A study of frost growth and roughness on a cold flat plate under forced convection.

The growth and roughness of frost formed on a cold flat plate under forced convection was characterized through experimental observations and analysis. The results indicated that the evolution of frost roughness was mainly determined by the complex structures of frost crystals appearing on the top of the frost layer. The frost crystal type, frost crystal size, and frost crystal distribution were strongly affected by environmental conditions and collectively determined the formation of frost surface roughness. Frost roughness parameters, including frost root-mean-square height and frost surface skewness, were measured by utilizing a 3-D photogrammetric method. The growth of frost roughness under various environmental conditions were then assessed. Both frost root-mean-square height and surface skewness increased with time until reaching a maximum value (up to 0.69 mm and 2.34, respectively), before starting to decrease. The frost surface skewness peak, the maximum frost root-mean-square height, and the time for the appearance of frost root-mean-square height typically varied linearly with the surrounding environmental conditions. The experimental investigation also showed that frost roughness varied over both time and location on the test surfaces. The frost equivalent sand-grain roughness height, calculated by using the frost root-mean-square height and frost surface skewness, was applied to represent the unevenness of frost roughness on cold substrates. Through an ANOVA test on the data of the frost equivalent sand-grain roughness height, air humidity and air temperature with p-values less than 0.05 were indicated to be the key factors determining the changes in the variation of the frost roughness in space. Empirical correlations were developed to describe the relationships between the appearance of the frost roughness peak and environmental conditions. Finally, based on the frost roughness data collected in this study, a predictive model of frost roughness parameters, including frost root-mean-square height, frost surface skewness, and frost equivalent sand-grain roughness height, was established with the R-square values of 0.57, 0.65, and 0.53, respectively.