Mechanisms of dropwise condensation on aluminum coated surfaces

Dropwise condensation (DWC) is a complex phase-change phenomenon involving the formation of randomly distributed droplets on the condensing surface. The promotion of DWC instead of the traditional filmwise condensation (FWC) is a promising solution to enhance the efficiency of heat exchangers by increasing the condensation heat transfer coefficient. The interaction between the condensing fluid and the surface (wettability) is important in defining the condensation mode. On metallic surfaces widely employed in heat transfer applications, the condensing process occurs in filmwise mode. Ideally, an engineered surface designed to achieve high DWC heat transfer coefficients should present low contact angle hysteresis and low thermal resistance. Among the different available techniques to modify the surface wettability, hybrid organic-inorganic sol-gel silica coatings functionalized with hydrophobic moieties (phenyl or methyl groups) have been identified as a feasible solution to promote DWC on metallic surfaces. In the present paper, different aluminum sol-gel coated surfaces have been tested during DWC of steam in saturated conditions. The realized coatings have been characterized by means of dynamic contact angles and coating thickness measurements. Condensation tests have been performed using a two-phase thermosyphon loop operating in steady-state conditions that allows visualization of the condensation process and simultaneous heat transfer measurements. Heat transfer coefficients have been measured by varying the heat flux, at 106 °C saturation temperature and with vapor velocity equal to 2.7 m s-1. A high-speed camera is used for the visualization of the DWC process.