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Research Papers: Electronic Cooling

Fluid Flow and Heat Transfer in a Horizontal Channel With Divergent Top Wall and Heated From Below

[+] Author and Article Information
C. S. Yang

Department of Computer Science and Information Engineering, Far East University, Hsin-Shih, Tainan County 744, Taiwan

D. Z. Jeng

Aeronautics Systems Research Division, Chung-Shan Institute of Science and Technology, Taichung 407, Taiwan

C. W. Liu, C. G. Liu

Institute of Aeronautics and Astronautics and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan

C. Gau1

Institute of Aeronautics and Astronautics and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwangauc@mail.ncku.edu.tw

1

Corresponding author.

J. Heat Transfer 132(8), 081403 (Jun 10, 2010) (8 pages) doi:10.1115/1.4001606 History: Received February 12, 2009; Revised February 10, 2010; Published June 10, 2010; Online June 10, 2010

Abstract

Secondary flow structure and its enhancement on the heat transfer in a horizontal divergent channel have been studied. The bottom wall is horizontal and is heated uniformly while the opposite top wall is insulated and inclined with respect to the horizontal plane so as to create a divergent angle of 3 deg. At the entrance of the channel, the aspect ratios for the width to the height and the channel length to the height are 6.67 and 15, respectively. The Reynolds number ranges from 100 to 2000 and the buoyancy parameter $Gr/Re2$ from 0 to 405. Both flow visualization and temperature fluctuation measurements at different locations are made to indicate the flow structure and oscillation of the secondary flow. The adverse pressure gradient in the divergent channel causes a thicker heated layer in the bottom and earlier initiation of secondary flow. Interaction between neighboring vortices and plumes becomes more severe and highly unstable. This precludes the formation of steady two-dimensional longitudinal vortex rolls in the downstream and leads to an earlier and larger enhancement of the heat transfer than the case of the parallel-plate channel. The effects of the buoyancy parameter and the divergence of the channel on the secondary flow structure and the Nusselt number are presented and discussed.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

Figure 1

Schematic for (a) the experimental setup, which includes (1) wind tunnel, (2) smoke generator, (3) dc power supply, (4) Plexiglas channel, (5) camera, (6) laser, (7) rotating mirror, (8) data acquisition system, (9) personal computer, and (10) compressor to blow smoke from smoke generator into the channel; and (b) the horizontal divergent channel, which includes (1) slot for smoke injection, (2) stainless steel foil, and (3) seven rows of thermocouples in the transverse direction

Figure 2

Flow structure in a divergent channel with Gr/Re2=20 and Re=500 at different locations (end view)

Figure 3

Flow structure in a parallel-plate channel with Gr/Re2=20 and Re=500 at different locations (end view)

Figure 4

Flow structure in a divergent channel with Gr/Re2=60 and Re=500 at different locations

Figure 5

Flow structure in a parallel-plate channel with Gr/Re2=450 and Re=105 at different locations

Figure 6

Flow structure in a divergent channel with Gr/Re2=450 and Re=105 at different locations

Figure 7

Flow structure in a divergent channel with Gr/Re2=10 and Re=1000; (a) top view, photograph is taken in a plane close to the bottom wall; ((b)–(f)) end view

Figure 8

Comparison of temperature fluctuations between the case of divergent channel and the case of a parallel-plate channel with Gr/Re2=20 and Re=500 at (a) x=15 cm, (b) x=25 cm, (c) x=31 cm, (d) x=36 cm, and (e) x=44 cm (the dotted lines are the mean temperature fluctuations in the parallel-plate channel, and the inlet temperature for both parallel-plate channel and the divergent channel is 27°C)

Figure 9

Temperature fluctuations in a horizontal divergent channel with Gr/Re2=60 and Re=500 at (a) x=5 cm, (b) x=15 cm, (c) x=25 cm, (d) x=31 cm, (e) x=36 cm, and (f) x=44 cm

Figure 10

Comparison of temperature fluctuations between the case of divergent channel and the case of the parallel-plate channel with Gr/Re2=10 and Re=1000 at (a) x=5 cm, (b) x=15 cm, (c) x=25 cm, (d) x=31 cm, (e) x=36 cm, and (f) x=44 cm ( ∗ means temperature fluctuations in the parallel-plate channel)

Figure 11

Comparison of the Nusselt number distributions at different Gr/Re2 between the case of divergent channel and the case of parallel-plate channel at (a) Re=500 and (b) Re=1000

Figure 12

Normalization of the Nusselt numbers for the case of the divergent channel

Figure 13

The normalized Nusselt number Nu/Re0.4 distributions at different Gr/Re2 for the divergent channel

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