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Research Papers: Heat Transfer Enhancement

The Effectiveness of Secondary Flow Produced by Vortex Generators Mounted on Both Surfaces of the Fin to Enhance Heat Transfer in a Flat Tube Bank Fin Heat Exchanger

[+] Author and Article Information
Liang-Bi Wang

e-mail: lbwang@mail.lzjtu.cn
Key Laboratory of Railway Vehicle
Thermal Engineering,
Lanzhou Jiaotong University,
Ministry of Education,
Lanzhou, 730070, PRC;
Department of Mechanical Engineering,
Lanzhou Jiaotong University,
Lanzhou, Gansu, 730070, PRC

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the Journal of Heat Transfer. Manuscript received April 4, 2011; final manuscript received November 9, 2012; published online March 20, 2013. Assoc. Editor: Oronzio Manca.

J. Heat Transfer 135(4), 041902 (Mar 20, 2013) (11 pages) Paper No: HT-11-1191; doi: 10.1115/1.4023037 History: Received April 04, 2011; Revised November 09, 2012

Secondary flow is the flow in the cross section normal to the main flow. It plays an important role on the enhanced heat transfer and in the applications in other fields. Secondary flow can greatly enhance the convective heat transfer. In order to find the effectiveness of secondary flow for heat transfer enhancement, a nondimensional parameter, Se, based on the absolute vorticity flux is reported to specify the intensity of secondary flow. Its physical meaning is the ratio of inertial force to viscous force induced by secondary flow. As an example, the effectiveness of secondary flow was numerically studied for a flat tube bank fin heat exchanger with vortex generators (VGs) mounted on both surfaces of the fin. The contributions of VGs are investigated for the enhancements of secondary flow intensity, convective heat transfer, and pressure drop. The method is demonstrated using Se to find out the optimum configurations of VGs. The results reveal that close relationships exist not only between the span-average nondimensional intensity of secondary flow and the span-average Nusselt number but also between the volume average nondimensional intensity of secondary flow and the overall average Nusselt number. For the configuration studied, a ratio of Nusselt number enhancement to the friction factor enhancement increases with increasing the enhancement of secondary flow intensity. As a supplement to traditional criteria on a good performance heat transfer surface, the nondimensional intensity of secondary flow can be used clearly for an optimum value of VG parameter.

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Figures

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Fig. 1

Schematic view of a flat tube bank fin heat exchanger. (a) VGs mounted on both surfaces. (b) Without VGs, plain.

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Fig. 2

Schematic view of the three-dimensional simulation domain. (a) Perspective view. (b) Top view. (c) Side view.

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Fig. 3

Grid system used for the simulation domain. (a) Schematic view of the three-dimensional grid. (b) Top view of the grid on the x y plane. (c) Top view of grid near the VG. (d) Front view of the cross section.

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Fig. 10

Relative increasing rates of Sem, Num, and f induced by VGs

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Fig. 11

Effectiveness of secondary flow produced by VGs to heat transfer enhancement with H = 2.25 mm

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Fig. 5

Comparisons of numerical and experimental results, (a) Nu of the plain fin, (b) f of the plain fin, (c) Nus of the fin with VGs, (d) f of the fin with VGs

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Fig. 12

The effect of Δx/Lx on heat transfer performance and efficiency of secondary flow under different Re

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Fig. 13

The effect of H on heat transfer performance and efficiency of secondary flow under different Re

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Fig. 6

Relationships between Ses and Nus

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Fig. 7

Comparisons of Ses and Nus in the channels with VGs and without VGs and the contributions of VGs to Ses and Nus

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Fig. 8

Relationships between Sem, Num, and f

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Fig. 4

The effect of grids on Nus

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Fig. 9

Contributions of VGs to Sem, Num, and f

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