The drag force acting on a spherical non-evaporating or evaporating particle immersed into a rarefied plasma flow

Abstract

A previous drag force study for a spherical particle immersed into a rarefied plasma flow has been improved upon, based on a more detailed analysis concerning the interaction between the charged sphere and ions and electrons. Revised analytical expressions are presented for the drag force acting on a non-evaporating or evaporating particle for the extreme case of the free-molecule flow regime and a thin plasma sheath. Both metallic and non-metallic limiting cases of the particle materials (with infinite and zero electrical conductivity, respectively) are considered. It is shown that the ion component of the drag force is appreciably less than the previous result and that the drag coefficient for a non-evaporating particle becomes identical to that derived for the case of non-ionized gases. The drag force on a spherical particle is directly proportional to the square of the particle radius and to the relative velocity between the particle and the bulk plasma at low speed ratios. Evaporation enhances the drag force acting on a particle due to the additional contribution to the drag force caused by the reaction of non-uniform evaporated mass flux outflowing from the evaporating particle. This evaporation-added drag force is more significant for particle materials with lower latent heat of evaporation and lower vapour-molecular mass. For the case without evaporation, there is almost no difference between metallic and non-metallic particles in their drag forces. However, a marked difference may exist between metallic and non-metallic evaporating particles in their drag forces because the different floating potential distributions on the surfaces of the two types of particles cause different distributions of the local heat fluxes along the particle surfaces.