Effect of nanoparticles in nanofluid on thermal performance in a miniaturethermosyphonZhen Hua Liu, Xue Fei Yang, and Guang Liang Guo Citation: Journal of Applied Physics 102, 013526 (2007); doi: 10.1063/1.2748348 View online: http://dx.doi.org/10.1063/1.2748348 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/102/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effects of alignment, pH, surfactant, and solvent on heat transfer nanofluids containing Fe2O3 and CuOnanoparticles J. Appl. Phys. 111, 064308 (2012); 10.1063/1.3694676 Effects of nanoparticle layering on nanofluid and base fluid pool boiling heat transfer from a horizontal surfaceunder atmospheric pressure J. Appl. Phys. 107, 114302 (2010); 10.1063/1.3342584 The interface effect of carbon nanotube suspension on the thermal performance of a two-phase closedthermosyphon J. Appl. Phys. 100, 104909 (2006); 10.1063/1.2357705 Effects of nanoparticle deposition on surface wettability influencing boiling heat transfer in nanofluids Appl. Phys. Lett. 89, 153107 (2006); 10.1063/1.2360892 Effect of nanoparticles on critical heat flux of water in pool boiling heat transfer Appl. Phys. Lett. 83, 3374 (2003); 10.1063/1.1619206

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

147.226.7.162 On: Wed, 19 Nov 2014 15:35:26

Effect of nanoparticles in nanofluid on thermal performancein a miniature thermosyphon

Zhen Hua Liu,a� Xue Fei Yang, and Guang Liang GuoSchool of Mechanical Engineering, Shanghai Jiaotong University, Shanghai 200240,People’s Republic of China

�Received 13 February 2007; accepted 28 April 2007; published online 12 July 2007�

An experiment was performed to investigate the effect of nanoparticles in the nanofluid on thethermal performance in a miniature thermosyphon. The nanofluids consisted of de-ionized water andCuO nanoparticles having an average size of 30 nm. The experimental results show that thewater-CuO nanofluids can greatly enhance the boiling heat transfer performance of the evaporatorin thermosyphon compared with that using water at subatmospheric pressure conditions. A muchlower and more uniform wall temperature of the thermosyphon can be obtained by substituting thenanofluids for water. Boiling heat transfer coefficients and the critical heat flux �CHF� of thenanofluids in the evaporator of the thermosyphon have significant increase compared with those ofde-ionized water. There was an optimal mass concentration which was estimated to be 1.0 wt % toachieve the maximum heat transfer performance. Operating pressure has very remarkable influenceson both the heat transfer coefficients and the CHF of nanofluids, which greatly increase with thedecrease of the test pressure. The heat transfer coefficient and the CHF can increase, respectively,about 160% and 120% at the pressure of 7.45 kPa compared with those of water. The experimentalstudy confirmed that the heat transfer performance of the miniature thermosyphon can evidently bestrengthened by using water-CuO nanofluids. © 2007 American Institute of Physics.�DOI: 10.1063/1.2748348�

I. INTRODUCTION

The past decade or more has witnessed a considerablefocus on exploring the heat transfer performances of nano-fluids which are suspensions consisting of base fluids andsolid particles with their sizes smaller than 100 nm. Most ofthe researches have demonstrated the potential of the nano-fluid in the heat transfer enhancement and the nanofluid ispredicted as an alternative working liquid in the heat transferengineering which can meet the challenge of increasing de-mand of heat removal.

The base fluids used are primarily the traditional fluidssuch as water, engine oil, and ethylene glycol. Nanoparticlescommonly used include metals such as Ag, Al, Au, Cu, andFe, oxides such as Al2O3, CuO, SiO2, and TiO2, and othermaterials such as SiC, carbon nanotubes, and diamond nano-particles. Besides the above two components, a low concen-tration of dispersants are used sometimes to help form a uni-form and stable suspension. Generally two methods are usedfor the preparation of nanofluids including the mixingmethod and the gas condensation method. The mixingmethod is to disperse nanoparticles directly into the basefluids while the gas condensation method is to use inert gasesto cool the vapor of nanoparticles which then drop into thebase fluids.

