The influence of cold surface roughness on the initial stage of frosting in a wide temperature range

Abstract

The initial frosting phenomenon is a discontinuous phase-change nucleation process. The temperature and surface conditions of cold surfaces exert a decisive influence on this phenomenon, particularly during the initial frosting stage. With the advancement of aerospace and energy transportation technologies, increasing attention has been paid to frosting phenomena and frost suppression methods at low temperatures (-100°C to -30°C) and ultra-low temperatures (-273°C to -100°C). For cold surfaces at different temperatures, surface roughness exerts differential effects on the frosting process due to variations in frosting modes. In this study, an experimental investigation was conducted on the initial frosting behavior of copper surfaces with varying roughness (Ra = 2.1∼57.8 μm) over a wide temperature range (-190°C to -30°C), and the influence of roughness on frost crystal coverage was quantitatively analyzed. It was found that under the cold surface conditions examined herein, three frosting modes were observed, which are cold surface sublimation frosting, air boundary layer condensation frosting, and air boundary layer sublimation frosting. Four types of frost crystal morphologies were identified, which are hexagonal prism (feather), branch (pine needle), cluster (shrub), and floc (grape).The effect of roughness on frost crystal coverage exhibits stage-specific characteristics: in the stage dominated by cold surface deposition, high roughness significantly promotes frost crystal growth; in the stage dominated by boundary layer deposition, low roughness inhibits water vapor mass transfer; under extremely low temperature conditions, the influence of roughness becomes random. Universal patterns across different roughness intervals are as follows: in the high roughness range (Ra = 57.8∼39.6 μm), cold surfaces at -30∼-50°C within the roughness range of Ra = 53.8∼57.8 μm exhibit the maximum frost crystal coverage. In the medium roughness range (Ra = 35.5∼20.3 μm), frost crystal coverage on most (>80%) cold surfaces at different temperatures is independent of roughness. In the low roughness range (Ra = 18.5∼2.1 μm), frost crystal coverage at -30°C decreases with decreasing surface roughness.