Effect of steam velocity during dropwise condensation

•Dropwise condensation of steam was investigated on a vertical surface.•Heat transfer coefficients and droplet departing diameters were measured at varying the vapor velocity.•A new equation was proposed to account for the effect of the steam velocity on the departing radius.•The effect of vapor velocity was included in the model of the heat transfer coefficient. Dropwise condensation (DWC) is a complex phenomenon involving droplets nucleation, coalescence and motion. Starting from the nanoscale up to the macroscale, DWC involves millions of droplets per square meter. The maximum dimension assumed by a droplet before sliding is characterized by the departing radius: it is the radius at which the droplet starts to sweep through the surface as a consequence of the acting forces (gravity force, adhesion force, drag force induced by the flowing vapor). In the literature, very few works investigate the effect of vapor velocity on the heat transfer coefficient (HTC) and on the droplet departing radius during DWC. Furthermore, the effect of vapor velocity is not accounted for in available DWC models. In the present paper, DWC of steam has been promoted on an aluminum sol-gel coated surface. Heat transfer coefficients and droplets departing diameters have been measured at 107 °C saturation temperature, heat flux of 335 kW m−2 and average vapor velocity between 2.7 m s−1 and 11 m s−1. A method for the estimation of the droplet departing radius in presence of non-negligible vapor velocity is here proposed. The equation accounting for vapor velocity has been included in the model by Miljkovic et al. [1] for heat transfer coefficient prediction during DWC and has been assessed using the present data and two other datasets from independent laboratories.