The Investigation on Flow Boiling Heat Transfer of R134a in Micro-channels

Transcription

1 Journal of Thermal Science Vol.24, No.5 (2015) DOI: /s Article ID: (2015) The Investigation on Flow Boiling Heat Transfer of R134a in Micro-channels LI Xuejiao 1,2, JIA Li 1,2* 1. Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing, China 2. Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale, Beijing , China Science Press and Institute of Engineering Thermophysics, CAS and Springer-Verlag Berlin Heidelberg 2015 The present study reports an experimental evaluation of heat transfer characteristic of R134a flow boiling in micro-channel heat sink. The heat sink is composed of 30 parallel rectangular micro-channels with cross-sectional dimensions of 500μm width and depth, as well as total length 30mm. Experiments were conducted with heat flux up to W/cm 2, mass velocity ranging from to kg/m 2 s, vapor quality ranging from 0.06 to 0.9. The wall temperature of heat sink heated could be controlled at around 50. Heat transfer coefficient could be up to 180 kw/m 2 K. Two dominating flow patterns were observed by analyzing boiling curves. The heat transfer characteristics of nucleate boiling and convective boiling were presented in the study. Revised correlations of R134a flow boiling in micro-channel heat sink were carried out with the consideration of nucleate boiling and convective boiling mechanisms. Keywords: flow boiling, micro-channel, R134a, correlation, IGBT Introduction Power electronic devices have been widely used in the traction system of high-speed rail, metro vehicle, switching supplies and other power conversion systems. To meet the challenge, high drawbar power (about 1MW for each train general) is needed to keep high-speed trains moving. Traction converter is a key technology of providing high-speed train running power. Currently traction converter adopts IGBT (Insulated Gate Bipolar Transistor) as the mainstream technology to convert DC to AC for driving traction motor. As power elements switch continually with high voltage superimposing, IGBT generates a lot of heat, which absorbed by the environment and encapsulation [1]. The heat flux of IGBT devices is now at the level from 100 to 150 W/cm 2. With the increasing of current capacity and frequency, it is projected to increase to 500 W/cm 2 in the next generation [2, 3, 4]. Since the elements in IGBT are sensitive to temperature, the IGBT module operation is strongly dependent on the adopted thermal management solutions. An IGBT device generates a lot of heat in normal operation because of its power conversion inefficiency. High heat flux could lead to unacceptably high temperature in IGBT. Inapposite ways would lead IGBT to be out of safe operating area (SOA) and the module failure [5]. Therefore, thermal management of IGBT has been placed on a critical position. The primary thermal management methods of IGBT on power electronic devices are air-cooling heat sink and liquid-cooling plates. Takahisa Hitachi et al[6] proceeded an experiment of direct liquid cooling for IGBT modules. They designed a Received: June 2015 JIA Li: Professor This research was supported by National Natural Science Foundation of China (No ).

2 LI Xuejiao et al. The Investigation on Flow Boiling Heat Transfer of R134a in Micro-channels 453 Nomenclature Bo boiling number, Greek symbols Bd bond number viscosity C D specific heat thickness D channel diameter density G mass velocity surface tension g gravitational acceleration Subscripts h heat transfer coefficient c conductive h fd latent heat CB convective boiling H micro-channel height exp experimental k thermal conductivity f liquid L micro-channel length g gas m mass flow rate in inlet Nu Nusselt number NB nucleate boiling q heat flux out outlet Re Reynolds number pre predictive T Temperature sat saturated w micro-channel width x vapor quality copper baseplate with needle fins contacted to the chip of IGBT directly and coolant fluid flowed through needle fins and chip and took away heat generated by IGBT. Jeremy and Campbell et al [7] presented a two-phase cooling method using R134a refrigerant to dissipate the heat energy generated by converters. They submerged power electronics in an R134a bath and tested simultaneous operation with a mock automotive air-conditioner. The experimental results show that the techniques in these studies could maintain IGBT in a reliable temperature range. Compared with air cooling and liquid flow cooling, the approaches above-mentioned can improve the heat transfer ability, but it is difficult in designing practical products for various kinds of IGBT. Without sealing components together, the