Investigation of Non-equilbrium Effects in Plasma Jets

Plasma jets are found in many technological applications such as medicine, arc welding, plasma cutting, waste treatment, nanopowder fabrication and in inductively coupled plasma (ICP) facilities to test thermal protection materials for reentry vehicles. In many circumstances, plasma jets are modeled assuming Local Thermodynamic Equilibrium (LTE). The argument in support of this hypothesis is that operating conditions (e.g. pressure) are such that collisions between free-electrons and heavy-particles (e.g. atoms and molecules) are frequent enough to ensure that LTE prevails. However, these assumptions cannot be justified a priory and should be confirmed by simulations accounting for Non-LTE (NLTE) effects, an understanding of which is also crucial for design and correct interpretation of experiments.

The goal of this work is to investigate NLTE effects in plasma jets. Particular attention is devoted to capturing inherently unsteady features such as hydrodynamic instabilities and turbulence. To this purpose the Navier-Stokes equations are discretized using high-order Finite Volume schemes along with explicit or semi-implicit time-integration methods to minimize dissipation. Boundary conditions are treated using Local One-Dimensional Inviscid (LODI) relations augmented with multi-dimensional and viscous effects to suppress spurious reflections. NLTE kinetic processes (e.g. ionization/three-body recombination) are treated using either multi-temperature and/or collisional-radiative models. Simulations are performed at different conditions (e.g. pressure, Reynolds number) to assess the importance of non-equilibrium and compared against LTE results to quantify the errors on quantities of interest such as temperature and population distributions of excited electronic states.