During hypersonic flight, thermally protective materials are subjected to complex, and often coupled thermal, chemical and mechanical loads in extreme conditions. The predictive modeling of gas-surface interactions and material response of ablative materials is essential to enabling a more robust and tailorable class of thermal protection system (TPS) materials for hypersonic vehicle design. Enhanced chemical reactions at the surface of a vehicle lead directly to strong non-continuum behavior in the hot gas near the surface of the vehicle. This phenomenon, referred to as continuum breakdown, requires the development of high-fidelity computational models that can provide accurate predictions of surface heating and fluid/structure interactions during hypersonic flight. Our group has established a set of parameters based on Generalized Chapman-Enskog Theory for rapid identification of continuum breakdown in reacting flows, as part of a larger effort to construct hybrid continuum/rarefied computational tools. In the first part of this talk, I will demonstrate how the Generalized Chapman-Enskog Theory establishes an important connection among gas-phase chemical reactions, surface chemical reactions, and diffusion-driven continuum breakdown relevant for modern hypersonic flight vehicles and ablative thermal protection systems. While surface chemical reactions are found to be paramount in near-surface continuum breakdown, they also play a crucial role in surface heating and material response. In the second part of this talk, I will address how recent advances in computational modeling and new insights from material characterization and experiments have provided an unprecedented level of detail behind surface chemical reactions, namely oxidation processes, for carbon-based and UHTC materials. With these recent advances, computational models can now quantify the oxidation rates for a known TPS material microstructure coupled with the flow. An integrated computational approach is presented using the carbon preform microstructure obtained from X-ray micro-tomography within direct simulation Monte Carlo (DSMC) to construct and validate detailed surface chemistry models for vitreous carbon and ZrC matrix materials. The influence of oxidation-induced degradation on the thermal, chemical and mechanical properties of carbon-based TPS materials is also discussed.

Kelly Stephani is an Associate Professor in the Department of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign, and Affiliate Professor of the Department of Aerospace Engineering and the Materials Research Laboratory. She received her Ph.D. in Aerospace Engineering from The University of Texas at Austin in 2012, and was a Postdoctoral Research Fellow at the University of Michigan in the Department of Aerospace Engineering before joining the faculty of UIUC in 2014. Prof. Stephani is an Associate Fellow of the American Institute of Aeronautics and Astronautics, Associate Director of the Center for Hypersonics and Entry Systems Studies, Co-Director of the University Consortium for Applied Hypersonics, and serves on the National Academies Board on Army Research and Development. She is recipient of the NASA Early Career Faculty (ECF) Award and AFRL Summer Faculty Fellowship in 2015, AFOSR Young Investigator Program (YIP) Award in 2017, and the Presidential Early Career Award for Scientists and Engineers (PECASE) in 2019.