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 working at atmospheric pressure has been developed, using the customized computational fluid dynamic (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 argon plasmas under the assumptions of LTE and laminar flow. In order to evaluate the importance of various 3–D effects on calculated plasma temperature and flow fields, comparisons of our 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. Three–dimensional results concerning different coil shapes are presented. Calculations have also been carried out for some real torches specifically designed for particular applications, with the aim of validating the code. In addition, an improved version of the 3–D model has been used to simulate thermal history and trajectory of metallic and ceramic powders injected in the discharge through a carrier gas, taking into account the plasma–particle interaction. Moreover, comparisons of calculated side–on emission intensity profiles obtained from 3–D temperature results with experimental data coming from optical emission spectroscopy measurements have been carried out, for a 300 W, 40 MHz argon radio frequency inductively coupled plasma operated at atmospheric pressure, for some characteristic Ar–I wavelengths.