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

THE EFFECTS OF MICROCHANNEL GEOMETRY ON HEAT TRANFER BEHAVIORS FOR TWO PHASE FLOW BY NUMERICAL SIMULATION NGHIÊN C ỨU ẢNH HƯỞNG CỦA HÌNH DÁNG HÌNH HỌC KÊNH MICRO ĐẾN CÁC ĐẶC TÍNH TRUYỀN

THE EFFECTS OF MICROCHANNEL GEOMETRY ON HEAT TRANFER BEHAVIORS FOR TWO PHASE FLOW BY NUMERICAL SIMULATION

NGHIÊN C ỨU ẢNH HƯỞNG CỦA HÌNH DÁNG HÌNH HỌC KÊNH MICRO ĐẾN CÁC ĐẶC TÍNH TRUYỀN NHIỆT CHO DÒNG CHẢY HAI PHA BẰNG PHƯƠNG

PHÁP MÔ PH ỎNG SỐ

Batan Le1, Thanhtrung Dang1a, Tronghieu Nguyen1, Minhhung Doan1, Quochoai Nguyen1, Maicuong Bui1, Vanhien Nguyen1, Thanhxuan Nguyen1, and Jyh-tong Teng2

1Ho Chi Minh City University of Technology and Education, Vietnam

2Chung Yuan Christian University, Taiwan

atrungdang@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 T ẮT

Bài báo này nghiên c ứu về ảnh hưởng của hình dáng hình học kênh micro đến các đặc tính truy ền nhiệt cho dòng chảy hai pha bằng phương pháp mô phỏng Nghiên cứu này đã chỉ

ra được hình dáng hình học tối ưu đó là kênh có tiết diện hình tròn Thêm vào đó, các kết quả đạt được từ nghiên cứu này cũng rất phù hợp với thực nghiệm và các nghiên cứu có liên quan

T ừ khóa: Kênh micro, Hai pha, Nhiệt độ, Vận tốc, Mô phỏng số

1 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,

when thermodynamic equilibrium quality xe increase (in case xe <0.2); However, it will increase in case xe >0.2 For the small diameter 1,39mm, the heat transfer coefficient increase when thermodynamic equilibrium quality xe increase (in case xe >0, low mass velocity); However, it will decrease rapidly (in case xe >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 xe (in case xe >0) Yan and Lin [8] developed the pipe shaped evaporator, with 28 parallel pines, diameter of 2 mm, refrigerant of R-134a, this evaporator could dissipate about 2 W/cm 2 They showed that, the heat transfer coefficient decrease steady when increasing the thermodynamic equilibrium quality xe (in case xe >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

2 STRUCTURE DESIGN

The parallel microchannel heat exchangers using different microchannel cross-sections are illustrated in Figure 1 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

Figure 1 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 cross-section with a width of 10 mm, a length of 19.5 mm and a depth of 1 mm The distance between two microchannel is 500 µm The thickness of the substrate is 2 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 2 shows the dimensions of a microchannel heat exchanger Table 1 presents the summary of microchannel dimensions for differences cross section

W

H

W

H

W 1

H

W 2

R

Figure 2 The dimensions of a microchannel heat exchanger

Table 1 The summary of microchannel dimensions for different cross-section

Trapezoidal W1= 125 µm

W2= 500 µm

800 µm

3 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, V-shape 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 2

Table 2 General parameters for cases under study

1 Rectangular cross-section: W=500µm, H=500µm Heat power: Psource=176W

Inlet temperature: Tin=30 °C Ambiant temperature: Tamb=30 °C Source temperature: Ts=120 °C Mass flow rate: m = 0.3 g/s Cross-section area: A=0.25 mm2

2 Trapezoidal cross-section

W1= 125 µm, W2= 500 µm, H= 800 µm

3 Triangle cross-section: W= 500 µm, H=1 mm

4 Circle cross-section: R= 500 µm

Figure 3 shows the location of full vaporization of the water in rectangular cross-sectioned 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

Figure 3 The location of full evaporation of the water, for case 1

Figure 4 The temperature distribution for the middle slice of the channel

Figures 4 and 5 show the thermal field and the curve of temperature during the length of channel (for case 1 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 6 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%

Figure 5 The curve of temperature during the length of channel, for case 1 and middle

slice of the channel

Figure 6 Comparison between numerical simulation and experimental data

Simuation results

Experiment results

Temperature

X - Direction

X - Direction Heat flux

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 7 It is easy to recognize that the case 4 with circular cross-section 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 combustor for a

micro-reformer integrated with a micro-evaporator, Journal of Power Sources, Volume 182,

Issue 2, 1 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