Transient modeling of heat, mass and momentum transfer of an evaporating cerium nitrate solution droplet with a surrounding shell in a rf thermal argon–oxygen plasma under reduced pressure

A model was developed to study the evaporation of a solution droplet surrounded by a porous crust in a stagnant rf Ar–O2 thermal plasma under reduced pressure. This model considered a three phase system: a liquid core of dissolved Ce(NO3)3 · 6H2O in water, a porous crust of homogeneously precipitated spherical crystals of equal size containing water vapor, and an Ar–O2 plasma under reduced pressure. The model was solved considering a receding solution/crust interface in an ALE frame using temperature and composition dependant thermophysical properties. Darcy flow with a Knudsen correction to account for the gaseous flow through a porous media composed of nano-sized crystals was employed. The strength of the solid/liquid interface was calculated by computing the strength of liquid bridges formed at this interface. This value was compared to the pressure build-up caused by solvent evaporation and the point of crust breakage was determined at different operating conditions. The effects of plasma gas temperature, pressure and composition, droplet size, size of precipitated crystals and crust porosity on crust bursting were studied. The results showed that crust bursting occurred for all the conditions analyzed and that plasma temperature, droplet size and the size of the precipitated crystals had a significant effect on pressure build-up.