coolant liquid is easy to divulge, leading to the damage of IGBT. Compared with the methods mentioned above, flow boiling in the micro-channel heat sink could provide outstanding heat transfer performance, stable operating condition and simple manufacture method [8, 9]. Taking advantage of the latent heat absorbed during working fluid evaporation in microchannels, two phase flow boiling in micro-channels provides high heat transfer coefficients, meanwhile as low boiling point and latent heat liberated of cryogenic fluid during boiling. Two-phase flow system could operate at lower mass flow rates with lower power input. Do Nascimento et al [10] carried the study on experimental evaluation of heat sink for R134a flow boiling in microscale channels. The experiments were conducted at the conditions of heat flux up to 310 kw/m 2 and mass velocities ranging from 400 to 1500 kg/m 2 s. The heat transfer coefficients could reach 36 kw/m 2 K. R. Ali et al[11] conducted a series of experiments to measure the heat transfer coefficients in a circular micro-channel with R134a as working fluid. Mass velocity was varied from 175 kg/m 2 s to 500 kg/m 2 s and heat flux ranged from 5 kw/m 2 to 60 kw/m 2. The results show that R134a has a better performance than R245fa in micro-channel flow boiling. In spite of these advantages, the complex heat transfer mechanism of cryogenic fluid flow boiling in micro-channels has limited consideration for IGBT thermal management [12, 13, 14]. The primary experimental objective of this work is to study the IGBT cooling by micro-channel heat sink with cryogenic fluid as working fluid. Heat transfer characteristics of R134a flow boiling in micro-channel heat sink were studied with variation of heat fluxes, vapor qualities and mass velocities. By analyzing R134a saturated boiling phenomenon in experiments, the correlations based on the mechanisms of cryogenic fluid flow boiling in microchannel heat sink were suggested. Experimental investigation Encapsulation of IGBT As the packaging of IGBT has great impact on its heat dissipation, the construction of encapsulation for IGBT should be rigorously designed. Fig.1 shows layers constituting of IGBT package in an inverter.

3 454 J. Therm. Sci., Vol.24, No.5, 2015 Fig. 1 Mounting details of IGBT The silicon chip is soldered to the direct bond copper (DBC) layer and DBC is soldered to the copper baseplate for supporting IGBTs and diodes. For reducing thermal resistance between the heat sink and baseplate, thermal grease is used to connect them. Considering the thermal properties of IGBT, every layer of the packaging was defined an identical temperature. The junction temperature (T j ) is used to describe the temperature inside the chip, which is relevant to the losses and safe operating area, meanwhile it is the temperature assumed homogenous cooling temperature for the device. In most situations T j should not exceed 150. The case temperature (T c ) is defined as the allowed temperature range of the module case in operation. The devices operating under T j =150 usually have a case temperature less than 125. Heat sink temperature (T s ) is defined as the surface temperature of heat sink pasted on the mounted IGBT. The critical T s can be calculated with the thermal resistances shown in Fig.2. Fig. 2 Thermal resistance of IGBT module and heat sink In most cases, thermal conductivity of grease used in IGBT is in the range of W/m K and the thickness of thermal grease is about 100 microns, with the heat flux of IGBT increasing to 80W/cm 2 and the temperature difference between T c and T w. In conclusion, the T w in this experiment is better to be controlled under 120. Experimental facility The experimental system is comprised of test circuit, cooling water circuit, heating system and data acquisition system, shown in Fig.3. Refrigerant circuit shown comprises a pump driving R134a from the fluid reservoir through the closed loop, the filter, visual liquid mirror and Rota meter. Liquid flow rate is set through the pump controller based on the Rota meter installed on the downstream of pump. The sight glass installed before the inlet of test section is used to check the status of cryogenic fluid. Before starting experiment, test loop should be vacuumed for 6 hours by vacuum pump to ensure the vacuum situation. The test section is a brass heat sink with 30 microchannels and each channel has designed dimension of 500μm width and depth, and 30mm length. For eliminating flow instability, two flumes (10mm 10mm 30mm) were machined at both ends of micro-channels. In order to reduce thermal resistance between heat sink and heater, a concave (24mm 30mm) is manufactured on the back of heat sink and the heater was implemented into the concave, shown in Fig.4. The thickness between heater Fig. 3 Experimental test loop

