Comparative study on heat transfer characteristics of nanofluidic thermosyphon and grooved heat pipe

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1 Journal of Mechanical Science and Technology 25 (6) (2011) 1391~ DOI /s Comparative study on heat transfer characteristics of nanofluidic thermosyphon and grooved heat pipe Dong-Ryun Shin 1, Seok-Ho Rhi 2,*, Taek-Kyu Lim 2 and Ju-Chan Jang 2 1 Department of Mechanical Engineering, Myeongji University, san 38-2 Namdong, Cheoin-gu, Yongin, Gyeonggido, , Korea 2 School of Mechanical Engineering, Chungbuk National University, Cheongju, Chungbuk, , Korea (Manuscript Received February 9, 2010; Revised December 4, 2010; Accepted March 11, 2011) Abstract The present study used TiO 2 -nanofluid with different volume ratios as the working fluids of a therrmosyphon and grooved heat pipe and investigated various parameters such as volume concentration of nanoparticles, orientation, heat flux, and cooling media. Further, the present study used nanofluids and dispersed TiO 2 -nanoparticles into pure water with each cross-blended concentration of 0.05%, 0.1%, 0.5%, and 1%. The authors observed the best heat transfer performance in the 0.05% concentration with thermosyphon. The present study presents the enhancement of heat transfer performance with TiO 2 -nanofluids, and fabricated a heat pipe from a straight stainless steel tube with an outer diameter and length of 10 and 500 mm, respectively. At the optimum condition for the pure refrigerant, the thermosyphon with 0.05% TiO 2 -nanoparticle concentration gave 1.40 times higher efficiency than that of pure water. Keywords: Heat pipe; Thermosyphon; Nanofluids; Heat transfer Introduction Lee and Mital did the first comprehensive analysis of the wickless heat pipe (the so-called two-phase closed thermosyphon (TS)) with conventional working fluid [1]. Many researchers [1-5] have carried out experiments or analytical studies to determine the effects of several related parameters such as the amount of working fluid, evaporator-condenser length ratio (L + ), mean operating pressure, heat flux, heat transfer coefficient, and type of working fluid to obtain the TS performance in various geometric variables such as diameter, thickness, length, and orientation. The heat pipe s main concern remains how much the heat the pipe can transfer from heat source to sink. The design of this most important factor must maximize the heat transfer ability with various influence parameters mentioned above for the thermosyphon. The heat pipe does not differ greatly from the thermosyphon, or the wickless heat pipe. Usually, in the heat pipe, the heat transfer limitation, due to the capillary driving force, represents the most crucial factor in improving heat pipe performance. Pumping pressure, friction loss toward length, and inclination angle influence the maximization of heat transfer among operation limits [6, 7]. This paper was recommended for publication in revised form by Associate Editor Ji Hwan Jeong * Corresponding author. Tel.: , Fax.: address: rhi@chungbuk.ac.kr KSME & Springer 2011 A few researchers [8-15] have investigated various nanofluids such as gold, silver, and silica in heat pipes. They tried to find improved thermal performance, in terms of thermal resistance of heat pipes with various nanoparticle concentrations. Tsai et al. [8] reported on the reduction of thermal resistance due to the resultant smaller bubble size. Ma et al. [9] reported the experimental results of oscillating heat pipe (OHP) with nanofluids to develop a high-performance cooling device. They reported that the temperature difference between the evaporator and the condenser could decrease from 40.9 to 24.3 for 80 W input power using diamond nanofluid. With no change in the structure of the heat pipe such as wick, container, orientation, or length, the working fluid signifies the main solution to improve the heat transfer performance of heat pipes. The right selection of working fluid can lead to a decrease in the thermal resistance of the heat pipe. Some studies have applied nanofluids, a new type of heat transfer fluid, as the working fluid for heat pipes. A nanofluid represents the suspension of nanosized particles in a conventional host fluid [8-15]. We believe Choi first called fluids with particles of nanometer dimensions nano-fluids [16]. The term nanofluid refers to a two-phase dispersed mixture constituted of nanoparticles, extremely fine metallic particles of 100 nm or less. The thermal properties of such a nanofluid appear much improved when compared with a conventional base fluid. In fact, some experimental data [16-19] show that even with a relatively low

