VAPOR VELOCITY AND DROPLET DYNAMICS DURING DROPWISE CONDENSATION OF STEAM FLOWING OVER HYDROPHILIC SURFACES

Dropwise condensation (DWC) is a complex phase-change process that involves the nucleation, growth and removal of randomly distributed drops on the condensing surface. It is widely established that the promotion of dropwise condensation can significantly improve the heat transfer coefficient (HTC) as compared to filmwise condensation (FWC). The interaction between the condensing fluid and the surface (wettability) defines the condensation mode. Low wettability coatings with small contact angle hysteresis and low thermal resistance are a possible solution to obtain high heat transfer coefficients during DWC on metals. In energy applications, the condensing steam usually has a non-negligible velocity, but experimental data taken with flowing vapor are rare in the literature. Therefore, the investigation of DWC in presence of steam velocity would help to understand the physical mechanisms governing the phenomenon and to develop comprehensive dropwise condensation models. In the present study, hydrophilic (advancing contact angle θa < 90°) sol-gel coated aluminum samples with reduced contact angle hysteresis (Δθ < 30°) were tested during DWC of pure steam. The engineered surfaces were characterized by dynamic contact angles and film thickness measurements. The experimental apparatus used for DWC investigation is a thermosyphon loop operating in steady-state conditions. The test rig is equipped with an optical system for the study of the droplet population and droplet dynamics. Heat transfer measurements and droplet population analyses were carried out at constant saturation temperature (~ 107.5 °C) while varying the heat flux in the range 290-1020 kW m-2 and increasing the inlet vapor velocity from 3 m s-1 to 13.5 m s-1. As a further step, the collected experimental data were compared against DWC models accounting for the effect of vapor velocity.