Dropwise-to-filmwise transition during condensation of steam on hydrophilic surfaces

• Dropwise-to-filmwise transition is studied on hydrophilic coated samples. • Heat transfer and video analysis are performed at varying wall subcooling. • Higher surface wettability enhances the HTC during dropwise condensation. • Transition from DWC to FWC occurs at lower subcooling on more hydrophilic surfaces. • A numerical model to predict DWC heat transfer and DWC/FWC transition is proposed. Dropwise condensation (DWC) is a two-phase heat transfer process that is expected to achieve heat transfer coefficients several times higher as compared to filmwise condensation (FWC). Usually, low-wettability coatings are required to promote DWC on metallic surfaces. However, recent studies suggest that hydrophilic (wettable) surfaces with low contact angle hysteresis can further increase the heat transfer coefficient (HTC), potentially leading to heat transfer processes with higher effectiveness. Despite this potential, the ability of such surfaces to sustain enhanced DWC without flooding remains unclear. Moreover, the underlying mechanisms governing the transition mechanism from dropwise to filmwise condensation, which is likely to occur on hydrophilic surfaces, remain poorly understood. While some studies have addressed this phenomenon, conclusive findings are still lacking. This paper aims to address these unclear aspects of DWC by investigating pure steam condensation on samples with varying wettability at a constant saturation temperature (107 °C) while adjusting the coolant medium temperature from 20 °C to 95 °C. Heat transfer measurements and high-speed imaging were employed to provide a detailed analysis of how the saturation-to-wall temperature difference, and consequently heat flux, affects the condensation mode. The results demonstrate that DWC can be effectively sustained on hydrophilic surfaces with enhanced droplet mobility, achieving HTC values up to 70 % higher than those observed for DWC on hydrophobic surfaces. However, on hydrophilic surfaces, the transition from DWC to FWC occurs at lower wall subcooling values. As a further step, a simplified model for predicting the condensation mode and the heat flux during DWC is proposed and validated using the present experimental data. The model is found to accurately predict the effect of surface wettability and subcooling on the DWC HTC, with a mean deviation between calculated and measured values lower than 12 %. Furthermore, the model enables the successful identification of the subcooling range over which DWC can be sustained, based on the surface properties and operating conditions.