2 1392 D.-R. Shin et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1391~1398 concentration of particles, i.e., from 1~5% in volume, the effective thermal conductivity of the mixture increases by almost 20% compared to that of the base fluid. Such an increase depends mainly on several factors such as the form and size of the particles and their concentration, and the thermal properties of the base-fluid, as well as those of the particles. The main advantage of nanofluids is to enhance the heat-transfer characteristics of the original fluid. The present study tested mainly water and TiO 2 -nanofluid with different volume concentrations. Few researchers have investigated heat pipes with nanofluids. Comparing the thermal resistance value between nanofluids and pure water revealed a reduction of 50-80% or more [10, 11], or similar, or worse results than distilled water [7]. Some research results mentioned that the critical heat flux (CHF) can increase when compared with pure water [14, 15]. The working fluid inside the heat pipe, a thermodynamic device, undergoes a thermodynamic operational cycle [20-22]. Khalkhali et al. [21] analyzed the working cycle of heat pipes using temperature-entropy diagrams. They developed a thermodynamic model of conventional cylindrical heat pipes based on the second law of thermodynamics. They also tried to investigate the effects of various heat pipe parameters on the entropy generation. Zuo et al. [22] reported that heat pipe dimensions must be thermally compatible with the heat pipe materials to establish the thermodynamic cycle. They proposed a dimensionless number related to comparisons with previous experimental and numerical studies. The present study compared thermosyphon and grooved heat pipes with TiO 2 -nanofluid in terms of particle volume concentrations, charged amounts, and orientation. 2. Experiments The present two-phase closed thermosyphon (TS) and grooved heat pipe (GHP) systems using nanofluids represent potential applications for future real industries. Therefore, it is very important to select the right component combination for optimum performance and reliability in terms of working fluids, charged amount, and orientation. The experimental apparatus, illustrated in Fig. 1, mainly consists of the main thermosyphon (TS), grooved heat pipe (GHP) assembly, the cooling system in the condenser section for wick and wickless heat pipes, the heat generation section, the charging system, and nanofluids as the working fluid. The physical characteristics of the main GHP and TS assembly used in the study are divided into three parts: the evaporation section with the evaporator, the transporting sections, and the condenser section with water jacket. The coolant for both heat pipes circulated through the cooling water jacket, where heat was removed from the condenser section by flowing water forced convection, and then to a constant temperature water circulation bath. We set the water bath to the experimental temperature and held it constant throughout the tests. The power supply and measurement system used an electrical resistance heater powered by a DC Fig. 1. Experimental setup. Fig. 2. Particle distribution in nanofluids. power transformer. As shown in Fig. 1, we manufactured the TS and GHP from stainless steel tubes with a 12 mm O.D. and a wall thickness of 1 mm. Figs. 1 and 2 show the TS and GHP design. We designed, machined, and extruded the grooves of the GHP with 1 mm in depth, 1 mm width at the top, and 1.3 mm at the bottom. Each groove has a taper shape starting from the groove tip. The total length of the test heat pipe was about 500 mm with the bend at the beginning. As shown in Fig. 1, the three working sections of the heat pipe each measure 167 mm long. From the top ends of the tubes, we made the water jacket from brass tubing with 30 mm O.D. The overall length of the condenser measured 167 mm. We made a temporary seal of the test heat pipe using a vacuum pump (PJ KODIVAC). To measure the temperature distribution over the length of the heat pipe, we used nine K-type thermocouples (Φ=0.25mm)

