Using patterned surface wettability to enhance air-side heat transfer through frozen water droplet vortex generators-part I: Experimental study

Abstract

• Frozen water droplets can serve as vortex generators (VGs) to increase air-side heat transfer. • The VGs are formed via the naturally-occurring processes of condensation and freezing. • Patterned surface wettability encourages coalescence in predetermined locations. • A circular moat helps concentrate water inside a circular region prior to freezing. • The average height of the frozen droplets after the third defrost cycle was 1.25–3.20 mm. • The VG height is greater than the boundary layer thickness in the entrance region. In this study, a novel technique for deploying hemispherical vortex generators (VGs) on a refrigerator evaporator for air-side heat transfer enhancement is investigated. The dome VGs are formed via the naturally-occurring processes of condensation and freezing. By using patterned surface wettability to collect condensate and encourage coalescence in predetermined locations, it was found that large frozen droplets could be formed in various configurations on the fin surface, which in turn could then serve as VGs during the early frost growth period. These findings were examined experimentally here using a flat hydrophobic-coated aluminum plate (Part I) and were then simulated numerically in a follow-up study (Part II). Various geometries of staggered circles were studied to test the efficacy of the idea. The circular regions had an inner diameter of 3.175 mm (0.125 in) and were surrounded by a microchannel “moat” to promote the coalescence and trapping of condensate into a single droplet inside the circle. Surface testing was performed inside an environmental chamber via a one-hour condensation period ( T w ∼1°C) followed by three one-hour frost growth periods with ten minutes of defrosting in between. The following experimental conditions were used during testing: RH=60–80%, T air = 20°C–24°C, and T w = −3°C to –12°C. By varying the microchannel width (∼100–500 µm) and the wettability of the inner area, large frozen droplets were formed which remained visible for 40–60 min during the third frost cycle. The average diameter and height of the frozen droplets at the end of the third cycle were 3.20–3.90 mm and 1.25–3.20 mm, respectively.