Non-condensable gas (NCG) is well known for degrading condensation heat transfer due to the accumulation of NCG near the gas-liquid interface. It has been found that a small amount of NCG results in a significant reduction of heat transfer performance. In the present work, dropwise condensation heat and mass transfer mechanism of steam-air mixture were studied on a vertical plate experimentally and theoretically. Considering the dynamic interaction of condensate and gas-vapor diffusion layer, the study focused on the interfacial effect on heat and mass transfer of condensation in the presence of NCG. Comparison of growth rates of new nucleated droplets in different regions showed the enhancement of mass transfer by the gas phase perturbation. Taking advantage of visualization, the influences of droplet curvature, departure movement and transversal suction effect on mass transfer in the interfacial mass diffusion layer were investigated. The numerical simulation subsequently revealed the mixing characteristic of steam and air in the mass diffusion layer which was corroborated by the visual results inspected by PIV technology. Due to the relative motion of condensed droplet and steam-air mixture vapor, eddy flow occurred in the gas phase resulting in a perpendicular velocity of the bulk vapor to the condensing surface and a perpendicular velocity of the accumulated NCG to the vapor bulk which enhanced the heat and mass transfer in dropwise condensation.

The study provides an insight into the disturbance of the diffusion boundary layer by droplet departure movement, as well as the need to design specific surface to promote the droplet departure movement and achieve enhanced heat and mass transfer during dropwise condensation in the presence of NCG.

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