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Effect of evaporation section and condensation section length on thermal performance of flat plate heat pipe
Applied Thermal Engineering 31 (2011) 2367e2373
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Effect of evaporation section and condensation section length on thermal performance of flat plate heat pipe Shuangfeng Wang*, Jinjian Chen, Yanxin Hu, Wei Zhang Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2011 Accepted 31 March 2011 Available online 8 April 2011
Flat plate heat pipes (FPHPs) are one of the available technologies to deal with the high density electronic cooling problem due to their high thermal conductivity, reliability, and low weight penalty. A series of experiments were performed to investigate the effect of evaporation and condensation length on thermal performance of flat plate heat pipes. In the experiments, the FPHP had heat transfer length of 255 mm and width of 25 mm, and pure water was used as the working fluid. The results show that comparing to vapor chamber, the FPHP could realize long-distance heat transfer; comparing to the traditional heat pipe, the FPHP has large area contact with heat sources; the thermal resistance decreased and the heat transfer limit increased with the increase of evaporation section length; the FPHP would dry out at a lower heating power with the increase of condensation section length, which indicated that the heat transfer limit decreased, but the evaporator temperature also decreased; when the condensation section length approached to evaporation section length, the FPHP had a better thermal performance. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Flat plate heat pipes Evaporation section length Condensation section length Heat transfer limit Evaporator temperature
1. Introduction With increasing heat dissipation of electronic chips, the cooling problem becomes more and more challenging. Especially, the double chips or multi chips setting on one PCB (Printed Circuit Board), the heat flux is so high that the traditional heat pipe can not solve the heat dissipation problem. Then the high-capacity flat plate heat pipe (FPHP) has found applications in high-power chips, such as LED (Light Emitting Diode), CPUs (Central Processing Units) and GPUs (Graphic Processing Units). Flat plate heat pipes are one of the available technologies to deal with the high density electronic cooling problem due to their high thermal conductivity, reliability, and low weight penalty [1]. Experimental investigations on flat heat pipes have been conducted by some researchers. Sun et al. [2] presented an approximate method for calculating the effective length of a flat plate heat pipe when a strip heater is partially covering the evaporator section. The results show that for a specific width of the strip heater, the optimal position to achieve a minimum effective length is found when the heater is placed symmetrically at the centre of the evaporator section. In this position, a higher value of the capillary heat transport limit can be achieved as compared to the case where the heater is placed on one side of the evaporator section. Schmalhofer and Faghri [3] considered the block heater * Corresponding author. Tel.: þ86 20 22236929. E-mail address: [email protected] (S. Wang). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.03.037
covering half the circumferential length of a circular heat pipe. An approximate method was introduced to calculate the effective heat transport length that can be used to evaluate the maximum capillary heat transfer limit. The results show that the experimental capillary limits determined for a copper-water heat pipe in circumferential and block heating modes were compared with the analytical values calculated. The analytical predictions were within 25% of the experimental limits. Prasher [4] put forward a conduction based model to assess the heat transport capability of heat pipes and vapor chamber for various configurations. It was shown that the conduction model compared very well with experiments conducted in remote cooling mode (where the heat sink is at the end of the vapor chamber) and active cooling mode (where the heat sink is on the top of the vapor chamber). A flat rectangular heat pipe with heat transfer length of 130 mm was considered in his experiments. The paper also discusses how thermal designers as customers and heat pipe suppliers as vendors can interact for optimized design of heat pipes and vapor chamber. Tan et al. [5] introduced an analytical approach to study the liquid flow performances inside the wick structure of a flat plate heat pipe under different heat source conditions. The results with line, strip and discrete heating conditions on the heat pipe were presented. With the comparable results, the liquid flow model is competent to predict the liquid flow distribution in the wick structure of the heat pipe under different heating conditions. Koito et al. [6] investigated heat transfer characteristics of heat sinks with flat plate heat pipe. This study focused particularly on
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Nomenclature L Le Lc La Qh Qt RFPHP Te Tc ΔT
heat transfer length (mm) length of evaporation section (mm) length of condensation section (mm) length of adiabatic section (mm) heating power (W) heat transport by FPHP (W) thermal resistance of FPHP ( C/W) temperature of evaporator ( C) temperature of condenser ( C) temperature difference in outlet and inlet ( C)
Subscripts c condensation section e evaporation section l liquid v vapor
sintered heat pipe can realizes long-distance heat transfer, but it is cylindrical shape, when taking account of cooling compact electronic equipment, such kind of heat pipe has some disadvantages. For one thing, it is necessary to add a specific thermal interface to make a close contact with the heat source, because most objects to be cooled have a flat thermal contact surface. This added thermal interface increases the thermal resistance between the heat source and evaporator. For the other thing, it also increases the thickness of evaporator, which has a strict limit for compact enclosure of electronic devices. Thus, in order to solve this problem, we proposed a strip sintered FPHP. This FPHP has a large contact area and can contact with the chips directly, moreover, this FPHP can realizes long-distance heat transfer. In this paper, a strip sintered FPHP is proposed and tested. The main intension of this work is to investigate the effect of evaporation section length and condensation section length on thermal performance of the FPHP. As a result, the maximum value of heat transfer performance can be achieved. It is expected that the result will provide useful information for its installation in applications. 2. Experimental setup
