Frost formation is an important phenomenon in the cryogenic industries. In some instances, frost deposition is deleterious and leads to energy waste. The phenomenon of frost formation is a complicated process in which heat and mass transfer are at work simultaneously.
This paper deals with an experimental investigation and a theoretical work of the effect of the air humidity, velocity, temperature and plate wall temperature on frost formation rate on a horizontal flat plate, held at subfreezing temperature.
The frost thickness increased with decreasing plate wall temperature and with increasing air humidity. The frost density was high at the high air humidity, velocity and plate wall temperature. Due to the increasing resistance of the frost layer to heat flow, the surface temperature of the frost increased during the run. A theoretical model based on molecular diffusion of water vapor at the frost surface and quasi-steady state assumption, by using Dietenbergers'(1983) thermal conductivity equation, showed good agreement with the experimental data.
The values of the local heat and mass transfer coefficients under frosting condition were larger than those estimated by the smooth wall Nusselt number equation and Chilton-Colburn analogy. The reasons for this fact were considered to be due to the interference of heat and mass transfer and the frost surface roughness.
Quasi-steady state overall heat transfer coefficient between humid air and plate wall covered with frost was found to increase with increasing air humidity and velocity.
