on

kodoneesia,

ZnO nanoparticlesZnO nanouids

anothetheatio

urgenth signhose cu

combined to formulate nanouids have been evaluated over thelast decade to determine the variation in thermophysical proper-ties, with particular emphasis on measurements of thermalconductivity and viscosity [10e15]. A number of publications onthe thermal properties of nanouids consisting of metallic or

spherical-shaped TiO2 nanoparticles dispersed in deionized waterusing a transient hot wire technique. Their results demonstratedthat the thermal conductivity was inuenced by the volumefraction as well as the shape and size of the particles. From theliterature above, it is evident that uids containing nanoparticleshave substantially higher thermal conductivities than their baseuids.

However, for heat transfer applications, the enhancement ofthermal conductivity is not the only concern; the real worth of the

* Corresponding author. Tel.: 62 217694975; fax: 62 217515171.

Contents lists available at

International Journal

w.e

International Journal of Thermal Sciences 63 (2013) 125e132E-mail addresses: rosari.saleh@ui.ac.id, rosari.saleh@gmail.com (R. Saleh).and it is also well known that uids typically have lower thermalconductivity compared to crystalline solids [4]. Therefore, uidscontaining suspended solid particles can be reasonably expected tohave higher thermal conductivities than pure uids. The idea ofusing nanouids, dened as liquids with nanometer-sized particlesuspensions, was rst introduced by Choi in 1995 [5]. It has beenshown that when solid nanometer-sized particles are suspended inuid, the enhancement of thermal conductivity can be signicant.This enhancement can improve the efciency of uids used in heattransfer applications [6e9].

A wide variety of nanoparticles and base uids that can be

CuO nanoparticles in distilled water exhibited as much as a 20%increase in effective thermal conductivity over that of the baseuid (distilled water) at room temperature. Similar behavior wasalso observed in another study of 33 nm Al2O3 nanoparticlesdispersed in distilled water [17]. Moreover, it has been shown thatthe enhancement of thermal conductivity tends to increase withincreased temperature. Das et al. [18] showed in their experimentusing a temperature-oscillating technique that the thermalconductivity increased not only with increasing temperature butalso with increasing particle volume fraction. Murshed et al. [19]studied the thermal conductivities of both rod-shaped andThermal conductivityHeat pipe

1. Introduction

In recent times, there has been anelds for a new cooling medium wittransfer performance compared to t1290-0729/$ e see front matter 2012 Elsevier Mashttp://dx.doi.org/10.1016/j.ijthermalsci.2012.07.011the measured enhancement. The nanouids used in conductivity measurements were further employedas the working medium for a conventional screen-mesh wick heat pipe. The experiments were per-formed to measure the temperature distribution and thermal resistance of the heat pipe. The resultsshowed temperature distribution and thermal resistance to decrease as the concentration and thecrystallite size of the nanoparticle increased.

2012 Elsevier Masson SAS. All rights reserved.

need inmany industrialicantly improved heatrrently available [1e3]

nonmetallic nanoparticles have shown a large enhancement in thethermal conductivity of nanouids. Eastman et al. [16] rst re-ported the enhancement of thermal conductivity by 40% when 0.3vol% Cu nanoparticles were dispersed in ethylene glycol. In addi-tion, they reported that nanouids consisting of 4 vol% of 36 nmKeywords:enhancement of the nanouid demonstrated a nonlinear relationship with respect to volume fractionand crystallite size, with increases in the volume fraction and crystallite size both resulting in increases inAvailable online 22 August 2012 tion was used to disperse the nanoparticles in ethylene glycol as the base uid. The thermal conductivityExperimental investigation of thermal cperformance of ZnO nanouids

Rosari Saleh a,*, Nandy Putra b, Suhendro Purbo PraaDepartemen Fisika, Fakultas MIPA-Universitas Indonesia, Kampus UI Depok 16426, InbHeat Transfer Laboratory, Department of Mechanical Engineering, University of Indon

a r t i c l e i n f o

Article history:Received 21 March 2011Received in revised form21 July 2012Accepted 23 July 2012

a b s t r a c t

Nanouids consisting of nover the last view years forwork investigated the synprocess. Chemical precipit

journal homepage: wwson SAS. All rights reserved.ductivity and heat pipe thermal

so a, Wayan Nata Septiadi b

siaKampus UI Depok 16426, Indonesia

particles dispersed in heat transfer carrier uid have received attentionir enormous potential to improve the efciency of heat transfer uids. Thissis of ZnO nanoparticle-based thermal uids prepared using a two-stepn was used for the synthesis of the ZnO powders, and ultrasonic irradia-

