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A liquid metal cooling system for the thermal management of high power LEDs
International Communications in Heat and Mass Transfer 37 (2010) 788–791
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
A liquid metal cooling system for the thermal management of high power LEDs☆ Yueguang Deng, Jing Liu ⁎ Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Available online 12 May 2010 Keywords: Light-emitting diode (LED) Liquid metal cooling Heat dissipation
a b s t r a c t An active cooling solution using liquid metal as the coolant was proposed for high power light emitting diodes (LEDs). The typical thermal-physical properties of liquid metal were presented. Then a series of experiments under different operation conditions were performed to evaluate the heat dissipation performance of the liquid metal cooling system, and the results were compared with that of water. In order to better understand the cooling capability of liquid metal cooling system, a theoretical thermal resistance model was established and discussed. Both the experiments and theoretical analysis indicated that liquid metal cooling was a powerful way for heat dissipation of high power LEDs, and the fabrication of practical liquid metal cooling devices was feasible and useful. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction High brightness Light Emitting Diodes (LEDs) is rapidly emerging as a new generation of lighting source for its many distinctive advantages, such as long work-life, low-power consumption, environmental-friendly characteristics and so on [1]. However, because of the optical performance and reliability of LED are greatly affected by junction temperature, which should be kept under 120 °C [2], heat dissipation reserves to be a big challenge for manufactures to design high power LEDs, especially in high brightness lighting fields. So far, many heat dissipation solutions have been investigated for the thermal management of LED, mainly from package level to system level [2]. The package level thermal management, which includes thermal material research [3–5], package design optimization [1]. And theoretical simulation analysis [6] is important to determine the packaging thermal resistance of LED. As to high power LEDs, system level thermal management with external active cooling is necessary and of crucial importance. Currently, fin-heat sink [7,8] is still the mainstream method in industry due to its highest reliability and lowest cost. Meanwhile, heat pipe [9] is becoming a good option for emerging high power LEDs. As to extremely high flux heat dissipation requirements, water cooling is widely studied. But relatively large system volume, coolant leakage and evaporation problems are the main disadvantages for its practical application in heat dissipation of LEDs. Except for that, some novel and advanced methods also emerged, such as micro-jet array cooling [10–12], micro-channel cooler [13], electrohydrodynamic approach [14], thermoelectric cooling [15] and piezoelectric fan [16]. But these strategies often involve complex design process, reliability, cost issues or poor cooling ☆ Communicated by P. Cheng and W. Q. Tao. ⁎ Corresponding author. E-mail address: [email protected] (J. Liu). 0735-1933/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2010.04.011
capability, which are the main obstacles for their commercialization and utilization. Recently, liquid metal cooling was identified as an effective method for heat dissipation of many high heat flux devices. That is mainly because liquid metal owns superior thermo-physical properties and the ability to be driven efficiently by a silent, nonmoving pump [17,18]. Up till now, no detailed theoretical or experimental results on liquid metal cooling of high power LEDs are reported in literatures. In this paper, a LED cooling system using liquid metal as the coolant was proposed. A prototypical liquid metal cooling system was fabricated. The experiments and theoretical analysis indicated that liquid metal cooling was a powerful way for heat dissipation of LEDs, especially for high heat flux applications, and the fabrication of practical liquid metal cooling devices for high power LEDs was feasible and useful. 2. Experimental set up Up to now, the best candidates of liquid metal suitable for LED cooling system are gallium and its alloys, for their excellent properties of low melting points, high thermal conductivity, non-flammable and non-toxic activities, low vapor pressure, and high boiling point etc. [17]. In the experiments, GaIn20 (Ga 80%, In 20%), which has the melting point of about 16°C, was adopted as the coolant of the liquid metal cooling system. Its thermal-physical properties, such as melting point, density, thermal conductivity and heat capacity were measured in the former experiment and shown in Table 1 [19]. The whole experimental platform consists of two parts: the LED heat source and the liquid cooling system (Fig. 1). The LED heat source was simulated using a square heating block, which had a total heating power of 100 W and heat dissipation area of 5cm × 5cm. In order to reduce the heat loss of the heating block, all of the non-heat dissipation surfaces were wrapped with insulating cotton cloth. As to the liquid cooling system, it was comprised of a cold plate, a fan-
Y. Deng, J. Liu / International Communications in Heat and Mass Transfer 37 (2010) 788–791
released respectively. From Eq.(1), it can be calculated that the system thermal resistance with liquid metal as coolant was only 0.13 °C/W, which was about 59% of that based on water. As to a typical liquid cooling system, convective heat transfer coefficient is often used to evaluate the heat dissipation performance. And it can be determined by:
