Three dimensional modelling of inductively coupled plasma torches: comparison with experiments and applications

A three-dimensional model for the simulation of inductively coupled plasma torches (ICPTs) working at atmospheric pressure has been developed at the University of Bologna, using customized CFD commercial code FLUENT. The helicoidal coil is taken into account in its actual 3-D shape, showing its effects on the plasma discharge for various geometric, electric and operating conditions without axisymmetric hypotheses of simplification. The electromagnetic equations are solved in their vector potential form, while the steady flow and energy equations are solved for optically thin plasmas under the assumptions of LTE and laminar flow. Simulations have been performed for Ar and Ar/H2 plasmas, treating, in the latter case, diffusion as suggested in and making use of transport and thermodynamic coefficient published. In order to evaluate the importance of various 3-D effects on calculated plasma temperature and flow fields, comparisons of our new results with the ones obtainable from 2-D models and from an improved 2-D model that includes 3-D coil effects are presented. The effects of changing inlet gas flow rates, direction of the swirl velocity component, axial length and number of turns of the coil and the net amount of power dissipated in the discharge are evidenced, in order to give useful hints for avoiding the formation of a hot temperature spot in the confinement tube wall due to the axial displacement of the plasma fireball. Threedimensional results concerning different coil shapes including planar coil configuration are also presented. Metallic and ceramic particle axial injection in the discharge through a carrier gas by means of a probe is simulated as well, taking into account the energy and momentum transfer between the continuous and the discrete phase. In order to test the numerical codes, 3-D simulation results for an existing torch designed for applications in atmospheric plasma spraying of materials, are compared with experimental measurements carriied out by means of an enthalpy probe technique. Moreover, results coming from 3-D modelling concerning a torch configuration designed and realized for plasma assisted chemical synthesis and deposition of pure silica, are presented. In addition, temperature results from 3-D numerical simulation of a 0.3 kW, 40 MHz argon radio frequency inductively coupled plasma operated at atmospheric pressure are used to reconstruct side-on emission intensity profiles for some characteristic Ar-I wavelengths, and than compared with the ones obtained from direct emission spectroscopy measurements.