Air-Carbon Boundary Layer Chemistry for Hypersonic Ablation

Although advancements in the fundamental understanding of hypersonic flow physics have been made in recent years, boundary layer chemistry for ablative thermal protection systems, involving air-CO/CO2 gas-phase reactions, remains highly uncertain. This uncertainty prevents rigorous validation of CFD simulations with modern ablation experiments, which measure species within the boundary layer. We propose a balance of molecular beam experiments (Timothy K. Minton, Montana State University), theoretical chemistry (George C. Schatz, Northwestern University), and hypersonic flow simulations (Thomas E. Schwartzentruber, University of Minnesota) to investigate boundary layer chemistry. Our objective is to formulate new/updated/validated air-CO/CO2 reaction rate models for use in CFD and carry out validation studies with existing and on-going high-enthalpy ablation experiments. This research will advance hypersonic CFD simulations. We propose that a new chemistry-focused approach is required to make advances beyond the current state-of-the-art. Although the chemistry of the air-CO/CO2 boundary layer is in many respects unique, there is significant experience among Minton and Schatz in studies of hyperthermal and high temperature reaction dynamics and in the use of direct simulation Monte Carlo (DSMC) calculations by Schwartzentruber to simulate reacting gases in strong nonequilibrium conditions. The detailed dynamical information at the atomistic level that will come from the experiment-theory collaboration between Minton and Schatz will anchor the whole effort. Crossed molecular beams experiments can provide product branching and differential cross sections for important boundary layer reactions, which will serve as benchmarks for theory. The theoretical calculations will aid the interpretation of and receive validation from the experimental data. Validated theoretical calculations will then provide detailed dynamical information, as well as trustworthy rate data for situations that go beyond the experimental conditions. This information will be directly incorporated into the models being developed by Schwartzentruber. The proposed project will provide input to the boundary layer model from experimental and theoretical results. Crossed molecular beams data on O + CO, O + CO2, O + N2, O + NO, N + O2, and N + CO will be used to validate theoretical calculations which will, in turn, be used to predict rate coefficients and product internal state distributions for conditions relevant to NASA EDL environments. The challenging reactions of carbon radicals (C, C2, C3) with O and O2 will be studied preliminarily by theory to evaluate the potential importance of these reactions in boundary layer chemistry and point the way to possible future improvements in the model. The research will be targeted through the guidance of hypersonic flow simulations by Schwartzentruber. The overall approach, based on the investigation of individual reactions, should lead to a dramatic reduction in uncertainty for air-CO/CO2 models and a consequent transformation in predictive ability for hypersonic flow applications of interest to NASA.