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Experimental study of electrohydrodynamically augmented pool boiling heat transfer on smooth and enhanced tubes
.•• ELSEVIER
Journal of
ELECTROSTATICS Journal of Electrostatics 40&41 (1997) 597-602
Experimental study ofelectrohydrodynamically augmented pool boiling heat transfer on smooth and enhanced tubes J. Seyed-Yagoobi~, J.T. Hardesty', P. Raghupathi', and J.E. Bryan" ~Department of Mechanical Engineering Texas A&M University College Station, Texas 77843-3123, U.S.A. The electrohydrodynamically enhanced heat transfer in pool boiling was studied using R-123 as the working fluid at 4°C pool temperature. Both a smooth tube and an enhanced tube were considered. Significant enhancements in heat transfer were achieved with a negligible power consumption for establishing the electric fields. I. INTRODUCTION The enhancement of boiling heat transfer is of crucial importance to many industries including the HVAC&R, power, process and aerospace industries. The electrohydrodynamieally (EHD) enhanced boiling process is considered a promising candidate in the field. The EHD enhancement of boiling heat transfer features several distinct advantages over conventional methods. One important benefit is the possibility of varying the boiling heat transfer by simply changing the applied voltage. Significant enhancements in boiling heat transfer earl be achieved with EHD. Furthermore, the EHD boiling process contains no moving parts and the electric power input is negligible. The concept of EHD enhancement of boiling heat transfer has been researched actively recently (e.g., Kawahira et al., 1990; Ogata et al., 1992; Karayiatmis et al., 1993; Ogata and Yabe, 1993a and 1993b; Singh et al., 1993; and Seyed-Yagoobi et al., 1996). These studies investigated the effect of EHD on boiling at a pool temperature of approximately 25 *C limiting their relevance to industrial applications. The main objective of this study was to investigate the effect of electric field presence on the pool boiling heat transfer of R-123 (HCFC-123) refrigerant, an ozonefriendly substitute for R-11 (HCFC-11), at a pool temperature of 4 ° C. Furthermore, a smooth tube and an enhanced tube were considered in this study in order to broaden the applicability of the results. 2. ELECTROHYDRODYNAMIC PHENOMENA To clarify the effects of an electric field on pool-boiling, different mechanisms of electrically induced liquid and vapor motion must be explained. The electric body force density acting on the molecules of a dielectric fluid in the presence of an electric field consists of three terms (Pohl, 1951). 0304-3886/97/$17.00 © Published by Elsevier Science B.V. All rights reserved. S0304-3886(97)00109-5
PII
598
J Seyed-Yagoobi et al./Journal of Electrostatics 40&41 (1997) 597-602
i
P
(1)
where q, E, e, and p are the electric charge density, electric field strength, fluid electric permittivity, and fluid density, respectively. The three terms in the above equation stand for three different kinds of force densities acting on the liquid. The first term represents the Coulomb force, which is the acting force on the free charges in an electric field. The Coulomb force is expected to be negligible in this study due to the lack of free charges in the fluid (the current measured is on the order of I~A). The second term stands for the dielectrophoretic force, which is created by a local change of the permittivity in an electric field. A change in the permittivity occurs, for example, at the interface of a vapor bubble with a liquid. The third term is called the electrostriction term. The electrostriction force occurs primarily when a non-uniform electric field is applied on a dielectric fluid. The dielectrophoretie force and the electrostriction force are beth forces which act on polarized charges and are both defined as polarization forces. The polarization force is based on the fact that the molecules of the fluid align themselves along the electric field. The pole headed toward the higher electric field is pulled more strongly than the other pole. This unbalanced force pulls a dipole to the high electric field area. 3. EXPERIMENTAL APPARATUS The single-tube pool boiling apparatus consists of a rectangular stainless steel chamber which houses a condenser in the upper half and an evaporator in the lower half (see Figure 1). A large sight glass in the lower front of the chamber allows for the observation of the experiments. The chamber stands in an environmentally con~olled enclosure, where the temperature can be varied from 0 to 30 °C, in order to match the desired pool temperature. During experiments, the lower half of the chamber is filled with 4.5 liters of refrigerant.