the effect of the thermal resistance of the heat pipe for a changing heat flux input, temperature of the cooling air, and orientation angle. The useful data were presented for the improvement of the vapor chamber. Koito et al. [7] also carried out a thorough experimental and numerical analysis of heat transfer in flat plate heat pipes with a single axi-symmetrical heat source. It was shown that the small effective thermal conductivity of the saturated wick (8.32 W/mK) causes the largest temperature gradient and particularly in the wick region near the heat source. Xuan et al. [8] study performance and mechanism of a flat plate heat pipe in which a layer of sintered copper powder is applied to the heated surface of the heat pipe to enhance evaporation process, by means of both experimental and theoretical approaches. The results show that the porous sintered layer on the heated surface can enhance evaporation process and improve performance of the FPHP. The model and simulation method developed in this article have been verified by the experimental results. Tournier et al. [9] assumed an isothermal condition in the liquidevapor interface to develop a 2D heat pipe transient analysis model. The mass flow rate at the evaporator and condenser were assumed to be the same in this model. The numerical instabilities during the phase change were suppressed by employing a mushy-cell temperature of 2 108 K [10]. The calculated steady-state water vapor and wall axial temperature profiles and the transient power throughput are in good agreement with measurements. Gu et al. [11] designed an FPHP with ceramic wick and made experimental measurement in two cases: The condenser section is located on the top plate with the maximum heat flux 6 104 W/m2, and the condenser section is located at the bottom plate with the maximum heat flux 2.5 104 W/m2. Purdue [12] produced interesting results suggesting a sensitivity of the liquid coolant flow from condenser to evaporator that is dependent on the porosity of the wick. Their data show that high porosity wicks tend to degrade the liquid flow back toward the evaporator. In recsent years, there are some new requirements for heat dissipation of electronic chips in applications. For example, there are many electronic devices at one PCB, so that the space of PCB is limit, and then the heat sink cannot be installed on the top of electronic chips. At this moment, a heat pipe which can realizes long-distance heat transfer will be needed to solve this cooling problem. It’s required that the evaporator of heat pipe contact with the chips, and the heat sink is installed at the condenser where is outside of the PCB. In this way, the heat can transfer by the heat pipe from chips to the heat sink. Typically, the total length of traditional FPHP is short, so it’s not suitable for this cooling problem. In addition, the universal
A copper tube with an internal and external diameter of 18 mm and 19 mm, respectively, was used as the material in this experiment. The copper tube was pressed to thickness of 5 mm after sintering a layer of copper powder (the porosity of capillary was 50% 5%, and the wick thickness was 1 mm). Moreover, the mesh support structure with thickness of 2 mm was installed in middle of FPHP. The sealing was welded by the argon arc welding technology. The vacuum was established by using a vacuum pump, and then the working fluid was filled in the FPHP (the filling ratio was about 50%, the uncertainty was 5%). A long strip FPHP was successfully fabricated, as show in Fig. 1(a), the total length of FPHP is 255 mm the width is 25 mm. The experimental apparatus of FPHP is shown in Fig. 1(c). The operational orientation was horizontal heating mode, where the FPHP was set in the horizontal direction. It was because the engineering application most requires the FPHP using in horizontal direction. The heating block was a copper block which was heated with two heating rods. These heating rods were connected to the transformer, which supplied heating power by adjusting the current and voltage. The length of evaporation section was controlled by the length of heating block which width was 20 mm, and the length was 20 mm, 30 mm, 40 mm, 50 mm and 70 mm respectively (the uncertainty was 1 mm). The condensation section was cooled by water (25 0.1 C). The length of condensation section was controlled by a tank whose length could change as 20 mm, 40 mm, 50 mm, 60 mm, 80 mm and 100 mm respectively (the uncertainty was 1 mm), and the plexiglass is used as the material. The OMEGA K-type thermocouples (the uncertainty was 0.1 C) were installed to measure the wall temperature at different positions of FPHP. The detailed location of thermocouples was shown in Fig.1(b). There was insulation cotton packaged outside at adiabatic section in order to avoid heat loss. All tests were conducted at an ambient temperature of 25 1 C. Based on the application requirements, the FPHP must operate normally. Thus, when the heat transfer limit (dry-out) appeared, the experiment would be stopped. The thermal resistance of FPHPs was equal to temperature difference between condensation section and evaporation section divided by heating power. RFPHP ¼ (Te Tc)/Qh (RFPHP : thermal resistance of FPHP; Te : average temperature of T1, T2, T3 and T4 approximately instead of evaporator temperature; Tc: average temperature of T7, T8, T9 and T10 approximately instead of condensation section temperature; Qh: heating power). The contact resistance between heating sources and FPHP was reduced by the
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Fig. 1. Experimental setup.
