Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV THE EFFECTS OF MICROCHANNEL GEOMETRY ON HEAT TRANFER BEHAVIORS FOR TWO PHASE FLOW BY NUMERICAL SIMULATION NGHIÊN CỨU ẢNH HƯỞNG CỦA HÌNH DÁNG HÌNH HỌC KÊNH MICRO ĐẾN CÁC ĐẶC TÍNH TRUYỀN NHIỆT CHO DÒNG CHẢY HAI PHA BẰNG PHƯƠNG PHÁP MÔ PHỎNG SỐ Batan Le1, Thanhtrung Dang1a, Tronghieu Nguyen1, Minhhung Doan1, Quochoai Nguyen1, Maicuong Bui1, Vanhien Nguyen1, Thanhxuan Nguyen1, and Jyh-tong Teng2 Ho Chi Minh City University of Technology and Education, Vietnam Chung Yuan Christian University, Taiwan a trungdang@hcmute.edu.vn ABSTRACT In this paper, the effects of microchannel geometry on heat transfer behaviors for two phase flow were numerically investigated The optimal approach for searching the best performances geometry of microchannels is the circular cross-section In addition, the results obtained from this study were in good agreement with experimental data and relative papers Keywords: Microchannel, Two phase, Temperature, Velocity, Numerical simulation TÓM TẮT Bài báo nghiên cứu ảnh hưởng hình dáng hình học kênh micro đến đặc tính truyền nhiệt cho dòng chảy hai pha phương pháp mô Nghiên cứu hình dáng hình học tối ưu kênh có tiết diện hình tròn Thêm vào đó, kết đạt từ nghiên cứu phù hợp với thực nghiệm nghiên cứu có liên quan Từ khóa: Kênh micro, Hai pha, Nhiệt độ, Vận tốc, Mô số INTRODUCTION One of the most important topics in this century is energy saving and environmental protection In the conventional heat exchangers, they have very big size and low heat transfer efficiency Hence, it necessarily becomes to replace the traditional big size heat exchangers by the small size microchannel heat exchangers which giving higher heat transfer efficiency Thus, these microchannel heat exchangers make the heat transfer efficiency could be improved quickly as well as the reciprocation of the whole system increased due to their high heat flux and compacted heat exchangers Related to microchannel heat exchangers, there are some related researches which will be reviewed below Tsukamoto and Imai [1] designed a high heat flux V-shaped micro-evaporator that could achieve 125 W/cm2 for water inlet temperature of 900C and flow rate of 1.0 mL/min The measured pressure drop was less than 1000 Pa A new micro-combustor configuration for a micro-reformer integrated with a micro-evaporator was studied by Kim and Kwon [2] The micro-combustion was simulated by using FLUENT 6.2 The measured and predicted temperature distributions across the micro-combustor walls indicated that heat generated in the micro-combustor was effectively dissipated Tuo and Hrnjak [3, 4] tried to increase the performance index of microchannel evaporator Increasing the performance index of microchannel evaporator was also investigated by Shi and coworkers [5] In their research, they tried to improve the effect of manifolds Kew and Cornwell [6] designed the single pipe evaporators, diameter of 1.39–3.69 mm, with refrigerant of R-141b They showed that, for the 631 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV big diameter (2.87 and 3.69 mm), the heat transfer coefficient decrease steady or not change when thermodynamic equilibrium quality x e increase (in case x e 0.2 For the small diameter 1,39mm, the heat transfer coefficient increase when thermodynamic equilibrium quality x e increase (in case x e >0, low mass velocity); However, it will decrease rapidly (in case x e >0, high mass velocity) Ravigururajan [7] developed the rectangular shaped microchannel evaporator, with 54 parallel channels, dimension of 0.27 x 1.0 mm, refrigerant of R-124, this evaporator could dissipate about 300 W They showed that, the heat transfer coefficient decrease steady when increasing the thermodynamic equilibrium quality x e (in case x e >0) Yan and Lin [8] developed the pipe shaped evaporator, with 28 parallel pines, diameter of mm, refrigerant of R-134a, this evaporator could dissipate about W/cm2 They showed that, the heat transfer coefficient decrease steady when increasing the thermodynamic equilibrium quality x e (in case x e >0) and was effected by heat flux, refrigerant saturation temperature, mass velocity Subsequent to the above literature reviews, it is important to clearly understand the effects of microchannel geometry on heat transfer