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A Particle Image Velocimetry Study of Flow Structure in an Offset-Strip Array with Delta-Wing Vortex Generators

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Title: A Particle Image Velocimetry Study of Flow Structure in an Offset-Strip Array with Delta-Wing Vortex Generators
Author(s): Smotrys, M.L.; Dutton, J.C.; Jacobi, A.M.
Subject(s): air-side heat transfer
Abstract: The potential for heat transfer enhancement from the combination of spanwise and streamwise vorticity has been investigated using a vortex-enhanced interrupted fin (VEIF) array. The VEIF consists of a fin with two delta-wing vortex generators (VGs) located symmetrically about the fin’s spanwise centerline. The VEIF was placed in an offset-strip fin array, and the array was thus called the Two-VG array. Spanwise vortices were created by the offset-strip array, and streamwise vortices were introduced to the flow by the VGs. The Two-VG array and a baseline offset-strip array were examined over the Reynolds number range 1025 £ Re £ 2450. The flow structure through the array was investigated using dye-in-water flow visualization and particle image velocimetry (PIV), and the results were compared to naphthalene sublimation heat transfer data obtained in a separate study. The Two-VG array showed heat transfer enhancement at Re £ 1025. At these Reynolds numbers, spanwise vortices are not dominant in the offset-strip array. The introduction of streamwise vortices by the VGs enhances the heat transfer behavior of the baseline array by increasing the mixing of freestream fluid with the boundary layer fluid of the fins. Shear layer instabilities in the wake of the baseline array occur at Re = 1230. The onset of these oscillations at Re = 1230 corresponds to a decrease in heat transfer near the exit of the Two-VG array, where no oscillations or instabilities occur. Spanwise vortex shedding begins at the back of the array as the Reynolds number increases past 1550 in the baseline array, and past 1610 in the Two-VG array. As the Reynolds number is increased through Re = 1780, the heat transfer decrement persists, and moves upstream through the array. The onset of spanwise vortex shedding similarly moves upstream with increasing Reynolds number. The heat transfer behavior of the Two-VG array within the Reynolds number range 1230 £ Re £ 1780 suggests that the interaction of spanwise and streamwise vorticity is destructive. An increase in Reynolds number to Re = 2450, however, showed improved heat transfer behavior, and, thus, a beneficial interaction of spanwise and streamwise vorticity. Spanwise vortex shedding occurred throughout the baseline and Two-VG arrays at Re = 2450. Spanwise flow unsteadiness in the Two-VG array began at fin 3 for Re = 2450. At the lower Reynolds numbers investigated, flow unsteadiness did not begin before fin 6.
Issue Date: 2001-06
Publisher: Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report: Air Conditioning and Refrigeration Center TR-182
Genre: Technical Report
Type: Text
Language: English
URI: http://hdl.handle.net/2142/12035
Sponsor: Air Conditioning and Refrigeration Project 104
Date Available in IDEALS: 2009-06-04
 

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