components u s e d i n t h i s s t u d y a r e g i v e n i n t h e n e x t s e c t i o n . 2.2 D e t a i l s o f A p p a r a t u s Components 2.2.1 M a i n T e s t S e c t i o n The main t e s t s e c t i o n was m a n u f a c t u r e d i n t h e UBC M e c h a n i c a l E n g i n e e r i n g W o r k s h i p f r o m a s e c t i o n o f 2 1/2 i n c h o.d. t h i c k w a l l (0.25 i n c h ) t y p e 316 s t a i n l e s s s t e e l t u b i n g . Two i d e n t i c a l s e c t i o n s o f t u b i n g 18 i n c h e s l o n g were o b t a i n e d . C i r c u l a r f l a n g e s were w e l d e d t o one end o f e a c h t u b e and s q u a r e f l a n g e s t o t h e o t h e r . The c i r c u l a r f l a n g e s were d r i l l e d t h r o u g h and t a p p e d t o a c c e p t 1/2 i n c h NPT f i t -t i n g s . The s q u a r e f l a n g e s had been b o r e d t o t h e i . d . o f t h e t u b e and a c o n c e n t r i c O - r i n g g r o o v e m i l l e d i n t h e f l a n g e f a c e . The c i r c u l a r f l a n g e s were d e s i g n e d t o a c t as s u p p o r t s and as a d a p t o r s f o r t h e f l o w l o o p t u b i n g . The s q u a r e f l a n g e s were d r i l l e d t h r o u g h w i t h f o u r m o u n t i n g h o l e s and a r e b o l t e d t o t h e t e s t s e c t i o n b l o c k . T e f l o n O - r i n g s were u s e d between t h e f l a n g e s and t h e t e s t s e c t i o n b l o c k . The e n t i r e a s s e m b l y was d e s i g n e d t o be s y m m e t r i c a l so t h a t t h e t e s t s e c t i o n b l o c k c a n be mounted w i t h t h e o p t i c a l p a t h i n e i t h e r o f two o r i e n t a t i o n s . The t e s t s e c t i o n b l o c k c a n a l s o be i n v e r t e d i f r e q u i r e d . C o p p e r t u b e s 3 i n c h e s i n d i a m e t e r were a l s o s l i p p e d o v e r e a c h t u b e s e c t i o n . The a n n u l u s f o r m e d was sealed with sp l i t rubber bushings and was orig inal ly intended to be used as a system heat exchanger. These heat exchangers were only used during f i l l i n g of the test loop as coolers but were used to insulate the test section in some data runs. The test section block is a rectangular block of type 304 stainless steel 3 inches by 4 inches by 7 inches high. The 7 inch length was bored to match the i . d . of the main test section tubes. The upper and lower faces were d r i l l e d and taped 5/8 NC to accept the four mounting bolts . A 1 inch diameter optical path is bored through the 4 inch width at mid-height of the test section block. The viewing path is counterbored to accept the 1 1/2 inch diameter by 3/4 inch thick viewing ports. The viewing ports are optical quality tempered glass. A circumferential O-ring groove is cut at mid-depth of the window boss. This design results in more uniform stress distribution in the glass window. The window is a press f i t into the test block past the Teflon O-ring and is held in place by a threaded plug bored with a 1 inch diameter optical path. The stress produced by the mounting plug is distributed more evenly over the glass surface by a soft fibre washer. Details of the porthole assembly are shown in Figure (8 ). The test section block is also bored through the 3 inch face into the main flow passage. The holes are tapped 1/2 inch NPT for the Conax transducer glands. 33 2.2.2 F l o w l o o p t u b i n g a n d V a l v e s A l l t u b i n g u s e d i n t h e t e s t l o o p was t y p e 316 s t a i n l e s s s t e e l . O n e - h a l f i n c h o.d. t h i n w a l l t u b i n g was u s e d f o r t h e main f l o w l o o p and 0.25 i n c h o.d. t h i n w a l l t u b i n g was u s e d t o c o n n e c t a l l a u x i l i a r y e q u i p m e n t . I n i t i a l -l y a l l 1/2 i n c h o.d. t u b i n g was 0.027 i n c h w a l l t h i c k n e s s b u t a change t o 0.035 i n c h w a l l t h i c k n e s s t u b i n g was r e q u i r e d f o r t h e p a r t s o f t h e l o o p f r e q u e n t l y t a k e n a p a r t as t h e t h i n w a l l t u b i n g d i d n o t s e a l w e l l i n t h e f e r r u l e t y p e f i t t i n g s . No l e a k a g e p r o b l e m s o c c u r r e d w i t h t h e s m a l l e r o.d. a u x i l i a r y t u b i n g . A l l f r e q u e n t l y d i s a s s e m b l e d f i t t i n g s i n t h e a p p a r a t u s were t y p e 316 s t a i n l e s s s t e e l . L a c k o f a v a i l -a b i l i t y o f s p e c i a l f i t t i n g s i n s t a i n l e s s s t e e l f o r c e d t h e ; use o f some c a d m i u m - p l a t e d m i l d s t e e l f i t t i n g s . Cadmium p l a t e d f i t t i n g s were n o t a c c e p t a b l e f o r t h e f r e q u e n t l y d i s a s s e m b l e d s e c t i o n s as t h e p l a t i n g was e a s i l y s p a l l e d o f f and e x p o s e d t h e m i l d s t e e l . A l l v a l v e s u s e d were W o r c h e s t e r b a l l v a l v e s o f t y p e 316 s t a i n l e s s s t e e l w i t h D e l r i n s e a t s . T h e s e v a l v e s f e a t u r e d c o m p l e t e l y u n o b s t r u c t e d f l o w when f u l l y open and e a s y i n - l i n e d i s a s s e m b l y . The v a l v e s had o n l y a v e r y s m a l l l e a k a g e r a t e t h r o u g h t h e s e a t e v e n w i t h t h e v a p o r - l i k e s u p e r c r i t i c a l f l u i d w i t h b u l k t e m p e r a t u r e s as h i g h as 105 deg F. C o m p r e s s i o n work d u r i n g r a p i d c h a n g es i n b u l k s y s t e m p r e s s u r e c o u l d c a u s e d i f f e r e n t i a l h e a t i n g o f t h e v a l v e body and a l l o w l e a k a g e t o a t m o s p h e r e . T h i s l e a k a g e was m i n i m i z e d by c i r c u l a t i n g t h e t e s t f l u i d d u r i n g any l a r g e s y s t e m p r e s s u r e c h a n g e s . 34 2.2.3 F l o w C o n d i t i o n i n g and T r a n s i t i o n S e c t i o n One o f t h e p r i m a r y g o a l s o f t h i s e x p e r i m e n t a l i n v e s t i g a t i o n was t o p r o v i d e an e x p l a n a t i o n o f t h e mechanisms o f h e a t t r a n s f e r i n t h e n e a r c r i t i c a l r e g i o n . I t was t h e r e -f o r e f e l t t h a t v i s u a l i z a t i o n o f t h e r e s u l t a n t f l o w f i e l d s was n e c e s s a r y . The f l o w a p p r o a c h i n g t h e h e a t e d c y l i n d e r was a l s o r e q u i r e d t o be u n i f o r m i n t e m p e r a t u r e as w e l l as v e l o c i t y . The c o n n e c t i n g t u b i n g f r o m the- pump was 1/2 i n c h d i a m e t e r t u b i n g and t h e f l o w e n t e r e d t h e l a r g e r t e s t s e c t i o n as a w e l l o r g a n i z e d j e t . I t was n e c e s s a r y t o b r e a k up t h i s j e t and p r o v i d e u n i f o r m f l o w t h r o u g h t h e one i n c h s q u a r e t e s t s e c t i o n i n s e r t . The f l o w e n t e r e d t h e 2 i n c h d i a m e t e r t e s t s e c t i o n and was f i r s t p a s s e d t h r o u g h a s e r i e s o f t i g h t l y p a c k e d t e f l o n - w o o l pads t o b r e a k up t h e j e t i n t h e f i r s t 4 i n c h e s o f t h e t e s t s e c t i o n . The f l o w was a l l o w e d t o s e t t l e f o r a p p r o x i m a t e l y 1 i n c h and t h e n p a s s e d t h r o u g h a 10 i n c h l e n g t h o f 1/16 i n c h s q u a r e honeycomb t o remove t h e mean c r o s s m o t i o n . The f l o w was a g a i n a l l o w e d t o s e t t l e f o r a n o t h e r i n c h and t h e n p a s s e d t h r o u g h a s e r i e s o f t h r e e 1000-mesh s t a i n l e s s s t e e l s c r e e n s e a c h s e p a r a t e d by 1/2 i n c h . The Re b a s e d on s c r e e n w i r e d i a m e t e r was f r o m 10 t o 75 and t h e f l o w c o n t a i n e d o n l y m i n o r t e m p e r a t u r e and v e l o c i t y d i s t u r b a n c e s . The o u t l e t o f t h e s c r e e n s e c t i o n l e d d i r e c t l y i n t o t h e t r a n s i t i o n i n s e r t w h i c h s h a p e d t h e f l o w f r o m t h e c i r c u l a r c r o s s - s e c t i o n c o n d i t i o n i n g s e c t i o n t o t h e p a r a l l e l - s i d e d v i e w i n g r e g i o n . A cutaway v i e w o f t h e f l o w c o n d i t i o n i n g and t r a n s i t i o n s e c t i o n s i s shown as F i g u r e ( 7 ) . The t r a n s i t i o n i n s e r t was made o f v e r y l i g h t aluminum t u b i n g as t h e p r e s s u r e on e a c h s i d e o f t h e t u b i n g was e s s e n t i a l l y t h e same. The t r a n s i t i o n i n s e r t has o p t i c a l - q u a l i t y f l a t g l a s s windows s e t f l u s h w i t h t h e a p p r o a c h i n g c o n t o u r and p a r a l l e l t o t h e h i g h p r e s s u r e p o r t -h o l e s . I n i t i a l l y a l a r g e amount o f c i r c u l a t i o n was p r e s e n t between t h e t e s t s e c t i o n b l o c k and t h e t r a n s i t i o n i n s e r t due t o t h e p r e s s u r e d i f f e r e n t i a l d e v e l o p e d as t h e f l u i d a c c e l -e r a t e d t h r o u g h t h e t r a n s i t i o n s e c t i o n . T h i s c i r c u l a t i o n p r o b l e m hampered f l o w v i s u a l i z a t i o n b u t was c u r e d by c e m e n t i n g t h e t r a n s i t i o n i n s e r t i n t o t h e h i g h p r e s s u r e t e s t s e c t i o n b l o c k w i t h S i l i c o n e RTV-5. The RTV-5 cement does n o t h a r d e n r i g i d l y and t h e i n s e r t c o u l d s t i l l be p r e s s e d o u t o f t h e t e s t s e c t i o n b l o c k i f r e q u i r e d . A v e r y f i n e g r o o v e a r o u n d t h e t r a n s i t i o n i n s e r t a t window l e v e l and a s c r a t c h e x t e n d i n g t o t h e t o p o f t h e i n s e r t a l l o w e d p r e s s u r e e q u a l i z a t i o n w i t h o u t g r o s s f l u i d movement. The low v i s c o s i t y o f c a r b o n d i o x i d e i n t h e c r i t i c a l r e g i o n a l l o w e d l a r g e c i r c u l a t i o n due t o v e r y s m a l l d e n s i t y d i f f e r e n c e s . The q u a l i t y o f t h e f l u i d a f t e r p a s s i n g t h r o u g h t h e f l o w c o n d i t i o n i n g s e c t i o n was m o n i t o r e d by v i e w i n g t h e s c h l i e r e n image o f t h e f l o w p a s t t h e c y l i n d e r . The s e n s i t i v -i t y o f t h e s c h l i e r e n was v a r i a b l e and an i d e a o f t h e u n i f o r m -i t y o f t h e f l u i d was e a s i l y o b t a i n e d . The c o n d i t i o n i n g s e c t i o n was e v o l v e d by r e p e a t e d t e s t s and m o d i f i c a t i o n s u n t i l a s a t i s f a c t o r y q u a l i t y o f f l o w was r e a c h e d . The a v a i l a b l e d e s i g n c r i t e r i a f o r f l o w c o n d i t i o n i n g s e c t i o n s [33] and [34] a p p l y o n l y t o c o n s t a n t p r o p e r t y f l u i d s and t h e d i s t i n c t clumps o f h i g h d e n s i t y f l u i d e n t e r i n g t h e f l o w c o n d i t i o n i n g s e c t i o n r e q u i r e d t h a t m o d i f i c a t i o n s be made t o t h e c o n s t a n t p r o p e r t y f l u i d s p e c i f i c a t i o n s . A s e c o n d i n d i c a t i o n o f t h e q u a l i t y o f t h e f r e e - s t r e a m f l u i d was o b t a i n e d by m e a s u r i n g t h e rms component o f t h e s u p p l y v o l t a g e t o t h e h e a t e d c y l i n d e T h i s rms component was i n t e r p e r t e d as t h e combined e f f e c t o f V e l o c i t y f l u c t u a t i o n s and n o n - u n i f o r m f l u i d t e m p e r a t u r e . F o r a l l f i n a l d a t a r u n s t h e combined e f f e c t o f t h e t u r b u l e n t d i s t u r b a n c e s was l e s s t h a n 0.002 v o l t s rms on a s i g n a l v o l t a g e o f f r o m 1.0 t o 5.0 v o l t s . The rms s i g n a l o r q u a l i t y i n d e x r e m a i n e d a l m o s t c o n s t a n t w i t h c y l i n d e r t e m p e r a t u r e i n c r e a s e . I s o l a t e d d i s t u r b a n c e s up t o 0.04 v o l t s rms on a b a s e v o l t a g e o f 4.5 v o l t s were o c c a s i o n a l l y o b s e r v e d i n t h e s u b c r i t i c a l b o i l i n g t e s t s b u t t h e s e d i s t u r b a n c e s were a t t r i b u t e d t o t h e o s c i l l a t i n g c o n d i t i o n s d u r i n g t h e c o -e x i s t i n g f i l m a nd n u c l e a t e b o i l i n g . 2.2.4 C i r c u l a t i n g Pump The c i r c u l a t i n g pump u s e d i n t h i s s t u d y was a s t a i n l e s s s t e e l c a n n e d - r o t o r f i x e d rpm i m p e l l e r t y p e pump. The pump was m a n u f a c t u r e d by t h e CHEMPUMP d i v i s i o n o f CRANE CO. and i s a u n i q u e d e s i g n d e v e l o p e d t o e l i m i n a t e c o n t a m i n a t i o n and ; f o r l e a k p r o o f a p p l i c a t i o n s . The pump has o n l y one mo v i n g p a r t , a combined r o t o r - i m p e l l e r a s s e m b l y 37 d r i v e n by t h e m a g n e t i c f i e l d o f an i n d u c t i o n m o t o r . No e x t e r n a l l u b r i c a n t i s u s e d as t h e b e a r i n g s a r e l u b r i c a t e d by a s e c o n d a r y c i r c u l a t i o n o f t h e t e s t f l u i d . The pump was c a p a b l e o f o p e r a t i n g a t a b a s e p r e s s u r e o f up t o 2500 p s i a and i s r a t e d 49.5 U.S. gpm a t a head o f 138 f e e t o f w a t e r . I n t h e p r e s e n t s t u d y t h e f l o w r a t e t h r o u g h t h e t e s t s e c t i o n was v a r i e d by a b y p a s s a r r a n g e m e n t and t h e r e m a i n d e r o f t h e pump d i s c h a r g e r e -c i r c u l a t e d and c o o l e d i n a b y p a s s l o o p . The pump o p e r a t e s on 440 v o l t 3 p h a s e ac power a t 3450 rpm. The pump i n l e t and d i s c h a r g e were m o d i f i e d t o a c c e p t 1/2 i n c h NPT s t a i n l e s s s t e e l c o n n e c t o r s t o s u i t t h e p r e s e n t t e s t f a c i l i t y . Due t o t h e l a r g e volume o f r e c i r c u l a t e d f l u i d t h e pump s u p p l i e d enough f r i c t i o n h e a t i n g t o r a i s e t h e f l u i d b u l k t e m p e r a t u r e t o any d e s i r e d v a l u e and e l i m i n a t e d any ne e d f o r b u l k f l u i d h e a t e r s . No l e a k s , e x c e s s i v e v i b r a t i o n , o r o t h e r common o p e r a t i n g p r o b l e m s were n o t e d w i t h t h e pumping c i r c u i t u s e d ' i n t h i s s t u d y . 2.2.5 Heat E x c h a n g e r s As i t was n e c e s s a r y t o c o n t r o l t h e f l u i d b u l k t e m p e r a t u r e w i t h i n c l o s e l i m i t s a v a r i a b l e f l o w h e a t exchanger" was r e q u i r e d . I t was f o u n d f o r f o r c e d f l o w d a t a r u n s t h a t t h e pump running, a t f i x e d rpm s u p p l i e d enough h e a t i n g t o r a i s e t h e b u l k t e m p e r a t u r e t o any r e q u i r e d v a l u e . I n f a c t , t h e h e a t e x c h a n g e r a p p l i c a t i o n was f o r . c o o l i n g o n l y . I n f r e e c o n v e c t i o n d a t a r u n s no h e a t e x c h a n g e r s c o u l d be u s e d a t any p o i n t i n t h e a p p a r a t u s as t h e y g e n e r a t e d s t r o n g c o n v e c t i v e f l o w d i s t u r b a n c e s . The a m b i e n t t e m p e r a t u r e i n t h e l a b o r a t o r y was b e l o w t h e c r i t i c a l t e m p e r a t u r e and i t was f e l t t h a t c o o l i n g t h e f l u i d t o a t e m p e r a t u r e v e r y n e a r a m b i e n t w o u l d p r o v i d e s u f f i c i e n t s u b c o o l i n g t o i l l u s t r a t e t h e i n f l u e n c e o f t h e c r i t i c a l and p s e u d o - c r i t i c a l t e m p e r a t u r e s . The c o o l i n g mechanism s e l e c t e d was f o r c e d f l o w w a t e r h e a t e x c h a n g e r s u s i n g t h e d o m e s t i c c o l d w a t e r s u p p l y . T h e s e e x c h a n g e r s were o f v e r y s i m p l e d e s i g n ; a 1 1/2 i n c h d i a m e t e r s e c t i o n o f t h i n w a l l aluminum t u b i n g was s l i p p e d o v e r a s t r a i g h t s e c t i o n o f t h e 1/2 i n c h d i a m e t e r f l o w l o o p t u b i n g and t h e ends o f t h e r e s u l t i n g a n n u l u s s e a l e d w i t h r u b b e r p l u g s b o r e d t o s u i t t h e f l o w l o o p t u b i n g . The r u b b e r p l u g s were f o r c e d i n t o p l a c e and n o n - h a r d e n i n g s i l i c o n RTV-5 cement was u s e d t o p r e v e n t any l e a k s . The t h i n w a l l aluminum t u b i n g was b o r e d t o a c c e p t 1/4 i n c h p i p e t h r e a d c o n n e c t o r s , a g a i n s e a l e d w i t h s i l i c o n e RTV-5, and t h e w a t e r s u p p l y d e l i v e r e d v i a a v a r i a b l e f l o w v a l v e . T h e r e were t h r e e h e a t e x c h a n g e r s o f t h i s d e s i g n on t h e a p p a r a t u s ; two on t h e main f l o w l o o p and one on t h e b y p a s s l o o p . The w a t e r s u p p l y t o t h e e x c h a n g e r s was p i p e d i n p a r a l l e l and a l l u s e d a common d r a i n . F l o w c o u l d t h e r e f o r e be v a r i e d t o any o f t h e e x c h a n g e r s i n any c o m b i n a t i o n . F o r most d a t a r u n s t h e b y p a s s - l o o p h e a t e x c h a n g e r was on f u l l and a s u i t a b l e f l o w r a t e was a d j u s t e d t h r o u g h t h e two m a i n -l o o p e x c h a n g e r s . A s e n s i t i v e b a l a n c e was n e c e s s a r y between t h e two m a i n - l o o p e x c h a n g e r s as o t h e r w i s e t h e f l u i d a p p r o a c h -39 i n g t h e p r o b e w o u l d c o n t a i n clumps o f c o l d e r f l u i d f r o m t h e t u b i n g w a l l i n t h e e x c h a n g e r s . The l a r g e h e a t e x c h a n g e r s on t h e main f l o w l o o p c o n d i t i o n i n g s e c t i o n were u s e d o n l y as i n s u l a t o r s o r t h e r m a l c u s h i o n s t o m a i n t a i n t h e f l o w l o o p t e m p e r a t u r e . The l a r g e h e a t e x c h a n g e r s on t h e mai n t e s t s e c t i o n c o u l d n o t be "used t o c o n t r o l b u l k f l u i d t e m p e r a t u r e f o r e i t h e r f r e e o r f o r c e d c o n v e c t i o n d a t a r u n s . The l o w e r h e a t e x c h a n g e r was l o c a t e d a r o u n d t h e f l o w c o n d i t i o n i n g s e c t i o n and any c o o l i n g f r o m t h e h e a t e x c h a n g e r was o n l y t r a n s f e r r e d t o t h e f l u i d i n t h e o u t e r p a s s a g e s o f t h e honey-comb s e c t i o n . The lumps o f c o l d e r f l u i d d i d n o t b r e a k down i n t h e f i n e s c r e e n s , and c a u s e l a r g e f l u c t u a t i o n s when c a r r i e d p a s t t h e h e a t e d c y l i n d e r . The c o n v e c t i v e c u r r e n t s g e n e r a t e d a r e so s t r o n g t h a t t h e e n t i r e v e r t i c a l t e s t s e c t i o n b e g i n s -J t o a c t as a n a t u r a l c i r c u l a t i o n l o o p . C o o l i n g on t h e u p p e r main t e s t s e c t i o n h e a t e x c h a n g e r had t h e same e f f e c t , clumps o f c o l d f l u i d sank p a s t t h e h e a t e d c y l i n d e r and c a u s e d an i n d u c e d c i r c u l a t i o n i n t h e t e s t s e c t i o n . C o l d f l u i d sank down a l o n g t h e w a l l r e g i o n o f t h e t e s t s e c t i o n and f o r c e d t h e l i g h t e r f l u i d up t h r o u g h t h e c e n t e r s e c t i o n . T h i s c i r c u -l a t i o n was e a s i l y n o t e d on t h e s c h l i e r e n image o f t h e t e s t s e c t i o n . The h e a t e x c h a n g e r s d i d n o t p e r f o r m as w e l l as c o n s t a n t p r o p e r t y f l u i d c a l c u l a t i o n s p r e d i c t e d b u t t h i s was e x p e c t e d as i t i s one o f t h e p r o b l e m s a s s o c i a t e d w i t h a p p l i -c a t i o n s u s i n g a s u p e r c r i t i c a l f l u i d . The s i m p l e d e s i g n gave 40 no o p e r a t i n g p r o b l e m s and w o u l d p e r m i t e a s y a d d i t i o n o f more h e a t e x c h a n g e r c a p a c i t y on any s t r a i g h t r u n o f t u b i n g i f r e q u i r e d . 2.2.6 T r a n s d u c e r G l a n d s E l e c t r i c a l a c c e s s t o t h e i n t e r i o r o f t h e h i g h p r e s s u r e t e s t chamber t o s u p p l y power t o t h e h e a t e d c y l i n d e r was p r o v i d e d by two 2 - p o r t Conax t r a n s d u c e r g l a n d s . E a c h t r a n s d u c e r g l a n d had two p a s s a g e s b o r e d t o s e a l on l / 1 6 t h i n c h d i a m e t e r e l e c t r i c a l l e a d s . S e a l i n g was a c c o m p l i s h e d by f o r c i n g a s t e e l r o d a g a i n s t a t e f l o n p l u g t h u s d e f o r m i n g t h e t e f l o n p l u g a r o u n d t h e t r a n s d u c e r l e a d s . S o l i d c o p p e r r o d s were p a s s e d t h r o u g h one p o r t o f e a c h t r a n s d u c e r g l a n d t o s u p p l y power t o t h e p r o b e and a p r e s s u r e t a p was f e d t h r o u g h ± one t r a n s d u c e r g l a n d w h i l e one s h e a t h e d t h e r m o c o u p l e was f e d t h r o u g h t h e o t h e r g l a n d . The c o p p e r r o d s , t h e p r e s s u r e t a p t u b i n g , and t h e t h e r m o c o u p l e s h e a t h were a l l 1/16th i n c h d i a m e t e r and t h e power l e a d s were i s o l a t e d f r o m t h e p r e s s u r e and t e m p e r a t u r e t a p s by 2 - p o r t c e r a m i c i n s u l a t o r s . No l e a k s were n o t e d a t any t i m e a r o u n d t h e s e i n s t r u m e n t f i t t i n g s . 