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Heat transfer characteristics and LED heat sink application of aluminum plate oscillating heat pipes
Applied Thermal Engineering 31 (2011) 2221e2229
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Heat transfer characteristics and LED heat sink application of aluminum plate oscillating heat pipes Zirong Lin a, Shuangfeng Wang a, *, Jiepeng Huo a, Yanxin Hu a, Jinjian Chen a, Winston Zhang b, Eton Lee b a b
Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, South China University of Technology, Guangzhou 510640, China Novark Technology Inc, Shenzhen, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2010 Accepted 5 March 2011 Available online 21 March 2011
An experimental study was carried out to investigate the heat transfer characteristics of aluminum plate oscillating heat pipes (OHPs), which consisted of parallel and square channels. Sizes, different crosssections and different number of turns were considered. In the experiments, acetone was used as working fluid. The study on the effect of heating mode orientations, cooling conditions and internal structures had been done through visualization observation and thermal performance tests. The flow visualization showed that the aluminum plate OHPs can maintain the heat transfer characteristics of liquid and vapor slug oscillation as well as the conventional tubular OHPs. The flow pattern changed and OHPs’ thermal performance improved with the increase of heating power. The trend in one-way direction circulation of working fluid emerged. The tests showed that the gravity greatly influenced the thermal performance of plate OHPs. Increasing the cooling temperature decreased the thermal resistance of plate OHPs. Increasing the number of turns and area of channel cross-section could improve the heat transport capability of plate OHPs. A heat sink with a plate OHP was developed for LED (Light Emitting Diode) cooling, TDP (Thermal Design Power) of which was 64 W. The result showed that the temperature of the LED significantly decreased while being cooled by natural convection when a plate OHP was used in LED heat sink. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Aluminum plate oscillating heat pipes Heat transfer characteristics Flow visualization Heat transport capability LED cooling
1. Introduction Oscillating heat pipes (OHPs) have attracted the attention of many researchers as viable candidates for enhancing heat transfer through passive, two-phase heat transfer mechanism. OHP technology is predicted as one of the most promising solution for compact cooling due to its simple design, small size and excellent thermal performance. Generally, a conventional OHP consists of tubes/channels of capillary dimensions arranged in a serpentine manner and joined end to end as shown in Fig. 1(a). In fact, such a loop serpentine tubular structure is not beneficial for OHPs being used in a heat sink design assembly. For example, the combination of heat pipes and fins become complicated; a groove metal plate is required to be installed at the interface between the evaporation section and the heating section for improving the heat dissipation. With the aim of exploring potential applications of OHP technology, in 2002, Khandekar [1] et al. first designed the integral structure which considered OHP as a part of thermal spreader, and explained the operational characteristics and performance limits of * 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.2011.03.003
flat plate OHPs, where ethanol and water was used as working fluid. In 2007, they also carried out the experimental study of embedded structures in an aluminum substrate and estimated the effective thermal conductivity of OHPs as 2500 W/mK [2]. In 2006, Xu [3] et al. investigated the heat transfer characteristics of aluminum plate OHPs by using HFC-134a and Butane as working fluid. The lowest thermal resistance was 0.05 C/W. In 2007, Vassilev [4] et al. designed an OHP exchanger with flat plate evaporator and condenser. They found that the maximum heat throughput was 1400 W using water as working fluid. The performance of methanol was quite close to water, even able to work against gravity. In 2008, Yang [5] et al. presented an experimental study on two flat plate OHPs in a thermal spreader configuration. When ethanol was used as working fluid, the influence of various operating parameters was investigated and successful operation at all orientations with respect to gravity was achieved. In 2009, Fumoto [6] et al. used selfrewetting fluids to enhance the heat transport capability of a flat plate OHP constructed by aluminum tubes. Qu [7] et al. made a thermal performance comparison between the embedded circular and square capillary type. Quan [8] et al. studied the heat transfer characteristics of plate OHPs with different cross-section shapes. In summary, most of the researches focused on copper plate
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Fig. 1. Comparison of different OHP structures. (a) A loop serpentine tubular OHP. (b) An aluminum plate OHP with interconnected rectangular channels.
OHPs. Even the material and working fluid did not meet the high reliability requirement in some cases. In addition, a few samples cannot be applied in industry-specific problems directly for the lack of sufficient experimental data. According to the issues and reviews mentioned above, a novel OHP structure is developed as shown in Fig. 1(b). A series of parallel interconnected rectangular channels are manufactured as a meandering closed loop in a metal plate. Considering the cost, an aluminum plate was selected as the material of the thermal spreader as compared to copper. In addition, aluminum is more suitable for excavating micro-channels, as harder than copper. Such a closed loop structure of OHP built into an aluminum plate can reduce the contact resistance, simplify the manufacturing process, and make the size of OHPs become smaller. It is beneficial to apply OHPs to the electrics surface directly for compact cooling. In this study, a series of experiments, including flow visualization and thermal performance tests, were performed to investigate the heat transfer characteristics of the novel aluminum plate OHPs. Then, a heat sink with a plate OHP was developed for LED (64 W) cooling.
Fig. 2. Experimental apparatus of aluminum plate OHPs.
