MECHANISM OF GAS-LIQUID EXCHANGE IN MICROBUBBLE EMISSION BOILING *T. Furusho, K. Yuki, R. Kibushi, N. Unno and K. Suzuki 2 Tokyo University of Science-Yamaguchi, Daigaku-dori --, Sanyo-Onoda, Yamaguchi, 76-0884 Japan *f60@ed.tusy.ac.jp 2 Tokyo University of Science, Yamasaki 264, Noda, Chiba, 278-80 Japan ABSTRACT Microbubble emission boiling (MEB) is one of the boiling heat transfer phenomena observed at highly subcooled condition. The maximum heat flux of MEB is above MW/m 2 with water and it is higher than the ordinary critical heat flux. Therefore, boiling heat transfer with MEB is a promising cooling technology for an inverter of the electric vehicle. This study aims at evaluating the mechanism of MEB. To clarify the mechanism of gas-liquid exchange in MEB, the temperature field of fluid around the heat transfer surface is measured in MEB. In order to measure the averaged temperature field and root mean square (RMS) of the temperature fluctuation of water close to the heat transfer surface, we used a thermocouple tree that enables to measure fluid temperatures at five locations simultaneously. The diameter of the thermocouple is 0.2 mm and the sampling speed is 2 ms. The measurement area is 30 mm in x-direction 0 mm in z-direction centerd on the heat transfer surface. The diameter of the heat transfer surface is 0 mm. The distance of each thermocouple is 2.0 mm and the thermocouple tree is traverse above the heat transfer surface at the interval of.0 mm. It is previously examined that the thermocouple tree does not affect the boiling curve and the behaviour of the bubbles. The obtained time averaged temperature field in MEB reveals that there is a high temperature area close the outside of the heat transfer surface. On the other hand, the averaged fluid temperature on the center of the heat transfer surface is lower than that on the outside of the heat transfer surface. This is because the center of heat transfer surface is partially covering with a thin vapor film. Furthermore, the RMS of the temperature field on the outside of the heat transfer surface is also higher than that on the outside of the heat transfer surface. As a result, the gas-liquid exchange in the MEB regime is enhanced in outside of the heat transfer surface. Moreover, the RMS of the temperature fields increases with the increase in the liquid subcooling. This means that the gas-liquid exchange becomes more active with increasing the liquid subcooling. INTRODUCTION In highly subcooled boiling, the heat flux increases beyond the ordinary critical heat flux by occurrence of Microbubble Emission Boiling (MEB) in the beginning of the transition boiling [, 2]. For example, in the previous studies, it was clarified that MEB was occurred in cases of both a flat plate and a thin wire, and it was induced at highly subcooled conditions above 20 K for distilled water at an atmospheric pressure. In the MEB regime, the maximum heat flux exceeded 0 MW/m 2 at the flat plate heat transfer surface [3], and the maximum heat flux exceeded 4 MW/m 2 in case of heating the thin wire [4]. Therefore, MEB can work as heat transfer enhancement technology for the cooling system utilizing the boiling heat transfer. From this point of view, Suzuki et al. have evaluated the effect of pressure [], properties of the heat transfer surface [6] and the coolant [7, 8] on MEB. On the other hand, mechanism of MEB was discussed based on visualization of vapor bubble collapse process by Ueno et al. [9, 0] and microlayer model by Tsuruta et al. []. Ueno et al. focused on the process of vapor bubble condensation in subcooled pool boiling. As a result, vapor bubbles repeat rapid expansion and rapid condensation on a very short time scale of less than /0,000 s. Moreover, the surrounding liquid was supplied from the upper part of the heat transfer surface. In addition, Tsuruta et al., compared the typical experimental results of the thin wire with the theoretical estimation and indicated that there was a possibility to express the heat transfer of MEB by the microlayer model developed for the critical heat flux. However, the mechanism of the gas-liquid exchange of MEB based on temperature field around the heat transfer surface has not been discussed yet. In the present study, the mechanism of gas-liquid exchange in the MEB regime is discussed by measuring the temperature field of fluid around the heat transfer surface in MEB. In addition, the temperature distribution of the heat transfer surface is also evaluated in the MEB regime in order to compare the surface temperature with the distribution of the average temperature of fluid and the temperature fluctuation. Consequently, the gas - liquid exchange mechanism on the heat transfer surface is discussed. EXPERIMENTAL SETUP AND CONDITION An experimental setup for subcooled boiling test is shown in Fig.. The vessel for the pool boiling experiment is made of a stainless steel circular pipe of 20 mm in diameter and 70 mm in height. The vessel has three observation windows, which enables the observation of the behavior of boiling bubbles with a high speed video camera. A heat transfer copper block is installed at the back side of the bottom plate of the vessel. The copper block consists of cylindrical part of 2 mm in diameter and a trapezoidal-like base. The top surface of the cylindrical part, which is a heat transfer surface, is polished with a #00 emery paper. Five cartridge heaters are inserted into the bottom of the base of the copper block. The maximum power of the heaters is 4000 W (800 W ). The location of thermocouple is shown in Fig. 2.