thermal paste which has high coefficient of thermal conductivity. Because the FPHP was cooled by water, and the water directly contacted with the FPHP, so the thermal contact resistance between the FPHP and cooling sources very small or even negligible. There was insulation cotton packaged outside at heating block in order to avoid heat loss. Heat balance equation calculates as follow: Qh (heating power) ¼ Qt (heat transport by FPHP) þ Ql (heat loss). The heat transport by FPHP under the different heating power: Qt ¼ rwCwuwΔT,where, rw is the density of water, Cw is the specific heat, uw is volume flow rate (the uncertainty was 1%), ΔT is temperature difference in outlet and inlet (the uncertainty was 0.1 C which determined by the thermocouples). As the uncertainties of flow rate and temperature difference of the water were provided, the uncertainty of Qt could be calculated. 3. Phenomenon of dry out As shown in Fig. 2(a), when the evaporation section length was 50 mm, and condensation section length was 50 mm, the temperature of FPHP curve increased with the increased of the heating power. Moreover, the temperatures of evaporation section (T1, T2, T3 and T4) kept rising and without a stable trend at heating power of 160W, and the temperatures of condensation section (T9 and T10) kept dropping
slowly. This phenomenon was partial dry out, while the transfer limit appeared on FPHP. As can be seen in Fig. 3(c), the thermal resistance of FPHP decreased with the heating power increased, and the minimum thermal resistance reduced to 0.008 C/W at 140unit W, but the thermal resistance raised at 160unit W, which corresponded to the phenomenon of partial dry out. In order to analyze the phenomenon, the thermal resistance and heat transport curve was used to describe the thermal performance of FPHP. Comparing to the Fig. 2(a), the evaporation section temperatures sharply raise at the heating power of 120unit W, this phenomenon was dry out. As shown in Fig. 2(b), the temperature of FPHP curve raise with the heating power increased, at condensation section length of 100 mm. When the heating power of 120unit W, the temperatures of evaporation section (T1 and T2) sharply raised and the temperatures of condensation section (T9 and T10) reduced. While the heat transfer limit appeared on FPHP. 4. Results and discussion 4.1. Effect of evaporation section length In this experiment, several evaporation section lengths were tested at the same condensation section length. As can be seen in
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Fig. 3(a), as the heating power increased, the thermal resistance of FPHP decreased before the FPHP dried out. When Lc was 20 mm, Le was 20 mm and 30 mm, the thermal resistance of FHPH decreased before 180unit W, and then as the heating power increased to 200unit W, the thermal resistance increased, the FPHP dried out at 200unit W. When Le was 50 mm, the thermal resistance decreased before 200unit W, and then the FPHP dried out at 220unit W. These trends also appeared at other condensation section lengths, for example, the condensation section length at 40 mm (Le is 30 mm, 40 mm and 50 mm, respectively), as shown in Fig. 3(b); and condensation section length at 50 mm (Le is 30 mm, 50 mm and 70 mm, respectively) as shown in Fig. 3(c). It showed that there is an optimal point for FPHP operation. And with the evaporation section length increased, the thermal resistance decreased. It was because that at the same heating power, the heat flux increased with the evaporation section length decreased, such that the partial evaporation (evaporation occur in a portion of the evaporator) more intense, and then there was more vapor produced in evaporator, so the pressure difference between evaporation and condensation increased. Which was unbeneficial to assists the condensed working fluid to flow back to the evaporation section. Therefore, the temperature difference between evaporation and condensation increased, so the thermal resistance increased. On the contrary, when the evaporation section length increased, the heat flux decreased, so that the evaporation was not so intense, it was
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beneficial to assists the condensed working fluid to flow back to the evaporation section, so that the thermal resistance reduced. As can be seen in Fig. 4, when the condensation section length was 50 mm, the evaporation section length was changed to 30 mm, 50 mm and 70 mm, respectively. With the evaporation section
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These trends also appeared at other condensation section lengths, for example, the condensation section length at 20 mm (Le is 20 mm, 30 mm and 50 mm, respectively), as shown in Fig. 4(a); and the condensation section length at 40 mm (Le is 30 mm, 40 mm and 50 mm, respectively), as shown in Fig. 4(b). It was also revealed that the FPHP achieved the maximum heat transport capability (Qmax) of 132.2unit W at the heating power of 140W, when the length of evaporation section and condensation section both were 50 mm. Qmax 100% ）was up The efficiency of heat transfer （it defined as Qh to 94.4%, it showed the strip sintered flat plate heat pipe has a superior thermal performance. In conclusion, based on experimental observations and analysis, at the same condensation section length, with the increase of evaporation section length, the thermal resistance decreased. Besides, due to the thermal resistance decreased, the heat transfer limit increased.
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In this experiment, several condensation section lengths were tested at the same evaporation section length. For example, when the evaporation section length was 50 mm, the condensation section length was changed to 20 mm, 40 mm, 50 mm, 60 mm, 80 mm, and 100 mm, respectively. Fig. 5 shows the thermal resistance at different condensation section length. When the condensation section length was 100 mm, as the heating power increased, thermal resistance decreased and reached the minimum point at 100unit W. While the heating power increased to 120unit W, FPHP dried out and thermal resistance raised obviously. The same situation occurred in condensation section length at 80 mm. However, by changing condensation section length to 60 mm, the FPHP dried out at 140unit W. Moreover, FPHP dried out at 160unit W with condensation section length of 50 mm, while Lc equaled to 40 mm, FPHP dried out at 180unit W, and Lc equaled to 20 mm, FPHP dried out at 220unit W. It showed that at the same evaporation section length, decreasing condensation section length would make the FPHP dry out at a higher heating power. In other words, increasing condensation section length would make the FPHP dry out at a lower heating power, which indicated that the heat transfer limit decreased. It was related to the capillary limit. The FPHP with a bigger condensation section length (at high heating power) allows for vapor from evaporator to condense more rapidly. But due to the capillary limit, the condensate liquid could not flow rapidly back to the evaporator,
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length increased, the thermal resistance decreased, the heat transfer limit increased. Fig. 4(c) shows that at the evaporation section length of 30 mm and 50 mm, the heat transfer limit appeared at 160unit W. However, while evaporation section length increased to 70 mm, the heat transfer limit reached to 180unit W.
