Effects of crystal formation on the initial frost thickness and density on cold surfaces

Abstract

In refrigeration and air-source heat pump systems, water vapor in moist air circulating across the heat exchanger fins condenses into liquid nuclei when the surface temperature is below the dew point. The supercooled liquid droplets grow and coalesce until a liquid-to-solid phase transition occurs, preceding the growth of crystals on the ice beads and the subsequent development of a frost layer within and on top of the crystals. This paper describes a comprehensive, three-stage model for frost nucleation, in which a novel crystal growth model stage was integrated with droplet growth and frost layer growth models. The overall model was experimentally validated for both hydrophobic and hydrophilic surface types and various environmental conditions focused on surface temperatures between 0 and −10 °C, relative humidities between 70 and 90%, and air temperatures between 0 and 20 °C. Model results indicated that the surface contact angle affected the ice beads' shape and size. The air's water vapor content affected the frost layer's thickness and density. If the humidity increased, the mass transfer potential increased, prompting the crystal growth–frost growth transition to occur quickly and at high density values. The cold surface temperature also affected the stage transitions. At −9 °C surface temperature, crystal growth lasted approximately 2 min while it increased to 21 min at −5 °C. Frost thickness was initially higher at −9 °C due to a shorter droplet growth stage, but the thickness increase slowed during frost layer growth, resulting in a temporarily higher thickness at −5 °C.