RESEARCH PAPERS: Micro/Nanoscale Heat Transfer

Convective Transport in Nanofluids

[+] Author and Article Information
J. Buongiorno1

Nuclear Science and Engineering Department, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307


Tel: (617)253-7316; e-mail: jacopo@mit.edu

J. Heat Transfer 128(3), 240-250 (Aug 15, 2005) (11 pages) doi:10.1115/1.2150834 History: Received March 07, 2005; Revised August 15, 2005

Nanofluids are engineered colloids made of a base fluid and nanoparticles (1100nm). Nanofluids have higher thermal conductivity and single-phase heat transfer coefficients than their base fluids. In particular, the heat transfer coefficient increases appear to go beyond the mere thermal-conductivity effect, and cannot be predicted by traditional pure-fluid correlations such as Dittus-Boelter’s. In the nanofluid literature this behavior is generally attributed to thermal dispersion and intensified turbulence, brought about by nanoparticle motion. To test the validity of this assumption, we have considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid. These are inertia, Brownian diffusion, thermophoresis, diffusiophoresis, Magnus effect, fluid drainage, and gravity. We concluded that, of these seven, only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids. Based on this finding, we developed a two-component four-equation nonhomogeneous equilibrium model for mass, momentum, and heat transport in nanofluids. A nondimensional analysis of the equations suggests that energy transfer by nanoparticle dispersion is negligible, and thus cannot explain the abnormal heat transfer coefficient increases. Furthermore, a comparison of the nanoparticle and turbulent eddy time and length scales clearly indicates that the nanoparticles move homogeneously with the fluid in the presence of turbulent eddies, so an effect on turbulence intensity is also doubtful. Thus, we propose an alternative explanation for the abnormal heat transfer coefficient increases: the nanofluid properties may vary significantly within the boundary layer because of the effect of the temperature gradient and thermophoresis. For a heated fluid, these effects can result in a significant decrease of viscosity within the boundary layer, thus leading to heat transfer enhancement. A correlation structure that captures these effects is proposed.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Heat transfer data for water-based nanofluids with oxide nanoparticles (7) and metal nanoparticles (13). Note the deviation from the Dittus-Boelter correlation, despite the fact that the nanofluid properties were used in defining Nu, Re, and Pr in both studies: (a) alumina and titania nanoparticles; and (b) copper nanoparticles.

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Figure 2

Flow structure near the wall

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Figure 3

Variation of the nanoparticle volumetric fraction within the laminar sublayer

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Figure 4

Heat transfer in alumina/water nanofluids: (a) ϕb=0; (b) ϕb=0.01; and (c) ϕb=0.03

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Figure 5

Heat transfer in titania/water nanofluids: (a) ϕb=0.01; and (b) ϕb=0.03



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