Modelling and diagnostics of plasma chemical processes in mixed-gas arcs

Abstract

The characterisation of thermal plasmas derived from gas mixtures is considered. The assumption of local chemical equilibrium is used to derive the combined diffusion coefficient formulation, which greatly simplifies the treatment of diffusion in mixtures of non-reacting homonuclear gases. The formulation is applied to the modelling of demixing in free-burning arcs. A two-dimensional numerical model is used to assess the relative importance of different demixing processes in argon-helium and argon-nitrogen arcs. Demixing is found to strongly alter the composition of these arcs. The predicted arc compositions are shown to agree well with measurements performed by spectroscopic techniques. A comparison of laser-induced fluorescence measurements of atomic hydrogen density in an argon-hydrogen arc with calculations shows that the assumption of local chemical equilibrium is not valid in the fringes of such an arc.