Nomenclature A Heat transfer area c Heat capacity h Convective heat transfer coefficient m Mass flow Q Released heat Rcontact Contact thermal resistance Rconvective Convective thermal resistance Rcapacity Capacity thermal resistance Rradiator Radiator thermal resistance Rsys System thermal resistance Tambient Ambient temperature Tcold plate Temperature of cold plate substrate T Temperature of LED substrate ―substrate Tf Mean fluid temperature in the cold plate Tin Inlet fluid temperature of the cold plate
Q = hAðTcold
cooled radiator and a driving pump. The cold plate was made of copper alloy to reduce the conductive thermal resistance, and a thin layer of thermal grease was used to improve thermal conductivity and reduce surface roughness between the heating block and the cold plate. Though liquid metal can be driven by an electromagnetic pump without moving parts, a peristaltic pump was adopted in this experiment for convenient comparison with water. Based on our recent experiment, if an electromagnetic pump is adopted, the consumed power would be less than 2W which shows excellent energy-saving characteristic. The fan-cooled radiator had a radiation area of 0.38m2 and a 24 V–7.2 W cooling fan to cool the heated fluid. When the system is operating, the liquid metal would circulate and absorb the heat of LED substrate in cold plate and then dissipate the heat to the environment in the fan-cooled radiator. Five thermocouples were set so as to measure the temperatures of inlet and outlet of cold plate, heat dissipation surface of heating block, substrate of cold plate, and the environment, respectively. 3. Results 3.1. Heat dissipation performance on 100 W heat load Fig. 2 shows the temperature curves of the LED substrate when the system was started with water and liquid metal as the coolant respectively. The heating power was 100 W, the volume flow was 10.8 ml/s, and the ambient temperature was 24 °C. As can be seen from Fig. 2, under the same heat load, when using liquid metal as the coolant, the temperature rise of LED substrate could be only about 13 °C, which was much lower than that based on water (about 22 °C). The system thermal resistance can be defined as: Rsys =
Tsubstrate −Tambient Q
ð1Þ
where, Rsys, Tsubstrate, Tambient, Q are the system thermal resistance, temperature of LED substrate, ambient temperature and the heat
Table 1 Thermal-physical properties of liquid metal and water.
Melting point (°C) Density (kg/m3) Thermal conductivity (W/(m·K)) Heat capacity (J/(kg·K)) a
33.28 °C b29.8 °C c20 °C.
789
Ga
GaIn20
Water
29.8 6093a 29.28b 409.9b
16 6335c 26.58c 403.5c
0 998.2c 0.599c 4183c
plate −
― Tf Þ
ð2Þ
where, h is the convective heat transfer coefficient, A is the heat transfer area, Tcoldplate is the temperature of cold plate substrate, and ― Tf the mean fluid temperature in the cold plate (can be calculated as the mean fluid temperature of inlet and outlet here). From Eq. (2), it can be calculated that the convective heat transfer coefficient of water in the cold plate was about 3675 W/(m2 °C), while using liquid metal as coolant, the convective heat transfer coefficient could be as high as 9343W/(m2 °C). Therefore, much higher convective heat transfer coefficient is the main reason for the superior cooling capability of liquid metal when compared with water. In this work, the comparison focused on the cooling capability evaluation under the same flow condition, thus it was performed based on the same volume flow but not the same pump power. However, if comparing the cooling capability under the same pump power which means the volume flow of liquid metal would be smaller, the liquid metal could also show higher convective coefficient and lower convective thermal resistance. That condition has already been investigated before on a microchannel cold plate [19], which therefore will not be detailed here. 3.2. System thermal resistance evaluation under different volume flow During the heat dissipation process, the heat generated by the LED would first pass through the thermal interface material to the cold plate, and then it would be taken away by the cooled coolant through convection process. Finally, the heat would be dissipated to the ambient air when the heated fluid flows through the fan-cooled radiator. Therefore, quantitative description of each thermal resistance in these series of heat transfer processes is of great value for system design and evaluation. The thermal resistance of a typical liquid cooling system can be divided into four individual thermal resistances, namely: contact thermal resistance, convective thermal resistance, capacity thermal resistance, and the radiator thermal resistance. The definition of these thermal resistances can be expressed as: Rcontact = ðTsubstrate −Tcold Rconvective = ðTcold
plate Þ = Q
―
plate − Tf Þ = Q
ð3Þ ð4Þ
― Rcapacity = ð Tf −Tin Þ = Q
ð5Þ
Rradiator = ðTin −Tambient Þ = Q
ð6Þ
where, Rcontact, Rconvective, Rcapacity, and Rradiator are contact thermal resistance, convective thermal resistance, capacity thermal resistance, and the radiator thermal resistance respectively. Tin is the inlet fluid ― temperature of the cold plate, and Tf is the mean fluid temperature of inlet and outlet of cold plate. The calculated individual thermal resistances of the cooling system with water and liquid metal as coolant were shown in Fig. 3. As can be seen from Fig. 3, under the same volume flow, the overall system thermal resistance of the liquid metal cooling system is smaller than that based on water. When using water as coolant, the convective thermal resistance accounts for the largest proportion of the total thermal resistance, which is mainly due to the reason that the convective heat transfer coefficient of water is relatively low. However, if using liquid metal as the coolant, the convective thermal
790
Y. Deng, J. Liu / International Communications in Heat and Mass Transfer 37 (2010) 788–791
Fig. 1. The schematic of experimental platform.
