Effect of moving boundaries on the modeling of heat and mass transfer from an evaporating spherical drop

The effect of unsteadiness of the energy and vapor transport within the gas phase from an evaporating drop is studied by solving a moving boundary problem taking into account the effect of drop temperature variation and radius shrinking. The effect of convection is also taken into account in a simplified way by means of the film theory approach, which yields a double moving boundary problem. A proper change of the reference system leads to the numerical solution of a partial differential equation system with fixed boundaries. A comparison with the commonly adopted quasi-steady model allows to point out the effect of ambient temperature and pressure, convection and chemical species, by quantifying the discrepancies between the two predictions for sixteen different compounds, Reynolds number ranging between 0 and 20 and gas pressure up to 20 bar. The simplified approach used in this paper was chosen to maintain the same simplifying assumptions of the widely used quasi-steady model, with the only exception of the drop shrinking. Therefore, the discrepancies between the two predictions can solely be ascribed to the unsteadiness caused by the interface movement, allowing to quantitatively point out this specific effect.