Figure 1. Pool Boiling Apparatus
d. Seyed- Yagoobi et al. /Journal of Electrostatics 40&41 (1997) 597-602
599
Three copper-constantan thermocouples measure temperatures inside the chamber. One measures the temperature at the bottom of the pool and another measures the temperature at the top of the pool. From these two measurements, typically 0.2 "C apart, an average pool temperature is calculated. The third thennocouple measures the vapor temperature. The condenser is a helically bent plain copper tube. An external chiller pumps coolant through the condenser tube. The boiling tube is a single, water-heated, horizontal tube. The water temperature and flow rate are varied in order to control the heat flux and the temperature drop across the length of the tube. This temperature drop is limited to 1 "C in order to make the assumption of constant heat flux along the length of the tube. The temperature of the water flowing through the boiling tube is maintained by a PID controlled heater. The flow rate is measured with an electronic turbine flowmeter, and the water inlet and outlet temperatures of the boiling tube are measured with thermistors calibrated to an accuracy of + 0.05"C. The heat flux, q', from the boiling tube to the refrigerant pool and the heat transfer coefficient, h, are calculated according to the following two equations q// = m cp h = qH/ AT
(Twi - T~,) / A
(2) (3)
where m, ce, T~, Two, and A are the water flow rate, water specific heat, water inlet and outlet temperatures, and the outer surface area of the boiling tube. The AT, tube superheat temperature, is the difference between the tube outer surface temperature and the pool temperature. Two boiling tubes are considered for this study. One is a smooth tube, and the other is a low fin enhanced tube with 19 fins per inch. The outer diameter of the smooth tube is 1.91 cm and the tube is 0.14 cm thick. The low fin enhanced tube has a maximum diameter of 1.9 cm, an internal diameter of 1.42 cm, and a fin base diameter of 1.62 cm. The fm base area is used as the area of enhanced tube in equation (3). The most serious problem of a heat transfer apparatus is the method of measuring surface temperatures. The copper-constantan thermocouples (Type Omega, Special Limits of Error) are soldered into little craters on the surface of the boiling tube. The welded head of each thermocouple is sitting in this crater and touches the surface. To avoid disturbances by the eleotric field, the thermocouple wires are guided through the inside of the boiling tube. Twelve thermocouples aro soldered into the boiling tube, in three different cross sections. In each cross section, the thermocouples are installed at 90" angles around the tube. The heat transfer coefficient reported here for given operating conditions is the average of all the twelve local heat transfer coefficients determined from equation (3). The electrode design is a construction of eight straight brass wires along the boiling tube. The wires are 0.16 cm in diameter and are evenly spaced around the tube. The distance between the wires and the grounded evaporator tube is 0.30 cm. At the beginning of every experiment, all gases were evacuated from the system by establishing a vacuum of 40 Pa. During an experiment, the pressure and the saturation temperature were kept constant. Data were sampled and averaged over a ten minute interval at given operating conditions. The experiments were conducted at 0, 5, and 10 kV.
600
J.. Seyed-Yagoobi et al./Journal of Electrostatics 40&41 (1997) 597-602
4. RESULTS The heat flux and average heat transfer coefficient as a function of the boiling tube outer surface superheat, AT, for the smooth tube are shown in Figures 2 and 3, respectively. The data are provided at 0, 5, and 10 kV voltage levels. Similar results are illustrated in Figures 4 and 5 for the enhanced tube. From these figures it is evident that application of the electric field results in significant augmentation of pool boiling heat transfer for both smooth and enhanced tubes. The range of AT considered in this study corresponds to the upper portion of the free convection regime and the lower portion of nucleate boiling regime. It was observed that even at low wall superheats, there were isolated active nucleation sights for both smooth and enhanced tubes. As expected the number of nucleation sights increased drastically with the increase in the wall superheat indicating the domination of the nucleate boiling regime over the free convection regime. The enhancements in the heat transfer at the free convection dominated regime (low AT) is attributed primarily to the turbulence and sweeping flow generated by the electric forces. At high wall superheat levels, where the nucleate boiling dominated, the electric forces affected the bubble departure diameter and frequency. This resulted in significant augmentation seen in the pool boiling heat transfer. Currently, there is an ongoing work to quantify the effects of the electric forces on the above bubble parameters. Because of the presence of a small thermal gradient within the pool, it is quite probable that charges are induced within the volume of the fluid adding to the complexity of the problem. Figure 6 summarizes the results in terms of the enhancement ratio. According to this figure, when comparing the results of the enhanced tube at 0 kV and 10 kV to the corresponding results of the smooth tube at 0 kV and 10 kV, slightly better enhancements are achieved with the enhanced tube versus smooth tube. This is especially true at high heat flux values. On the other hand, the enhancement ratios are very significant when comparing the smooth or enhanced tube results at 10 kV to the smooth tube results at 0 kV. This confirms the expected enhancements from the application of the EHD technology to pool boiling process.
