An investigation of heat and mass transfer by free convection from humid air to a metal plate under frosting conditions

Abstract

The results of a theoretical and experimental investigation of heat and mass transfer by free convection from humid air to a flat vertical plate are presented. The domain of interest in the experimental investigation included a range of plate temperatures from 10° F to minus 100° F, and a range of chamber temperatures from 20° F to 120° F. Experimental data are presented for the temperature and concentration boundary layer profiles, frost-air interface temperature, frost growth rate, and total heat transfer. Convective coefficients and frost thermal conductivities were obtained from their definitions as given by Fourier's Law of conductive heat transfer and Newton's Law of convective cooling. The heat transfer due to radiation was accounted for by using the Stefan-Boltzman relation. Chamber humidity was consistently found to be an influential factor in the test results. Maximum frost densities, growth rates, and heat transfer rates were obtained for those tests conducted at chamber condition which had the highest relative humidity at a major plate-chamber temperature difference. In the majority of tests the rate of heat transfer reached an asymptotic condition after approximately 120 to 150 minutes. Frost proved to be an effective insulator after this condition was reached. The initial formation of frost on the plate caused a roughening of the surface, which in turn increased the effective heat transfer area. Consequently, there is an increase in the heat transfer convective coefficient during the first stages of frost formation. Frost thermal conductivity increased with frost density, and this increase tends to override the effect of an increase in frost thickness..