Numerical investigation of transport phenomena of arc plasma in argon-oxygen gas mixture

Abstract

Mixtures of gases are utilized widely in a variety of arc welding processes to improve the arc property and thus the welding quality. This research focuses on the properties of the arc plasma in argon-oxygen gas mixture and a two dimensional (2D) axial symmetrical model is developed to investigate the transport phenomena in such arc, with diffusion treated by using combined diffusion coefficient approach. The gas mixture is applied to conventional tungsten inert gas (TIG) arc torch and a double-gas-shielding arc torch, respectively. Two different mass fractions of oxygen, i.e. 5% and 20%, in input gas mixture, are considered for the TIG arc, whereas that of 20% is taken into account for the outer shielding gas in the double-gas-shielding arc. The influence of the arc current on the demixing in the two types of arc is highlighted. The effect of the oxygen addition to argon on the TIG arc temperature is exhibited. It is found that for the TIG arcs, the oxygen concentrates in the regions close to the arc axis and to the electrodes, and increased current reduces the oxygen concentration near the cathode tip, but facilitates the mass transfer of the oxygen near the arc axis towards the anode; while for the double-gas-shielding arcs, the oxygen concentrates markedly in the periphery region of the arc and diffuses slightly towards the arc axis, with much lower mass fraction of the oxygen near the anode, for all the cases considered. Adding oxygen to the argon constricts the arc columns near the anode as compared with those in the pure argon for TIG arc. The oxygen mass fraction distribution 0.1 mm above the anode is uneven for the both types of the arc, implying nonuniform distribution of the oxygen reacting with the surface of the molten anode.