3-D Modeling of Thermal Plasmas (RF and Transferred Arc) for the Design of Sources and Industrial Processes

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. 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. In addition, the gas injection section of an industrial TEKNA PL-35 plasma torch is included in the model without geometry simplifications, refining the mesh at the injection points, in order to perform a more realistic simulation of the inlet region of the discharge, taking into account turbulence effects using the Reynolds Stress Model. 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. Three-dimensional 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 and the effect of particle turbulent dispersion. 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 the experimental measurements by means of an enthalpy probe technique. Moreover, results coming from 3-D modeling 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. The behavior transferred arc thermal plasma sources operating at atmospheric pressure for the treatment of a substrate material (for waste treatment purposes and for metallic substrate cutting or hardening) is also investigated by means of a 3-D time-dependent numerical model, using a customized version of the CFD commercial code FLUENT©. Unsteady flow and heat transfer equations are solved with coupled electromagnetic ones, for an Ar optically thin plasma under conditions of laminar flow and LTE. The transient effects of an imposed external magnetic field on the shape of the single torch arc are investigated. The importance of fully investigating plasma velocity and temperature fields in high power twin torch transferred arc systems designed for waste treatment purposes is outlined with reference to a plasma source designed and operated by Centro Sviluppo Materiali (CSM S.p.A.) in Castel Romano, Rome. All simulations are performed over a network cluster of double processor calculators in order to use the full capabilities of the 3-D modeling in a time-dependent framework.