Treatments of diffusion in thermal plasmas

Treatments of diffusion that have been applied to the modelling of thermal plasmas are reviewed. Eight such treatments are identified. Three allow cal culation of the diffusion of each individual species present; the other fiv e require the species derived from a given gas to be combined. The accuracy of each of the treatments is assessed through comparison with full multico mponent diffusion calculations and experimental results. Of the first group , the self-consistent effective binary diffusion coefficient method is foun d to be the most accurate under conditions relevant to thermal plasmas, and its use is recommended in cases when the diffusion of individual species i s calculated. In cases in which the species can be combined into gases, the combined diffusion coefficient formulation, which gives the same results a s the full multicomponent diffusion coefficient treatment under conditions in which local chemical equilibrium applies, is found to be significantly m ore accurate than other models.