Numerical and experimental investigation of the temperature distribution of a microwave air plasma jet: Model development and validation

This study presents the development of a robust and computationally efficient CFD model for microwave plasma torches, designed with scalability toward industrial applications. Particular focus was placed on validating the temperature distribution in the plasma jet, as it is an important factor in the electrification of high-temperature processes. The three-dimensional model considers conservation of mass, momentum, and energy, as well as turbulence, species transport with chemical reactions, turbulence–chemistry interaction, and radiation. Electromagnetic effects were neglected, and the plasma source was represented as a volumetric heat source with a defined energy distribution. The initial calculations and validation were conducted for a microwave air-plasma. The conducted parameter studies revealed that the turbulent Prandtl number has a significant influence on the temperature profile in the plasma jet. Reducing the turbulent Prandtl number to 0.1 improved the agreement with the experimental measurements by enhancing the turbulent thermal conductivity and homogenizing the temperature profile in the jet. The analysis of the velocity distribution reveals a complex vortex structure, with the tangential velocity component dominating before the volumetric heat source. Due to the thermal expansion of the gas, the axial velocity increases significantly and the vortex structure disappears. The velocity was found to be highly sensitive to the temperature gradients due to its density dependence. Additionally, endothermic dissociation reactions influenced temperature profile in the torch, while NO x formation, particularly thermal NO and subsequent NO 2 formation, emerged as relevant processes in air plasmas. Overall, the model provides valuable insights and can be applied for plasma-assisted high-temperature processes. • Temperature measurement in a microwave air plasma jet and uncertainty analysis. • Validation of open-jet temperature profiles under varied operating conditions. • Adjustment of model parameters (e.g. turbulent Prandtl number) affecting temperature. • Model-based study of velocity and species distribution in a microwave discharge.