Theoretical and experimental study of electron and heavy particle temperatures in a transferred arc

Abstract

Summary form only given. It is well known, that deviations from thermodynamic and chemical equilibrium have to be expected in the plasma of free burning arcs in particular near the electrodes and in the outer arc fringes. However, in most simulations of arc applications the validity of the Local Thermodynamic Equilibrium is assumed because of the reduced model complexity. The latter includes not only the reduced number of equations to be solved but also required boundary conditions and material data. Aim of the present paper is to discuss features of a nonequilibrium arc description. A model of a free burning arc considering thermodynamic and chemical non-equilibrium in the whole simulation area will be presented including experimental validation by optical emission spectroscopy. As an example of reduced complexity the tungsten inert gas (TIG) arc using argon as the shielding gas and a cooled copper workpiece is considered. This arc is applied as a heat source in many applications like TIG welding because of a good stabilization by the shielding gas, the high heating efficiency, and relatively low equipment costs. The arc model is based on a two-fluid description of electrons and heavy particles. A simplified plasma chemistry model of argon is considered. The simulation includes the arc plasma, the energy balance in the electrodes and the relevant processes of the arc-electrode interaction at the tungsten cathode and the copper anode. Calculations have been done in pure argon at 1 bar and for the arc current 200 A. The results are compared with simulations assuming local thermodynamic equilibrium. In a corresponding experimental setup the arc radiation is observed over the cross section at different distances from the electrodes with a 0.5 m spectrometer and an ICCD camera. The absolutely calibrated radiance of atomic and ionic argon lines is recorded. Assuming rotational symmetry of the arc and optical thin radiation, emission coefficients of these lines are deduced from Abel inversion. The radial profiles of temperatures and line emission coefficients predicted by the non-equilibrium model are in a good agreement with the experimental observations. .