CFD evaluation of seeded and unseeded air MHD accelerators

This paper presents a numerical study of the acceleration of seeded and unseeded air using Magnetohydrodynamics (MHD). Issues in the proper physical treatment of high temperature ionizing flow fields are addressed, with specific emphasis upon: (1) Thermal Nonequilibrium, (2) Finite Rate Chemistry, (3) Seed particle modeling and (4) Transport properties. Calculations involving seeded air use Potassium Carbonate as the seed substance. Multiphase effects in seeding of the flow are addressed in the context of recent experiments in this area. The settling of seed particles and finite rate heat transfer to seed particles are considered in CFD codes based on approximate Riemann solvers in one and two dimensions. Seeded air is modeled using equilibrium models with simple estimates of nonequilibrium ionization. Unseeded air calculations are performed also using an air model comprising of 11 chemical species and a two-temperature model of thermal nonequilibrium. Analytical estimates and correlations betweent the equilibrium and nonequilibrium models are used for validation purposes. Results presented here mostly pertain to steady flows, with some unsteady shock tube simulations. Navier-Stokes equations are decoupled from Maxwell equations with the assumption that the Magnetic Reynolds number is low. Thermodynamic and transport properties are evaluated from well documented curve fits. INTRODUCTION The flow of conducting air in the presence of electromagnetic fields can probably now be considered as a classical problem. Numerous analytical and computational studies of this problem are available in the litera*Member Technical Staff, AIAA member tprofessor, Dept. of Mechanical and Aerospace Engrg. XProfessor and Chairman, Dept. of Mechanical and Aerospace Eww Copyright @American Institute of Aeronautics and Astronautics, Inc., 2000. All rights reserved ture, each of which take recourse to certain assumptions which limit their range of applicability. Predominantly, these assumptions include one or more of: (1) Zero,‘constant or infinite electrical conductivity. (2) Constant distributions of Electric and Magnetic fields. (3) Perfect gas, constant gas properties. (4) Thermal equilibrium, or a limited treatment of electron temperature using the Saha equation. (5) In seeded flows, uniform seed distribution, and instantaneous seed vaporization and ionization. In the present work, we try to address each of these issues in such a depth as to provide insight into the currently available numerical models and to comprehensively model the physics of MHD flows with as few assumptions as possible. Primary motivation comes from recent experiments performed at the University of Texas at Arlington [13] in conjunction with the MARIAH program funded by NASA [2]. The conventional use of seeded gases in Magnetohydrodynamics (MHD) based acceleration devices poses various problem when MHD is to be used for hypersonic testing. It has been proposed by several authors (e.g. [23]) that MHD augmentation can greatly enhance the current testing capabilities of hypersonic facilities. Easily ionizable alkali seed substances are used to enhance the electrical conductivity of gases at low temperatures. The introduction of a seed material results in various problems, the more important of which are: (1) The nonuniformities in the injection process, which can result in nonsmooth flow conditions, (2) The expense involved in the seeding process, (3) The contamination of the test gas by the seed material, and, (4) Possibility of corrosion of test models by seed particles. These factors have their relative penalties depending on the way in which the MHD process is implemented. Solid seed particles have a tendency to settle down rapidly to the bottom of the channel, and take finite time to evaporate. Seed solution sprayed into the tun-