ASHRAE OR-16-C032

Abstract

<em>The goal of this project was to assess the potential benefits of using a highly-controlled surface wettability to preferentially condense and locate water droplets on a heat transfer surface during the early stages of frost growth and to more completely drain melted frost during a defrost cycle. A thinner, denser frost layer may lead to improved air-side heat tranfer, and the retention of less water following defrosting might also be used to slow frost growth and lengthen the operational cycle. A baseline aluminum surface was compared with a surface with an ultra-thin hydrophobic coating, three surfaces laser etched to produce a surface microtexture in addition to the hydrophobic coating, and a fourth that was micro-milled. The results have been mixed. Contact angle measurements and spray testing data have shown some benefit and the possibility for preferred drainage patterns on the enhanced surfaces. During the frosting cycles, each surface exhibited a uniform layer of frost growth. In both the second and third frosting cycles, frozen water droplets increased the retained mass for all surfaces. Photographs of the frosting surfaces show water droplet retention along preferential patterns for some of the laser-etched surfaces. However, frost thickness and mass measurements do not present conclusive evidence that preferred drainage patterns improved frosting performance.</em>