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and then there was less working liquid at evaporator, which could made the FPHP dry out. So that the FPHP with bigger condensation section length would dry out at a lower heating power. The maximum heat transport (Qmax) can be calculated, while the heat transfer limit appeared on FPHP. It’s better to solve the heat dissipation problem at high power with the Qmax greater. Based on the application requirements, the evaporator temperature of electronic chip is usually not higher than 70 C, such as high power LED cooling. Therefore, the FPHP can achieve a better overall performance (while the Qmax is bigger, at the same time, the evaporator temperature is lower). As shown in Fig. 6, the heat transport of FPHP decreased with the condensation section length increased. It’s worth noting, increasing the condensation section length could make the thermal resistance and the Qmax decreasing, but could make the FPHP operating at a lower temperature, as shown in Fig. 7, the evaporator temperature reduced with the condensation section length increased. Fig. 8 generalized the relationship between condensation section length, Qmax and Te. As can be seen in Fig. 8(a), when the evaporation section length was 50 mm, the condensation section length was 20 mm, FPHP had a bigger Qmax of 192.5unit W, but the Te closed to 100 C, which could not satisfy the electronic chip temperature requirements. Moreover, when the condensation section length was 80 mm and 100 mm, FPHP had a lower Te, but the Qmax was lower too, so that FPHP was easy to dry out and then
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could not satisfy the high power cooling requirements too. However, when the condensation section length within 40 mme60 mm, FPHP had a bigger Qmax (from 100unit W to 150unit W), and the evaporator temperature was lower (from 50 C to 60 C), such that the FPHP achieved a better overall performance. The same trend appeared in condition with evaporation section length of 30 mm and 40 mm, when the condensation section length approached to the evaporation section length, the FPHP had a bigger Qmax and a lower Te, as shown in Fig. 8(b) and Fig. 8(c).Thus
S. Wang et al. / Applied Thermal Engineering 31 (2011) 2367e2373
we could find that FPHP achieved a better overall performance when the condensation section length approached to evaporation section length. Therefore, the condensation section length should be consistent with evaporation section length on the application. 5. Conclusions A series of experiments were performed to investigate the effect of the evaporation section and condensation section length on thermal performance of FPHPs. In the experiments, FPHPs has the heat transfer length (L) of 255 mm and the width of 25 mm. The evaporation section length was 20 mm, 30 mm, 40 mm, 50 mm and 70 mm respectively. The condensation section length was 20 mm, 40 mm, 50 mm, 60 mm, 80 mm and 100 mm, respectively. The FPHP were set to operate at horizontal heating mode. Several conclusions can be summarized as follows: (1) The FPHP could realize long-distance heat transfer and large area contact, so that the heat source with different sizes or multi heat sources can be installed on the FPHP. The total heat transfer length reaches 255 mm and the width is 25 mm. Moreover, the FPHP achieved the maximum heat transport capability of 132.2unit W (the heat transport efficiency reached 94.4%) at heating power of 140unit W, when the length of evaporation section and condensation section both were 50 mm. It shows that the FPHP has very good thermal performance. (2) The evaporation section length had significant effect on thermal performance of FPHP. At the same length of condensation section, when the evaporation section length increased, the thermal resistance of FPHP decreased (see Fig. 3), however, the heat transfer limit increased (see Fig. 4). (3) The condensation section length had significant effect on thermal performance of FPHP too. When at the same length of evaporation section, with the condensation section length increasing, the FPHP would dry out at a lower heating power (see Fig. 5), and the Qmax reduced (see Fig. 6), but the evaporator temperature also decreased (see Fig. 7). (4) When the condensation section length approached to evaporation section length, the FPHP had better overall performance
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(see Fig. 8). Therefore, the condensation section length should be consistent with evaporation section length in applications. Acknowledgements The authors deeply appreciate the financial support offered by NSFC (Granted No.50876033), National Fund of Guang Dong Province Joint Key Projects（U0834002）and Novark Technology INC. References [1] A. Faghri, Heat Pipe Science and Technology. Taylor and Francis, Washington, DC, 1995. [2] K.H. Sun, C.Y. Liu, K.C. Leong, The effective length of a flat plate heat pipe covered partially by a strip heater on the evaporator section, Heat Recovery Systems & CHP 15 (1995) 383e388. [3] J. Schmalhofer, A. Faghri, A study of circumferentially-heated and blockheated heat pipes I. Experimental analysis and generalized analytical prediction of capillary limits, International Journal of Heat and Mass Transfer 36 (1993) 201e212. [4] Ravi S. Prasher, A simplified conduction based modeling scheme for design sensitivity study of thermal solution utilizing heat pipe and vapor chamber technology, Journal of Electronic Packaging 125 (2003) 378e385. [5] B.K. Tan, T.N. Wong, K.T. Ooi, A study of liquid flow in a flat plate heat pipe under localized heating, International Journal of Thermal Sciences 49 (2010) 99e108. [6] Y. Koito, K. Motomatsu, H. Imura, M. Mochizuki, Y. Saito, Fundamental investigations on heat transfer characteristics of heat sinks with a vapour chamber, in: Proceedings of the 7th International Heat Pipe Symposium (2003), pp. 247e251. [7] Y. Koito, H. Imura, M. Mochizuki, Y. Saito, S. Torii, Numerical analysis and experimental verification on thermal fluid phenomena in vapour chamber, Applied Thermal Engineering 26 (2006) 1669e1676. [8] Yimin Xuan, Yuping Hong, Qiang Li, Investigation on transient behaviors of flat plate heat pipes, Experimental Thermal and Fluid Science 28 (2004) 249e255. [9] J.M. Tournier, M.S. El-Genk, A heat pipe transient analysis model, International Journal of Heat and Mass Transfer 37 (1994) 753e762. [10] J.M. Tournier, M.S. El-Genk, Transient analysis of the start-up of a water heat pipe from a frozen state, Numerical Heat Transfer Part A-applications 28 (1995) 461e486. [11] C.B. Gu, L.C. Chow, K. Baker, Thermal performance of a flat plate heat pipe of dielectric liquid FC-70 for direct contact electronic cooling, in: Proceedings of the 7th International Heat Pipe (1996), pp. 872e876. [12] U. Vadakkan, S. Garimella, C. Sobhan, Characterization of the performance of flat heat pipes for electronics cooling, in: Proceedings of the International Mechanical Engineering Congress and Exposition, Orlando, FL (2000).