3 D.-R. Shin et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1391~ Fig. 3. Comparison between experiments and theoretical models on thermal conductivity of nanofluids. Fig. 4. Effect of quantity of working fluid. (Fig. 1). We specified the temperature accuracy induced from thermocouples as a maximum of ± 0.05% of readings for the K-type thermocouple at the range of -200~1370 o C. As shown in Fig. 1, a special heater was designed and manufactured. The wire resistance heater could supply up to 350 W of maximum power. The pure distilled water and nanofluids as the working fluids were prepared to fill into the TS and GHP. Nanofluids used in this work were particle dispersed fluids with TiO 2 -nano particles of a size range of nm, supplied by NanoANP Co, Korea. We used pure distilled water as the base liquid. Nanoparticles were dispersed into the pure water and the mixture was sonicated continuously for 16-20h in an ultrasonic bath (DaeRyun Science Inc., Korea). Fig. 2 shows the normal distribution of nanoparticles dispersed in the nanofluid with 2.0% volume concentration of TiO 2. The working fluid in a heat pipe has a significant effect on its performance. Thus, the present experiment considered the various conditions of fluids to measure the temperature distribution of the heat pipe, and to calculate the thermal resistance. In the present experimental study, we recorded the readings of the power and the temperatures using a regulated DC power supply (±0.03% of readings for voltage and current), and the MX-100 data acquisition system (±0.05% of readings for temperatures). Using the experimental uncertainty analysis, we determined the accumulated uncertainty error for thermal resistance as ±1.65%. The data analysis shown in this paper is based on average values from fluctuating temperatures. 3. Results and discussion Fig. 3 shows the thermal conductivities of TiO 2 -nanofluids as a function of volume fraction (α) of nanoparticles. Deionized water was used as the base fluid for nanofluids. k and ko in the figure represent thermal conductivity of the nanofluid and the base fluid, respectively. Fig. 3 also shows the slightly improved thermal conductivity of the present TiO 2 -nanofluid, but the theoretical calculations based on the known correlation from previous researchers, are overestimated. About 24% enhancement of thermal conductivity occurs with 1.0% volume fraction of TiO 2 nanoparticles. Since experimental results have been reported on the relationship of thermal conductivity of nanofluids with several factors such as the stability of suspension of nanoparticles, nanoparticle size, and viscosity of base fluids, the new theories should include the effects of those factors. We presume that the compensation of the effects of those factors determines the thermal conductivity of nanofluids. Further required experimental study on the compensation of the effects of those factors will provide insight into the mechanism of thermal transport in nanofluids. The quantity of the working fluid in a heat pipe would directly affect the heat transfer performance of the system. The effect of the charged amount represents an important constraint in the operation of a heat pipe. The present study defines the charging ratio of the working fluid inside a heat pipe as the ratio of the volume of working fluid at the ambient condition to the inside volume. Imura et al. [2] suggested a useful dimensionless formula in terms of the average temperature of the top inside, bottominside, and adiabatic wall and the critical heat flux. The present study defines the quantity of the working fluid inside a heat pipe as the ratio between the charged volume of working fluid inside the heat pipe and the heat pipe s total volume. As shown in Fig. 4, the optimum value of the filling charge ranges from 30% to 50% of the total volume. It was noticed from the experimental study that all other parameters have little or no effect. At the same time, there exists a minimum value of the filling charge which depends mainly on heat flux, the type of working fluid, and the internal volume of the system. Fig. 4 shows the best heat transfer performance with the TS and the GHP placed in a 32% charged amount of pure water. Therefore, we did the following experiments with a 32% charged amount. Fig. 5 shows the effect of nanofluids on thermal resistance with various volume concentrations of nanoparticles. Thermal

4 1394 D.-R. Shin et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1391~1398 Fig. 5. Thermal resistance on nanoparticle volume concentration. (a) 30 o (b) 60 o (c) 90 o Fig. 6. Effect of heat pipe orientations on heat transfer performance. resistance suddenly decreased to 1/3 compared with that of the pure water. For both TS and GHP, thermal resistance was the lowest at a 0.05% concentration of TiO 2. The figure further shows a large decrease of thermal resistance of the GHP and the TS with nanofluids as compared with pure water. The thermal resistance of the present heat pipes decreased to 85% for the GHP and 67% for the TS (from 0.56 to o C/W with 0.05%TiO 2 -nanofluid for the GHP; 0.4 to 0.13 o C/W with 0.5% TiO 2 -nanofluid