A Study of Droplet-Behavior Transition on Superhydrophobic Surfaces for Efficiency Enhancement of Condensation Heat Transfer.

Enhancement in heat-transfer performance via dropwise condensation on superhydrophobic surfaces is much greater than that realized via generic condensation on a regular surface. However, if the supersaturation level during condensation increases above a specific value, water may seep to greater depths between structures. This may lead to attached condensation, which reduces condensation heat-transfer efficiency below that of ordinary surfaces. Therefore, it is critical to avoid the occurrence of supersaturation when superhydrophobic surfaces are employed in condenser design. The proposed study presents a simple method for regulating supersaturation on the laboratory scale. Experiments concerning droplet behavior on a superhydrophobic plate were performed to investigate droplet detachment and attachment in accordance with the surface and droplet temperatures. Results obtained have been represented as a ″droplet-behavior map″, which clearly depicts boundaries dividing the detachment and attachment regions. The supersaturation threshold obtained from the said map has been compared against results obtained from condensation heat-transfer experiments performed in an actual condenser environment. As observed, the two results demonstrate excellent agreement. Although superhydrophobicity of surfaces remains unchanged at room temperature, changes may occur in the extent of the supersaturation section, which improves condensation heat-transfer performance, depending on the surface-structure complexity. Therefore, droplet-behavior mapping has been used in this study to determine the available supersaturation section in accordance with the variation in surface roughness. Results confirm that the available supersaturation region increases with increasing surface roughness and structural complexity. Therefore, prior to applying superhydrophobicity to condensers, droplet-behavior mapping must be performed to avoid operation under the supersaturation conditions, which causes attached condensation.