resistance would be greatly reduced since it has very high convective heat transfer coefficient, and the radiator thermal resistance begins to become the bottleneck of the whole cooling system. What is more, it can be found that the capacity thermal resistance of the water based system is small, while in the liquid metal cooling system it is very evident, which is mainly because that the heat capacity of liquid metal is relatively smaller and it would be much easier to get a temperature rise. Therefore, the volume flow design is of great importance for liquid metal cooling system. Though liquid metal could have smaller convective thermal resistance, the system heat dissipation performance might be deteriorated for high capacity thermal resistance resulted from insufficient volume flow.
4. Discussion Considering from the thermal-physical property, the best advantage of liquid metal lies in its very high thermal conductivity, therefore it could result in a greater convective heat transfer coefficient and a smaller convective thermal resistance when compared with water. However, as to a typical liquid cooling system, convective thermal resistance is only a part of the system thermal resistance, and the radiator thermal resistance and capacity thermal resistance also play a very important role in the overall system cooling performance. Therefore, quantitatively evaluating each thermal resistance and eliminating the bottlenecks of the heat transfer process so as to minimize the overall thermal resistance is very important for
3.3. System temperature response on instantaneous thermal shock When power supply fluctuation happens, very high heat flux would be generated in a short period. Therefore, the cooling system must have the ability to stand instantaneous thermal shock. Fig. 4 shows the system temperature response when a heat load of 150W was suddenly applied and continued 30 seconds on the cooling system. As can be seen from Fig. 4, under the same thermal shock, the liquid metal cooling system has a smaller temperature rise. That means that although the liquid metal has smaller heat capacity, it can have better heat dissipation performance under instantaneous thermal shock since it has smaller system thermal resistance. Therefore, when using liquid metal as the coolant, the cooling system could stand higher thermal shock and operate more stably.
Fig. 2. The temperature of LED substrate versus time when the heat load is 100 W, (a) (b).
Fig. 3. System thermal resistance evaluation versus volume flow with (a) water and (b) liquid metal as the coolant (TR in the figure means “Thermal resistance”).
Y. Deng, J. Liu / International Communications in Heat and Mass Transfer 37 (2010) 788–791
791
5. Conclusion A liquid metal cooling system for heat dissipation of high power LEDs was proposed and investigated in this study. The result indicated that liquid metal cooling system could have much higher cooling capability than that based on water. The thermal resistance analysis showed that higher convective heat transfer coefficient was the main reason for liquid metal to be an excellent coolant for heat dissipation, but the system design was also very important. Considering both the advantages of high cooling performance and the energy saving characteristic, the liquid metal cooling system for high power LEDs is quite feasible and useful.
Acknowledgements
Fig. 4. System temperature response on instantaneous thermal shock.
the design and optimization of liquid metal LED cooling system. From Eqs. (1) and (3)–(6), the system thermal resistance can be expressed as: Rsys = Rcontact + Rconvective + Rcapacity + Rradiator :
ð7Þ
The contact thermal resistance is mainly determined by the contact surface condition and the property of thermal interface material. In this experiment, it only takes a small proportion of the whole system thermal resistance. The convective thermal resistance is closely related to the convective heat transfer coefficient in the cold plate. Because liquid metal owns much higher convective coefficient, the convective thermal resistance would be much smaller. Therefore, the liquid metal cooling is very suitable for heat dissipation of LEDs, especially for extremely high flux power applications. The relationship between the convective thermal resistance and the convective coefficient can be expressed as: Rconvective = 1 = ðhAÞ:
ð8Þ
Because liquid metal has much higher convective heat transfer coefficient than that of water, the convective thermal resistance of cooling system could be greatly reduced when using liquid metal as the coolant. The capacity resistance is resulted from the temperature rise when liquid metal flows through the cold plate. It is determined by the heat capacity and the mass flow of coolant. The relationship can be expressed as: Rcapacity = 1 = ð2mcÞ
ð9Þ
where, m is the mass flow and c is the heat capacity. Because liquid metal has high thermal conductivity and low heat capacity, it is easy to get a temperature rise so the capacity thermal resistance could be very evident. Therefore, appropriate flow design is of great importance to ensure a small capacity thermal resistance of the liquid metal cooling system. The radiator thermal resistance is used to quantitatively evaluate the cooling capability of the fan-cooled radiator. The closer the fluid outlet temperature to the ambient temperature, the smaller thermal resistance would be. An ideal radiator could have the fluid outlet temperature equal to the ambient temperature, therefore has no thermal resistance. But most of the actual radiators could not achieve that performance for the limitation of the system volume and cost. Under a certain fluid volume flow, the radiator thermal resistance is mainly related to the radiation area and the cooling capability of the fan. Therefore, in practical liquid metal cooling system design, the volume flow could be first determined by the capacity thermal resistance, and then it is in accordance with the heat load to determine the design of the radiator so as to achieve a reasonable overall thermal resistance.