4 LI Xuejiao et al. The Investigation on Flow Boiling Heat Transfer of R134a in Micro-channels 455 accuracy of measuring instruments provided by manufactures (temperature sensors, mass flow meter, pressure sensors) is listed in Table 1, which also includes the maximum experimental uncertainty in the heat transfer coefficient with standard error analysis. Fig. 4 Picture of heat sink back and micro-channels is merely 1mm that is benefit for cooling heater and measuring T w accurately. Three channels were carved on the concave bottom for thermocouples fixing up. Silver paste was applied in channels for reducing thermal resistance between thermocouples and heat sink heated wall. Data reduction The local heat transfer coefficient h is calculated from: q h (1) AT w Tsat Where T sat is the liquid saturation temperature and T W is the temperature of channel wall corrected based on T s. The local heat sink wall temperature is obtained from the thermal couples installed in the concave back of heat sink. The relation between T W and T s is shown as follows: q Tw Ts (2) k is the thickness from heat sink heated surface to the surface of micro-channels bottom. A is the total heated area in the micro-channels: A=N(w+2H)L (3) Where N is the number of micro-channels in heat sink, w, H, and L are the micro-channel width, depth and length respectively. Heat flux q for heating heat sink is defined as: dt q kac (4) c Where k is thermal conductivity coefficient of brass, A c is the area of heater in the concave of heat sink back. dt is the temperature difference of two thermal couples mounted along the heater at a distance of one centimeter, hence c in equation (4) is one centimeter. In experiments, thermodynamic vapor quality is used to describe the vapor quality of flow boiling in micro-channels which is defined as: 1 q Li x CpTsat Tin h fg m L (5) An uncertainty analysis was carried out to evaluate quality and reproducibility of the experimental data. The Table 1 Uncertainties in measured and calculated parameters Parameter Instrument/measurement range Uncertainty Channel diameter 0.5mm ±0.002mm Absolute pressure Pressure sensor(0-20bar) ±0.05% Mass flow rate Rota meter ±1.5% Temperature Thermal couples ±0.2 Heat flux ±1.5% Mass flux ±2% Heat transfer coefficient <±12% Experimental results and discussion Heat sink temperature One of the most important parameters in IGBT cooling is the temperature of heat sink wall (T w ) which is influenced by the T j. Fig.5 shows the variation of T w as a function of heat flux (q). T w decreases with the increasing of heat flux from 18.7 W/cm 2 to 37 W/cm 2 except with mass velocity of kg/m 2 s. T w decreases to which is much lower than designed T w (120 ) at 26 W/cm 2 with mass velocity of kg/m 2 s. With heat flux exceeding 37 W/cm 2, T w varies as a wavy line while the wave crest and wave valley appears at 50W/cm 2 and W/cm 2 respectively. In the range of 50 W/cm 2 to W/cm 2, T w is lower than 40.65, which is superior to the designed 120. For high mass velocity, T w shows slight fluctuation with variation of heat fluxes. While under low mass velocity, T w presents much more unstable. Fig. 5 Variation of heat sink wall temperatures with heat flux

5 456 J. Therm. Sci., Vol.24, No.5, 2015 Boiling heat transfer Boiling curves of R134a flow boiling in micro-channel heat sink for different mass velocities were given in Fig.6 and Fig.7. Fig.6 shows the variation of heat transfer coefficient h with the increasing of heat flux (from 18.7 W/cm 2 to W/cm 2 ) under different mass velocities from kg/m 2 s to kg/m 2 s. It can be seen that the heat transfer coefficient increases with the increasing of heat flux at most mass velocity, while decreasing with low mass velocity at high heat flux. When the heat flux is 26 W/cm 2, heat transfer coefficient could reach W/m 2 K. With the increasing q, h increases to W/m 2 K at the mass velocity of kg/m 2 s. The value of h is superior to that of conventional channels. It is easily to observe that heat transfer coefficient increases monotonically with the mass velocity at all range heat flux. Fig.7 shows variation of heat transfer coefficient with the vapor quality from 0.06 to 0.9. It can be seen that heat transfer coefficient increases with the mass flux at Fig. 6 Heat transfer coefficient as a function of heat flux for different mass velocities Fig. 7 Heat transfer coefficient as a function of vapor quality for different mass velocities most vapor qualities. But this phenomenon is not observed at low