THEORETICAL AND EXPERIMENTAL STUDIES ON THE PSEUDO-DROPWISE CONDENSATION OF A BINARY VAPOR MIXTURE

When a water-ethanol binary mixture condenses on a flat plate, one obs erves that the liquid film condensate rises locally and eventually for ms many droplets on the film. Usually, filmwise condensation is expect ed because both substances are completely soluble in each other and th ey wet a copper plate well. This paper presents the droplet growth mec hanism during so-called pseudo-dropwise condensation. Instability anal ysis is used to determine the transition from filmwise condensation to pseudo-dropwise condensation theoretically. In a stress balance at th e vapor-liquid interface, the analysis considers not only the surface tension itself but also the surface tension variation due to changes i n temperature and concentration, assuming saturation conditions at the interface. Numerical results indicate that the Marangoni effect plays a more important role than the absolute value of the surface tension in pseudo-dropwise condensation. The change in surface tension with te mperature is nor always negative; it becomes positive for certain mixt ures due to the dependence on concentration Pseudo-dropwise condensati on is only realized when surface tension increases with temperature. T his analysis qualitatively predicts the critical Marangoni number expe rimentally observed during water-ethanol mixture condensation.