Cfd Based Optimisation of the Superhydrophobic Functional Surface in Dropwise Condensation

Dropwise condensation processes, where condensation occurs through small droplets on a solid surface, has been demonstrated to significantly improve heat transfer rates in comparison to film-wise condensation (where a whole surface is covered by a thin film of liquid). Dropwise condensation usually takes place on superhydrophobic surfaces, and one of the main engineering and scientific challenges is to develop such a surface to allow condensation, and removal of the droplets, to occur in a continuous manner. Experimentally validated CFD analysis of droplets on superhydrophobic surfaces is undertaken to describe the mechanism of self propelled droplets ’jumping’ from the surface after coalescence, the process by which two droplets merge during contact to form a single droplet. Owing to the larger diameter of the newly created droplet, it starts to push against the surface and spread beyond static contact angle usually observed between the fluid and the solid surface in equilibrium. Spreading beyond the equilibrium state creates a reaction at solid-liquid interface and shifts the momentum upwards, generating unbalanced kinetic energy and propelling the droplet. This allows the liquid to overcome the adhesion forces and dewet the surface separating droplet from surface. Optimisation of droplets radius and contact angle to obtain high ’jumping’ droplet velocity, hence improving droplets evacuation, was performed using design of experiment and surface response techniques. CFD-based optimisation is proven to be a reliable and fast predictive modelling tool.