A study of combined heat and mass transfer by free convection from a controlled environment to a plate at -310°F

Abstract

An experimental study of heat and mass transfer in free convection from a controlled environment to a plate at liquid nitrogen temperature (-310°F) has been made. The temperature of the environment was varied from 36°F to 118°F and the specific humidity was varied from 15 to 476 grains of moisture per pound of dry air. Tests were conducted on both horizontal and vertical positions of the plate. The total heat transfer rate from the plate was calculated and then the heat transfer rate by convection was computed. Frost surface temperature, mass and thickness data were measured experimentally. Temperature and concentration values inside the boundary layer adjacent to the plate were also measured. Using Newton's Law of convective cooling and Fourier's Law of conductive heat transfer, the values of convective coefficients and frost thermal conductivities were calculated. The heat transfer from the plate was found to be a function of the chamber temperature and humidity. For the range of temperature and humidity under consideration, the heat transfer rate was found to be minimum at some intermediate temperature and humidity. This was due to the fact that at lower temperatures liquid oxygen was seen to flow along the plate prohibiting normal growth of the frost. The energy was transported through the frost layer not by simple conduction but by some other mechanisms like internal diffusion of water vapor within the frost layer. The temperature boundary layer profile was in good agreement with a parabolic profile. Average Nusselt, Prandtl and Grashof numbers for heat transfer were calculated from the experimental data for both vertical and horizontal positons of the plate. In the computation of the above mentioned dimensionless groups, the property values were determined at a mean film temperature, which was an arithmetic mean of the frost surface temperature and the temperature at the outer edge of the boundary layer. The correlation obtained between these dimensionless parameters were in fairly good agreement with the equations available in the literature.