Frost formation from general-low to ultra-low temperatures: A review

Abstract

The frosting at ultra-low temperatures can cause significant degradation of the thermal performance of heat exchangers, which also leads to a decrease in the energy efficiency of air-source heat pumps. In this paper, the temperature terminologies for frosting were classified, and the frosting mechanism and frost properties were investigated from general-low to ultra-low temperatures. The dominant phase change mechanisms at general-low and ultra-low temperatures were condensation-freezing and desublimation, respectively. The frost layer growth mechanism at ultra-low temperatures can be classified into the formation of ice nuclei, growth of ice particles, and deposition of a frost layer in the initial stage of frosting. The frost morphology at ultra-low temperatures showed wisteria or shrub-shaped frost, which was not observed at general-low temperatures. Frost properties are determined depending on frosting factors such as cooling surface temperature, air temperature, absolute humidity, and air velocity, while the thermal conductivity of frost was determined by frost porosity. At ultra-low temperatures, some unusual phenomena were observed that were not observed at general-low temperatures. When the cooling surface temperature was at ultra-low temperatures, the fog was formed near the cooling surface along with the frost, and this fog was dominantly influenced by the frost surface temperature. In addition, unique phenomena such as thread-shaped frost and leading-edge frost concentrations were observed at ultra-low temperatures, and there was a clear difference in the frosting mechanism based on the temperature range.