Numerical analysis on effects of various cathode and nozzle geometries on plasma characteristics of transferred arc torches for waste treatment

A numerical model is presented to investigate the influences of cathode and nozzle geometries on the characteristics of transferred arc torches for waste melting process. The arc plasma is described in the atmospheric condition by a two dimensional magnetohydrodynamic (MHD) model with the assumptions of steady state, axisymmetry, local thermodynamic equilibrium (LTE) and optically thin plasma. In order to simulate the realistic torch configurations with complex nozzle and cathode arrangements, the unstructured triangular and/or quadru-lateral grid systems are used with a finite volume discretization, gradient reconstruction procedure and a SIMPLE-like pressure-correction algorithm. For the self-consistent prediction of temperature profiles on the anode workpiece and heat transfer rates to it, the energy conservation and current continuity equations are solved not only in the thermal plasma region but also in the anode region with the special treatment of electric conductivity and energy balance at the interface of plasma and anode material. By the suggested numerical model, the temperature and flow field distributions of argon plasmas and the heat flux rates to anode material are calculated for the various nozzle length and diameter and for a wide range of vertex angle of the cone-shaped cathode. As a result of these calculations, optimum geometrical design parameters of transferred plasma torch are obtained for waste melting process. The predicted results are validated by comparing with some experimental data.