UC Davis

During hypersonic flight, a vehicle heat shield encounters harsh aerothermal environments and undergoes complex surface chemical and physical processes that impact its mass loss, shape change, and surface heating responses. Thermal protection system design requires credible ablation models that describe the gas-surface interactions under these extreme conditions. An uncertainty quantification methodology was implemented to characterize legacy and recently developed air/carbon surface ablation models for nonequilibrium high-enthalpy flow conditions.

Validation assessments compared model predictions of carbon monoxide, a primary ablation product, quantities in the boundary layer with measured laser absorption spectrometry data. Experiments were conducted at the hypersonic reflected shock tunnel at Sandia National Laboratories. Simulations were performed to model the graphite cylindrical test articles that were preheated to surface temperatures ranging from 1250-2150 K for 4 km/s freestream conditions. Sensitivity analyses assessed the influence of epistemic reaction rate uncertainties on predicted gas species quantities at the surface and in the boundary layer. Dominant rate uncertainties were propagated through the model to quantify the comparison error between predicted and measured carbon monoxide quantities. Results show near agreement between finite-rate and equilibrium model predictions, which overpredict carbon monoxide number density for analyzed experimental conditions.

An approach to identify reaction- and diffusion-limited ablation regimes through a Damköhler analysis is employed to compare ablation model predictions between 3 and 4 km/s freestream shock tunnel test conditions. The predicted surface ablation regimes for each test condition agree with observed ablation rate trends between finite-rate and equilibrium models. Companion sensitivity analyses further characterized model behavior differences by identifying dominant reaction rate and chemical pathways in predicting gas species quantities for each test case. The demonstrated analysis framework enables researchers to interrogate complex models to understand potential chemical pathways as part of the ablation process. It also provides a flexible validation methodology as more experimental data becomes available and models continue to be developed and matured.