A multi-fluid stagnation-flow plasma model with self-consistent treatment of the collisional sheath

A two-temperature, multi-fluid model of a plasma in stagnation flow against a cooled, electrically biased surface is presented in this paper. The model couples bulk fluid motion, species diffusion and convection, electron and bulk energy equations, and net finite-rate ionization with Poisson's equation for the electric field in a generalized formulation. Application of the model to argon flow reveals important interactions between thermal, hydrodynamic, chemical and electrical boundary layers, with implications to current-limiting regimes of arcjet operation. Our analysis also examines the response of a planar, Langmuir probe in contact with a collisional, flowing plasma. Determinations of current-voltage behavior compare well to simple theory, including dependence on incident plasma velocity. Departures from this theory arise from boundary-layer perturbations near the electrode surface, away from free-stream conditions. The computational model incorporates a finite-rate catalytic recombination of ions and electrons at the electrode surface together with a specified current. These boundary conditions determine electron and ion fluxes at the surface consistent with mass and charge conservation. While the value of the net recombination rate is unknown, the dependence of calculated sheath strength on this rate is discussed.