This work is partially supported by the National Natural Science Foundation of China.
References [1] S. Gao, J. Hong, S. Shin, Y. Lee, S. Choi, S. Yi, Design optimization on the heat transfer and mechanical reliability of High Brightness Light Emitting Diodes (HBLED) package, 58th Electronic Components and Technology Conference, Orlando, 2008, pp. 798–803. [2] M. Arik, C. Becker, S. Weaver, J. Petroski, Thermal management of LEDs: package to system, 3rd International Conference on Solid State Lighting, San Diego, 2003, pp. 64–75. [3] C. Zweben, Advances in LED packaging and thermal management materials, 12th Conference on Light-Emitting Diodes: Research, Manufacturing, and Applications XI, San Jose, 2008, p. 91018. [4] Z. Kai, M.M.F. Yuen, D.G.W. Xiao, Y.Y. Fu, P. Chan, Directly synthesizing CNT-TIM on aluminum alloy heat sink for HB-LED thermal management, 58th Electronic Components and Technology Conference, Orlando, 2008, pp. 1659–1663. [5] M.L. Cao, F.Y. Pang, M.C. Zhang, Z.J. Cheng, Y. Gao, P.G. He, L.K. Pan, Z. Sun, Improved heat-dissipating silicone by nano-materials for LED packaging, 2nd IEEE International Nanoelectronics Conference, Shanghai, 2008, pp. 758–760. [6] T. Cheng, X.B. Luo, S.Y. Huang, Z.Y. Gan, S. Liu, Simulation on thermal characteristics of LED chips for design optimization, International Conference on Electronic Packaging Technology and High Density Packaging, Shanghai, 2008 pp. 464–467. [7] X.B. Luo, W. Xiong, T. Cheng, S. Liu, Temperature estimation of high-power light emitting diode street lamp by a multi-chip analytical solution, IET Optoelectron. 3 (5) (2009) 225–232. [8] A. Christensen, M. Ha, S. Graham, Thermal management methods for compact high power LED arrays, 7th International Conference on Solid State Lighting, San Diego, 2007, 66690Z.1 66690Z.19. [9] L. Kim, J.H. Choi, S.H. Jang, M.W. Shin, Thermal analysis of LED array system with heat pipe, 6th Symposium of the Korean Society of Thermophysical Properties, Seoul, 2006, pp. 21–25. [10] S. Liu, J.H. Yang, Z.Y. Gan, X.B. Luo, Structural optimization of a microjet based cooling system for high power LEDs, Int. J. Therm. Sci. 47 (8) (2008) 1086–1095. [11] X.B. Luo, W. Chen, R.X. Sun, S. Liu, Experimental and numerical investigation of a microjet-based cooling system for high power LEDs, Heat Transf, Eng. 29 (9) (2008) 774–781. [12] X.B. Luo, S. Liu, A microjet array cooling system for thermal management of highbrightness LEDs, IEEE Trans, Adv. Packag. 30 (3) (2007) 475–484. [13] L.L. Yuan, S. Liu, M.X. Chen, X.B. Luo, Thermal analysis of high power LED array packaging with microchannel cooler, 7th International Conference on Electronics Packaging Technology, Shanghai, 2006, pp. 574–577. [14] S.W. Chau, C.H. Lin, C.H. Yeh, C. Yang, Study on the cooling enhancement of LED heat sources via an electrohydrodynamic approach, 33rd Annual Conference of the IEEE Industrial Electronics Society, Taipei, 2007, pp. 2934–2937. [15] J.H. Cheng, C.K. Liu, Y.L. Chao, R.M. Tain, Cooling performance of silicon-based thermoelectric device on high power LED, 24th International Conference on Thermoelectrics (ICT), Clemson, 2005, pp. 53–56. [16] T. Acikalin, S.V. Garimella, J. Petroski, A. Raman, Optimal design of miniature piezoelectric fans for cooling light emitting diodes, 9th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, Las Vegas, 2004, pp. 663–671. [17] K.Q. Ma, J. Liu, Liquid metal cooling in thermal management of computer chips, Front. Energy Power Eng. China 1 (4) (2007) 384–402. [18] K.Q. Ma, J. Liu, Heat-driven liquid metal cooling device for the thermal management of a computer chip, J. Phys. D Appl. Phys. 40 (15) (2007) 4722–4729. [19] Y.G. Deng, J. Liu, Y.X. Zhou, Liquid metal based mini/micro channel cooling devices, Seventh International ASME Conference on Nanochannels, Microchannels and Minichannels, Pohang, South Korea, 2009, June 22–24.