2. Experimental setup In order to understand the operation mechanism of aluminum plate OHPs, visualization experiments were carried out for internal flow observation. The test mode was vertical bottom heating. A piece of glass sheet covered on the aluminum plate with a series Table 1 Experimental parameters of aluminum plate OHPs. Spreader Open/closed Cross-section Cross-section Turns Aluminum plate No. circulation shape size (mm) size (mm) 1 2 3 4 5 6
closed closed closed closed closed closed
rectangle rectangle rectangle rectangle rectangle rectangle
1.5 2 2 1.5 2 1
1.5 2 2 1.5 2 1
4 6 10 11 20 33
120 120 120 250 180 180
30 4 60 4 80 4 60 4 120 4 120 4
Z. Lin et al. / Applied Thermal Engineering 31 (2011) 2221e2229
of parallel interconnected rectangular channels in a closed loop serpentine manner. When clamping the glass, the plastic washer was filled into the slit of edges to prevent leakages. The flow visualization experimental apparatus was shown in Fig. 2. The goal was to visualize the bubble formation, movement, and growth, which were expected to impact the OHP’s thermal performance. Based on the consideration of compatibility, acetone was used as working fluid. In the experiment, as shown in Fig. 2, the length of evaporation section was 20 mm. A copper block heater was used as the heat
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source attached to the bottom of plate OHPs. Heating power was varied by regulating different voltage and current through the heater. The condensation section (20 mm) was cooled by water module (25 0.05 C). In the liquid filling process, internal channels of the aluminum plate OHP was first exhausted and the working fluid was fully filled into the tube under the pressure difference. Liquid filling ratio (FR) was controlled around 50 5% by the second vacuum, which assisted in discharging the excess liquid. Temperature at different sections was measured using K-type thermocouples. The detailed location of thermocouples was shown
Fig. 3. Start-up observations (growth of bubbles in vertical mode, spreader No.2).
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in Fig. 2. T1 to T12 were installed to measure the average temperature of evaporation section, adiabatic section and condensation section at different heating powers. Besides the visualization experiments, several aluminum plate OHPs were fabricated for thermal performance tests. The details experimental parameters of aluminum plate OHPs were shown in Table 1. The test apparatus and process followed that mentioned above. The test would be stopped when the average temperature of evaporation section was over 100 C, usually the limiting operating temperature for electronic devices. This was also a consideration of the seal broken due to the larger vapor pressure of acetone. The thermal resistance of aluminum plate OHPs was used as a criterion for thermal performance comparison. It was assumed to be equal to temperature difference between condensation section and evaporation section divided by heating power. ROHP ¼ (Te Tc)/Qh (ROHP: thermal resistance of OHP; Te: average temperature of T1, T2, T3 and T4; Tc: average temperature of T9, T10, T11 and T12; Qh: heating power).
3. Flow visualization After the second vacuum for liquid filling control, the temperature of whole aluminum plate OHP would drop. Liquid and vapor slugs showed the random distribution between the parallel channels due to capillary action. The proportion of vapor slug in both side channels connected with the liquid injection tube, increased significantly due to more liquid loss by the second vacuum. Most liquid slugs located in the bottom of channels in vertical mode due to gravity effect. When heating power input was lower (10 W), bubbles would first emerge in both side channels at evaporation section temperature of around 30 C, as larger proportion of vapor slug resulting in low flow resistance. It was also noted that bubbles mainly grew up in sharp corner of the square cross-section. The record photos were shown in Fig. 3(A and B). As the heating power increased, more bubbles emerged and grew up in the sharp corner. The growth rate of bubbles increased. As shown in Fig. 3(C and D), a typical phenomenon was observed that the subsequent one caught up with the previous one at
Fig. 4. Thin film evaporation in the evaporation section (1 second interval per photo, vertical mode, spreader No.2).
Z. Lin et al. / Applied Thermal Engineering 31 (2011) 2221e2229
a higher jet speed. Several initial small spherical bubbles merged into a big vapor column. The vapor column would rise to the condensation section due to pressure difference. As mentioned above, the intense boiling of working fluid in both side channels caused pressure imbalance among all connected channels, at the same time, which resulted in the liquid and vapor slug intermittent oscillation. As shown in Fig. 3(E and F), the local oscillation in both side channels gradually shifted to continuous
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and interactive oscillation in all parallel channels, at evaporation section temperature from 45 C to 55 C. When the continuous and interactive oscillation became stable, as shown in Fig. 4, the intense evaporation made a large vapor area emerged in the evaporation section. A large number of bubbles grew in this vapor area. There was a thin film between the wall and the vapor slug, which caused high heat transfer efficiency [10,11]. According to the mark a, b, c, d, e and f, three bubbles grew up in the
Fig. 5. Bubbles operation through the condensation section (vertical mode, spreader No.2).
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Fig. 8. Effect of cooling conditions on the performance of the plate OHPs.
channels. The visualization observation found that the trend of one-way direction circulation occurred in spreader No.1 at 25 W, the same trend in spreader No.2 and No.3 at 30 W. In contrast, only the U-shaped up and down oscillation could be observed in Fig. 6. Comparison of temperature record between conventional tubular and plate OHPs. (a) Temperature record of plate OHPs with power steps (Spreader No.4). (b) Temperature record of conventional tubular OHPs with power steps [9].
channel in 5 seconds. The distance of a tracking bubble was about 10 mm. As shown in Fig. 5, the shrink and burst of bubbles in condensation section could be observed as a typical phenomenon. The condensed working fluid turned into liquid film and flowed back to the evaporation section with the help of gravity and inertia oscillation. The heat transfer of plate OHP was achieved just through this internal evaporation and condensation of working fluid. As the heating power increased, the running distance of vapor columns spraying from the evaporation section to the condensation section also increased. As shown in Fig. 5, when the spraying distance of vapor columns was more than the length of the plate OHP, the oscillation of vapor slugs crossed the condensation section. Then, the intermittent one-way direction circulation replaced the U-shaped up and down oscillation in the connecting
Fig. 7. Effect of orientations on the performance of the plate OHPs.
