Numerical study of evaporation-condensation in a vertical diffusion gap

The present work deals with mass transfer between a vertical falling film over a heated plate and a condensing film over a parallel cooled plate in a diffusion gap. This is typically the case of the distillation process in a diffusion still. The governing equations for mass, momentum, and energy are considered for the evaporating film, the diffusion gap, and the condensing film, together with the boundary and interfacial conditions. The local similar technique is used to solve the problem numerically and to get the velocity and temperature distributions in the gap. A comprehensive analysis of the effect of evaporating temperature, condensing temperature, and diffusion gap width over the diffusion mass flow rate and evaporation heat transfer coefficient are carried out. Performance charts of air and helium diffusion gaps are given. Additionally, the analytical results are experimentally validated.