The condensation characteristics of individual droplets during dropwise condensation

Abstract

Recently, nonwetting surfaces have attracted explosive attention in the community of dropwise condensation. Prediction models have been used to improve the fundamental understanding of dropwise condensation heat transfer. However, the multiscale heat transfer characteristics of individual droplets and the quantitative heat transfer evaluation of the droplet growth on different condensing surfaces are rarely carried out for dropwise condensation, which are focused on in this work. Based on the droplet heat transfer models, we consider three respective groups of nonwetting surfaces (hydrophobic surfaces, structured superhydrophobic surfaces, and slippery surfaces) in a pure vapor environment, as well as the presence of non-condensable gas (NCG). We first elucidate the dynamic roles that the thermal resistances have in the intrinsically multiscale droplets during condensation. The resulting heat transfer characteristics of droplets are understood simultaneously. We highlight that two critical sizes of the droplet significantly characterizes the condensation behaviors of droplets, and three regions are defined to characterize the dependence of the droplet size on the dominant thermal driving loss. Subsequently, the droplet size distribution is considered to further understand the role of dynamically growing droplets on the total thermal resistance. In the presence of NCG, over the whole size range of droplets, the dynamic roles of the thermal resistances and the heat transfer characteristics are significantly changed due to the resulting diffusion resistance.