This study focuses on the flow boiling heat transfer of R134a refrigerant inside Multi-microchannels heat sink. The test section is composed of 27 parallel ...
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8 International Conference on Multiphase Flow ICMF 2013, Jeju, Korea, May 26 - 31, 2013
Flow Boiling Heat Transfer of R134a in Multi-Microchannel Heat Sink Phubate Thiangthama, Omid Mahianb, Somchai Wongwisesa* a
Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab (FUTURE) Department of b Mechanical Engineering, King Monkut’s University of Technology Thonburi, Bangkik 10140, Thailand Department of Mechanical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
Abstract This study focuses on the flow boiling heat transfer of R134a refrigerant inside Multi-microchannels heat sink. The test section is composed of 27 parallel microchannels, 382 μm width, 470 μm height, 416 μm-thick fins and 40 mm length. The ranges of heat flux and mass velocities in this work are 18 to 175 kW/m2 and 1000 to 3000 kg/m2s, respectively. The results show that the heat transfer coefficient in fully developed boiling is independent of mass flow rate while the inlet temperature of flow has important effects on the coefficient.
1. Introduction About three decades ago, microchannels were proposed as efficient devices to cool the electronic devices and other equipment which work with high powers [1, 2]. Relatively, extensive studies have been done on in microchannels under two- phase flow conditions. Here, some of the most recent experiments concerning the flow boiling in microchannels where the working fluid is a refrigerant are reviewed briefly. Lee and Mudawar [3, 4] investigated the heat transfer and pressure characteristics of R 134a through a microchannel heat sink. The authors [3] presented two correlations to estimate the pressure drop in the microchannel for both laminar and turbulent regimes of vapor (the flow regime of liquid is laminar) by considering the simultaneously effects of liquid inertia, viscous force and surface tension. Later on, they [4] presented three correlations to calculate the heat transfer coefficient based on three different thermodynamic equilibrium quality of the working fluid. Wojtan et al. [5] estimated the critical heat flux in two microchannel tubes with inner diameters of 0.5 and 0.8 mm where two working fluids including R-134a and R-245fa are utilized. In another work, Kuan and Kandlikar[6] examined the effects of stability of flow boiling of on critical heat flux in a microchannel including six parallel channels where the working fluid is R-123.
Costa-Patry et al. [7, 8] investigated also the pressure drop and heat transfer characteristics due to flow boiling of R236fa and R245fa in a microchannel with 35 local heaters. The flow boiling of R-134a refrigerant in a multimicrochannel heat sink including 100 channels is studied by Madhour et al.[9], where the range of heat flux is between 2.57 - 189 W/cm2 and the mass flux between 205 to 1000 kg/m2s while the saturation temperature is 63°C. Bortolin et al.[10] investigated the flow boiling of a HFC-245fa in a single circular channel with inner diameter of 0.96mm where heat flux changes from 5 to 85 kW/ m2 with a vapor quality that varies between 0.05 - 0.8. They performed the experiments with different mass fluxes between 200 - 400 kg/m2 s. Saraceno et al. [11] in their experiments on the flow boiling of FC-72 refrigerant in microchannels concluded that with increasing the heat flux applied to the microchannel, the heat transfer coefficient increases while the vapor quality has no significant effect on the heat transfer coefficient. In this work, the ranges of mass fluxes and heat fluxes are 10002000 kg /m2 s and 10-150 kW/ m2. The main aim of this study is to investigate the flow boiling of R-134a refrigerant in a microchannel. Experiments are performed in a copper microchannels heat sink having 27 parallel rectangular channels with a channel depth 470 µm,
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8 International Conference on Multiphase Flow ICMF 2013, Jeju, Korea, May 26 - 31, 2013
channel width 382 µm. The effects of important operating parameters such as the mass velocity, heat flux and the inlet subcooled temperature on heat transfer characteristic of flow boiling in microchannels are also investigated .
Flow rate data Temperature and Pressure data ΔP
Camera T
P
T
2. Experimental setup P
DC Power supply controller
Electrical pre-heater
V A V A
T
Cartridge heaters
Thermocouple measurement positions Plate heat exchanger
P
-------+1.38 kg/s
A plate-type heat exchanger is located at the outlet of the test section to condense the vapor created in the test section before the fluid flows back to the reservoir to recirculate in the loop.
T
FUTURE Lab.