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
A liquid metal cooling system for the thermal management of high power LEDs☆ Yueguang Deng, Jing Liu ⁎ Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Available online 12 May 2010 Keywords: Light-emitting diode (LED) Liquid metal cooling Heat dissipation
a b s t r a c t An active cooling solution using liquid metal as the coolant was proposed for high power light emitting diodes (LEDs). The typical thermal-physical properties of liquid metal were presented. Then a series of experiments under different operation conditions were performed to evaluate the heat dissipation performance of the liquid metal cooling system, and the results were compared with that of water. In order to better understand the cooling capability of liquid metal cooling system, a theoretical thermal resistance model was established and discussed. Both the experiments and theoretical analysis indicated that liquid metal cooling was a powerful way for heat dissipation of high power LEDs, and the fabrication of practical liquid metal cooling devices was feasible and useful. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction High brightness Light Emitting Diodes (LEDs) is rapidly emerging as a new generation of lighting source for its many distinctive advantages, such as long work-life, low-power consumption, environmental-friendly characteristics and so on [1]. However, because of the optical performance and reliability of LED are greatly affected by junction temperature, which should be kept under 120 °C [2], heat dissipation reserves to be a big challenge for manufactures to design high power LEDs, especially in high brightness lighting fields. So far, many heat dissipation solutions have been investigated for the thermal management of LED, mainly from package level to system level [2]. The package level thermal management, which includes thermal material research [3–5], package design optimization [1]. And theoretical simulation analysis [6] is important to determine the packaging thermal resistance of LED. As to high power LEDs, system level thermal management with external active cooling is necessary and of crucial importance. Currently, fin-heat sink [7,8] is still the mainstream method in industry due to its highest reliability and lowest cost. Meanwhile, heat pipe [9] is becoming a good option for emerging high power LEDs. As to extremely high flux heat dissipation requirements, water cooling is widely studied. But relatively large system volume, coolant leakage and evaporation problems are the main disadvantages for its practical application in heat dissipation of LEDs. Except for that, some novel and advanced methods also emerged, such as micro-jet array cooling [10–12], micro-channel cooler [13], electrohydrodynamic approach [14], thermoelectric cooling [15] and piezoelectric fan [16]. But these strategies often involve complex design process, reliability, cost issues or poor cooling ☆ Communicated by P. Cheng and W. Q. Tao. ⁎ Corresponding author. E-mail address: [email protected] (J. Liu). 0735-1933/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2010.04.011
capability, which are the main obstacles for their commercialization and utilization. Recently, liquid metal cooling was identified as an effective method for heat dissipation of many high heat flux devices. That is mainly because liquid metal owns superior thermo-physical properties and the ability to be driven efficiently by a silent, nonmoving pump [17,18]. Up till now, no detailed theoretical or experimental results on liquid metal cooling of high power LEDs are reported in literatures. In this paper, a LED cooling system using liquid metal as the coolant was proposed. A prototypical liquid metal cooling system was fabricated. The experiments and theoretical analysis indicated that liquid metal cooling was a powerful way for heat dissipation of LEDs, especially for high heat flux applications, and the fabrication of practical liquid metal cooling devices for high power LEDs was feasible and useful. 2. Experimental set up Up to now, the best candidates of liquid metal suitable for LED cooling system are gallium and its alloys, for their excellent properties of low melting points, high thermal conductivity, non-flammable and non-toxic activities, low vapor pressure, and high boiling point etc. [17]. In the experiments, GaIn20 (Ga 80%, In 20%), which has the melting point of about 16°C, was adopted as the coolant of the liquid metal cooling system. Its thermal-physical properties, such as melting point, density, thermal conductivity and heat capacity were measured in the former experiment and shown in Table 1 [19]. The whole experimental platform consists of two parts: the LED heat source and the liquid cooling system (Fig. 1). The LED heat source was simulated using a square heating block, which had a total heating power of 100 W and heat dissipation area of 5cm × 5cm. In order to reduce the heat loss of the heating block, all of the non-heat dissipation surfaces were wrapped with insulating cotton cloth. As to the liquid cooling system, it was comprised of a cold plate, a fan-
Y. Deng, J. Liu / International Communications in Heat and Mass Transfer 37 (2010) 788–791
released respectively. From Eq.(1), it can be calculated that the system thermal resistance with liquid metal as coolant was only 0.13 °C/W, which was about 59% of that based on water. As to a typical liquid cooling system, convective heat transfer coefficient is often used to evaluate the heat dissipation performance. And it can be determined by:
