Enhancement of water droplet evaporation by radiation absorption

Abstract

Water droplet evaporation is relevant to fire suppression, industrial cooling processes, and many other technologically important applications. A mathematical model of unsteady evaporation of a water droplet is developed. The droplet is assumed to be spherical and semitransparent to radiation. A radiative transfer model based on geometrical optics theory is used to calculate the local volumetric rate of radiation absorption. Published spectral absorption coefficient data for water are used in performing the calculations. The effects of thermal expansion and temperature-dependent thermophysical properties on the evaporation process are accounted for. The internal circulation in the droplet due to the externally imposed flow is accounted for through an effective thermal conductivity of the water droplet. The model predictions are compared with available computational results and experimental data. The results of calculations show that the absorption of radiation and thermal expansion significantly affect the lifetime of a droplet. The results also reveal that neglect of radiation absorption by a droplet underpredicts the rate of water evaporation and greatly overpredicts the droplet lifetime for droplet diameters greater than 500 μ m .