Growth and redistribution in a droplet cloud interacting with radiation

Abstract

Previous work on the growth and redistribution of a droplet distribution interacting with a radiation field has been extended. With no other macroscopic heat and mass transfer processes the supersaturation is shown to be non-zero when the Lewis number, Le, of the surrounding vapour-gas mixture differs from unity. Redistribution of mass with evaporation of the smallest droplets then occurs for radiative cooling and Le < 1, a condition satisfied for water aerosols. Exact and approximate expressions are given for the wavelength-averaged absorption efficiencies of water droplets. Calculations of changes to water droplet distributions were performed using exact methods and simplified moment equations. The latter are adequate when radiative redistribution leads to a common radius and there is no initial transient. The conditions for no mass transfer to the aerosol and a constant total water density and temperature are discussed. In the latter case with radiative cooling, which could possibly apply to reactor situations, a considerable widening of the size distribution takes place on a long timescale.