Coupling of the electromagnetic and heat transfer phenomena in a non-transferred arc plasma torch is generally based on a current density profile and a temperature imposed on the cathode surface. However, it is not possible to observe the current density profile experimentally. To eliminate this boundary condition and be able to predict the arc dynamics in the plasma torch, the electrodes were included in the computational domain, the arc current was imposed on the rear surface of the cathode, and the electromagnetism and energy conservation equations for the fluid and the electrodes were coupled. The solution of this system of equations was implemented in a CFD computer code to model various plasma torch operating conditions. The model predictions for various arc currents were consistent and indicated that such a model could be applied with confidence to plasma torches of different geometries, such as cascaded-anode plasma torches.

This paper describes the development of a numerical model and explains how it is used to investigate arc-cathode interactions in a plasma arc torch. The model is based on magnetohydrodynamic (MHD) theory and couples Navier-Stokes equations for a nonisothermal fluid with Maxwell’s equations for electromagnetic fields. The equations account for the internal geometry of the torch as well as arc current and gas type and flow rate. They are solved using CFD code and relevant boundary conditions and are shown to provide insight on arc dynamics and the effect of cathode shape on arc behavior.