Abstract
A mathematical model of a free-burning TIG electric arc with non-consumable electrodes using a flat anode and argon shielding gas is presented. Differential equations describing the conservation of mass, momentum and energy are solved together with Maxwell's equations describing the electromagnetic field. The dependence of transport coefficients on temperature is taken into account. The gas flow is assumed to be laminar and the partially ionized plasma is assumed to be in local thermodynamic equilibrium. The mathematical model can cope with a broad range of operating conditions. The model is used to demonstrate the strong influence that the velocity and temperature of the flow of gas entering the top of the electric arc in the region of the cathode can have on the arc column. In particular, it is shown that cathode flows of strength sufficient to produce a significant constriction of the electric arc need to be assumed in order to account for experimentally measured electric fields in the arc column as well as the total voltage drop for 10 mm arcs. The use of this model also shows the part played by the cathode spot and its location in the nature of the electric arc column. In particular, two complementary techniques for studying the arc column are highlighted. In the first, a strictly stable static arc is needed in order to employ the spectroscopic technique of temperature measurement. In contrast, when the position of the arc is less stable, a rapid measurement with an electric probe is indicated in order to measure the electric field. The arc model described by Ducharme et al (1993) will then yield the temperature field. The model presented here produces the results for the temperature distribution and for the electric field based on the use of appropriate boundary conditions.
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