Evaporation of a droplet on a heated spherical particle

Abstract

A three-dimensional, CLSVOF-based numerical model was developed to study the hydrodynamics of water droplets of various diameters impacting a heated solid particle. The temperature of the particle was set to be above the Leidenfrost temperature of the fluid, such that the influence of several key parameters on the dynamics of film boiling of the droplet could be examined. The simulation results were validated against experimental observations, where it was found that the numerical model could satisfactorily reproduce the dynamics of the droplet. The spread of the droplets upon impact was found to be dependent on the Weber number, with surface tension and viscous forces then acting to recoil the droplet. The rate of droplet recoil was found to be highly dependent on the Reynolds number, as fluid advection tends to enhance the rate of heat transfer within the droplet and the evaporation at the solid–liquid contact line. Eventually, evaporation causes build-up of vapour pressure at the bottom of the liquid, and the droplet lifts-off from the heated particle. It was found that the onset of the droplet lift-off could be estimated through the first-order vibration of a freely oscillating droplet, particularly in cases with low values of Weber number. Finally, the rate of evaporation of the droplet was found to be highly dependent on the capillary length of the fluid and the stability of the vapour layer formation underneath the droplet.