Drag effect of steam flow on droplet removal during dropwise condensation at different surface inclinations

Dropwise condensation is a quasi-cyclic process characterized by the nucleation, growth, and removal of discrete liquid droplets on a subcooled surface. The removal of condensate is a critical aspect, usually achieved by exploiting the gravity force, the drag force of vapor or the surface wettability gradient. This paper presents an experimental study of the vapor drag action on condensate removal, with focus on droplet's departing radius (rmax). Specifically, for the experimental campaign, vapor velocity was varied from 3 to 14 m s−1 considering three different surface inclinations: vertical, 45° inclined, and horizontal. The results showed that, as the velocity increases, the difference in departing radii among the three different configurations decreases and, consequently, the difference in heat transfer coefficients decreases too. In fact, at the highest vapor velocity (~14 m s−1), rmax was almost equal for all the inclinations leading to similar heat transfer coefficients (~120 kW m−2 K−1). Interestingly, on a horizontal surface considering vapor velocity equal to 3 m s−1, despite the lack of gravity's contribution to droplet removal, no transition to filmwise condensation was observed.