2.2.7 V e l o c i t y Measurement F l o w v e l o c i t y t h r o u g h t h e t e s t s e c t i o n was c a l c u l a t e d f r o m t h e d i f f e r e n t i a l p r e s s u r e d e v e l o p e d a c r o s s a v e n t u r i m e t e r downstream o f t h e t e s t s e c t i o n i n t h e main f l o w l o o p t u b i n g . The d i f f e r e n t i a l p r e s s u r e was m easured on a B a r t o n model 200 D i f f e r e n t i a l P r e s s u r e M e t e r , c a l i b r a t e d t o measure a d i f f e r -e n t i a l p r e s s u r e o f f r o m 0 t o 10 p s i a on a w o r k i n g p r e s s u r e up t o 3000 p s i a . The B a r t o n M e t e r i s a b e l l o w s - t y p e i n s t r u m e n t f e a t u r i n g i n t e r c h a n g e a b l e d i f f e r e n t i a l p r e s s u r e r a n g e s . A l l p a r t s o f t h e m e t e r i n c o n t a c t w i t h t h e t e s t f l u i d were t y p e 3.6 s t a i n l e s s s t e e l and s t a i n l e s s s t e e l t u b i n g was u s e d t o c o n n e c t t h e m e t e r , t h r o u g h i s o l a t i n g v a l v e s , t o t h e v e n t u r i m e t e r . The d i f f e r e n t i a l p r e s s u r e m e t e r was c a l i b r a t e d a t a t m o s p h e r i c b a s e p r e s s u r e u s i n g c a r b o n d i o x i d e gas i n e q u i l i b r i u m w i t h a m e r c u r y c o l u m n . The l i n e a r c a l i b r a t i o n c u r v e o b t a i n e d was assumed t o h o l d a t a l l d a t a r u n p r e s s u r e s . The v e n t u r i m e t e r was m a n u f a c t u r e d i n t h e w o r k s h i p a t t h e UBC M e c h a n i c a l E n g i n e e r i n g D e p a r t m e n t and was d e s i g n e d t o t h e s t a n d a r d ASME s p e c i f i c a t i o n s as d e s c r i b e d i n [ 3 5 ] . '• The v e n t u r i m e t e r was made i n two p a r t s ; an i n n e r f l o w s e c t i o n and an o u t e r s l e e v e c o n t a i n i n g t h e p r e s s u r e t a p s . The r a t i o o f t h r o a t d i a m e t e r t o u p s t r e a m d i a m e t e r was 0.5, t h e t h r o a t d i a m e t e r was 0.25 i n c h e s and t h e d i s c h a r g e c o e f f i c i e n t was t a k e n as 0.9 6 f o r t h e R e y n o l d s numbers, b a s e d on t h r o a t d i a m e t e r , u s e d i n t h e d a t a r u n s . The v e n t u r i m e t e r was i n -s t a l l e d w i t h o v e r 100 t u b e d i a m e t e r s o f s t r a i g h t t u b i n g u p s t r e a m o f t h e t h r o a t and a b u l k f l u i d t e m p e r a t u r e p r o b e was l o c a t e d 10 t u b e d i a m e t e r s downstream o f t h e t h r o a t . T h e s e d i s t a n c e s e x c e e d t h e minimum q u o t e d by [36] f o r a c c u r a t e v e n t u r i m e t e r measurements. V e l o c i t y was c a l c u l a t e d a s s u m i n g i n c o m p r e s s i b l e , c o n s t a n t p r o p e r t y f l o w t h r o u g h t h e v e n t u r i m e t e r and t h e f l o w 42 l o o p . The maximum f l o w v e l o c i t y o b t a i n e d t h r o u g h t h e m e t e r t h r o a t was c a l c u l a t e d a t l e s s t h a n 30% o f t h e s p e e d o f s o u n d i n t h e f l u i d a t t h e c o n d i t i o n s t e s t e d . P r o b a b l e a c c u r a c y o f t h e v e l o c i t y d e t e r m i n a t i o n i s d i s c u s s e d b e l o w i n A p p e n d i x ( I I I ) . 2.2.8 T e m p e r a t u r e Measurement F l u i d b u l k t e m p e r a t u r e was m e a s u r e d by e i t h e r o f two c o p p e r - c o n s t a n t a n t h e r m o c o u p l e s , s e t i n s t a i n l e s s s t e e l s h e a t h s , and a r r a n g e d as shown i n t h e c i r c u i t o f F i g u r e ( 1 0 ) . The t h e r m o c o u p l e s h e a t h s were l / 1 6 t h i n c h o.d. s e l e c t e d t o s e a l i n one p o r t o f a Conax t r a n s d u c e r g l a n d . The t h e r m o c o u p l e was e l e c t r i c a l l y i n s u l a t e d f r o m t h e s h e a t h by powdered c e r a m i c and t h e s h e a t h s w i t h t h e r m o c o u p l e s were p u r c h a s e d as s e a l e d u n i t s f r o m t h e Conax Company. One t h e r m o c o u p l e was l o c a t e d i n t h e main t e s t s e c t i o n one i n c h above t h e h e a t e d c y l i n d e r , i n t h e f l u i d s t r e a m , b u t n o t i n t h e h e a t e d wake. The s e c o n d t h e r m o c o u p l e was l o c a t e d 10 t u b e d i a m e t e r s downstream o f t h e v e n t u r i m e t e r t h r o a t . The r e m a i n d e r o f t h e t e m p e r a t u r e measur-i n g c i r c u i t c o n s i s t e d o f a c o p p e r - c o n s t a n t a n c o l d j u n c t i o n and s w i t c h i n g t o r e a d e i t h e r t h e r m o c o u p l e p o t e n t i a l on a L e e d s and N o r t h r u p M o d e l K-5 p o t e n t i o m e t e r . The s l i g h t i m b a l a n c e o f t h e c i r c u i t due t o s m a l l t e m p e r a t u r e c h a n g e s d u r i n g a t e s t r u n was measured on a Pye Scalamp g a l v a n o m e t e r c a l i b r a t e d t o r e a d t e m p e r a t u r e d i f f e r e n c e d i r e c t l y . The s t a n d a r d c e l l u s e d f o r r e f e r e n c e v o l t a g e was a PYE Cadmium r e f e r e n c e c e l l . The v o l t a g e s o u r c e u s e d t o o b t a i n s t a b l e i n p u t power t o t h e m e a s u r i n g c i r c u i t was one c e l l o f a 6 v o l t l e a d - a c i d s t o r a g e b a t t e r y . The c o l d j u n c t i o n was immersed i n a t h e r m a l f l a s k c o n t a i n i n g a mix-t u r e o f d i s t i l l e d w a t e r and i c e . The t h e r m o c o u p l e s were c a l i b r a t e d i n a C a l o r a c o n s t a n t t e m p e r a t u r e b a t h f i l l e d w i t h d i s t i l l e d w a t e r . C a l i b r a t i o n was by c o m p a r i s o n w i t h two P r e c i s i o n 18 i n c h m e r c u r y - i n - g l a s s t h e r m o m e t e r s p r e v i o u s l y c a l i b r a t e d [37] a g a i n s t a Dymec M o d e l 2801A q u a r t z - c r y s t a l -4 d i g i t a l - t h e r m o m e t e r a c c u r a t e t o 10 deg C. A f t e r two s e p a r a t e c a l i b r a t i o n s o v e r an e i g h t month p e r i o d i t was f o u n d t h a t b o t h t h e r m o c o u p l e p o t e n t i a l s were w i t h i n 1% o f t h e s t a n d a r d t a b u l a t e d v a l u e s f o r c o p p e r - c o n s t a n t a n and t h i s s t a n d a r d c a l i b r a t i o n was u s e d f o r t e m p e r a t u r e d e t e r m i n a t i o n . 2.2.9 P r e s s u r e Measurement System b u l k p r e s s u r e was m easured by a 0 t o 1500 p s i a H e i s e B o u r d o n t u b e p r e s s u r e gauge, f a c t o r y c e r t i f i e d a g a i n s t a d e a d w e i g h t t e s t e r t o be a c c u r a t e t o 0.1% o f t h e f u l l s c a l e r e a d i n g . The b u l k p r e s s u r e s i g n a l was t r a n s f e r r e d t o t h e p r e s s u r e gauge by a l / 1 6 t h i n c h d i a m e t e r s t a i n l e s s s t e e l t u b e p a s s i n g o u t o f t h e h i g h p r e s s u r e t e s t b l o c k t h r o u g h one p o r t o f a Canax t r a n s d u c e r g l a n d . The s y s t e m p r e s s u r e p r o b e was l o c a t e d i n t h e t e s t s e c t i o n a p p r o x i m a t e l y t h r e e i n c h e s above t h e h e a t e d c y l i n d e r b u t e x t e n d e d o n l y 44 t o t h e edge o f t h e t e s t s e c t i o n i n s e r t so t h a t i t was immersed i n s t a g n a n t f l u i d and m e asured o n l y s t a t i c p r e s s u r e . 2.2.10 Hot W i r e Anemometer Power S u p p l y The e x p e r i m e n t a l h e a t e d c y l i n d e r , o r p r o b e , was m a i n t a i n e d a t c o n s t a n t t e m p e r a t u r e by c u r r e n t s u p p l i e d f r o m a ThermoSystems I n c . M o d e l 1010-A W i r e Anemometer S e t f a c t o r y m o d i f i e d t o p r o v i d e 100 w a t t s o f power and r e t a i n n o r m a l h o t w i r e o p e r a t i n g c h a r a c t e r i s t i c s . Hot w i r e o r h o t f i l m o p e r a t i o n s u s u a l l y r e q u i r e l e s s t h a n 1 w a t t o f power t o m a i n -t a i n t h e h e a t e d p r o b e s a t c o n s t a n t t e m p e r a t u r e b u t t h e l a r g e power r e q u i r e m e n t i n t h e p r e s e n t s t u d y was due t o t h e l a r g e s u r f a c e a r e a o f t h e p r o b e and t o t h e v e r y h i g h h e a t t r a n s f e r c o e f f i c i e n t s a t t a i n e d i n n e a r - c r i t i c a l c a r b o n d i o x i d e . The anemometer c i r c u i t c o n s i s t e d o f a w h e a t s t o n e b r i d g e a r r a n g e -ment w h i c h had t h e t e s t p r o b e as one v a r i a b l e l e g and a d e c a d e r e s i s t a n c e box as t h e o t h e r v a r i a b l e l e g . The b r i d g e h a d a f e e d b a c k l o o p w h i c h s u p p l i e d s u f f i c i e n t power t o r a i s e t h e h e a t e d c y l i n d e r r e s i s t a n c e so t h a t t h e b r i d g e was i n b a l a n c e . V a r y i n g t h e d e c a d e r e s i s t a n c e t h e r e f o r e v a r i e d t h e h e a t e d c y l i n d e r o p e r a t i n g r e s i s t a n c e and t h e r e f o r e t h e h e a t e d c y l i n d e r t e m p e r a t u r e . The r a t e o f r e s i s t a n c e change w i t h t e m p e r a t u r e change f o r t h e t e s t c y l i n d e r s was d e t e r m i n e d e x p e r i m e n t a l l y u s i n g t h e b u i l t - i n r e s i s t a n c e m e a s u r i n g c i r c u i t o f t h e anemometer s e t . The d e c a d e r e s i s t a n c e o f t h e Thermo-45 Systems I n c . anemometer s e t was d e s i g n e d t o r e a d p r o b e o p e r a t -i n g r e s i s t a n c e d i r e c t l y . The anemometer s e t was d e s i g n e d f o r use w i t h p r o b e s o f o p e r a t i n g r e s i s t a n c e between 0.10 and 3 9 . 9 9 ohms.. O p e r a t i n g r e s i s t a n c e was v a r i a b l e o v e r t h e e n t i r e r a n g e i n 0.01 ohm s t e p s . T h e s e s t e p s o f 0.01 ohms c o r r e s p o n d t o a p r o b e t e m p e r a t u r e v a r i a t i o n o f a p p r o x i m a t e l y 2 deg F a t low t e m p e r a t u r e d i f f e r e n c e s . I n n o r m a l h o t w i r e o r h o t f i l m anemometry a p p l i c a -t i o n s t h e o v e r h e a t r a t i o i s f i x e d and a f t e r p r o b e c a l i b r a t i o n a t t h i s o v e r h e a t , t h e power r e q u i r e d t o m a i n t a i n t h e h e a t e d s e n s o r a t t e m p e r a t u r e i s i n t e r p r e t e d as a v e l o c i t y . F l u c t u -a t i o n s i n t h e mean power l e v e l a r e i n t e r p r e t e d as t u r b u l e n t d i s t u r b a n c e s . I n t h e p r e s e n t s t u d y t h e o v e r h e a t r a t i o was v a r i e d w i t h t h e p r o b e i n a c o n s t a n t v e l o c i t y s t r e a m and t h e ! power r e q u i r e d t o keep t h e h e a t e d c y l i n d e r a t t h e e l e v a t e d t e m p e r a t u r e s was meas u r e d . The c u r r e n t f l o w i n g t o t h e p r o b e was me a s u r e d on t h e b u i l t - i n d . c . c u r r e n t m e t e r s o f t h e anemometer s e t and t h e power d i s s i p a t e d was c a l c u l a t e d u s i n g t h e p r o b e o p e r a t i n g r e s i s t a n c e . The o v e r h e a t r a t i o was u s e d t o c a l c u l a t e t h e c y l i n d e r t e m p e r a t u r e u s i n g t h e e x p e r i m e n t a l l y d e t e r m i n e d c o e f f i c i e n t o f r e s i s t a n c e change w i t h t e m p e r a t u r e . The rms component o f t h e p r o b e s u p p l y v o l t a g e was a l s o m o n i - -t o r e d and i n t e r p r e t e d as t h e combined e f f e c t o f t u r b u l e n t f l u c t u a t i o n s and n o n - u n i f o r m t e m p e r a t u r e i n t h e b u l k f l u i d . The rms component was me a s u r e d u s i n g a H e w l e t t P a c k a r d M o d e l 3400A rms v o l t m e t e r c a p a b l e o f m e a s u r i n g t o 0.001 v o l t s rms f u l l s c a l e w i t h a time- c o n s t a n t o f 2 s e c o n d s . : 46 The use o f a h o t w i r e anemometer power s u p p l y t o h e a t a t e s t p r o b e and p r o v i d e h e a t t r a n s f e r d a t a h a d many a d v a n t a g e s . The t e s t p r o b e t e m p e r a t u r e was known q u i t e a c c u r a t e l y f r o m t h e o p e r a t i n g r e s i s t a n c e and t h e i n s t a n t a n e o u s power d i s s i p a t e d by t h e h e a t e d c y l i n d e r c o u l d be m o n i t o r e d . S h a r p c h a nges i n h e a t t r a n s f e r r a t e , s u c h as e n c o u n t e r e d i n t r a n s i e n t f i l m b o i l i n g , were i n d i c a t e d o n l y by a d e c r e a s e i n power s u p p l i e d t o t h e p r o b e . R e g i o n s i n w h i c h c o n s t a n t f l u x h e a t i n g w o u l d have d e s t r o y e d t h e t e s t p r o b e were i n v e s t i g a t e d s u c c e s s f u l l y . 2.2.11 P r o b e s and H e a t e d C y l i n d e r s I t was o r i g i n a l l y i n t e n d e d t o use a m o d i f i e d h o t f i l m p r o b e as t h e h e a t e d c y l i n d e r f o r t h e p r e s e n t s t u d y . Hot * f i l m p r o b e s u s u a l l y c o n s i s t o f a t h i n l a y e r o f p l a t i n u m ( s e l e c t e d f o r t h e h i g h c o e f f i c i e n t o f r e s i s t a n c e change w i t h t e m p e r a t u r e ) d e p o s i t e d on a q u a r t z c y l i n d e r and c o a t e d w i t h an e l e c t r i c a l l y i n s u l a t i n g f i l m o f q u a r t z ( s e l e c t e d b e c a u s e o f t h e r e a s o n a b l e t h e r m a l c o n d u c t i v i t y and low e l e c t r i c a l c o n d u c t i v i t y ) . U s u a l h o t f i l m p r o b e s a r e a p p r o x i m a t e l y 0.010 i n c h e s i n d i a m e t e r and t h e c y l i n d e r i s a b o u t 0.10 i n c h e s l o n g . The r e q u i r e d h e a t e d c y l i n d e r was t o be a p p r o x i m a t e l y 0.010 i n c h e s i n d i a m e t e r and 0.6 i n c h e s l o n g and have a c o l d r e s i s t a n c e o f 6 t o 10 ohmns. Two s u c h p r o b e s were p r o d u c e d by ThermoSystems I n c . and i n s t a l l e d i n t h e e x p e r i m e n t a l a p p a r a t u s . Q u a l i t a t i v e d a t a was t a k e n and p r o b e c a l i b r a t i o n 47 was c o m p l e t e d f o r one p r o b e and t h e p r o b e f a i l e d . The s e c o n d p r o b e f a i l e d d u r i n g c a l i b r a t i o n . B o t h f a i l u r e s were due t o d i f f e r e n t i a l t h e r m a l e x p a n s i o n between t h e o u t e r q u a r t z s h i e l d and t h e p l a t i n u m l a y e r w h i c h c a u s e d l a r g e a r e a s o f t h e p l a t i n u m t o s p a l l o f f . S e v e r a l a d d i t i o n a l p r o b e s were d e s i g n e d and b u i l t by ThermoSystems b u t a l l f a i l e d d u r i n g c a l i b r a t i o n . D u r i n g t h i s p e r i o d a n i c h r o m e w i r e p r o b e was m a n u f a c t u r e d by t h e a u t h o r and u s e d t o o b t a i n i n i t i a l d a t a . B e c a u s e o f t h e low c o e f f i c i e n t o f r e s i s t a n c e change w i t h t e m p e r a t u r e d a t a p o i n t s were r a t h e r f a r a p a r t . ThermoSystems d e v e l o p e d a p r o b e w i t h a p l a t i n u m f i l m on a p y r e x c y l i n d e r and c o a t e d t h e p l a t i n u m w i t h g o l d . A p r o b e o f t h i s c o n s t r u c t i o n was c a l i b r a t e d and i n i t i a l d a t a t a k e n . The p r o b e f a i l e d d u r i n g an a t t e m p t t o • t r i g g e r an o s c i l l a t i n g f l o w s i t u a t i o n b u t was e a s i l y r e p a i r e d . A s e c o n d g o l d - c o a t e d p l a t i n u m on p y r e x p r o b e was s u p p l i e d and u s e d f o r a l l f u r t h e r e x p e r i m e n t a l d a t a . D a t a p r e s e n t e d were t a k e n u s i n g t h e s e c o n d g o l d c o a t e d p r o b e b u t F i g u r e (11) shows t h e d a t a f r o m t h r e e d i f f e r e n t d a t a r u n s w i t h two d i f f e r e n t p r o b e s and i l l u s t r a t e s t h e r e p r o d u c i b i l i t y o f t h e d a t a . The h e a t e d c y l i n d e r u s e d was c a l i b r a t e d by i m m e r s i o n i n an o i l b a t h i n s i d e t h e c a l o r a c o n s t a n t t e m p e r a t u r e w a t e r b a t h and t h e r e s i s t a n c e m e a s u r e d , u s i n g t h e anemometer measur-i n g c i r c u i t , a t v a r i o u s t e m p e r a t u r e s f r o m 23 t o 98 deg C, The b e s t f i t l i n e a r c o e f f i c i e n t o f r e s i s t a n c e change w i t h t e m p e r a t u r e o b t a i n e d was assumed t o h o l d o v e r t h e e n t i r e 48 r a n g e o f c y l i n d e r t e m p e r a t u r e s s t u d i e d (0. t o 200 deg C ) . T h e . h e a t e d c y l i n d e r u s e d i s shown i n F i g u r e (12). The p r o b e d i a m e t e r was m e a s u r e d i n f o u r l o c a t i o n s u s i n g b o t h a v e r n i e r e y e p i e c e on a low power m i c r o s c o p e and t h e m i c r o -m e t e r s t a g e a d j u s t m e n t on a n o t h e r m i c r o s c o p e . The d i a m e t e r was u n i f o r m a l o n g t h e h e a t e d c y l i n d e r l e n g t h and a p h o t o -m i c r o g r a p h o f t h e p r o b e s u r f a c e i s shown as F i g u r e (12). H e a t e d c y l i n d e r l e n g t h was assumed f r o m m i d p o i n t o f t h e s u p p o r t s as i n d i c a t e d i n F i g u r e (12). 2.2.12 P h o t o g r a p h i c E q u i p m e n t and S c h l i e r e n S y s t e m The s c h l i e r e n s y s t e m u s e d i n t h i s s t u d y was a l e n s t y p e s y s t e m u s i n g v e r t i c a l k n i f e edges as t h e s o u r c e and s c h l i e r e n c u t o f f s . The s y s t e m was a r r a n g e d as shown i n F i g u r e (14). The k n i f e edge s l i t w i d t h , t h e k n i f e edge s p a c i n g , and t h e l e n s s p a c i n g and o r i e n t a t i o n were d e t e r m i n e d by t r i a l and e r r o r t o g i v e a c l e a r image o f t h e f l u i d i m m e d i a t e l y s u r r o u n d i n g t h e h o r i z o n t a l c y l i n d e r . The s e c o n d c u t o f f was mounted on a h o r i z o n t a l s l i d e and o n l y one k n i f e edge was u s e d t o d e f l e c t t h e l i g h t beam. The amount o f i n t e r -f e r e n c e o f t h i s s e c o n d c u t o f f c o u l d be v a r i e d and t h e s e n s i -t i v i t y o f t h e s y s t e m was t h e r e f o r e e a s i l y v a r i a b l e . T e s t s were made w i t h b o t h h o r i z o n t a l and v e r t i c a l c u t o f f s and w i t h a c o l o r s c h l i e r e n s y s t e m b u t t h e c l e a r e s t images were o b t a i n e d w i t h t h e v e r t i c a l c u t o f f , b l a c k and w h i t e s y s t e m . The image p r o d u c e d c o u l d be p r o j e c t e d on a s c r e e n a t any s c a l e up t o 1000 x f u l l s i z e . A l l s t i l l p h o t o g r a p h s were t a k e n by p r o j e c t i n g t h e f l o w f i e l d image d i r e c t l y on t h e f i l m p l a n e o f a P e n t a x S p o t m a t i c 35mm camera. Maximum s h u t t e r s p e e d a v a i l a b l e was 1/1000 s e c and t h i s was s u f f i c -i e n t t o g i v e c l e a r i n d i c a t i o n o f t h e f l o w p a t t e r n a t any g i v e n i n s t a n t . H i g h s p e e d m o v i e s were a l s o t a k e n by p r o -j e c t i n g t h e image d i r e c t l y on t h e f i l m p l a n e o f a HYCAM. r o t a t i n g p r i s m 16mm r o l l c amera. The HYCAM camera was c a p a b l e o f f i l m i n g a t up t o 10,000 p p s . T h i s l a r g e f i l m i n g s p e e d was p o s s i b l e b e c a u s e a r o t a t i n g 16 s i d e d p r i s m was u s e d as t h e s h u t t e r . The camera i s d r i v e n by a d . c . mo t o r and f i l m -i n g s p e e d i s a d i r e c t ' f u n c t i o n o f a p p l i e d v o l t a g e . The camera had a b u i l t - i n t i m i n g l i g h t w h i c h marks a p u l s e on t h e f i l m m a r g i n . The l i g h t was t r i g g e r e d by a s e l f c o n t a i n e d s i g n a l g e n e r a t o r and f o r t h e p r e s e n t s t u d y t i m i n g marks were p u t on t h e f i l m e v e r y 1/1000 s e c . The HYCAM had an a c c e l e r a t i o n l a g b u t t h e t i m i n g marks p r o v i d e d an e a s y r e f e r e n c e t o r e l a t e a c t u a l m o t i o n t o f i l m v i e w i n g s p e e d . A l l f i l m s t a k e n f o r t h i s p r o j e c t were t a k e n a t 5000 p p s . 