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4th Thermal and Fluids Engineering Conference

ISSN: 2379-1748

CREATING VORTEX GENERATORS USING PREFERENTIAL SURFACE WETTABILITY PATTERNING FOR AIR-SIDE HEAT TRANSFER ENHANCEMENT

Andrew E. Koopman
Department of Mechanical and Manufacturing Engineering, Miami University, 56 Garland Hall, 650 East High Street, Oxford, OH 45056 USA

Christian J. L. Hermes
Department of Mechanical Engineering, POLO Research Laboratories, Federal University of Santa Catarina, 88040535 Florianópolis, SC, Brazil

Andrew D. Sommers
Department of Mechanical and Manufacturing Engineering, Miami University, 56 Garland Hall, 650 East High Street, Oxford, OH 45056 USA

DOI: 10.1615/TFEC2019.epa.027602
pages 1279-1288


KEY WORDS: hemispherical vortex generator, dimple, surface wettability, frost, hydrophobic, hydrophilic, microchannels, heat exchanger

Abstract

A new technique for the deployment of hemispherical vortex generators (VGs) on aluminum is reported. The technique utilizes the naturally-occurring condensation within a fin-and-tube heat exchanger. The objective was to examine the effectiveness of using patterned surface wettability to collect condensate in predetermined locations on the fin surface during the early frost growth period for the formation of large frozen droplets, which could serve as VGs to help enhance air-side heat transfer at airflow rates typical to domestic refrigeration.
To date, various geometries of staggered circles on a hydrophobic-coated background have been studied. Surrounding these regions was a circular microchannel, which was milled into the surface to promote the coalescence of the "trapped" condensate into a single droplet. These circular regions had an inner diameter of 0.125". Surface testing was performed which included a one-hour condensation period (Tw ~ 1°C) followed by three one-hour frost growth periods (Tw = -3 to -12°C) with five minutes of defrosting in between. By varying the channel width (~ 100-500μm) and having the inner area be either hydrophobic or hydrophilic, large frozen droplets were formed, which remained visible for 40-60 minutes during the third frost cycle. The average diameter and height of these frozen droplets at the end of the third cycle were 3.15-3.90 mm and 1.25-3.20 mm, respectively, which indicates that the disturbance height of the VGs was comparable to the local boundary layer thickness in the entrance region. The following experimental conditions were examined during testing: relative humidity 60-80%, air temperatures (Ta) 20°C-24°C, and surface temperatures (Tw) -3°C to -12°C.

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