Two-Temperature Chemical Non-equilibrium Modeling of Argon DC Arc Plasma Torch

A 2D two-temperature chemical non-equilibrium model has been developed to investigate the plasma characteristics inside a DC arc plasma torch of argon. In this study the chemical kinetic model is carefully examined in order to properly capture the spatial variations of plasma characteristics, especially near the electrodes and arc fringe, where large gradients of plasma parameters exist. Two different sets of chemical kinetic processes are adopted in the calculation. The first chemical kinetic model[model (a)]includes the ground-state argon atoms, excited argon 4s state, atomic ions, molecular ions, and electrons, while the second model[model (b)]only considers the ground-state argon atoms, excited argon 4s state, atomic ions, and electrons. The predicted exit temperature and arc voltage by model (a) are in better agreement with experimental measurements. A delayed anode attachment with a higher arc voltage occurs in model (a) due to the low electron density in the upstream region of arc-anode attachment. Compared with the wide arc-anode attachment zone of model (b), model (a) exhibits a more constricted anode arc root with higher current density, which leads to a higher heat flux and temperature on the anode. This phenomenon may be explained by the rapid loss of electrons outside the arc-anode attachment zone due to the dissociative recombination of argon molecular ions. Moreover, model (a) produces a longer arc column with slightly higher maximum temperature and velocity on the axis, which is caused by its larger input energy.