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Effect of evaporation section and condensation section length on thermal performance of flat plate heat pipe Shuangfeng Wang*, Jinjian Chen, Yanxin Hu, Wei Zhang Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2011 Accepted 31 March 2011 Available online 8 April 2011
Flat plate heat pipes (FPHPs) are one of the available technologies to deal with the high density electronic cooling problem due to their high thermal conductivity, reliability, and low weight penalty. A series of experiments were performed to investigate the effect of evaporation and condensation length on thermal performance of flat plate heat pipes. In the experiments, the FPHP had heat transfer length of 255 mm and width of 25 mm, and pure water was used as the working fluid. The results show that comparing to vapor chamber, the FPHP could realize long-distance heat transfer; comparing to the traditional heat pipe, the FPHP has large area contact with heat sources; the thermal resistance decreased and the heat transfer limit increased with the increase of evaporation section length; the FPHP would dry out at a lower heating power with the increase of condensation section length, which indicated that the heat transfer limit decreased, but the evaporator temperature also decreased; when the condensation section length approached to evaporation section length, the FPHP had a better thermal performance. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Flat plate heat pipes Evaporation section length Condensation section length Heat transfer limit Evaporator temperature
1. Introduction With increasing heat dissipation of electronic chips, the cooling problem becomes more and more challenging. Especially, the double chips or multi chips setting on one PCB (Printed Circuit Board), the heat flux is so high that the traditional heat pipe can not solve the heat dissipation problem. Then the high-capacity flat plate heat pipe (FPHP) has found applications in high-power chips, such as LED (Light Emitting Diode), CPUs (Central Processing Units) and GPUs (Graphic Processing Units). Flat plate heat pipes are one of the available technologies to deal with the high density electronic cooling problem due to their high thermal conductivity, reliability, and low weight penalty [1]. Experimental investigations on flat heat pipes have been conducted by some researchers. Sun et al. [2] presented an approximate method for calculating the effective length of a flat plate heat pipe when a strip heater is partially covering the evaporator section. The results show that for a specific width of the strip heater, the optimal position to achieve a minimum effective length is found when the heater is placed symmetrically at the centre of the evaporator section. In this position, a higher value of the capillary heat transport limit can be achieved as compared to the case where the heater is placed on one side of the evaporator section. Schmalhofer and Faghri [3] considered the block heater * Corresponding author. Tel.: þ86 20 22236929. E-mail address: [email protected] (S. Wang). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.03.037
covering half the circumferential length of a circular heat pipe. An approximate method was introduced to calculate the effective heat transport length that can be used to evaluate the maximum capillary heat transfer limit. The results show that the experimental capillary limits determined for a copper-water heat pipe in circumferential and block heating modes were compared with the analytical values calculated. The analytical predictions were within 25% of the experimental limits. Prasher [4] put forward a conduction based model to assess the heat transport capability of heat pipes and vapor chamber for various configurations. It was shown that the conduction model compared very well with experiments conducted in remote cooling mode (where the heat sink is at the end of the vapor chamber) and active cooling mode (where the heat sink is on the top of the vapor chamber). A flat rectangular heat pipe with heat transfer length of 130 mm was considered in his experiments. The paper also discusses how thermal designers as customers and heat pipe suppliers as vendors can interact for optimized design of heat pipes and vapor chamber. Tan et al. [5] introduced an analytical approach to study the liquid flow performances inside the wick structure of a flat plate heat pipe under different heat source conditions. The results with line, strip and discrete heating conditions on the heat pipe were presented. With the comparable results, the liquid flow model is competent to predict the liquid flow distribution in the wick structure of the heat pipe under different heating conditions. Koito et al. [6] investigated heat transfer characteristics of heat sinks with flat plate heat pipe. This study focused particularly on
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Nomenclature L Le Lc La Qh Qt RFPHP Te Tc ΔT
heat transfer length (mm) length of evaporation section (mm) length of condensation section (mm) length of adiabatic section (mm) heating power (W) heat transport by FPHP (W) thermal resistance of FPHP ( C/W) temperature of evaporator ( C) temperature of condenser ( C) temperature difference in outlet and inlet ( C)
Subscripts c condensation section e evaporation section l liquid v vapor
sintered heat pipe can realizes long-distance heat transfer, but it is cylindrical shape, when taking account of cooling compact electronic equipment, such kind of heat pipe has some disadvantages. For one thing, it is necessary to add a specific thermal interface to make a close contact with the heat source, because most objects to be cooled have a flat thermal contact surface. This added thermal interface increases the thermal resistance between the heat source and evaporator. For the other thing, it also increases the thickness of evaporator, which has a strict limit for compact enclosure of electronic devices. Thus, in order to solve this problem, we proposed a strip sintered FPHP. This FPHP has a large contact area and can contact with the chips directly, moreover, this FPHP can realizes long-distance heat transfer. In this paper, a strip sintered FPHP is proposed and tested. The main intension of this work is to investigate the effect of evaporation section length and condensation section length on thermal performance of the FPHP. As a result, the maximum value of heat transfer performance can be achieved. It is expected that the result will provide useful information for its installation in applications. 2. Experimental setup
