Evaporation of Leidenfrost droplets on microtextured substrates

A thin thermally insulating vapor layer beneath a levitating Leidenfrost droplet adversely affects the heat transfer from the underlying hot substrate during cooling applications. In this work, we present a theoretical model to determine the total heat transfer to a Leidenfrost droplet on microtextured substrates, taking into account the curved shape of the liquid-vapor interface. The profile of the vapor gap and vapor flow field beneath a Leidenfrost droplet on a microtextured surface is shown to be dependent on the substrate morphology. The shape of the liquid-vapor interface beneath the Leidenfrost droplet, in turn, influences the rate of evaporation on a microtextured substrate. We determine the variation of the minimum and maximum vapor gap over micro-pillared surfaces with different substrate permeability for various droplet volumes and wall superheat. The heat transferred to the droplet is via conduction across the vapor gap beneath the droplet and convection from the ambient air around the droplet. We determine the total evaporation time of a Leidenfrost droplet over a micro-textured substrate using the curved interface model and the conventional flat vapor gap interface model and compare with that obtained from experiments. The overprediction of the average rate of evaporation of a Leidenfrost droplet by the flat interface model ranges from-59 % for tall and sparse pillars (marked by high substrate permeability) to -29 % for short pillars (with low substrate permeability). We show that the average rate of evaporation of the LF droplet obtained using the curved interface model agrees reasonably (within 9%-23 %) with that observed in the experiments.

Automated summary

In this study, a theoretical model is proposed to determine the total heat transfer to a Leidenfrost droplet on microtextured substrates, considering the curved shape of the liquid-vapor interface. The results show that the vapor gap and flow field beneath the Leidenfrost droplet on a microtextured substrate depend on the substrate morphology, and the shape of the liquid-vapor interface influences the evaporation rate on the substrate.