Dropwise condensation from moist air over a hydrophobic metallic substrate

•Model proposed for moist air dropwise condensation on non-wetting surfaces.•Effect of humidity ratio on interfacial heat transfer coefficient is explicitly derived.•Coalescence and instabilities are shown to be unaffected by noncondensable gases.•Yield of condensate is enhanced for higher contact angle and lower hysteresis.•Application to an optimized atmospheric water harvester is demonstrated. Moist air condensation is of significant scientific and societal interest especially for harvesting potable water from atmospheric air. Dropwise condensation of water vapor present in moist air is preferred when compared to filmwise condensation because of high transport coefficients that augment condensation rate of the distillate. The efficacy and sustainability of dropwise condensation depends on various operating parameters, for instance, temperature of moist air, relative humidity, subcooling of the substrate, thermophysical properties of moist air and physico-chemical properties of the condensing substrate. In this context, a mathematical model of dropwise condensation of moist air over a textured surface has been developed in the present work. Jointly, an experimental study of moist air condensation over a specially prepared superhydrophobic surface has been carried out for model validation. By suitably deriving the correction factor for the interfacial resistance to account for the presence of noncondensable gases, good agreement has been obtained between the model and experiments. Thereafter, simulations have been performed over a range of conditions from the viewpoint of maximizing the condensate output. Results of the study show that a vertical substrate with a high equilibrium contact angle and low contact angle hysteresis increases drainage of water drops. Hence, there is an increase in the water productivity of the surface. In addition, the condensation rate increases with increase in relative humidity, degree of subcooling and saturation temperature of moist air. The results of the study can be utilized towards designing devices for water harvesting from the atmosphere.