Relaxation Time and Steady Evaporation Rate of Freely Falling Raindrops

Abstract

Relaxation times and equilibrium temperatures and evaporation rates of stationary and falling water drops are predicted using beat and mass transfer analyses. Except possibly for very large drops the relaxation time is small and steady-state evaporation takes place over most of the lifetime of a drop. The steady-state temperature is equal to the wet-bulb temperature for both the stationary and the translating cases. The form of the ventilation coefficient measured by Kinzer and Gunn for low Reynolds number is predicted by an asymptotic theory of Acrivos and Taylor. Limiting analyses for heat transfer to spheres from fluids with high and low Prandtl numbers bracket the high Reynolds number data. Analytical expressions are suggested for predicting mass transfer rates at various Reynolds numbers.