SciVerse ScienceDirect

of Thermal Sciences

lsevier .com/locate/ i j ts

of Tnanouid as a cooling medium can only be examined underconvective conditions. Tsai et al. [20] synthesized gold (Au) nano-particles of different sizes and dispersed these nanoparticles in anaqueous solution. The Au nanouid was then employed asa working medium for a conventional circular heat pipe. Themeasured results showed that the thermal resistance of the heatpipe with the nanoparticle solution was lower compared to thatwith deionized water. Kang et al. [21] investigated the thermal

Nomenclature

l X-ray wavelength (A) average crystallite size (nm)D, b full-width at half maximum (rad)Q, q diffraction angle (rad)2 Q, 2 q x-ray diffraction peak position (degree)a, b, c lattice parameter (A)h lattice strainF(R) KubelkaeMunk functionR reectance (%)Eg energy gap (eV)kR relative values of thermal conductivityReea, Rth thermal resistance (C/W)T temperature (C)DT temperature difference (C)Q input power (W)

subscriptsd dryinge evaporator sectiona adiabatic sectioneea between evaporator section and adiabatic sectionth thermal

R. Saleh et al. / International Journal126performance of various particle sizes of silver (Ag) nanoparticlesdispersed in deionized water using a grooved heat pipe. It wasobserved that when more Ag nanoparticles were dispersed in thedeionizedwater, the increase in the heat pipewall temperaturewassmaller than that for a pure water-lled heat pipe under variousheat loads. In addition, maximum reductions of 50 and 80% of thethermal resistance were achieved when nanoparticle sizes of 10and 35 nm, respectively, were added to the deionized water. Sha-fahi et al. [22,23] studied the thermal performance of CuO, Al2O3and TiO2nanouids in cylindrical and at-shaped heat pipes. Theyshowed that the presence of nanoparticles in the working uid ledto a reduction in the speed of the liquid, a smaller temperaturedifference along the heat pipe and substantial enhancement in thethermal performance of the heat pipe.

The present work reports the synthesis of ZnO nanoparticlesusing a co-precipitation method and the dispersal of these nano-particles in ethylene glycol without the addition of any surfactant.Nanoparticles were characterized by x-ray diffraction (XRD),energy dispersive x-ray (EDX), thermal analysis TGeDTA, UVeVisspectroscopy and infrared absorption spectroscopy (FTIR). Theobserved structural and optical properties of the ZnO nanoparticleswere determined to be correlated with those in the uid samplesby performing FTIR and UVeVis measurements on the diluted ZnOnanoparticles in ethylene glycol. The thermal conductivity behav-iors of the synthesized nanouids were measured as a function ofparticle volume fraction. In addition, ZnO nanouid was employedas theworking uid for a conventional screen-mesh wick heat pipe.In this experiment, the temperature distribution and the heat pipethermal resistance were further investigated.2. Experimental procedure

2.1. Synthesis of ZnO nanoparticles

For the synthesis of the undopedZnO nanoparticles in this study,ZnSO4$7H2O and 25% aqueous ammonia or NaOH were used,procured from Aldrich. All of the chemicals were GR grade withoutfurther purication. Analytical-grade ZnSO4$7H2O from Aldrichwas dissolved in deionized water to obtain a nal concentration of0.1 M. In this solution, 0.1 M of NaOH solution was added graduallyuntil the nal pH of the solution reached 13. The solution was thenstirred at a temperature of 80 C for 30 min until a milky whitesolution was obtained. Subsequently, the solution was washedseveral times with deionized water and anhydrous alcohol toremove the by-product sodium sulfate (NaSO4), and the solutionwas then ltered. The mixture was aged at room temperature for24 h. To obtain different crystallite sizes, the mixture was dried at200 or 600 C for 4 h.