for the TS) compared with different Fig. 7. Thermal resistance variation on supplied power. nanoparticle solutions. This occurs because the included nanoparticles can cause various phenomena such as particle collisions, large bubble formations induced from bubble nucleation, creating very smaller size bubbles, and producing a large number of bubbles. The results of the present study indicate the high thermal potential of nanofluids as a potential fluid to replace the conventional working fluids in heat pipes. Figs. 6(a)-(c) show the heat transfer performance in terms of temperature of the GHP and the TS on various working fluids including nanofluids with various heat pipe orientations (30 in Fig. 6(a), 60 in Fig. 6(b), and 90 in Fig. 6(c)). As the figures show, at the lower angle (inclined to horizontal state), the TS has a better performance than the GHP. Compared with the pure water system, the nanofluidic system shows about 50% reduced overall temperature difference ( T h-c ). When the orientation of the system changes to a vertical state (90 in Fig. 6(c)), the GHP shows a better performance than the TS. In any orientation, GHP with pure water shows the worst performance in terms of T h-c. The inclination signifies the most important parameter in the system s operation; in the case of 30, the GHP shows lower T h-c than the TS. We observed this trend in the 60 system. Fig. 6(c) does not show a big difference between the GHP and the TS in the vertical state (90 ). For the TS, the system shows a low temperature difference with 0.5 and 1%. Also, in the case of the GHP, we observed that the system with 1% shows the best heat transfer performance. Both systems with pure water show a higher temperature difference compared with the TiO 2 system. Fig. 7 shows the thermal resistance of the system. We decreased the thermal resistance while increasing the nanoparticle concentration. As Fig. 7 also shows, in the case of the low concentration (0.05 and 0.1%), the grooved heat pipe shows a better performance as compared with the thermosyphon. The GHP with 0.5% TiO 2 -nanofluid shows higher thermal resistance. As Fig. 7 reveals, we obtained the lowest thermal resistance from the GHP with 0.05% TiO 2 -nanofluid. GHP with pure water showed a serious operational state. Eventually, the

5 D.-R. Shin et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1391~ (a) Q = 70 W Fig. 10. Effect of post nanofluidic heat pipe with pure water on Q. (b) Q = 100 W Fig. 8. Temperature profile with different working fluids. Fig. 9. Effect of transient temperature variation on different nanoparticle concentrations. present system with TiO 2 -nanofluid as the working fluid shows a better performance than the system with water. The grooved heat pipe system shows a slightly lower thermal resistance. The thermal resistances of a GHP and TS containing pure water reached 0.5 /W for the TS and 0.6 /W for the GHP, respectively. As shown in Fig. 7, the thermal resistance of a heat pipe containing TiO 2 -nanofluid was much lower than that of pure water in various range of a heat flux. The thermal resistance of the TS and the GHP with nanofluids decreased to 30% that of pure water TS and GHP. Figs. 8(a) and (b) show the temperature profile along the system with 70 and 110 W. The GHP system was observed to operate in lower temperature conditions compared with the TS. Fig. 8 shows the temperature profile for the present two systems (GHP and TS) with different working fluids. As shown, the system temperatures with the nanofluid in each position were lower than the pure water. Fig. 9 shows transient temperature variation with different TiO 2 -volume concentration. The difference between these trials likely occurs due to the difference in nanoparticle volume concentration. Increasing concentration leads to stable system operation. In the case of 0.05 and 0.1% TiO 2 -nanofluid, the GHP reached a critical heat flux, which sustains heat transfer ability. This can be explained with surface wettability [26]. Fig. 10 shows the post-nanofluid experiments. We performed a set of experiments to examine the boiling characteristics as the saturation pressure moves toward ambient. As revealed in Fig. 10, the post-nanofluid experimental data using the GHP reveal that the post-nanofluid heat pipe performance resembles the results of the 1% TiO 2 -nanofluid system. The wall temperature differences of GHP and TS with nanofluids were lower than those of pure water-filled heat pipes. The reason for the heat pipe thermal enhancement for the post GHP and TS can be explained with the particle deposition on the surface by higher wettability. This