Condensation on Superhydrophobic Surfaces in Vapor Shear Flow

In a condensing environment, the heat transfer through superhydrophobic (SH) surfaces is significantly affected by the size of the condensed drops, wetting state and resulting drop mobility on the surface. To maintain dropwise condensation and accompanying high heat transfer rates, condensed drops may be removed using an external force, such as gravity or vapor shear. Without removal, condensate remains on the surface eventually forming a film, dramatically inhibiting heat transfer. This experimental study focuses on predicting drop departure diameter and condensation heat transfer for drops exposed to vapor shear flow on SH surfaces. A facility that provides air with high moisture content at an elevated temperature is passed over a cooled SH surface for condensation to occur. The condensation process is imaged through time using multiple cameras and drop departure diameters are measured as a function of the vapor flow speed. Measured drop departure diameter is compared to theoretical calculations of departure by equating the drop adhesion force to the fluid drag force acting on the drop. Experiments were performed and the drop departure diameter was determined for shear flows ranging from 0.5 to 1.5 m/s. The influence of the surface solid fraction and pitch (spacing between micro- or nanoscale features) was also quantified over typical ranges of these parameters for realizable SH surfaces. The model results show good agreement with the experimental values and enable predictions that can be further integrated into an overall heat transfer model of condensing flows over SH surfaces.