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Current Applied Physics 6 (2006) 1068–1071

Investigation on characteristics of thermal conductivityenhancement of nanofluids

Y.J. Hwang, Y.C. Ahn, H.S. Shin, C.G. Lee, G.T. Kim, H.S. Park, J.K. Lee *

Department of Mechanical Engineering, Pusan National University, San 30, Jangjeon-Dong, Kumjung-Ku, Busan, 609-735, Republic of Korea

Received 4 September 2004; received in revised form 2 April 2005Available online 19 August 2005

Abstract

It has been shown that a nanofluid consisting of nanoparticles dispersed in base fluid has much higher effective thermal conduc-tivity than pure fluid. In this study, four kinds of nanofluids such as multiwalled carbon nanotube (MWCNT) in water, CuO inwater, SiO2 in water, and CuO in ethylene glycol, are produced. Their thermal conductivities are measured by a transient hot-wiremethod. The thermal conductivity enhancement of water-based MWCNT nanofluid is increased up to 11.3% at a volume fraction of0.01. The measured thermal conductivities of MWCNT nanofluids are higher than those calculated with Hamilton–Crosser modeldue to neglecting solid–liquid interaction at the interface. The results show that the thermal conductivity enhancement of nanofluidsdepends on the thermal conductivities of both particles and the base fluid.� 2005 Elsevier B.V. All rights reserved.

PACS: 82.70.k; 47.17

Keywords: Nanofluids; Nanoparticle; Thermal conductivity; Zeta potential; Transient hot-wire method

1. Introduction

Heating or cooling fluids are important to manyindustrial sectors, including transportation, energy sup-ply and production and electronics. The thermal con-ductivity of these fluids plays a vital role in thedevelopment of energy-efficient heat transfer equipment.However, conventional heat transfer fluids have poorheat transfer properties compared to most solids. Infact, an amount of studies of the effective thermal con-ductivity of suspensions consisting of solid particleshave been conducted. However, these studies have been

1567-1739/$ - see front matter � 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.cap.2005.07.021

* Corresponding author. Tel.: +82 51 510 2455; fax: +82 51 5125236.

E-mail address: jklee@pusan.ac.kr (J.K. Lee).

confined to those produced with millimeter or microme-ter-sized particles. The lack of stability of suspensionsthat involve coarse-grained particles is undoubtedly aprimary reason why the fluids with dispersed milli- ormicro-sized particles have not been applied to anyindustrial sectors. In the recent researches, suspensionscontaining a small amount of metal, nonmetal nanopar-ticles or carbon nanotubes have been reported to havesubstantially higher thermal conductivities than thoseof the base fluids [1–3]. Mechanisms of the thermal con-ductivity enhancement in nanofluids also have beenstudied [4,5]. To measure the thermal conductivity offluid, a transient hot-wire method is widely used. Inthe present study, we produce suspensions with CuO,SiO2 and multiwalled carbon nanotubes (MWCNTs)with base fluids of DI-water, ethylene glycol. The ther-mal conductivities of these nanofluids are measured withthe transient hot-wire method.

Table 1Properties of materials for producing nanofluid

Properties Particles

Carbon nanotubes (multiwalled) CuO SiO2 H2O Ethylene glycol

Specific gravity 2.6 6.32 2.22 1 1.11Thermal conductivity (W/mK) �3000 76.5 1.38 0.613 0.252Average size Length: 10–50 lm 33 nm 12 nm – –

Diameter: 10–30 nm

Y.J. Hwang et al. / Current Applied Physics 6 (2006) 1068–1071 1069

2. Materials

Table 1 shows the properties of materials for pre-paring nanofluids. The thermal conductivities ofMWCNTs, CuO nanoparticles and SiO2 nanoparticlesare 3000 W/mK [6], 76.5 W/mK, and 1.38 W/mK,respectively. The thermal conductivities of base fluids,DI-water and ethylene glycol, are 0.613 W/mK and0.252 W/mK, respectively.

Fig. 1 shows the photographs of the test particles.MWCNTs have fibrous morphologies and the averagelength and diameter are 10–50 lm and 10–30 nm,respectively. The average diameters of CuO and SiO2

nanoparticles are 35.4 nm and 7 nm, respectively. Mor-phologies of these particles are spherical.

An ultrasonic disruptor is used to produce nanofl-uids. After 2 h intensive sonication, the stable suspen-

Fig. 1. Photographs o

sions are obtained in case of CuO and SiO2

nanofluids. Since MWCNTs are entangled and agglom-erated in aqueous suspension, sodium dodecyl sulfate(SDS) is used as a surfactant when producing MWCNTnanofluids.

3. Measuring thermal conductivity of nanofluid

Fig. 2 shows schematic diagram of transient hot-wiresystem for thermal conductivity measurement of carbonnanofluids. Transient hot-wire method for measuringthermal conductivity of a low viscosity fluid is well-known. In this study, transient hot-wire method formeasuring electrically conducting fluid [7] because theparticles, used in this experiment, are electrically con-ductive. Teflon coated platinum wire, which diameter

f test particles.

