Ablation Chemistry Model Evaluation via Hypersonic Shock Tunnel and Laser Absorption Spectroscopy

Experiments were conducted in Sandia National Laboratories’ hypersonic shock tunnel, measuring carbon monoxide (CO) temperatures and concentrations in an environment targeting finite-rate chemistry. Samples were exposed to flow conditions in air at a freestream velocity of 2.7 km/s, a pressure of 180 Pa, a translational–rotational temperature of 170 K, and a Mach number of 9.8. The total enthalpy was 3.8 MJ/kg. The samples were resistively preheated to wall temperatures ranging between approximately 1770 and 2200 K, with two different types of graphite investigated. Measurements were conducted downstream of a graphite wedge model. A tunable diode laser absorption spectroscopy diagnostic capable of resolving thermal nonequilibrium was developed to measure CO concentrations and temperatures. Additionally, molecular tagging velocimetry provided the velocity field over most of the test article. The experiments were simulated with computational fluid dynamics using equilibrium and air–carbon ablation (ACA) gas–surface kinetics mechanisms. For the observed experimental conditions, each model overpredicted CO concentrations at the measurement location. Concentrations predicted by the equilibrium model were the greatest, providing evidence that the surface was in a reaction-limited or transitional ablation regime. The ACA model predicted CO concentrations that were within a factor of 1.3 to 2.7 times the experiments as the surface temperature decreased from 2150 K through to 1770 K.