Figure 1 presents a schematic diagram of the experimental set-up. The working fluid is pumped from the tank towards the test section using a magnetic gear pump. The refrigerant passes from a filter/dryer and then is preheated by an electrical pre-heater to reach the desirable temperature before entering the microchannel. To measure the mass flow rate a Coriolis type mass flow meter is installed before the pre-heater. After heating, the refrigerant enters the microchannel. The microchannels is made of copper with 27 parallel rectangular channels. The microchannel has a depth of 470 µm, a channel width of 382 µm, a wall thickness of 416.5 µm and 40 mm long as shown in the fig. 3 (a). To measure the temperature at different points of the microchannel, twelve sheath thermocouples (K-Type) with an accuracy of 0.1 oC are mounted in the bottom of microchannels. To collect the data of the wall temperature distribution along the test section, the average temperatures from the both sides of the test piece are calculated.The pressure drop between the inlet and outlet of the microchannel is measured using a differential transmitter having a range within 0-500 kPa and the inlet pressure is measured by a pressure transmitter. Twelve cartridge heaters are installed vertically in the bottom of the test section to heat the microchannel by maximum power of 2.4 kW. The electrical power input to the cartridge heaters is controlled manually by a DC power supply unit.
Coriolis mass flow meter
1.2
T
1.0
0.8
0.6
By pass Receiver tank
NI Compact DAQ
T
0.4
0.2
RTD pt 100
Flow meter
Computer
Filter/drier Condensing Unit Water Pump Magnetic micro gear pump
Heater
Subcool Ethylene glycol tank
Fig.1. Schematic diagram of the experimental test loop
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8 International Conference on Multiphase Flow ICMF 2013, Jeju, Korea, May 26 - 31, 2013
W fin , H fin and are the channel
Where Wch ,
width, the fin width, the fin height and the fin efficiency respectively. qb is the heat flux at the base and obtained as follows:
qb
V I Ab (2)
V I is the electrical power input which applied to the cartridge heater and Ab is the The term of
cross section base area. The fin efficiency by assuming adiabatic fin tip condition is given by [12] Transparent polycarbonate plate
tanh(m H ch ) m H ch (3)
Tf Where m is the fin parameter defined as
H fin
Tw,b W fin
Wch
h 2 Wf Lf ks Wf Lf
m
tc
(4)
Ttc
in which Lf is the fin length. The heat transfer coefficient is defined as
Copper microchannel test piece
h
q
qw Tw Tfl (3)
(a) Photograph of heat sink (b) Schematic of the
Where T fl is the liquid saturation temperature in
microchannel
the two phase region. Tw is the local microchannel
Fig.2. Microchannels heat sink
wall temperature that is calculated using the assumption of 1-D heat conduction from the temperature plane to the of channel base.
3. Data reduction The heat flux on the wall ( qw ) of microchannel is
Tw Ttc
qb tc ks
obtained by the following relation:
qw
(4)
qb(Wch W fin )
In the above,
Wch 2 H fin (1)
tc is
distance from microchannels
base to thermocouple installed position.As it is seen the Eq.(1) and Eqs(3-5) are coupled. To obtain the heat transfer coefficient an initial value for the fin
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8 International Conference on Multiphase Flow ICMF 2013, Jeju, Korea, May 26 - 31, 2013
microchannel Qelect IV and the heat absorbed
efficiency is assumed and then with trial and error method the corrected value of efficiency is obtained.
by the refrigerant Qref mc p Tout Tin does not exceed than 5%. This indicates that the heat losses from the system are negligible. This is valid for different mass flow rates of the working fluid.
4. Results and discussions First of all, to investigate the accuracy of the experimental set up, the variations of Nusselt number with Reynolds number are investigated for single phase flow of R-134a under the turbulent developing regime. Figure 3 presents the comparison between the data obtained by the experimental set up and correlations available in the literature [13-17]. A good agreement is observed between the present data and the results of former works.
40 Mass flow rate (kg/s) 35 30 25 Qref (W)
+5%
0.014 0.016 0.018 0.020 0.022
-5%
20 15 10
300 5
Turbulent developing correlation:
Nusselt Number
250
0
Sieder and Tate (1936) Hausen (1943) Philips (1987) Gnielski (1976) Debray (2001) Present data
200
150
0
5
10
15
20 25 Qelect (W)
30
35
40
Fig. 4. Energy balance between electrical input powers to measured heat transfer rate
100
50
0 4000
4500
5000
5500
6000
6500
7000
7500
Reynolds Number
Fig. 3. Comparison between the data obtained by the experimental set up and correlations available in the literature for single phase flow. To estimate the heat loss to the surroundings, in the first, the energy balance obtained across the test section assembly is evaluated for single-phase fluid flow. A series of single phase experiments are conducted with different mass velocities and heat fluxes supplied to the test section. Figure 4 is plotted to show the amount of dissipated heat. As seen from the figure, the difference between the heat flux applied to the
8000
Figure 5 is drawn to show that the mass flux has no significance effect on the heat transfer coefficient at different heat fluxes applied to the microchannel. Although, with increasing the mass flux the heat transfer coefficient increases slightly. Furthermore, Fig. 5 also shows the effect of heat flux on the heat transfer coefficient. The results indicate that the average heat transfer coefficient increases with heat flux. This happens because the nucleate boiling is the main mechanism of heat transfer in the presented system. The number of bubbles increases with an increase in heat flux, so the flow becomes more turbulent. This enhances the rate of heat transfer and hence the average heat transfer coefficient increases.
