A. B. Murphy

CSIRO Telecommunications and Industrial Physics
P.O. Box 218, Lindfield NSW 2070
tony.murphy@tip.csiro.au

The treatment of diffusion causes substantial difficulties in thermal plasma modelling. Other transport coefficients, such as viscosity and thermal conductivity, can each be described by one value for a given temperature and composition. To treat diffusion, in contrast, an ordinary diffusion coefficient has to be calculated for each pair of species and a thermal diffusion coefficient for each species.

This difficulty has led to the adoption of a number of approximate treatments of diffusion. Most of these were developed for non-ionised gases, and their application to plasmas leads to serious inaccuracies [1,2]. Two treatments have recently been developed that address these problems. One, the combined diffusion coefficient formulation [3], is applicable to mixtures of non-reacting homonuclear gases. Under local chemical equilibrium conditions, all the species (e.g., N2, N+, N2+ that make up a given gas (e.g., nitrogen) can be treated together. For a mixture of two gases, this allows all the ordinary and thermal diffusion coefficients to be replaced by just two, without loss of accuracy. The other treatment, the self-consistent effective binary diffusion method [4], is more generally applicable, but requires each species to be treated separately.

In this paper, the various treatments of diffusion will be reviewed, with particular emphasis on the combined diffusion coefficient formulation. Results obtained using this formulation will be compared with those given by the approximate treatments for processes relevant to thermal plasma processing, such as demixing in a free-burning arc [5], and the vaporisation of metal electrodes and particles in a plasma.

[1] A. B. Murphy, A comparison of treatments of diffusion in thermal plasmas, J. Phys.D: Appl. Phys., 29 (1996) 1922--1932.
[2] M. D'esilets, P. Proulx and G. Soucy, Modeling of multicomponent diffusion in high temperature flows, Int. J. Heat Mass Transfer, 40 (1997) 4273-4278.
[3] A. B. Murphy, Diffusion in equilibrium mixtures of ionized gases, Phys. Rev. E, 48 (1993) 3594-3603; Erratum, ibid., 50, (1994) 5145-5146.
[4] J. D. Ramshaw and C. H. Chang, Ambipolar diffusion in multicomponent plasmas, Plasma Chem. Plasma Process., 11 (1991) 395-403.
[5] A. B. Murphy, Demixing in free-burning arcs, Phys. Rev. E, 55 (1997) 7473-7494.