80 ¸
6O
~,-5 IN| ~---lOWJ
1~-sw/ U--lOW I
a-"
i' 2
20
I
0 0.0
3.0
I
i
I
i
6.0
9.0
12.0
15.0
~T('C)
Figure 2. Heat flux versus wall superheat for smooth tube
0 18.0
0.0
I ,.0
I e.O
I 9.0 AT(%'~
I 12.0
I 15.0
18.0
Figure 3. Heat transfer coefficient versus wall superheat for smooth tube
601
.1.. Seyed- Yagoobi et al, /Journal o f Electrostatics 40&41 (1997) 597-602
The electrode design selected for this study was not an optimum design. However, this simple straight wire electrode design was selected from the application point of view. One draw back observed with this design was that the bubbles created at the bottom of the tube were held under the tube by these electrodes. The electrode wires at the side of the tube pushed the rising bubbles back, acting against their buoyancy force. Particularly at high voltage and heat flux levels, a static vapor line of accumulated bubbles was held between the bottom electrodes by the electric forces. Previous researchers interpreted this effect as "bubble growth on the electrode wires". Proper electrode design can eliminate this problem (see Seyed-Yagoobi et al., 1996). B
e
!,
cr
2O 0 0.0
2
) 3.0
t 6.0
,
J
I
g.o
12.0
15.0
Ar¢~
0 18.0
0.0
Figure 4. Heat flux versus wall superheat for e ~ e e d tube
I 3.0
t 8.0
I ILO
Ar¢c)
t I~..0
I
1~c.0
1B.0
Figure 5. Heat transfer coefficient versus wall superheat for enhanced tube
S.0
4.0 J
3.0
i
2.0 t
................
1.0 0.0
b
t
i
i
20
40
eo
80
Figure 6. Comparison of different cases Comparison of the smooth tube data at a pool temperature of 4 "C reported here with those reported by Seyed-Yagoobi et al. (1996) at 27 "C for the same working fluid of R-123 revealed that the heat transfer is significantly higher at 4 "C compared to 27 "C. For example, at a wall superheat of 9 "C the boiling heat transfer coefficients at 0 kV and 10 kV are approximately 2.5 times higher at a pool temperature of 4 "C compared to a pool temperature of 27 "C. This is because of the combined effects of the pool temperature on the thermal and electrical properties of the working fluid. Specifically, lowering the pool temperature changes the latent heat, viscosity, surface tension, density, and specific heat of the fluid. These changes alter the bubble departure diameter and bubble generation frequency (Carey, 1992). Lowering the pool temperature also
602
J Seyed-Yagoobi et al. /Journal of Electrostatics 40&41 (1997) 597-602
changes the electric conductivity and permittivity of the fluid affecting its electric relaxation time. To achieve significant enhancements in the pool boiling heat transfer with application of the electric field, the electric relaxation time of the working fluid must be on the same order of magnitude (or smaller) as the bubble generation time. The EHD power consumption of all the experiments was on the order of 0.01 percent of the heat transfer power in the boiler indicating a negligible power consumption with the EHD. The maximum uncertainty for the measured heat transfer coefficients was 6.5 percent. The uncertainties in the voltage measurements were below _+0.1 percent. 5. ACKNOWLEDGEMENTS This work was partially supported by the American Society of Heating, Refrigeration, and Air Conditioning Engineers and the NASA-Johnson Space Center. The refrigerant was provided by DuPont. REFERENCES
1. V.P. Carey, Liquid-Vapor Phase-Change Phenomena, Hemisphere Publishing Corporation, Bristol, Pennsylvania, U.S.A., 1992. 2. T.G. Karayiaunis, ILK. A1-Dadah, R.W. James, and P.H.G. Allen, Electrohydrodynamie Boiling Heat Transfer Enhancement in Heat Exchangers, ASME Paper No. 93-WA/HT-41, 1993. 3. H. Kawahira, J. Kubo, T. Yokoyama, and J. Ogata, The Effect of an Electric Field on Boiling Heat Transfer of Refrigerant-ll--Boiling on a Single Tube, IEEE Transactions on lndustry Applications, Vol. 26, No. 2, pp. 359-365, 1990. 4. J. Ogata, Y. Iwafuji, Y. Shimada, and T. Yamazaki, Boiling Heat Transfer Enhancement in Tube-Bundle Evaporators Utilizing Electric Field Effects, BA-92-52, ASHRAE Transactions: Symposia, Vol. 98, pp. 435-444, 1992. 5. J. Ogata and A. Yabe, Basic Study on the Enhancement of Nucleate Boiling Heat Transfer by Applying Electric Fields, Int. J. Heat Mass Transfer, Vol. 36, No. 3, pp. 775-782, 1993a. 6. J. Ogata and A. Yabe, Augmentation of Boiling Heat Transfer by Utilizing the EHD Effeet-EHD Behaviour of Boiling Bubbles and Heat Transfer Characteristics, lnt. J. Heat and Mass Transfer, Vol. 36, No. 3, pp. 783-791, 1993b. 7. H.A. Pohl, The Motion and Precipitation of Suspensoids in Divergent Electric Fields, J. of Applied Physics, Vol. 22, No. 7, pp. 869-871, 1951. 