| A new analytical method is proposed for gas-solid mass transfer calculations under plasma conditions. Plasma-particle mass transfer has been a difficult subject in the past primarily due to the lack of an effective means to eliminate the problems posed by steep temperature gradients that are typically encountered in plasma processing operations. The Z-potential method proposed herein fully accounts for the extreme temperature variations in the particle vicinity and permits use of the well-known isothermal expression Sh = 2.0. The method is also very convenient to use and greatly facilitates the study of phenomena of possible importance in plasma-particle mass transfer.;The effect of thermal diffusion is investigated using the Z-potential method. The results of some illustrative calculations suggest that thermal diffusion can make a considerable contribution to the overall mass transfer in plasma-particle systems. However, the Dufour effect, which is the reciprocal phenomenon of thermal diffusion, is observed to be far less significant.;The Z-potential approach is also found to be extremely useful in ascertaining the Knudsen discontinuum effect on plasma-particle mass transfer. A predictive model for the Knudsen effect is developed by incorporating the Z-potential approach into Maxwell's jump theory. Case studies on self-diffusion in argo and nitrogen plasma systems indicate that, under conditions typically encountered in thermal plasma processing of fine particles, the Knudsen effect is significant and depends strongly on the particle size and the surface conditions. The Knudsen effect can also enhance the plasma-particle mass transfer rate in certain situations.;In light of the revelations of the above study, a method is developed for the calculation of the gas-solid heat and mass transfer rates in the presence of Knudsen effect in rarefied, multicomponent gas mixtures. Results of illustrative computations on some binary systems indicate the need to use this simple, yet more rigorous, calculation method instead of existing single gas expressions.;A computer program is also developed to predict the in-flight velocity, temperature and size of a particle injected into a non-reactive plasma. The program is capable of accounting for the effect of steep temperature gradients, the Knudsen effect and the effect of evaporation. |
