Investigation of plasma swirl dynamics and effects of secondary gas injection in a vortex gas stabilized DC arc plasma

A numerical analysis of plasma gas swirl is performed to gain an understanding of fluid motion and the contact pattern of the plasma with a secondary gas. The secondary gas is injected into the plasma torch to either quench the hot plasma or alternatively to facilitate the heating of this secondary gas. A three-dimensional, transient fluid model is implemented in the OpenFOAM solver to examine the arc and plasma dynamics. The gases (plasma gas as well as secondary quench gases) are injected via 2 sets of 4 tangential ports spaced apart at a specified distance. The tangential injection create a plasma gas vortex that facilitates the swirling of the arc inside the torch. The plasma swirl propagates in the downstream direction where it mixes with the quench gas. The simulation results reveal that the operating arc current has a significant impact on the decay of plasma gas swirl in the downstream direction. At high arc currents, the rise in the joule heating enhances the cathode jet which suppresses the velocity gradients of the incoming swirl. A high inlet gas flow rates (e.g. Q = 18 slpm) have been observed to retain the inlet swirl intensity after passing through the arc column compared to the case with low gas flow rate (e.g. Q = 4.5 slpm). We have also examined the impact of secondary gas swirl on the quenching behaviour plasma torch plume by feeding the secondary gas in a co-current and counter-current with respect to the plasma gas rotational direction. The present simulation results help in understanding the effect of key process parameters controlling the flow and mixing behaviours in vortex stabilized plasma torch.