IHTC16-21954 电子冷却微通道气体节流的实验研究

Proceedings of the 16th International Heat Transfer Conference, IHTC-16August 10-15, 2018, Beijing, ChinaIHTC16-21954*Corresponding Author: jhweng@EXPERIMENTAL INVESTIGATION OF GAS THROTTLING INJianhua Weng 1*1, Geng Hui 2,Xiaoyu Cui 2, Tongyun Zhang 1, Tenghui Liu 11Shanghai University of Electric Power, 2103 Ping Liang Rd. Yangpu District, Shanghai 200090, China 2University of Shanghai for Science and Technology, 516 Jun Gong Rd. Yangpu District, Shanghai 200093,ChinaABSTRACTThis experimental investigation was intended to evaluate the capability of micro-coolers designed with microchannel for electronic cooling. Two prototypes of micro-cooler were fabricated for the experiments. Both of them had a counter-flow heat exchanger, which consisted of 7 layers of microchannel. Each layer had 6 microchannels. The throttling process was carried out with gas flowing through microchannels of smaller size. The two prototypes of micro-cooler had similar configuration but different cross-sectional area of microchannels for the heat exchanger and the throttle. In the experiments the temperature of several locations along the surfaces of the micro-coolers was measured. Experiments with different gas inlet pressure and temperature were carried out and experimental results were obtained. It showed that for the throttling process the higher the inlet gas pressure, the larger the temperature drop. And for inlet temperature, the lower the inlet gas temperature, the larger the temperature drop. From the experiment results, the cooled gas after throttling could be used for cooling electronic devices with high-heat-flux. It can be considered as one of the prospective electronic cooling methods for further study.KEY WORDS: Electronic cooling, Heat dissipation, Throttling process, Microchannels1. INTRODUCTIONAs heat flux from electronic devices continues to rise, heat dissipation for these devices remains a great challenge. New technologies have been proposed to remove the heat of high-heat-flux, such as microchannels, spray cooling, etc. [1]. The Joule-Thomson(J-T) cryocooler has been used for cooling infrared sensors on satellites and cryosurgery for some time. The J-T cryocooler usually consists of a counter-flow heat exchanger, a throttling (capillary) device and a refrigerating cavity[2]. The J-T cryocooler can be an open cycle or a closed cycle[3]. The open cycle cryocooler uses a gas reservoir, and it has the advantages of simplicity, small size, low weight, and absence from mechanical noise or vibration[4].The working fluid used in J-T crycoolers includes argon, CO 2, N 2, etc.. The closed cycle cryocooler has a compressor, and mixed refrigerants are considered to be used as working fluid[5]. Derking et al.[6] investigated a cooling system using a gas mixture of methane, ethane and isobutene, with cold end temperature reaching at 150K and cooling power of 46mW. Ardhapurkar et al.[7]used mixture of N 2 and CO 2 as working fluid in a J-T cryocooler. Naer et al.[8] applied hydrocarbon mixtures in a closed micro cooler system for the temperature range of -73 to -183℃. In commercial application, Hampson-type J-T cryocooler has been widely adopted and the conventional one has a large mandrel at the center of the cryocooler. Since Tuckerman and Pease [9] used microchannels to remove heat with high-heat-flux, heat transfer and fluid flow in microchannles has been studied extensively. Colgan et al.[10] designed a Si microchannel cooler for cooling high power chips. The test showed the designed cooler demonstrated the capability of heat removal with heat flux greater than 300W/cm 2. Wei et al. [11]fabricated a stacked microchannel heat sink with two layers of microchannels, and the total thermal resistance could be as low as0.09℃/(W/cm 2). To minimize the size of cryocoolers, Little[12] introduced microchannels into these small cooling devices. Besides small size, these cryocoolers also have the features of fast response and low noise. They could easily be integrated into other instruments. Lerou et al.[13] designed 8 types of cooler and fabricated 14 prototypes. One of the configurations had the microchannel with 2.2mm in width and 0.5mm in depth, and with a net cooling power of 20mW at about 100K. Hatwar et at.[14] fabricated an open cycle miniature J-T refrigerator with a single microchannel, using low temperature co-fired ceramic technique. The dimension of the microchannel was 1mm in width and 0.25mm in depth. Using N 2 gas as working fluid, temperature drops of 22 to 46K were achieved with the inlet gas pressure ranging from 40 to 80bar. Wang et al.