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RESEARCH PAPERS: Natural and Mixed Convection

# Thermal Conductivity of Metal-Oxide Nanofluids: Particle Size Dependence and Effect of Laser Irradiation

[+] Author and Article Information
Sang Hyun Kim, Sun Rock Choi

Department of Mechanical Engineering, POSTECH, Pohang, 790-784 Korea

Dongsik Kim1

Department of Mechanical Engineering, POSTECH, Pohang, 790-784 Koreadskim87@postech.ac.kr

1

Corresponding author.

J. Heat Transfer 129(3), 298-307 (May 25, 2006) (10 pages) doi:10.1115/1.2427071 History: Received October 10, 2005; Revised May 25, 2006

## Abstract

The thermal conductivity of water- and ethylene glycol-based nanofluids containing alumina, zinc-oxide, and titanium-dioxide nanoparticles is measured using the transient hot-wire method. Measurements are performed by varying the particle size and volume fraction, providing a set of consistent experimental data over a wide range of colloidal conditions. Emphasis is placed on the effect of the suspended particle size on the effective thermal conductivity. Also, the effect of laser-pulse irradiation, i.e., the particle size change by laser ablation, is examined for $ZnO$ nanofluids. The results show that the thermal-conductivity enhancement ratio relative to the base fluid increases linearly with decreasing the particle size but no existing empirical or theoretical correlation can explain the behavior. It is also demonstrated that high-power laser irradiation can lead to substantial enhancement in the effective thermal conductivity although only a small fraction of the particles are fragmented.

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## Figures

Figure 1

Typical TEM images of nanoparticles

Figure 2

Schematic diagram of the experimental setup for thermal conductivity measurement

Figure 3

Schematic diagram of the laser fragmentation process for nanofluid size reduction

Figure 4

TEM images of ZnO∕water nanofluids for average particle sizes of (a) 60, (b) 30, and (c)10nm

Figure 5

Absorbance peak as a function of particle mean diameter

Figure 6

Thermal-conductivity enhancement by Al2O3∕water and by Al2O3∕EG nanofluids as a function of volume fraction

Figure 7

Thermal-conductivity ratio: (a)ZnO∕water and (b)ZnO∕EG nanofluids

Figure 8

Thermal-conductivity ratio: (a)TiO2∕water and (b)TiO2∕EG nanofluids

Figure 9

Variation of thermal-conductivity ratio with particle diameter: (a)ZnO∕water and (b)ZnO∕EG nanofluids (comparison with a theoretical model (see Ref. 9)

Figure 10

Variation of thermal-conductivity ratio with particle diameter: (a)TiO2∕water and (b)TiO2∕EG nanofluids (comparison with a theoretical model (see Ref. 9)

Figure 11

Normalized thermal conductivity of 60nmZnO∕water nanofluids after laser irradiation at different laser fluences (36,000 pulses) and two concentrations. The thermal conductivity has been normalized by the value before laser irradiation.

Figure 12

TEM images of ZnO nanoparticles after laser irradiation at laser fluences (a) 310 and (b)610mJ∕cm2

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