The first observation of the anomalous thermal charac-teristics of the suspension by suspending nanosized particlesinto traditional fluids can date back to 1993 reported by Ma-suda et al.1 It is Choi,2 however, who first used “nanofluid”

to define such suspension. Since then, researchers have at-tached great importance to the heat transfer performances ofnanofluids such as the forced convective heat transfer in thetubes and the pool boiling heat transfer on the plat surface.

Nanofluid �nanoparticle suspension�, as a kind of func-tional fluid, has many unique characteristics. It is an innova-tive research to use nanofluid technology in traditional ther-mal engineering field. Choi et al.,3,4 Lee et al.,5 and Das etal.6 have reported that the stable nanofluids have potentialhigh effective thermal conductivity. So far, the studies ofnanofluids on the thermal engineering mainly focus on thesingle phase convective heat transfer of nanofluids flowing intubes to enhance the forced convective heat transfer by useof increased effective thermal conductivity.7,8 In recent years,some studies on phase-change heat transfer of nanofluidshave been reported, but these studies are limited and almostfocused on pool boiling heat transfer at atmosphericpressures.9–13

On the other hand, miniature heat pipe technology risesas a heat transfer technology accompanied by the develop-ment of electronics, communication, and computing tech-nologies. These years, the exponential growth of these tech-nologies and their devices through miniaturization and anenhanced rate of operation and storage of data have broughtabout serious problems in the thermal management of thesedevices. Electronic devices allow normal working tempera-ture less than 80 °C, in general. If the temperature goes be-yond this limit, the capability of their components will dropsignificantly; even the devices cannot work normally.

Enlightened by the enhanced heat transfer of nanofluids,some researchers applied nanofluids in heat pipes to enhance

a�Author to whom correspondence should be addressed; FAX: 0086-21-62933086; electronic mail: liuzhenh@sjtu.edu.cn

JOURNAL OF APPLIED PHYSICS 102, 013526 �2007�

0021-8979/2007/102�1�/013526/8/$23.00 © 2007 American Institute of Physics102, 013526-1

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

147.226.7.162 On: Wed, 19 Nov 2014 15:35:26

their heat transfer performance. The little data we have col-lected draw an illegible picture for us to understand its heattransfer behavior.

Xue and Fan14 investigated the effect of carbon nanotubesuspension on the heat transfer performance of a thermosy-phon �gravity assisted heat pipe�. In comparison with thebase fluid of de-ionized water, the thermosyphon filled withcarbon nanotube suspension had a higher evaporator walltemperature and thermal resistance of thermosyphon. There-fore, the addition of carbon nanotubes deteriorated the heattransfer performance of the thermosyphon.

Tsai et al.15 measured the heat transfer performance ofthe heat pipe with an aqueous solution of various sized goldnanoparticles. The heat pipe was a vertical copper tube withmesh screen distributed inside. Contrary to the results pre-sented by Xue and Fan,14 there was a significant reduction inthe thermal resistance of the heat pipe with gold-de-ionizedwater suspension. Smaller and well-dispersed nanoparticleswere simultaneously recommended for a better heat transferperformance.

The enhancement of heat transfer performance but in anoscillation heat pipe was also observed by Ma and Fang16

with diamond nanoparticles added directly into the high per-formance liquid chromatography-grade water. The nanofluidsignificantly enhanced the heat transport capability in theoscillation heat pipe.

Silver nanoparticles were employed in a grooved circu-lar heat pipe by Shung and Kang.17 The measured resultsalso showed that the thermal resistances of the heat pipedecreased as the silver nanoparticle size and concentrationincreased, in the range of the input power of 30–60 W.

All of studies mentioned above simulated actual heatpipe operating conditions including a fixed condensing con-dition, while the operating pressure changed unceasinglywith the change of the input power in a test run. Therefore,these results are almost the qualitative comparisons of thewall temperatures and the heat resistances between the nano-fluids and water at some special condensing conditions andgeometric conditions. No fundamental data on the thermalperformances of the heat pipes at a fixed operating pressurewere proposed.