8. J. Seyed-Yagoobi, C.A. Geppert, and L.M. Gepport, ElectrohydrodynamieaUy Enhanced Heat Transfer in Pool Boiling, Transactions of ASME, Journal of Heat Transfer, Vol. 118, pp. 233-237, 1996. 9. A. Singh, A. Kumar, S. Dessiatoun, M.A. Faani, M.M. Ohadi, and A.I., Ansari, Compound EI-ID-Enhaneed Pool Boiling of R-123 in a Liquid-to-Refrigerant Heat Exchanger, ASME Paper No. 93-WA/HT-40, 1993.
Journal of
ELECTROSTATICS Journal of Electrostatics 40&41 (1997) 597-602
Experimental study ofelectrohydrodynamically augmented pool boiling heat transfer on smooth and enhanced tubes J. Seyed-Yagoobi~, J.T. Hardesty', P. Raghupathi', and J.E. Bryan" ~Department of Mechanical Engineering Texas A&M University College Station, Texas 77843-3123, U.S.A. The electrohydrodynamically enhanced heat transfer in pool boiling was studied using R-123 as the working fluid at 4°C pool temperature. Both a smooth tube and an enhanced tube were considered. Significant enhancements in heat transfer were achieved with a negligible power consumption for establishing the electric fields. I. INTRODUCTION The enhancement of boiling heat transfer is of crucial importance to many industries including the HVAC&R, power, process and aerospace industries. The electrohydrodynamieally (EHD) enhanced boiling process is considered a promising candidate in the field. The EHD enhancement of boiling heat transfer features several distinct advantages over conventional methods. One important benefit is the possibility of varying the boiling heat transfer by simply changing the applied voltage. Significant enhancements in boiling heat transfer earl be achieved with EHD. Furthermore, the EHD boiling process contains no moving parts and the electric power input is negligible. The concept of EHD enhancement of boiling heat transfer has been researched actively recently (e.g., Kawahira et al., 1990; Ogata et al., 1992; Karayiatmis et al., 1993; Ogata and Yabe, 1993a and 1993b; Singh et al., 1993; and Seyed-Yagoobi et al., 1996). These studies investigated the effect of EHD on boiling at a pool temperature of approximately 25 *C limiting their relevance to industrial applications. The main objective of this study was to investigate the effect of electric field presence on the pool boiling heat transfer of R-123 (HCFC-123) refrigerant, an ozonefriendly substitute for R-11 (HCFC-11), at a pool temperature of 4 ° C. Furthermore, a smooth tube and an enhanced tube were considered in this study in order to broaden the applicability of the results. 2. ELECTROHYDRODYNAMIC PHENOMENA To clarify the effects of an electric field on pool-boiling, different mechanisms of electrically induced liquid and vapor motion must be explained. The electric body force density acting on the molecules of a dielectric fluid in the presence of an electric field consists of three terms (Pohl, 1951). 0304-3886/97/$17.00 © Published by Elsevier Science B.V. All rights reserved. S0304-3886(97)00109-5
PII
598
J Seyed-Yagoobi et al./Journal of Electrostatics 40&41 (1997) 597-602
i
P
(1)
where q, E, e, and p are the electric charge density, electric field strength, fluid electric permittivity, and fluid density, respectively. The three terms in the above equation stand for three different kinds of force densities acting on the liquid. The first term represents the Coulomb force, which is the acting force on the free charges in an electric field. The Coulomb force is expected to be negligible in this study due to the lack of free charges in the fluid (the current measured is on the order of I~A). The second term stands for the dielectrophoretic force, which is created by a local change of the permittivity in an electric field. A change in the permittivity occurs, for example, at the interface of a vapor bubble with a liquid. The third term is called the electrostriction term. The electrostriction force occurs primarily when a non-uniform electric field is applied on a dielectric fluid. The dielectrophoretie force and the electrostriction force are beth forces which act on polarized charges and are both