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On the Anomalous Convective Heat Transfer Enhancement in Nanofluids: A Theoretical Answer to the Nanofluids Controversy

[+] Author and Article Information
M. Mochizuki

Department of Mechanical Engineering,
Shizuoka University,
3-5-1 Johoku, Naka-ku,
Hamamatsu 432-8561, Japan

A. Nakayama

Department of Mechanical Engineering,
Shizuoka University,
3-5-1 Johoku, Naka-ku,
Hamamatsu 432-8561, Japan;
School of Civil Engineering and Architecture,
Wuhan Polytechnic University,
Wuhan, Hubei 430023, China
e-mail: tmanaka@ipc.shizuoka.ac.jp

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received June 5, 2012; final manuscript received January 3, 2013; published online April 11, 2013. Assoc. Editor: Oronzio Manca.

J. Heat Transfer 135(5), 054504 (Apr 11, 2013) (9 pages) Paper No: HT-12-1270; doi: 10.1115/1.4023539 History: Received June 05, 2012; Revised January 03, 2013

A theoretical answer to the controversial issue on the anomalous convective heat transfer in nanofluids has been provided, exploiting the Buongiorno model for convective heat transfer in nanofluids with modifications to fully account for the effects of nanoparticle volume fraction distributions on the continuity, momentum, and energy equations. A set of exact solutions have been obtained for hydrodynamically and thermally fully developed laminar nanofluid flows in channels and tubes, subject to constant heat flux. From the solutions, it has been concluded that the anomalous heat transfer rate, exceeding the rate expected from the increase in thermal conductivity, is possible in such cases as titania–water nanofluids in a channel, alumina–water nanofluids in a tube and also titania–water nanofluids in a tube. Moreover, the maximum Nusselt number based on the bulk mean nanofluid thermal conductivity is captured when the ratio of Brownian and thermophoretic diffusivities is around 0.5, which can be exploited for designing nanoparticles for high-energy carriers.

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Figures

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Fig. 1

Physical models: (a) channel flow and (b) tube flow

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Fig. 6

Heat transfer characteristics in alumina–water nanofluids in a tube, NuB

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Fig. 7

Heat transfer characteristics in titania–water nanofluids in a tube, NuB

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Fig. 8

Comparison of the heat transfer coefficients of nanofluids in a tube

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Fig. 2

Effects of nanoparticle volume fraction on velocity, temperature and volume fraction profiles in a tube with NBT = 0.2 and γ = 0: (a) velocity profiles; (b) temperature profiles; and (c) volume fraction profiles

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Fig. 3

Effects of the ratio of Brownian and thermophoretic diffusivities NBT on the pressure gradients: (a) channel and (b) tube

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Fig. 4

Heat transfer characteristics in alumina–water nanofluids in a channel: (a) NuB; (b) hDh/kw; and (c) kw/kB

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Fig. 5

Heat transfer characteristics in titania–water nanofluids in a channel: (a) NuB; (b) hDh/kw; and (c) kw/kB

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