[15] fabricated and tested a polymer-based J-T micro cryogenic cooler. It contained a polymer heat exchanger and a silicon/glass J-T valve.This paper presents the experimental results of two J-T micro-coolers using microchannel both for the heat exchanger and throttling device. And gas of CO 2 and argon were used as working fluid.2. GAS FLOWING AND THROTTLING IN MICROCHANNELS2.1 Microchannels For gas flowing in microchannels, four flow regimes can be defined by Knudsen number: LKn λ= (1)Based on the value of Kn, the four flow regimes are continuum flow(Kn<10-3), slip flow(10-3<Kn<10-1), transition flow(10-1<Kn<10) and free molecular flow(Kn>10), respectively[16]. For continuum flow, the flow mathematically still can be described by Navier-Stokes equations with classical no-slip boundary conditions. For example, gas of CO 2 with pressure of 1atm and temperature of 300K flowing in a channel with cross section of 0.3mm by 0.4mm, the Kn is calculated to be less than 9×10-5, and the flow can be considered as continuum flow.2.2 Gas Throttling When a real gas passes through a capillary tube or a valve, its pressure decreases. The enthalpy usually is considered as approximately constant for such a throttling process. After throttling, the gas temperature might decrease, remain unchanged, or even increase, depending on the value of J-T coefficient[17]： h p ⎪⎪⎭⎫ ⎝⎛∂∂=T JT μ (2) At room temperature, gases such as CO 2, N 2 and argon cool down during the throttling process, as their J-T coefficients are positive. Situations are similar when gas flowing through a microchannel. Pressure of the gas will decrease and its temperature will drop if its J-T coefficient is positive.3. EXPERIMENTAL SETUP AND TEST PROTOTYPES3.1 Experimental Setup The experimental setup was an open system as shown in Fig. 1. Gas of CO 2 or argon came from a cylinder tank, and its flow rate was measured by a flowmeter. The pressure of the gas was regulated by a valve close to the outlet of the tank. The test prototype was enclosed by a chamber with a vacuum pump withdrawing the air inside the chamber, so the prototype was thermally isolated with the environment. After the throttling process in the microchannel of the test prototype, the low temperature gas was eventually discharged to the environment through a pipe. Pressure and temperature at inlet and outlet of the prototype as well as temperature of several points along the surface of the prototype (see Fig.2) were measured. All the measured data were collected by a data acquisition system. The range and accuracy of instruments used in the experiment are listed in Table 1.3.2 Test Prototypes The prototype consisted of 7 layers for high and low pressure gas. Fig.3a shows the plate for high pressure gas of prototype A, which had 6 microchannels for the heat exchanger and one microchannel for the throttling process. Fig.3b shows the plate for low pressure gas of prototype A. The plates were made of stainless steel. For prototype A, the cross-section dimension of microchannel for thethe top and bottom layer, see Fig.4), while that for the throttling microchannel was 0.1mm by 0.1mm.1-Gas Cylinder 2-Inlet Valve 3-Gas Filter and Drier 4-Mass Flowmeter5-High Pressure Transducer 6-Data Acquisition System 7-Prototype and Vacuum Chamber8-Low Pressure Transducer 9-Vacuum Gauge 10-Vacuum Pump 11-Outlet ValveFig. 1 Schematic of the experimental setupFig. 2a Temperature measurement points on the surface of prototype AFig. 2b Temperature measurement points on the surface of prototype BTable 1 Measurement range and accuracy of instrumentsMeasurement Instrument Range AccuracyThermocouple for prototype surface measurement-73~215℃ ±0.2℃ Thermocouple for gas measurement at inlet andoutlet-200-400℃ ±0.1℃ High Pressure Transducer 0-10MPa 0.2 GradeLow Pressure Transducer 0-10kPa 0.2 GradeMass Flowmeter 0-300SLPM 1%The length of throttling microchannel was 40mm. There was one microchannel for throttling on each layerfor high pressure gas. For prototype B, the cross-section dimension of microchannel for the heat exchangerbottom layer), and that for the throttling channel was 0.15mm by 0.1mm. The length of throttling microchannel also was 40mm. There were 6 microchannels for the throttling on each layer for high pressure gas. Layer arrangement at the section of heat exchanger for both prototype A and prototype B is shown in Fig.4. Except for the top and bottom layer, each