Nomenclature A Heat transfer area c Heat capacity h Convective heat transfer coefficient m Mass flow Q Released heat Rcontact Contact thermal resistance Rconvective Convective thermal resistance Rcapacity Capacity thermal resistance Rradiator Radiator thermal resistance Rsys System thermal resistance Tambient Ambient temperature Tcold plate Temperature of cold plate substrate T Temperature of LED substrate ―substrate Tf Mean fluid temperature in the cold plate Tin Inlet fluid temperature of the cold plate
Q = hAðTcold
cooled radiator and a driving pump. The cold plate was made of copper alloy to reduce the conductive thermal resistance, and a thin layer of thermal grease was used to improve thermal conductivity and reduce surface roughness between the heating block and the cold plate. Though liquid metal can be driven by an electromagnetic pump without moving parts, a peristaltic pump was adopted in this experiment for convenient comparison with water. Based on our recent experiment, if an electromagnetic pump is adopted, the consumed power would be less than 2W which shows excellent energy-saving characteristic. The fan-cooled radiator had a radiation area of 0.38m2 and a 24 V–7.2 W cooling fan to cool the heated fluid. When the system is operating, the liquid metal would circulate and absorb the heat of LED substrate in cold plate and then dissipate the heat to the environment in the fan-cooled radiator. Five thermocouples were set so as to measure the temperatures of inlet and outlet of cold plate, heat dissipation surface of heating block, substrate of cold plate, and the environment, respectively. 3. Results 3.1. Heat dissipation performance on 100 W heat load Fig. 2 shows the temperature curves of the LED substrate when the system was started with water and liquid metal as the coolant respectively. The heating power was 100 W, the volume flow was 10.8 ml/s, and the ambient temperature was 24 °C. As can be seen from Fig. 2, under the same heat load, when using liquid metal as the coolant, the temperature rise of LED substrate could be only about 13 °C, which was much lower than that based on water (about 22 °C). The system thermal resistance can be defined as: Rsys =
Tsubstrate −Tambient Q
ð1Þ
where, Rsys, Tsubstrate, Tambient, Q are the system thermal resistance, temperature of LED substrate, ambient temperature and the heat
Table 1 Thermal-physical properties of liquid metal and water.
Melting point (°C) Density (kg/m3) Thermal conductivity (W/(m·K)) Heat capacity (J/(kg·K)) a
33.28 °C b29.8 °C c20 °C.
789
Ga
GaIn20
Water
29.8 6093a 29.28b 409.9b
16 6335c 26.58c 403.5c
0 998.2c 0.599c 4183c
plate −
― Tf Þ
ð2Þ
where, h is the convective heat transfer coefficient, A is the heat transfer area, Tcoldplate is the temperature of cold plate substrate, and ― Tf the mean fluid temperature in the cold plate (can be calculated as the mean fluid temperature of inlet and outlet here). From Eq. (2), it can be calculated that the convective heat transfer coefficient of water in the cold plate was about 3675 W/(m2 °C), while using liquid metal as coolant, the convective heat transfer coefficient could be as high as 9343W/(m2 °C). Therefore, much higher convective heat transfer coefficient is the main reason for the superior cooling capability of liquid metal when compared with water. In this work, the comparison focused on the cooling capability evaluation under the same flow condition, thus it was performed based on the same volume flow but not the same pump power. However, if comparing the cooling capability under the same pump power which means the volume flow of liquid metal would be smaller, the liquid metal could also show higher convective coefficient and lower convective thermal resistance. That condition has already been investigated before on a microchannel cold plate [19], which therefore will not be detailed here. 3.2. System thermal resistance evaluation under different volume flow During the heat dissipation process, the heat generated by the LED would first pass through the thermal interface material to the cold plate, and then it would be taken away by the cooled coolant through convection process. Finally, the heat would be dissipated to the ambient air when the heated fluid flows through the fan-cooled radiator. Therefore, quantitative description of each thermal resistance in these series of heat transfer processes is of great value for system design and evaluation. The thermal resistance of a typical liquid cooling system can be divided into four individual thermal resistances, namely: contact thermal resistance, convective thermal resistance, capacity thermal resistance, and the radiator thermal resistance. The definition of these thermal resistances can be expressed as: Rcontact = ðTsubstrate −Tcold Rconvective = ðTcold
plate Þ = Q
―
plate − Tf Þ = Q
ð3Þ ð4Þ
― Rcapacity = ð Tf −Tin Þ = Q
ð5Þ
Rradiator = ðTin −Tambient Þ = Q
ð6Þ
where, Rcontact, Rconvective, Rcapacity, and Rradiator are contact thermal resistance, convective thermal resistance, capacity thermal resistance, and the radiator thermal resistance respectively. Tin is the inlet fluid ― temperature of the cold plate, and Tf is the mean fluid temperature of inlet and outlet of cold plate. The calculated individual thermal resistances of the cooling system with water and liquid metal as coolant were shown in Fig. 3. As can be seen from Fig. 3, under the same volume flow, the overall system thermal resistance of the liquid metal cooling system is smaller than that based on water. When using water as coolant, the convective thermal resistance accounts for the largest proportion of the total thermal resistance, which is mainly due to the reason that the convective heat transfer coefficient of water is relatively low. However, if using liquid metal as the coolant, the convective thermal
790
Y. Deng, J. Liu / International Communications in Heat and Mass Transfer 37 (2010) 788–791
Fig. 1. The schematic of experimental platform.
