Role of substrate thermal conductivity and vapor pressure in dropwise condensation

The role of constriction resistance is examined experimentally using hydrophobic gold-plated copper, aluminum, carbon steel, and stainless steel condenser surfaces over a range of surface subcooling and vapor pressure. It is confirmed that the condensation heat transfer coefficient depends on the substrate thermal conductivity (five-folds lower for stainless steel compared to copper) and the tendency of a hydrophobic surface to maintain a small liquid-film resistance is controlled by the droplet growth and coalescence rate. These are accompanied by an increase in the departing droplet radius and the surface coverage with decreasing substrate thermal conductivity. Numerical thermal simulations isolate the role of constriction resistance from the role of the droplet distribution. Additionally, based on recently available molecular dynamics results, the liquid-vapor interfacial resistance is modeled as a vapor temperature jump using the vapor mean free path and the predicted pressure dependence is in good agreement with the experimental results.