This work aimed at a more fundamental understandingof the application of nanofluid in a miniature thermosyphon�gravity assisted heat pipe�. CuO-water nanofluid was usedas the working fluid. The experiments were carried out atsome fixed subatmospheric pressures. The study focused onthe boiling heat transfer performance of the evaporator sec-tion because the main heat resistance in thermosyphon isfrom the evaporator. A better heat transfer performance in theevaporator was found by substituting nanofluid for de-ionized water. The effects of the mass concentration of thenanoparticles in the nanofluids and the system operatingpressure on the heat transfer performance of the evaporatorwere investigated and discussed. The experimental resultsare useful for designing a miniature thermosyphon usingnanofluids as working liquids.

II. EXPERIMENTAL APPARATUS AND PROCEDURES

In the present study, CuO-water nanofluid was used asthe working fluid. De-ionized water was employed as thebase fluid. CuO nanoparticles were commercial productswith an average size of 30 nm. According to our past studies,in order to prevent the occurrence of a sorption layer formedby nanoparticles on the heat transfer surface, no surfactantwas added into the suspensions. CuO-water nanofluid wasprepared by directly dispersing CuO nanoparticles into thebase water. The nanofluid, then, was sonicated continuouslyfor about 12 h in a supersonic water bath. By this process,the stability and uniformity of the nanofluid could be guar-anteed for several days. After that, nanoparticles depositedgradually. In the experiment, however, the nanofluid couldmaintain good uniformity due to the disturbance effect of thebuoyant force and boiling bubbles. The mass concentrationsof the CuO nanoparticles in the present experiment were 0.1,0.5, 1.0, and 2.0 wt %, respectively. Figure 1 shows thetransmission electron microscopy �TEM� photograph of thenanofluid with the concentration of 1.0 wt %.

The surface tension of the nanofluid was measured usingthe maximum bubble pressure method. The surface tensionof the nanofluid decreased about 15% compared with that ofwater and the mass concentration of particles has no mean-ingful effect on the surface tension at 25 °C. The thermalconductivity of the nanofluids was measured using a thermalconductivity analyzer �Sweden, Carnas Company, Hot diskAB�. In the present experimental range, the thermal conduc-tivity of the nanofluid increased linearly with the increase ofthe mass concentration of the nanoparticles. The thermalconductivity of nanofluid with 1.0 wt % mass concentrationincreased about 2% compared with that of water at 25 °C.The viscosity and density of the nanofluid have hardlychange compared with those of water. We will discuss laterthat the thermal conductivity is not an important influencingfactor for the significant enhancement of the heat transferperformance of the thermosyphon.

The working liquid was infused into the evaporator fromthe liquid-filling devices and the filling ratio of the workingliquid �the ratio of the liquid volume to the evaporator vol-ume� was fixed at 50% in all tests. It is according to previous

FIG. 1. TEM photograph of CuO nanoparticle suspension with the concen-tration of 1.0 wt %.

013526-2 Liu, Yang, and Guo J. Appl. Phys. 102, 013526 �2007�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

147.226.7.162 On: Wed, 19 Nov 2014 15:35:26

researches18,19 that the best filling ratio ranged from 40% to60%. In this range the filling ratio has no effect to the heattransfer characteristics.

The experimental apparatus is schematically shown inFig. 2. It mainly consisted of a test section, an electricalheater, a dc power supply, a stainless steel box, charging anddischarging lines, a graduated metering tube, a pressure sta-bilizer, and a vacuum pump. The test section that acted as athermosyphon was vertically fixed. It was a copper tube withan inner diameter of 6 mm, a wall thickness of 1 mm, and atotal length of 350 mm. Its bottom evaporator section, theadiabatic section, and the upper condenser section were 100,100, and 150 mm long, respectively. The evaporator sectionwas heated by the electrical heater which was coiled onto itsouter surface. The electrical heater was linked to the dcpower supply. The evaporator and the electrical heater wereput together in a stainless steel box which was charged withasbestos for the thermal insulation. The adiabatic section wasinsulated by a thick teflon tube. A cooling jacket with itsouter diameter of 18 mm was closely welded to the outersurface of the condenser section. Cooling tap water flowedacross the jacket so as to control the heat removal by varyingits flow rate. Charging and discharging lines were used forcharging working fluid and discharging uncondensable gases,respectively, before each test run. A high vacuum bulletvalve was used to combine the test section with these auxil-iary lines. Charging lines were connected to the graduatedmetering tube which was used to measure the charging vol-