vapor qualities. With the increasing vapor qualities, heat transfer coefficient increases at a slower rate. Heat transfer coefficient is affected obviously by heat flux, owing to heat transfer mechanism under different flow patterns. At low vapor qualities, nucleate boiling is the dominant heat transfer mechanism. The influence of mass velocity is weak. However, at high vapor qualities, convection flow boiling is dominant flow boiling pattern in which annular flow appears mostly [15]. Boiling regime It is widely known that saturated flow boiling in channels is governed by two mechanisms: nucleate boiling and forced convection boiling [16]. In the nucleate boiling dominant region, liquid near the heated channel wall is superheated, thus a sufficient degree is formed to sustain the nucleation and the growth of vapor bubbles and small bubbles will be stirred in liquid tempestuously [17]. With the increasing of heat flux, small bubbles coalesce into a long bubble, meanwhile, vapor quality is effected by heat flux intensively. As a result, long bubbles coalesce into a single longer bubble or transform to annular flow which represents convection boiling regime with the increasing heat flux and vapor quality [18, 19]. Fig.8 (a) shows the picture of heat transfer coefficient with different flow pattern from Tailian Chen s work [20]. Tailian Chen and Suresh V. Garimella studied flow boiling of FC-77 in silicon micro-channel heat sink by using visualization and five temperature sensors located along the midline of the heat sink. They found that the heat transfer coefficient varies as the same tendency over the heat flux range for each flowing rate. They divided the boiling curves into five distinct boiling regimes labeled a though e. Boiling regime a represents the initiation of flow boiling at the lowest heat fluxes. As heat flux is increased, regime b appears with flow pattern of bubbly flow, slug flow, elongated slugs, and then annular flow. As heat flux increasing, flow instability occurrences in regime c with wispy-annular flow and churn flow represents convective flow boiling. That heat transfer coefficient decreases with heat flux at high heat flux range indicates channel dry-out at regime d and e. In the present work, heat transfer coefficient is in the same variation tendency with the increasing of heat flux. Based on analysis of Fig.8 (a), boiling curves could be defined with three regimes: nucleate boiling regime, convective boiling regime and dry-out regime. Nucleate boiling and convective boiling were the dominant heat transfer mechanisms in saturated flow boiling in micro-channels, which were regime I and II respectively. Regime III represented dry-out in flow boiling which was risky for operational device. As shown in

6 LI Xuejiao et al. The Investigation on Flow Boiling Heat Transfer of R134a in Micro-channels 457 Fig.8, it could be observed that mass velocity played little impact on heat transfer coefficient in regime I. Curves of heat transfer coefficients at different mass velocities were similar. In regime I, heat transfer coefficients decreased slightly with the heat flux. In regime II, heat transfer coefficient was affected by both mass velocity and heat flux. The highest heat transfer coeffi- cient at kg/m 2 s was kw/m 2 K, which was 19.45% higher than that at mass velocity of kg/m 2 s. Heat transfer coefficient fluctuates significantly at low mass velocity ( kg/m 2 s), shown in Fig.8 (b). There were two wave crests and one trough along the curve. Based on heat transfer mechanism analyzed above, wave crests represented fully developed nucleate boiling and convective boiling respectively. The trough represented bubbly flow, slug flow and elongated slugs. For different mass velocities, diverse flow patterns arose at different heat fluxes and sustain different periods. Table.2 shows heat transfer coefficient of two wave crests and trough as well as the comparison of heat transfer coefficients of nucleate boiling in the experiment. Table.2 shows that the heat transfer coefficient at kg/m 2 s had the lowest percentage compared with nucleate boiling at elongated slug regime and increased more rapidly to convention boiling regime than that at higher mass velocity. Heat transfer coefficient between nucleate boiling and convention boiling had smaller difference than that at other mass velocity. However, heat transfer coefficient at high mass velocity shows remarkable performance at convection boiling regime in comparison with nucleate boiling regime. The phenomenon indicates that high mass velocity is skilled at high heat flux, while heat sink with low mass velocity can act at nucleate boiling regime