3. EXPERIMENTAL PROCEDURE 3.1 I n i t i a l P r e p a r a t i o n and C a l i b r a t i o n o f t h e A p p a r a t u s The components o f t h e f l o w l o o p were a s s e m b l e d and t h e l o o p f i l l e d w i t h d i s t i l l e d w a t e r . The l o o p was p r e s s u r e t e s t e d a t v a r i o u s p r e s s u r e s up t o 2000 p s i a . P r e s s u r e was r e l e a s e d a t i n t e r v a l s and l e a k i n g f i t t i n g s t i g h t e n e d . The e n t i r e a p p a r a t u s was d i s a s s e m b l e d and a l l components e x c e p t f o r t h e main pump were t a k e n t o a c o m m e r c i a l p l a t i n g s h o p . The components were d e g r e a s e d as f o r chromium p l a t i n g u s i n g a h o t e t h y l e n e t r i c h l o r s o l u t i o n and r e t u r n e d i n s e a l e d p l a s t i c b a g s . The main pump was d e g r e a s e d i n p l a c e w i t h s o l u t i o n s o f a l c o h o l and c o o l e t h y l e n e t r i c h l o r and t h e pump body f l u s h e d w i t h a c a r b o n d i o x i d e s t r e a m . The f l o w l o o p was r e a s s e m b l e d u n d e r s t e r i l e c o n d i t i o n s and f i l l e d w i t h c a r b o n d i o x i d e gas t o p r e v e n t c o n t a m i n a t i o n . P r i o r t o any d a t a r u n s t h e f l o w l o o p was e v a c u a t e d t o a p r e s s u r e b e l o w 4 p s i a and p u r g e d by r e p e a t e d f i l l i n g s w i t h l i q u i d c a r b o n d i o x i d e . The s y s t e m was a g a i n e v a c u a t e d and f i l l e d w i t h l i q u i d c a r b o n d i o x i d e . The f r e e p i s t o n a c c u m u l a t o r s were f u l l y e x t e n d e d by t h e c a r b o n d i o x i d e , c o m p r e s s e d a i r was i n t r o d u c e d i n t o t h e o t h e r s i d e o f t h e p i s -t o n and t h e c o n t e n t s o f t h e a c c u m u l a t o r s f o r c e d i n t o t h e main f l o w l o o p . T h i s r a i s e d t h e f l o w l o o p p r e s s u r e t o j u s t u n d e r t h e c r i t i c a l v a l u e so t h e p r e s s u r e w o u l d be b l o c k e d i n 51 t h e m ain f l o w l o o p and t h e a c c u m u l a t o r s a g a i n a l l o w e d t o f i l l w i t h l i q u i d c a r b o n d i o x i d e f r o m t h e s u p p l y t a n k s . The f l o w l o o p and a c c u m u l a t o r p r e s s u r e was a l l o w e d t o e q u a l i z e and c o m p r e s s e d a i r a g a i n u s e d t o b u i l d t h e p r e s s u r e i n t h e f l o w l o o p . By r e p e a t i n g t h e p r o c e d u r e t h e p r e s s u r e i n t h e main f l o w l o o p c o u l d be b u i l t up t o any r e q u i r e d t e s t p r e s s u r e up t o 1500 p s i a . D u r i n g t h e f i l l i n g p r o c e d u r e a l l h e a t e x c h a n g e r s were f u l l y on and t h e f l u i d was c i r c u l a t e d t h r o u g h t h e f l o w l o o p t o remove t h e h e a t o f c o m p r e s s i o n . The a c c u m u l a t o r s were l e f t v e r y n e a r l y f u l l o f c a r b o n d i o x i d e and w i t h a c u s h i o n o f c o m p r e s s e d a i r b e h i n d t h e p i s t o n , t o e a s i l y v a r y t h e s y s t e m p r e s s u r e w i t h o u t r e l e a s i n g c a r b o n d i o x i d e . The d e g r e a s e d f l o w l o o p had b een a s s e m b l e d u n d e r s t e r i l e c o n d i t i o n s b u t a s m a l l amount o f o i l was f o u n d t o be i i n s o l u t i o n i n t h e s u p e r c r i t i c a l c a r b o n d i o x i d e . T h i s o i l w o u l d p r e c i p i t a t e o u t on t h e h e a t e d p r o b e g r e a t l y c h a n g i n g t h e h e a t t r a n s f e r c h a r a c t e r i s t i c s . I t was f o u n d t h a t t h e o i l c o u l d be removed f r o m t h e s y s t e m by s l i g h t l y o p e n i n g a f i t t i n g and a l l o w i n g c a r b o n d i o x i d e t o s o l i d i f y on t h e t u b i n g . The o i l w o u l d come o u t o f s o l u t i o n i n t h e s o l i d c a r b o n d i o x i d e and c o u l d be e a s i l y removed f r o m t h e t u b i n g e x t e r i o r . T h i s method o f r e m o v i n g t h e d i s s o l v e d o i l was u s e d a f t e r e a c h r e -a s s e m b l y o r whenever o i l was n o t e d i n t h e s y s t e m . I t was l a t e r f o u n d t h a t t h e s o u r c e o f t h e o i l i n s o l u t i o n was t h a t t h e c a r b o n d i o x i d e , w h i c h i s a p o w e r f u l o r g a n i c s o l v e n t i n t h e v i c i n i t y o f t h e c r i t i c a l p o i n t , was l e a c h i n g t h e o i l o u t o f 52 t h e m i c a r t a p r o b e s u p p o r t and f r o m t h e i n s u l a t i o n on t h e power s u p p l y w i r e s . When t h e p r o p e r T hermo-systems I n c . p r o b e was i n s t a l l e d i n t h e t e s t s e c t i o n , t h e power s u p p l y w i r e s were c h a n g e d t o s o l i d c o p p e r r o d c o v e r e d w i t h T e f l o n s p a g g e t t i and t h e m i c a r t a p r o b e s u p p o r t was r e p l a c e d w i t h an aluminum s u p p o r t ( e l e c t r i c a l i n s u l a t i o n was n o t n e c e s s a r y ) . V e r y s m a l l amounts o f o i l s t i l l a p p e a r e d i n t h e s y s t e m o v e r l o n g p e r i o d s o f t i m e b u t were removed by t h e method d e s c r i b e d a b o v e . As p a r t o f t h e i n i t i a l p r e p a r a t i o n o f t h e a p p a r a t u s • t h e o p t i c a l b e n c h was s e t up and a l i g n e d . The p r o p e r s i z e and o r i e n t a t i o n o f t h e k n i f e edges u s e d i n t h e S c h l i e r e n were f o u n d by t r i a l and e r r o r u n t i l t h e b e s t image was o b t a i n e d on t h e v i e w i n g s c r e e n . The a l i g n m e n t p r o c e d u r e c o n s i s t e d o f > e n s u r i n g t h a t t h e image o f t h e f i l a m e n t s o u r c e was l o c a t e d s h a r p l y on t h e p l a n e o f t h e f i r s t c u t o f f k n i f e edge, t h e n c h e c k i n g t h a t t h e f i r s t s c h l i e r e n l e n s was l o c a t e d so t h a t t h e c u t o f f s l i t was a t t h e f o c a l p o i n t and t h e r e f o r e e n s u r i n g t h a t t h e beam o f l i g h t e n t e r i n g t h e o p t i c a l p a t h t h r o u g h t h e t e s t s e c t i o n was c o l l i m a t e d and h o r i z o n t a l . The s e c o n d s c h l i e r e n l e n s was l o c a t e d t o o b t a i n a s h a r p image on t h e v i e w i n g s c r e e n . The s e c o n d c u t o f f s l i t was p o s i t i o n e d on t h e o p t i c a l b e n c h so t h a t i t was i n t h e p l a n e o f t h e f o c a l p o i n t o f t h e s e c o n d l e n s . The s e n s i t i v i t y o f t h e s y s t e m c o u l d be v a r i e d by m o ving t h e s l i t p e r p e n d i c u l a r t o t h e o p t i c a l a x i s so t h a t more o f t h e c u t o f f came i n t o u s e . Removing t h e s e c o n d s l i t c o m p l e t e l y r e s u l t e d , i n a shadowgraph t y p e o f o p t i c a l s e t - u p w h i c h i n some i n s t a n c e s gave more i n s i g h t i n t o t h e f l o w p a t t e r n s a p p e a r i n g on t h e s c r e e n . T h i s i n i t i a l a l i g n m e n t was c a r r i e d o u t as a c h e c k a f t e r e v e r y m a j o r t e a r d o w n o f t h e a p p a r a t u s as a s l i g h t change i n t h e f i r s t s e t o f components c o u l d c a u s e t h e l o s s o f a l a r g e amount o f l i g h t . The a l i g n m e n t was c a r r i e d o u t w i t h t h e o p t i c a l p a t h f i l l e d w i t h l i q u i d c a r b o n d i o x i d e , above t h e c r i t i c a l p o i n t , t o m i n i m i z e any e f f e c t s o f r e f r a c t i o n a t t h e v a r i o u s s u r f a c e s i n t h e o p t i c a l p a t h . As t h e h o t f i l m p r o b e m a n u f a c t u r e r was h a v i n g g r e a t d i f f i c u l t y i n p r o d u c i n g a s t a b l e h e a t e d c y l i n d e r a t t h i s i n i t i a l s t a g e o f t h e e x p e r i m e n t a l work i t was d e c i d e d t o do t h e i n i t i a l work w i t h a h e a t e d c y l i n d e r p r o b e m a n u f a c t u r e d by t h e a u t h o r . T h i s p r o b e c o n s i s t e d o f 0.00 3 i n c h d i a m e t e r n i c h r o m e w i r e s t r e t c h e d a c r o s s two c o p p e r s u p p o r t r o d s w h i c h were mounted on an i n s u l a t i n g m i c a r t a b a s e . A l t h o u g h t h i s p r o b e has an a s p e c t r a t i o o f g r e a t e r t h a n 500 t h e c y l i n d e r d i a m e t e r was s m a l l and i n t e r p r e t a t i o n o f t h e f l o w f i e l d s n e a r t h e h e a t e d c y l i n d e r was d i f f i c u l t . L a r g e r d i a m e t e r n i c h r o m e c y l i n d e r s c o u l d n o t be u s e d b e c a u s e t h e r e s u l t i n g l o w e r r e s i s t a n c e s were n o t c o m p a t i b l e w i t h t h e power s u p p l i e s a v a i l a b l e . M a t e r i a l s o t h e r t h a n n i c h r o m e c o u l d n o t be u s e d b e c a u s e o f t h i s same r e s i s t a n c e r e q u i r e m e n t . N i c h r o m e had t h e d i s a d v a n t a g e t h a t t h e low, n o n - l i n e a r , c o e f f i c i e n t o f r e s i s t a n c e change w i t h t e m p e r a t u r e r e s u l t e d i n p r o b e t e m p e r a -t u r e d i f f e r e n c e s b e i n g v a r i a b l e o n l y i n s t e p s l a r g e r t h a n 5 54 deg.F and r a i s e d some q u e s t i o n a b o u t t h e a c t u a l w i r e t e m p er-a t u r e . E v e n s o , t h e n i c h r o m e p r o b e s were v e r y v a l u a b l e i n i n i t i a l d a t a r u n s d e s i g n e d t o d e t e r m i n e t h e h e a t e x c h a n g e r r e q u i r e m e n t s and p o s i t i o n s , t h e l i m i t s o f t e m p e r a t u r e and p r e s s u r e c o n t r o l p o s s i b l e , and i n t h e d e v e l o p m e n t o f p h o t o g r a p h i c t e c h n i q u e s . D u r i n g a l l d a t a r u n s t h e a c c u m u l a t o r s were open t o t h e s y s t e m . p r e s s u r e as i t was f o u n d t h a t good p r e s s u r e c o n t r o l c o u l d be o b t a i n e d by v a r y i n g o n l y t h e p r e s s u r e on t h e c o m p r e s s e d a i r s i d e o f t h e a c c u m u l a t o r s . The a c c u m u l a t o r s a l s o , a c t e d as a c u s h i o n t o a b s o r b s m a l l volume changes i n t h e main s y s t e m . S t e a d y s t a t e c o n d i t i o n s f o r t h i s e x p e r i m e n t a l work were d e f i n e d as a p r e s s u r e change o f n o t more t h a n ±2 p s i a :' and a b u l k t e m p e r a t u r e change o f l e s s t h a n ±0.5 deg F d u r i n g any d a t a r u n . E x c e p t i n t h e i m m e d i a t e v i c i n i t y o f t h e c r i t i c a l t e m p e r a t u r e and p r e s s u r e i t was p o s s i b l e t o m a i n t a i n s t e a d y s t a t e c o n d i t i o n s f o r p e r i o d s o f one h o u r o r more w i t h i n 30 p s i a and 5 deg F o f t h e c r i t i c a l p o r . i t s t e a d y s t a t e c o n -d i t i o n s were o n l y p o s s i b l e f o r a p p r o x i m a t e l y o n e - h a l f h o u r and t h e f u l l a l l o w a b l e t e m p e r a t u r e r a n g e was u s e d . T h i s d i f f i c u l t y i s due t o t h e w i d e l y v a r i a b l e f l u i d p r o p e r t i e s a t s t a t e s so c l o s e t o t h e c r i t i c a l p o i n t . The d e v i s e d manner of- d a t a t a k i n g t o m i n i m i z e t h e s e b u l k c o n d i t i o n f l u c t u a t i o n s i s d e s c r i b e d below. B u l k f l u i d t e m p e r a t u r e was c o n t r o l l e d by v a r y i n g t h e f l o w r a t e t o any o f t h e t h r e e c o u n t e r - f l o w w a t e r t o c a r b o n d i o x i d e h e a t e x c h a n g e r s l o c a t e d i n t h e f l o w l o o p . S t e a d y s t a t e c o n d i t i o n s f o r f o r c e d c o n v e c t i o n d a t a r u n s r e q u i r e d a d e l i c a t e b a l a n c e between t h e w a t e r f l o w r a t e s t o t h e s e h e a t e x c h a n g e r s t o p r o d u c e a u n i f o r m f l o w w h i c h was w i t h o u t clumps o f unmixed c o o l e d f l u i d d i r e c t l y f r o m t h e w a l l s o f t h e h e a t e x c h a n g e r s . As t h e s c h l i e r e n s y s t e m u s e d i n t h i s s t u d y was e a s i l y a d j u s t a b l e f o r s e n s i t i v i t y o b s e r -v a t i o n o f t h e f l o w p a s t t h e u n h e a t e d c y l i n d e r was u s e d t o d e t e r m i n e t h e t h e r m a l q u a l i t y o f t h e f l o w . I t was r e q u i r e d t h a t t h e f l u i d t e m p e r a t u r e as i n d i c a t e d by t h e two thermo-c o u p l e s i n t h e f l o w be u n i f o r m and t h e r e be no w e l l o r g a n i z e d clumps o f c o l d e r f l u i d i n t h e f l o w . A t b e s t s c h l i e r e n s y s t e m s g i v e a q u a l i t a t i v e p i c t u r e o f t h e t e m p e r a t u r e d i f f e r e n c e s b u t by o b s e r v i n g t h e s h a d i n g o f l i g h t r e s u l t i n g f r o m a known t e m p e r a t u r e d i f f e r e n c e i t was p o s s i b l e t o e s t i -mate when t h e t e m p e r a t u r e d i s t r i b u t i o n i n t h e f l o w i n g f l u i d was w i t h i n 0.5 deg F o f t h e b u l k f l u i d t e m p e r a t u r e . 3.2 P r o c e d u r e i n F r e e C o n v e c t i o n Measurements F o l l o w i n g t h e s t a n d a r d t e c h n i q u e s u s e d i n c o n s t a n t p r o p e r t y s i t u a t i o n s i t was o r i g i n a l l y i n t e n d e d t o h o l d t h e f l u i d a t t h e r e q u i r e d b u l k t e m p e r a t u r e and p r e s s u r e c o n d i t i o n by h e a t e x c h a n g e r s a d j a c e n t t o t h e t e s t s e c t i o n . However, i t was f o u n d t h a t t h i s p r o c e d u r e r e s u l t e d i n l a r g e measurement e r r o r s b e c a u s e o r g a n i z e d clumps o f f l u i d f r o m t h e n e a r - w a l l r e g i o n s o f t h e h e a t e x c h a n g e r s w o u l d c i r c u l a t e t h r o u g h t h e t e s t s e c t i o n g r e a t l y a f f e c t i n g t h e f r e e c o n v e c t i o n f l o w p a t t e r n s . I n some i n s t a n c e s t h e e f f e c t w o u l d be t o i n d u c e a v a r i a t i o n i n t h e m o n i t o r e d power d i s s i p a t i o n f r o m t h e h e a t e d p r o b e o f as much as 75% o f t h e u n d i s t u r b e d r e a d i n g . T h e s e d i f f e r e n c e s w o u l d o c c u r w i t h t h e h e a t e x c h a n g e r f l u i d o n l y 5 t o 10 d e g F h o t t e r o r c o l d e r t h a n t h e b u l k f l u i d t e m p e r a t u r e . The clumps o f f l u i d and t h e s u b s e q u e n t b r e a k -down o f t h e o r d e r e d f r e e c o n v e c t i o n f l o w c o u l d be e a s i l y o b s e r v e d on t h e v i e w i n g s c r e e n . A f t e r many a t t e m p t s t o c o n t r o l t h e f l u i d b u l k t e m p e r a t u r e by s u c h t e c h n i q u e s as m i x i n g t h e f l u i d and p a s s i n g i t t h r o u g h an e x t e r n a l h e a t e x c h a n g e r u n t i l t h e f l u i d i n t h e e n t i r e l o o p was a t a u n i f o r m t e m p e r a t u r e , i t was f o u n d t h a t t h e o n l y method o f b u l k t e m p e r a t u r e c o n t r o l w h i c h w o u l d y i e l d r e p r o d u c i b l e and c o n s i s t e n t r e s u l t s was t o h e a t t h e e n t i r e room and a p p a r a t u s t o t h e d e s i r e d b u l k f l u i d t e m p e r a t u r e . F a i l u r e t o do t h i s w o u l d r e s u l t i n l a r g e c u r r e n t f l u c t u a t i o n s ; f o r example, w i t h a room t e m p e r a t u r e o f 77 deg F and a w e l l m i x e d f l u i d b u l k t e m p e r a t u r e o f 80.5 deg F i t was f o u n d t h a t t h e c u r r e n t d i s s i p a t e d by t h e p r o b e c o u l d o s c i l l a t e f r o m 0.35 amps t o 0.48 amps a t t h e same o p e r a t i n g r e s i s t a n c e o v e r a two m i n u t e p e r i o d . The l a r g e r c u r r e n t r e a d i n g s o c c u r r e d when a clump o f f l u i d a t room t e m p e r a t u r e f r o m t h e s i d e o f t h e t e s t chamber w o u l d s i n k p a s t 1 t h e p r o b e and c a u s e more h e a t t r a n s f e r b o t h by i t s m o t i o n and by t h e l a r g e r t e m p e r a t u r e d i f f e r e n t i a l between t h e clump and t h e h e a t e d p r o b e . A l l f r e e c o n v e c t i o n d a t a r u n s r e p o r t e d i n t h i s work a r e t a k e n w i t h t h e b u l k t e m p e r a t u r e o f t h e b u l k f l u i d e q u a l t o t h e e n v i r o n m e n t t e m p e r a t u r e . I t was a l s o n o t e d t h a t e v e n w i t h w e l l m i x e d f l u i d a t room t e m p e r a t u r e clumps o f d i f f e r e n t d e n s i t y f l u i d w o u l d c o n t i n u e t o c i r c u l a t e f o r a p e r i o d o f a b o u t 15 m i n u t e s a f t e r t h e pump had been s h u t o f f . T h e r e clumps were p r o b a b l y f o r m e d on t h e s e c t i o n s o f t h e pump n e a r t h e warm b e a r i n g s and c o n t i n u e d t o c i r c u l a t e a t odd i n t e r v a l s w e l l a f t e r any o r g a n i z e d f l u i d m o t i o n c a u s e d by t h e pump had d i e d o u t . A l l f r e e c o n v e c t i o n d a t a r u n s r e p o r t e d h e r e were t a k e n a minimum o f 1/2 h o u r a f t e r t h e pump shutdown. A s t a n d a r d i z e d p r o c e d u r e f o r a l l f r e e c o n v e c t i o n d a t a r u n s was e v e n t u a l l y d e v e l o p e d . I n i t i a l l y t h e l a b o r a t o r y was r a i s e d t o t h e r e q u i r e d b u l k f l u i d t e m p e r a t u r e and t h e h e a t e x c h a n g e r s f i l l e d w i t h w a t e r and b l o c k e d c l o s e d . The pump was t h e n o p e r a t e d t o c i r c u l a t e t h e f l u i d and t o h e a t t h e f l u i d t o t h e r e q u i r e d t e m p e r a t u r e . The p r e s s u r e was a d j u s t e d t o a p p r o x i m a t e l y 10 p s i a above t h e d e s i r e d p r e s s u r e and t h e f l u i d c i r c u l a t e d u n t i l t h e e n t i r e a p p a r a t u s , i n c l u d i n g t h e w a t e r i n t h e h e a t e x c h a n g e r s , was a t t h e r e q u i r e d t e m p e r a t u r e . The pump was s t o p p e d and t h e a p p a r a t u s a l l o w e d t o s t a n d a t t h e p r o p e r t e m p e r a t u r e and p r e s s u r e u n t i l a t l e a s t one h a l f h o u r had p a s s e d . F i n a l a d j u s t m e n t s o f l e s s t h a n 5 p s i a were c a r r i e d o u t d u r i n g t h i s t i m e by v e n t i n g c o m p r e s s e d a i r f r o m 5 8 the accumulators. The Thermo-System power supply had been on ; in the standby mode at least one hour before the test was to begin to allow the components to reach operating temperature. It had been noted that i n i t i a l shifts observed in the probe cold resistance were due to shifts in the decade resistances in the power supply. The probe cold resistance was checked using the b u i l t - i n c i rcu i t in the power supply. An overheat of approximately 10% was set on the resistance decade and the probe turned on. The balance of the wheatstone bridge was adjusted using the nul l meter integral with the power supply and the feedback c i rcu i t tuned for maximum frequency response by adjusting the variable capacitance in the power supply. The power supply was returned to the standby position and the cold resistance checked again. Assuming that the two ' determinations of cold resistance agreed, the power supply resistance decade was set to an overheat of 0.01 ohms and the power supplied to the probe. The nul l balance was adjusted so that the current indicated by the panel meter was just that required to keep the heated cylinder at the resistance selected on the resistance decade. The probe operating resistance, current, the output voltage of the bridge, and the rms voltage fluctuation level, were a l l recorded for each data point. The temperature of the bulk was monitored continuously on the galvanometer. The system bulk pressure was checked to ensure that i t did not vary by more than +2 psia during the data run. The probe overheat was increased in 0.01 ohm steps (correspond-59 i n g to approximately 2 deg F probe temperature steps) and the bridge balance checked and adjusted at each data p o i n t . I f a 0.01 ohm overheat change d i d not r e s u l t i n a d i s c e r n a b l e change i n probe c u r r e n t l a r g e r steps were used. The overheat r a t i o was increased u n t i l the maximum p e r m i s s i b l e probe temperature to prevent probe damage (400 deg F) was reached. A d d i t i o n a l data p o i n t s were then taken i n random order through-out the range p r e v i o u s l y covered as a r e p r o d u c i b i l i t y check. Photographs were taken at operating p o i n t s of i n t e r e s t and the data was a l s o included" f o r these operating c o n d i t i o n s . A computer program was w r i t t e n to o b t a i n heat t r a n s f e r r a t e , heat t r a n s f e r c o e f f i c i e n t , and temperature d i f f e r e n c e d i r e c t l y from the raw data of overheat r a t i o and probe c u r r e n t . Both t a b u l a t e d and p l o t t e d r e s u l t s of heat t r a n s f e r r a t e and heat t r a n s f e r c o e f f i c i e n t vs temperature d i f f e r e n c e were obtained d i r e c t l y . 