layer consisted of 2 plates, and microchannels were formed when two plates for high pressure or two for low pressure were mounted together. An electric heater was mounted at the cold end of the prototype, as shown in Fig.5.1-Inlet 2-Entrance Section 3-Outlet 4-Microchannels for heat exchanger5-Microchannel for throttling 6-ChamberFig.3a Microchannels for heat exchanger and microchannel for throttling process, high pressure, prototype AFig.3b Microchannles for heat exchanger, low pressure, prototype AFig.4 Layer arrangement at the section of heat exchanger, for both prototype A and prototype BFig.5 Location of the heater4. RESULTS AND DISCUSSIONSExperiments of gas flowing through both prototype A and B were carried out. Fig.6-9 show the measured temperature along the prototype surface for gas of CO2. Fig.6-7 show at the beginning all the temperature decreased but a few minutes later the temperature gradually reached steady values. Fig.8-9 indicate the lowest temperature of the measurement points was located at the end of the prototypes, which was just after the throttling microchannels. The thermocouple (TC) no. 2 to 5 for both prototypes were located on the surface ofheat exchanger. The gas flowing through microchannels before throttling was cooled by the low temperature gas after the throttling process, which was flowing in the opposite direction, and the walls of microchannels were cooled as well. Besides, the temperature decrease of TC no.2 to 5 was also partly caused by the heat conduction along the microchannel wall as the temperature at the end of the prototypes was low.Fig.6 Temperature varies with time with inlet gas pressure 3MPa and temperature 12℃,Prototype A, gas of CO2Fig.7 Temperature varies with time with inlet gas pressure 1.4MPa and temperature 12℃,Prototype B, gas of CO2Fig.8 Readings of thermocouples on surface of prototype A, steady state, gas of CO2Fig.9 Readings of thermocouples on surface of prototype B, steady state, gas of CO2Among the measurement points, the temperature of TC no. 11 for prototype A was the lowest temperature. The temperature of TC11 for different inlet conditions are listed in Table 2. It shows at the same inlet temperature, the temperature of TC11 with higher gas inlet pressure was lower than that with lower gas inlet pressure. For example, with CO2inlet gas temperature at 12℃, the reading of TC11 was -45.03℃for 4.2MPa inlet gas pressure at steady state, while that of TC11 was -26.68℃ for 3MPa inlet gas pressure. It also shows at the same inlet pressure, the temperature of TC11 with lower inlet gas temperature was lower than that with higher inlet gas temperature, e.g. with CO2 inlet gas pressure at 3MPa, the reading of TC11 was -26.68℃ for 12℃ inlet gas temperature at steady state, while that of TC11 was -13.65℃ for 21℃ inlet gas temperature.Table 2 Readings of thermocouple no.11 for prototype A at different inlet temperature and pressure, gas of CO2Inlet Temperature/℃Inlet Pressure/MPa Reading of TC11/℃12 3 -26.684.2 -45.0321 3 -13.654.2 -26.09For prototype B, the lowest temperature among the measurement points was located at the same place as prototype A. However, the TC no. for this point for prototype B experiment was no. 9, instead of no. 11. Table 3 shows the readings of TC9 at different CO2 inlet gas pressure but same inlet gas temperature. The results were the same as that of prototype A, which was that at the same inlet temperature the lowest temperature among measurement points with higher inlet gas pressure was lower than that with lower inlet gas pressure. This also was true for gas of argon as shown in Table 4. However, as mentioned above, the prototype B had larger cross section area for gas throttling comparing with prototype A, which resulted in larger flow rate for prototype B. Therefore, for prototype B with CO2 gas at inlet pressure of 1.4MPa, the lowest measured temperature could be as low as -33.46℃, while for prototype A at even higher inlet pressure of 3MPa that was -26.68℃ at same inlet gas temperature, as listed in Table 2.Table 3 Readings of thermocouple no.9 for prototype B at different inlet pressure, gas of CO2 Inlet Temperature/℃Reading of TC9/℃12 1 -14.461.4 -33.46Table 4 Readings of thermocouple no.9 for prototype B at different inlet pressure, gas of argon Inlet Temperature/℃Inlet Pressure/MPa Reading of TC9/℃16 3 -17.875.3 -43.957.3 -51.26When the cold end of prototype B was heated by an electric heater (see Fig. 5), the temperature reading of TC9 increased as shown in Fig. 10. When