particle deposition can create a large number of nucleates, which can improve the bubble formation rate [26]. Figs. 11 and 12 show that in heat pipe operation cycles, the working state of the heat pipe changes continuously. As shown in Fig. 11, nanofluid was deposited in the heat pipe inner surface and the working fluid after experiments looks like clear water close to pure water. This means that the particle volume concentration of the working fluid differs from its initial concentration state. Also, the heat pipe s working fluid shows different working situation in three different working sections of heat pipe. Fig. 13 shows the effect of particle volume concentration in the TS and the GHP on system stability. As shown, 110 Watts were supplied into the TS and the GHP and the volume concentration increased from 0 to 1% of TiO 2. The temperature

6 1396 D.-R. Shin et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1391~1398 (a) Thermosyphon Fig. 11. Working fluid variation along the heat pipe working cycle [21, 22]. (b) Grooved heat pipe Fig. 13. Effect of nanoparticle concentration in TS. Fig. 12. Temperature fluctuation along the heat pipe working cycle [21, 22]. difference, ΔT h-c and temperature fluctuation between the entrance and the end of the condenser varied. In the case of the TS, ΔT h-c did not vary greatly and ΔT h-c increased with 0.05% nanofluid. When comparing the GHP with the TS, ΔT h-c increased with increasing volume concentration, and the amplitude and wave period of temperature fluctuation rose. In the evaporator section, three thermocouples were installed at the exit, middle, and bottom positions of the evaporator. As Fig. 13 shows, the temperature difference of the evaporator varied greatly with large fluctuations and differences. With the increasing concentration, the temperature difference also increased and showed chaotic variation in 0.1%. Also, the evaporator temperatures with the GHP varied with the large wave period, and the temperature difference increased. Fig. 14 shows the effect of supplied heat flux. With increased heat flux, we observed that the heater surface temperature decreased, but the system stability increased only slightly. In the operating cycle, in the evaporator section, the particle motion related to temperature behavior was stabilized with increasing heat flux. In the case of TS, the temperature fluctuation in both sections decreased with increasing heat flux, but the GHP with increased heat flux revealed a severe temperature fluctuation in the condenser section. We attribute this possibly to the wick structure to prevent particle motion in the reciprocal working cycle from the evaporator to the condenser derived by capillary force. 4. Conclusions The present study investigated the thermal enhancement of thermosyphon and heat pipe performance using TiO2- nanofluid as the working fluid. The results of the performance test of this comparative study and concluding remarks follow. At a lower angle, the TS shows better performance than the GHP. Compared with the pure water system, the nanofluidic system shows about a 50% reduced overall temperature difference ( T h-c ). Thermal resistance decreased with increased nanoparticle concentration. The grooved heat-pipe system shows slightly lower thermal resistance. The thermal resistances of TS and GHP with nanofluids decreased to 30% of pure water TS and GHP. The system temperatures with nanofluid in each position along the heat pipes were lower than those of the pure water. The post-nanofluid experimental data with GHP show that the post-nanofluid heat pipe performance resembles the results

7 D.-R. Shin et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1391~ The heat pipe systems in the present study worked in the thermodynamic operation cycle and under different temperature behaviors in the three sections. Acknowledgment This work was supported by the research grant of the Chungbuk National University in References (a) Thermosyphon (b) Grooved heat pipe Fig. 14. Effect of Q with TS & GHP. of the 1% TiO 2 nanofluid system. In this investigation, the thermal performance enhancement of wick and wickless heat pipe varied with driving parameters. This signifies the attractiveness of nanofluids as a cooling or energy transfer fluid for devices with high energy density. [1] Y. Lee and U. Mital, A two-phase closed thermosyphon, Int. J. of Heat and Mass Transfer, 15 (1972) [2] H. Imura, K. Sasaguchi and H. Kozai, Critical heat flux in a two phase closed thermosyphon, Int. J. of Heat Transfer, 26 (8) (1993) [3] T. Ma, X. Liu and J. Wu, Flow pattern and operation limits in two-phase closed