Experiments of dropwise condensation on wettability controlled surfaces

During filmwise condensation, the liquid layer adjacent to the wall introduces a thermal resistance that adversely affects the heat transfer. On the contrary, the dropwise condensation mode leads to higher thermal performance by inducing the breakage of the liquid film and replacing it with a big amount of randomly distributed droplets. The potential of dropwise condensation is based on the increase of the condensation heat transfer coefficients by several times compared to those measured during filmwise condensation. The surface properties, and in particular the surface wettability, play a crucial role in determining the condensation mode (e.g. hydrophobic surfaces are expected to promote dropwise condensation). The present paper aims to present and discuss experimental data taken during filmwise and dropwise condensation of pure steam over vertical surfaces. Surfaces with different wettability have been realized on aluminum substrates and then characterized by means of contact angle measurements using the standard sessile drop method. The test rig used for the experimental campaign is a two-phase thermosyphon loop that allows measurement of heat transfer coefficients and simultaneous visualization of the condensation process. Heat transfer coefficients have been measured at around 107°C saturation temperature with three different aluminum samples. A high-speed camera has been used to analyze the condensation regimes.