R1

RwR3

R2

Voltage output Data acquisition

system

Platinum Wire

⇒

Fig. 2. Schematic diagram of transient hot-wire system for thermalconductivity measurement of nanofluids.

0.00 0.25 0.50 0.75 1.00

0

10

20

30

40

Measured Data: MWCNT+Water Measured Data: MWCNT+Ethylene glycol

----- Hamilton-Crosser Model: MWCNT+Water

Δk/

k o x 1

00 (%

)

Volume Fraction (%)

Fig. 3. Comparison between the values calculated with Hamilton–Crosser model and the experimental results for water-based MWCNTnanofluids as a function of MWCNT volume fractions.

1070 Y.J. Hwang et al. / Current Applied Physics 6 (2006) 1068–1071

is 76 lm and the thickness of Teflon insulation layer is17 lm, is used for a hot wire in the measurement system.Initially the platinum wire immersed in media is kept atequilibrium with surroundings. When a uniform voltageis supplied to the circuit, the electric resistance of theplatinum wire rises with the temperature of the wireand the voltage output is measured by an A/D convert-ing system at a sampling rate of 10 times per second. Therelation between the electric resistance and the tempera-ture of platinum wire is well-known [8]. The measureddata of temperature rise is linear against logarithmictime interval. The thermal conductivity is calculatedfrom the slope of the rise in the wire�s temperatureagainst logarithmic time interval by the following equa-tion [7]:

k ¼ q4pðT 2 � T 1Þ

lnt2

t1

� �. ð1Þ

The value of k is the thermal conductivity of fluid. T isthe temperature of the wire at time t.

4. Thermal conductivity of nanofluid

Fig. 3 depicts the thermal conductivity enhancementsof DI-water based MWCNT nanofluid as a function ofvolume fraction of MWCNTs. In this paper, Dk is thethermal conductivity enhancement of nanofluid, andko is the thermal conductivity of base fluid. The thermalconductivity of nanofluid is enhanced up to 11.3% at avolume fraction of 0.01. Uncertainty of the thermal con-ductivity of nanofluid is increased with increasing parti-cle volume fraction. The uncertainties of the thermalconductivity of MWCNT nanofluid are 0.51% and2.19% at a volume fraction of 0.0025 and 0.01, respec-tively. It is believed that it is difficult to obtain highlystable suspension which has good reproducible proper-

ties when the MWCNTs volume fraction increases. Itis shown that the comparison between the experimentaldata for MWCNT in deionized water and the values cal-culated from Hamilton–Crosser model [9]. The resultshows that the measured thermal conductivities aregreater than those calculated from Hamilton–Crossermodel. Hamilton–Crosser model is known as followingequation [9]:

ke

ko

¼ kp þ ðn� 1Þko � ðn� 1Þ/ðko � kpÞkp þ ðn� 1Þko þ /ðko � kpÞ

. ð2Þ

The notations of e, p, and o represent nanofluid, particleand base fluid, respectively. / and k represent the vol-ume fraction and thermal conductivity, respectively.

In suspensions, the liquid molecules close to a particlesurface are known to form a solid-like layer at the inter-face. This solid-like structure of liquid molecules mayplay an important role in heat transport from solid wallto adjacent liquid [10]. Since these interactions are notconsidered in conventional models for predictingthermal conductivity of suspension, they always under-estimate the thermal conductivity enhancement of nano-fluid. The weakness of this model has been presentedelsewhere [1–3,11].

Fig. 4 shows the thermal conductivities of variousnanofluids. For water-based nanofluid, MWCNT nano-fluid has the highest thermal conductivity whereas SiO2

nanofluid has the lowest thermal conductivity. This re-sult shows that the thermal conductivity enhancementof nanofluid is higher when the suspended nanoparticleshave higher thermal conductivity. In case of CuO nano-fluids, the thermal conductivity enhancement of ethyleneglycol-based nanofluid is higher than that of water-based nanofluid. This result shows that nanofluid ismore effective for a working fluid which has lower ther-mal conductivity.

0

2

4

6

8

10

12

Particle concentration: 1 vol%

Ethylene Glycol + CuO

Water+SiO2Water+CuOWater+MWCNT

Δk/

k o x

100

(%)

Fig. 4. Thermal conductivity enhancement of four kinds of nanofluidsat a particle volume fraction of 1%.

Y.J. Hwang et al. / Current Applied Physics 6 (2006) 1068–1071 1071

5. Conclusions

To investigate the thermal conductivity of nanofluid,the thermal conductivity of four kinds of nanofluidssuch as MWCNTs in water, CuO in water, SiO2 inwater, and CuO in ethylene glycol, are measured by atransient hot-wire method. The thermal conductivityof MWCNT nanofluids are almost linearly increasedwith increasing particle volume fraction and the mea-sured thermal conductivity of MWCNT nanofluids arehigher than the calculated values with Hamilton–Cros-

ser model due to neglecting solid–liquid interaction atthe interface. The thermal conductivity enhancementof water-based MWCNT nanofluid is increased up to11.3% at a volume fraction of 0.01. The results showthat the thermal conductivity of nanofluids depends onthe thermal conductivity of the suspended particlesand base fluids.

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