as well as convention boiling regime. The results show that flow boiling patterns transform promptly at low mass velocities, while behaving stably at high mass velocities. Since dry-out regime is close to convective boiling regime, heat sinks operated at nucleate boiling is more safe for cooling electrical devices at low mass velocities. On the other hand, because convective boiling has good heat transfer performance at high mass velocity, nucleate boiling needs less power supply for pumping fluid. Experimental correlation in micro-channels Fig. 8 Comparison of boiling regimes represented by T. Chen and S.V.Garimella[20] Comparison of heat transfer models and correlations Several popular correlations were examined for accuracy in predicting the present heat transfer data and they are summarized in Table 3. Table 2 Elongated slugs and convention boiling compared with nucleate boiling Mass velocity (kg/m 2 s) Nucleate boiling Elongated slugs Convection boiling h (W/m 2 K) q (W/cm 2 ) h (W/m 2 K) q (W/cm 2 ) Compared with nucleate boiling % h (W/m 2 K) q (W/cm 2 ) Compared with nucleate boiling %

7 458 J. Therm. Sci., Vol.24, No.5, 2015 Table 3 Previous correlations for boiling in micro-channels No. Reference correlation remarks 1 Lazarek and Black[21] k l htp 30ReD h Bo Dh G(kg/m 2 s): q(w/cm 2 ): D(mm):3.1 Workingfluid:R113 2 Warrier[22] 3 Agostini and Bontemps[23] 4 Li and Wu[24] 5 S. G. Kandlikar Balasubramanian[25] 6 Bertsch[26] h tp = Eh sp E = Bo (1855Bo)x k l hsp 0.023Re fo Prf Dh tp H H h q G x x h 28q G x x 0.43 tp k g l htp 334Bo Bd Re f, Bd Dh 2 l g Dh h tp = larger of {h tp,nbd, h tp,cbd } h tp,nbd = Co 0.2 (1x) 0.8 h lo +1058Bo 0.7 (1x) 0.8 F Fl h lo h tp,cbd = 1.136Co 0.9 (1x) 0.8 h lo Bo 0.7 (1x) 0.8 F Fl h lo h tp = Rh cb +Sh nb h cb = h sp,fo (1x)+h sp,go x E = 1+80(x 2 x 6 )exp(0.6n conf ) h nb = h tp,cooper, S = 1x N conf Dh Re Pr fo f, 2, 3.66 L k f hsp fo 2 gf gd D h D 3 h h Refo Prf L Dh Rego Prg 3.66 L k GD GD, Re, Re D D 3 h f g h Rego Prg L g h h sp, fo 2 fo go h G(kg/m 2 s): q(w/cm 2 ): D(mm):0.75 Working fluid:fc84 G(kg/m 2 s): q(w/cm 2 ): x: D(mm):2.01 Working fluid:r134a G(kg/m 2 s): q w (W/cm 2 ):0-115 D(mm): Working fluid: water, refrigerants, FC-77, ethanol, propane G(kg/m 2 s): q(w/cm 2 ): x: D(mm):3-25 Working fluid:r113, R141b, R123 D(mm): Working fluid: Water, refrigerants, FC-77, nitrogen, 3899 data points The correlations in Table.3 were derived for specific fluids and specific ranges of operating conditions. Lazarek et al [21] measured saturated boiling of R113 in a round tube with the internal diameter of 0.31cm. The heat transfer coefficient correlation was taken as a relatively simple form: Nu=30Re Bo (6) The above equation showed that the nucleate boiling is the dominant heat transfer mechanism. Pujol and Stenning [27] indicated that with Bo number increases the quality at which the transition from nucleate dominated to nucleate suppressed (annular evaporative) boiling also increases. Their experimental results implied that the critical quality can be obtained before the transition to nucleate suppressed boiling for high values of Bo number. Gopinath R. Warrier et al [22] performed subcooled and saturated nucleate boiling experiments in 0.75mm rectangular channels using FC-84 working fluid. They developed a correlation for saturated flow boiling heat transfer only dependent on the parameters Bo number and x. Based on previous studies [28, 29, 30] with a fixed heat flux and low mass vapor qualities(x<0.3), heat transfer coefficient decreases rapidly with increasing x, while at high values of x (x>0.3), heat transfer coefficient becomes independent of x. Agostini and Bontemps [23] presented an experimental study of ascendant forced flow boiling in mini-channels with R134a. The hydraulic diameter of channel was 2.01mm. They found heat transfer coefficient is greater than that previously reported in the literature for conventional tubes and most of experimental data points belong to nucleate boiling regime (with or without dry-out). Therefore, they considered that to correlate the heat transfer coefficient with G, q and x is possible. Li and Wu [24] collected more than 3700 data points, including water, refrigerants, FC-77, ethanol and propane, covering wide range of operational conditions, as well as with the diameter of micro-channel from 0.16mm to 3.1mm. They considered the difference Bond