defined as polarization forces. The polarization force is based on the fact that the molecules of the fluid align themselves along the electric field. The pole headed toward the higher electric field is pulled more strongly than the other pole. This unbalanced force pulls a dipole to the high electric field area. 3. EXPERIMENTAL APPARATUS The single-tube pool boiling apparatus consists of a rectangular stainless steel chamber which houses a condenser in the upper half and an evaporator in the lower half (see Figure 1). A large sight glass in the lower front of the chamber allows for the observation of the experiments. The chamber stands in an environmentally con~olled enclosure, where the temperature can be varied from 0 to 30 °C, in order to match the desired pool temperature. During experiments, the lower half of the chamber is filled with 4.5 liters of refrigerant.
Figure 1. Pool Boiling Apparatus
d. Seyed- Yagoobi et al. /Journal of Electrostatics 40&41 (1997) 597-602
599
Three copper-constantan thermocouples measure temperatures inside the chamber. One measures the temperature at the bottom of the pool and another measures the temperature at the top of the pool. From these two measurements, typically 0.2 "C apart, an average pool temperature is calculated. The third thennocouple measures the vapor temperature. The condenser is a helically bent plain copper tube. An external chiller pumps coolant through the condenser tube. The boiling tube is a single, water-heated, horizontal tube. The water temperature and flow rate are varied in order to control the heat flux and the temperature drop across the length of the tube. This temperature drop is limited to 1 "C in order to make the assumption of constant heat flux along the length of the tube. The temperature of the water flowing through the boiling tube is maintained by a PID controlled heater. The flow rate is measured with an electronic turbine flowmeter, and the water inlet and outlet temperatures of the boiling tube are measured with thermistors calibrated to an accuracy of + 0.05"C. The heat flux, q', from the boiling tube to the refrigerant pool and the heat transfer coefficient, h, are calculated according to the following two equations q// = m cp h = qH/ AT
(Twi - T~,) / A
(2) (3)
where m, ce, T~, Two, and A are the water flow rate, water specific heat, water inlet and outlet temperatures, and the outer surface area of the boiling tube. The AT, tube superheat temperature, is the difference between the tube outer surface temperature and the pool temperature. Two boiling tubes are considered for this study. One is a smooth tube, and the other is a low fin enhanced tube with 19 fins per inch. The outer diameter of the smooth tube is 1.91 cm and the tube is 0.14 cm thick. The low fin enhanced tube has a maximum diameter of 1.9 cm, an internal diameter of 1.42 cm, and a fin base diameter of 1.62 cm. The fm base area is used as the area of enhanced tube in equation (3). The most serious problem of a heat transfer apparatus is the method of measuring surface temperatures. The copper-constantan thermocouples (Type Omega, Special Limits of Error) are soldered into little craters on the surface of the boiling tube. The welded head of each thermocouple is sitting in this crater and touches the surface. To avoid disturbances by the eleotric field, the thermocouple wires are guided through the inside of the boiling tube. Twelve thermocouples aro soldered into the boiling tube, in three different cross sections. In each cross section, the thermocouples are installed at 90" angles around the tube. The heat transfer coefficient reported here for given operating conditions is the average of all the twelve local heat transfer coefficients determined from equation (3). The electrode design is a construction of eight straight brass wires along the boiling tube. The wires are 0.16 cm in diameter and are evenly spaced around the tube. The distance between the wires and the grounded evaporator tube is 0.30 cm. At the beginning of every experiment, all gases were evacuated from the system by establishing a vacuum of 40 Pa. During an experiment, the pressure and the saturation temperature were kept constant. Data were sampled and averaged over a ten minute interval at given operating conditions. The experiments were conducted at 0, 5, and 10 kV.