2.2. Characterization of ZnO nanoparticles

The structural characterization was conducted using a standardx-ray diffractometer (Philips PW1710) with monochromatic CueKa(l 1.54060A) radiation operated at 40 kV and 20 mA in the rangeof 10 to 80. The calibration of the diffractometer was performedusing Si powder. The XRD patterns of the nanoparticles wereveried by comparison with the JCPDS data. The average size ofcrystallites size was estimated using the Scherrer peak-broadening method: 0.89 l/D cos Q, where D is the X-raywavelength, D is line-broadening at half-height and Q is the Braggangle of the particles. The average crystallite size is obtained fromthe (002) peak, corresponding to the ZnO wurtzite structure.Elemental analyses of the samples were conducted through energydispersive x-ray spectroscopy (EDX) using a scanning microscope.Simultaneous thermogravimetric and differential thermal analysis(TGeDTA) measurements were performed using a Setaram TAG 24.The samples were heated from room temperature up to 800 C at10 C min1. To study the bonding conguration, infrared absorp-tion measurements were performed using a Shimadzu Fouriertransform spectrometer. Optical characterizations were performedby measuring the diffuse reectance spectroscopy. All spectra weretaken in the range of 200e800 nm using a Shimadzu UVeVisspectrophotometer with an integrating sphere attachment andaSpectralon reectance standard.

2.3. Synthesis of ZnO nanouids

ZnO nanouids were prepared using a two-step procedure bydissolving ZnO nanoparticles in ethylene glycol (EG) as the baseuid. As no surfactant was added in the preparation of the nano-uids, it is expected that the nanoparticles would form clusters inthe nanouid samples. To improve the dispersion of particles in theuid samples, the nanouids were stirred using a magnetic stirrerand sonicated using an ultrasonic processor under continuouspulse for 2 h. Five weight concentrations of nanouids wereprepared by dissolving 25, 50, 75, 100 and 500 mg of ZnO nano-particles in 100 ml of ethylene glycol.

2.4. Measurement of thermal conductivity of the nanouids

The thermal conductivity of the nanouids was measured usinga KD2 Pro thermal properties analyzer from Decagon Devices, Inc,which has a sensor with a length of 60 mm and a diameter of1.28 mm. This KD2 thermal analyzer was used successfully by Wen

hermal Sciences 63 (2013) 125e132and Ding [24] and Putra et al. [25] in their workwith nanouids and

by other researchers [26]. The sensor must be inserted completelyinto the medium to obtain measurements. The interior of thesensor contains an integrated heating element and thermo-resistor,and it is connected to a microprocessor to control and conduct themeasurements. The nanouid sample was held in a glass beakerlocated in a thermostat bath to ensure that all of the measurementswere at a constant temperature. At least ve measurements weretaken for each nanouid to ensure the realiability of themeasurements within 2%. The measurements were performed onthe nanouids at room temperature.

2.5. Measurement of ZnO nanouid heat pipe thermal performance

The heat pipes used in our experiments were constructed usingstraight copper tubing with an outer diameter of 8 mm, an innerdiameter of 7.44mm and length of 200mm. Thewicks for each heatpipe weremade from a stainless steel wire screenmesh with awirediameter of 56.5 mm and 67.42 strands per mm. The effect of uidloading on the performance of the heat pipes was tested usingpipes that each had four-layer screen-mesh wicks.

The experimental system (Fig. 1a) was composed of a coolingsystem, a test section, a power supply, a measurement system anda data acquisition system (National Instrument NI 9211). Heat wasapplied to the heat pipein a 60-mm-long evaporator section usinga exible electrical heater that was connected to the DC powersupply. To minimize heat loss, the evaporator and adiabatic sectionwere wrapped with several layers of glass wool and isolated usingpolyurethane. The condenser section of the heat pipe was insertedhorizontally into the cooling chamber. The coolant circulatedthrough the cooling chamber, where heat was removed from the

condenser section to the thermostatic circulating bath (TCB). TheTCB was set to the required temperature and held at a constanttemperature throughout the tests.

The surfaces of the heat pipes tested here were equipped withve K-type thermocouples mounted along the wall to measure thesurface temperature distribution in the evaporator, adiabatic andcondenser sections (Fig. 1b). Three thermocouples were attached tothe evaporator, two were attached to the adiabatic section and theothers were attached to the condenser section. All thermocoupleshad diameters of 0.05 mm and were calibrated against a digitalthermometer. The uncertainty in temperature measurements was0.1 C. The thermocouples were connected to a high-precisiondata acquisition system. The uncertainty of the geometrical sizeof the tested heat pipe was 0.5%. The maximum uncertainties ofwall heat ux and the thermal resistance were within 2% and 3%,respectively.

