A modified pseudo-equilibrium model competing with kinetic models to determine the composition of a two-temperature SF6 atmosphere plasma
Résumé
This paper is devoted to calculation of the non-equilibrium composition in a SF6 thermal plasma at atmospheric pressure. Non-equilibrium thermal plasmas are characterized by heavy species temperatures Th below 9000 K with electron temperatures at the maximum three times higher than Th when the latter is below 4000 K. Different theories have been used based on either multi-temperature plasmas, Saha-Potapov modified by André et al, van de Sanden et al, Cliteur et al, or kinetic calculations or the pseudo-equilibrium model, recently developed. This model gives results similar to those of kinetic calculations for N2 and H2 plasmas but with calculation times two orders magnitude faster. Pseudo-equilibrium calculation takes into account the reactions with low activation energies instead of ionization reactions, while keeping all the species present in the kinetic calculation. First, the theories are compared in a case already studied in the literature by Cliteur: a heavy species temperature Th at 6000 K, with the electron temperature Te varying between 6000 and 15 000 K. Comparison of the results shows that the multi-temperature calculations, except those of Cliteur, are far from kinetic especially for ne and nF-. In addition, the pseudo-equilibrium model fits rather well with the kinetic calculations as long as molecular species are present in the plasma. Second, to calculate the composition of non-equilibrium thermal plasmas the ratio Te/Th is assumed to vary as the logarithm of the electron densities ratio ne/ncmax, nemax being the electron density over which equilibrium prevails, i.e. 1023 m-3. For kinetic reactions where electrons are involved (in the direct reaction while heavy species intervene in the reverse reaction), a temperature T* between Te and Th is defined. T* is calculated as a function of the electron flux to that of heavy species. The variation of T* with Th is smoother than that of Te. In such conditions again, there is an excellent agreement between kinetic and pseudo-equilibrium calculations performed at T*, which is not the case for multi-temperature calculations. These results demonstrate that the pseudo-equilibrium calculation developed for thermal plasma simple forming gases such as N2 and H2 can also be applied to more complex gases such as SF6.