3.3 Procedure I n Forced Convection Measurements An experimental technique, designed to minimize ex-perimental e r r o r s , was a l s o developed f o r the forced convection data runs. F l u i d bulk pressure was adjusted to s l i g h t l y under the r e q u i r e d t e s t pressure, the pump was s t a r t e d and the flow r a t e through the t e s t s e c t i o n adjusted to the d e s i r e d value. The heat exchangers were adjusted u n t i l the bulk f l u i d was being very slowly heated (less than 1 deg F per hour) and the loop allowed to s t a b i l i z e . Minor c o r r e c t i o n s were made to bulk p r e s s u r e ( l e s s t h a n 5 p s i a ) and t o t h e f l o w r a t e ( l e s s t h a n 5% o f d i f f e r e n t i a l p r e s s u r e ) . The l o o p was o p e r a t e d u n t i l t h e b u l k f l u i d t e m p e r a t u r e r e a c h e d t h e a l l o w a b l e l o w e r l i m i t o f t h e t e s t v a l u e and t h e p r o b e c o l d r e s i s t a n c e c h e c k e d . D a t a was t a k e n u n t i l t h e b u l k t e m p e r a t u r e h a d i n c r e a s e d t o j u s t o v e r t h e r e q u i r e d v a l u e and t h e h e a t e x c h a n g e r f l o w r a t e i n c r e a s e d . A t y p i c a l d a t a r u n a t a d e s i g n b u l k t e m p e r a t u r e o f s a y 80.5 deg F w o u l d be s t a r t e d when t h e b u l k f l u i d t e m p e r a t u r e r e a c h e d 80.0 deg F and t h e h e a t e x c h a n g e r f l o w r a t e m o d i f i e d s l i g h t l y when t h e b u l k t e m p e r a t u r e r e a c h e d 80.5 d eg F. The b u l k t e m p e r a t u r e w o u l d c o n t i n u e t o r i s e and d a t a w o u l d be t a k e n u n t i l t h e b u l k t e m p e r a t u r e r e a c h e d 80.9 deg F. The b u l k t e m p e r a t u r e w o u l d b e g i n t o d r o p and t h e d a t a w o u l d be t a k e n u n t i l t h e b u l k t e m p e r a t u r e r e a c h e d 80.2 deg F. In some d a t a r u n s t h e b u l k t e m p e r a t u r e w o u l d o n l y r i s e by 0.5 deg F d u r i n g t h e e n t i r e d a t a r u n . D a t a p o i n t s were t a k e n f i r s t i n i n c r e a s i n g c y l i n d e r t e m p e r a t u r e s t e p s , t h e n i n d e c r e a s i n g c y l i n d e r t e m p e r a t u r e s t e p s , t h e n i n d e c r e a s i n g c y l i n d e r t e m p e r a t u r e s t e p s and f i n a l l y i n random o r d e r . D a t a p o i n t s shown t h e r e f o r e c o v e r t h e e n t i r e v a r i a t i o n i n b u l k t e m p e r a t u r e d u r i n g a d a t a r u n . No d i s c e r n a b l e c hange was a l l o w e d i n e i t h e r b u l k p r e s s u r e o r d i f f e r e n t i a l p r e s s u r e d u r i n g a d a t a r u n . P r o b e c o l d r e s i s t a n c e was c h e c k e d e v e r y 10 d a t a p o i n t s and no v a r i a t i o n g r e a t e r t h a n 0.005 ohm was a l l o w e d d u r i n g a d a t a r u n . Flow f i e l d s were m o n i t o r e d d u r i n g a l l d a t a r u n s . P h o t o g r a p h s were t a k e n d u r i n g some d a t a r u n s and s p e c i a l d u p l i c a t e d a t a r u n s were made f o r o t h e r p h o t o -g r a p h s . I n a l l c a s e s t h e d a t a o b t a i n e d d u r i n g t h e p h o t o -g r a p h s h a ve been i n c l u d e d as a c h e c k on t h e o r i g i n a l d a t a . A t y p i c a l d a t a r u n w o u l d c o n s i s t o f 20 s t e p s o f 0.01 ohms' (2 deg F ) , 30 s t e p s o f 0.02 ohms (4 t o 5 deg F ) , 20 s t e p s o f 0.05 ohms (10-12 deg F) u n t i l t h e maximum c y l i n d e r t e m p e r a t u r e was r e a c h e d and a f u r t h e r 30 d a t a p o i n t s t a k e n o v e r t h e e n t i r e t e m p e r a t u r e r a n g e i n random o r d e r . A l l f o r c e d c o n v e c t i o n d a t a r u n s p r e s e n t e d a r e c o n t i n u o u s d a t a r u n s e x t e n d i n g o v e r a t l e a s t one h o u r . T y p i c a l f o r c e d c o n v e c t i o n r u n s were d u p l i c a t e d on s u c c e s s i v e d a y s and t h e d a t a were c o m p l e t e l y r e p r o d u c i b l e . 62 EXPERIMENTAL RESULTS 4.1 E f f e c t o f F r e e - s t r e a m V e l o c i t y T y p i c a l e x p e r i m e n t a l r e s u l t s i l l u s t r a t i n g t h e e f f e c t s o f f r e e - s t r e a m v e l o c i t y on h e a t t r a n s f e r and h e a t t r a n s f e r c o e f f i c i e n t as a f u n c t i o n o f t e m p e r a t u r e d i f f e r e n c e between t h e h e a t e d c y l i n d e r and t h e b u l k f l u i d a r e g r a p h i c a l l y summa-r i z e d i n F i g u r e s (15) t o ( 2 6 ) . The m a j o r i t y o f r e s u l t s were o b t a i n e d w i t h b u l k f l u i d b e l o w t h e p s e u d o - c r i t i c a l t e m p e r a t u r e , and h e a t e d c y l i n d e r t e m p e r a t u r e s b o t h above and b e l o w t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . R e s u l t s p r e s e n t e d c o v e r a v a r i e t y o f s u p e r c r i t i c a l t h e r m o d y n a m i c s t a t e s , and f r e e c o n v e c t i o n s t u d i e s have been i n c l u d e d f o r c o m p a r i s o n p u r p o s e s . F o r c o n -v e n i e n c e , f r e e - s t r e a m v e l o c i t y i s e x p r e s s e d i n t e r m s o f an e q u i v a l e n t c o n s t a n t p r o p e r t y R e y n o l d s number (Re*) b a s e d on t h e c o n s t a n t f r e e - s t r e a m f l u i d p r o p e r t i e s and t h e c y l i n d e r d i a m e t e r . F i g u r e s (15) and (16) show t h e v e l o c i t y e f f e c t s i n f l u i d w i t h a b u l k t e m p e r a t u r e o f 80 deg F and b u l k p r e s s u r e o f 1100 p s i a ( s u p e r c r i t i c a l l i q u i d - l i k e r e g i o n ) . N o t e t h a t w h enever t h e t e m p e r a t u r e d i f f e r e n c e i s above 10 deg F t h e f l u i d v e r y n e a r t h e h e a t e d c y l i n d e r w i l l be above t h e p s e u d o -c r i t i c a l t e m p e r a t u r e and have v a p o r - l i k e p r o p e r t i e s . F o r a l l v e l o c i t i e s t e s t e d t h e h e a t t r a n s f e r c o e f f i c i e n t ( F i g . 16) shows a peak, w h i c h i s s l i g h t l y v e l o c i t y d e p e n d e n t i n b o t h m a g n i t u d e and p o s i t i o n , when t h e c y l i n d e r t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . I n c r e a s i n g v e l o c i t y s l i g h t l y s h i f t s t h e h e a t t r a n s f e r c o e f f i c i e n t peak t o a h i g h e r c y l i n d e r t e m p e r a t u r e and i n c r e a s e s t h e e f f e c t i v e w i d t h and maximum v a l u e o f t h e i m p r o v e d h e a t t r a n s f e r c o e f f i c i e n t . E v e n t h o u g h t h e f i l m c o e f f i c i e n t shows t h i s d e f i n i t e n o n - c o n s t a n t b e h a v i o u r , t h e h e a t t r a n s f e r r a t e r e m a i n s a s m o o t h l y i n c r e a s i n g f u n c t i o n o f i n c r e a s i n g v e l o c i t y and t e m p e r a t u r e d i f f e r e n c e . A t any g i v e n t e m p e r a t u r e d i f f e r e n c e t h e h e a t t r a n s f e r r a t e i s s h a r p l y i n c r e a s e d o v e r t h e f r e e c o n v e c t i o n v a l u e by even a v e r y s m a l l f r e e - s t r e a m v e l o c i t y . F u r t h e r i n c r e a s e i n f r e e - s t r e a m v e l o c i t y c a u s e s l e s s o f an improvement i n h e a t t r a n s f e r r a t e . A t l a r g e r t e m p e r a t u r e d i f f e r e n c e s t h e h e a t t r a n s f e r c o e f f i c i e n t s f o r v a r i o u s Re* a p p r o a c h c o n s t a n t v a l u e s . The marked changes i n t h e f l o w f i e l d r e s u l t i n g f r o m f r e e - s t r e a m v e l o c i t y c h a n g e s a r e i l l u s t r a t e d i n t h e s e r i e s o f s c h l i e r e n p h o t o g r a p h s o f F i g u r e (35) . The p h o t o g r a p h s show t h e w e l l o r d e r e d f r e e c o n v e c t i o n f l o w f i e l d w i t h a h o t s h e e t o f f l u i d r i s i n g i n columns o f h o t t e r f l u i d t o a t u r b u -l e n t plume and t h e p r o g r e s s i v e breakdown o f t h i s w e l l o r d e r e d f l o w w i t h i n c r e a s i n g f r e e - s t r e a m v e l o c i t y . The e f f e c t s on t h e f l o w f i e l d o f e v e n a v e r y s m a l l v e l o c i t y s u c h as c a u s e d by c i r c u l a t i o n due t o d e n s i t y g r a d i e n t s i n t h e t e s t s e c t i o n a r e shown i n F i g u r e ( 3 6 ) . A l s o shown i n F i g u r e (36) i s t h e s m a l l e f f e c t on t h e f l o w f i e l d due t o v i b r a t i o n i n d u c e d by t h e c i r c u l a t i n g pump. 64 F i g u r e s (17) and (18) i l l u s t r a t e t h e e f f e c t s o f v e l o c i t y when t h e b u l k f l u i d t e m p e r a t u r e i s j u s t u n d e r t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . A t 87 deg F and 1100 p s i a t h e h e a t t r a n s f e r c o e f f i c i e n t e x h i b i t s a v e l o c i t y - i n f l u e n c e d maximum when t h e c y l i n d e r t e m p e r a t u r e i s s l i g h t l y above t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . The peak h e a t t r a n s f e r c o e f f i c i e n t a t t a i n e d was s l i g h t l y l a r g e r t h a n w i t h a b u l k t e m p e r a t u r e o f 8 0 deg F b u t t h e h e a t t r a n s f e r r a t e i s a l m o s t i d e n t i c a l f o r a l l v e l o c i t i e s t e s t e d . The h e a t t r a n s f e r r a t e r e m a i n s a s m o o t h l y i n c r e a s i n g f u n c t i o n o f i n c r e a s i n g t e m p e r-a t u r e d i f f e r e n c e and v e l o c i t y . The f l o w f i e l d s have t h e same a p p e a r a n c e f o r e i t h e r b u l k t e m p e r a t u r e and s t r o n g l y r e s e m b l e t h e p h o t o g r a p h s o f F i g u r e ( 3 5 ) . The d i s t i n c t i o n between t h e w e l l o r d e r e d f r e e and f o r c e d c o n v e c t i o n p a t t e r n s * i s r e t a i n e d and ev e n l e s s t h e r m a l d i s t u r b a n c e i n t h e t e s t s e c t i o n i s r e q u i r e d t o c a u s e t h e f r e e c o n v e c t i o n f l o w p a t t e r n t o b r e a k down ( s i m i l a r t o t h a t shown i n F i g u r e (36)) . F i g u r e s (19) and (20) show t h e e f f e c t s o f v e l o c i t y a l s o a t 1100 p s i a b u l k p r e s s u r e when t h e b u l k f l u i d t e m p e r-a t u r e i s r a i s e d t o 91 deg F ( j u s t o v e r t h e p s e u d o - c r i t i c a l t e m p e r a t u r e o f 90 deg F f o r 1100 p s i a ) . Now t h e h e a t t r a n s f e r c o e f f i c i e n t shows o n l y a d e c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e d i f f e r e n c e f o r any v e l o c i t y t e s t e d . The i n f l u -e n c e o f v e l o c i t y on h e a t t r a n s f e r r a t e i s n o t as s t r o n g as 65 w i t h l o w e r b u l k t e m p e r a t u r e and t h e h e a t t r a n s f e r r a t e f o r any t y p i c a l t e m p e r a t u r e d i f f e r e n c e and Re* i s l e s s t h a n h a l f t h a t o b s e r v e d w i t h f l u i d o n l y 4 deg F c o o l e r and a t t h e same b u l k p r e s s u r e . The e n t i r e b u l k f l u i d i s above t h e p s e u d o - c r i t i c a l t e m p e r a t u r e and t h e r e f o r e v a p o r - l i k e p r o p e r t i e s b u t t h e q u a l i t a t i v e n a t u r e o f t h e f l o w f i e l d s i s t h e same. R e l a t i v e l y s t r o n g (by c o n s t a n t p r o p e r t y s t a n d a r d s ) p r o p e r t y ' v a r i a t i o n s s t i l l e x i s t f r o m t h e h e a t e d f l u i d n e a r t h e c y l i n d e r t o t h e b u l k f l u i d and t h e same f l o w p a t t e r n s a p p e a r b u t . t h e d i s t i n c t i o n between h o t and c o l d f l u i d i s n o t as marked. The h e a t t r a n s f e r r a t e i s a g a i n a s m o o t h l y i n c r e a s i n g f u n c t i o n o f i n c r e a s i n g v e l o c i t y and t e m p e r a t u r e d i f f e r e n c e b u t t h e dependence on v e l o c i t y and t e m p e r a t u r e d i f f e r e n c e i s n o t n e a r l y as s t r o n g as w i t h t h e l i q u i d - l i k e ' b u l k f l u i d . I n o r d e r t o i s o l a t e w h e t h e r t h e p e a k s i n h e a t t r a n s f e r c o e f f i c i e n t were c o n n e c t e d w i t h t h e c r i t i c a l o r p s e u d o - c r i t i c a l t e m p e r a t u r e a s e r i e s o f d a t a r u n s was done a t o t h e r b u l k p r e s s u r e s . F i g u r e s (21,) and (22) show t h e e f f e c t s o f v e l o c i t y on h e a t t r a n s f e r r a t e and f i l m c o e f f i c i e n t f o r f l u i d a t 1300 p s i a and w i t h a b u l k t e m p e r a t u r e o f 8 0 deg F. The peak h e a t t r a n s f e r c o e f f i c i e n t i s v e l o c i t y i n f l u e n c e d and a p p e a r s when t h e c y l i n d e r t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o -c r i t i c a l t e m p e r a t u r e f o r 1300 p s i a (105 deg F ) . The peak h e a t t r a n s f e r c o e f f i c i e n t o c c u r s o v e r a b r o a d e r r a n g e a t t h i s i n c r e a s e d p r e s s u r e b u t t h e h e a t t r a n s f e r r a t e r e m a i n s a s m o o t h l y i n c r e a s i n g f u n c t i o n o f v e l o c i t y and t e m p e r a t u r e d i f f e r e n c e . S i m i l a r t r e n d s o c c u r a t a b u l k p r e s s u r e o f 1300 p s i a when t h e b u l k f l u i d t e m p e r a t u r e i s r a i s e d t o 86 deg F as shown i n F i g u r e s (23) and (24) . The h e a t t r a n s f e r r a t e r e m a i n s a s m o o t h l y i n c r e a s i n g f u n c t i o n o f i n c r e a s i n g v e l o c i t y and t e m p e r a t u r e d i f f e r e n c e and t h e h e a t t r a n s f e r c o e f f i c i e n t s t i l l e x h i b i t s a s h a r p peak when t h e c y l i n d e r t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . N ote t h a t t h e f r e e c o n v e c t i o n h e a t t r a n s f e r r a t e s a r e v e r y s i m i l a r w i t h t h e b u l k f l u i d a t 80 o r 86 deg F, b u t t h e f o r c e d c o n v e c t i o n h e a t t r a n s f e r r a t e i s more s t r o n g l y i n c r e a s e d by i n c r e a s i n g v e l o c i t y i n t h e c o l d e r b u l k f l u i d . The s e r i e s o f p h o t o g r a p h s shown as F i g u r e (36-A) i l l u s t r a t e s t h e f r e e c o n v e c t i o n f l o w f i e l d and t h e c h a r a c t e r i s t i c change i n d u c e d by v a r y i n g t h e f r e e - s t r e a m v e l o c i t y f o r t h e b u l k c o n d i t i o n s shown i n F i g u r e s (21) and ( 2 2 ) . The f l o w f i e l d s a r e v e r y s i m i l a r t o t h o s e p r o d u c e d by i d e n t i c a l c y l i n d e r t e m p e r a t u r e and f r e e - s t r e a m v e l o c i t i e s i n t h e warmer b u l k f l u i d shown i n F i g u r e s (23) and ( 2 4 ) . The f l o w f i e l d s a t 1100 p s i a b u l k p r e s s u r e a r e a l s o v e r y s i m i l a r t o t h o s e a t 1300 p s i a b u l k p r e s s u r e . The f r e e c o n v e c t i o n f l o w f i e l d i n a l l c a s e s c o n s i s t s o f a s h e e t o f h e a t e d f l u i d d i v i d e d i n t o h o t t e r v e r t i c a l columns w h i c h r i s e t o a t u r b u l e n t plume. The s p a c i n g between t h e t u r b u l e n t plume and t h e h e a t e d c y l i n d e r d e c r e a s e s w i t h i n c r e a s i n g c y l i n d e r t e m p e r a t u r e , i t i s o n l y s l i g h t l y a f f e c t e d by b u l k t e m p e r a t u r e and p r e s -s u r e c h a n g e s i n t h e s u p e r c r i t i c a l f l u i d . The s p a c i n g and i n t e n s i t y o f t h e v e r t i c a l columns a p p e a r s t o be r e l a t e d t o t h e n e a r n e s s o f t h e b u l k f l u i d t e m p e r a t u r e t o t h e p s e u d o -c r i t i c a l t e m p e r a t u r e . The columns become c l o s e r t o g e t h e r and more d i s t i n c t as t h e b u l k t e m p e r a t u r e a p p r o a c h e s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . The f o r c e d c o n v e c t i o n f l o w s show q u a l i t a t i v e l y t h e same e f f e c t s due t o i n c r e a s i n g c y l i n d e r t e m p e r a t u r e and i n c r e a s i n g v e l o c i t y a t d i f f e r e n t b u l k s t a t e s . S h e e t s o f f l u i d a r e r e p e a t e d l y t o r n a t d e f i n i t e i n t e r v a l s f r o m t h e h e a t e d c y l i n d e r by t h e a p p r o a c h -i n g f l u i d . The i n t e r v a l d e c r e a s e s w i t h i n c r e a s i n g f r e e -s t r e a m v e l o c i t y . The shape and o r i e n t a t i o n o f t h e h e a t e d l a y e r s i s a c o m p l i c a t e d f u n c t i o n o f c y l i n d e r and b u l k t e m per-a t u r e , s y s t e m p r e s s u r e and t h e f r e e - s t r e a m Re*. The v e r t i c a l ' s p a c i n g between t h e h e a t e d l a y e r s c o r r e s p o n d s r o u g h l y t o t h a t i n d i c a t e d f r o m Karmen v o r t e x a n a l y s i s , b u t t h e h o r i z o n -t a l v a r i a t i o n s seem t o be d e p e n d e n t on c y l i n d e r t e m p e r a t u r e . F o r f u r t h e r c o m p a r i s o n , t h e e f f e c t s o f v e l o c i t y on h e a t t r a n s f e r r a t e and h e a t t r a n s f e r c o e f f i c i e n t as a f u n c t i o n o f t e m p e r a t u r e d i f f e r e n c e a r e i n c l u d e d f o r a b u l k t e m p e r a t u r e o f 80 deg F and a b u l k p r e s s u r e o f 1500 p s i a i n F i g u r e s (25) and ( 2 6 ) . The h e a t t r a n s f e r r a t e i s s t i l l a s m o o t h l y i n c r e a s i n g f u n c t i o n o f i n c r e a s i n g v e l o c i t y and t e m p e r a t u r e d i f f e r e n c e . The h e a t t r a n s f e r c o e f f i c i e n t s t i l l shows a b r o a d s h a l l o w peak when t h e c y l i n d e r t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . The f r e e c o n v e c t i o n h e a t t r a n s f e r r a t e i n b u l k f l u i d a t 80 deg F i s o n l y s l i g h t l y i n c r e a s e d by t h e p r e s s u r e i n c r e a s e f r o m 1100 t o 1300 t o 1500 p s i a . The f o r c e d c o n v e c t i o n h e a t t r a n s f e r r a t e i s s t r o n g l y a f f e c t e d by t h e d i f f e r e n t b u l k p r e s s u r e s . The e f f e c t o f i n c r e a s i n g v e l o c i t y on t h e f l o w f i e l d s i s shown by a q u a l i t i t i v e c o m p a r i s o n o f F i g u r e s ( 3 5 ) , (36-A) and ( 3 6 - B ) . The s t r o n g s i m i l a r i t y o f t h e v e l o c i t y e f f e c t on a l l s u p e r c r i t i c a l f l o w f i e l d s examined i s e v i d e n t . A d e t a i l e d c o m p a r i s o n o f F i g u r e s ( 1 6 ) , (22) and (26) w i l l show t h a t t h e e f f e c t o f i n c r e a s i n g p r e s s u r e i s t o i n c r e a s e t h e h e a t t r a n s f e r c o e f f i c i e n t when t h e c y l i n d e r t e m p e r a t u r e i s above t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . When t h e b u l k f l u i d and t h e h e a t e d f l u i d n e a r t h e c y l i n d e r a r e b o t h b e l o w t h e p s e u d o - c r i t i c a l t e m p e r a t u r e t h e h e a t t r a n s f e r c o e f f i c i e n t i s h i g h e s t n e a r t h e c r i t i c a l p r e s s u r e . When t h e b u l k f l u i d i s l i q u i d - l i k e and t h e h e a t e d c y l i n d e r i s above t h e p s e u d o - c r i t i c a l t e m p e r a t u r e ( t h e r e f o r e v a p o r - l i k e f l u i d a d j a c e n t t o t h e c y l i n d e r ) t h e h e a t t r a n s f e r c o e f f i c i e n t i n c r e a s e s w i t h i n c r e a s i n g p r e s s u r e b eyond t h e c r i t i c a l p r e s s u r e . The f l o w f i e l d s do n o t show any s h a r p change as t h e h e a t e d c y l i n d e r t e m p e r a t u r e p a s s e s t h r o u g h t h e p s e u d o -c r i t i c a l t e m p e r a t u r e . The i n f l u e n c e o f t h e p s e u d o - c r i t i c a l t e m p e r a t u r e on t h e f i l m c o e f f i c i e n t i s n o t d i s c e r n a b l e i n f r e e c o n v e c t i o n b u t i n c r e a s e s w i t h i n c r e a s i n g f r e e - s t r e a m v e l o c i t y . The i n i t i a l d e c r e a s e i n h e a t t r a n s f e r c o e f f i c i e n t w i t h i n c r e a s i n g p r e s s u r e i s s m a l l and doe s n o t a p p e a r t o 69 a f f e c t t h e o v e r a l l h e a t t r a n s f e r r a t e e v e n a t t h e h i g h e s t Re* t e s t e d . The f a c t t h a t t h e h e a t t r a n s f e r r a t e i n l i q u i d -l i k e f l u i d i s i n c r e a s e d more by a v e l o c i t y i n c r e a s e o n l y when t h e f l u i d v e r y n e a r t h e h e a t e d c y l i n d e r i s i n t h e v a p o r - l i k e c o n d i t i o n , s u g g e s t s t h a t t h e f o r c e d c o n v e c t i o n h e a t t r a n s f e r mechanism i s a i d e d by t h e r a p i d d i s p e r s i o n o f t h e h e a t e d v a p o r - l i k e f l u i d i n t o t h e l i q u i d - l i k e f r e e - s t r e a m . The s e c o n d a r y h e a t t r a n s f e r mechanism i s v e l o c i t y d e p e n d e n t and a p p e a r s as a r a p i d l y o s c i l l a t i n g (5,000 t o 20,000 c p s ) r e g i o n b e h i n d t h e h e a t e d c y l i n d e r i n t h e h i g h s p e e d m o t i o n p i c t u r e s . The e x t e n t o f t h e o s c i l l a t i n g zone has been i n d i c a t e d i n F i g u r e s (35) and ( 3 6 - B ) . 