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8 International Conference on Multiphase Flow ICMF 2013, Jeju, Korea, May 26 - 31, 2013
10000
10000
Tsub,in = 20 oC
9000
G = 2000 kgm-2s-1 = 2550 = 3000
8000
G = 2550 kgm-2s-1 Tsub,in = 14 oC
9000
= 20 = 25
7000 h (W/m2K)
h (W/m2K)
8000
6000
7000 6000
5000
5000 4000
4000 3000 0
20
40
60
80
100
120
q"w (kw/m2)
Fig. 5. Variation of heat transfer coefficient with wall heat for varying the mass flux
Figure 6 displays the variations of heat transfer coefficient versus heat flux applied to the system for different inlet temperatures. As shown, with an increase in the inlet temperature the heat transfer coefficient increases. As known, the heat transfer coefficient is a function of velocity and temperature of the working fluid. Therefore, in the present case the higher inlet temperature results in higher heat transfer coefficient.
3000 0
20
40
60
80
100
q"w (kw/m2)
Fig. 6. Variation of heat transfer coefficient with wall heat for varying the inlet subcooled temperature
5. Conclusions An experimental study is conducted to show the effects of mass flux and inlet temperature in a microchannel with the size of 382 μm width, 470 μm height, 416 μm-thick fins and 40 mm length. The results shows that the mass flux has no considerable effect on the heat transfer coefficient. Also, it is found the inlet temperature of the refrigerant has a significant effect on the heat transfer coefficient. References [1] Tuckerman DB, PeaseRFW High performance heat sink for VISI. IEEE Electron Device Lett 4 (1981) 126–129. [2] Tao Dong , Zhaochu Yang , Qincheng Bi , Yulong Zhang, Freon R141b flow boiling in silicon microchannel heat sinks: experimental investigation, Heat Mass Transfer 44 (2008) 315– 324.
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[3] Lee, J., Mudawar, I. Two-phase flow in highheat-flux micro-channel heat sink for refrigeration cooling applications: Part I - Pressure drop characteristics, International Journal of Heat and Mass Transfer 48 (2005) 928-940. [4] Lee, J., Mudawar, I. 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 48 (2005) 941-955. [5] Wojtan, L., Revellin, R., Thome, J.R., Investigation of saturated critical heat flux in a single, uniformly heated microchannel, Experimental Thermal and Fluid Science 30 (2006 ) 765-774 [6] Kuan, W.K., Kandlikar, S.G., Experimental study and model on critical heat flux of refrigerant123 and water in microchannels, Journal of Heat Transfer 130 (2008), art. no. 034503. [7] Costa-Patry, E., Olivier, J., Nichita, B.A., Michel, B., Thome, J.R., Two-phase flow of refrigerants in 85μm-wide multi-microchannels: Part I - Pressure drop, International Journal of Heat and Fluid Flow 32 (2011) 451-463. [8] Costa-Patry, E., Olivier, J., Michel, B., Thome, J.R., Two-phase flow of refrigerants in 85μm-wide multi-microchannels: Part II - Heat transfer with 35 local heaters, International Journal of Heat and Fluid Flow 32 (2011) 464-476. [9] Madhour, Y., Olivier, J., Costa-Patry, E., Paredes, S., Michel, B., Thome, J.R., Flow boiling of R134a in a multi-microchannel heat sink with hotspot heaters for energy-efficient microelectronic CPU cooling applications, IEEE Transactions on Components, Packaging and Manufacturing Technology 1 (6) , art. no. 5766718 , (2011) 873883. [10] Bortolin, S., Del Col, D., Rossetto, L. , Flow boiling of R245fa in a single circular microchannel, Heat Transfer Engineering 32 (2011) 1160-1172. [11] Saraceno, L., Celata, G.P., Furrer, M., Mariani, A., Zummo, G., Flow boiling heat transfer of refrigerant FC-72 in microchannels, International Journal of Thermal Sciences 53 (2012) 35-41
[12] F. P. Incropera, D. P. Dewitt, T. L. Bergman, A. S. Lavine, Fundamentals of heat and mass transfer, 6th ed. New York: Wiley (2005) [13] Sieder, E.N. and G.E. Tate, Heat transfer and pressure drop of liquids in tubes, Industrial and Engineering Chemistry, 28 (1936) 1429-1435 [14] Hausen , H., VDI Beih. Verfahrenstech, 4 (1943) 91 [15] Gnielinski, V., New equation for heat and mass transfer in turbulent pipe and channel flow, International chemical engineering 16 (1976) 359368 [16] Richard J. Phillips, MicroChannel heat sinks, The lincoln laboratory journal 1(1988) 31 [17] Debray, F., France, J.P., Maitre T., Renaud, S., Measure des coefficient de transfert thermique par convection forcée en mini-canaux. Mechanical & Indrustries 2 (2001) 443-454