the heating power increased, the reading of TC9 increased as well. From the experiment, with gas of argon as working fluid prototype B could dissipate 4W heat while the cold end temperature at the measurement point was below 6℃. The heating power in this experiment was much larger than those reported by other references[12,13], indicating the prototypes with current configurations could dissipate larger heat from electronic devices. Moreover, for most electronic devices, as their outside surface temperature are allowed much higher than several Celsius degree, more amount of heat can be dissipated with prototype B.Fig. 10 Reading of thermocouple no.9 varies with heating power5. CONCLUSIONSExperiments with two types of J-T micro-cooler were carried out to investigate their capability of electronic cooling. Both prototypes were fabricated with 7 layers (including the top and bottom layer) of microchannels. Gas of CO2 and argon were used as working fluid. Experiment results showed that for either CO2 or argon the higher the inlet gas pressure, the larger the temperature drop. And the lower the gas inlet temperature, the larger the temperature drop. The configuration of the prototype can be optimized to further increase the capability of heat removal. Hence, with further study gas throttling in microchannels with multilayer can be considered as a prospective method for cooling electronic devices with high-heat-flux.ACKNOWLEDGMENTThis research work was supported by Shanghai Natural Science Foundation, Grant No. 14ZR1429100. NOMENCLATUREKn Knudsen number ( - )L characteristic length (m)λmean free path (m)μJT Joule-Thompson coefficient (K/kPa)REFERENCES[1] Ebadian, M. A., Lin, C. X., “A review of high-heat-flux heat removal technologies,” J. of Heat Transfer, 133, pp.110801(1-11),(2011).[2] Ng, K. C., Xue, H., Wang, J.B., “Experimental and numerical study on a miniature Joule-Thomson cooler for steady-statecharacteristics, ”Int. J. Heat Mass Transf., 45, pp.609-618, (2002).[3] Maytal, B. Z., Nellis, G. F., Klein, S. A., et al., “Elevated-pressure mixed-coolants Joule-Thomson cryocooling,”Cryogenics,46, pp.55-67, (2006).[4] Timmerhaus, K. D., “ Cryocooler Development, ”AICHE Journal, 42(11), pp. 3203-3211, (1996).[5] Gong, M. Q., Wu, J. F., Yan, B., et al., “Study on a miniature mix-gases Joule-Thomson cooler driven by an oil-lubricatedmini-compressor for 120K temperature ranges,” Physics Procedia, 67, pp.405-410, (2015).[6] Derking, J. H., Vermeer, C. H., Tirolien, T., et al., “A mixed gas miniature Joule-Thomson cooling system,”Cryogenics, 57,pp.26-30, (2013).[7] Ardhapurkar, P. M., Sridharan, A., Atrey, M. D., “Experimental investigation on temperature profile and pressure drop in two-phase heat exchanger for mixed refrigerant Joule-Thomson cryocooler,”Applied Thermal Engineering, 66, pp.94-103, (2014).[8] Naer, V., Rozhentsev, A., “Application of hydrocarbon mixtures in small refrigerating and cryogenic machines, ”Int. J. ofRefigeration, 25, pp. 836-847, (2002).[9] Tuckerman, D. B., Pease, R. F. W. , “High performance heat sink for VLSI,” IEEE Electron Dev. Lett., EDL-2, pp. 126-129,(1981).[10] Colgan, E. G., Furman, B., Gaynes, M., et al., “A practical implementation of silicon microchannel coolers for high powerchips,”21st IEEE SEMI-THERM Symposium, pp. 1-7, (2005).[11] Wei, X., Joshi, Y., Patterson, M. K., “Experimental and numerical study of a stacked microchannel heat sink for liquid coolingof microelectronic devices, ”J. of Heat Transfer, 129, pp.1432-1444, (2007).[12] Little, W. A., “Microminiature refrigeration,”Advances in Cryogenic Engineering: Transactions of the Cryogenic EngineeringConference, 53: pp.597-605, (2008).[13] Lerou, P.P.P.M., Venhorst, G.C.F., Veenstra, T.T., et al., “All-micromachined Joule-Thomson cold stage,”Crycocoolers, 14,pp.437-441, (2007).[14] Hatwar, R., Ambad, S., Kumbhalkar, M. D., et al., “Development of a miniature Joule-Thomson refrigerator using LTCC(LowTemperature Co-fired Ceramic) technique,”Proceedings of 1st Int. Sym. on Physics and Technology of Sensors, pp.145-148, (2012).[15] Wang, Y. D., Lewis, R., Lin, M., et al., “Wafer-level processing for polymer-based planar micro cryogenic coolers,”25th IEEEInt. Conf. on Micro Electro Mechanical Systems, pp.341-344, (2012).[16] Kandlikar, S.,Garimella, S., Li,D., et al., Heat Transfer and Fluid Flow in Minichannels and Microchannels, Oxford: ElsevierInc., (2006).[17] Cengel, Y. A., Boles, M. A., Thermodynamics: An Engineering Approach, Fourth Ed.,New York: McGraw-Hill, (2002).。