8 LI Xuejiao et al. The Investigation on Flow Boiling Heat Transfer of R134a in Micro-channels 459 number and liquid Reynolds number for conventional channels and mini/micro-channels. According to their experimental data points, they found that the higher Bond numbers are inclined to have higher Nu values. This non-dimensional number presents the relative importance of surface tension, body force, viscous force and inertia force in saturated-flow boiling in micro/mini-channels. Kandlikar and Balasubramanian [25] modified flow boiling correlation based on large diameter tubes developed by Kandlikar [31, 32] for using the laminar single-phase heat transfer coefficient for all liquid flowing. The correlation used the nucleate boiling as the dominant part of the original correlation and identified different conditions by Reynold number. Bertsch et al [26] developed a composite correlation which included nucleate boiling and convective boiling heat transfer terms, while accounting for the effect of bubble confinement in small channels. They were following the basic form of Chen correlation [33]. In their correlation, S is a suppression factor accounting for dry-out as the vapor quality increases in the nucleate boiling. Factor F accounts for the enhanced convection due to higher flow velocities with increasing vapor qualities in convective boiling. Comparison of measured and predicted saturated boiling local heat transfer coefficients is shown in Fig.8. Fig.9 compares the measured heat transfer coefficient with predictions based on correlations mentioned in Table.2. The predictive accuracy of a correlation is measured by the mean absolute error, defined as: 1 hpred h exp MAE 100% N (7) h exp Fig. 9 Comparison between trends in the measurements and those from the proposed correlation (a) Lazarek and Black [21], (b) Warrier [22],(c) Agostini and Bontemps [23], (d) Li and Wu [24] (e) S.G.Kandlikar Balasubramanian [25] (f) Bertsch [26]

9 460 J. Therm. Sci., Vol.24, No.5, 2015 It is clear that none of the correlations correctly predict the experimental data obtained in this study. All correlations predict a much lower h value. Fig.9 indicates that nucleate boiling data assembles closely while convection boiling data distributes approximate linear. The distribution of boiling data reveals that nucleate boiling heat transfer coefficients have less alteration than convection boiling in all operational range. The phenomenon illustrates that mass velocity and heat flux have important effects on convection flow boiling, also physical properties affect nucleate boiling greatly. Among the correlations mentioned above, the correlations suggested by Kandlikar and Bertsch are both composed with nucleate boiling equation and convection boiling equation, but they have larger errors than other correlations due to imprecise definition of mechanism in two flow patterns. Comparing those correlations with the experimental database, the best prediction was obtained by the method proposed by Li and Wu whose correlations were based on physical properties of working fluids. In consideration of above factors, a correlations for R134a nucleate boiling and convection boiling in micro-channels are proposed in section 4.2. Suggested correlation The primary purpose of this study is to develop a simple method to predict the heat transfer coefficient for cryogenic fluid saturated boiling in micro-channels flows with high accuracy. An alternative strategy adopted in the present study is to utilize the general functional forms of Li and Lazarek respectively. Considering the cryogenic fluid properties in nucleate boiling by introducing Bond number and vapor quality in convective boiling, the following relations were proposed to predict the heat transfer coefficient for the nucleate boiling dominant regime: k f hnb 189Bo Bd Re f (8a) D And for the convective boiling dominant regime: Re k f hcb f Bo x (8b) D Notice that the index numbers of Bond number Bd and Reynold number Re in correlation (8a) are both smaller than correlation of Li and Wu. It is because that the surface tension, body force and inertia force are more important than the mass velocity and heat flux in nucleate boiling dominant regime in micro-channels for cryogenic fluid. Correlation (8b) is developed on the correlation of Lazarek and Black with the term of (1-x) added deliberately. The term of (1-x) accounts for thin film evaporated in micro-channel convective boiling regime. While on the contrary, the index numbers of Boiling number Bo and Reynolds number Re are larger than correlation of Lazarek and Black, since R134a is more sensitive to the mass velocity and heat flux than R113 in micro-channels convective boiling. Making a comparison between correlation (8a) and correlation (8b), it can be found that the index numbers of Bo and Re in correlation (8a) are smaller than that in correlation (8b). The result shows that convective boiling is more sensitive