Simulations of high-enthalpy gases in an Electric Arc Heater – Study of Equilibrium vs. Non- Equilibrium assumptions and Uncertainty Quantifications of an Arc Jet

Abstract

Space exploration has experienced exceptional advances the last years since the NASA’s Apollo missions landed man on the moon. In spite of all the discoveries and advances in computational power, the heat flux on the surface of atmospheric entry vehicles remains as a high risk problem that has to be studied in a detailed manner. In order to avoid possible inaccuracies in the calculation of the heat flux, it is of vital importance to understand complex multiphysics phenomena that combine the chemical aspect of both the materials and the hypersonic flows. Since the man has stepped into space, the space vehicles that have been developed are commonly exposed to tremendous aerodynamic heating, requiring an accurate study of thermal protection systems (TPS). In order to test and understand the material response, ground test facilities for the study of TPS have been developed. The most useful facilities are plasma wind tunnels being able to provide high enthalpy flow for a long period. The NASA Ames Research Center (ARC) Jet Complex segmented arc heater capabilities, where plasma flows with high chemical purity are generated, will be reviewed. In these tunnels, gas (typically air) is heated directly by a continuous electrical arc between two sets of electrodes. Large amount of energy is transferred from electric to gas enthalpy by making the gas conductive. The high-enthalpy gas generated in the constrictor passes through a converging/diverging nozzle thus achieving high Mach number speeds. The gases exit the nozzle into large vacuum chamber test sections. Flows in these facilities produce a close approximation of the surface temperature, pressure and gas enthalpy found in high velocity, hypersonic flows experienced by vehicles during atmospheric entry. While the consideration of chemical equilibrium or not along the nozzle has been investigated recently, the state of the gas in the constrictor has not been studied and will be a subject of this project. The operations of the 20 to 60 MW Arc Jet tunnels are expensive and do not lend themselves to detailed scientific investigations. Recently, a mini Arc Jet test capability, mARC, is being developed at ARC. This laboratory capability will enable fundamental studies and can be instrumented for extracted fundamental quantities that will enable better understanding of Arc Jet performance. In this work we simulate the mARC facility and investigate the flow in the constrictor and through the nozzle. There are many experimental uncertainties that may affect the heat flux at the test article placed in the facility. In order to quantify the margins and reduce the uncertainties, Uncertainty Quantification (UQ) methods will be used to define these margins. Sources of uncertainties can be of different origins. Some of them are always present and are inherent to nature itself, so they cannot be eliminated and are irreducible. Others, are just matter of the lack of knowledge and are reducible. UQ techniques will permit to build a sensitivity analysis which will allow us to determine the influence of experimental variables on the expected output heat load.