thermosyphon, Proc. of 6 th IHPC, Grenoble (1987) [4] K. T. Feldman and R. Srinivasn, Investigation of heat transfer limits in two-phase closed thermosyphon, Proc. of 5 th IHPC, Tsukuba (1984) [5] G. Bartsch and J. Unk, A contribution to calculating the optimum quantity for filling a closed two-phase thermosyphon, Proc. of 6 th IHPC, Grenoble (1987) [6] Y. Lee and A. Bedrossian, The characteristics of heat exchangers using heat pipes or thermosyphon, Int. J. of Heat and Mass Transfer, 21 (1978) [7] J. C. Jang, S. H. Rhi and C. G. Lee, Heat transfer characteristics on toroidal convection loop with nanofluids, KSME Journal B, 33 (2009) [8] C. Y. Tsai, H. T. Chien, P. P. Ding, B. Chan, T. Y. Luh, P. H. Chen, Effect of structural character of gold nanoparticles in nanofluid on heat pipe thermal performance, Material Letters, 58 (2004) [9] H. B. Ma, C. Wilson, B. Borgmeyer, K. Park, Q. Yu, S. U. S. Choi and M. Tirumala, Effect of nanofluid on the heat transport capability in an oscillating heat pipe, Applied Physics Letters, 88 (14) (2006) [10] C. Y. Tsai, H. T. Chien, B. Chan, P. H. Chen, P. P. Ding and T. Y. Luh, Effect of structural character of gold nanoparticles in nanofluid on heat pipe thermal performance, Materials Letters, 58 (2004) [11] S. W. Kang, W. C. Wei, S. H. Tsai and S. Y. Yang, Experimental investigation of silver nano-fluid on heat pipe thermal performance, Applied Thermal Engineering, 26 (2006) [12] S. H. Rhi, K. Cha, K.W. Lee and Y. Lee, A two-phase loop thermosyphons with nanofluids, Proceedings of the 13th International Heat Pipe Conference, [13] H. B. Ma, C. Wilson, B. Borgmeyer, K. Park, Q. Yu, S. U. S. Choi and M. Tirumala, An experimental investigation of heat transport capability in a nanofluid oscillating heat pipe, Journal of Heat Transfer, 128 (2006) [14] S. M. You, J. H. Kim and K. H. Kim, Effect of nanoparticles on critical heat flux of water in pool boiling heat

8 1398 D.-R. Shin et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1391~1398 transfer, Applied Physics Letters, 83 (16) (2003) [15] P. Vassallo, R. Kumar and S. D Amico, Pool boiling heat transfer experiments in silica water nano-fluids, International Journal of Heat and Mass Transfer, 47 (2004) [16] S. Lee, S. U. S. Choi, J. A. Eastman and S. Lee, Measuring thermal conductivity of fluids containing oxide nanoparticles, Transaction of ASME, 121 (1999) [17] S. U. S. Choi, X. Wang and W. Xu, Thermal conductivity of nano-particle-fluid mixture, Journal of Thermophysics and Heat Transfer, 13 (4) (1999) [18] H. E. Patel, S. K. Das, T. Sundararajan, A. S. Nair, B. George and T. Pradeep, Thermal conductivities of naked and monolayer protected metal nano-particles based nano-fluids: manifestation of anomalous enhancement and chemical effects, Applied Physics Letters, 83 (14) (2003) [19] Y. Xuan and Q. Li, Heat transfer enhancement of nanofluids, International Journal of Heat and Fluid Flow (2000) [20] K. C. Cheng, Some observations on carnot cycle as the genesis of the heat pipe and thermosyphon, Int. Journal of Mechanical Engineering Education, 28 (1) (2000) [21] Z. J. Zuo and A. Faghri, A network thermodynamic analysis of the heat Pipe, Int. J. Heat Mass Transfer, 41 (11) (1998) [22] H. Khalkhali, A. Faghri and Z. J. Zuo, Entropy generation in a heat pipe system, Applied Thermal Engineering, 19 (1999) [23] J. C. Maxwell, A treatise on electricity and magnetism, oxford university press, Cambridge, UK, 2 (1904) 435. [24] R. L. Hamilton and O. K. Crosser, Thermal conductivity of heterogeneous two component systems, I & EC Fundamentals, 2 (1962) 187. [25] D. J. Jeffery, Proc. Roy. Soc. London, Ser. A, 335 (1973) 355. [26] S. J. Kim, I. C. Bang, J. Buongiorno, L. W. and Hub, Surface wettability change during pool boiling of nanofluids and its effect on critical heat flux, Int. J. of Heat and Mass Transfer, 50 (19-20) (2007) Dong-Ryun Shin is a chairman in the institute of Korea Filter Co., He received a Ph.D degree from the Myeongji University, Korea. His research interests include heat pipes, heat exchangers and, thermal design, automotive engineering. Seok-Ho Rhi is an Associate Professor in Chungbuk National University, He received a Ph.D degree from the University of Ottawa, Canada. His interests include heat pipes, heat exchangers and, thermoelectric modules. Taek-Kyu Lim is a graduate student in the School of Mechanical Engineering, Chungbuk National University. He is working on heat pipe systems, CFD and heat exchangers. Ju-Chan Jang is a graduate student in the School of Mechanical Engineering, Chungbuk National University. He is working on heat pipe systems, and electric vehicle battery cooling system.