to them while nucleate boiling is controlled by physical properties primarily, although both the mass velocity and heat flux have influence on nucleate boiling and convective boiling. Fig.10 shows the comparison of predictions from the suggested correlations. Fig. 10 Comparison of predictions of suggested correlations with experimental data For the nucleate boiling (Eq. 8a) and convective boiling (Eq. 8b) dominant regime the MAE is 6.3% and 8.8% respectively. Therefore, the proposed correlations show good predictive accuracy as compared with previous correlations for predicting saturated flow boiling in micro-channels with R134a. Conclusions The experimental study on flow boiling of R134a in micro-channel heat sink was presented. The test section was fabricated with brass, 30 micro-channels of 500μm width and depth, 30mm length. Experiments were conducted with the heat flux up to W/cm 2 s, mass velocity ranging from to kg/m 2 s, vapor quality ranging from 0.06 to 0.9. Major conclusions from the current study are given as follows: 1. The wall temperature of heat sink with micro-channels could be maintained under 30 with the mass velocity more than 1000 kg/m 2 s at 80W/cm 2. With the lower mass velocities, the wall temperature of heat sink with micro-channels is far less than The effects of mass velocity, heat flux and vapor quality on heat transfer coefficients of micro-channel

10 LI Xuejiao et al. The Investigation on Flow Boiling Heat Transfer of R134a in Micro-channels 461 heat sink were obtained. Heat transfer coefficient increases with heat flux for all range of mass velocity, but it has shown the opposite trend with vapor quality. The heat transfer coefficient could reach W/m 2 K with 80.1W/cm 2 at the mass velocity of kg/m Two dominant flow boiling patterns, nucleate boiling and convective boiling, were observed by the boiling curves. The results show that flow boiling patterns transform promptly at low mass velocities, while behaving stably at high mass velocities. 4. Proposed correlations were carried out with prediction error less than 9%, with considering the mechanisms of R134a nucleate boiling and convective boiling in micro-channels. Acknowledgement This research was supported by National Natural Science Foundation of China (No ). References [1] Mertens, R. G., Chow, L., Sundaram, K. B., Cregger, R. B., Rini, D. P., Turek, L., Saarloos, B. A., (2007), Spray Cooling of IGBT Devices, Journal of Electronic Packaging, Vol. 129, No. 3, pp [2] Bhunia, A., Chandrasekaran, S., Chen, C. L., (2007), Performance Improvement of a Power Conversion Module by Liquid Micro-jet Impingement Cooling, Journal of Components and Packaging Technologies, Vol. 30, No. 2, pp [3] Pautsch, A. G., Gowda, A., Stevanovic, L., Beaupre, R., (2009), Double-Sided Microchannel Cooling of a Power Electronics Module Using Power Overlay, Proceedings of ASME 2009 InterPACK Conference collocated with the ASME 2009 Summer Heat Transfer Conference and the ASME rd International Conference on Energy Sustainability, San Francisco, California, USA, July 19 23, pp [4] Mudawar, I., Bharathan, D., Kelly, K., Narumanchi, S., (2009), Two-phase Spray Cooling of Hybrid Vehicle Electronics, Journal of Components and Packaging Technologies, Vol. 32, No. 2, pp [5] Rahimo, M., Kopta, A., Eicher, S., Schlapbach, U., Linder, S., (2004), Switching-Self-Clamping-Mode" SSCM", a Breakthrough in SOA Performance for High Voltage IGBTs and Diodes, Proceedings of the 16th International Symposium on Power Semiconductor Devices and ICs (ISPSD 10), Kitakyushu, Japan, May 24 27, pp [6] Hitachi, T., Gohara, H. and Nagaune, F., (2012), Direct Liquid Cooling IGBT Module for Automotive Applications, Power Semiconductor Contributing in Energy and Environment Region, Vol. 58, No. 2, pp. 55. [7] Campbell, J. B., Tolbert, L. M., Ayers, C. W., Ozpineci, B., Lowe, K. T., (2007), Two-phase Cooling Method Using the R134a Refrigerant to Cool Power Electronic Devices, Journal of Industry Applications, Vol. 4, No. 33, pp [8] Kim, D. W., Rahim, E., Bar-Cohen, A., Han, B., (2010), Direct Submount Cooling of High-power LEDs, Journal of Components and Packaging Technologies, Vol. 33, No. 4, pp [9] Bar-Cohen, A., Rahim, E., (2009), Modeling and Prediction of Two-phase Microgap Channel Heat Transfer Characteristics, Journal of Heat Transfer Engineering, Vol. 30, No. 8, pp [10] do Nascimento, F. J., Leão, H. L. S. L., Ribatski, G., (2013), An Experimental Study on Flow Boiling Heat Transfer of R134a in a Microchannel-based Heat Sink, Journal of Experimental Thermal and Fluid Science, Vol. 45, pp [11] Ali, R., Palm, B., Maqbool, M. H., (2012), Flow Boiling Heat Transfer of