Non-condensable gas is well known for degrading condensation heat transfer especially in natural convection and laminar flow because of condensate is impermeable to the non-condensable gas, as a result of mass flow of the vapor in condensing process, the non-condensable gas accumulates near to the interface of the condensate and gas phase as the main resistance in condensation. It has been found that a very small amount of non-condensable gas can result in a significant reduction in heat transfer.

Filmwise and dropwise condensation heat and mass transfer characteristics of saturated steam-air mixture were studied on the vertical plate experimentally and theoretically. The main objective is to investigate the interfacial effect on heat and mass transfer process of condensation in the presence of non-condensable gas by considering the dynamic interaction of condensate motion and gas-vapor diffusion. The influences of the droplet growth process, the droplet curvature, the droplet falling process and the transversal suction effect on mass transfer properties within the interfacial mass diffusion layer were investigated experimentally. The numerical simulation results revealed the mixing characteristic of the steam and air within the mass diffusion layer. From the above results, the dominate mechanism on the heat and mass transfer of dropwise condensation of steam-air mixture vapor is confirmed clearly.

Key words: dropwise condensation; non-condensable gas; heat and mass transfer enhancement; interface effect