the effect of the thermal resistance of the heat pipe for a changing heat flux input, temperature of the cooling air, and orientation angle. The useful data were presented for the improvement of the vapor chamber. Koito et al. [7] also carried out a thorough experimental and numerical analysis of heat transfer in flat plate heat pipes with a single axi-symmetrical heat source. It was shown that the small effective thermal conductivity of the saturated wick (8.32 W/mK) causes the largest temperature gradient and particularly in the wick region near the heat source. Xuan et al. [8] study performance and mechanism of a flat plate heat pipe in which a layer of sintered copper powder is applied to the heated surface of the heat pipe to enhance evaporation process, by means of both experimental and theoretical approaches. The results show that the porous sintered layer on the heated surface can enhance evaporation process and improve performance of the FPHP. The model and simulation method developed in this article have been verified by the experimental results. Tournier et al. [9] assumed an isothermal condition in the liquidevapor interface to develop a 2D heat pipe transient analysis model. The mass flow rate at the evaporator and condenser were assumed to be the same in this model. The numerical instabilities during the phase change were suppressed by employing a mushy-cell temperature of 2 108 K [10]. The calculated steady-state water vapor and wall axial temperature profiles and the transient power throughput are in good agreement with measurements. Gu et al. [11] designed an FPHP with ceramic wick and made experimental measurement in two cases: The condenser section is located on the top plate with the maximum heat flux 6 104 W/m2, and the condenser section is located at the bottom plate with the maximum heat flux 2.5 104 W/m2. Purdue [12] produced interesting results suggesting a sensitivity of the liquid coolant flow from condenser to evaporator that is dependent on the porosity of the wick. Their data show that high porosity wicks tend to degrade the liquid flow back toward the evaporator. In recsent years, there are some new requirements for heat dissipation of electronic chips in applications. For example, there are many electronic devices at one PCB, so that the space of PCB is limit, and then the heat sink cannot be installed on the top of electronic chips. At this moment, a heat pipe which can realizes long-distance heat transfer will be needed to solve this cooling problem. It’s required that the evaporator of heat pipe contact with the chips, and the heat sink is installed at the condenser where is outside of the PCB. In this way, the heat can transfer by the heat pipe from chips to the heat sink. Typically, the total length of traditional FPHP is short, so it’s not suitable for this cooling problem. In addition, the universal
A copper tube with an internal and external diameter of 18 mm and 19 mm, respectively, was used as the material in this experiment. The copper tube was pressed to thickness of 5 mm after sintering a layer of copper powder (the porosity of capillary was 50% 5%, and the wick thickness was 1 mm). Moreover, the mesh support structure with thickness of 2 mm was installed in middle of FPHP. The sealing was welded by the argon arc welding technology. The vacuum was established by using a vacuum pump, and then the working fluid was filled in the FPHP (the filling ratio was about 50%, the uncertainty was 5%). A long strip FPHP was successfully fabricated, as show in Fig. 1(a), the total length of FPHP is 255 mm the width is 25 mm. The experimental apparatus of FPHP is shown in Fig. 1(c). The operational orientation was horizontal heating mode, where the FPHP was set in the horizontal direction. It was because the engineering application most requires the FPHP using in horizontal direction. The heating block was a copper block which was heated with two heating rods. These heating rods were connected to the transformer, which supplied heating power by adjusting the current and voltage. The length of evaporation section was controlled by the length of heating block which width was 20 mm, and the length was 20 mm, 30 mm, 40 mm, 50 mm and 70 mm respectively (the uncertainty was 1 mm). The condensation section was cooled by water (25 0.1 C). The length of condensation section was controlled by a tank whose length could change as 20 mm, 40 mm, 50 mm, 60 mm, 80 mm and 100 mm respectively (the uncertainty was 1 mm), and the plexiglass is used as the material. The OMEGA K-type thermocouples (the uncertainty was 0.1 C) were installed to measure the wall temperature at different positions of FPHP. The detailed location of thermocouples was shown in Fig.1(b). There was insulation cotton packaged outside at adiabatic section in order to avoid heat loss. All tests were conducted at an ambient temperature of 25 1 C. Based on the application requirements, the FPHP must operate normally. Thus, when the heat transfer limit (dry-out) appeared, the experiment would be stopped. The thermal resistance of FPHPs was equal to temperature difference between condensation section and evaporation section divided by heating power. RFPHP ¼ (Te Tc)/Qh (RFPHP : thermal resistance of FPHP; Te : average temperature of T1, T2, T3 and T4 approximately instead of evaporator temperature; Tc: average temperature of T7, T8, T9 and T10 approximately instead of condensation section temperature; Qh: heating power). The contact resistance between heating sources and FPHP was reduced by the
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Fig. 1. Experimental setup.