Fig. 9. Thermal performance comparison of different aluminum plate OHPs.
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spreader No.4 in the test range of heating power. The vapor column was able to be jetted up about 175 mm, 70% of the whole height. It proved that there was an effective heat transfer length of plate OHPs for efficient heat transfer through the circular flow in certain heating power input. As shown in Fig. 6(a), this was a typical operating temperature curve of plate OHPs. When heating power increased, there was temperature oscillation to prevent the temperature rise. The period of bubbles growth was from 10 W to 20 W. The period of vapor columns spraying was from 30 W to 40 W. At this time, the temperature of evaporation section nearly remained the same. This was considered as the plate OHP start-up and operating well. In conclusion, the results of visualization experiments found that the aluminum plate OHPs maintained the heat transfer characteristics of conventional tubular OHPs. Their heat transfer also mainly depended on the liquid and vapor slug oscillation. The difference was that the temperature oscillation was weakened under the temperature equalization effect of aluminum plate. A typical temperature oscillation of the conventional tubular OHPs was shown in Fig. 6(b). The comparison of the operating temperature curves in Fig. 6 proved that aluminum plate OHPs were more beneficial to electronic cooling. It was because electronic devices required a stable operating temperature.
Fig. 10. Effect of inclination angles on the performance of the plate OHPs.
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4. Effect of heating mode orientation The experimental results at different heating modes were shown in Fig. 7. In the experiment, the evaporator (a copper block with heating rods) and condenser (water-cooling module) were placed about 10 mm from both ends, and the evaporation and condensation section were set to 20 mm. The inlet of cooling water was 25 C. The data showed that the gravity influenced the thermal performance of plate OHPs. An aluminum plate OHP at vertical bottom heating mode exhibited the lowest thermal resistance, while the top heating mode had the worst performance. Thermal performance in horizontal and side heating mode were in the midrange due to gravity having a minimal effect. It was able to be explained by pressure difference distribution. When a plate OHP is operating, an approximately saturated pressure distribution of the working fluid varied along the meandering parallel channel. Gravity played an important role in these pressure distributions. If horizontal heating mode was considered as a baseline, the greatest pressure difference would appear in vertical bottom heating mode, while the lowest one would be in vertical top heating mode. As
Fig. 11. A LED heat sink design of aluminum plate OHPs.
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Table 2 Comparison on different assembly of LED heat sinks under natural convection. Inclination angle/operation mode
Base contacted with LED
Temperature of LED ( C)
Thermal resistance of base plate ( C/W)
Ambient temperature ( C)
Horizontal mode 30 60 Vertical mode Horizontal mode Vertical mode
spreader No.5 spreader No.5 spreader No.5 spreader No.5 aluminum plate aluminum plate
66.70 63.86 61.84 61.40 68.62 68.96
0.15 0.13 0.12 0.11 0.15 0.19
32 32 32 32 32 32
mentioned in flow visualization, the liquid and vapor slug oscillation was determined by pressure difference. Therefore, the thermal performance of plate OHPs was a function of the heating mode orientations. 5. Effect of cooling condition The effect of cooling condition on thermal performance of aluminum plate OHPs were shown in Fig. 8. It maintained the same experimental condition in Section 4, and varied the temperature of cooling water from 15 C to 35 C. The results found that the higher cooling temperature resulted in the lower thermal resistance at low heating power. The performance difference among the cooling conditions reduced as heating power increased. In addition, according to data recorded in Sections 4 and 5, the heat transport capability of aluminum plate OHPs was better than the aluminum plate in the same size regardless of the heating mode orientations. As shown in Fig. 8, when using vertical bottom heating mode, the thermal resistance of spreader No.2 was 0.18 C/W at 110 W. Compared with the same size aluminum plate in Fig. 7, the thermal resistance decreased by about 82.5%. 6. Effect of internal structure The effect of internal structure in aluminum plate OHPs was shown in Fig. 9. According to thermal performance comparison of the spreader No.1, No.2, and No.3, it was found that increasing the area of flow channels could improve the heat transport capability of an aluminum plate OHP, as flow resistance decreased. As can be seen in Fig. 9, even in horizontal heating mode, spreader No.3 still remained at a relatively low thermal resistance state. When the heat flux input increased to 43750 W/m2, the thermal resistance of spreader No.3 was 0.8 C/W. According to thermal performance comparison of the spreader No.2 and No.4, it was found that, as a major factor, length influenced the heat transport capability of aluminum plate OHPs. The greater length of OHP resulted in the worse heat transport capability. While judging from thermal performance comparison of the spreader No.1 and No.4, increasing the density of channels was able to reduce the adverse effect from the length increase. This was because increasing the number of turns was beneficial to enhance the instantaneous imbalance among the meandering parallel channels, which induced the oscillation of working fluid. 7. LED heat sink design with plate OHP According to the heat transfer characteristics of aluminum plate OHPs discussed in sections above, a unique heat sink with a plate OHP was developed for LED (Light Emitting Diode) cooling, TDP (Thermal Design Power) of which was 64 W. The heating area was about 120 120 mm with four LED chips. Based on the performance test in Section 6, the size of spreader No.5 and 6 with different cross-sections and different number of turns were