behaviors for two phase flow in order to get an optimal design For the present study, four heat exchangers with differences of cross sections such as rectangular, trapezoidal, circle, V-shape will be discussed STRUCTURE DESIGN The parallel microchannel heat exchangers using different microchannel cross-sections are illustrated in Figure It consists of manifolds and microchannels: all microchannels are connected by manifolds The water firstly from the inlet manifold flows through microchannels, then going out of the system by outlet manifold During its journey, it receives amount of heat - which supplied by the outside sources - to become vapor at the outlet manifold W2 H W Rectangular R H H W1 Trapezoidal W Triangular Circle Figure A parallel microchannel heat exchanger and different microchannel cross-sections The material used for the substrate of heat exchangers is aluminum, with the thermal conductivity of 237 W/(mK), density of 2,700 kg/ m3, and specific heat at constant pressure of 904 J/(kgK) For each microchannel heat exchanger, the top side has 20 microchannels The length of each microchannel is 120 mm In a microchannel heat exchanger, all channels are connected by manifolds The manifolds of the heat exchangers have a rectangular crosssection with a width of 10 mm, a length of 19.5 mm and a depth of mm The distance between two microchannel is 500 µm The thickness of the substrate is mm To seal the microchannels, the layer of PMMA (polymethyl methacrylate) was bonded on the top side of the substrate The PMMA has the thermal conductivity of 0.19 W/(mK) and density of 1,420 kg/m3 The Figure shows the dimensions of a microchannel heat exchanger Table presents the summary of microchannel dimensions for differences cross section 632 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV Figure The dimensions of a microchannel heat exchanger Table The summary of microchannel dimensions for different cross-section W H R Rectangular 500 µm 500 µm Trapezoidal W = 125 µm W = 500 µm 800 µm Triangular 500 µm mm µm Circle RESULTS AND DISCUSSION As described above, finding the best performanced cross-section of microchannels for two phase flow is the important task to determine the optimal design of two phase flow microchannel heat exchangers In this study, for the simulation, four microchannel heat exchangers with differences type of cross sections such as rectangular, trapezoidal, circle, Vshape will be evaluated In order to study the effects of microchannel geometry on heat transfer behaviors for two phase flow, all numerical simulation conditions or the four microchannel heat exchangers were kept the same excepting changing the cross-section Throughout the paper, four cases of simulation were discussed: the first one for the Rectangular cross-section (case 1), the second for the Trapezoidal cross-section (case 2), and the third for the Triangle cross-section (case 3) and the last one for the Circle cross-section (case 4) The general parameters for these two cases are summarized in Table Case Table General parameters for cases under study Variable parameters Fixed parameters Rectangular cross-section: W=500µm, H=500µm Heat power: P source =176W Inlet temperature: T in =30 °C Trapezoidal cross-section Ambiant temperature: T amb =30 °C W = 125 µm, W = 500 µm, H= 800 µm Source temperature: T s =120 °C Triangle cross-section: W= 500 µm, H=1 mm Mass flow rate: m = 0.3 g/s Circle cross-section: R= 500 µm Cross-section area: A=0.25 mm2 Figure shows the location of full vaporization of the water in rectangular crosssectioned microchannels It is observed that the flow rate in middle channels is larger than in marginal channels, so the full vaporization of middle channels is slower than that obtained from the marginal channels, leading to the vaporization profile is parabolic shape For the others case of 2-4, the results are almost the same 633 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV Figure The location of full evaporation of the water, for case Figure The temperature distribution for the middle slice of the channel Figures and show the thermal field and the curve of temperature during the length of channel (for case and middle slice of the channel), respectively They show that the maximum temperature of the sample is about 130 °C Whereas, the