4.2 E f f e c t o f B u l k T e m p e r a t u r e -The e f f e c t s o f b u l k t e m p e r a t u r e v a r i a t i o n on h e a t t r a n s -f e r r a t e and h e a t t r a n s f e r c o e f f i c i e n t i n f o r c e d c o n v e c t i o n a r e g r a p h i c a l l y summarized i n F i g u r e s (27) t o ( 3 2 ) . B u l k t e m p e r a t u r e v a r i a t i o n s c a u s e o n l y v e r y m i n o r h e a t t r a n s f e r r a t e v a r i a t i o n s f o r t h e f r e e c o n v e c t i o n c a s e i n s t i l l , u n i f o r m , f l u i d . T h i s e f f e c t c a n be n o t e d by a d e t a i l e d c o m p a r i s o n o f F i g u r e s ( 1 5 ) , ( 1 7 ) , and ( 1 9 ) . F i g u r e s (27) and (28) show t h e e f f e c t o f b u l k t e m p er-a t u r e v a r i a t i o n f r o m 80 t o 105 deg F i n b u l k f l u i d a t 1100 p s i a f o r a Re* o f 300. The h e a t t r a n s f e r r a t e shows a l a r g e d e c r e a s e f o r a g i v e n t e m p e r a t u r e d i f f e r e n c e when t h e b u l k f l u i d t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e and t h e f r e e s t r e a m f l u i d becomes v a p o r - l i k e . A t any g i v e n t e m p e r a t u r e d i f f e r e n c e t h e h e a t t r a n s f e r r a t e c h a n g e s by l e s s t h a n 5% f o r a b u l k t e m p e r a t u r e i n c r e a s e f r o m 80 t o 86 deg F ( l i q u i d - l i k e f r e e - s t r e a m f l u i d ) b u t d e c r e a s e s by 50% when t h e b u l k t e m p e r a t u r e i s r a i s e d t o 90 deg F (a v a p o r -l i k e f r e e - s t r e a m f l u i d ) . A f u r t h e r i n c r e a s e t o 105 deg F ( a l s o i n t h e v a p o r - l i k e r e g i o n ) o n l y d e c r e a s e s t h e h e a t t r a n s f e r r a t e by 10%. F o r t h e l i q u i d - l i k e f r e e - s t r e a m (80 and 86 deg F) t h e h e a t t r a n s f e r c o e f f i c i e n t shows a maximum when t h e c y l i n d e r t e m p e r a t u r e e x c e e d s t h e p s e d u o c r i t i c a l t e m p e r a t u r e . The h e a t t r a n s f e r c o e f f i c i e n t i n t h e v a p o r -l i k e f l u i d a t 90 deg F shows o n l y a d e c r e a s e f r o m t h e i n i t i a l peak v a l u e w i t h i n c r e a s i n g t e m p e r a t u r e d i f f e r e n c e . When t h e b u l k t e m p e r a t u r e i s r a i s e d t o 105 deg F t h e h e a t t r a n s f e r c o e f f i c i e n t i s v e r y n e a r l y i n d e p e n d e n t o f t e m p e r a t u r e d i f f e r e n c e . The e f f e c t o f b u l k t e m p e r a t u r e v a r i a t i o n a t a h i g h e r Re* i s e v e n more c l e a r l y shown i n F i g u r e s (29) and ( 3 0 ) . F i g u r e (29) shows t h e s h a r p d e c r e a s e (40 t o 50%) i n h e a t t r a n s f e r r a t e o c c u r r i n g when t h e b u l k t e m p e r a t u r e i s r a i s e d so t h a t t h e b u l k f l u i d i s i n t h e v a p o r - l i k e r e g i o n . F i g u r e (30) v e r y c l e a r l y shows t h e peak h e a t t r a n s f e r c o e f f i c i e n t o c c u r r i n g w i t h l i q u i d - l i k e f r e e - s t r e a m f l u i d when t h e c y l i n d e r t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o - c r i t i c a l temper-a t u r e . The h e a t t r a n s f e r c o e f f i c i e n t a g a i n shows o n l y a d e t e r i o r a t i o n f r o m a maximum v a l u e when t h e b u l k f r e e - s t r e a m f l u i d i s i n t h e v a p o r - l i k e r e g i o n . 71 F i g u r e s (31) and (32) a l s o summarize t h e e f f e c t o f b u l k t e m p e r a t u r e v a r i a t i o n on h e a t t r a n s f e r r a t e and f i l m c o e f f i c i e n t i n f o r c e d f l o w (Re*=600) b u t f o r a b u l k p r e s s u r e o f 1300 p s i a . The f l u i d i n a l l c a s e s i s s t i l l i n t h e l i q u i d -l i k e r e g i o n as t h e p s e u d o - c r i t i c a l t e m p e r a t u r e f o r 1300 p s i a i s a p p r o x i m a t e l y 105 deg F. A more g r a d u a l d e c r e a s e i n h e a t t r a n s f e r r a t e w i t h i n c r e a s i n g b u l k t e m p e r a t u r e c a n be n o t e d a t t h i s i n c r e a s e d p r e s s u r e . F i g u r e (32) shows t h e v a r i a t i o n i n h e a t t r a n s f e r c o e f f i c i e n t w i t h i n c r e a s i n g b u l k t e m p e r a t u r e and t h e peak h e a t t r a n s f e r c o e f f i c i e n t o c c u r s when t h e c y l i n d e r t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e f o r 1300 p s i a . The h e a t t r a n s f e r r a t e a t 105 deg F b u l k t e m p e r a t u r e and 1300 p s i a b u l k p r e s s u r e i s a i d e d by t h e l a r g e t h e r m a l c o n d u c t i v i t y o f t h e b u l k f l u i d b u t i s d e c r e a s e d by t h e f a c t t h a t t h e b u l k f l u i d i s v e r y n e a r l y i n t h e v a p o r - l i k e s t a t e and p r o p e r t y v a r i a t i o n s t h r o u g h t h e h e a t e d f l u i d l a y e r s a r e n o t as s t r o n g as w i t h c o l d e r b u l k f l u i d . A d e t a i l e d c o m p a r i s o n o f F i g u r e s (27) t o (32) shows t h a t t h e r e l a t i o n s h i p o f t h e b u l k f l u i d t e m p e r a t u r e t o t h e p s e u d o - c r i t i c a l t e m p e r a t u r e d e t e r m i n e s t h e m a g n i t u d e o f t h e peak h e a t t r a n s f e r c o e f f i c i e n t . S h a r p e s t p e a k s i n h e a t t r a n s -f e r c o e f f i c i e n t o c c u r w i t h l i q u i d - l i k e b u l k f l u i d c l o s e t o t h e c r i t i c a l t e m p e r a t u r e . H e a t t r a n s f e r c o e f f i c i e n t s i n v a p o r - l i k e b u l k f l u i d r a p i d l y d e c r e a s e t o a c o n s t a n t v a l u e . 72 Heat t r a n s f e r r a t e s r e m a i n s m o o t h l y i n c r e a s i n g f u n c t i o n s o f i n c r e a s i n g f r e e - s t r e a m Re* and t e m p e r a t u r e d i f f e r e n c e . 4.3 E f f e c t o f B u l k P r e s s u r e The e f f e c t s o f b u l k p r e s s u r e v a r i a t i o n i n s u p e r -c r i t i c a l f r e e c o n v e c t i o n a r e s m a l l and c a n be o b t a i n e d by a d e t a i l e d c o m p a r i s o n o f F i g u r e s ( 1 5 ) , ( 2 1 ) , and (25) a t 80 deg F b u l k t e m p e r a t u r e and F i g u r e s (17) and (23) a t 86 deg F b u l k t e m p e r a t u r e . The e f f e c t o f i n c r e a s i n g p r e s s u r e was t o i n c r e a s e t h e h e a t t r a n s f e r r a t e s l i g h t l y and t h i s e f f e c t becomes s t r o n g e r w i t h i n c r e a s i n g t e m p e r a t u r e d i f f e r e n c e . Maximum improvement n o t e d i n f r e e c o n v e c t i o n h e a t t r a n s f e r r a t e f o r a p r e s s u r e i n c r e a s e f r o m 1100 t o 1500 p s i a was l e s s t h a n 25%. F i g u r e (33) shows t h e e f f e c t o f i n c r e a s i n g p r e s s u r e on h e a t t r a n s f e r r a t e i n f o r c e d c o n v e c t i o n (Re*=600) i n c l u d i n g s u b c r i t i c a l and s u p e r c r i t i c a l p r e s s u r e (1000 p s i a ) t h e peak n u c l e a t e h e a t f l u x o c c u r r i n g a t a t e m p e r a t u r e d i f f e r e n c e o f 5 deg F i s c l e a r l y v i s i b l e . F i g u r e (38) shows t h e e f f e c t o f i n c r e a s i n g c y l i n d e r t e m p e r a t u r e on t h e f r e e c o n v e c t i o n s u b -c r i t i c a l and s u p e r c r i t i c a l f l o w f i e l d s . A t a t e m p e r a t u r e d i f f e r e n c e l e s s t h a n 6 deg F n u c l e a t e b o i l i n g was o b s e r v e d o v e r t h e e n t i r e c y l i n d e r ( F i g . 3 8 - a ) , f u r t h e r i n c r e a s e i n t e m p e r a t u r e d i f f e r e n c e r e s u l t s i n some f i l m b o i l i n g a t random l o c a t i o n s on t h e h e a t e d c y l i n d e r . F i l m b o i l i n g c o v e r s a v a r i a b l e amount o f t h e c y l i n d e r and s m a l l c h a n g e s i n h e a t t r a n s f e r r a t e , d e p e n d e n t on t h e p e r c e n t a g e o f t h e c y l i n d e r i n f i l m b o i l i n g , were o b s e r v e d . F i l m b o i l i n g showed 73 no p r e f e r e n c e f o r any p a r t o f t h e c y l i n d e r and has been p h o t o g r a p h e d a t v a r i o u s l o c a t i o n s a l o n g t h e c y l i n d e r l e n g t h . ( F i g . 38-b) The amount o f c y l i n d e r a r e a c o v e r e d w i t h f i l m b o i l i n g i n c r e a s e d w i t h i n c r e a s i n g t e m p e r a t u r e d i f f e r e n c e u n t i l a t e m p e r a t u r e d i f f e r e n c e o f 50 deg F was a t t a i n e d . S t a b l e f i l m b o i l i n g was o b s e r v e d a t a l l h i g h e r t e m p e r a t u r e d i f f e r e n c e s ( F i g . 38-c) F i g u r e s ( 3 8 - d ) , ( 3 8 - e ) , and ( 3 8 - f ) show t h e c o r r e s p o n d i n g f r e e c o n v e c t i o n f l o w f i e l d s f o r i n c r e a s i n g t e m p e r a t u r e d i f f e r e n c e a t s u p e r c r i t i c a l p r e s s u r e (1100 p s i a ) . F i g u r e (39) shows t h e e f f e c t o f i n c r e a s i n g v e l o c i t y a t s u b -c r i t i c a l p r e s s u r e a t t h e maximum t e m p e r a t u r e d i f f e r e n c e a t t a i n e d i n t h e p r e s e n t work. F i g u r e (33) a l s o shows t h a t t h e e f f e c t o f p r e s s u r e ' on h e a t t r a n s f e r r a t e f o r f o r c e d c o n v e c t i o n i s s m a l l u n t i l t h e c y l i n d e r t e m p e r a t u r e e x c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a -t u r e . A t l o w e r t e m p e r a t u r e d i f f e r e n c e s t h e h e a t t r a n s f e r r a t e i s v e r y n e a r l y i n d e p e n d e n t o f t h e s u p e r c r i t i c a l p r e s s u r e and t h e f l u i d v e r y n e a r t h e h e a t e d c y l i n d e r and i n t h e f r e e s t r e a m i s i n t h e l i q u i d - l i k e c o n d i t i o n . T h i s i s shown e v e n more c l e a r l y i n F i g u r e (34) where i t c a n be n o t e d t h a t f o r a g i v e n t e m p e r a t u r e d i f f e r e n c e , l e s s t h a n t h e p s e u d o - c r i t i c a l t e m p e r a t u r e , t h e h e a t t r a n s f e r c o e f f i c i e n t d e c r e a s e s s l i g h t l y w i t h i n c r e a s i n g p r e s s u r e . When t h e c y l i n d e r t e m p e r a t u r e e x c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e v a p o r - l i k e f l u i d i s i n c o n t a c t w i t h t h e h e a t e d c y l i n d e r and t h e e f f e c t o f i n -c r e a s i n g p r e s s u r e i s t o i n c r e a s e t h e h e a t t r a n s f e r r a t e . I n s h o r t , i n c r e a s i n g s u p e r c r i t i c a l p r e s s u r e i n c r e a s e s t h e h e a t t r a n s f e r r a t e s i g n i f i c a n t l y o n l y when t h e h e a t e d c y l i n d e r i s i n c o n t a c t w i t h v a p o r - l i k e f l u i d ( t h a t i s , a b o v e t h e p s e u d o c r i t i c a l t e m p e r a t u r e ) and i s i n a l i q u i d - l i k e f r e e -s t r e a m . F i g u r e (34) a l s o shows t h a t t h e h e a t t r a n s f e r c o e f f i c i e n t i n f o r c e d f l o w r a p i d l y a p p r o a c h e s a c o n s t a n t v a l u e , n e a r l y i n d e p e n d e n t o f p r e s s u r e , when t h e t e m p e r a t u r e d i f f e r e n c e e x c e e d s 100 d e g F. F i g u r e (40) shows t h e f l o w f i e l d v a r i a t i o n w i t h t e m p e r a t u r e d i f f e r e n c e and p r e s s u r e v a r i a t i o n f o r f o r c e d c o n v e c t i o n (Re*=650). 4 . 4 I n t e r p r e t a t i o n o f P h o t o g r a p h s S e l e c t e d t y p i c a l 35 mm p h o t o g r a p h s have been r e p r o -d u c e d above ( F i g . 35 t o 40) t o i l l u s t r a t e t h e f l o w f i e l d v a r i a t i o n s n o t e d . A d d i t i o n a l o b s e r v a t i o n s b a s e d on d i r e c t v i s u a l o b s e r v a t i o n o f t h e f l o w f i e l d s w h i l e s y s t e m a t i c a l l y v a r y i n g b u l k t e m p e r a t u r e , b u l k p r e s s u r e , c y l i n d e r t e m p e r a t u r e and f r e e - s t r e a m v e l o c i t y a r e d e s c r i b e d b e l o w . A s e r i e s o f 16 mm h i g h s p e e d m o t i o n p i c t u r e s was a l s o t a k e n a t 5000 pps i n o r d e r t o g a i n i n s i g h t i n t o t h e t i m e d e p e n d e n c e o f t h e e x t r e m e l y r a p i d h e a t t r a n s f e r mechansims w h i c h were n o t e d . A l i s t i n g o f t h e o p e r a t i n g c o n d i t i o n s and a b r i e f summary o f t h e o b s e r v a t i o n s o f e a c h f i l m a r e c o n t a i n e d i n A p p e n d i x (I I ) . 75 The f l o w f i e l d s p h o t o g r a p h e d by t h e h i g h s p e e d f i l m s were o b s e r v e d t o be e s s e n t i a l l y t h e same as shown i n t h e s t i l l p h o t o g r a p h s r e p r o d u c e d a b o v e . The t i m e d e p e n d e n c e o f t h e v a r i o u s h e a t t r a n s f e r mechanisms c o u l d be d e t e r m i n e d by p r o j e c t i n g t h e h i g h s p e e d f i l m s a t a known s l o w e r r a t e o r by a frame by fram e e x a m i n a t i o n . The main v a l u e o f t h e m o v i e s was t o o b s e r v e t h e h e a t t r a n s f e r mechanisms p r o j e c t e d i n s l o w m o t i o n . S e l e c t e d s e c t i o n s o f t h e s i x t e e n f i l m s have been combined i n t o a summary f i l m w h i c h i s a v a i l a b l e f o r v i e w i n g on r e q u e s t . The r e p e a t e d t e a r i n g o f h e a t e d f l u i d l a y e r s f r o m t h e c y l i n d e r i n t h e s u p e r c r i t i c a l f l u i d a t low Re* and t h e r a p i d l y o s c i l l a t i n g r e g i o n b e h i n d t h e h e a t e d c y l i n d e r c a n be c l e a r l y i d e n t i f i e d i n t h e f i l m s and an e s t i m a t e o f f r e -q u ency o b t a i n e d . The t e a r i n g o f h e a t e d f l u i d l a y e r s i s d i r e c t l y v e l o c i t y d e p e n d e n t and o c c u r s a t r o u g h l y t h e f r e -q u ency p r e d i c t e d f o r a Karman V o r t e x s t r e e t . A t h i g h e r Re* t h e s h e e t s o f f l u i d a r e t o r n o f f much f a s t e r t h a n t h e camera c a n f o l l o w and o n l y t h e s e c o n d a r y mechanism o f r a p i d l y (5000 t o 20,000 c p s ) o s c i l l a t i n g clumps o f v a p o r - l i k e f l u i d i s v i s i b l e . O b s e r v a t i o n s f r o m t h e h i g h s p e e d m o v i e s show t h a t t h e r a t e o f b u b b l e g e n e r a t i o n i n n u c l e a t e b o i l i n g i s much s l o w e r (0.01X) t h a n t h e movement o f h e a t e d clumps o f f l u i d a t s u p e r c r i t i c a l p r e s s u r e . O t h e r o b s e r v a t i o n s f r o m t h e m o v i e s s u g g e s t t h a t t h e h e a t t r a n s f e r mechanism a t t h e c o n d i t i o n o f maximum h e a t t r a n s f e r c o e f f i c i e n t i s t h e same as a t much h i g h e r t e m p e r a -t u r e d i f f e r e n c e s . O n l y one mechanism c o u l d be o b s e r v e d i n t h e s u p e r c r i t i c a l f l u i d i n c o n t r a s t t o t h e two d i s t i n c t mechanisms a c t i v e i n t h e s u b c r i t i c a l f l u i d , ( n u c l e a t e b u b b l g e n e r a t i o n and v a p o r - f i l m f o r m a t i o n . ) The s u p e r c r i t i c a l h e a t t r a n s f e r mechanism i n l i q u i d -l i k e f r e e - s t r e a m f l u i d c a n be d e s c r i b e d as s t a b l e f i l m b o i l i n g w i t h o u t a d i s t i n c t i n t e r f a c e between t h e l i q u i d - l i k and v a p o r - l i k e f l u i d b u t a i d e d by t h e t r a n s p o r t o f v a p o r -l i k e f l u i d f r o m t h e h e a t e d c y l i n d e r i n t o t h e wake. 77 DISCUSSION OF RESULTS 5.1 Free Convection Free c o n v e c t i o n heat t r a n s f e r measurements have shown t h a t the heat t r a n s f e r c o e f f i c i e n t s o b t a i n e d i n f r e e c o n v e c t i o n can reach l a r g e peak v a l u e s when the c y l i n d e r temperature j u s t exceeds t h e " p s e u d o - c r i t i c a l temperature f o r the t e s t p r e s s u r e . The -heat t r a n s f e r r a t e however remained a smoothly i n c r e a s i n g f u n c t i o n of i n c r e a s i n g temperature d i f f e r e n c e . Peak v a l u e s of heat t r a n s f e r c o e f f i c i e n t o c c u r r e d whenever the bulk f l u i d temperature was below the p s e u d o - c r i t i c a l temperature, but the heat t r a n s f e r c o e f f i c i e n t s showed o n l y d e t e r i o r a t i o n from a maximum value when the bulk f l u i d temperature exceeded the p s e u d o - c r i t i c a l temperature. R e s u l t s obtained i n the present work are compared with the experimental r e s u l t s obtained by Knapp [18] and G o l d s t e i n and Aung [20] f o r very n e a r l y , the same bulk con-d i t i o n s i n F i g u r e (41). The heat t r a n s f e r r a t e , i n c o n t r a s t to t h a t observed by Knapp, was a smoothly i n c r e a s i n g f u n c t i o n of temperature d i f f e r e n c e and d i d not show the sharp i n c r e a s e with the appearance of a b u b b l e - l i k e flow noted by Knapp ( F i g . 