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Research Papers

# Single-Phase Thermal Transport of Nanofluids in a Minichannel

[+] Author and Article Information
Dong Liu1

Department of Mechanical Engineering, University of Houston, Houston, TX 77004-4006dongliu@uh.edu

Leyuan Yu

Department of Mechanical Engineering, University of Houston, Houston, TX 77004-4006

1

Corresponding author.

J. Heat Transfer 133(3), 031009 (Nov 16, 2010) (11 pages) doi:10.1115/1.4002462 History: Received February 15, 2010; Revised May 06, 2010; Published November 16, 2010; Online November 16, 2010

## Abstract

Nanofluids have been proposed as a promising candidate for advanced heat transfer fluids in a variety of important engineering applications ranging from energy storage and electronics cooling to thermal processing of materials. In spite of the extensive studies in the literature, a consensus is lacking on if and how the dispersed nanoparticles alter the thermal transport in convective flows. In this work, an experimental investigation was conducted to study single-phase forced convection of $Al2O3$-water nanofluid in a circular minichannel with a 1.09 mm inner diameter. The friction factor and convection heat transfer coefficients were measured for nanofluids of various volume concentrations (up to 5%) and were compared with those of the base fluid. The Reynolds number (Re) varied from 600 to 4500, covering the laminar, transition, and early fully developed turbulent regions. It was found that in the laminar region, the nanofluids exhibit pronounced entrance region behaviors possibly due to the flattening of the velocity profile caused by the flow-induced particle migration. Three new observations were made for nanofluids in the transition and turbulent regions: (1) The onset of transition to turbulence is delayed; (2) both the friction factor and the convective heat transfer coefficient are below those of water at the same Re in the transition flow; and (3) once fully developed turbulence is established, the difference in the flow and heat transfer of nanofluids and water will diminish. A simple scaling analysis was used to show that these behaviors may be attributed to the variation in the relative size of the nanoparticle with respect to the turbulent microscales at different Re. The results from this work suggest that the particle-fluid interaction has a significant impact on the flow physics of nanofluids, especially in the transition and turbulent regions. Consequently, as a heat transfer fluid, nanofluids should be used in either the laminar flow or the fully developed turbulent flow at sufficiently high Re in order to yield enhanced heat transfer performance.

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## Figures

Figure 1

Effective particle size versus sonication time (0.1% Al2O3 nanofluid, pH=3)

Figure 2

DLS measurement of effective particle size (0.1% Al2O3 nanofluid, pH=3)

Figure 3

Schematic of the experimental apparatus

Figure 4

Effective viscosity of nanofluid at various volume concentrations at 25°C

Figure 5

Effective thermal conductivity of nanofluid at various temperatures (error bar ±5%)

Figure 6

Friction factor versus Reynolds number (water)

Figure 7

Local Nusselt number versus the inverse of Graetz number (water, heat flux q″=6.5 kW/m2)

Figure 8

Average Nusselt number versus Reynolds number (water, heat flux q″=6.5 kW/m2)

Figure 9

Friction factor versus Reynolds number for nanofluids at various volume concentrations

Figure 10

Local convective heat transfer of nanofluids (heat flux q″=6.5 kW/m2)

Figure 11

Average Nusselt number versus Reynolds number in the laminar flow region (nanofluids, heat flux q″=6.5 kW/m2)

Figure 12

Average Nusselt number versus Reynolds number in the transition and turbulent flow regions (nanofluids, heat flux q″=6.5 kW/m2)

Figure 13

Average Nusselt number versus Reynolds number over the entire flow range (nanofluids, heat flux q″=6.5 kW/m2)

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