Understanding plasma fluid dynamics inside plasma torches through advanced modeling

The aim of this work is to investigate by means of a 2-D and 3-D FLUENT© based numerical model the behaviour of different types of transferred arc dual gas plasma torches used for cutting of metallic materials, putting into evidence the physical reasons for the industrial success of various design and process solutions appeared over the last years, such as: secondary gas swirl injections with different directions, various different approaches for the geometry of the plasma chamber, the effect of externally superimposed magnetic fields. Flow and heat transfer equations are solved with coupled electromagnetic ones, for a LTE optically thin plasma, while turbulence phenomena are taken into account by means of a k-ε RNG model. Simulations include a prediction of the thermal behaviour of the solid components of the torch head, including electrode and hafnium insert, thermal histories and trajectories of hafnium oxide particles emitted from it under idealized conditions and the efficiency of nozzle and electrode cooling systems in various operating conditions including gas mixtures (O2/air, H35/N2, N2/N2). Radiation is included in the calculation of heat transfer to the surfaces of the components using a customized Discrete Ordinate (DO) model. Results obtained from the simulations of one of the plasma torches here considered are also compared with experimental measurements in order to perform a validation of the model for what concerns pressure values in different regions of the device, and the gas mass flow rate during the working phase. For this purpose suitably modified plasma torches with measurement systems have been designed and used during realistic cutting conditions.