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TECHNICAL PAPERS: Heat Transfer Enhancement

A Numerical Study of Flow and Heat Transfer Enhancement Using an Array of Delta-Winglet Vortex Generators in a Fin-and-Tube Heat Exchanger

[+] Author and Article Information
A. Joardar1

Department of Mechanical Science and Engineering, University of Illinois, 158 Mechanical Engineering Building MC-244, 1206 West Green Street, Urbana, Il 61801joardar@uiuc.edu

A. M. Jacobi

Department of Mechanical Science and Engineering, University of Illinois, 158 Mechanical Engineering Building MC-244, 1206 West Green Street, Urbana, Il 61801

See http://www.eia.doe.gov/

1

Corresponding author.

J. Heat Transfer 129(9), 1156-1167 (Dec 06, 2006) (12 pages) doi:10.1115/1.2740308 History: Received September 14, 2006; Revised December 06, 2006

This work is aimed at assessing the potential of winglet-type vortex generator (VG) “arrays” for multirow inline-tube heat exchangers with an emphasis on providing fundamental understanding of the relation between local flow behavior and heat transfer enhancement mechanisms. Three different winglet configurations in common-flow-up arrangement are analyzed in the seven-row compact fin-and-tube heat exchanger: (a) single–VG pair; (b) a 3VG-inline array (alternating tube row); and (c) a 3VG-staggered array. The numerical study involves three-dimensional time-dependent modeling of unsteady laminar flow (330Re850) and conjugate heat transfer in the computational domain, which is set up to model the entire fin length in the air flow direction. It was found that the impingement of winglet redirected flow on the downstream tube is an important heat transfer augmentation mechanism for the common-flow-up arrangement of vortex generators in the inline-tube geometry. At Re=850 with a constant tube-wall temperature, the 3VG-inline-array configuration achieves enhancements up to 32% in total heat flux and 74% in j factor over the baseline case, with an associated pressure-drop increase of about 41%. The numerical results for the integral heat transfer quantities agree well with the available experimental measurements.

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Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Arrangement of vortex generators (3-row) on the fin-and-tube assembly: (a) schematic diagram showing core region of a plate fin-and-tube heat exchanger; (b) winglet vortex generator dimensions and the placement with respect to the tube; (c) coordinate system and computational domain comprising of single row of inline tubes mounted with winglets; and (d) typical computational mesh (all dimensions are in mm)

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Figure 2

Various configurations of the winglet pairs: (a) 1VG leading edge; (b) 3VG (alternate tube) inline array; and (c) 3VG staggered array

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Figure 3

Computed and measured (from Ref. 6) friction factor and Colburn j factor at different air-side Reynolds number for baseline and 3VG array configuration

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Figure 4

Pathlines downstream of trailing edge of winglets showing the so called “outflow vortex pair”

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Figure 5

3D iso-vorticity (magnitude) surfaces plotted in a selected range to identify the horseshoe vortex systems occurring at the junction of fin and tube for: (a) 3VG-inline array; and (b) baseline configurations

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Figure 6

Streamlines on the cross-sectional planes (top half only) at different axial locations coinciding with the indicated tube centers at Re=850: (a) single-VG-pair; and (b) 3VG-inline-array configuration

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Figure 7

Computed circulation (integral of vorticity magnitude) on the cross-stream planes at different axial locations for Re=850: (a) total circulation; and (b) net circulation for the enhanced configurations obtained by subtracting the baseline circulation from the total and is indicative of the winglet induced vortex strength

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Figure 8

Limiting streamlines on a plane close to the fin mounted with the vortex generators at Re=850 showing the singular points for flow in the vicinity of: (a) first tube; and (b) second tube: A=saddle point of separation, B=nodal point of attachment, and E=focus

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Figure 9

Local heat flux distribution on the VG fin (z=0) for: (a) 3VG-inline-array configuration; and (b) baseline configuration at Re=850; (c) line-weighted average heat flux distribution at different axial locations on the VG fin for all four configurations

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Figure 10

Distribution of span-averaged tube surface heat flux along tube circumference for baseline and winglet—enhanced configurations at Re=850 (a) tube with VG, both leading and interior; and (b) tube downstream of VG

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Figure 11

Steady-state area-averaged total tube surface heat flux for baseline and different winglet—enhanced configurations at Re=850

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