maximum temperature of fluid at the end of the channel is about 119 °C Figure shows the comparison between the simulation and experiment about the temperature in the case of inputs: heat power of 176W, ambient temperature of =30 °C, source temperature of 30 °C, mass flow rate of 0.3 g/s, cross-section area of 0.25 mm2, substrate thickness of 900 µm, depth of 500 µm It is observed that the numerical results are in good agreement with experimental results: the maximum percentage error is less than 0.3% 634 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV Temperature X - Direction Figure The curve of temperature during the length of channel, for case and middle slice of the channel Simuation results Experiment results Figure Comparison between numerical simulation and experimental data Heat flux X - Direction Figure The comparision of heat flux for all of cases 635 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV In addition, to determine the best performanced cross-section of microchannels for two phase flow, the heat fluxes (x-direction) of the middle slice of the channel for case 2-4 are also carried out by using COMSOL Merging these results, the comparison of heat flux for all of cases is shown in the Figure It is easy to recognize that the case with circular crosssection has higher heat flux than those obtained from others cases The maximum heat flux is about 1.152 x 108 W/ m2 CONCLUSION The numerical simulation has been done on four microchannel heat exchangers with differences type of cross sections to find out the effects of microchannel geometry on heat transfer behaviors for two phase flow In the study, it indicates that the microchannel heat exchanger with the circle cross-section is the best choice for designing There was less than 0.3% error between simulation and experiment; the results obtained from this study were in good agreement with relative papers Besides, maximum heat flux is about 1.152 x 108 W/ m2 The maximum temperature at the end of the channel is about 119 °C REFERENCES [1] T Tsukamoto and R Imai, Thermal characteristics of a high heat flux micro-evaporator, Experimental Thermal and Fluid Science, Vol 30, Issue 8, August 2006, pp 837-842 [2] K.B Kim and O.C Kwon, Studies on a two-staged micro-combustor for a microreformer integrated with a micro-evaporator, Journal of Power Sources, Volume 182, Issue 2, August 2008, pp 609-615 [3] Hanfei Tuo and Pega Hrnjak, Effect of the header pressure drop induced flow maldistribution on the microchannel evaporator performance, International Journal of Refrigeration, Volume 36, Issue 8, December 2013, pp 2176-2186 [4] Hanfei Tuo and Pega Hrnjak, New approach to improve performance by venting periodic reverse vapor flow in microchannel evaporator, International Journal of Refrigeration, Volume 36, Issue 8, December 2013, pp 2187-2195 [5] Junye Shi, Xiaohua Qu, Zhaogang Qi, Jiangping Chen, Investigating performance of microchannel evaporators with different manifold structures, International Journal of Refrigeration, Volume 34, Issue 1, January 2011, pp 292-302 [6] P.A Kew, K Cornwell, Correlations for the prediction of boiling heat transfer in small diameter channels, Appl Therm Eng., Vol 17, 1997, pp.705–715 [7] T.S Ravigururajan, Impact of channel geometry on twophase flow heat transfer characteristics of refrigerants in microchannel heat exchangers, J Heat Transfer, Vol 120, 1998, pp 485–491 [8] Y.Y Yan, T.F Lin, Evaporation heat transfer and pressure drop of refrigerant R-134a in a small pipe, Int J Heat Mass Transfer, Vol 41, 1998, pp 4183–4194 AUTHOR’S INFORMATION Thanhtrung Dang HCMC University of Technology and Education trungdang@hcmute.edu.vn 0913.606261 636 ... addition, to determine the best performanced cross-section of microchannels for two phase flow, the heat fluxes (x-direction) of the middle slice of the channel for case 2-4 are also carried out by. .. out the effects of microchannel geometry on heat transfer behaviors for two phase flow In the study, it indicates that the microchannel heat exchanger with the circle cross-section is the best choice... DISCUSSION As described above, finding the best performanced cross-section of microchannels for two phase flow is the important task to determine the optimal design of two phase flow microchannel heat