4). No b u b b l e - l i k e flow was observed i n the present study and the heat t r a n s f e r r a t e was observed to be c o n s t a n t l y above t h a t obtained by G o l d s t e i n and Aung. T h i s was expected 78 b e c a u s e o f t h e s m a l l e r h e a t e d c y l i n d e r u s e d i n t h e p r e s e n t s t u d y . The f l o w f i e l d o b s e r v e d a t a l l t e m p e r a t u r e d i f f e r e n c e s seemed q u i t e s i m i l a r t o t h a t d e s c r i b e d by G o l d s t e i n and Aung f o r low t e m p e r a t u r e d i f f e r e n c e s , and e x c e p t f o r an i n s t a b i l i t y due t o s m a l l d i s t u r b a n c e s t h e f l o w f i e l d s were r e p r o d u c i b l e t i m e a f t e r t i m e . W i t h t h e i n i t i a l n i c h r o m e p r o b e a t v e r y l a r g e t e m p e r a t u r e d i f f e r e n c e s ( g r e a t e r t h a n 1000 deg F) t h e o s c i l l a t i n g r e g i o n above t h e h e a t e d c y l i n d e r was o b s e r v e d t o a t t a c h t o t h e h e a t e d c y l i n d e r and an u n u s u a l e f f e c t c o u l d o c c u r . The w i r e m i g h t s t a r t t o b u z z , w i t h a u d i b l e i n t e n s i t y and v e r y q u i c k l y , a f t e r l a r g e c u r r e n t f l u c t u a t i o n s , b r e a k . I t was f e l t t h a t t h e w i r e v i b r a t i o n was a c o m p l i c a t e d i n t e r -a c t i o n o f t h e h e a t t r a n s f e r p r o c e s s , t h e n o n - l i n e a r c o e f f i c i e n t o f r e s i s t i v i t y w i t h t e m p e r a t u r e o f t h e n i c h r o m e , and t h e p r o b e ' c o n s t r u c t i o n d e t a i l s . L a r g e i n c r e a s e s i n h e a t t r a n s f e r were n o t e d d u r i n g t h e v i b r a t i o n b u t no b u b b l e - l i k e f l o w was o b s e r v e d a r o u n d t h e h e a t e d w i r e . I t was n o t p o s s i b l e t o r a i s e t h e p l a t i n u m - o n - q u a r t z - c y l i n d e r t e m p e r a t u r e above 400 deg F b u t t h i s was much g r e a t e r t h a n t h e t e m p e r a t u r e p r e v i o u s l y o b s e r v e d t o i n i t i a t e b u b b l e - l i k e f l o w . I t i s f e l t t h a t t h e d e s c r e p a n c y i n h e a t t r a n s f e r r a t e vs t e m p e r a t u r e d i f f e r e n c e o b s e r v e d by Knapp c a n p o s s i b l y be e x p l a i n e d by t h e n o n - l i n e a r v a r i a t i o n o f e l e c t r i c a l r e s i s t i v i t y w i t h i n c r e a s i n g t e m p e r a t u r e shown by c e r t a i n n i c h r o m e a l l o y s . F i g u r e (4 2) shows t h e a c t u a l r e s i s t a n c e v s t e m p e r a t u r e b e h a v i o u r shown by N i c h r o m e V i n t h e f u l l y a n n e a l e d c o n d i t i o n . N o t e t h a t f o r a g i v e n r e s i s t a n c e 79 r a t i o (or g i v e n v o l t a g e i m b a l a n c e o f a W h e a t s t o n e b r i d g e m e a s u r i n g c i r c u i t ) t h e w i r e c a n have two o r more s u r f a c e t e m p e r a t u r e s . Knapp measured v o l t a g e i m b a l a n c e and i n f e r r e d a w i r e t e m p e r a t u r e by a s s u m i n g a c o n s t a n t , e x p e r i m e n t a l l y d e t e r m i n e d c o e f f i c i e n t o f r e s i s t i v i t y change w i t h t e m p e r a t u r e . I t i s s u g g e s t e d t h a t t h e a p p e a r a n c e o f t h e b u b b l e - l i k e f l o w n o t e d by Knapp a c t u a l l y o c c u r r e d a t a much h i g h e r t e m p e r a t u r e d i f f e r e n c e t h a n r e p o r t e d . F u r t h e r r e s u l t s w h i c h t e n d t o s u p p o r t t h i s s u g g e s -t i o n i n c l u d e t h e f a c t t h a t t h e o c c u r r a n c e o f t h e b u b b l e - l i k e f l o w o b s e r v e d by G o l d s t e i n o c c u r r e d a t r o u g h l y t h e same h e a t f l u x b u t a t much h i g h e r c y l i n d e r t e m p e r a t u r e s . The p r e s e n t r e s u l t s t e n d t o c o n f i r m G o l d s t e i n ' s d a t a t h a t h e a t t r a n s f e r r a t e i s a smooth f u n c t i o n o f t e m p e r a t u r e d i f f e r e n c e and t h a t no b u b b l e - l i k e f l o w a p p e a r s a t l e a s t t o t e m p e r a t u r e d i f f e r e n -c e s up t o 350 deg F. The o r i g i n a l work done by S a b e r s k y on t h e b u b b l e - l i k e s u p e r c r i t i c a l h e a t t r a n s f e r mechanism i n F r e o n (12) a l s o i n d i c a t e d t h a t t h e a p p e a r a n c e o f t h e b u b b l e -l i k e f l o w was n o t marked by a s h a r p i n c r e a s e i n h e a t t r a n s f e r r a t e . Knapp a l s o c o n c l u d e d t h a t a h o r i z o n t a l c y l i n d e r w i t h t h e b u b b l e - l i k e f l o w w o u l d have a h e a t f l u x more t h a n f i v e t i m e s t h a t f r o m e i t h e r a h o r i z o n t a l s t r i p o r a s h o r t v e r t i c a l w a l l i n u n d e r i d e n t i c a l c o n d i t i o n s . H o l t [ 1 7 ] , i n s u p e r c r i t i c a l w a t e r , and K a t o [26] i n s u p e r c r i t i c a l c a r b o n d i o x i d e f o u n d t h a t t h e h e a t f l u x f r o m a f l a t s t r i p was v e r y n e a r l y t h e same as f r o m a h o r i z o n t a l c y l i n d e r . 80 I t i s a l s o suggested t h a t the b u b b l e - l i k e mechanism p r e v i o u s l y observed may not be an improved mechanism of heat t r a n s f e r . The b u b b l e - l i k e flow has o n l y been noted when the f l u i d near the c y l i n d e r i s i n the v a p o r - l i k e c o n d i t i o n and the f r e e - s t r e a m i s l i q u i d - l i k e . The v a p o r - l i k e f l u i d , even i f r e o r g a n i s e d i n t o a b u b b l e - l i k e r e g i o n , has i n s u l a t i n g p r o p e r t i e s very much l i k e the s u b c r i t i c a l vapor f i l m which covers the c y l i n d e r i n s t a b l e f i l m b o i l i n g . T h i s i d e a t h a t the bubble-l i k e mechanism may not r e s u l t i n a l a r g e i n c r e a s e i n heat t r a n s f e r r a t e i s supported by the data of Sabersky [12] and G o l d s t e i n [ 2 0 ] . However a b u b b l e - l i k e mechanism of heat t r a n s f e r could not be observed with the present c o n s t a n t temperature c y l i n d e r a t the bulk c o n d i t i o n s and c y l i n d e r temperature d i f f e r e n c e s p r e v i o u s l y r e p o r t e d . The p r e s e n t work i n d i c a t e s t h a t the b u b b l e - l i k e flow i s a very e l u s i v e phenomenon as i t c o u l d not be found with the p r e s e n t experimental apparatus. I t should be emphasized t h a t the p r e s e n t study, i n a r e l a t i v e l y l a r g e volume t e s t chamber, has shown the f r e e c o n v e c t i o n heat t r a n s f e r r a t e and flow f i e l d s t o be g r e a t l y a f f e c t e d by c i r c u l a t i n g d i s t u r b a n c e s caused by minor temper-atur e v a r i a t i o n s i n the bulk f l u i d through the t e s t loop. The unusual l a r g e i n c r e a s e s i n heat t r a n s f e r r a t e o c c a s i o n a l l y observed i n s u p e r c r i t i c a l Freon by Doughty and Drake [13] appeared very s i m i l a r to those t r a c e d , by d i r e c t flow f i e l d o b s e r v a t i o n , t o c i r c u l a t i n g d i s t u r b a n c e s i n the p r e s e n t study. 81 S m a l l d i f f e r e n c e s i n measured h e a t t r a n s f e r r a t e between d i f f e r e n t i n v e s t i g a t o r s w o r k i n g a t e s s e n t i a l l y t h e same b u l k c o n d i t i o n s a r e a l s o p r o b a b l y due t o c o n t a i n e r g e o m e t r y e f f e c t s . 5.2 F o r c e d C o n v e c t i o n H e a t T r a n s f e r The p u r p o s e o f t h e p r e s e n t s t u d y was t o d e t e r m i n e t h e v a r i a b l e s a f f e c t i n g f o r c e d c o n v e c t i o n h e a t t r a n s f e r i n n e a r - c r i t i c a l c a r b o n d i o x i d e and t o a t t e m p t t o u n d e r s t a n d t h e mechanism o f h e a t t r a n s f e r i n t h e r e g i o n where s t r o n g p r o p e r t y v a r i a t i o n s c a n be e x p e c t e d i n t h e t h e r m a l b o u n d a r y l a y e r o v e r a h e a t e d c y l i n d e r . P h o t o g r a p h s o f t h e h e a t e d f l o w f i e l d s have been p r e s e n t e d f o r v a r i o u s o p e r a t i n g c o n d i t i o n s and t h e h e a t t r a n s f e r p r o c e s s d e s c r i b e d b o t h as i t a p p e a r s f r o m d i r e c t o b s e r v a t i o n and as i n t h e h i g h s p e e d m o v i e s . E x p e r i m e n t a l r e s u l t s i n f o r c e d c o n v e c t i o n show t h a t h i g h h e a t t r a n s f e r r a t e s a r e p o s s i b l e w i t h v e r y low f r e e -s t r e a m v e l o c i t i e s n o r m a l t o a h e a t e d c y l i n d e r i n s u p e r c r i t i c a l c a r b o n d i o x i d e . C o n t r a r y t o i n i t i a l e x p e c t a t i o n s no u n u s u a l mechanisms o f h e a t t r a n s f e r have been o b s e r v e d . The e f f e c t o f even a s l i g h t v e l o c i t y however, was t o b r e a k up t h e w e l l o r d e r e d f r e e c o n v e c t i o n f l o w and t o g r e a t l y i n c r e a s e t h e h e a t t r a n s f e r r a t e . H eat t r a n s f e r i n n e a r - c r i t i c a l d i o x i d e c a n be r o u g h l y d i v i d e d i n t o two d i s t i n c t c l a s s e s ; s u b c r i t i c a l w i t h t h e w e l l e s t a b l i s h e d r e g i m e s o f n u c l e a t e , t r a n s i e n t f i l m and n u c l e a t e , 82 and s t a b l e f i l m b o i l i n g , and s u p e r c r i t i c a l i n w h i c h o n l y one b a s i c h e a t t r a n s f e r m e c h a n i s m h a s b e e n o b s e r v e d . F o r c e d f l o w a t s u b c r i t i c a l p r e s s u r e s h a s two d i s t i n c t h e a t t r a n s f e r mechan-i s m s ; t h e f i r s t c o n s i s t s o f d i s t i n c t v a p o r b u b b l e s w h i c h e x t r a c t h e a t f r o m t h e s u r f a c e i n t h e f o r m o f a h e a t o f v a p o r -i z a t i o n and c a r r y t h i s h e a t i n t o t h e n o r m a l c o n v e c t i o n o f h e a t e d f l u i d o v e r t h e c y l i n d e r , t h e s e c o n d m e c h a n i s m c o n s i s t s o f a v a p o r f i l m o v e r p a r t o r a l l o f t h e h e a t e d c y l i n d e r w h i c h a l s o t a k e s h e a t o f v a p o r i z a t i o n f r o m t h e s u r f a c e b u t i n s u l a t e s t h e s u r f a c e f r o m t h e f r e e - s t r e a m f l o w . When o n l y p a r t o f t h e h e a t e d s u r f a c e i s c o v e r e d b y a v a p o r b l a n k e t t h e h e a t t r a n s f e r c o e f f i c i e n t d e c r e a s e s s h a r p l y w i t h i n c r e a s i n g t e m p e r a t u r e d i f f e r e n c e , r e s u l t i n g i n d e s t r u c -t i o n o f a c o n s t a n t c u r r e n t h e a t e r . The p r e s e n t c o n s t a n t t e m p e r a t u r e c y l i n d e r v e r y c l e a r l y i n d i c a t e d t h e d e t e r i o r a t i o n i n h e a t t r a n s f e r c o e f f i c i e n t as t h e v a p o r f i l m b e g a n t o i n s u l a t e t h e h e a t e d c y l i n d e r f r o m t h e f r e e - s t r e a m f l u i d . O t h e r o b s e r v a t i o n s , s u c h a s t h e rms component o f t h e c y l i n d e r v o l t a g e , i n d i c a t e t h e t r a n s i e n t f i l m b o i l i n g r e g i m e t o be v e r y u n s t a b l e . The h e a t t r a n s f e r r a t e a t a c o n s t a n t t e m p e r a t u r e d i f f e r e n c e may v a r y b y 10% w i t h t i m e . The v a r i a t i o n d i s a p p e a r s when t h e e n t i r e c y l i n d e r i s c o v e r e d b y a s t a b l e v a p o r - b l a n k e t . I n c o n t r a s t , when t h e b u l k p r e s s u r e i s i n c r e a s e d t o j u s t o v e r t h e c r i t i c a l v a l u e t h e h e a t t r a n s f e r m e c h a n i s m c h a n g e s d r a s t i c a l l y and becomes much l i k e c o n s t a n t p r o p e r t y f o r c e d c o n -v e c t i o n e x c e p t t h a t t h e f l u i d l a y e r s w h i c h r o l l up i n t o Karmen 83 v o r t i c e s a r e r e g i o n s o f h e a t e d v a p o r - l i k e f l u i d , and clumps o f f l u i d c a n be t r a n s p o r t e d i n t o t h e main f r e e - s t r e a m s t i l l m a i n t a i n i n g p a r t o f t h e i r i d e n t i t y . The f l o w most r e s e m b l e s f i l m b o i l i n g w i t h no d i s t i n c t i n t e r f a c e between t h e l i q u i d -l i k e and v a p o r - l i k e f l u i d b u t a i d e d by clumps o f h o t t e r f l u i d f r o m t h e h e a t e d s u r f a c e w h i c h t r a n s f e r h e a t i n t o t h e wake b e h i n d t h e c y l i n d e r . Measurements o f t h e rms component o f t h e c y l i n d e r power s u p p l y v o l t a g e show t h a t t h e s u p e r -c r i t i c a l h e a t t r a n s f e r mechanism i s n o t t i m e - d e p e n d e n t l i k e t r a n s i e n t f i l m b o i l i n g a t t h e same t e m p e r a t u r e d i f f e r e n c e s . H eat t r a n s f e r i n s u p e r c r i t i c a l c a r b o n d i o x i d e c a n a l s o be d i v i d e d i n t o two b r o a d c l a s s e s ; l i q u i d - l i k e f r e e -s t r e a m f l u i d , and v a p o r - l i k e f r e e - s t r e a m f l u i d . When t h e f r e e s t r e a m f l u i d i s l i q u i d - l i k e , peak h e a t t r a n s f e r c o e f f i c -i e n t s c a n be e x p e c t e d when t h e h e a t e d c y l i n d e r t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . The same mechanism a p p e a r s t o o p e r a t e a t a l l t e m p e r a t u r e d i f f e r e n c e s and t h e peak h e a t t r a n s f e r c o e f f i c i e n t c a n be due t o t h e l a r g e v a l u e s o f t h e t r a n s p o r t p r o p e r t i e s o c c u r r i n g i n t h e f l u i d v e r y n e a r t h e h e a t e d c y l i n d e r . The e x t r a h e a t t r a n s -f e r r e d by t h e clumps o f h e a t e d f l u i d t r a n s p o r t e d d i r e c t l y f r o m t h e h e a t e d s u r f a c e i n t o t h e c y l i n d e r wake i n c r e a s e s w i t h t h e d e n s i t y d i f f e r e n c e between t h e l i q u i d - l i k e and v a p o r - l i k e f l u i d ( t h e r e f o r e w i t h i n c r e a s i n g t e m p e r a t u r e ) . The f o r c e d c o n v e c t i o n p r o c e s s i s more v e l o c i t y d e p e n d e n t w i t h i n c r e a s i n g t e m p e r a t u r e d i f f e r e n c e and t h i s i s i n agreement w i t h t h e i d e a t h a t t h e e x t r a h e a t t r a n s f e r i s s u p p l i e d by t h e clumps o f v a p o r - l i k e f l u i d . T h i s enhancement mechanism i s s i m i l a r t o t h e i d e a o f a t u r b u l e n t eddy d i f f u s i v i t y f i r s t t r e a t e d by D i e s s l e r [ 4 ] i n s u p e r c r i t i c a l w a t e r and more r e c e n t l y by J a c k s o n e t a l . [51] i n s u p e r c r i t i c a l c a r b o n d i o x i d e . S t r i c t l y s p e a k i n g t h e i d e a o f a t u r b u l e n t eddy d i f f u s i v i t y i s v a l i d o n l y i n t h e f r e e - s t r e a m and d o e s n o t a p p l y t o f l u i d i n t h e s u b l a y e r a d j a c e n t t o t h e h e a t e d s u r f a c e . I n t h e p r e s e n t c a s e t h e h e a t e d clumps o f f l u i d seem t o be t r a n s p o r t e d f r o m t h e s u b l a y e r i n t o t h e f r e e - s t r e a m and t h e mechanism a p p e a r s s i m i l a r t o t h a t d e s c r i b e d by Hauptmann [30] i n w h i c h t h e h e a t e d c l u s t e r s a f f e c t e d t h e t u r b u l e n c e l e v e l o f f l o w o v e r a f l a t p l a t e t h r o u g h t h e s u b l a y e r . Hauptmann o b s e r v e d t h a t c l u s t e r g e n e t a t i o n was s t r o n g e s t w i t h l i q u i d - l i k e f r e e -s t r e a m f l u i d when t h e p l a t e t e m p e r a t u r e was above t h e p s e u d o -c r i t i c a l and t h a t t h e c l u s t e r s seemed t o have a d e f i n i t e t i m e b a s e . The i d e a o f p a c k e t s o f v a p o r - l i k e f l u i d b e i n g d i r e c t l y t r a n s p o r t e d i n t o t h e f r e e - s t r e a m has b e en r e c e n t l y p o s t u l a t e d by Graham [32] as a p e n e t r a t i o n m o d e l . P r e s e n t r e s u l t s seem t o r e - i n f o r c e s u c h a m o d e l . b u t t h e i n t e n s i t y o f clump movement was o b s e r v e d t o be r e l a t e d t o t h e p r o x i m i t y o f t h e b u l k f l u i d and c y l i n d e r t e m p e r a t u r e s t o t h e p s e u d o -c r i t i c a l t e m p e r a t u r e . The m i x i n g a c t i o n o f t h e h e a t e d clumps i s l a r g e s t i n c o l d e r b u l k f l u i d w i t h t h e c y l i n d e r much h o t t e r t h a n t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . 85 S h a r p e s t p e a k s i n h e a t i n h e a t t r a n s f e r c o e f f i c i e n t c a n be e x p e c t e d w i t h l i q u i d - l i k e b u l k f l u i d n e a r t h e c r i t i c a l p r e s s u r e when t h e c y l i n d e r t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . I n c r e a s i n g p r e s s u r e d e c r e a s e s and b r o a d e n s t h e peak h e a t t r a n s f e r c o e f f i c i e n t and t h e h e a t t r a n s f e r r a t e d o e s n o t show an i n c r e a s e u n t i l t h e p s e u d o -c r i t i c a l t e m p e r a t u r e has been e x c e e d e d and v a p o r - l i k e f l u i d i s i n c o n t a c t w i t h t h e h e a t e d c y l i n d e r . I n c r e a s e i n b u l k t e m p e r a t u r e w i l l r e s u l t i n o n l y a v e r y s m a l l d e c r e a s e i n h e a t t r a n s f e r r a t e u n t i l t h e f r e e - s t r e a m becomes v a p o r - l i k e . F o r c e d c o n v e c t i o n s u p e r c r i t i c a l h e a t t r a n s f e r w i t h a v a p o r - l i k e f r e e - s t r e a m i s n o t n e a r l y as e f f e c t i v e as w i t h t h e l i q u i d - l i k e f r e e - s t r e a m . H e a t t r a n s f e r r a t e s a r e l e s s t h a n h a l f as much as w i t h b u l k f l u i d o n l y 5 t o 10 deg F c o l d e r f o r t h e same Re* and t e m p e r a t u r e d i f f e r e n c e . Heat t r a n s f e r c o e f f i c i e n t s c a n be e x p e c t e d t o show o n l y d e t e r i o r -a t i o n w i t h i n c r e a s i n g t e m p e r a t u r e d i f f e r e n c e i n v a p o r - l i k e b u l k f l u i d , and t h e amount o f d e t e r i o r a t i o n i s s h a r p e s t a t p r e s s u r e s c l o s e s t t o t h e c r i t i c a l . A t p r e s s u r e s f u r t h e r f r o m t h e c r i t i c a l t h e h e a t t r a n s f e r c o e f f i c i e n t i n v a p o r -l i k e f l u i d i s v e r y n e a r l y i n d e p e n d e n t o f t e m p e r a t u r e d i f f e r e n c e . The o b s e r v e d f l o w f i e l d s a r e v e r y s i m i l a r i n b o t h l i q u i d - l i k e and v a p o r - l i k e f l u i d and show v e r y l i t t l e v a r i -a t i o n w i t h t e m p e r a t u r e d i f f e r e n c e i n e i t h e r c a s e . The i n t e n s i t y o f t h e m i x i n g i n t h e wake and t h e amount o f 86 o s c i l l a t i o n n o t e d b e h i n d t h e c y l i n d e r a r e b u l k t e m p e r a t u r e , b u l k p r e s s u r e , and v e l o c i t y d e p e n d e n t b u t a l l s u p e r c r i t i c a l f l o w f i e l d s were o b s e r v e d t o be r e m a r k e d l y s i m i l a r . The e x p r e s s i o n f o r f o r c e d c o n v e c t i v e h e a t t r a n s f e r c o e f f i c i e n t d e v e l o p e d by P e t u k o v [ 1 0 ] f o r f l o w i n s i d e h e a t e d t u b e s t r e a t s t h e Nu as a f u n c t i o n o f t h e f l u i d Re and P r and d e t e r m i n e s c o n s t a n t s t o f i t t h e e x p e r i m e n t a l d a t a . The p h y s i c a l p r o p e r t i e s o f s u p e r c r i t i c a l c a r b o n d i o x i d e were u s e d t o e v a l u a t e t h e d i m e n s i o n l e s s g r o u p s and t h e p r e d i c t e d h e a t t r a n s f e r c o e f f i c i e n t v a r i a t i o n w i t h t e m p e r a t u r e a t d i f f e r e n t b u l k p r e s s u r e s q u a l i t a t i v e l y r e s e m b l e s p r e s e n t r e s u l t s . An e x p r e s s i o n f o r a t u r b u l e n t f o r c e d c o n v e c t i o n h e a t t r a n s f e r c o e f f i c i e n t i n t h e same f o r m as t h a t d e v e l o p e d by P e t u k o v c o u l d be j u s t i f i e d i n t h e p r e s e n t work and s u i t -a b l e c o n s t a n t s d e t e r m i n e d t o f i t t h e e x p e r i m e n t a l d a t a . However, s u c h an e x p r e s s i o n w o u l d be o f v e r y l i m i t e d u s e f u l -n e s s as i t w o u l d a p p l y o n l y t o t h e p r e s e n t s i t u a t i o n s . O b s e r v a t i o n s o f t h e h e a t e d f l o w f i e l d a l s o s u g g e s t t h a t t h e enhancement e f f e c t on h e a t t r a n s f e r r a t e due t o t h e d i f f u s i o n clumps o f h e a t e d f l u i d must be i n c l u d e d i n any a n a l y t i c a l model o f t h e f l o w s i t u a t i o n . No f u r t h e r d e v e l o p m e n t on a t h e o r e t i c a l model o f f o r c e d c o n v e c t i o n i n t h e n e a r - c r i t i c a l r e g i o n has b een a t t e m p t e d as i t i s f e l t t h a t t h e p r o b l e m may b e s t be h a n d l e d by an e x t e n s i o n o f two-phase t e c h n i q u e s t o i n c l u d e v a r i a b l e f l u i d p r o p e r t i e s i n e a c h p h a s e and t o d a t e even t h e s i m p l e s t two-phase p r o b l e m has n o t been c o m p l e t e l y s o l v e d . 87 CONCLUSIONS 6.1 G e n e r a l C o n c l u s i o n s a b o u t t h e h e a t t r a n s f e r c o e f f i c i e n t , t h e h e a t t r a n s f e r r a t e b e h a v i o u r and t h e h e a t t r a n s f e r mechan-i s m i n n e a r - c r i t i c a l c a r b o n d i o x i d e have been drawn on t h e b a s i s o f t h e e x p e r i m e n t a l r e s u l t s p r e s e n t e d and a s t u d y o f t h e f l o w f i e l d s b o t h by d i r e c t o b s e r v a t i o n and h i g h s p e e d p h o t o g r a p h y . S t r i c t l y s p e a k i n g , t h e c o n c l u s i o n s drawn a r e v a l i d o n l y f o r f o r c e d c o n v e c t i o n n o r m a l t o a h e a t e d c y l i n d e r m a i n t a i n e d a t v a r i o u s c o n s t a n t t e m p e r a t u r e d i f f e r e n c e s i n c a r b o n d i o x i d e . I t i s f e l t t h a t t h e c o n c l u s i o n s c a n q u a l i -t a t i v e l y be e x t e n d e d t o t h e n e a r - c r i t i c a l b e h a v i o u r o f o t h e r f l u i d s as t o t h e s t r o n g i n f l u e n c e o f v e l o c i t y and t h e p s e u d o -c r i t i c a l t e m p e r a t u r e on t h e mechanism o f h e a t t r a n s f e r . 