Air-Carbon Ablation Model for Hypersonic Flight from Molecular Beam Data

Abstract

Recent molecular beam experiments of high velocity O, N, and O<sub>2</sub> impacting carbon material at high temperature produced detailed surface chemistry data relevant for carbon ablation processes. New data on O and N reactions with carbon has been published using a continuous molecular beam with lower velocity (2000 m/s) and approximately 500 times higher beam flux than previous pulsed-beam experiments. This data is interpreted to construct a new air-carbon ablation model for use in modeling carbon heat shield ablation. The new model comprises 20 reaction mechanisms describing reactions between impinging O, N, and O<sub>2</sub> species with carbon and producing scattered products including desorbed O and N, O<sub>2</sub> and N<sub>2</sub> formed by surface-catalyzed recombination, as well as CO, CO<sub>2</sub>, and CN. The new model includes surface-coverage dependent reactions and exhibits non-Arrhenius reaction probability in agreement with experimental observations. All reaction mechanisms and rate coefficients are described in detail and each is supported by experimental evidence or theory. The model predicts pressure effects and is tested for a wide range of temperatures and pressures relevant to hypersonic flight. Model results are shown to agree well with available data and are shown to have significant differences compared to other models from the literature. <br>

Development of a Nitridation Gas-Surface Boundary Condition for High-Fidelity Hypersonic Simulations

“…Though the FRC model does not accurately calculate the mass blowing rate measured in the Plasmatron experiments, the model takes into account the surface recombination reaction, which was not identified in [7]. Inclusion of both nitridation and recombination reactions allows for accurate model calibration through direct flow simulations such as those performed in the current work, rather than relying on 0D simulations as in [5].…”

“…where R is the set of surface reactions, k r , the rate coefficient for reaction r, [X j ] = ρ j /(N A m j ), the concentration of species j, ν j , its stoichiometric coefficient, and N A , Avogadro's number. The FRC model present in MUTATION++ has been further extended to use the chemical mechanism developed at the University of Minnesota for nitridation and nitrogen recombination [5]: the reactions considered are listed in table 1. The model was built from molecular beam experiment data obtained in low pressure conditions [4] and from Plasmatron data at a pressure of 1500 Pa [7].…”

“…The GSI module of MUTATION++ has been used to model catalytic and ablative materials, both assuming thermal equilibrium and non-equilibrium (state-to-state catalysis or two temperature ablation) of the gas near the surface [1,3]. In this work, we extend the library to use a finite-rate chemistry model derived from beam experiments [4], for describing, among other reactions, surface nitridation and nitrogen recombination [5].…”