Concluding Remarks and Perspectives

Abstract

Dropwise condensation is vapor-to-liquid phase-change in the form of discrete drops on or underneath horizontal and inclined substrates. The process is hierarchical in the sense that it occurs over a wide range of length and timescales. A mathematical model of dropwise condensation underneath textured surfaces, horizontal (with or without wettability gradient) and inclined, is reported. The model starts from the formation of drops at the atomic scale at randomized nucleation sites and follows its growth by direct condensation and coalescence, till the drop is large enough to fall-off or slide away. In the model, nucleation sites are randomly distributed over the substrate. Growth rate at each nucleation site is derived on the basis that vapor condenses on the free surface of the drop and releases latent heat that is transferred through the liquid drop to the cold substrate. A simple model of coalescence has been adopted in this work. The stability criterion is developed as a force balance equation at the level of a drop. Transport parameters of a sliding drop are determined using a CFD model and presented in the form of correlations. Performing simulation of the complete cycle of dropwise condensation, the spatiotemporal distribution of drops is obtained, from which local and area-averaged heat transfer rates as a function of time are predicted. Predictions of numerical simulation are compared against experimentally derived condensation patterns and heat fluxes. The limitations of the model and suggestions for possible improvement are discussed.