3. Results and discussion

The typical XRD patterns of the nanoparticles dried at temper-atures of 200 and 600 C are shown in Fig. 2a. Structural changes inthe nanoparticles could be observed with increasing dryingtemperature. The spectra are almost equal to the typical XRDspectra of ZnO nanoparticles reported from other experiments[27,28]. In all of the XRD patterns, nine peaks were observed atapproximately 2 Q 32.12, 34.48, 36.6, 47.76, 56.84, 63.02, 66.95,68.23 and 69.24, which correspond to the Braggs reection plane(100), (002), (101), (102), (110), (103), (200), (112) and (201),respectively. These peaks are in excellent agreement with thestandard JCPDS le for ZnO and are indexed as the hexagonal

5

6

4

0

a

R. Saleh et al. / International Journal of Thermal Sciences 63 (2013) 125e132 1271

2

7

80 mV thermocouple input24-bit data input

24 watt power input

3

10 40 100T1 T2 T3

bFig. 1. a. Schematic of the experimental set-up for thermal performance of a heat pipe: 1. Tsystem; 8. Computer for data processing; 9. DC Power supply; 10. Insulating material; 11. TTo TCB

9

10

8

11

130 200 mmT4 T5CB; 2. Flow meter; 3. Thermometer; 4. Water reservoir; 5. Heat pipe; 6. Heater; 7. DAQhermocouple. b. Thermocouple distribution on the tested heat pipe.

decreased slightly with increasing drying temperature, indicatingthe enhancement of crystallinity. The average crystallite size andstrain of the ZnO samples were calculated from the XRD data usingthe equation [30] b cosq 0.89 l/ h sin q, where l, b, q, h and represent the wavelength of the X-ray radiation, FWHM,diffraction angle, strain and average crystallite size, respectively.Using this equation, we found that the average crystallite sizeincreased from 18 to 23 nm, while the strain decreased as thedrying temperature increased from 200 to 600 C (Table 1). Theresults of the XRD measurements are consistent with the resultsobtained from the EDX spectra in Fig. 2b. In addition to an oxygenpeak at 0.6 keV, Zn signals at approximately 1.01, 8.7 and 9.5 keVwere observed, which are indicative of the successful synthesis ofZnO nanoparticles, as already conrmed by the XRD data.

To reveal any changes that occurred during thermal treatment ofthe precursor, TGeDTA analysis was carried out at atmospheretemperatures from room temperature to 600 C in (see Fig. 3).According to the TG curve, the precursor lost its weight in two

a

R. Saleh et al. / International Journal of Thermal Sciences 63 (2013) 125e132128wurtzite structure of ZnO having space group P63mc. No other peaksrelated to the presence of impurities were found. The values of thelattice parameters calculated from the XRD data using the Rietveldrenement analysis are shown in Table 1. The lattice parametersincreased from a b 3.218 A and c 5.155 A to a b 3.233 Aand c 5.173 A as the drying temperature increased from 200 to600 C. These results are consistent with those of bulk ZnO [29].However, a careful analysis of the peak positions suggested a smallshift in the value toward a lower 2 Q with increasing dryingtemperature, indicating the presence of compressive strain in thesamples. It was also shown for all samples that the reection peaksbecame sharper and that the full-width at half maximum (FWHM)

b

Fig. 2. a. XRD patterns of ZnO nanoparticles synthesized with different dryingtemperatures. b. EDX spectra of ZnO nanoparticles synthesized with a dryingtemperature of 600 C.

Table. 1Lattice parameter, average crystallite size and strain of ZnO nanoparticles synthe-sized with different dry temperatures.

Dry temperatureTd (C)

Lattice parameters Average crystallite size Strain

a b (A) c (A) (nm) h (102)200 3.218 5.155 18 2.724600 3.233 5.173 23 2.264major steps. The rst major step was in the temperature rangebelow 100 C and indicated a loss of 3.2 wt.%, revealing the dehy-drogenation of surface-adsorbed water molecules. The second stepof weight loss (2.6 wt.%), appearing in the range of 100e220 C,could be associated with diffusion of OH ions or hydrogen atomsout of the ZnO network. A further outward diffusion of OH ionsor H atoms was observed from the slight decrease of the TGcurve at temperatures higher than 220 C. The total weight lossin the entire thermal analysis study was less than 8 wt.%,indicating that the majority contributed to ZnO formation. TheDTA curve of the precursor obtained during thermal treatmentexhibited two endothermic peaks, corresponding to the twomajor weight loss steps of the TGA curve.

