Axisymmetric Water Jet Impingement on Superheated Superhydrophobic Surfaces*

Superhydrophobic (SH) surfaces, characterized by the presence of air cavities between microstructures, possess self-cleaning properties and present an attractive solution to fouling during jet impingement but also impede heat transfer. Previous research shows that film boiling occurs on superheated SH surfaces, rather than more effective nucleate boiling, as a thin insulating vapor film forms between the surface and the liquid due to these cavities. Differences between heat transfer on SH and smooth surfaces during liquid jet impingement were explored experimentally. A smooth silicon wafer or a microstructured, Teflon-coated SH surface was clamped to an aluminum block heated to temperatures up to 330C. An axisymmetric water jet impinged and cooled the surface while a high-speed camera recorded boiling behavior. Heat flux and surface temperature for various radial locations were recorded via embedded thermocouples in the block. Boiling curves specifying Leidenfrost temperature and overall local heat transfer coefficients were determined. An equation to model heat transfer as a function of surface superheat and wettability, jet Reynolds number, radial distance and impingement angle is developed.