Fig. Experimental setup for subcooled pool boiling test Fig.2 Location of the thermocouple at cylindrical part of Cu block Fig.3 Thermocouple Tree Three sheathed K-type thermocouples of 0. mm in diameter are embedded at the central axis and the location of 3 mm from the central axis in a radial direction. The locations of the thermocouples for each is 3. mm, 4.6 mm and 7.6 mm apart from the heat transfer surface. The surface temperature and the temperature gradient are estimated by extrapolating the temperature distribution of the temperature data when each temperature is in a steady state. The heat flux is calculated by Fourier s low of heat conduction. The base part of the copper block is covered with thermal insulations. The heat loss leaked from the side of the cylindrical part is estimated to be approximately 0 percent of the axial heat flow in the MEB regime. Distilled water is used as a cooling liquid and its volume in vessel is 4 liters. The temperature of the liquid is controlled at a predetermined temperature within +-. K using a U-shape heater, a cooling coil and a stirrer. In this study, the liquid subcoolings are 30 K, 40 K and 0 K under the atmospheric condition. The temperature field of fluid around the heat transfer surface is measured with a thermocouple tree (hereafter, TCT.) that enables to measure fluid temperatures at five locations simultaneously. The TCT detail shown in Fig. 3. The diameter of the thermocouple is 0.2 mm and the sampling speed is 2 ms. The TCT working range is shown in Fig. 4. The measurement area is 30 mm in x-direction 0 mm in z-direction centerd on the heat transfer surface. The thermocouples arranged at equal spaces of 2.0 mm and the TCT. is traversed above the heat transfer surface with.0 mm step. The temperature field is measured at the middle region of nucleate boiling, around the critical Fig.4 Thermocouple tree working range heat flux, lower heat flux side of MEB, and higher heat flux side of MEB. EXPERIMENTAL RESULTS Figs. show the boiling curve at the liquid subcoolings of (a) 30 K, (b) 40 K and (c) 0 K. The higher heat flux data of in the MEB regime at the liquid subcooling of 0 K could not be measured due to the limit of the allowable temperature of the sealant used between the heat transfer block and the bottom plate. In the legend of Fig., center is the data at the center of the heat transfer surface, 3 mm is the data at 3.0 mm away from the center in the radial direction, NB means Nucleate boiling, lower HF MEB means lower heat flux case of MEB, and higher HF MEB means higher heat flux case of MEB. In Fig., the reference data without the TCT and the data with the TCT are in good agreement
0 ΔT sub =30K P.W. refelence 0. lower HFMEB center lower HFMEB 3mm Higher HF MEB center Higher HF MEB 3mm 20 40 60 80 00 0 ΔT sub =40K P.W. refelencce 0. lower HF MEB center lower HFMEB 3mm Higher HF MEB center Higher HF MEB 3mm 20 40 60 80 00 0 0. ΔT sub =0K P.W. refelence lower HF MEB center lower HF MEB 3mm 20 40 60 80 00 (c) ΔT sub = 0 K Fig. Boiling curve especially for the nucleate boiling data, the CHF data, and the MEB data in lower heat flux case. It is previously confirmed that the TCT does not affect the boiling curve and the behavior of the bubbles.the change in the temperature of the heat transfer surface during the experiment with TCT is within 3.3 K at the maximum. From this fact, it is considered that the existence of the (c) ΔT sub = 0 K Fig.6 Averaged temperature fields around the heat transfer surface in case of lower heat flux of MEB TCT does not affect the boiling phenomenon even when the TCT is the closest to the heat transfer surface. This figure proves that the temperature at the center of the heat transfer surface is higher than that near the edge of the heat transfer surface in both the higher and lower heat flux case of the MEB regime and that the heat flux near the edge is much higher than that at the center. Figs. 6 show the average temperature fields at the lower heat flux case of MEB at the liquid subcoolings of (a) 30 K, (b) 40 K and (c) 0 K. Figs. 7 show the average temperature field at higher heat flux of MEB at the liquid subcoolings of (a) 30 K and (b) 40 K. Focusing on the averaged temperature field around the heat transfer surface, the temperature near the center of the heat transfer surface is lower than that near the outside of the heat transfer surface. The temperature difference at the lower heat flux of MEB is approximately 0 K at the liquid subcooling of 30 K, 4 K at the liquid subcooling of 40 K and 8 K at the liquid subcooling of 0 K.