resistance would be greatly reduced since it has very high convective heat transfer coefficient, and the radiator thermal resistance begins to become the bottleneck of the whole cooling system. What is more, it can be found that the capacity thermal resistance of the water based system is small, while in the liquid metal cooling system it is very evident, which is mainly because that the heat capacity of liquid metal is relatively smaller and it would be much easier to get a temperature rise. Therefore, the volume flow design is of great importance for liquid metal cooling system. Though liquid metal could have smaller convective thermal resistance, the system heat dissipation performance might be deteriorated for high capacity thermal resistance resulted from insufficient volume flow.
4. Discussion Considering from the thermal-physical property, the best advantage of liquid metal lies in its very high thermal conductivity, therefore it could result in a greater convective heat transfer coefficient and a smaller convective thermal resistance when compared with water. However, as to a typical liquid cooling system, convective thermal resistance is only a part of the system thermal resistance, and the radiator thermal resistance and capacity thermal resistance also play a very important role in the overall system cooling performance. Therefore, quantitatively evaluating each thermal resistance and eliminating the bottlenecks of the heat transfer process so as to minimize the overall thermal resistance is very important for
3.3. System temperature response on instantaneous thermal shock When power supply fluctuation happens, very high heat flux would be generated in a short period. Therefore, the cooling system must have the ability to stand instantaneous thermal shock. Fig. 4 shows the system temperature response when a heat load of 150W was suddenly applied and continued 30 seconds on the cooling system. As can be seen from Fig. 4, under the same thermal shock, the liquid metal cooling system has a smaller temperature rise. That means that although the liquid metal has smaller heat capacity, it can have better heat dissipation performance under instantaneous thermal shock since it has smaller system thermal resistance. Therefore, when using liquid metal as the coolant, the cooling system could stand higher thermal shock and operate more stably.
Fig. 2. The temperature of LED substrate versus time when the heat load is 100 W, (a) (b).
Fig. 3. System thermal resistance evaluation versus volume flow with (a) water and (b) liquid metal as the coolant (TR in the figure means “Thermal resistance”).
Y. Deng, J. Liu / International Communications in Heat and Mass Transfer 37 (2010) 788–791
791
5. Conclusion A liquid metal cooling system for heat dissipation of high power LEDs was proposed and investigated in this study. The result indicated that liquid metal cooling system could have much higher cooling capability than that based on water. The thermal resistance analysis showed that higher convective heat transfer coefficient was the main reason for liquid metal to be an excellent coolant for heat dissipation, but the system design was also very important. Considering both the advantages of high cooling performance and the energy saving characteristic, the liquid metal cooling system for high power LEDs is quite feasible and useful.
Acknowledgements
Fig. 4. System temperature response on instantaneous thermal shock.
the design and optimization of liquid metal LED cooling system. From Eqs. (1) and (3)–(6), the system thermal resistance can be expressed as: Rsys = Rcontact + Rconvective + Rcapacity + Rradiator :
ð7Þ
The contact thermal resistance is mainly determined by the contact surface condition and the property of thermal interface material. In this experiment, it only takes a small proportion of the whole system thermal resistance. The convective thermal resistance is closely related to the convective heat transfer coefficient in the cold plate. Because liquid metal owns much higher convective coefficient, the convective thermal resistance would be much smaller. Therefore, the liquid metal cooling is very suitable for heat dissipation of LEDs, especially for extremely high flux power applications. The relationship between the convective thermal resistance and the convective coefficient can be expressed as: Rconvective = 1 = ðhAÞ:
ð8Þ
Because liquid metal has much higher convective heat transfer coefficient than that of water, the convective thermal resistance of cooling system could be greatly reduced when using liquid metal as the coolant. The capacity resistance is resulted from the temperature rise when liquid metal flows through the cold plate. It is determined by the heat capacity and the mass flow of coolant. The relationship can be expressed as: Rcapacity = 1 = ð2mcÞ
ð9Þ
where, m is the mass flow and c is the heat capacity. Because liquid metal has high thermal conductivity and low heat capacity, it is easy to get a temperature rise so the capacity thermal resistance could be very evident. Therefore, appropriate flow design is of great importance to ensure a small capacity thermal resistance of the liquid metal cooling system. The radiator thermal resistance is used to quantitatively evaluate the cooling capability of the fan-cooled radiator. The closer the fluid outlet temperature to the ambient temperature, the smaller thermal resistance would be. An ideal radiator could have the fluid outlet temperature equal to the ambient temperature, therefore has no thermal resistance. But most of the actual radiators could not achieve that performance for the limitation of the system volume and cost. Under a certain fluid volume flow, the radiator thermal resistance is mainly related to the radiation area and the cooling capability of the fan. Therefore, in practical liquid metal cooling system design, the volume flow could be first determined by the capacity thermal resistance, and then it is in accordance with the heat load to determine the design of the radiator so as to achieve a reasonable overall thermal resistance.