ume of the working liquid. Discharging lines were connectedto the pressure stabilizer which can decrease the pressurefluctuation during the vacuum pumping process because ofits large volume. The vacuum pump was used to vacuumpumping of the test section before the test run.

Four thermocouples were welded at the outer surface ofthe evaporator section, one to that of the adiabatic sectionand three to that of the condenser section to measure thetemperature distributions along the wall. Layout of the eightthermocouples is shown in Fig. 3. A pressure transducer lo-cated at the central position of the adiabatic section was usedto measure the operating pressure in the test section.

In the test run, the vacuum pumping process and pre-heating of liquid process were reparably carried out to driveout any dissolved gases in the thermosyphon.

The test run was performed under three steady operatingpressures of 19.97, 12.38, and 7.45 kPa. During each run, theflow rate of the cooling water was very carefully controlledto help stabilize the operating pressure at each power input.When the evaporator wall temperatures could keep stable fora long term, signals from the thermocouples and the pressuretransducer were sampled into a data acquisition system�Agilent-34950A�. The voltage and the current of the dcpower supply were also recorded.

In the run, the heating power was gradually increased byan increment of 5%. When the measured wall temperatureincreased abruptly and could not hold a steady state, whichindicated that a dry-out phenomenon occurred on the wall,the electric power supply was instantly switched off. Then,the run was restarted from the former steady output powerand the power was then increased in an increment of 1% ofthe former power. When the measured wall temperaturesonce again increased abruptly and could not hold a steadystate, the electric power supply was instantly switched offand the test was stopped. The critical heat flux �CHF� valuewas determined from the electric power of the former time.

In the experiment, a thermocouple was used to measurethe saturated steam temperature and the pressure transducermentioned above was matched to measure the saturatedsteam pressure inside the adiabatic section. Experimental re-sult shows that the deviations between the measured satu-rated steam temperature and the calculated saturated steamtemperature corresponding to the measured pressure wereless than 0.2 K in all runs. In each run, the measured tem-perature from the thermocouple was used as the saturatedsteam temperature to calculate the wall superheat.

The wall heat flux of the evaporator, q, was calculatedby

q = �UI�/�2�roL� , �1�

where U was the voltage, I was the current, ro was the outerradius of the evaporator, and L was the heated length of the

FIG. 2. Schematic diagram of the experimental apparatus. �1� Fixed splint.�2� Fixed bolt. �3� Electrical heater. �4� Stainless box. �5� Teflon layer. �6�Pressure transducer. �7� Thermosyphon. �8� Cooling jacket. �9� Flange. �10�Vacuum bullet valve. �11� Three-way valve. �12� Valve. �13� Graduatedmetering tube. �14� Valve. �15� Vessel. �16� Acquisition system. �17� Currentmeter. �18� Voltage meter.

FIG. 3. Location of the thermocouples in the test section.

013526-3 Liu, Yang, and Guo J. Appl. Phys. 102, 013526 �2007�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

147.226.7.162 On: Wed, 19 Nov 2014 15:35:26

evaporator section. The steady thermal diffusion equationwas adopted to obtain the inner average wall temperature ofthe test section, Tw,

Tw = To +qro

�ln

ri

ro, �2�

where To was measured average outer wall temperature oftest section, � was the thermal conductivity of the coppertube, and ri was the inner radius of the copper tube. Theaverage wall superheat, �T, was calculated by the equation

�T = Tw − Ts, �3�

with Ts the saturated temperature of the saturated vapor atthe adiabatic section. The average heat transfer coefficient ofthe evaporator section, he, was calculated by the equation

he = q/�T . �4�

In the preparatory experiment, the heat loss from theevaporator and adiabatic sections were carefully estimatedby an energy balance relation and it was not over 2% of theinput power.

