Abstract
Dropwise condensation (DWC) is a two-phase heat transfer process that has been identified as more efficient than filmwise condensation (FWC). To promote DWC on metallic surfaces, low wettability coatings are usually required. However, it has recently been suggested that DWC on hydrophilic (wettable) surfaces with low contact angle hysteresis should result in a further increase in the heat transfer coefficient (HTC). An open issue is related to the ability of hydrophilic surfaces to promote and sustain DWC without flooding at high values of heat flux. In this work, this question is addressed by studying condensation of steam on samples of different wettability at constant saturation temperature (107 °C), while varying the coolant medium temperature from 20 °C to 95 °C. Heat transfer data and high-speed videos have been used to provide a comprehensive description of the effects of the saturation-to-wall temperature difference (and thus heat flux) on the condensation mode. From the results it can be concluded that DWC can be successfully promoted on hydrophilic surfaces with improved droplet mobility, leading to values of HTC higher than those obtained during DWC on hydrophobic surfaces. However, for hydrophilic surfaces, the transition from DWC to FWC occurs at lower wall subcooling values.