DRAG FORCE ACTING ON AN EVAPORATING PARTICLE IMMERSED IN A RAREFIED PLASMA-FLOW

Analytical expressions are presented for the drag force acting on an e vaporating or nonevaporating particle immersed in a plasma flow for th e extreme case of free-molecule flow regime and thin plasma sheath. It is shown that the drag force on a spherical particle is proportional to the square of the particle radius and to the relative velocity betw een the particle and the bulk plasma at low speed ratios. The existenc e of a relative velocity between the particle and the plasma results i n a nonuniform heat flux distribution with its maximum value at the fr ontal stagnation point of the sphere. This nonuniform distribution of the local heat flux density causes a nonuniform distribution of the lo cal evaporated-mass flux and vapor reaction force around the surface o f an evaporating particle, and thus induces an additional force on the particle. Consequently, the drag force acting on an evaporating parti cle is always greater than that on a nonevaporating one. This addition al drag force due to particle evaporation is more significant for nonm etallic particles and for particle materials with lower latent heat of evaporation and lower vapor molecular mass. It increases with increas ing plasma temperature and with decreasing gas pressure at the high pl asma temperatures associated with appreciable gas ionization. The drag ratio increases with increasing electron/heavy-particle temperature r atio at high electron temperatures for a two-temperature plasma.