Non-equilibrium modelling of free-burning argon arc in different anode sheath regimes

Abstract

In this paper, an anode sheath model coupled with two-dimension arc column model is performed to study the physical process in the near-anode region. In the anode sheath model, two different physical mechanisms of ion and electron dynamics, for positive and negative anode sheath voltages, are considered. The effect of the arc column on the anode is also considered. The arc column model accounts for the thermal non-equilibrium and ionization non-equilibrium. The diffusion equations and generalized Ohm’s law are used to analyze the effect of diffusion of plasma species near the anode region. The simulation results from this model are presented for an atmospheric free-burning argon arc with arc currents of 50 A and 150 A. The simulation shows that: (1) both positive and negative anode sheath voltages are obtained for I = 50 and 150 A. The distributions of electric potential in front of the anode surface predicted by the model are in good agreement with the experimental data. (2) In the negative anode sheath regime, the diffusion current density exceeds the total current density, which makes the electric field reverse in front of anode surface. In the positive anode sheath regime, the diffusion current becomes negative in the near-anode region, which makes the ohmic current increase rapidly in the near-anode region. (3) There is a significant interaction, which is positive feedback, between the diffusion current and anode sheath voltage in both positive and negative anode sheath modes. (4) The thermal conduction from the arc column plays a major role in the heat transfer to the anode for the negative anode sheath mode while it becomes much smaller for the positive anode sheath mode.