thermal paste which has high coefficient of thermal conductivity. Because the FPHP was cooled by water, and the water directly contacted with the FPHP, so the thermal contact resistance between the FPHP and cooling sources very small or even negligible. There was insulation cotton packaged outside at heating block in order to avoid heat loss. Heat balance equation calculates as follow: Qh (heating power) ¼ Qt (heat transport by FPHP) þ Ql (heat loss). The heat transport by FPHP under the different heating power: Qt ¼ rwCwuwΔT,where, rw is the density of water, Cw is the specific heat, uw is volume flow rate (the uncertainty was 1%), ΔT is temperature difference in outlet and inlet (the uncertainty was 0.1 C which determined by the thermocouples). As the uncertainties of flow rate and temperature difference of the water were provided, the uncertainty of Qt could be calculated. 3. Phenomenon of dry out As shown in Fig. 2(a), when the evaporation section length was 50 mm, and condensation section length was 50 mm, the temperature of FPHP curve increased with the increased of the heating power. Moreover, the temperatures of evaporation section (T1, T2, T3 and T4) kept rising and without a stable trend at heating power of 160W, and the temperatures of condensation section (T9 and T10) kept dropping
slowly. This phenomenon was partial dry out, while the transfer limit appeared on FPHP. As can be seen in Fig. 3(c), the thermal resistance of FPHP decreased with the heating power increased, and the minimum thermal resistance reduced to 0.008 C/W at 140unit W, but the thermal resistance raised at 160unit W, which corresponded to the phenomenon of partial dry out. In order to analyze the phenomenon, the thermal resistance and heat transport curve was used to describe the thermal performance of FPHP. Comparing to the Fig. 2(a), the evaporation section temperatures sharply raise at the heating power of 120unit W, this phenomenon was dry out. As shown in Fig. 2(b), the temperature of FPHP curve raise with the heating power increased, at condensation section length of 100 mm. When the heating power of 120unit W, the temperatures of evaporation section (T1 and T2) sharply raised and the temperatures of condensation section (T9 and T10) reduced. While the heat transfer limit appeared on FPHP. 4. Results and discussion 4.1. Effect of evaporation section length In this experiment, several evaporation section lengths were tested at the same condensation section length. As can be seen in
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Fig. 3(a), as the heating power increased, the thermal resistance of FPHP decreased before the FPHP dried out. When Lc was 20 mm, Le was 20 mm and 30 mm, the thermal resistance of FHPH decreased before 180unit W, and then as the heating power increased to 200unit W, the thermal resistance increased, the FPHP dried out at 200unit W. When Le was 50 mm, the thermal resistance decreased before 200unit W, and then the FPHP dried out at 220unit W. These trends also appeared at other condensation section lengths, for example, the condensation section length at 40 mm (Le is 30 mm, 40 mm and 50 mm, respectively), as shown in Fig. 3(b); and condensation section length at 50 mm (Le is 30 mm, 50 mm and 70 mm, respectively) as shown in Fig. 3(c). It showed that there is an optimal point for FPHP operation. And with the evaporation section length increased, the thermal resistance decreased. It was because that at the same heating power, the heat flux increased with the evaporation section length decreased, such that the partial evaporation (evaporation occur in a portion of the evaporator) more intense, and then there was more vapor produced in evaporator, so the pressure difference between evaporation and condensation increased. Which was unbeneficial to assists the condensed working fluid to flow back to the evaporation section. Therefore, the temperature difference between evaporation and condensation increased, so the thermal resistance increased. On the contrary, when the evaporation section length increased, the heat flux decreased, so that the evaporation was not so intense, it was
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beneficial to assists the condensed working fluid to flow back to the evaporation section, so that the thermal resistance reduced. As can be seen in Fig. 4, when the condensation section length was 50 mm, the evaporation section length was changed to 30 mm, 50 mm and 70 mm, respectively. With the evaporation section
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These trends also appeared at other condensation section lengths, for example, the condensation section length at 20 mm (Le is 20 mm, 30 mm and 50 mm, respectively), as shown in Fig. 4(a); and the condensation section length at 40 mm (Le is 30 mm, 40 mm and 50 mm, respectively), as shown in Fig. 4(b). It was also revealed that the FPHP achieved the maximum heat transport capability (Qmax) of 132.2unit W at the heating power of 140W, when the length of evaporation section and condensation section both were 50 mm. Qmax 100% ）was up The efficiency of heat transfer （it defined as Qh to 94.4%, it showed the strip sintered flat plate heat pipe has a superior thermal performance. In conclusion, based on experimental observations and analysis, at the same condensation section length, with the increase of evaporation section length, the thermal resistance decreased. Besides, due to the thermal resistance decreased, the heat transfer limit increased.
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In this experiment, several condensation section lengths were tested at the same evaporation section length. For example, when the evaporation section length was 50 mm, the condensation section length was changed to 20 mm, 40 mm, 50 mm, 60 mm, 80 mm, and 100 mm, respectively. Fig. 5 shows the thermal resistance at different condensation section length. When the condensation section length was 100 mm, as the heating power increased, thermal resistance decreased and reached the minimum point at 100unit W. While the heating power increased to 120unit W, FPHP dried out and thermal resistance raised obviously. The same situation occurred in condensation section length at 80 mm. However, by changing condensation section length to 60 mm, the FPHP dried out at 140unit W. Moreover, FPHP dried out at 160unit W with condensation section length of 50 mm, while Lc equaled to 40 mm, FPHP dried out at 180unit W, and Lc equaled to 20 mm, FPHP dried out at 220unit W. It showed that at the same evaporation section length, decreasing condensation section length would make the FPHP dry out at a higher heating power. In other words, increasing condensation section length would make the FPHP dry out at a lower heating power, which indicated that the heat transfer limit decreased. It was related to the capillary limit. The FPHP with a bigger condensation section length (at high heating power) allows for vapor from evaporator to condense more rapidly. But due to the capillary limit, the condensate liquid could not flow rapidly back to the evaporator,
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length increased, the thermal resistance decreased, the heat transfer limit increased. Fig. 4(c) shows that at the evaporation section length of 30 mm and 50 mm, the heat transfer limit appeared at 160unit W. However, while evaporation section length increased to 70 mm, the heat transfer limit reached to 180unit W.
