Effects of anode nozzle geometry on thermal plasma characteristics generated by non-transferred torches for material processing

The thermal plasma characteristics produced by the non-transferred torches with various anode nozzle geometries, such as a tubular nozzle and a stepped nozzle, operating at an atmospheric pressure are calculated and measured by a numerical simulation and an optical emission spectroscopy, respectively. On the basis of the assumption of local thermodynamic equilibrium (LTE) and optically thin plasmas, the temperature distributions of argon thermal plasmas are determined by the Abel inversion and Boltzmann plot methods for the measured intensity of Ar I lines. On the other hand, their velocity distributions are deduced from the measured temperatures by means of power balance equations. For the numerical simulation, two-dimensional magnetohydrodynamic (MHD) equations are employed with a K-epsilon turbulence model. As a numerical scheme, the finite volume discretization and SIMPLE-like pressure correction algorithm are adopted in an unstructured triangular grid system for reflecting the complicated nozzle geometry. The thermal plasma properties produced with different nozzle types and sizes, such as tubular nozzles of various diameters and stepped nozzles of different step positions and diameters, are compared between the experimental and numerical results. Furthermore, from the obtained information on the effects of anode nozzle geometry on thermal plasma characteristics, the optimum design conditions of nozzle type and geometry are determined for the non-transferred plasma torches to be used as heat sources for material processing, like plasma spraying, synthesis, and decomposition.