This work is partially supported by the National Natural Science Foundation of China.
References [1] S. Gao, J. Hong, S. Shin, Y. Lee, S. Choi, S. Yi, Design optimization on the heat transfer and mechanical reliability of High Brightness Light Emitting Diodes (HBLED) package, 58th Electronic Components and Technology Conference, Orlando, 2008, pp. 798–803. [2] M. Arik, C. Becker, S. Weaver, J. Petroski, Thermal management of LEDs: package to system, 3rd International Conference on Solid State Lighting, San Diego, 2003, pp. 64–75. [3] C. Zweben, Advances in LED packaging and thermal management materials, 12th Conference on Light-Emitting Diodes: Research, Manufacturing, and Applications XI, San Jose, 2008, p. 91018. [4] Z. Kai, M.M.F. Yuen, D.G.W. Xiao, Y.Y. Fu, P. Chan, Directly synthesizing CNT-TIM on aluminum alloy heat sink for HB-LED thermal management, 58th Electronic Components and Technology Conference, Orlando, 2008, pp. 1659–1663. [5] M.L. Cao, F.Y. Pang, M.C. Zhang, Z.J. Cheng, Y. Gao, P.G. He, L.K. Pan, Z. Sun, Improved heat-dissipating silicone by nano-materials for LED packaging, 2nd IEEE International Nanoelectronics Conference, Shanghai, 2008, pp. 758–760. [6] T. Cheng, X.B. Luo, S.Y. Huang, Z.Y. Gan, S. Liu, Simulation on thermal characteristics of LED chips for design optimization, International Conference on Electronic Packaging Technology and High Density Packaging, Shanghai, 2008 pp. 464–467. [7] X.B. Luo, W. Xiong, T. Cheng, S. Liu, Temperature estimation of high-power light emitting diode street lamp by a multi-chip analytical solution, IET Optoelectron. 3 (5) (2009) 225–232. [8] A. Christensen, M. Ha, S. Graham, Thermal management methods for compact high power LED arrays, 7th International Conference on Solid State Lighting, San Diego, 2007, 66690Z.1 66690Z.19. [9] L. Kim, J.H. Choi, S.H. Jang, M.W. Shin, Thermal analysis of LED array system with heat pipe, 6th Symposium of the Korean Society of Thermophysical Properties, Seoul, 2006, pp. 21–25. [10] S. Liu, J.H. Yang, Z.Y. Gan, X.B. Luo, Structural optimization of a microjet based cooling system for high power LEDs, Int. J. Therm. Sci. 47 (8) (2008) 1086–1095. [11] X.B. Luo, W. Chen, R.X. Sun, S. Liu, Experimental and numerical investigation of a microjet-based cooling system for high power LEDs, Heat Transf, Eng. 29 (9) (2008) 774–781. [12] X.B. Luo, S. Liu, A microjet array cooling system for thermal management of highbrightness LEDs, IEEE Trans, Adv. Packag. 30 (3) (2007) 475–484. [13] L.L. Yuan, S. Liu, M.X. Chen, X.B. Luo, Thermal analysis of high power LED array packaging with microchannel cooler, 7th International Conference on Electronics Packaging Technology, Shanghai, 2006, pp. 574–577. [14] S.W. Chau, C.H. Lin, C.H. Yeh, C. Yang, Study on the cooling enhancement of LED heat sources via an electrohydrodynamic approach, 33rd Annual Conference of the IEEE Industrial Electronics Society, Taipei, 2007, pp. 2934–2937. [15] J.H. Cheng, C.K. Liu, Y.L. Chao, R.M. Tain, Cooling performance of silicon-based thermoelectric device on high power LED, 24th International Conference on Thermoelectrics (ICT), Clemson, 2005, pp. 53–56. [16] T. Acikalin, S.V. Garimella, J. Petroski, A. Raman, Optimal design of miniature piezoelectric fans for cooling light emitting diodes, 9th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, Las Vegas, 2004, pp. 663–671. [17] K.Q. Ma, J. Liu, Liquid metal cooling in thermal management of computer chips, Front. Energy Power Eng. China 1 (4) (2007) 384–402. [18] K.Q. Ma, J. Liu, Heat-driven liquid metal cooling device for the thermal management of a computer chip, J. Phys. D Appl. Phys. 40 (15) (2007) 4722–4729. [19] Y.G. Deng, J. Liu, Y.X. Zhou, Liquid metal based mini/micro channel cooling devices, Seventh International ASME Conference on Nanochannels, Microchannels and Minichannels, Pohang, South Korea, 2009, June 22–24.