2 2 2 2 2 2
Thermal resistance of LED heat sink ( C/W) 0.54 0.50 0.47 0.46 0.57 0.58
designed to one and half times of spreader No.3. As the LED was installed at a certain inclination angle, the effect of inclination angles on plate OHPs first was investigated. As can be seen in Fig. 10, the highest thermal performance occurred at vertical bottom heating mode, decreasing continuously as the device was turned towards horizontal. However, the performance remained nearly comparable from vertical position to about 60 . As a result, spreader No.5 was adopted for LED heat sink design due to its lower thermal resistance. The LED heat sink design of aluminum plate OHP was conceived. As shown in Fig. 11, an area not in contact with the chips was reserved as the condensation section, according to the result in forced convection. It was used to maintain the operation pressure difference of OHPs. In addition, a mirror with a tilt angle was installed near the condensation section for gathering light. It also formed a sufficient space for the LED power and circuit layout. The thermal performance of LED heat sink with a plate OHP was tested and compared with one without a plate OHP under natural convection. The compared data of different assembly were shown in Table 2 when TDP of LED up to 64 W. The LED heat sinks with plate OHP had better performance than those using aluminum plates in the same size regardless of inclination angle variation, accord with the results in Fig. 10. The temperature of LED decreased by about 7 C as inclination angle increased to 60 . Therefore, aluminum plate OHPs used in the LED heat sink should be considered as an effective solution for natural convection cooling. 8. Conclusions Heat transfer characteristics of aluminum plate oscillating heat pipes (OHPs) consisting of parallel square channels were studied. The cross-sections and number of turns were experimentally studied in a parametric fashion. Using the best design found from the experiments, a LED heat sink design with a plate OHP was developed. Several conclusions were summarized as follows: (1) The flow visualization showed that the aluminum plate OHPs maintained the heat transfer characteristics of liquid and vapor slug oscillation similar to tubular OHPs. The flow pattern changed and OHPs’ thermal performance improved with the increase of heating power. The trend in one-way direction circulation of working fluid emerged. The difference was that the temperature oscillation of plate OHPs was weakened, which was more beneficial to electronic cooling than conventional tubular OHPs. (2) The tests showed that the gravity greatly influenced the thermal performance of plate OHPs. Increasing the cooling temperature decreased the thermal resistance of plate OHPs. Increasing the number of turns and area of the channel crosssection could improve the heat transport capability of plate OHPs. (3) The temperature of a LED Light decreased significantly when a plate OHP was applied in its heat sink. The heat sink with
Z. Lin et al. / Applied Thermal Engineering 31 (2011) 2221e2229
a plate OHP was considered as an effective solution for LED cooling. 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. Thanks for the constructive suggestions from A.A. Merrikh and G.Refai-Ahmed (Life Fellow ASME) in Advanced Micro Devices (AMD), Inc. References [1] S. Khandekar, M. Schneider, P. Schäfer, et al., Thermofluid dynamic study of flat-plate closed-loop pulsating heat pipes, Microscale Thermophysical Engineering 6 (4) (2002) 303e317. [2] S. Khandekar, G. Ashish. Embedded pulsating heat pipe radiators. 14th International Heat Pipe Conference (14th IHPC), Florianópolis, Brazil, April 22e27, 2007. [3] G.P. Xu, S.B Liang, M. Vogel, et al. Thermal characterization of pulsating heat pipes. 10th Intersociety Conference on Thermal and Thermomechanical Phenomena and Emerging Technologies in Electronic Systems (ITherm 2006), San Diego, CA, United states, May 30eJune 2, 2006, pp. 552e556.
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[4] M. Vassilev, Y. Avenas, C. Schaeffer, et al. Experimental study of a pulsating heat pipe with combined circular and square section channels. Industry Applications Conference 42nd Annual Meeting (IAS), New Orleans, LA, United states, September 2e27, 2007, pp. 1419e1425. [5] H.H. Yang, S. Khandekar, M. Groll, Performance characteristics of pulsating heat pipes as integral thermal spreaders, International Journal of Thermal Sciences 48 (2009) 815e824. [6] K. Fumoto, M. Kawaji, T. Kawanami. Effect of self-rewetting fluids on pulsating heat pipe thermal performance, Proceedings of the ASME 2009 2nd Micro/ Nanoscale Heat & Mass Transfer International Conference MNHMT2009, December 18e21, 2009, Shanghai, China, 2010, 3, pp. 381e387. [7] W. Qu, B. Yang. Performances of flat plate pulsating heat pipes, Proceedings of the ASME 2009 2nd Micro/Nanoscale Heat & Mass Transfer International Conference MNHMT2009 December 18-21, 2009, Shanghai, China, 2010, 3, pp. 403e410. [8] L. Quan, L. Jia. Experimental study on heat transfer characteristic of plate pulsating heat pipe, Proceedings of the ASME 2009 2nd Micro/Nanoscale Heat & Mass Transfer International Conference MNHMT2009 December 18e21, 2009, Shanghai, China, 2010, 3, pp. 361e366. [9] Z.R. Lin, S.F. Wang, J.J. Chen, et al., Experimental study on effective range of miniature oscillating heat pipes, Applied Thermal Engineering 31 (5) (2011) 880e886. [10] H.H. Sait, H.B. Ma, An experimental investigation of thin film evaporation, Nanoscale and Microscale Thermophysical Engineering 13 (4) (2009) 218e227. [11] H.B. Ma, P. Cheng, B. Borgmeyer, et al., Fluid flow and heat transfer in the evaporating thin film region, Microfluidics and Nanofluidics 3 (4) (2008) 237e243.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Heat transfer characteristics and LED heat sink application of aluminum plate oscillating heat pipes Zirong Lin a, Shuangfeng Wang a, *, Jiepeng Huo a, Yanxin Hu a, Jinjian Chen a, Winston Zhang b, Eton Lee b a b
Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, South China University of Technology, Guangzhou 510640, China Novark Technology Inc, Shenzhen, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2010 Accepted 5 March 2011 Available online 21 March 2011
An experimental study was carried out to investigate the heat transfer characteristics of aluminum plate oscillating heat pipes (OHPs), which consisted of parallel and square channels. Sizes, different crosssections and different number of turns were considered. In the experiments, acetone was used as working fluid. The study on the effect of heating mode orientations, cooling conditions and internal structures had been done through visualization observation and thermal performance tests. The flow visualization showed that the aluminum plate OHPs can maintain the heat transfer characteristics of liquid and vapor slug oscillation as well as the conventional tubular OHPs. The flow pattern changed and OHPs’ thermal performance improved with the increase of heating power. The trend in one-way direction circulation of working fluid emerged. The tests showed that the gravity greatly influenced the thermal performance of plate OHPs. Increasing the cooling temperature decreased the thermal resistance of plate OHPs. Increasing the number of turns and area of channel cross-section could improve the heat transport capability of plate OHPs. A heat sink with a plate OHP was developed for LED (Light Emitting Diode) cooling, TDP (Thermal Design Power) of which was 64 W. The result showed that the temperature of the LED significantly decreased while being cooled by natural convection when a plate OHP was used in LED heat sink. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Aluminum plate oscillating heat pipes Heat transfer characteristics Flow visualization Heat transport capability LED cooling
1. Introduction Oscillating heat pipes (OHPs) have attracted the attention of many researchers as viable candidates for enhancing heat transfer through passive, two-phase heat transfer mechanism. OHP technology is predicted as one of the most promising solution for compact cooling due to its simple design, small size and excellent thermal performance. Generally, a conventional OHP consists of tubes/channels of capillary dimensions arranged in a serpentine manner and joined end to end as shown in Fig. 1(a). In fact, such a loop serpentine tubular structure is not beneficial for OHPs being used in a heat sink design assembly. For example, the combination of heat pipes and fins become complicated; a groove metal plate is required to be installed at the interface between the evaporation section and the heating section for improving the heat dissipation. With the aim of exploring potential applications of OHP technology, in 2002, Khandekar [1] et al. first designed the integral structure which considered OHP as a part of thermal spreader, and explained the operational characteristics and performance limits of * 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.2011.03.003
flat plate OHPs, where ethanol and water was used as working fluid. In 2007, they also carried out the experimental study of embedded structures in an aluminum substrate and estimated the effective thermal conductivity of OHPs as 2500 W/mK [2]. In 2006, Xu [3] et al. investigated the heat transfer characteristics of aluminum plate OHPs by using HFC-134a and Butane as working fluid. The lowest thermal resistance was 0.05 C/W. In 2007, Vassilev [4] et al. designed an OHP exchanger with flat plate evaporator and condenser. They found that the maximum heat throughput was 1400 W using water as working fluid. The performance of methanol was quite close to water, even able to work against gravity. In 2008, Yang [5] et al. presented an experimental study on two flat plate OHPs in a thermal spreader configuration. When ethanol was used as working fluid, the influence of various operating parameters was investigated and successful operation at all orientations with respect to gravity was achieved. In 2009, Fumoto [6] et al. used selfrewetting fluids to enhance the heat transport capability of a flat plate OHP constructed by aluminum tubes. Qu [7] et al. made a thermal performance comparison between the embedded circular and square capillary type. Quan [8] et al. studied the heat transfer characteristics of plate OHPs with different cross-section shapes. In summary, most of the researches focused on copper plate
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Z. Lin et al. / Applied Thermal Engineering 31 (2011) 2221e2229
Fig. 1. Comparison of different OHP structures. (a) A loop serpentine tubular OHP. (b) An aluminum plate OHP with interconnected rectangular channels.
OHPs. Even the material and working fluid did not meet the high reliability requirement in some cases. In addition, a few samples cannot be applied in industry-specific problems directly for the lack of sufficient experimental data. According to the issues and reviews mentioned above, a novel OHP structure is developed as shown in Fig. 1(b). A series of parallel interconnected rectangular channels are manufactured as a meandering closed loop in a metal plate. Considering the cost, an aluminum plate was selected as the material of the thermal spreader as compared to copper. In addition, aluminum is more suitable for excavating micro-channels, as harder than copper. Such a closed loop structure of OHP built into an aluminum plate can reduce the contact resistance, simplify the manufacturing process, and make the size of OHPs become smaller. It is beneficial to apply OHPs to the electrics surface directly for compact cooling. In this study, a series of experiments, including flow visualization and thermal performance tests, were performed to investigate the heat transfer characteristics of the novel aluminum plate OHPs. Then, a heat sink with a plate OHP was developed for LED (64 W) cooling.
Fig. 2. Experimental apparatus of aluminum plate OHPs.