600
J.. Seyed-Yagoobi et al./Journal of Electrostatics 40&41 (1997) 597-602
4. RESULTS The heat flux and average heat transfer coefficient as a function of the boiling tube outer surface superheat, AT, for the smooth tube are shown in Figures 2 and 3, respectively. The data are provided at 0, 5, and 10 kV voltage levels. Similar results are illustrated in Figures 4 and 5 for the enhanced tube. From these figures it is evident that application of the electric field results in significant augmentation of pool boiling heat transfer for both smooth and enhanced tubes. The range of AT considered in this study corresponds to the upper portion of the free convection regime and the lower portion of nucleate boiling regime. It was observed that even at low wall superheats, there were isolated active nucleation sights for both smooth and enhanced tubes. As expected the number of nucleation sights increased drastically with the increase in the wall superheat indicating the domination of the nucleate boiling regime over the free convection regime. The enhancements in the heat transfer at the free convection dominated regime (low AT) is attributed primarily to the turbulence and sweeping flow generated by the electric forces. At high wall superheat levels, where the nucleate boiling dominated, the electric forces affected the bubble departure diameter and frequency. This resulted in significant augmentation seen in the pool boiling heat transfer. Currently, there is an ongoing work to quantify the effects of the electric forces on the above bubble parameters. Because of the presence of a small thermal gradient within the pool, it is quite probable that charges are induced within the volume of the fluid adding to the complexity of the problem. Figure 6 summarizes the results in terms of the enhancement ratio. According to this figure, when comparing the results of the enhanced tube at 0 kV and 10 kV to the corresponding results of the smooth tube at 0 kV and 10 kV, slightly better enhancements are achieved with the enhanced tube versus smooth tube. This is especially true at high heat flux values. On the other hand, the enhancement ratios are very significant when comparing the smooth or enhanced tube results at 10 kV to the smooth tube results at 0 kV. This confirms the expected enhancements from the application of the EHD technology to pool boiling process.
80 ¸
6O
~,-5 IN| ~---lOWJ
1~-sw/ U--lOW I
a-"
i' 2
20
I
0 0.0
3.0
I
i
I
i
6.0
9.0
12.0
15.0
~T('C)
Figure 2. Heat flux versus wall superheat for smooth tube
0 18.0
0.0
I ,.0
I e.O
I 9.0 AT(%'~
I 12.0
I 15.0
18.0
Figure 3. Heat transfer coefficient versus wall superheat for smooth tube
601
.1.. Seyed- Yagoobi et al, /Journal o f Electrostatics 40&41 (1997) 597-602
The electrode design selected for this study was not an optimum design. However, this simple straight wire electrode design was selected from the application point of view. One draw back observed with this design was that the bubbles created at the bottom of the tube were held under the tube by these electrodes. The electrode wires at the side of the tube pushed the rising bubbles back, acting against their buoyancy force. Particularly at high voltage and heat flux levels, a static vapor line of accumulated bubbles was held between the bottom electrodes by the electric forces. Previous researchers interpreted this effect as "bubble growth on the electrode wires". Proper electrode design can eliminate this problem (see Seyed-Yagoobi et al., 1996). B
e
!,
cr
2O 0 0.0
2
) 3.0
t 6.0
,
J
I
g.o
12.0
15.0
Ar¢~
0 18.0
0.0
Figure 4. Heat flux versus wall superheat for e ~ e e d tube
I 3.0
t 8.0
I ILO
Ar¢c)
t I~..0
I
1~c.0
1B.0
Figure 5. Heat transfer coefficient versus wall superheat for enhanced tube
S.0
4.0 J
3.0
i
2.0 t
................