6.2 F r e e C o n v e c t i o n Heat T r a n s f e r I n f r e e c o n v e c t i o n t h e h e a t t r a n s f e r c o e f f i c i e n t e x h i b i t s peak v a l u e s when t h e c y l i n d e r t e m p e r a t u r e j u s t ex-c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e b u t t h e h e a t t r a n s f e r r a t e i s a s m o o t h l y i n c r e a s i n g f u n c t i o n o f t e m p e r a t u r e d i f f e r -e n c e . U n u s u a l h e a t t r a n s f e r mechanisms p r e v i o u s l y r e -p o r t e d were n o t o b s e r v e d b u t t h e h e a t t r a n s f e r r a t e and t h e f r e e c o n v e c t i o n f l o w f i e l d s a r e e x t r e m e l y s e n s i t i v e t o s m a l l d i s t u r b a n c e s i n t h e t e s t f l u i d p r o p e r t i e s t o n o n - u n i f o r m t e m p e r a t u r e . The f r e e c o n v e c t i o n h e a t t r a n s f e r c o e f f i c i e n t i s a c o m p l i c a t e d f u n c t i o n o f c y l i n d e r t e m p e r a t u r e , t h e r e l a t i o n -s h i p o f t h e b u l k f l u i d t e m p e r a t u r e t o t h e p s e u d o - c r i t i c a l t e m p e r a t u r e , and t o a l e s s e r e x t e n t b u l k f l u i d t e m p e r a t u r e and p r e s s u r e . L a r g e s t h e a t t r a n s f e r c o e f f i c i e n t s c a n be e x p e c t e d i n l i q u i d - l i k e b u l k f l u i d , n e a r e s t t o t h e c r i t i c a l p r e s s u r e , and when t h e c y l i n d e r t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . A p o s s i b l e e x p l a n a t i o n f o r t h e l a r g e v a r i a t i o n s i n h e a t t r a n s f e r r a t e p r e v i o u s l y o b s e r v e d by o t h e r w o r k e r s i s t h e n o n - l i n e a r b e h a v i o u r o f t h e c o e f f i c i e n t o f r e s i s t i v i t y w i t h t e m p e r a t u r e o f n i c h r o m e w i r e . 6.3 F o r c e d C o n v e c t i o n H e a t T r a n s f e r 6.3.1 F o r c e d C o n v e c t i o n - G e n e r a l The f o r c e d c o n v e c t i o n h e a t t r a n s f e r c o e f f i c i e n t was m e a s u r e d as a s t r o n g and c o m p l i c a t e d f u n c t i o n o f f r e e -s t r e a m v e l o c i t y , h e a t e d c y l i n d e r t e m p e r a t u r e , t h e p r o x i m i t y o f b u l k t e m p e r a t u r e t o p s e u d o - c r i t i c a l t e m p e r a t u r e , and t o a l e s s e r e x t e n t b u l k f l u i d p r e s s u r e . The h e a t t r a n s f e r c o e f f i c i e n t showed l a r g e peak v a l u e s i n p a r t s o f t h e n e a r -c r i t i c a l r e g i o n b u t t h e h e a t t r a n s f e r r a t e was a l w a y s a 89 s m o o t h l y i n c r e a s i n g f u n c t i o n o f i n c r e a s i n g v e l o c i t y and temper-a t u r e d i f f e r e n c e . The v e r y l a r g e h e a t t r a n s f e r r a t e s m e a s u r e d i n f r e e c o n v e c t i o n c a n be g r e a t l y i n c r e a s e d by e v e n a s m a l l f r e e -s t r e a m v e l o c i t y . 6.3.2 E f f e c t o f V e l o c i t y The e f f e c t o f v e l o c i t y i s t o i n c r e a s e t h e h e a t t r a n s f e r r a t e and h e a t t r a n s f e r c o e f f i c i e n t f o r a l l s u p e r -c r i t i c a l b u l k p r e s s u r e s , b u l k t e m p e r a t u r e s , and t e m p e r a t u r e d i f f e r e n c e s i n v e s t i g a t e d . The e f f e c t o f v e l o c i t y on h e a t t r a n s f e r r a t e i s l a r g e s t a t h i g h e r t e m p e r a t u r e d i f f e r e n c e s i n l i q u i d - l i k e f r e e - s t r e a m f l u i d b u t o n l y when t h e h e a t e d c y l i n d e r i s h o t t e r t h a n t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . The s t r o n g e s t e f f e c t o f v e l o c i t y on h e a t t r a n s f e r c o e f f i c i e n t o c c u r s i n l i q u i d - l i k e b u l k f l u i d n e a r e s t t h e c r i t i c a l p r e s s u r e and when t h e c y l i n d e r t e m p e r a t u r e j u s t e x c e e d s t h e p s e u d o -c r i t i c a l t e m p e r a t u r e . The e f f e c t s o f e v e n d o u b l i n g t h e f r e e -s t r e a m v e l o c i t y a r e s m a l l when b o t h t h e c y l i n d e r t e m p e r a t u r e and b u l k t e m p e r a t u r e a r e above o r be l o w t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . 6.3.3 E f f e c t o f B u l k T e m p e r a t u r e When t h e b u l k f l u i d t e m p e r a t u r e i s be l o w t h e p s e u d o -c r i t i c a l t e m p e r a t u r e , t h e h e a t t r a n s f e r c o e f f i c i e n t show a maximum a t a c y l i n d e r t e m p e r a t u r e s l i g h t l y above t h e p s e u d o -90 c r i t i c a l t e m p e r a t u r e . When t h e b u l k t e m p e r a t u r e i s above t h e p s e u d o - t e m p e r a t u r e t h e h e a t t r a n s f e r c o e f f i c i e n t shows o n l y d e t e r i o r a t i o n w i t h i n c r e a s i n g c y l i n d e r t e m p e r a t u r e . The p e a k s i n h e a t t r a n s f e r c o e f f i c i e n t become l a r g e r w i t h i n c r e a s -i n g v e l o c i t y and s h i f t s l i g h t l y f u r t h e r above t h e p s e u d o -c r i t i c a l t e m p e r a t u r e . A t a g i v e n s u p e r c r i t i c a l p r e s s u r e , t h e h e a t t r a n s f e r r a t e , a t a c o n s t a n t t e m p e r a t u r e d i f f e r e n c e , d e c r e a s e s l i g h t l y as t h e b u l k t e m p e r a t u r e a p p r o a c h e s t h e p s e u d o - c r i t i c a l t e m p e r a t u r e and d e c r e a s e d s h a r p l y as t h e b u l k t e m p e r a t u r e i n c r e a s e b e y o n d t h e p s e u d o - c r i t i c a l t e m p e r a t u r e . 6.3.4 E f f e c t o f B u l k P r e s s u r e E x c e p t f o r a n a r r o w r a n g e o f c y l i n d e r t e m p e r a t u r e s v e r y n e a r t o t h e peak n u c l e a t e h e a t f l u x , t h e h e a t t r a n s f e r r a t e i s l a r g e r a t s u p e r c r i t i c a l p r e s s u r e s t h a n a t s u b c r i t i c a l p r e s s u r e s f o r t h e same b u l k t e m p e r a t u r e and f r e e - s t r e a m v e l o c i t y . A t s u p e r c r i t i c a l p r e s s u r e , whenever t h e h e a t e d c y l i n d e r i s h o t enough t o be i n c o n t a c t w i t h v a p o r - l i k e f l u i d , t h e h e a t t r a n s f e r r a t e i n c r e a s e s w i t h i n c r e a s i n g p r e s s u r e . The h e a t t r a n s f e r c o e f f i c i e n t d e c r e a s e s s l i g h t l y w i t h i n c r e a s -i n g p r e s s u r e when t h e c y l i n d e r t e m p e r a t u r e i s n o t h o t enough t o be i n c o n t a c t w i t h v a p o r - l i k e f l u i d . 6.3.5 H e a t T r a n s f e r Mechanisms The h e a t t r a n s f e r mechanism i s s u p e r c r i t i c a l c a r b o n d i o x i d e a p p e a r s t o be n o r m a l f o r c e d c o n v e c t i o n w i t h no c l e a r d i s t i n c t i o n between t h e l i q u i d - l i k e and v a p o r - l i k e f l u i d b u t a i d e d by clumps o f v a p o r - l i k e f l u i d w h i c h d i f f u s e i n t o t h e f r e e - s t r e a m . The l a r g e v a l u e s o f h e a t t r a n s f e r c o e f f i c i e n t and t h e l o c a t i o n o f t h e peak h e a t t r a n s f e r c o e f f i c i e n t c a n p r o b a b l y be e x p l a i n e d i n t e r m s o f t h e l a r g e p r o p e r t y v a r i a t i o n s . P r e -d i c t i o n s o f h e a t t r a n s f e r r a t e a p p e a r p o s s i b l e w i t h s u i t a b l y s e l e c t e d mean p r o p e r t y v a l u e s b u t an enhancement f a c t o r , d e p e n d e n t on t h e p r o x i m i t y o f t h e b u l k t e m p e r a t u r e t o t h e p s e u d o - c r i t i c a l t e m p e r a t u r e , s h o u l d be i n c l u d e d i n any a n a -l y t i c a l approach.. 92 t / * / -/ 60 70 80 9 0 100 110 120 TEMPERATURE [°F~ F i g u r e 1 Summary o f N e a r - C r i t i c a l R e g i o n T e r m i n o l o g y I H -ro K> -3 <^ H -O OJ l-l O 13 ro rt < OJ l-i H -OJ rt H -O cn H -a ro OJ H I o i-i H -rt H -n n OJ i i cr o t> D H -O X H -ro PRANDTL NUMBER [ p r ] ( x 2 ) DENSITY (LB/FT 3 ) e6 o DYNAMIC CM O CM VISCOSITY 130 94 F i g u r e 3 P r o p e r t y V a r i a t i o n s w h i c h w o u l d o c c u r i n S u p e r -c r i t i c a l C 0 2 f o r t h e F r e e C o n v e c t i o n T e m p e r a t u r e D i s t r i b u t i o n D e v e l o p e d i n A i r 95 B u b b l e - L i k e F l o w = 260 deg F F i g u r e 4 B u b b l e - L i k e Heat T r a n s f e r M echanism P r e v i o u s l y O b s e r v e d by Knapp WORKER GEOMETRY MATERIAL Tb RANGE Pb RANGE TEMPERATURE DIFFERENCE RANGE KNAPP & SABERSKY 1965. GOLDSTEIN & AUNG 1966 NISHIKAWA & MIYABE (Data=YAMAGATA) 1962-65 DANIELS & BRAMALL 1965 DUBROVINA & SKRIPOV 1964-65 HORIZONTAL CYLINDER SHORT VERTICAL WALL HORIZONTAL STRIP VERTICAL CYLINDER 0.010" NICHROME 49-137°F 1100-1500 psia HORIZONTAL CYLINDER 0.015" PLATINUM 49-136°F 1075-1300 psia HORIZONTAL CYLINDER 0.008" NICHROME 79-155°F 1050-1470 psia HORIZONTAL CYLINDER HORIZONTAL CYLINDER VERTICAL CYLINDER NISHIWAKI, KATO, KERATA HORIZONTAL CYLINDER 1967 SHORT VERTICAL WALL MONEL 68-104°F 865-1265 psia 0.001" PLATINUM 88-99°F 0.080"ST. STEEL 60-103°F 900-1275 psia 1176 psia 0-400°F 0-1500°F (most to 400°F) 0-800°F (most to 200°F) 0-200°F 0-0.5°F (Measured along isotherms) 0-80°F 0-90°F (most 0-35°F) ALL ABOVE WORK WAS MAINLY WITH THE CIRCULAR CYLINDER GEOMETRY USING SHORT WALLS, OR VERTICAL CYLINDERS AS A CHECK AND WAS IN FREE CONVECTION. IN FORCED CONVECTION: NISHIWAKI, KATO, HIRATA 1967 HORIZONTAL CYLINDER IN FORCED CROSSFLOW (one flow rate was used but a Reynolds number variation due to property - variations was noted) 0.057" ST. STEEL 82-92°F 1176 psia 0-30°F (most 0-20°F) Figure 5 Summary of Previous Work With Heated Cylinders in Supercri t ical Fluids ~ COMPRESSED AIR SUPPLY COMPRESSED ? AIR EXHAUST VACUUM PUMP CANNED ROTOR PUMP SYSTEM EXHAUST X <*0 F i g u r e 6 S c h e m a t i c o f N e a r - C r i t i c a l C a r b o n D i o x i d e F o r c e d F l o w L o o p f F i g u r e 7 Cutaway V i e w o f T e s t S e c t i o n A r r a n g e m e n t F i g u r e 8 S e c t i o n o f T e s t S e c t i o n B l o c k 100 F i g u r e 9 G e n e r a l L a y o u t o f E x p e r i m e n t a l E q u i p m e n t TEST SECTION T/C (1) COPPER SELECTOR SWITCH VENTURI METER T/C(2) COLD N ICE BATH 6 6 PYE GALVANOMETER L & N K-5 POTENTIOMETER CONSTANTAN F i g u r e 10 T h e r m o c o u p l e M e a s u r i n g C i r c u i t o I—1 Tb=80°F ?h- moo PS-IA 300 • • • • • • •43° n O • 40 80 120 TEMPE 200 2 DIFFERENCE Figure 11 R e p r o d u c i b i l i t y of Data F i g u r e 13 F l o w F i e l d A r o u n d P r o b e and S u p p o r t s i n F r e e C o n v e c t i o n MIRROR TEST SECTION BLOCK SOURCE CONDENSER ^SCHLIEREN LENS F i g u r e 14 A r r a n g e m e n t o f S c h l i e r e n S y s t e m ADJUSTABLE CUTOFF . _\ 1 SCHLIEREN LENS SCREEN OR CAMERA HEATED CYLINDER MIRROR I 1 CN CN o CN 00 I I i o o x -1 1 1 1 1 1 1 + Tb :80°F Re* O Pb : 1100 PSIA o o o ° o o °^-°1050-o o 600 o A o o A A • D n O O A • a D D — 3 0 0 - o o o A A A • a • • D o A n • • o ° A A o A r\ A • n o *± o A A „ • O A • _ O A o ° A D ° ° A • oo D . • • • • D • • • • • ° CD 0 • • 1 1 40 80 120 160 200 240 280 TEMPERATURE DIFFERENCE [°F] 320 F i g u r e 15 E f f e c t o f V e l o c i t y on H e a t T r a n s f e r R a t e i n CO., a t 80 deg F a n d 1100 p s i a 2 I 1 A 1-CN U. CM or h- o CN 2 «o I— O LU O LU o TPC c5b o ° o A*A o I I Tb = 80°F Pb = 1100 PSIA Re A O A ° 0 1050 A A a • n > A — 600 • •••300 O .... o o ° O A A" A • • n • • o A A • 8 0 • f V o O o A A • O o o o 40 9 O O 9 • — I 120 160 200 240 280 TEMPERATURE DIFFERENCE [°F] Figure 16 E f f e c t of V e l o c i t y on Heat Transfer C o e f f i c i e n t i n CO, at 80 deg F and 1100 ps i a z 80 320 Tb = 87 F Re Pb =1100 PSIA 625 . • • • ' + + * . — 310 • • T ' v v • • V . • • T v • • v • • V v v v v - ®—0 V „ v O 9 9 © o © " 0 40 80 120 160 200 240 280 320 TEMPERATURE DIFFERENCE [°F] r e 17 E f f e c t o f V e l o c i t y on H e a t T r a n s f e r R a t e i n CO, a t 87 deg F and 1100 p s i a 2 CN O CN V Tb ~-87 °F |~y • Pb= 1 100 PS IA ^ V • • V _ • v v * • ^ ' ^ v „ ^ • ^  •  ^  Re v v v I v I T j — 3 1 0 ® * * » o @ a „ v — - 1 5 5 • " ° # • • • ® 9 9 9 -0 4 0 80 120 160 200 240 280 320 T E M P E R A T U R E D I F F E R E N C E [ ° F ] F i g u r e 18 E f f e c t o f V e l o c i t y on H e a t T r a n s f e r C o e f f i c i e n t i n C 0 o a t 87 d e g F and 1100 p s i a 2 io 1 CN CN O CN Z> h-DQ i — ^ lO "o x *—^ o X " < LU 3Z Tb = 9 1 °F Pb = 1 1 0 0 PSIA A " D A • A O D • a Re A 600 • — 300 — 1 50 0 40 80 120 160 200 240 280 TEMPERATURE DIFFERENCE [°F] 320 F i g u r e 19 E f f e c t o f V e l o c i t y on H e a t T r a n s f e r R a t e 110Q p s i a i n CO a t 91 deg F and I 1 CN O CN CO I I V to Tb = Tpc A A I A A rtn A B • A A • D 9 9 9 9 ® Tb = 91 "F Pb = 1100 PSIA ' -R e * A — 6 0 0 ! -° — 3 00 • • • • • l 5 0 9 • • • • 0 A A 9 4 0 80 120 160 200 240 TEMPERATURE DIFFERENCE [ °F 280 320 F i g u r e 20 E f f e c t o f V e l o c i t y on H e a t T r a n s f e r C o e f f i c i e n t i n C 0 0 a t 91 deg F and 1100 p s i a z CN O CN Tb =80 °F Pb =1300 PSIA A A A * A A ° • " D A " * n n • A • A n • A D • • ' * ° A • * AO • • A-A° . • • 0 4 0 80 120 160 2 00 240 TEMPERATURE DIFFERENCE [°F] F i g u r e 21 E f f e c t . o f V e l o c i t y on H e a t T r a n s f e r R a t e i n C 0 o a t 80 1300 p s i a 2 •i 1 o CN CN h-L l _ cr X . O CN h-G Q T pc i i " to CN ,__ o X h-L U u L U O O —I o Tb = 80 °F Pb = 1300 PSIA A A ^ A .AA • n D ° n • • A • • @ • @ O Re* A — 6 0 0 N — 3 0 0 o © o o*>—0 0 40 80 120 160 200 240 280 320 T E M P E R A T U R E D I F F E R E N C E [ ° F ] F i g u r e 22 E f f e c t o f V e l o c i t y on H e a t T r a n s f e r C o e f f i c i e n t i n CO a t 80 d e q F and 1300 p s i a 2 C N O CN IT) o -I I I T ^ I | Re* T b = 8 6 ' ° F 0 - — — - 9 5 0 Pb = 1 3 0 0 PSIA o ° o o o ' A ^ 6 0 0 A A A o A o o o o o A o O A " A • • o A A . • A • * O A a £n i • i i i • . 0 4 0 80 120 160 2 00 240 2 80 TEMPERATURE D IFFERENCE [°F] F i g u r e 23 E f f e c t o f V e l o c i t y on H e a t T r a n s f e r R a t e i n C 0 „ a t 86 deg F and .1300 P s i a 2 H CD K> 3 H i P i ttv fD H O oo rt o o o H i TJ cn < H- CD (U I—1 0 o H-ft <^ O ffi CD CU rt Hi H 0) cn H i CD H o O CD H i H i H-O H-CD rt H-n o t\J> Ui r t oo cn Pi CD H m -o m 70 > H TO m Q m m O m FILM COEFFICIENT ( x i c f 2 ) [BTU/HRFT 2 oF 0 15 2 0 25 o oo o o o o o o to o to 00 o CJ ro o 9 ">r o e o o>>9 o o o o o 9 o o o o o o 9 9 t o o o o o o o o o > o o > o > o O Oi o o 70 CD "0 cr l l TJ" O cr II — 00 co Os o o -n "0 CO A A ' ' ' R e * T b = 80 °F — - - - - - - 600 Pb = 1500 PSIA A A A A O A " A • D • A • O • • A D . • 0 A D a • • • • A 9 O Am © • A n * o • A D a ° a ^ I I I ' l_ I I 3 4 0 80 120 160 2 0 0 240 2 8 0 32 T E M P E R A T U R E D I F F E R E N C E [ ° F ] are 25 E f f e c t o f V e l o c i t y on H e a t T r a n s f e r R a t e i n C0„ a t 80 deg'F' a n d 1500 p s i a 2 Tpc Tb = 80 ° F Pb =.1.500 PSIA A A A A A A ••••• D n • s 9 9 S 9 O 9 9 o 9 • 9 9 • 9 9 Re A A—600 n —300 • 9 9 9"^9"0 0 40 80 120 160 200 240 280 320 T E M P E R A T U R E D I F F E R E N C E [ ° F ] F i g u r e 26 E f f e c t o f V e l o c i t y on H e a t T r a n s f e r C e o f f i c i e n t i n CO, a t 80 d e g F and 1500 p s i a 2 to r 1 CN C N O CN cr CD i _i i I D o X *—^ o x -h-< L U Pb=1100 PSIA Re:300 • o H O ^ • o o • V 0 a o A A v • - V V * v V * V Tb [?] ••• 80 ••86 ... 9 0 •• 1 05 B O a o • V • V 0 4 0 80 120 160 2 0 0 240 TEMPERATURE DIFFERENCE [ ° F ] 280 320 F i g u r e 27 E f f e c t o f Bulk Temperature on Heat T r a n s f e r Rate a t 1100 p s i a (Re*=300) 00 H-iQ fD co H i H i (D O — rt fD O X- H i II u> rjd o C O H — ?r fD 3 TJ fD H PJ rt K-fD 0 a fD 0) rt i-3 Ui cn H i fD H n o fD H i H i H -O H -CD rt 0» rt o o o m m 70 > 70 m m 70 m O F I L M C O E F F I C I E N T ( x i o " 2 ) [ B T U / H R F T 2 O F ] 0 5 10 15 20 25 o oo o < < < < . < < o o o ro o o ro o ro oo o co ro o B CB < t < < • < < • Si o 70 ~0 o B CO o o o B o o " 0 CO o B <• • o B o — NO 00 00 o o a o C/t i—i o i i O 6TT F i g u r e 29 E f f e c t o f B u l k T e m p e r a t u r e on H e a t T r a n s f e r C o e f f i c i e n t a t 1100 p s i a (Re*=600) I 1 C N I-CN i— CO I I o X X Z) o CN 40 I— < LU X Pb - 1 100 PSIA Tb [ -F] Re =6 00 80 86 105 o •7 B - * v IO V J. 4 0 80 120 160 2 0 0 240 280 T E M P E R A T U R E D I F F E R E N C E [ ° F ] 320 to F i g u r e 30- E f f e c t : o f . B u l k - T e m p e r a t u r e on H e a t T r a n s f e r C o e f f i c i e n t a t 1100 p s i a (Re*=600) CN 8 Pb -- 130 0 PSIA o Re"=600 A A - — — 8 0 A o • D . A 8 6 • A ... n n O A O O o 105 • A O O * • n . o o ° A • O o A O O A • O ^ o ° • O h *h o A O A a O O 40 80 120 160 200 240 280 320 T E M P E R A T U R E D I F F E R E N C E [ ° F ] Figure 31 Effect of. Bulk Temperature on Heat Transfer Rate at 1300 psia (Re*=600) £ CN O CN Pb = 1 3 0 0 PSIA it Re - 6 0 0 A Poo00oo • • „ A A O ° ° ° 0 ° n * * ° ° ° O o ° • n * A - 8 0 ° °. o o o Q ° D - - 8 6 0 0 0 ° o _ T 0 5 40 80 120 160 200 240 280 32 T E M P E R A T U R E D I F F E R E N C E [ ° F ] F i g u r e 32 E f f e c t o f B u l k T e m p e r a t u r e on H e a t T r a n s f e r C o e f f i c i e n t a t 1300 p s i a (Re*=600) I 1 CN cr x CD i i O x X Z> CN o CN O < LU X Tb : 8 0 . 5 °F TV Re = 600 o -• o • • o • e Pb [PSIA] a 1 5 0 0 o 1 3 0 0 A _ 11 0 0 o © ~ 1 0 0 0 0 4 0 80 120 160 2 0 0 240 280 TEMPERATURE DIFFERENCE 320 4 ^ F i g u r e 33 E f f e c t o f B u l k P r e s s u r e on H e a t T r a n s f e r R a t e a t 80 deg F (Re*=600) o o o co o CN =d o a A 0 ° g on a ° O Tb^ B0.5 R ^ 6 0 0 O n • ° • O O • Pb [PSIA] • . . . . . 1500 o . . . . 1300 A 1100 — " 1000 • J n n o o S S 0 o • • • • m - -40 80 120 160 200 240 280 T E M P E R A T U R E D I F F E R E N C E [ ° F ] 320 to F i g u r e 34 E f f e c t o f Bulk P r e s s u r e on Heat T r a n s f e r C o e f f i c i e n t a t 80 deg F (Re*=600) 126 127 R e * = 6 0 0 R e * = 6 0 0 3 6 - A 1 3 0 0 p s i a 3 6 - B 1 5 0 0 p s i a F i g u r e 36 F l o w F i e l d V a r i a t i o n w i t h V e l o c i t y a t 80 d e g F a n d 1 3 0 0 , 1 5 0 0 p s i a F r e e C o n v e c t i o n - Pump O f f F r e e C o n v e c t i o n - Pump On A f t e r C i r c u l a t i n g D i s t u r b a n c e F i g u r e 37 F l o w F i e l d V a r i a t i o n w i t h C i r c u l a t i o n and Pump V i b r a t i o n - F r e e C o n v e c t i o n 1 29 F i g u r e 38 Flow F i e l d V a r i a t i o n w i t h C y l i n d e r T e m p e r a t u r e I n c r e a s e i n S u b c r i t i c a l and S u p e r c r i t i c a l F r e e C o n v e c t i o n 1 3 0 1 3 1 F i g u r e 40 F l o w F i e l d V a r i a t i o n w i t h P r e s s u r e and C y l i n d e r T e m p e r a t u r e V a r i a t i o n i n F o r c e d C o n v e c t i o n I 1 CN IT) CN CN ! I i o X 3 < U J in Q Knapp 7 7 °G ree n 80 • G o l d s t e i n 73 FREE CONVECTION P b=1100 P S I A a & 9 o ® a a • n a o ~ Q 9 a • 0 4 0 80 120 160 2 0 0 240 TEMPERATURE DIFFERENCE [ ° F ] 2 8 0 320 Figure 41-A Comparison of Experimental Free Convection Data with Other Workers NJ K"> 1 1 CM C N X o CM i i O X * — " o x — _ J L U r -< L U 80 Tb = 30 °F Pb = 1 5 0 0 PS a a a ° a A a » • —Knapp Free 9 9 9 9 A ° • * ' 9 A 3 _ 9 9 9 J. 80 120 160 2 0 0 240 2 80 320 TEMPERATURE DIFFERENCE [ ° F ] Figure 41-B Comparison of Experimental Free Convection Data with Other Workers M U ) CO o O h - o 8 o CN o o o Assumed L inear Actua 200 600 1 ooo TEMPERATURE 1400 °F 1800 F i g u r e 42 E l e c t r i c a l R e s i s t e n c e Change o f N i c h r o m e V w i t h T e m p e r a t u r e (Minimum C u r v e f o r A n n e a l e d N i c h r o m e W i r e From [40]) u> 135 REFERENCES 1. Schmidt, E . , Eckert, E . , G r i g u l l , U., "Heat Transfer By Liquids Near the C r i t i c a l State," AFF Translation No. 527, Headquarters Air Material Command, Wright F i e l d , Dayton, Ohio, February 21, 1946. 2. Powell, W.B., "Heat Transfer to Fluids in the Region of the C r i t i c a l Temperature," Progress Report No. 20-2 85, Jet Propulsion Laboratory, Cal i fornia Institute of Technology, A p r i l 1956. 3. Goldman, K . , "Special Heat Transfer Phenomena for Super-c r i t i c a l Fluids," NDA 2-31, 1956. 