Microwave plasma torch at atmospheric pressure : thermal transfer
Among the microwave plasma torches, the axial injection torch (TIA) has been used for several years to create chemically active species, in applications such as gas analysis, surface processing and gaseous waste treatments. Our study concerns the energy transferred from the plasma created by the torch at atmospheric pressure, which finds its interest in particular in the heating of helium in a dirigible balloon to achieve its rise in altitude. The TIA allows the coupling of microwave energy (2.45 GHz) with a gas injected axially at the nozzle's exit. The TIA produces a non equilibrium plasma with a high luminosity and a maximum density of charged particles at the nozzle's exit. Our study involves both experiment and modelling of this torch in order to understand the distribution of the electromagnetic field, the flow of gas / plasma system and the plasma-to-gas heat transfer.Measurements by optical emission spectroscopy in argon and helium allowed to determine the gas temperature (1500K vs 3000 K) based on experimental conditions (flow, power). They were estimated by fitting the N2 ro-vibrational spectra obtained from air, using the SPECAIR software. The measurement of electron density (of about a few 10^14 cm^-3) was performed in a helium plasma by Stark broadening of the Hβ. Experiments have a double objective : to obtain input data for the model and to validate its results. Modelling uses two modules of the COMSOL Multiphysics software: (i) a 3D electromagnetic module, which solves Maxwell's equations, (ii) a 2D hydrodynamic module, which solves the Navier-Stokes equations for the gas / plasma system taking into account the ions. A plasma module, which solves the fluid equations for electrons and ions is in development This modelling succeeded in predicting temperatures similar to those obtained experimentally and showed the influence of plasma upon flow and the gas temperature and the efficiency of heat transfer from the plasma to the gas.