Analysis of frost thickness and roughness growth from the perspective of frost crystal structure

Laboratory experiments were conducted to characterize frost thickness and roughness on a horizontal cold flat surface with respect to the type of frost crystal. Air velocities, free stream temperatures, wall temperatures, and relative humidities were varied from 0.5 to 1.5 m s(-1), 16 to 6 degrees C, and -25 to -15 degrees C, and 50 to 75%, respectively. Three-dimensional photogrammetry was employed for evaluating frost roughness characteristics. The results showed that the frost growth corresponded with changes of frost crystal formation. Feather and needle crystals created a thick and porous layer with a rough surface, while flake and irregular crystals formed a thin and dense frost layer with a relatively smooth surface. Frost root-mean-square height began to decrease sooner under higher air velocities, higher air temperatures, and higher wall temperatures. The maximum value of frost root-mean-square height was largest under lower air velocities, lower air temperatures, lower wall temperatures, and higher relative humidities. Larger frost surface skewness occurred under lower air velocities, lower air temperatures, higher wall temperatures, and higher relative humidities. The impacts of increasing relative humidity on the frost roughness parameters plateaued at higher relative humidities. (C) 2019 Elsevier Ltd and IIR. All rights reserved.