4. Deissler, R . G . , "Heat Transfer and Fluid Fr ic t ion for Ful ly Developed Turbulent Flow of Air and Supercrit ical Water with Variable Fluid Properties," Trans. ASME, Vol . 82, Series C, 176-182 (1961). 5. Dean, L . E . and L.M. Thompson. "Study of Heat Transfer to Liquid Nitrogen," ASME Paper No. 56-SA-4 (1956). 6. Hsu, Y.Y. and Smith, J . M . , "The Effect of Density Variation on Heat Transfer in the C r i t i c a l Region," Trans. ASME, Vol . 82, Series C, 176-182 (1961) 7. Fr i t sch , G . A . , and Grosh, R . J . , "Free Convective Heat Transfer to a Supercrit ical F lu id ," Proceedings of 1961  International Heat Transfer Conference, Part V, Paper 121, 1010-1016 (August 1961) . 8. Larson, J .R. and Schoenhals, R . J . , "Turbulent Free Con-vection in Near C r i t i c a l Water," ASME Paper 65-HT-57 (1965) . 9. Hauptmann, E . G . , Ph.D. Thesis, Cal i fornia Inst, of  Technology, Pasadena, C a l i f . (December 1965). 10. Petukov, B . S . , "Heat Transfer in a Single Phase Medium Under Supercrit ical Conditions," Translations of Teplofizika Vysokikh Temperatur, Vo l . 6, No. 4, 732-745, July-August, 1968. 11. Hendricks, R . C , Simoneau, R . J . Smith, R . V . , "Survey of Heat Transfer to Near C r i t i c a l Fluids," NASA Technical  Memorandum, NASA TMX 52612. 136 12. G r i f f i t h , J .D. and Sabersky, R . H . , "Convection in a Fluid at Supercrit ical Pressures," ARS Journal, Vol . 30, No. 3, 289-291 (1961). 13. Doughty, D.L. and Drake, R.M. J r . , "Free-convection Heat Transfer From a Horizontal Right Circular Cylinder to Freon 12 Near the C r i t i c a l State", Trans. ASME, Vol . 78, 1843-1850 (1956). 14. McAdams, W.H., Heat Transmission, 3rd ed. , McGraw-Hill, New York, 1954. 15. Schmidt, e., "Warmetransport durch naturliche Konvektion in Stoffen bei kritischem Zustand," Int. J . Heat Mass  Transfer, Vo l . 1, 92-101 (1960). 16. Graham, R.W. , Hendricks, R . C , and Ehlers, R . C , "Ana-l y t i c a l and Experimental Study of Pool Heating of Liquid Hydrogen Over a Range of Accelerations," NASA Technical  Note, NASA TN D-1883, February 1965. 17. Holt, V . E . , "An Experimental Investigation of High Flux Free Convection Heat Transfer to Water up to Near-C r i t i c a l Conditions," Argonne Nat. Lab. , Rep. ANL 6400 (August 1961). 18. Knapp, K.K. Ph.D. Thesis, Cal i fornia Inst, of Technology, Pasadena, C a l i f . (September 1964) . 19. Knapp, K.K. and Sabersky, R . H . , "Free Convection Heat Transfer to Carbon Dioxide Near the C r i t i c a l Point," International Journal of Heat and Mass Transfer, V o l . 9, No. 1, 1966, pp. 41-51. 20. Goldstein, R . J . and Aung, W., "Heat Transfer by Free Convection from a Horizontal Wire to Carbon Dioxide in the C r i t i c a l Region," Transactions of ASME, Vo l . 90, Series C, 1968, pp. 51-55. 21. Yamagata, K . , Nishikawa, K . , Hasegawa, S., F u j i i , T . , Miyabe, K . , and Ito, T . , "Free Convective Heat Transfer to a Supercritical Fluid (1st report)," Tech. Report of  Kyushu University, Vo l . 36, No. 1, pp. 14-19 (in Japanese). 22. Nishikawa, K . , and Miyabe, K . , "On the Boi l ing- l ike Phenomena at Supercrit ical Pressures," Memoirs of the  Faculty of Engineering at Kyusa University, Vo l . 25, No. 1, December 1965, pp. 1-25. 137 23. Nishikawa, N . , Shimomura, R. , Hatano, M . , Nagatomo, H . , "Investigation of Surface Boiling under Free Convection", B u l l , JSME, Vo l . 10, No. 37, 123-131 (1967). 24. Skripov, V.P.aand Dubrovina, E . N . , "Convective Heat Transfer in The Supercrit ical Region of Carbon Dioxide," Proceedings of Second All -Soviet Union Conference on  Heat and Mass Transfer, Vo l . 1, C. Gazley, J r . , J . P . Hartnett and E.R.G. Eckert, eds. Cal i fornia University Press, 1966, pp. 36-45. 25. Daniels, T .C . and Bramall, J .W. , "An Experimental Investigation of the Heating Mechanism of Carbon Dioxide Above the C r i t i c a l Point," Proc. Institute of  Mechanical Engineers, 1965-66. 26. Kato, H . , Nishiwaki, N. and Hirata, M . , "Studies on the Heat Transfer of Fluids at a Supercrit ical Pressure," Bulletin of JSME, Vo l . 11, No. 46, 1968, pp. 654-663. 27. G r i g u l l , U. and Abadzic, E . , "Heat Transfer from a Wire in the C r i t i c a l Region," Symposium on Heat Transfer  and Fluid Dynamics of Near C r i t i c a l Fluids , B r i s t o l , March 1968, Paper 8. 28. Baumeister, K . J . , and Simoneau, R . J . , "Saturated Film Poiling of Nitrogen from Atmospheric to the C r i t i c a l Pressure," NASA Technical Memorandum, NASA TM X-52615, (1969). : ! ~~ 29. Hasegawa, S. and Yoshioka, K . , "An Analysis for Free Convective Heat Transfer to Supercrit ical Fluids," Proceedings of the Third International Heat Transfer Conference, AIChE, Vol . 2, 214-222 (1966). 30. Hauptmann, E .H. and Sabersky, R . H . , "An Experimental Investigation of Forced Convective Heat Transfer to a Fluid in the Region of i t s C r i t i c a l Point," International  Journal of Heat and Mass Transfer, Vo l . 10, 1967, pp. 1499-1508. : 31. Simoneau, R . J . , and Williams, J . C . , "Laminar Couette Flow with Heat Transfer Near the Thermodynamic C r i t i c a l Point," Int. J . Heat Mass Transfer, Vo l . 12, 120-124 (1968). 32. Graham, R.W., "Penetration Model Explanation for Turbu-lent Forced-Convection Heat Transfer Observed in Near-C r i t i c a l Fluids," NASA TND-5522, 1969. 33. Dryden, H . L . , and Schubauer, G . B . , "The Use of Damping Screens for the Reduction of Wind Tunnel Turbulence," J . Aero, S c i . , Vo l . 14, 221-228 (1947). 138 34. Lumlet, J . L . , and C a r v e r , J . F . , "Reducing Water Tunnel Turbulence by Means of a Honeycomb," ASME Paper 67-FE-5, (1967). 35. "Fluid Meters - Their Theory and Application", Report fo ASME Research Committee on Fluid Meters, F i f th Edition 1959, ASME, New York 18, N.Y. 36. Sprenkle, R . E . , "Piping Arrangements for Acceptable Flowmeter Accuracy," Trans. ASME, Vo l . 67, 345-350 (1945). 37. Robbins, R . J . , M.A.Sc. Thesis, University of Br i t i sh  Columbia, Vancouver, Br i t i sh Columbia (April 1969). 38. Instruction Manual, Heat Flux System Model 1010 (Modified to 2.5 Amps) Thermo-Systerns Inc. , Minneapolis, Minnesota. 39. Advances in Heat Transfer - Volume 1, T . F . Irvine and J . P . Hartnett, eds. Academic Press, New York, N.Y. (1964). 40. Nichrome And Other High Nickel Al loys , Driver-Harris Company, Harrison, New Jersey, Catalog R-59. 41. Michels, A. and C. Michels, "Isotherms of CO., between 0° and 150° and Pressures from 16 to 250 atm," Proc. Roy. Soc. London/ A 153 (1935), pp. 201-214. 42. Michels, A . , B. Blaisse, and C. Michels, "The Isotherms of CO2 in the Neighborhood of the C r i t i c a l Point and Round the Coexistence Line," Proc. Roy. Soc. London, A 160 (1937), pp. 358-375. 43. Michels, A . , A. Botzen, and W. Schuurman, "The Viscosity of Carbon Dioxide between 0°C and 75°C and at Pressures up to 2000 Atmospheres," Physica, Vo l . 23 (1957), pp. 95-102. 44. Michels, A . , J . V . Sengers, and P.S. Van Der Gulik, "The Thermal Conductivity of Carbon Dioxide in the C r i t i c a l Region," Physica, Vo l . 2 8 (1962), pp. 1216-1237. 45. Ackerman, J .W., "Pseudo boiling Heat Transfer to Super-c r i t i c a l Water in Smooth and Ribbed Tubes," ASME- Paper 6 9-WA/HT-2, November 1969,. 46. Baumeister, K . J . and Hamill T . D . , "Laminar Flow Analysis of Film Boiling From A Horizontal Wire," NASA Technical  Note, NASA TN D-4035, July 1967. 1 3 9 47. Brodowicz, K. and Bialokoz, J . , "Free Convection Heat Transfer from a Vert ical Plate to Freon 12 Near the C r i t i c a l State," The Archive of Mechanical Engineering, Vol . X, No. 4, Warsaw (1963). 48. Bourke, P . J . and Denton, W.H., "An Unusual Phenomenon of Heat Transfer Near the C r i t i c a l Point," Memorandum, AERE 1946, United Kingdom Atomic Energy Authority, Harwell, Birkshire, 1967. 49. Cornelius, A . J . and Parker, J . D . , "Heat Transfer In-s tabi l i t i es Near the Thermodynamic C r i t i c a l Point", Proc. Heat Trans, and F l . Mech. Inst . , 317-329 (1965). 50. Hal l , W.B., Jackson, J .D. and Warson, A . , "A Review of Forced Convection Heat Transfer to Fluids at Supercrit ical Pressures," Symposium on Heat Transfer and Fluid Dynamics  of Near C r i t i c a l Fluids , B r i s t o l , March 1968, Paper 3. 51. H a l l , W.B., Jackson, J . D . , Khan, S .A. , "An Investigation of Forced Convection Heat Transfer to Supercritical Pressure Carbon Dioxide," Proceedings of the Third  International Heat Transfer Conference, AIChE, Vol . 1, 257-266 (1966). 52. Hasegawa, S. and Yoshioka, K . , "A Complete Study for Laminar Free Convective Heat Transfer to Supercrit ical Fluids Near the Transposed C r i t i c a l Point". 53. Hess, H . L . , and Kunz, H.R. , "A Study of Forced Convection Heat Transfer to Supercrit ical Hydrogen," J . of Heat  Transf. , Vo l . 87, 41-48 (1965). 54. P i t t s , D.R., Yen, H.H. and Jackson, T.W., "Transient Film Boiling on a Horizontal Wire," ASME Paper No. 68-HT-3. 55. P i t t s , C C . anf Leppert, G.,"The C r i t i c a l Heat Flux for Elec tr i ca l ly Heated Wires in Saturated Pool Boil ing," Int. J . Heat Mass Transfer, Vo l . 9, 365-377 (1966). 56. Sengers, J . V . and Sengers, A . L . , "The C r i t i c a l Region," Chemical and Engineering News, June 10, 1968. 57. Schnurr, N.M. , "Heat Transfer to Carbon Dioxide in the Immediate Vic in i ty of the C r i t i c a l Point,"-ASME Paper . No. 68-HT-32 58. Shiralkar, B . S . , and G r i f f i t h , P . , "Deterioration in Heat Transfer to fluids at Supercrit ical Pressure and High Heat Fluxes," J . of Heat Transfer, February 1969, pp. 27-36. APPENDIX I PROPERTIES OF NEAR-CRITICAL CARBON DIOXIDE D Y N A M I C V I S C O S I T Y O F C A R B O N D I O X I D E (Data taken f r o m [19]) * 1 1 1 1 I I L _ 50 100 150 200 250 300 350 400 TEMPERATURE - °F DYNAMIC VISCOSITY O F CARBON DIOXIDE (Data taken from [19]) 50 100 150 2 0 0 250 300 350 4 0 0 4 50 TEMPERATURE - °F D E N S I T Y O F C A R B O N D I O X I D E (Data taken f r o m [19]) ro E N T H A L P Y O F CARBON DIOXIDE (Data taken from [19]) T H E R M A L C O N D U C T I V I T Y O F C A R B O N D I O X I D E (Data taken f r o m [19]) 145 APPENDIX I I SUMMARY AND LISTING OF FILMS AND PHOTOGRAPHS A l i s t o f a l l h i g h s p e e d 16 mm b l a c k and w h i t e f i l m s t a k e n o v e r t h e c o u r s e o f t h e e x p e r i m e n t a l work i s c o n t a i n e d b elow. Segments o f f i l m s 2, 3, 4, 6, 7, 8, 12, and 13 have been combined i n t o a movie w h i c h summarizes t h e e x p e r i m e n t a l o b s e r v a t i o n s . A l l f i l m s were t a k e n w i t h t h e same f i e l d o f v i e w , t h e c e n t e r t w o - t h i r d s o f t h e h e a t e d c y l i n d e r , and a l l f i l m s were t a k e n a t 5000 p p s , w i t h t h e e x c e p t i o n o f f i l m 1 t a k e n a t 3000 pps and w i t h a l a r g e r f i e l d o f v i e w . / 146 APPENDIX II SUMMARY AND LISTING OF FILMS AND PHOTOGRAPHS Film P, T, T SUMMARY »T b b cy No. 1100 80 400 Free convection showing vapor columns r is ing from the cylinder and the break-of the laminar flow into the turbulent plume (3000 pps and a large f ie ld of view was taken). 1100 80 400 Forced convection (Re*=165) showing the tearing of f lu id from the cylinder. 1100 80 400 Forced convection (Re*=50) showing the breakdown of the free convection flow f ie ld with very small free-stream velocity. 1100 80 400 Forced convection (Re*=650) showing the effect of a larger free stream velocity on the heat transfer mechanism. The osc i l lat ing region behind the cylinder is v i s ib le . 1070 80 100 Free convection just under the c r i t i c a l pressure to show the stable film b o i l -ing in which the f lu id flows axial ly along the cylinder and rises in a vapor column. 1100 80 400 Free convection at 5000 pps to base a time rate on the speed of the r is ing vapor-like disturbances which rise from the heated cylinder. 1050 75 400 Free Convection stable film boil ing at the cylinder temperature used for Film 6 to show the difference between sub-c r i t i c a l and supercrit ical free convection. 1050 75 86 Free convection nucleate boil ing clearly showing the very rapid bubble generation and the dist inct bubble motion below the cylinder. Film P, No. cy 147 SUMMARY 9 1100 80 98 F o r c e d convection (Re*=50) to show the velocity effect at the conditions corres-ponding to maximum film coefficient. The flow is essentially as observed in Film 3 but shows some unsteady behaviour. 10 1100 90 400 Forced convection (Re*=165) to show the effect of operating with bulk f lu id very near the pseudo-critical temper-ature. The large property variations show up in the free stream and the average heat transfer rate is less than 80% of that with bulk f lu id 10 deg F colder, 11 1500 75 425 Free convection to show the effect of small disturbances generated by the motion i t s e l f on the flow s tab i l i ty . The probe had been in s t i l l f lu id at room temperature for two hours and was suddenly started at 425 deg F. The film was started after one minute of operation and shows the unstable nature of the free convection as the plume breaks down. Heat transfer rate increased by 15% during the breakdown. 12 1100 77 450 This film shows the developing flow f ie ld when the probe was suddenly turned on in s t i l l f l u i d . The camera had reached more than 3000 pps before the probe was started and a record of the e lec tr ica l response was also obtained during the transient. The transient time scale obtained from frame by frame examination shows the following; 0 to 3 msec . . . . thermal capacitance of the probe and f lu id in contact with the probe surface—a large voltage spike while the probe reaches temperature. 3 to 10 msec . . . . thickening of the heated layer about the cylinder and the layer starts to rise around the cylinder. 10 to 16 msec . . . . the f i r s t section of heated f luid breaks free of the cylinder and rises to 3 diameters above the cylinder 148 F i l m P. T, T SUMMARY No. b b c y 16 t o 2 0 0 msec . . . . t h e h e a t e d f l u i d r i s e s i n a plume and f i r s t becomes t u r b u l e n t a t 2 0 0 msec 200 t o 2 5 0 msec . . . . t h e f l o w f i e l d becomes f u l l y d e v e l o p e d w i t h t h e t u r b u l e n t plume o s c i l l a t i n g a b o u t a s t a b l e l o c a t i o n . 13 1000 80 85 F o r c e d c o n v e c t i o n b o i l i n g a t t h e l o c a -t i o n o f maximum f i l m c o e f f i c i e n t (peak n u c l e a t e h e a t f l u x c o n d i t i o n i n f o r c e d f l o w ) (Re*=300). 14 1000 80 85 F o r c e d c o n v e c t i o n b o i l i n g a t t h e peak n u c l e a t e h e a t f l u x b u t a t a much s l o w e r v e l o c i t y (Re*=70). 15 1000 80 425 F o r c e d c o n v e c t i o n b o i l i n g a t a much h i g h e r t e m p e r a t u r e d i f f e r e n c e t o p r o d u c e f i l m b o i l i n g (Re*=75) f o r c o m p a r i s o n w i t h t h e s u p e r c r i t i c a l c a s e i n F i l m 3 and 16. 16 1500 80 425 F o r c e d c o n v e c t i o n (Re*=75) t o i l l u s t r a t e t h e e f f e c t o f v e l o c i t y i n f l u i d above b u t away f r o m t h e c r i t i c a l p r e s s u r e . The f i l m i l l u s t r a t e s t h e s i m i l a r i t y o f a l l s u p e r c r i t i c a l f l o w s by c o m p a r i s o n w i t h F i l m s 3, 9 , 10. 149 APPENDIX III CALCULATIONS AND ERROR ANALYSIS Values of heat transfer rate, temperature difference, and heat transfer coefficient were calculated for each data point following the example shown below: A. Power Dissipated 2 P = I Rop (watts) = (.608x.608x4.06) = 1.50 watts % error in P• ^ = ^ + 2 $1 = ° - Q Q 4 + Q - 0 0 3 x 2 error xn e, p R o p + ^ I 4.06 .608 X = 0.1% + 2 x 0.5% = 1.1% The error in current is taken as typical of a l l operating points and is a combination of the rms fluctuation and the meter inaccuracy. B. Heat Transfer Rate Q = AREA = 3.1416 3x 4d 3x L (BTU/hr-ft 2) = 72290 BTU/hr-ft 2 . _ 6 Q 6 P fid ^ 6 L . c o ^ 0.00005 ^ 0.0005 « error i n Q; — = — + + — = 0.6% + - 0 Q 5 4 + - ^ y -= 1.1% + 1.0% + 0.8% = 2.9% C. T e m p e r a t u r e D i f f e r e n c e 150 = 1 + a A T A AT = i - 1.0) Rc a Rc ' A T = .000994 (I77! " i - 0 * = 9 5 ' 2 d e ^ F A m 6AT 6a ^ 6Rop ± 6Rc % e r r o r i n AT; -—-=• = — + • r + — ' AT a Rop Rc = 2% + .1% + 1% = 2.2% D. Heat T r a n s f e r C o e f f i c i e n t h = ( B T U / h r - f t 2 deg F) = 7 2 } 2 9 ° = 760 B T U / h r - f t 2 deg F • „ 6h 6Q , SAT % e r r o r m h; j - = ^ + — = 2.9% + 2.2% = 5.1% V a l u e s o f v e l o c i t y were c a l c u l a t e d f r o m t h e d i f f e r -e n t i a l p r e s s u r e d e v e l o p e d a c r o s s t h e v e n t u r i m e t e r . D i s c h a r g e c o e f f i c i e n t o f t h e v e n t u r i m e t e r was t a k e n as 0.96 f o r a l l r e a d i n g s . A t y p i c a l c a l c u a l t i o n i s r e p r q d u c e d b e l o w . E. F r e e - S t r e a m V e l o c i t y C, _ _ _ _ _ V + . K = /2g(P 0-P n = 1.01/2gl44Ap t h /—~4 1 L ' 1-6 P P i f i n a t y p i c a l c a s e a t 80 deg F, 1100 p s i a 151 Ap = 10.0 p s i g , p = 48 l b f t 3 V_, =1.01 /2x32.2x144x10 =1.01 /1850 = 43.0 f t / s e c t h ' 48 6(C, B) % e r r o r t h v ^ d 1 6p 1 6p_ i n V h ; V t h C b ' B 2 P 2 P t h 2.0% + .5% + 1.0% = 3.5% To c a l c u l a t e t h e v e l o c i t y p a s t t h e p r o b e ; ^ A s e c P AV 6V., 6A.. 6A o • TT P t h , t h , p % e r r o r i n V ; — * - = + + — £ — P V V.. A., A p t h t h p = 3.5% + 2% + 5% = 6% V a l u e s o f f r e e - s t r e a m R e y n o l d s number (Re*) were c a l c u l a t e d a s s u m i n g c o n s t a n t f r e e - s t r e a m p r o p e r t i e s and were b a s e d on c y l i n d e r d i a m e t e r . F. R e y n o l d s Number Vd Vdp . , 0. 0054 _ _ 48 l b 1 / l n = - — - - ~ where d = —=-= f t , p = - F r 3 ~ / y = - 1 4 0 l b v v it i t f t h r Re 1 5 2 Re* = 1 . 9 . 5 X . 0 0 5 4 x 4 8 x 3 6 0 0 1 2 x . l 4 0 = 1 0 7 5 % e r r o r i n Re*; cSRe* Re* 6V 6d 6p V d p = 6.% + 1 . 0 % + 2.% + 2% = 1 1 % Note - A t a g i v e n b u l k c o n d i t i o n , f r e e s t r e a m v e l o c i t i e s were v a r i e d by c h a n g i n g t h e d i f f e r e n t i a l p r e s s u r e by a f a c t o r o f 4 so t h a t w i t h i n a b u l k c o n d i t i o n % e r r o r i n Re* c hange G. B u l k P r e s s u r e The b u l k p r e s s u r e was m easured i n s t a g n a n t f l u i d i n t h e main t e s t s e c t i o n b l o c k u s i n g a H e i s e b o u r d o n gauge a c c u r a t e t o 1 p s i a i n f u l l s c a l e . Gauge p r e s s u r e c o u l d be r e a d t o 1 p s i a a t any r e a d i n g . 6P % e r r o r i n P r e s s u r e ; — = 0 . 1 % + 0 . 1 % = 0 . 2 % H. B u l k T e m p e r a t u r e B u l k t e m p e r a t u r e was d e t e r m i n e d f r o m t h e emf measured on a L e e d s and N o r t h r u p K-5 p o t e n t i m e t e r a c c u r a t e , t o 0.5?-,. The t h e r m o c o u p l e s were c a l i b r a t e d u s i n g t h e same c i r c u i t by d i r e c t i m m e r s i o n i n a h e a t e d b a t h and t h e s t a n d a r d c o p p e r -c o n s t a n t a n e m f - t e m p e r a t u r e t a b l e s a g r e e d w i t h i n 0.1 deg F i n 50 deg F. E r r o r s w o u l d be p r o p o r t i o n a t e l y s m a l l e r a t h i g h e r i s a p p r o x i m a t e l y <SRe* Re* 153 t e m p e r a t u r e d i f f e r e n c e s b u t maximum e r r o r c a n be e s t i m a t e d a s ; 6T % e r r o r i n t e m p e r a t u r e : — = 0.5% + 0 . 2 % = 0.7% The f l u i d was assumed u n i f o r m w i t h i n 0.2 deg F by a c o m p a r i s o n between t h e t h e r m o c o u p l e r e a d i n g s and by t h e d e n s i t y g r a d i e n t s shown on t h e s c h l i e r e n image. I. P r o p e r t y E v a l u a t i o n T r a n s p o r t p r o p e r t i e s were e v a l u a t e d f r o m smooth c u r v e s drawn by Knapp [18] t h r o u g h t h e p r o p e r t i e s e x p e r i m e n -t a l l y m easured by M i c h e l s e t a l . [41, 42, 43, 4 4 ] . P r o p e r t i e s have been assumed t o be a c c u r a t e l y p o r t r a y e d by t h e s e c u r v e s w h i c h a r e r e p o r o l u c e d i n Append].x I . The t v p i c a l — - e r r o r d e s c r i b e d above was c o n s i d e r e d p as an e v a l u a t i o n u n c e r t a i n t y o n l y . S p o t c h e c k s a t s e l e c t e d b u l k c o n d i t i o n s show good agreement w i t h p r o p e r t y e v a l u a t i o n s f r o m o t h e r t a b l e s o f n e a r - c r i t i c a l c a r b o n d i o x i d e p r o p e r t y v a r i a t i o n s [ 2 2 ] . 

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