A nonequilibrium plasmadynamics model for nitrogen/hydrogen arcjets.

摘要

Electrothermal arcjets offer significant cost advantage over conventional satellite propulsion systems. In these devices, the propellant is electrically heated, allowing for higher temperatures and specific impulse than chemical rockets. Despite the relative simplicity of the basic design, many complex physical processes are poorly understood. Less than 50% of the electrical power is converted to thrust kinetic power.;A numerical model is developed to study arcjet plasma flowfields. The model employs a modified set of Navier-Stokes equations, which includes separate energy equations for the electrons and heavy species. A thermal nonequilibrium, chemical equilibrium model is first developed, and then generalized to chemical nonequilibrium. A seven-species plasma of molecules, atoms, and ions is assumed for a variable mixture ratio of nitrogen and hydrogen. This permits simulation of various propellants, including hydrogen, ammonia, and hydrazine. Equations for charge continuity and Ohm's Law are employed to predict the arc current distribution. A thermal model for the nozzle is also included. The combined features of thermal nonequilibrium, chemical nonequilibrium, and propellant flexibility distinguish this model from previous research efforts.;Model calculations are presented for 1-2 kW-class arcjets operating with hydrogen and hydrazine propellants. Thermal equilibrium is predicted in the highly ionized arc core, while electron temperatures near the electrodes are an order of magnitude greater than heavy species temperatures. The thermal nonequilibrium enhances ionization and electrical conductivity, and thus governs the current attachment to the nozzle. A comparison of chemical equilibrium and nonequilibrium simulations indicates that ambipolar diffusion of electrons and ions also controls the current conduction.;The model is compared with experimental measurements. Respective calculations for specific impulse and exhaust velocities are within approximately 5% and 5-10% of experimental measurements, while heavy species exhaust temperatures are within 10% of ion and hydrogen atom translational temperatures. Electron temperatures are compared with electrostatic probe measurements that confirm the high degree of thermal nonequilibrium near the nozzle surface. Chemical nonequilibrium predictions for electron concentrations agree more favorably with experimental measurements than chemical equilibrium calculations. Thus, finite-rate chemistry models are required to capture accurately the plasma species distributions.