The structures of the samples were further investigated usinginfrared absorption measurements. Infrared spectra for samplesprepared under various drying temperatures are plotted in Fig. 4.For both spectra, the background signal was subtracted from theoriginal data. It can be observed that the intensity of the absorptionpeaks decreased with increasing drying temperatures. For all as-synthesized samples, the strong absorption peaks in the range of400e700 cm1 could be attributed to ZnO stretching modes [31].These stretching modes are indicative of successful synthesis ofnanocrystalline ZnO particles, as conrmed by XRD and EDXstudies. Similarly, the observed absorption peaks at approximately1646, 1390 and 1121 cm1correspond to the OH bending mode[32],CeOH plane bending and CeOH out-of-plane bending [32],respectively. The presence of hydrogen in the crystal growthFig. 3. TGeDTA curves of the ZnO precursor: (a) DTA and (b) TG.

Fig. 4. FTIR spectra of ZnO nanoparticles synthesized with different drying

b

R. Saleh et al. / International Journal of Tenvironment is not surprising, and it is very difcult to prevent itsincorporation into the crystal during the process of crystal growth[33]. A broad band in the region of 2900e3700 cm1 could beexplained as overlapping of physically absorbed water, OeHstretching modes and CeH stretching modes. CeH local vibra-tional modes between 2800 and 3100 cm1 have been observed ina number of semiconductors, such as amorphous silicon carbon(a-SiC:H) [34], GaAs [35] and GaN [36]. In these materials, thelocal vibrational modes are assigned to symmetric and antisym-metric CeH stretching modes.

Optical characterization was performed by measuring thediffuse reectance spectroscopy. The spectra were taken in therange of 200e800 nm. Fig. 5 shows the diffuse reectance spectra R

temperatures.as a function of wavelength for the samples shown in Fig. 2a.Because our samples were powders, the low reectance values

Fig. 5. UVeVis spectra of ZnO nanoparticles synthesized with different dryingtemperatures. The inset shows the correlated optical gap of the ZnO nanoparticles asa function of the drying temperature.a

hermal Sciences 63 (2013) 125e132 129in our spectra indicate high absorption in the correspondingwavelength regions. The data collected at room temperatureshowed a clear difference between the samples in the region of350e800 nm. The signicance of this was more visible afterapplying the KubelkaeMunk function F(R) given by the relationF(R) (1 R)2/2R, where R is themagnitude of reectance [37]. Theoptical gap (inset of Fig. 5) was estimated from the diffuse reec-tance spectra by plotting the square of the KubelkaeMunk functionF(R)2 as a function of energy. To obtain the optical gap, the linearpart of the F(R)2 curve was extrapolated until it intersected theenergy axis. The optical gap of ZnO dried at 200 C was determinedby the above method to be 3.46 eV, which is slightly higher thanthat for bulk ZnO (3.34 eV) [37]. The corresponding optical gapswere noted to shift to lower energies with increasing dryingtemperature. This variation follows the strain data obtained fromthe analysis of XRD patterns in a direct relationship, indicating thatthe variation in the optical gap is related to structural changesresulting from the variation of drying temperature. Variations inthe optical gap of ZnO have been observed by several authors

Fig. 6. a. FTIR spectra of the ZnO nanoparticles shown in Fig. 4 dispersed in ethyleneglycol. The weight concentration and average crystallite size are indicated in the plot.b. UVeVis spectra of the ZnO nanoparticles shown in Fig. 5 dispersed in ethyleneglycol. The weight concentration and average crystallite size are indicated in the plot.

in increase of absorption percentage, mainly for wavelength greaterthan 360 nm.

The thermal conductivity ratios of ZnO/EG nanouids withaverage crystallite sizes of 18 and 23 nmwere characterized and areplotted in Fig. 7. Thermal conductivity was measured immediatelyafter stirring and sonicating the nanouids. The relative values ofthermal conductivity kR were obtained by dividing the effectivethermal conductivity of the nanouid by the value of the thermalconductivity of the base uid. As expected, one can observe that thethermal conductivity increases with increasing volume fraction ofthe nanoparticles and with increasing average crystallite size. Atlower particle concentrations, the conductivity ratio showedenhancements of approximately 5.3% or 7% at 0.025%, but thesevalues increased to 14% or 15.5% at 0.5% for a ZnO/EG nanouidwithan average crystallite size of 18 or 23 nm, respectively. The exper-imental results showed a nonlinear relationship between thethermal conductivity and the particle volume fraction. However, inFig. 8, it can be observed that the measured thermal conductivitiesare greater than those reported by other researchers [9,39], i.e., thethermal conductivities of the nanouids increased nonlinearly asthe particle volume fraction increased. It was also observed that forthe nanouid with an average crystallite size of 23 nm, the increasein conductivity was larger than that of the nanouid with thesmaller crystallite size.

