KINETICS OF DROPLET EVAPORATION

This paper presents research on evaporation of droplets containing dissolved or dispersed solids. Experiments on evaporation kinetics of droplets of water, colloidal silica, sodium sulphate and skimmed milk were performed. The experimental procedure was similar to that proposed by Charlesworth and Marshall (1960, A.I.Ch.E. J. 6, 9-23). Individual droplets were suspended in a controlled air stream and their weight and temperature were measured as evaporation progressed. Video recording of the size and appearance of the droplets was made with magnification close to 100. The temperature was followed by a micro-thermocouple, while the mass was measured by using the deflection of a calibrated glass filament balance. Experimental data were compared with a computer simulation based on a model of heat and mass transfer, both within the droplet and from the droplet surface to surrounding air. Analytical and numerical solution of the set of equations was tested. A good agreement between experimental results and the numerical predictions for a variety of evaporation regimes is reported. Diffusion coefficients for three tested materials are determined. The model includes all stages of evaporation: initial heating and evaporation, quasi-equilibrium evaporation, crust formation and growth, boiling, and porous particle drying. Stage transition criteria are defined.