Abstract: Frosting is widely seen in nature and industry fields, and always plays negative effect. To accurate predict and analyze the gravity effect on the frosting process, frosting experiments at micro scale on horizontal, vertical and inverted cold plates under natural convection are carried out, with the surface temperature set at -25℃ and -15℃. The results show the droplet condensation and coalescence, the solidified liquid tip-growth and the frost layer growth stages end within 220 s with the frost thickness less than 450×10-6 m. When the gravity direction is consistent with the direction of dendrites growth, the gravity effect promotes the dendrites growth, but also causes the formation of large droplets and falling of dendrites. At the surface temperature of -15℃, the maximum diameter of droplets of the inverted cold plate formed in the droplet condensation and coalescence stage reaches 361.56×10-6 m. The periodic reverse melting and sudden falling of dendrites in the frost layer full growth stage lead to the sudden decrease and increase of frost thickness and fluctuation of the frost layer surface roughness. The difference between the peak and the trough of the roughness curve of the inverted cold plate at the surface temperature of -25℃ reaches 68.68×10-6 m. Contributions of this study could be used for predicting the specific frost characteristics on the surface of heat exchangers.

Key words: Frosting stage, Natural convection, Gravity effect, Frost thickness, Surface roughness, Reverse melting