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and then there was less working liquid at evaporator, which could made the FPHP dry out. So that the FPHP with bigger condensation section length would dry out at a lower heating power. The maximum heat transport (Qmax) can be calculated, while the heat transfer limit appeared on FPHP. It’s better to solve the heat dissipation problem at high power with the Qmax greater. Based on the application requirements, the evaporator temperature of electronic chip is usually not higher than 70 C, such as high power LED cooling. Therefore, the FPHP can achieve a better overall performance (while the Qmax is bigger, at the same time, the evaporator temperature is lower). As shown in Fig. 6, the heat transport of FPHP decreased with the condensation section length increased. It’s worth noting, increasing the condensation section length could make the thermal resistance and the Qmax decreasing, but could make the FPHP operating at a lower temperature, as shown in Fig. 7, the evaporator temperature reduced with the condensation section length increased. Fig. 8 generalized the relationship between condensation section length, Qmax and Te. As can be seen in Fig. 8(a), when the evaporation section length was 50 mm, the condensation section length was 20 mm, FPHP had a bigger Qmax of 192.5unit W, but the Te closed to 100 C, which could not satisfy the electronic chip temperature requirements. Moreover, when the condensation section length was 80 mm and 100 mm, FPHP had a lower Te, but the Qmax was lower too, so that FPHP was easy to dry out and then
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could not satisfy the high power cooling requirements too. However, when the condensation section length within 40 mme60 mm, FPHP had a bigger Qmax (from 100unit W to 150unit W), and the evaporator temperature was lower (from 50 C to 60 C), such that the FPHP achieved a better overall performance. The same trend appeared in condition with evaporation section length of 30 mm and 40 mm, when the condensation section length approached to the evaporation section length, the FPHP had a bigger Qmax and a lower Te, as shown in Fig. 8(b) and Fig. 8(c).Thus
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we could find that FPHP achieved a better overall performance when the condensation section length approached to evaporation section length. Therefore, the condensation section length should be consistent with evaporation section length on the application. 5. Conclusions A series of experiments were performed to investigate the effect of the evaporation section and condensation section length on thermal performance of FPHPs. In the experiments, FPHPs has the heat transfer length (L) of 255 mm and the width of 25 mm. The evaporation section length was 20 mm, 30 mm, 40 mm, 50 mm and 70 mm respectively. The condensation section length was 20 mm, 40 mm, 50 mm, 60 mm, 80 mm and 100 mm, respectively. The FPHP were set to operate at horizontal heating mode. Several conclusions can be summarized as follows: (1) The FPHP could realize long-distance heat transfer and large area contact, so that the heat source with different sizes or multi heat sources can be installed on the FPHP. The total heat transfer length reaches 255 mm and the width is 25 mm. Moreover, the FPHP achieved the maximum heat transport capability of 132.2unit W (the heat transport efficiency reached 94.4%) at heating power of 140unit W, when the length of evaporation section and condensation section both were 50 mm. It shows that the FPHP has very good thermal performance. (2) The evaporation section length had significant effect on thermal performance of FPHP. At the same length of condensation section, when the evaporation section length increased, the thermal resistance of FPHP decreased (see Fig. 3), however, the heat transfer limit increased (see Fig. 4). (3) The condensation section length had significant effect on thermal performance of FPHP too. When at the same length of evaporation section, with the condensation section length increasing, the FPHP would dry out at a lower heating power (see Fig. 5), and the Qmax reduced (see Fig. 6), but the evaporator temperature also decreased (see Fig. 7). (4) When the condensation section length approached to evaporation section length, the FPHP had better overall performance
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(see Fig. 8). Therefore, the condensation section length should be consistent with evaporation section length in applications. Acknowledgements The authors deeply appreciate the financial support offered by NSFC (Granted No.50876033), National Fund of Guang Dong Province Joint Key Projects（U0834002）and Novark Technology INC. References [1] A. Faghri, Heat Pipe Science and Technology. Taylor and Francis, Washington, DC, 1995. [2] K.H. Sun, C.Y. Liu, K.C. Leong, The effective length of a flat plate heat pipe covered partially by a strip heater on the evaporator section, Heat Recovery Systems & CHP 15 (1995) 383e388. [3] J. Schmalhofer, A. Faghri, A study of circumferentially-heated and blockheated heat pipes I. Experimental analysis and generalized analytical prediction of capillary limits, International Journal of Heat and Mass Transfer 36 (1993) 201e212. [4] Ravi S. Prasher, A simplified conduction based modeling scheme for design sensitivity study of thermal solution utilizing heat pipe and vapor chamber technology, Journal of Electronic Packaging 125 (2003) 378e385. [5] B.K. Tan, T.N. Wong, K.T. Ooi, A study of liquid flow in a flat plate heat pipe under localized heating, International Journal of Thermal Sciences 49 (2010) 99e108. [6] Y. Koito, K. Motomatsu, H. Imura, M. Mochizuki, Y. Saito, Fundamental investigations on heat transfer characteristics of heat sinks with a vapour chamber, in: Proceedings of the 7th International Heat Pipe Symposium (2003), pp. 247e251. [7] Y. Koito, H. Imura, M. Mochizuki, Y. Saito, S. Torii, Numerical analysis and experimental verification on thermal fluid phenomena in vapour chamber, Applied Thermal Engineering 26 (2006) 1669e1676. [8] Yimin Xuan, Yuping Hong, Qiang Li, Investigation on transient behaviors of flat plate heat pipes, Experimental Thermal and Fluid Science 28 (2004) 249e255. [9] J.M. Tournier, M.S. El-Genk, A heat pipe transient analysis model, International Journal of Heat and Mass Transfer 37 (1994) 753e762. [10] J.M. Tournier, M.S. El-Genk, Transient analysis of the start-up of a water heat pipe from a frozen state, Numerical Heat Transfer Part A-applications 28 (1995) 461e486. [11] C.B. Gu, L.C. Chow, K. Baker, Thermal performance of a flat plate heat pipe of dielectric liquid FC-70 for direct contact electronic cooling, in: Proceedings of the 7th International Heat Pipe (1996), pp. 872e876. [12] U. Vadakkan, S. Garimella, C. Sobhan, Characterization of the performance of flat heat pipes for electronics cooling, in: Proceedings of the International Mechanical Engineering Congress and Exposition, Orlando, FL (2000).