The change in heat transfer rate through the heat pipe alongwith differences between the average temperatures of the evapo-rator and condenser are shown in Figs. 9 and 10 for ZnO nanouids

Fig. 7. Relative value of thermal conductivity as a function of weight fraction for ZnOdispersed in ethylene glycol. Lines are provided for visual guidance.

R. Saleh et al. / International Journal of Thermal Sciences 63 (2013) 125e132130[28,38], some of whom explained the change in the optical gap onthe basis of variation of the average crystallite size. This explanationis also consistent with our results obtained from the XRD spectra(Table 1).

To connect the observed structural and optical properties of theZnO nanoparticles with those in the uid samples, we performedFTIR and UVeVis measurements on the diluted ZnO nanoparticlesin ethylene glycol (ZnO/EG). Fig. 6 (a) shows infrared absorptionspectra of ZnO/EG nanouids with average crystallite sizes of 18and 23 nm. The spectra of pure EG are also shown in the gure.Overall, the spectra of the ZnO/EG were similar to that of the baseuid (EG), indicating that the ZnO nanoparticles were welldispersed in their base uid. Fig. 6(b) shows the UVeVis absorptionspectra taken from ZnO/EG plotted in Fig. 6(a). For the lowestconcentrations (0.025 vol.%), the UVeVis absorption spectra ofZnO/EG nanouids with average crystallite sizes of 18 or 23 nmexhibited absorption peaks at approximately 348 or 359 nm,

respectively. Increasing the concentrations of ZnO/EG results only

Fig. 8. Comparison between the measured thermal conductivity of the ZnO nanouidsin the present work and the results from other researchers.Fig. 9. Average heat pipe temperature distributions for different weight fractions ofZnO nanoparticles synthesized with a drying temperature of 200 C dispersed in

ethylene glycol.

R. Saleh et al. / International Journal of Twith average crystallite sizes of 18 and 23 nm, respectively. For allmeasurements, the wall temperatures of the heat pipes decreasedalong the test section from the evaporator to the condenser sectionand increased with an increase in input power. As shown, the

Fig. 10. Average heat pipe temperature distributions for different weight fractions ofZnO nanoparticles synthesized with a drying temperature of 600 C dispersed inethylene glycol.

Fig. 11. Thermal resistances of the heat pipes shown in Figs. 9 and 10. Lines areprovided for visual guidance.temperature distributions of the heat pipe containing ethyleneglycol as a base uid were 88.6, 84.6, 70.8, 67.9 and 28.7 C,respectively. As the EG was replaced by 0.5 vol.% of ZnO/EG, theheat pipe wall temperature at the evaporator was signicantlyreduced from 86.6 to 80.7 C or from 86.5 to 82.6 C for ZnOnanouids with average crystallite sizes of 18 or 23 nm, respec-tively. Figs. 9 and 10 demonstrate that as the volume fraction of ZnOnanoparticles increased, the heat pipe wall temperature becamelower than that of the pipes lled with ethylene glycol. Upondispersing more nanoparticles in the working uid, the heat pipewall temperature decreased more than that of a pure base uid-lled heat pipe under various heat loads. These results indicatethat utilization of the ethylene glycol-based ZnO (EG/ZnO) nano-uid as the working uid enhanced the thermal performance of theheat pipe. In addition, it was also shown that both the volumefraction of nanoparticles and the average crystallite size of thenanoparticles inuenced the reduction of the wall temperature inthe evaporator section.