The maximum uncertainty of the operating pressure was5%. The maximum calibration error of the thermocoupleswas 0.2 K. The maximum uncertainty of the wall superheatwas about 4%. Truncation error of measurement was about1% and the instrument error was about 0.5%. Thus the maxi-mum uncertainties of heat flux and the heat transfer coeffi-cient were about 8% and 12%, respectively.

III. RESULTS AND DISCUSSION

A. Comparison of wall temperature distributionsof evaporator using both de-ionized waterand nanofluid

Figure 4�a� and 4�b� show, respectively, a sample of walltemperature distributions of the evaporator section using de-ionized water and nanofluid with the mass concentration ofnanoparticles of 1.0 wt %, at the operating pressures Ps

equaling to 7.45 kPa. As shown in these figures, the averageevaporator wall temperature with nanofluid is much lowerthan those with de-ionized water at the same power input.The presence of only 1.0 wt % of nanoparticles can signifi-cantly decrease the wall temperature of the evaporator sec-tion when the input power is below about 200 W. Hence, theheat transfer performance of the evaporator is greatly en-hanced and the thermal resistance of the thermosyphon isalso significantly reduced. For the present thermosyphon, themaximum power input for de-ionized water is about 200 Wwhile for nanofluid it can go up to more than 400 W. Whenthe input power is over 200 W, the data cannot be comparedto each other, because of the jump of the wall temperaturefor de-ionized water which indicated that a dry-out phenom-enon occurred on the wall. There is a remarkable onefoldincrease of the maximum heat removal capacity. Meanwhile,the decreasing trend of the average wall temperature of theevaporator is sped up with increasing power input. The re-duction amplitude of the wall temperatures is 7.4 °C for theinput power of 20 W, but it is increased to 24.8 °C for theinput power of 180 W. Therefore, the effect of nanoparticles

on the reduction of the wall temperature is more significantat high fluxes. In addition, seen from Fig. 4, a better walltemperature uniformity can also be obtained by using nano-fluid than de-ionized water. This can also contribute to asmaller heat resistance of the thermosyphon and better oper-ating condition.

B. The boiling heat transfer of nanofluids at differentoperating pressures

Figure 5�a�–5�c� shows the boiling curves of the averageheat transfer coefficient, he, versus the heat flux, q, for nano-fluids under three subatmospheric pressure conditions. Theoperating pressures are 7.45, 12.38, and 19.97 kPa, respec-tively and the range of nanofluid concentration,� was from0.1 to 2.0 wt %. For all mass concentrations tested, boilingcurves of the nanofluids move leftwards significantly com-pared with that of de-ionized water, and the heat transferenhancement effects are remarkable after using the nano-fluids.

The experimental results indicated that the mass concen-tration of nanofluids has great influence on the nucleate boil-ing heat transfer for every test pressure. At a fixed test pres-sure, the heat transfer coefficients of nanofluids are gradually

FIG. 4. Wall temperature distributions along the evaporator section at dif-ferent input powers. �a� De-ionized water. �b� Nanofluid with concentrationof 1.0 wt %.

013526-4 Liu, Yang, and Guo J. Appl. Phys. 102, 013526 �2007�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

147.226.7.162 On: Wed, 19 Nov 2014 15:35:26

enhanced with the increase of the mass concentration com-pared with that of water when the concentration is less than1.0 wt %. The maximum enhancement effect corresponds tothe mass concentration of 1.0 wt % and the heat transfer co-efficient could be double at subatmospheric pressures. How-ever, when the concentration is over 1.0 wt %, the heat trans-fer coefficient would have a slight worsening trend. It is clearthat there exists an optimum mass concentration for nano-fluids which corresponds to the maximum heat transfer en-

hancement and this optimum mass concentration is 1.0 wt %at all test pressures. At the optimum concentration of1.0 wt %, as compared with those of water, the heat transfercoefficients of nanofluids can maximally increase about 1.3and 1.6 times under the pressures of 19.97 and 7.45 kPa,respectively. Only 1.0 wt % of nanoparticles can surprisinglyenhance the heat transfer performance of the evaporator sec-tion. In addition, the heat transfer enhancement effect ofnanofluids has an increasing trend with increasing heat flux.