Refrigerants R134a and R245fa in a Horizontal Micro-channel, Journal of Experimental Heat Transfer, Vol. 25, No. 3, pp [12] Marcinichen, J. B., Thome, J. R., Michel, B., (2010), Cooling of Microprocessors with Micro-Evaporation: A Novel Two-phase Cooling Cycle, International Journal of Refrigeration, Vol. 33, No. 7, pp [13] Howes, J. C., Levett, D. B., Wilson, S. T., Marsala, J., Saums, D. L., (2008), Cooling of an IGBT Drive System with Vaporizable Dielectric Fluid (VDF), Proceedings of the 24th Semiconductor Thermal Measurement and Management Symposium, San Jose, CA, USA, March 16 20, pp [14] Hannemann, R., Marsala, J., Pitasi, M., (2004), Pumped Liquid Multiphase Cooling, Proceedings of International Mechanical Engineering Congress and Exposition, pp , Anaheim, CA. [15] Harirchian, T., Garimella, S. V., (2009), Effects of Channel Dimension, Heat Flux, and Mass Flux on Flow Boiling Regimes in Microchannels, International Journal of Multiphase Flow, Vol.35, No.4, pp [16] Madhour, Y., Olivier, J., Costa-Patry, E., Paredes, S., Michel, B., Thome, J. R., (2011), Flow Boiling of R134a in a Multi-microchannel Heat Sink with Hotspot Heaters for Energy-efficient Microelectronic CPU Cooling Applications, Journal of Packaging and Manufacturing Technology, Vol. 1, No. 6, pp [17] Kim, S. M., Mudawar, I., (2013), Universal Approach to Predicting Saturated Flow Boiling Heat Transfer in Mini/ micro-channels Part II. Two-phase Heat Transfer Coefficient, International Journal of Heat and Mass Transfer, Vol. 64, pp [18] Collier, J. G., Thome, J. R., (1994), Convective boiling

11 462 J. Therm. Sci., Vol.24, No.5, 2015 and condensation. Oxford university press, Oxford, UK. [19] Lee, J., Mudawar, I., (2005), Two-phase Flow in Highheat-flux Micro-channel Heat Sink for Refrigeration Cooling Applications: Part II Heat Transfer Characteristics, International Journal of Heat and Mass Transfer, Vol. 48, No. 5, pp [20] Chen, T., Garimella, S. V., (2011), Local Heat Transfer Distribution and Effect of Instabilities During Flow Boiling in a Silicon Microchannel Heat Sink, International Journal of Heat and Mass Transfer, Vol. 54, No. 15, pp [21] Lazarek, G. M., Black, S. H., (1982), Evaporative Heat Transfer, Pressure Drop and Critical Heat Flux in a Small Vertical Tube with R-113, International Journal of Heat and Mass Transfer, Vol. 25, No. 7, pp [22] Warrier, G. R., Dhir, V. K., Momoda, L. A., (2002), Heat Transfer and Pressure Drop in Narrow Rectangular Channels, Experimental Thermal and Fluid Science, Vol. 26, No. 1, pp [23] Agostini, B., Bontemps, A., (2005), Vertical Flow Boiling of Refrigerant R134a in Small Channels, International Journal of Heat and Fluid Flow, Vol. 26, No. 2, pp [24] Li, W., Wu, Z., (2010), A General Correlation for Evaporative Heat Transfer in Micro/mini-channels, International Journal of Heat and Mass Transfer, Vol. 53, No. 9, pp [25] Kandlikar, S. G., Balasubramanian, P., (2004), An Extension of the Flow Boiling Correlation to Transition, Laminar, and Deep Laminar Flows in Minichannels and Microchannels, Heat Transfer Engineering, Vol. 25,No. 3, pp [26] Bertsch, S. S., Groll, E. A., Garimella, S. V., (2009), A Composite Heat Transfer Correlation for Saturated Flow Boiling in Small Channels, International Journal of Heat and Mass Transfer, Vol. 52, No. 7, pp [27] Pujol, L., Stenning, A. H., (1968), Effect of Flow Direction on the Boiling Heat Transfer Coefficient in Vertical Tubes, In Proc. Int. Symp. Cocurrent Gas-Liquid Flow. Univ. Waterloo, Canada, pp [28] Wambsganss, M. W., France, D. M., Jendrzejczyk, J. A., Tran, T. N., (1993), Boiling Heat Transfer in a Horizontal Small-diameter Tube, Journal of Heat Transfer, Vol. 115, No. 4, pp [29] Tran, T. N., Wambsganss, M. W., Jendrzejczyk, J. A., France, D. M., (1993), Boiling Heat Transfer in a Small Horizontal Rectangular Channel (No. ANL/MCT/CP ; CONF ), Argonne National Lab., IL (United States). [30] Tran, T. N., Wambsganss, M. W., France, D. M., (1996), Small Circular-and Rectangular-channel Boiling with Two Refrigerants, International Journal of Multiphase Flow, Vol. 22,No. 3, pp [31] Kandlikar, S. G., (1990), A General Correlation for Saturated Two-phase Flow Boiling Heat Transfer Inside Horizontal and Vertical Tubes, Journal of Heat Transfer, Vol. 112, No. 1, pp [32] Kandlikar, S. G., (1991), A Model for Predicting the Two-phase Flow Boiling Heat Transfer Coefficient in Augmented Tube and Compact Heat Exchanger Geometries, Journal of Heat Transfer, Vol. 113, No. 1, pp [33] Chen, J. C., (1966), Correlation for Boiling Heat Transfer to Saturated Fluids in Convective Flow, Industrial & Engineering Chemistry Process Design and Development, Vol. 5, No. 3, pp