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Research Papers: Micro/Nanoscale Heat Transfer

Large Convective Heat Transfer Enhancement in Microchannels With a Train of Coflowing Immiscible or Colloidal Droplets

[+] Author and Article Information
Magnus Fischer

Laboratory of Thermodynamics in Emerging Technologies, Institute of Energy Technology, Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zürich, Switzerland

Damir Juric

Laboratoire d’Informatique pour la Mécanique et les Sciences de l’Ingénieur (LIMSI), Centre National de la Recherche Scientifique (CNRS), UPR 3251, BP 133, 91403 Orsay Cedex, France

Dimos Poulikakos1

Laboratory of Thermodynamics in Emerging Technologies, Institute of Energy Technology, Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zürich, Switzerlanddimos.poulikakos@ethz.ch

1

Corresponding author.

J. Heat Transfer 132(11), 112402 (Aug 13, 2010) (10 pages) doi:10.1115/1.4002031 History: Received November 30, 2009; Revised May 03, 2010; Published August 13, 2010; Online August 13, 2010

We show that heat transfer in microchannels can be considerably augmented by introducing droplets or slugs of an immiscible liquid into the main fluid flow. We numerically investigate the influence of differently shaped colloidal or simply pure immiscible droplets to the main liquid flow on the thermal transport in microchannels. Results of parametric studies on the influence of all major factors connected to microchannel heat transfer are presented. The effect of induced Marangoni flow at the liquid interfaces is also taken into account and quantified. The calculation of the multiphase, multispecies flow problem is performed, applying a front tracking method, extended to account for nanoparticle transport in the suspended phase when relevant. This study reveals that the use of a second suspended liquid (with or without nanoparticles) is an efficient way to significantly increase the thermal performance without unacceptably large pressure losses. In the case of slug-train coflow, the Nusselt number can be increased by as much as 400% compared with single liquid flow.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

The physical system under consideration. In the isothermal section of the microchannel, the droplets (liquid 1) are immersed into the base fluid (liquid 2).

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Figure 2

Local Nusselt number and temperature field in the axial direction for liquid-liquid flow with spherical droplets. The liquids used are water as the base fluid (liquid 1) and PAO and Al2O3 nanoparticles as the suspended liquid (liquid 2). The dashed line compares to the Nusselt number of a single fluid flow of water. The solid line shows the approximate solution for the Nusselt number (Eq. 24).

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Figure 14

Nusselt number versus pressure drop normalized by the pressure drop of one liquid flow (water)

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Figure 3

(a) Temperature field and (b) streamlines in a moving reference frame for nanofluid droplet-laden flow. The liquids used are water as the base fluid (liquid 1) and PAO and Al2O3 nanoparticles as the suspended liquid (liquid 2).

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Figure 4

Local Nusselt number and the temperature field in the axial direction for liquid-liquid flow with elongated droplets. The liquids used are water as the base fluid (liquid 1) and PAO and Al2O3 nanoparticles as the suspended liquid (liquid 2). The dashed line compares to the Nusselt number of a single liquid flow of water.

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Figure 5

(a) Temperature field), (b) streamlines in a moving reference frame, and (c) the particle concentration for liquid slug flow. The liquids used are water as the base fluid (liquid 1) and PAO and Al2O3 nanoparticles as the suspended liquid (liquid 2).

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Figure 6

Temporally averaged Nusselt number for different channel radii and velocities. The liquid in the droplet (liquid 1) is 5 cS silicone oil. It is surrounded by water (liquid 2). The dashed lines indicate the solution for aqueous single liquid flow.

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Figure 7

Nusselt number distribution for different cases with constant surface tension (σ=σ0) compared with calculations, where the Marangoni effect is included (σ=f(T)). The liquid in the droplet (liquid 1) is 5 cS silicone oil, surrounded by water (liquid 2).

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Figure 8

Velocity vectors in a moving reference frame (top) and temperature field (bottom) around a single spherical droplet

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Figure 9

The effect of nanoparticles on the temporally averaged local Nusselt number. In (a), the carrier liquid (liquid 2) is 5 cS silicone oil, and the liquid in the droplets (liquid 1) is water with and without nanoparticles. In (b), similar results are shown for slug flow, where the carrier liquid (liquid 1) is water and the liquid in the droplet (liquid 2) is PAO with and without nanoparticles.

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Figure 10

Bulk mean temperature Tb and the absolute value of the heat flux from the wall q∣wall,z=|−k(dT/dr)wall,z| in axial direction, as defined by Eqs. 22,23

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Figure 11

Temporally averaged Nusselt number distribution for different viscosity ratios μ1/μ2 and Reynolds numbers of the bulk fluid (liquid 2) Re2

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Figure 12

Temporally averaged Nusselt number distribution for differently shaped droplets. The fluids used for the droplet liquid (liquid 1) are (a) 5 cS silicone oil and (b) nanofluid of PAO and Al2O3 particles. In both cases, the base fluid (liquid 2) is water. The dashed line compares to the Nusselt number of single liquid aqueous flow.

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Figure 13

Temporally and spatially averaged Nusselt number versus pressure drop normalized by the pressure drop of one liquid water flow. (a) Nusselt number for differently shaped droplets, with and without nanoparticles. The liquids of the droplet (liquid 1) as well as their elongation (Δ) are given in the legend. (b) Nusselt number for spherical droplets with different viscosity ratios μ1/μ2. In (b), the liquids (liquid 1) are either PAO (+) or silicone oil (×). The dashed line separates high Reynolds number flow (Re2=50) from lower Reynolds number flow (Re2=5). In all cases shown here the base liquid (liquid 2) is water.

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