2. Experimental setup In order to understand the operation mechanism of aluminum plate OHPs, visualization experiments were carried out for internal flow observation. The test mode was vertical bottom heating. A piece of glass sheet covered on the aluminum plate with a series Table 1 Experimental parameters of aluminum plate OHPs. Spreader Open/closed Cross-section Cross-section Turns Aluminum plate No. circulation shape size (mm) size (mm) 1 2 3 4 5 6
closed closed closed closed closed closed
rectangle rectangle rectangle rectangle rectangle rectangle
1.5 2 2 1.5 2 1
1.5 2 2 1.5 2 1
4 6 10 11 20 33
120 120 120 250 180 180
30 4 60 4 80 4 60 4 120 4 120 4
Z. Lin et al. / Applied Thermal Engineering 31 (2011) 2221e2229
of parallel interconnected rectangular channels in a closed loop serpentine manner. When clamping the glass, the plastic washer was filled into the slit of edges to prevent leakages. The flow visualization experimental apparatus was shown in Fig. 2. The goal was to visualize the bubble formation, movement, and growth, which were expected to impact the OHP’s thermal performance. Based on the consideration of compatibility, acetone was used as working fluid. In the experiment, as shown in Fig. 2, the length of evaporation section was 20 mm. A copper block heater was used as the heat
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source attached to the bottom of plate OHPs. Heating power was varied by regulating different voltage and current through the heater. The condensation section (20 mm) was cooled by water module (25 0.05 C). In the liquid filling process, internal channels of the aluminum plate OHP was first exhausted and the working fluid was fully filled into the tube under the pressure difference. Liquid filling ratio (FR) was controlled around 50 5% by the second vacuum, which assisted in discharging the excess liquid. Temperature at different sections was measured using K-type thermocouples. The detailed location of thermocouples was shown
Fig. 3. Start-up observations (growth of bubbles in vertical mode, spreader No.2).
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in Fig. 2. T1 to T12 were installed to measure the average temperature of evaporation section, adiabatic section and condensation section at different heating powers. Besides the visualization experiments, several aluminum plate OHPs were fabricated for thermal performance tests. The details experimental parameters of aluminum plate OHPs were shown in Table 1. The test apparatus and process followed that mentioned above. The test would be stopped when the average temperature of evaporation section was over 100 C, usually the limiting operating temperature for electronic devices. This was also a consideration of the seal broken due to the larger vapor pressure of acetone. The thermal resistance of aluminum plate OHPs was used as a criterion for thermal performance comparison. It was assumed to be equal to temperature difference between condensation section and evaporation section divided by heating power. ROHP ¼ (Te Tc)/Qh (ROHP: thermal resistance of OHP; Te: average temperature of T1, T2, T3 and T4; Tc: average temperature of T9, T10, T11 and T12; Qh: heating power).
3. Flow visualization After the second vacuum for liquid filling control, the temperature of whole aluminum plate OHP would drop. Liquid and vapor slugs showed the random distribution between the parallel channels due to capillary action. The proportion of vapor slug in both side channels connected with the liquid injection tube, increased significantly due to more liquid loss by the second vacuum. Most liquid slugs located in the bottom of channels in vertical mode due to gravity effect. When heating power input was lower (10 W), bubbles would first emerge in both side channels at evaporation section temperature of around 30 C, as larger proportion of vapor slug resulting in low flow resistance. It was also noted that bubbles mainly grew up in sharp corner of the square cross-section. The record photos were shown in Fig. 3(A and B). As the heating power increased, more bubbles emerged and grew up in the sharp corner. The growth rate of bubbles increased. As shown in Fig. 3(C and D), a typical phenomenon was observed that the subsequent one caught up with the previous one at
Fig. 4. Thin film evaporation in the evaporation section (1 second interval per photo, vertical mode, spreader No.2).
Z. Lin et al. / Applied Thermal Engineering 31 (2011) 2221e2229
a higher jet speed. Several initial small spherical bubbles merged into a big vapor column. The vapor column would rise to the condensation section due to pressure difference. As mentioned above, the intense boiling of working fluid in both side channels caused pressure imbalance among all connected channels, at the same time, which resulted in the liquid and vapor slug intermittent oscillation. As shown in Fig. 3(E and F), the local oscillation in both side channels gradually shifted to continuous
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and interactive oscillation in all parallel channels, at evaporation section temperature from 45 C to 55 C. When the continuous and interactive oscillation became stable, as shown in Fig. 4, the intense evaporation made a large vapor area emerged in the evaporation section. A large number of bubbles grew in this vapor area. There was a thin film between the wall and the vapor slug, which caused high heat transfer efficiency [10,11]. According to the mark a, b, c, d, e and f, three bubbles grew up in the
Fig. 5. Bubbles operation through the condensation section (vertical mode, spreader No.2).
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Fig. 8. Effect of cooling conditions on the performance of the plate OHPs.
channels. The visualization observation found that the trend of one-way direction circulation occurred in spreader No.1 at 25 W, the same trend in spreader No.2 and No.3 at 30 W. In contrast, only the U-shaped up and down oscillation could be observed in Fig. 6. Comparison of temperature record between conventional tubular and plate OHPs. (a) Temperature record of plate OHPs with power steps (Spreader No.4). (b) Temperature record of conventional tubular OHPs with power steps [9].
channel in 5 seconds. The distance of a tracking bubble was about 10 mm. As shown in Fig. 5, the shrink and burst of bubbles in condensation section could be observed as a typical phenomenon. The condensed working fluid turned into liquid film and flowed back to the evaporation section with the help of gravity and inertia oscillation. The heat transfer of plate OHP was achieved just through this internal evaporation and condensation of working fluid. As the heating power increased, the running distance of vapor columns spraying from the evaporation section to the condensation section also increased. As shown in Fig. 5, when the spraying distance of vapor columns was more than the length of the plate OHP, the oscillation of vapor slugs crossed the condensation section. Then, the intermittent one-way direction circulation replaced the U-shaped up and down oscillation in the connecting
Fig. 7. Effect of orientations on the performance of the plate OHPs.
