Two-temperature modelling and optical emission spectroscopy of a constant current plasma arc welding process

In this work, a plasma arc welding (PAW) process operating in pure Ar with constant current at 40 A has been characterized by means of thermo-fluid-dynamic modelling under the assumption of local thermodynamic equilibrium (LTE) and of two-temperature thermal non-equilibrium modelling (2T), neglecting radiation reabsorption: thermal non-equilibrium is strongest in the fringes of the arc and upstream the plasma flow. The effect of radiation re-absorption has been then estimated using the discrete ordinate (DO) model for radiation transport, which requires the calculation of absorption coefficients as a function of wavelength and temperature, and it has been found that temperature profiles at the torch outlet obtained with this model are similar to those obtained neglecting reabsorption. Also, excitation temperature of Ar atoms has been measured using optical emission spectroscopy, exploiting a new method (called hybrid method) that combines features of the relative and absolute intensity methods. A good agreement has been verified between the electron temperature predicted by the 2T model and the measured excitation temperature in the whole investigated spatial region.