A comparison of treatments of diffusion in thermal plasmas

The different treatments of diffusion that have been used in published numerical models of plasmas in local chemical equilibrium (LCE) are examined. Four approximate formulations are identified. These are compared with the combined diffusion coefficient method, which is equivalent to the full multicomponent diffusion coefficient treatment when the assumption of LCE is made. Diffusion coefficients and diffusive mass fluxes are calculated using each formulation for a number of representive cases involving the diffusion of copper vapour and nitrogen in an argon plasma. The different formulations are further used in a two-dimensional numerical model of a free-burning are in a gas mixture, and the predicted compositions and temperatures are compared with measured values. Serious deficiencies are identified in the approximate formulations, both in the calculation of the diffusion coefficients and, more importantly, in the equation used to derive the diffusive mass flux from the diffusion coefficients. Use of the combined diffusion coefficient method is found to give good agreement with the measurements.