Fig. 9. Thermal performance comparison of different aluminum plate OHPs.
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spreader No.4 in the test range of heating power. The vapor column was able to be jetted up about 175 mm, 70% of the whole height. It proved that there was an effective heat transfer length of plate OHPs for efficient heat transfer through the circular flow in certain heating power input. As shown in Fig. 6(a), this was a typical operating temperature curve of plate OHPs. When heating power increased, there was temperature oscillation to prevent the temperature rise. The period of bubbles growth was from 10 W to 20 W. The period of vapor columns spraying was from 30 W to 40 W. At this time, the temperature of evaporation section nearly remained the same. This was considered as the plate OHP start-up and operating well. In conclusion, the results of visualization experiments found that the aluminum plate OHPs maintained the heat transfer characteristics of conventional tubular OHPs. Their heat transfer also mainly depended on the liquid and vapor slug oscillation. The difference was that the temperature oscillation was weakened under the temperature equalization effect of aluminum plate. A typical temperature oscillation of the conventional tubular OHPs was shown in Fig. 6(b). The comparison of the operating temperature curves in Fig. 6 proved that aluminum plate OHPs were more beneficial to electronic cooling. It was because electronic devices required a stable operating temperature.
Fig. 10. Effect of inclination angles on the performance of the plate OHPs.
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4. Effect of heating mode orientation The experimental results at different heating modes were shown in Fig. 7. In the experiment, the evaporator (a copper block with heating rods) and condenser (water-cooling module) were placed about 10 mm from both ends, and the evaporation and condensation section were set to 20 mm. The inlet of cooling water was 25 C. The data showed that the gravity influenced the thermal performance of plate OHPs. An aluminum plate OHP at vertical bottom heating mode exhibited the lowest thermal resistance, while the top heating mode had the worst performance. Thermal performance in horizontal and side heating mode were in the midrange due to gravity having a minimal effect. It was able to be explained by pressure difference distribution. When a plate OHP is operating, an approximately saturated pressure distribution of the working fluid varied along the meandering parallel channel. Gravity played an important role in these pressure distributions. If horizontal heating mode was considered as a baseline, the greatest pressure difference would appear in vertical bottom heating mode, while the lowest one would be in vertical top heating mode. As
Fig. 11. A LED heat sink design of aluminum plate OHPs.
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Table 2 Comparison on different assembly of LED heat sinks under natural convection. Inclination angle/operation mode
Base contacted with LED
Temperature of LED ( C)
Thermal resistance of base plate ( C/W)
Ambient temperature ( C)
Horizontal mode 30 60 Vertical mode Horizontal mode Vertical mode
spreader No.5 spreader No.5 spreader No.5 spreader No.5 aluminum plate aluminum plate
66.70 63.86 61.84 61.40 68.62 68.96
0.15 0.13 0.12 0.11 0.15 0.19
32 32 32 32 32 32
mentioned in flow visualization, the liquid and vapor slug oscillation was determined by pressure difference. Therefore, the thermal performance of plate OHPs was a function of the heating mode orientations. 5. Effect of cooling condition The effect of cooling condition on thermal performance of aluminum plate OHPs were shown in Fig. 8. It maintained the same experimental condition in Section 4, and varied the temperature of cooling water from 15 C to 35 C. The results found that the higher cooling temperature resulted in the lower thermal resistance at low heating power. The performance difference among the cooling conditions reduced as heating power increased. In addition, according to data recorded in Sections 4 and 5, the heat transport capability of aluminum plate OHPs was better than the aluminum plate in the same size regardless of the heating mode orientations. As shown in Fig. 8, when using vertical bottom heating mode, the thermal resistance of spreader No.2 was 0.18 C/W at 110 W. Compared with the same size aluminum plate in Fig. 7, the thermal resistance decreased by about 82.5%. 6. Effect of internal structure The effect of internal structure in aluminum plate OHPs was shown in Fig. 9. According to thermal performance comparison of the spreader No.1, No.2, and No.3, it was found that increasing the area of flow channels could improve the heat transport capability of an aluminum plate OHP, as flow resistance decreased. As can be seen in Fig. 9, even in horizontal heating mode, spreader No.3 still remained at a relatively low thermal resistance state. When the heat flux input increased to 43750 W/m2, the thermal resistance of spreader No.3 was 0.8 C/W. According to thermal performance comparison of the spreader No.2 and No.4, it was found that, as a major factor, length influenced the heat transport capability of aluminum plate OHPs. The greater length of OHP resulted in the worse heat transport capability. While judging from thermal performance comparison of the spreader No.1 and No.4, increasing the density of channels was able to reduce the adverse effect from the length increase. This was because increasing the number of turns was beneficial to enhance the instantaneous imbalance among the meandering parallel channels, which induced the oscillation of working fluid. 7. LED heat sink design with plate OHP According to the heat transfer characteristics of aluminum plate OHPs discussed in sections above, a unique heat sink with a plate OHP was developed for LED (Light Emitting Diode) cooling, TDP (Thermal Design Power) of which was 64 W. The heating area was about 120 120 mm with four LED chips. Based on the performance test in Section 6, the size of spreader No.5 and 6 with different cross-sections and different number of turns were
