Arc dynamics in a vortex-stabilized non-transferred plasma torch with a tangential gas feed

Abstract

The mode of anode arc attachments in a thermal plasma torch, such as constricted arc, multi-arc or diffused arc attachment, has operational as well as economic consequences on the use of plasma torches in industrial applications. The mode of arc attachment is strongly dependent on the gas flow dynamics, gas vortex formation and wall energy exchange. To gain more insight into arc attachment behaviour, this article presents a transient flow plasma model implemented using OpenFOAM computational fluid dynamics code to study the effect of gas swirl and arc rotation on modes of arc attachments. A tangentially fed plasma gas creates a gas swirl that helps to induce the arc rotation. The investigations were carried out at arc currents between 200–450 A and input gas flow rates in the range of 5–20 slpm. High gas swirl and low arc currents are found to favour constricted attachment on the anode surface. The local gas vortex driven by the plasma swirl creates hot spots inside the torch that dictate the arc attachment. The transition of arc attachment from the diffuse to constricted mode was simulated by imposing a step-change in the gas flow which shows the evolution of different arc attachment behaviour. During the transition from constricted to diffused attachment, the arc tends to undergo a transitionary multi-attachment mode at specific selected current and flow parameters.