The Interaction Between the Substrate and Frost Layer Through Condensate Distribution

Abstract

Microscopic observations of frost deposition on a variety of substrates having different contact angles, (polytetrafluoroethylene PTFE, kapton, glass and others) allow the quantification of substrate effects on frost structure during inception and growth. The deposition of water vapor at the beginning of the frosting process on a clean glass substrate is found to be as condensate (condensation frosting) rather than as ice (ablimation frosting) for a substrate temperatures above -33??C and an absolute humidity above 0.15 g/kg. The inception of "condensation frosting" (the condensation period and early frost growth period) is further examined microscopically as a function of air and substrate temperatures, absolute humidity, and substrate contact angle. The water distribution on the substrate at the end of the condensation period is found to be strongly dependent on substrate temperature, humidity ratio, and substrate contact angle. Colder substrates result in smaller more uniform droplets and substrates with lower contact angles result in shorter, larger diameter droplets with a larger percentage of the substrate covered. The effective density of the condensate on hydrophobic substrates is found to be lower than that on hydrophilic substrates. The structure and form of the ice immediately after freezing is substrate dependent. High-speed imaging of the freezing process is used to study the propagation of the freezing front in a droplet. The images show that a protrusion is formed at the top of the droplets during freezing. From observations, this protrusion is hypothesized to result from the convective condition at the droplet surface and the difference in specific volume between liquid and solid water. Additionally, the apparent ejection of water vapor during freezing of a droplet on a hydrophobic substrate was observed. This ejection of water vapor is thought to be caused by the wanning of the droplet caused by the release of latent heat. In contrast to trends observed during the early growth period, the growth rate of mature frost is found to decrease with substrate contact angle while frost density is found to increase. This behavior is explained in terms of the effect of substrate contact angle on the structure and form of the incipient frost, which constitutes the initial condition for further (mature) frost growth. A higher conductivity layer is formed on the hydrophilic than on the hydrophobic substrate. A model relating crystal orientation to conductivity is used to simulate the frost growth rate and density on the two different substrates and match the experimental data. Using similar reasoning, the higher conductivity frost formed on colder substrates is also explained.Air Conditioning and Refrigeration Project 10