An experimental study on the heat transfer and wettability characteristics of micro-structured surfaces during water vapor condensation under different pressure conditions

In this study, condensation characteristics of water vapor on micro-structured surfaces at different pressures and subcooling temperature has been investigated experimentally. This work represents a basic study in order to design a secondary evaporator section in a refrigeration cycle. A set of surfaces has been manufactured over copper substrates with one sample used as unstructured (smooth) reference surface and two micro-structured surfaces with longitudinal grooves having V-shape and square cross-sections. As the second step, the surfaces have been modified by using a polymer coating to achieve stronger hydrophobicity at the surface and hence to influence wettability such that increased heat transfer rates should be reached. The polymer coating has been created with a dip coating process by applying a mixture of perfluoroalkyl triethoxysilane. Concerning the wettability, a high-speed flow visualization study has been performed for the interpretation of heat transfer results. Experimental results showed that an increase in the droplet contact angle by applying the polymer coating over surfaces, while the largest droplet contact angle was obtained (130.9°±2.0°) for the surface with V-shaped micro-structuring compared to other surfaces. The comparison of heat transfer performance reveals an enhancement in heat transfer coefficient for the coated version of unstructured, square-grooved and V-grooved surface by 34.5%, 61.8%, and 73.4%, respectively.