comparative study on heat transfer characteristics … study on heat transfer characteristics of...

Journal of Mechanical Science and Technology 25 (6) (2011) 1391~1398

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-011-0409-9

Comparative study on heat transfer characteristics of nanofluidic thermosyphon

and grooved heat pipe† Dong-Ryun Shin1, Seok-Ho Rhi2,*, Taek-Kyu Lim2 and Ju-Chan Jang2

1Department of Mechanical Engineering, Myeongji University, san 38-2 Namdong, Cheoin-gu, Yongin, Gyeonggido, 449-728, Korea 2School of Mechanical Engineering, Chungbuk National University, Cheongju, Chungbuk, 151-742, Korea

(Manuscript Received February 9, 2010; Revised December 4, 2010; Accepted March 11, 2011)

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Abstract The present study used TiO2-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 TiO2-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 TiO2-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 ther-mosyphon with 0.05% TiO2-nanoparticle concentration gave 1.40 times higher efficiency than that of pure water.

Keywords: Heat pipe; Thermosyphon; Nanofluids; Heat transfer ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Lee and Mital did the first comprehensive analysis of the wickless heat pipe (the so-called two-phase closed thermosy-phon (TS)) with conventional working fluid [1]. Many re-searchers [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 con-cern 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 op-eration limits [6, 7].

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 ther-mal resistance of heat pipes with various nanoparticle concen-trations. 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 signi-fies the main solution to improve the heat transfer perform-ance 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 conven-tional host fluid [8-15].

We believe Choi first called fluids with particles of nano-meter dimensions ‘nano-fluids’ [16]. The term nanofluid re-fers to a two-phase dispersed mixture constituted of ‘nanopar-ticles,’ 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

† This paper was recommended for publication in revised form by Associate Editor Ji Hwan Jeong

*Corresponding author. Tel.: +82 43 261 2444, Fax.: +82 43 263 2441 E-mail address: [email protected]

© KSME & Springer 2011

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 ef-fective thermal conductivity of the mixture increases by al-most 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 proper-ties 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 TiO2-nanofluid with different volume con-centrations.

Few researchers have investigated heat pipes with nanoflu-ids. Comparing the thermal resistance value between nanoflu-ids 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 ther-modynamic 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 pro-posed a dimensionless number related to comparisons with previous experimental and numerical studies.

The present study compared thermosyphon and grooved heat pipes with TiO2-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 flu-ids, 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 cool-ing 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 sys-tem used an electrical resistance heater powered by a DC

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)

Fig. 1. Experimental setup.

Fig. 2. Particle distribution in nanofluids.

D.-R. Shin et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1391~1398 1393

(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 -200~1370oC.

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 nan-ofluids as the working fluids were prepared to fill into the TS and GHP. Nanofluids used in this work were particle dis-persed fluids with TiO2-nano particles of a size range of 27-56 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 TiO2. The working fluid in a heat pipe has a significant effect on its per-formance. Thus, the present experiment considered the vari-ous conditions of fluids to measure the temperature distribu-tion 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 tem-peratures). Using the experimental uncertainty analysis, we determined the accumulated uncertainty error for thermal re-sistance 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 TiO2-nanofluids as a function of volume fraction (α) of nanoparticles. Deion-ized 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 TiO2-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% vol-ume fraction of TiO2 nanoparticles. Since experimental results have been reported on the relationship of thermal conductivity of nanofluids with several factors such as the stability of sus-pension 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 nan-ofluids. Further required experimental study on the compensa-tion 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 di-rectly affect the heat transfer performance of the system. The effect of the charged amount represents an important con-straint in the operation of a heat pipe. The present study de-fines the charging ratio of the working fluid inside a heat pipe as the ratio of the volume of working fluid at the ambient con-dition to the inside volume.

Imura et al. [2] suggested a useful dimensionless formula in terms of the average temperature of the top inside, bottom-inside, 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 mini-mum 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

Fig. 3. Comparison between experiments and theoretical models on thermal conductivity of nanofluids.

Fig. 4. Effect of quantity of working fluid.


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 TiO2. 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 0.083oC/W with 0.05%TiO2-nanofluid for the GHP; 0.4 to 0.13oC/W with 0.5% TiO2-nanofluid for the TS) compared with different

nanoparticle solutions. This occurs because the included nanoparticles can cause various phenomena such as particle collisions, large bubble formations induced from bubble nu-cleation, creating very smaller size bubbles, and producing a large number of bubbles. The results of the present study indi-cate 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 (∆Th-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 ∆Th-c. The inclination signifies the most important parameter in the system’s operation; in the case of 30°, the GHP shows lower ∆Th-c than the TS. We ob-served 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 TiO2 system.

Fig. 7 shows the thermal resistance of the system. We de-creased the thermal resistance while increasing the nanoparti-cle 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% TiO2-nanofluid shows higher thermal resis-tance. As Fig. 7 reveals, we obtained the lowest thermal resis-tance from the GHP with 0.05% TiO2-nanofluid. GHP with pure water showed a serious operational state. Eventually, the

Fig. 5. Thermal resistance on nanoparticle volume concentration.

(a) 30o

(b) 60o

(c) 90o

Fig. 6. Effect of heat pipe orientations on heat transfer performance.

Fig. 7. Thermal resistance variation on supplied power.


present system with TiO2-nanofluid as the working fluid shows a better performance than the system with water. The grooved heat pipe system shows a slightly lower thermal resis-tance. 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 TiO2-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 sys-tems (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 TiO2-volume concentration. The difference between these trials likely occurs due to the difference in nanoparticle vol-ume concentration. Increasing concentration leads to stable system operation. In the case of 0.05 and 0.1% TiO2-nanofluid, the GHP reached a critical heat flux, which sustains heat trans-fer ability. This can be explained with surface wettability [26].

Fig. 10 shows the post-nanofluid experiments. We per-formed a set of experiments to examine the boiling character-istics 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% TiO2-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 parti-cle 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 con-centration increased from 0 to 1% of TiO2. The temperature

(a) Q = 70 W

(b) Q = 100 W

Fig. 8. Temperature profile with different working fluids.

Fig. 9. Effect of transient temperature variation on different nanoparti-cle concentrations.

Fig. 10. Effect of post nanofluidic heat pipe with pure water on Q.


difference, ΔTh-c and temperature fluctuation between the entrance and the end of the condenser varied. In the case of the TS, ΔTh-c did not vary greatly and ΔTh-c increased with 0.05% nanofluid. When comparing the GHP with the TS, ΔTh-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 in-creased 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 in-creased heat flux, we observed that the heater surface tempera-ture 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 fluc-

tuation in both sections decreased with increasing heat flux, but the GHP with increased heat flux revealed a severe tem-perature 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 dif-ference (∆Th-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

Time, Sec

(a) Thermosyphon

Time, Sec

(b) Grooved heat pipe Fig. 13. Effect of nanoparticle concentration in TS.

Fig. 11. Working fluid variation along the heat pipe working cycle [21, 22].

Fig. 12. Temperature fluctuation along the heat pipe working cycle [21, 22].


of the 1% TiO2 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.

The heat pipe systems in the present study worked in the thermodynamic operation cycle and under different tempera-ture behaviors in the three sections.

Acknowledgment

This work was supported by the research grant of the Chungbuk National University in 2009.

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Dong-Ryun Shin is a chairman in the institute of Korea Filter Co., He received a Ph.D degree from the Myeongji Uni-versity, 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 Univer-sity 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 elec-tric vehicle battery cooling system.

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