During the decay of a circuit-breaker arc, the plasma is subjected to strong blowing which can lead to deviations from chemical equilibrium. The intense convection may therefore be responsible for the presence of cold gas in the hot parts of the plasma. The cold particles then rapidly recombine with electrons, modifying the resistivity of the plasma. In order to study this phenomenon as it appears in circuit-breakers, we made a modelling of the extinction of an arc for a simplified geometry. The two-dimensional model that was set up was followed by a study of the kinetics of which enabled us to identify the various reaction processes governing the disappearance of electrons. The results show that convection acts on molecules which, at the edge of the discharge and for temperatures of between 4000 and 6000 K, are overpopulated with respect to equilibrium. Through charge exchange processes between and particles, the overpopulation of leads to an overpopulation of the ions. These ions mainly recombine with electrons, lowering the electron population and modifying the electrical conductivity of the plasma.
Studying the influence of thermal departures from equilibrium in SF 6 circuit-breakers, we develop a two-temperature kinetic model to calculate the composition. Such a kinetic approach has not been adopted until now for SF 6 plasma because of the complexity of chemical processes. Our model takes into account the collisional mechanisms responsible for the creation and disappearance of atoms and molecules through 19 species linked by 66 chemical reactions. To solve the conservation equations, the model uses the direct rates of reactions, that mainly proceed from the literature, and reverse rates, that are computed by two-temperature micro-reversibility laws. Thus, we point out the importance of the choice of the expression of Saha law, comparing Potapov and van de Sanden formulations of this law. We then discuss the impact of thermal departures from equilibrium on plasma composition, on 'mean path' of molecules before dissociation in the plasma, and on the reactions that govern the disappearance of electrons.Disappearance frequency of species A through reaction 'p' (s −1 ) ν ATotal disappearance frequency of species A (s −1 ) θ Ratio between electrons temperature and heavy particles one (θ = T e /T h ) hPlanck constant (J s) k B Boltzmann constant (J K −1
The demixing effect in a wall-stabilized arc leads to sulphur depletion in the central part of the arc where the ratio between fluorine concentration and sulphur concentration becomes higher than six. We have calculated material functions for different real or fictitious gases , with (specific heat, electrical and thermal conductivities, net emission coefficient) and used them in a one-dimensional model based on Patankar's algorithms. The electric field and the temperature radial profile have been calculated versus the current intensity using 5 mm and 3.2 mm arc diameters. Comparison with experimental results in the literature shows that the influence of demixing on the axis temperature is greater than the experimental error. Thus, we conclude that validating material functions of plasma by comparing experimental and theoretical values of temperature in wall-stabilized arcs needs to take into account demixing effects.
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