Modeling the boundary layer interactions between weakly ionized plasmas and cooled, planar electrodes in stagnation-point flows.

摘要

This thesis presents a two-temperature multifluid model of weakly ionized, near-thermal plasmas in stagnation-flow against cooled, electrically biased surfaces. The model includes coupling between bulk-fluid motion; species drift, diffusion and convection; electron- and bulk-energy transport; and finite-rate chemistry including net ionization. In addition, the model includes Poisson's equation for determination of the electric field and for resolution of the space-charge sheath. Application of the model to high-temperature argon flow reveals important interactions between thermal, hydrodynamic, chemical, and electrical boundary layers, with implications to current-limiting regimes. The analyses include results using a quasineutral approximation, results from the full formulation, and a detailed comparison between the two approaches. The model employs a global, finite, electron-ion recombination rate at the stagnation surface together with a specified current density. These boundary conditions determine the electron and ion fluxes at the surface consistent with mass and charge conservation. The derivation of this surface-recombination formulation is discussed, as well as the sensitivity of model predictions to the assumed heterogeneous rate coefficient.;Results from the quasineutral approximation demonstrate the importance of considering net convection, electrode cooling, and thermal non-equilibrium in the evaluation of plasma properties in the pre-sheath region. The effects of net fluid flow, neutral thermal gradients, and finite-rate chemistry are determined through parametric variation of specified conditions and model assumptions.;Inclusion of Poisson's equation and resolution of the ion-rich, collisional sheath allows investigation into the effects of specified conditions on the sheath structure. The analyses examine the response of a planar, electrostatic probe in contact with a collisional, flowing plasma. Determinations of current-voltage behavior compare well to simple theory, including dependence on freestream conditions. Departures from this theory arise from boundary-layer perturbations near the electrode surface. The model also correlates current-saturation and breakdown phenomena with changes in the sheath structure and ionization due to Joule heating for large cathode current densities. More quantitative comparisons of current-voltage conditions to experimental data for both cathode and anode reveal good agreement between the model predictions and the experimental measurements.