Fig.7 Averaged temperature fields around the heat transfer surface in case of higher heat flux of MEB Furthermore, the temperature difference at higher heat flux of MEB is approximately 2 K at the liquid subcooling of 30 K and 3 K at the liquid subcooling of 40 K. This fact is contrary to the finding based on Fig. that the surface temperature in the central region is higher than that in the outside region of the heat transfer surface, which suggests that there could be a thin vapor film that could not be caught at the temperature measurement at the height of.0 mm above the heat transfer surface. Figs. 8 show the RMS of the temperature fluctuation in the lower heat flux case of MEB at the liquid subcoolings of (a) 30 K, (b) 40 K and (c) 0 K. Figs. 9 show the RMS at higher heat flux side of MEB at the liquid subcoolings of (a) 30 K and (b) 40 K. The strong RMS area is spreads to the height of mm from the heat transfer surface at the liquid subcooling of 30 K. This is because the calm MEB [8] occurs due to the low subcooling, and there is a possibility that the high temperature liquid from the heat transfer surface flows upward compared with the other cases. In the higher heat flux regime of MEB, however, the RMS distribution is similar to the distribution of the average temperature at the liquid subcooling of 40 K, in which there is an obvious strong RMS area on the outside of the heat transfer surface. The RMS distribution at the liquid subcooling of 0 K is the similar to the liquid subcooling of 40 K. The RMS at the liquid subcooling of 0 K is much higher than that at the liquid subcooling of 40 K. This fact verifies that the gas-liquid exchange becomes more active on the outside of the heat transfer surface with increasing liquid subcooling. In the MEB regime, the central area of the heat transfer surface is (c) ΔT sub = 0 K Fig. 8 RMS around the heat transfer surface in case of lower heat flux of MEB partially covered with a thin vapor film, which results in the active gas-liquid exchange on the outside of the heat transfer surface. The temperature of the heat transfer surface at the center is 6 C in the lower heat flux case of MEB, while the temperature at 3.0 mm in the radial direction is 48 C. Moreover, the temperature in the higher heat flux case of MEB is 6 C, whereas 44 C at the same radial position. Although this temperature is higher than the wetting limit temperature of copper and water (approximately 30 C), it seems that the wetting limit temperature is increased by the influence of oxidation of the heat transfer surface [2]. During the experiment, the wetting limit temperature increases with the oxidation of the heat transfer surface. Therefore, the temperature of the outside of the heat transfer surface becomes lower than the wetting limit temperature at least because the gas-liquid exchange definitely occurs on the outside of the heat transfer surface.
Fig. 9 RMS around the heat transfer surface in case of higher heat flux of MEB CONCLUSION In this study aims at evaluated the mechanism of MEB, and to clarify the mechanism of gas-liquid exchange in MEB, the temperature field of fluid around the heat transfer surface was measured in MEB. In the MEB region, the temperature at the center of the heat transfer surface is higher than that near the edge of the heat transfer surface and that the heat flux near the edge is much higher than that at the center. In addition, a thin vapor film exists, and the gas-liquid exchange occurs around the outside of the heat transfer surface. And the temperature of the outside of the heat transfer surface becomes lower than the wetting limit temperature at least because the gas-liquid exchange definitely occurs on the outside of the heat transfer surface. In the future, we will improve the spatial temperature distribution accuracy of TCT. and reacquire the flow field with PIV to compare the temperature field and the flow field and elucidate the MEB gas-liquid exchange mechanism. REFERENCES () Inada, S., et al., A study on Boiling Curves in Subcooled Pool Boiling (3rd Report, Behaviors of bubble cluster and temperature fluctuations of heating surface), Trans. JSME (in Japanese), 47-422, 2030-204, 98. (2) Suzuki K., et al., Proc. 6th ASME/JSME Thermal Engineering Joint Conf. (2003), TED-AJ03-06, CD- ROM. (3) Kumagai, S., et al., Trans. JSME (in Japanese), Occurrence and Stability of Microbubble Emission Boiling, 8-46, 497-02, 992. (4) Shoji, M., Kanagawa University Engineering Laboratory Report, No.29, p.p. 2-9, 2006. () Suzuki, K., Aramaki, S., et al., Application of Microbubble Emission Boiling for Cooling Technology, st National Heat Transfer Symposium (in Japanese), E34, 204. (6) Suzuki, K., et al., 39th National Heat Transfer Symposium (in Japanese), E44, 2002, CD-ROM. (7) Suzuki K., Microbubble Emission Boiling of Alcohol-Water Mixtures, Proc. 3rd IASME/WSEAS Int. Conf. on Heat transfer, Thermal engineering and environment, p.p. 9-96, 200. (8) Furusho, T., et al., Fundamental study on subcooled boiling of binary mixture of anti-freezing liquid toward advanced cooling technology for a SiC-based on-vehicle inverter, th The Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic System 206, p.p.93-98, 206. (9) Ueno, I., et al., st National Heat Transfer Symposium (in Japanese), Effect of heat surface size on critical heat flux and collapsing of vapor bubbles in subcooled pool boiling, A2, 204, CD-ROM. (0) Ueno, I., et al., Effect of heated surface size on critical heat flux and collapsing of vapor bubbles in subcooled pool boiling, 2nd National Heat Transfer Symposium (in Japanese), D33, p.p. 426, 20, CD-ROM. () Tsuruta, T., et al., Study on Microbubble Emission Boiling Heat Transfer Based on Microlayer Model, Proc. JSME Thermal Engineering Conf. 204, No. 4-9, 204. (2) Nishio, S., Hirata, K., A study on Leiden Frost temperature, Trans. JSME (in Japanese), 43-374, 386-3867, 977.