C. The effect of the pressure and the nanofluidconcentration on the boiling heat transferperformance

Heat transfer coefficient enhanced ratios �defined as theratio of the heat transfer coefficient of nanofluid to that ofde-ionized water� against the heat flux at different operatingpressures are charted in Fig. 6. Here, he denotes the heattransfer coefficient of nanofluid while he,0 that of water. It isclear that the pressure has great influence on the heat transferenhancement of nanofluids as shown in Fig. 6. It is found theboiling heat transfer enhancement effects of nanofluids in-creased greatly with the decrease of the pressure. The heattransfer coefficient can increase about 1.3 times at the pres-sure of 19.97 kPa and about 1.6 times at the pressure of7.45 kPa, as compared with that of water. In addition, theenhancement ratio of the heat transfer coefficient increasesapproximately with increasing the heat flux though withsome initial declines.

Figure 7 shows the influence of nanofluids concentrationon the CHF at different pressures. In Fig. 7, nearly all valuesof the CHFs increase steeply at first at the low nanofluidconcentration and tend quickly to increase smoothly then.They finally reach nearly a constant. It means that there is aremarkable increase of the maximum heat transfer capabilityfor the thermosyphon when only a small amount of nanopar-ticles are added into the base fluid. The optimum mass con-centration corresponding to the maximum CHF enhancementeffect is 1.0 wt % and this value is the same as that corre-sponding to the maximum heat transfer coefficient enhance-ment effect.

FIG. 5. Effect of mass concentration of nanoparticles on boiling heat trans-fer coefficient at different operating pressures: �a� P=7.45 kPa; �b� P=12.38 kPa; �c� P=19.97 kPa.

FIG. 6. Effect of pressure on boiling heat transfer coefficient enhancementratio.

013526-5 Liu, Yang, and Guo J. Appl. Phys. 102, 013526 �2007�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

147.226.7.162 On: Wed, 19 Nov 2014 15:35:26

To investigate the enhancement effect of the CHF quan-titatively, the CHF enhancement ratio �defined as the ratio ofthe CHF of nanofluid to that of de-ionized water� against thenanofluid mass concentration is illustrated in Fig. 8. TheCHF of the nanofluids at different concentrations is qc whileqc,0 is the CHF of water. Similar with the trend shown in Fig.7, it is found that the CHF enhancement ratio increases rap-idly with the concentration and finally approaches a constantvalue at each pressure. Though with fluctuations in themiddle range of the mass concentration, it is noteworthy thatthe operating pressure seems to have negligible influence onthe CHF enhancement ratio. A slight 0.1 wt % of nanopar-ticles can increase the CHF of the base liquid by 60%–80%,and only 1.0 wt % of nanoparticles can increase the baseliquid by about 120% for all of the three operating pressures.

D. The total heat resistance of thermosyphon usingnanofluids

In general, for Newton fluids, the film condensing heattransfer in the condenser is greatly better than evaporatingheat transfer in the evaporator. The condensing heat transfercoefficient of the falling film on a vertical wall can be esti-mated by the well-known Nusselt correlation,

hc = 0.943��lg�l3��l − �v��hfg + 0.68cpl�Ts − Ti��

�lLc�Ts − Ti��1/4

,

�5�

where �l denotes the thermal conductivity of liquid, �l and �vthe density of liquid and vapor, hfg the latent heat of evapo-ration, cpl the specific heat, �l the viscosity of liquid, and Lc

the length of the condensing section.Figure 9 shows the comparison of the condensing heat

transfer coefficients for nanofluids between the experimentaldata and Nusselt correlation for the pressure of 7.45 kPa. It isfound that the condensing heat transfer of nanofluids withdifferent mass concentrations are almost the same as that ofwater and the experimental data are about less than the cal-culated values by about 15%–28%. Because the flow of thefalling film and vapor is countercurrent in the present ther-mosyphon, it is reasonable that the experimental data aresomewhat less than the calculated values for water.

