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- Numerical investigation of AC arc ignition on cold electrodes in atmospheric-pressure argonPublication . Santos, D. F. N.; Lisnyak, M; Almeida, N.; Benilova, L. G.; Benilov, M. S.Since experiments cannot clarify the mechanism of current transfer to non-thermionic arc cathodes, this can only be done by means of numerical modelling based on first principles and not relying on a priori assumptions. In this work, the first quarter-period after the ignition of an AC arc on cold electrodes in atmospheric-pressure argon is investigated by means of unified one-dimensional modelling, where the conservation and transport equations for all plasma species, the electron and heavy-particle energy equations, and the Poisson equation are solved in the whole interelectrode gap up to the electrode surfaces. Results are compared with those for DC discharges and analysed with the aim to clarify the role of different mechanisms of current transfer to non-thermionic arc cathodes. It is found that the glow-to-arc transition in the AC case occurs in a way substantially different from the quasi-stationary glow-to-arc transition. The dominant mechanisms of current transfer to the cathode during the AC arc ignition on cold electrodes are, subsequently, the displacement current, the ion current, and thermionic emission current. No indications of explosive emission are found. Electron emission from the impact of excited atoms can hardly be a dominant mechanism either. The introduction of the so-called field enhancement factor, which is used for description of field electron emission from cold cathodes in a vacuum, leads to computed cathode surface temperature values that are appreciably lower than the melting temperature of tungsten even in the quasi-stationary case. This means that pure tungsten cathodes of atmospheric-pressure argon arcs can operate without melting, in contradiction with experiments.
- Simulating changes in shape of thermionic cathodes during operation of high-pressure arc dischargesPublication . Cunha, M. D.; Kaufmann, H. T. C.; Santos, D. F. N.; Benilov, M. S.A numerical model of current transfer to thermionic cathodes of high-pressure arc discharges is developed with account of deviations from local thermodynamic equilibrium occurring near the cathode surface, in particular, of the near-cathode space-charge sheath, melting of the cathode, and motion of the molten metal under the effect of the plasma pressure, the Lorentz force, gravity, and surface tension. Modelling results are reported for a tungsten cathode of an atmospheric-pressure argon arc and the computed changes in the shape of the cathode closely resemble those observed in the experiment. The modelling has shown that the time scale of change of the cathode shape during arc operation is very sensitive to the temperature attained by the cathode. The fact that the computed time scales conform to those observed in the experiment indicate that the model of non-equilibrium near-cathode layers in high pressure arc discharges, employed in this work, predicts the cathode temperature for a given arc current with adequate accuracy. In contrast, modelling based on the assumption of local thermodynamic equilibrium in the whole arc plasma computation domain up to the cathode surface could hardly produce a similar agreement.
- Account of diffusion in local thermodynamic equilibrium and two-temperature plasma modelsPublication . Santos, D. F. N.; Lisnyak, M; Benilov, M. S.A self-consistent account of the effect of diffusion on charge transport in local thermodynamic equilibrium (LTE) and two-temperature (2T) ionization-equilibrium plasmas amounts to introducing into Ohm’s law, in addition to the conventional term proportional to the electric field (conduction current) and thermal-diffusion terms, also terms describing the diffusion due to plasma composition variations, which are proportional to the temperature gradient (or, in the case of 2T plasmas, to ∇Te and ∇Th) and to the plasma pressure gradient. These terms are calculated, with the use of the Stefan–Maxwell equations, for the particular case of 2T ionization-equilibrium atomic plasmas with singly charged ions. Also proposed is a simple way of approximate evaluation of reactive thermal conductivity in such plasmas. An online tool performing evaluation of the relevant coefficients for 2T argon, xenon, and mercury plasmas has been deployed on the internet. Representative modelling results show that the new form of Ohm’s law, when introduced into standard LTE or 2T models, may describe the electric field reversal in front of arc anodes, an effect that has been simulated previously only by means of (more complex) models taking into account deviations from ionization equilibrium.
- Simulation of pre-breakdown discharges in high-pressure air. I: The model and its application to corona inceptionPublication . Ferreira, N. G. C.; Santos, D. F. N.; Almeida, P. G. C.; Naidis, G. V.; Benilov, M. S.A ‘minimal’ kinetic model of plasmachemical processes in low-current discharges in high pressure air is formulated, which takes into account electrons, an effective species of positive ions, and three species of negative ions. The model is implemented as a part of numerical model of low-current quasi-stationary discharges in high-pressure air based on the use of stationary solvers, which offer important advantages in simulations of steady-state discharges compared to standard approaches that rely on time-dependent solvers. The model is validated by comparison of the computed inception voltage of corona discharges with several sets of experimental data on glow coronas. A good agreement with the experiment has been obtained for positive coronas between concentric cylinders in a wide range of pressures and diameters of the cylinders. The sensitivity of the computation results with respect to different factors is illustrated. Inception voltages of negative coronas, computed using the values of the secondary electron emission coefficient of 10−4 –10−3 , agree well with the experimental data. A simplified kinetic model for corona discharges in air, which does not include conservation equations for negative ion species, has been proposed and validated. Modelling of positive coronas in rod-to-plane electrode configuration has been performed and the computed inception voltage was compared with experimental data.