Based on the distribution of the wall temperature measure-ments, the thermal resistance between the evaporator section andthe adiabatic section can be calculated using the equation Rth DT/Q, where DT is the temperature difference between the evaporatorand adiabatic sections and Q is the input power. As Fig. 11 shows,the thermal resistance of the heat pipe using ZnO/EG is lower thanthat of the heat pipe using pure EG. Overall, the thermal resistanceof the heat pipe containing 23-nm-sized ZnO nanoparticle crys-tallites was lower than those using the smaller crystallite size ZnO(18 nm) or a higher input power. Increasing the volume fraction ofthe nanoparticles tended to decrease the thermal resistance evenfurther. This nding indicates that nanoparticles play an importantrole in reducing the thermal resistance in the heat pipe, as theincrease in the effective thermal conductivity of the working uidin the heat pipe has a correlation to the thermal resistance of theheat pipe.

4. Conclusion

In the present study, we reported our experimental investigationon the effective thermal conductivities of ethylene glycol-basednanouids containing low concentrations of ZnO nanoparticles. Inaddition, an experiment was performed to investigate the effect ofnanoparticles in a nanouid on the thermal performanceof a screen-mesh heat pipe. For these experiments, ZnO nanoparticles weresynthesized using a co-precipitation method and were dispersed inethylene glycol at concentrations from 0.025 to 0.5 vol.%. Theproperties of theZnOnanoparticleswere characterizedbyXRD, EDX,thermal analysis TGeDTA, UVeVis spectroscopy and infraredabsorption spectroscopy. The XRD patterns of the ZnO nanoparticlesagreed well with the standard XRD pattern of ZnOwith a hexagonalwurtzite structure. These results are consistent with the resultsobtained from EDX spectra. Moreover, all diffraction peaks revealedstrong peak intensities, indicating that the produced ZnO particleshad high crystallinity. The as-synthesized ZnO particles had averagecrystallite sizes of 18 or 23 nm. UVeVismeasurements showed a redshift in the optical absorption band with increasing average crys-tallite size. To connect the observed properties of the ZnO nano-particles with those in the uid samples, FTIR and UVeVismeasurements were conducted on the diluted ZnO nanoparticles inethylene glycol. The thermal conductivity behaviors of the synthe-sized nanouids were measured as a function of particle volumefraction. The experimental data revealed that nanouids containinga small fraction of nanoparticles had higher thermal conductivitiescompared to the base uid. The thermal conductivity increasedsignicantlywith increasing volume fraction of nanoparticles. It was

hermal Sciences 63 (2013) 125e132 131also demonstrated that the crystallite size inuenced the thermal

conductivity enhancement of the nanouid. At lower particleconcentrations, the conductivity ratio showed enhancements ofapproximately 5.3% until 15.5%. Moreover, it was found that theaverage crystallite size had an effect on the thermal conductivity.Thermal performance was evaluated with ZnO as the working uidfor a conventional screen-mesh wick heat pipe. In this experiment,both the temperature distribution and the heat pipe thermal resis-tance were evaluated, and the presence of nanoparticles in theworking uid led to a reduction of the heat pipe wall temperature.The heat pipe wall temperature at the evaporator was signicantlyreduced about 6 C. In addition, it was observed that the tempera-ture distribution and the heat pipe thermal resistance varied withthe particle volume fraction and the size of the ZnO nanoparticles.

dispersion of Al2O3, SiO2 and TiO2 ultra-ne particles, Netsu Bussei 4 (1993)227e233.

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[19] S.M.S. Murshed, K.C. Leong, C. Yang, Enhanced thermal conductivity ofTiO2dwater based nanouids, International Journal of Thermal Sciences 44(2005) 367e373.

[20] C.Y. Tsai, H.T. Chien, P.P. Ding, B. Chan, T.Y. Luh, P.H. Chen, Effect of structuralcharacter of gold nanoparticles in nanouid on heat pipe thermal perfor-mance, Materials Letters 58 (2003) 1461e1465.

[21] S.-W. Kang, W.-C. Wei, S.-H. Tsai, S.-Y. Yang, Experimental investigation ofsilver nano-uid on heat pipe thermal performance, Applied Thermal Engi-neering 26 (2006) 2377e2382.

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Experimental investigation of thermal conductivity and heat pipe thermal performance of ZnO nanofluids1. Introduction2. Experimental procedure2.1. Synthesis of ZnO nanoparticles2.2. Characterization of ZnO nanoparticles2.3. Synthesis of ZnO nanofluids2.4. Measurement of thermal conductivity of the nanofluids2.5. Measurement of ZnO nanofluid heat pipe thermal performance

3. Results and discussion4. ConclusionReferences