1.0 0.0
b
t
i
i
20
40
eo
80
Figure 6. Comparison of different cases Comparison of the smooth tube data at a pool temperature of 4 "C reported here with those reported by Seyed-Yagoobi et al. (1996) at 27 "C for the same working fluid of R-123 revealed that the heat transfer is significantly higher at 4 "C compared to 27 "C. For example, at a wall superheat of 9 "C the boiling heat transfer coefficients at 0 kV and 10 kV are approximately 2.5 times higher at a pool temperature of 4 "C compared to a pool temperature of 27 "C. This is because of the combined effects of the pool temperature on the thermal and electrical properties of the working fluid. Specifically, lowering the pool temperature changes the latent heat, viscosity, surface tension, density, and specific heat of the fluid. These changes alter the bubble departure diameter and bubble generation frequency (Carey, 1992). Lowering the pool temperature also
602
J Seyed-Yagoobi et al. /Journal of Electrostatics 40&41 (1997) 597-602
changes the electric conductivity and permittivity of the fluid affecting its electric relaxation time. To achieve significant enhancements in the pool boiling heat transfer with application of the electric field, the electric relaxation time of the working fluid must be on the same order of magnitude (or smaller) as the bubble generation time. The EHD power consumption of all the experiments was on the order of 0.01 percent of the heat transfer power in the boiler indicating a negligible power consumption with the EHD. The maximum uncertainty for the measured heat transfer coefficients was 6.5 percent. The uncertainties in the voltage measurements were below _+0.1 percent. 5. ACKNOWLEDGEMENTS This work was partially supported by the American Society of Heating, Refrigeration, and Air Conditioning Engineers and the NASA-Johnson Space Center. The refrigerant was provided by DuPont. REFERENCES
1. V.P. Carey, Liquid-Vapor Phase-Change Phenomena, Hemisphere Publishing Corporation, Bristol, Pennsylvania, U.S.A., 1992. 2. T.G. Karayiaunis, ILK. A1-Dadah, R.W. James, and P.H.G. Allen, Electrohydrodynamie Boiling Heat Transfer Enhancement in Heat Exchangers, ASME Paper No. 93-WA/HT-41, 1993. 3. H. Kawahira, J. Kubo, T. Yokoyama, and J. Ogata, The Effect of an Electric Field on Boiling Heat Transfer of Refrigerant-ll--Boiling on a Single Tube, IEEE Transactions on lndustry Applications, Vol. 26, No. 2, pp. 359-365, 1990. 4. J. Ogata, Y. Iwafuji, Y. Shimada, and T. Yamazaki, Boiling Heat Transfer Enhancement in Tube-Bundle Evaporators Utilizing Electric Field Effects, BA-92-52, ASHRAE Transactions: Symposia, Vol. 98, pp. 435-444, 1992. 5. J. Ogata and A. Yabe, Basic Study on the Enhancement of Nucleate Boiling Heat Transfer by Applying Electric Fields, Int. J. Heat Mass Transfer, Vol. 36, No. 3, pp. 775-782, 1993a. 6. J. Ogata and A. Yabe, Augmentation of Boiling Heat Transfer by Utilizing the EHD Effeet-EHD Behaviour of Boiling Bubbles and Heat Transfer Characteristics, lnt. J. Heat and Mass Transfer, Vol. 36, No. 3, pp. 783-791, 1993b. 7. H.A. Pohl, The Motion and Precipitation of Suspensoids in Divergent Electric Fields, J. of Applied Physics, Vol. 22, No. 7, pp. 869-871, 1951. 8. J. Seyed-Yagoobi, C.A. Geppert, and L.M. Gepport, ElectrohydrodynamieaUy Enhanced Heat Transfer in Pool Boiling, Transactions of ASME, Journal of Heat Transfer, Vol. 118, pp. 233-237, 1996. 9. A. Singh, A. Kumar, S. Dessiatoun, M.A. Faani, M.M. Ohadi, and A.I., Ansari, Compound EI-ID-Enhaneed Pool Boiling of R-123 in a Liquid-to-Refrigerant Heat Exchanger, ASME Paper No. 93-WA/HT-40, 1993.