2 2 2 2 2 2
Thermal resistance of LED heat sink ( C/W) 0.54 0.50 0.47 0.46 0.57 0.58
designed to one and half times of spreader No.3. As the LED was installed at a certain inclination angle, the effect of inclination angles on plate OHPs first was investigated. As can be seen in Fig. 10, the highest thermal performance occurred at vertical bottom heating mode, decreasing continuously as the device was turned towards horizontal. However, the performance remained nearly comparable from vertical position to about 60 . As a result, spreader No.5 was adopted for LED heat sink design due to its lower thermal resistance. The LED heat sink design of aluminum plate OHP was conceived. As shown in Fig. 11, an area not in contact with the chips was reserved as the condensation section, according to the result in forced convection. It was used to maintain the operation pressure difference of OHPs. In addition, a mirror with a tilt angle was installed near the condensation section for gathering light. It also formed a sufficient space for the LED power and circuit layout. The thermal performance of LED heat sink with a plate OHP was tested and compared with one without a plate OHP under natural convection. The compared data of different assembly were shown in Table 2 when TDP of LED up to 64 W. The LED heat sinks with plate OHP had better performance than those using aluminum plates in the same size regardless of inclination angle variation, accord with the results in Fig. 10. The temperature of LED decreased by about 7 C as inclination angle increased to 60 . Therefore, aluminum plate OHPs used in the LED heat sink should be considered as an effective solution for natural convection cooling. 8. Conclusions Heat transfer characteristics of aluminum plate oscillating heat pipes (OHPs) consisting of parallel square channels were studied. The cross-sections and number of turns were experimentally studied in a parametric fashion. Using the best design found from the experiments, a LED heat sink design with a plate OHP was developed. Several conclusions were summarized as follows: (1) The flow visualization showed that the aluminum plate OHPs maintained the heat transfer characteristics of liquid and vapor slug oscillation similar to tubular OHPs. The flow pattern changed and OHPs’ thermal performance improved with the increase of heating power. The trend in one-way direction circulation of working fluid emerged. The difference was that the temperature oscillation of plate OHPs was weakened, which was more beneficial to electronic cooling than conventional tubular OHPs. (2) The tests showed that the gravity greatly influenced the thermal performance of plate OHPs. Increasing the cooling temperature decreased the thermal resistance of plate OHPs. Increasing the number of turns and area of the channel crosssection could improve the heat transport capability of plate OHPs. (3) The temperature of a LED Light decreased significantly when a plate OHP was applied in its heat sink. The heat sink with
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a plate OHP was considered as an effective solution for LED cooling. 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. Thanks for the constructive suggestions from A.A. Merrikh and G.Refai-Ahmed (Life Fellow ASME) in Advanced Micro Devices (AMD), Inc. References [1] S. Khandekar, M. Schneider, P. Schäfer, et al., Thermofluid dynamic study of flat-plate closed-loop pulsating heat pipes, Microscale Thermophysical Engineering 6 (4) (2002) 303e317. [2] S. Khandekar, G. Ashish. Embedded pulsating heat pipe radiators. 14th International Heat Pipe Conference (14th IHPC), Florianópolis, Brazil, April 22e27, 2007. [3] G.P. Xu, S.B Liang, M. Vogel, et al. Thermal characterization of pulsating heat pipes. 10th Intersociety Conference on Thermal and Thermomechanical Phenomena and Emerging Technologies in Electronic Systems (ITherm 2006), San Diego, CA, United states, May 30eJune 2, 2006, pp. 552e556.
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[4] M. Vassilev, Y. Avenas, C. Schaeffer, et al. Experimental study of a pulsating heat pipe with combined circular and square section channels. Industry Applications Conference 42nd Annual Meeting (IAS), New Orleans, LA, United states, September 2e27, 2007, pp. 1419e1425. [5] H.H. Yang, S. Khandekar, M. Groll, Performance characteristics of pulsating heat pipes as integral thermal spreaders, International Journal of Thermal Sciences 48 (2009) 815e824. [6] K. Fumoto, M. Kawaji, T. Kawanami. Effect of self-rewetting fluids on pulsating heat pipe thermal performance, Proceedings of the ASME 2009 2nd Micro/ Nanoscale Heat & Mass Transfer International Conference MNHMT2009, December 18e21, 2009, Shanghai, China, 2010, 3, pp. 381e387. [7] W. Qu, B. Yang. Performances of flat plate pulsating heat pipes, Proceedings of the ASME 2009 2nd Micro/Nanoscale Heat & Mass Transfer International Conference MNHMT2009 December 18-21, 2009, Shanghai, China, 2010, 3, pp. 403e410. [8] L. Quan, L. Jia. Experimental study on heat transfer characteristic of plate pulsating heat pipe, Proceedings of the ASME 2009 2nd Micro/Nanoscale Heat & Mass Transfer International Conference MNHMT2009 December 18e21, 2009, Shanghai, China, 2010, 3, pp. 361e366. [9] Z.R. Lin, S.F. Wang, J.J. Chen, et al., Experimental study on effective range of miniature oscillating heat pipes, Applied Thermal Engineering 31 (5) (2011) 880e886. [10] H.H. Sait, H.B. Ma, An experimental investigation of thin film evaporation, Nanoscale and Microscale Thermophysical Engineering 13 (4) (2009) 218e227. [11] H.B. Ma, P. Cheng, B. Borgmeyer, et al., Fluid flow and heat transfer in the evaporating thin film region, Microfluidics and Nanofluidics 3 (4) (2008) 237e243.