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ACRC Technical Report 219PDF


Title:Vortex Generation as an Air-Side Enhancement Method for Frosted-Surface Heat Exchanger Performance
Author(s):Sommers, A.D.; Jacobi, A.M.
Subject(s):vortex generators
heat exchangers
air-side thermal performance
Abstract:Longitudinal vortex generation is a technique for enhancing heat transfer and can be accomplished by placing small flow manipulators on the fin surface of a heat exchanger. This technique is of particular benefit in plain-fin-and-tube heat exchangers where the fin pitch is large (5-10mm) and the air-side convective coefficient is small. In this study, a single row of delta-wing vortex generators is applied to a refrigerator evaporator with a fin spacing of 8.5 mm both along the leading edge and at a location halfway along the flow length for a total of 108 vortex generators. Heat transfer and pressure drop performance are measured before and after to determine the effectiveness of the vortex generator under frosting conditions. Under lightly frosted conditions, reductions in airside thermal resistance of 3.5% to 22.8% are achieved for face velocities of 0.45 m/s to 1.1 m/s. This heat transfer enhancement monotonically increases with air velocity and results in a small pressure drop penalty that is incommensurate with the achieved enhancement. Maximum frost accumulation in the enhanced heat exchanger is also examined for a single row of leading edge delta wings. Under these conditions, a reduction in the air-side thermal resistance is observed that falls within the uncertainty of the experiment. Finally, a second, denser array of 324 vortex generators is examined for the same evaporator where the delta wings are attached along four rows in an alternating single row, double row arrangement at core depth intervals of 50.8 mm (2 in). For Reynolds numbers between 500 and 1200, a reduction of 35.0% to 42.1% is observed in the air-side thermal resistance. Correspondingly, the heat transfer coefficient is observed to lie between 26-51 W/m2-K for the enhanced evaporator and between 16-26 W/m2-K for the baseline evaporator. Two different performance evaluation criteria are calculated and both show that the enhanced evaporator outperforms the baseline specimen for Reynolds numbers greater than approximately 700-750. Tests conducted under maximum frosting conditions reveal a diminished but statistically significant heat transfer enhancement.
Issue Date:2003-12
Publisher:Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report:Air Conditioning and Refrigeration Center TR-219
Genre:Technical Report
Sponsor:Air Conditioning and Refrigeration Project 121
Date Available in IDEALS:2009-06-12

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