If the particles have hardly been carried into vapor andpassed through the condenser, the experimental results men-tioned above are easy to explain. In the present study stage,it is still unclear if the particles were carried into vapor.

Figure 10 shows the comparison of the total heat resis-tance of thermosyphon, Rtot, respectively, using the nanofluidwith 1.0 wt % concentration and using water for the pressure

FIG. 7. Effect of mass concentration of nanoparticles on the CHF.

FIG. 8. Effect of mass concentration of nanoparticles on the CHF ratio.

FIG. 9. Condensing heat transfer of nanofluids at pressure of 7.45 kPa.

FIG. 10. Total heat resistances of thermosyphon using water and nanofluidwith �=1.0 wt %.

013526-6 Liu, Yang, and Guo J. Appl. Phys. 102, 013526 �2007�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

147.226.7.162 On: Wed, 19 Nov 2014 15:35:26

of 7.45 kPa. It is found that the total heat resistance usingnanofluid is significantly increased compared with that usingwater. The total heat resistance can increase about 30%–90%in the heat flux range using water as work liquid. At low heatflux, the total heat transfer enhancement effect is especiallyremarkable due to the fact that the heat resistance of theevaporator is governing.

When the heat flux is over 80 kW/m2, the thermosyphonusing water fails to work. However, the thermosyphon usingnanofluid can still work well and hold a low total heat resis-tance. Meanwhile, the change of the total heat resistance isslow in the whole heat flux range tested. This also is advan-tageous to the stable operation of the thermosyphon.

The total heat resistance of a heat pipe depends not onlyon both heat transfer coefficients in the evaporator and thecondenser, but also depends on both heat transfer areas of theevaporator and the condenser and the ratio of the both heattransfer areas. Therefore, the total heat resistances mentionedabove is only a special sample which is only from the presentstudy. However, qualitatively, it can be concluded that usingnanofluids can greatly decrease the total heat resistance ofthermosyphon and increase the work power.

A series of repeating tests were also carried out to findthe difference of heat transfer characteristics between thefresh nanofluids and old nanofluids. After the tests usingfresh nanofluid, the test system was laid up for about twoweeks and then the run was restarted. The repeating testsindicated that no meaningful difference of the heat transfercharacteristics was found between the fresh and old nano-fluids tests. The reason is because of the stirring effect of thebubbles for the suspension during boiling process. For an oldnanofluid in the evaporator, when it is reheated, the nanopar-ticles depositing on the heating surface can quickly spreadinto water and form uniform suspension again under effect ofthe buoyant force and boiling bubbles.

Summing the mentions above, it is confirmed that usingwater-CuO nanoparticle suspension with the mass concentra-tion of 1.0 wt % as the working fluid can significantlystrengthen the boiling heat transfer coefficient and the maxi-mum power of the evaporator in thermosyphon under lowpressure conditions. Nanofluid is a potential kind of workingfluid to enhance the heat transfer characteristics of heat pipe.

In the present study, the boiling heat transfer coefficientsand the CHFs of the nanofluids in the evaporator at lowpressure conditions have a dramatic increase. It is unclearwhy the test pressure has so great influence on the boilingheat transfer and the CHF of nanofluids. It cannot be ex-plained using the traditional heat transfer theory concerningthe boiling in a closed thermosyphon, such as the change ofthe fluid physical properties. It is a challenging problem forthe traditional heat transfer theory. In the present stage, thereason for heat transfer enhancement may be due to the bind-ing force effect between the nanoparticles and liquid, thechurning effect of the nanoparticles in the liquid film be-tween the wall and bubbles, and the effect of Brownian mo-tion of the particles in the base liquid.

IV. CONCLUSIONS

A research on a thermosyphon using CuO-water nano-fluid as the working fluid has been carried out. A comparisonof the heat transfer performance using the nanofluid againstde-ionized water was performed. In addition, effects of massconcentration of CuO nanoparticles and the system operatingpressure on the heat transfer performance of the thermosy-phon have been discussed. The experimental results aregiven as follows:

�1� The average evaporator wall temperature of the thermo-syphon using nanofluid is much lower than that usingde-ionized water. A better wall temperature uniformitycan also be guaranteed by substituting the nanofluid forde-ionized water.

