Effect of vapor velocity and surface inclination during dropwise condensation

Condensation is a phase-change process which can be exploited in several industrial applications. Nowadays, researchers are focusing their attention on the dropwise condensation (DWC) phenomenon, which involves the formation and the rapid removal of several droplets. The DWC mechanism allows to achieve heat transfer coefficients 5-6 times higher as compared to filmwise condensation (FWC), due to the reduction of the thermal resistance related to the continuous film of condensed vapor. This enables the possibility of designing heat exchangers with smaller heat transfer areas and smaller temperature difference between the fluids, with advantages from both the economic and the energetic point of view. Although in the literature there are many papers focused on the phenomenon of dropwise condensation, only a few of them investigate the effect of vapor velocity during DWC. Therefore, the aim of this master’s thesis is to address these aspects and in particular to study of the effect of vapor velocity during DWC on horizontal surfaces. First, DWC of steam is investigated at varying vapor velocities on a vertical surface, in terms of heat transfer coefficient and droplet departing radius. Second, the orientation of the condensing surface is varied from vertical to horizontal to study DWC without the effect of gravity on the surface renewal. In fact, in the case of the horizontal configuration, only the drag force due to the vapor flow promotes the droplets sweeping as gravity acts perpendicular to the surface. Therefore, the removal of condensate becomes more problematic (especially for low vapor flow rates), possibly leading to the transition from dropwise to filmwise mode. Finally, the experimental measurements obtained with the horizontal configuration are used to extend the validation of a model for the evaluation of the droplet departing radius in presence of vapor velocity to the case of droplets growing on horizontal surfaces.