FILM EVAPORATION OF A SPHERICAL DROPLET OVER A HOT SURFACE - FLUID-MECHANICS AND HEAT MASS-TRANSFER ANALYSIS

Film evaporation of a spherical droplet over a hot surface is investigated in this paper. In view of the radial evaporation-induced velocity at the liquid-gas interface, an improvement over the classical flow field solution of Stimson & Jeffery (1926) needs to be employed. In addition to the flow, the energy equation internally and externally to the droplet and the species equation in the gas phase are also solved. The boundary conditions at the droplet surface couple the temperature, species and flow field. Analytical expressions for the hydrodynamic force and its components (viscous and pressure) that the droplet experiences are obtained. It is shown that, depending on the droplet separation distance from the hot surface and the type of liquid, there may be a substantial temperature variation along the droplet surface. Furthermore, considering a quasi-steady approximation for the droplet regression rate and balancing at each time step the weight of the droplet with the hydrodynamic force it experiences, time histories are obtained numerically for various quantities of interest. Thus, it is predicted that the droplet moves away from the hot surface while evaporating and that the initially substantial temperature variation along the droplet surface decreases with time and diminishes towards the end of the droplet lifetime. It is also shown that the droplet surface temperature is more uniform at higher hot-surface temperature.