�2� The mass concentration of nanofluids has remarkableinfluence on the boiling heat transfer coefficient and theCHF of the nanofluids. The heat transfer coefficient andthe CHF increase with the increase of the concentrationwhen the concentration is less than 1.0 wt %. However,when the concentration exceeds 1.0 wt %, the CHF isbasically close to a constant, and the heat transfer dete-riorates gradually. There exists an optimum mass con-centration for nanofluids which corresponds to the maxi-mum heat transfer enhancement and this optimum massconcentration is 1.0 wt % at all test pressures.

�3� The pressure has very significant influence on the boil-ing heat transfer and the CHF of nanofluids, the heattransfer coefficient, and the CHF of nanofluids greatlyincrease with the decrease of the test pressure. The heattransfer coefficient and the CHF can increase, respec-tively, about 160% and 120% at the pressure of7.45 kPa.

�4� CuO nanoparticles suspension as a working fluid cansignificantly strengthen the heat transfer performanceand the maximum power of the thermosyphon under lowpressure conditions.

ACKNOWLEDGMENT

This study was supported by Key foundational researchproject of Science and Technology Bureau of Shanghai underGrant No. 04JC14049.

1H. Masuda, A. Ebata, K. Teramae, and N. Hishinuma, Netsu Bussei 7, 227�1993�.

2S. U. S. Choi, Proceedings of the 1995 ASME International MechanicalEngineering Congress and Exhibition, San Francisco, CA, 1995.

3S. U. S. Choi, Z. G. Zhang, W. Yu, F. E. Lockwood, and E. A. Grulke,Appl. Phys. Lett. 79, 2252 �2001�.

4J. A. Eastman, S. U. S. Choi, S. Li, W. Yu, and L. J. Thmpson, Appl. Phys.Lett. 78, 718 �2001�.

5S. Lee, S. U. S. Chois, S. Li, and J. A, J. Heat Transfer 121, 280 �1999�.6S. K. Das, N. Putra, P. Thlesen, and W. Roetzel, Int. J. Heat Mass Transfer46, 851 �2003�.

7Y. M. Xuan and Q. Li, Int. J. Heat Fluid Flow 21, 58 �2000�.8S. E. B. Maiga, C. T. Nguyen, N. Galanis, and G. Roy, SuperlatticesMicrostruct. 26, 543 �2004�.

9S. M. You, J. H. Kim, and K. H. Kim, Appl. Phys. Lett. 83, 3374 �2003�.10S. K. Das, N. Putra, and W. Roetzel, J. Heat Transfer 125, 567 �2003�.11P. Vassallo, R. Kuman, and S. D. Amico, Int. J. Heat Mass Transfer 47,

407 �2004�.12I. C. Bang and S. H. Chang, Int. J. Heat Mass Transfer 48, 2407 �2005�.

013526-7 Liu, Yang, and Guo J. Appl. Phys. 102, 013526 �2007�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

147.226.7.162 On: Wed, 19 Nov 2014 15:35:26

13D. S. Wen and Y. L. Ding, J. Nanopart. Res. 7, 265 �2005�.14H. S. Xue and J. R. Fan, J. Appl. Phys. 100, 1 �2006�.15C. Tsai, H. T. Chien, and P. P. Ding, Mater. Lett. 58, 1461 �2004�.16H. B. Ma and W. Fang, Appl. Phys. Lett. 88, 1 �2006�.

17S. K. Shung and S. W. Kang, Appl. Therm. Eng. 26, 2377 �2006�.18Y. Yao, M. Gao, and J. E. Beam, J. Thermophys. Heat Transfer 11, 158

�1997�.19R. Hopkins, A. Faghri, and D. Knustalev, J. Heat Transfer 12, 102 �1999�.

013526-8 Liu, Yang, and Guo J. Appl. Phys. 102, 013526 �2007�

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

147.226.7.162 On: Wed, 19 Nov 2014 15:35:26