Atomistic Modeling of the Decomposition of Charring Ablators

Abstract

The composition of the pyrolysis gases that are injected into the boundary layer gas from an ablative thermal protection system (TPS) has a significant impact on the aerothermodynamics near the TPS surface. Recent analysis revealed that predicted surface response (surface temperature) of the TPS is very sensitive to the initial composition of the pyrolysis gases in the pyrolysis zone and leads to large uncertainties. This sensitivity study used a recently developed high-fidelity numerical model ‐ one that accounts for the transport and reaction of the pyrolysis gases through the char in the surface ablation process, using both equilibrium and detailed finite-rate chemistry. For most carbon (PICA) or silicon (AVCOAT) based ablators the decomposing phase is a phenolic resin for which the underlying mechanism of thermal decomposition is unknown; the by-products of the thermal decomposition have also not been experimentally determined. The current study represents an attempt to obtain an improved fundamental understanding of the thermal decomposition process of phenolic resins, using reactive force field (ReaxFF/LAMMPS) atomistic simulations and characterize the composition of the resulting pyrolysis gas at various temperatures. In this study, decomposition pathways along with the composition of the resulting pyrolysis gases at different temperatures are determined for phenol and one other sub-structure of the PICA monomer, as a precursor for future studies on the thermal decomposition of more complex substances like PICA and other phenolic-resin based ablators. Results from these simulations are outlined in terms of detailing the decomposition pathway and by comparing the resulting composition of the pyrolysis gases against available experimental data. Potential use of these simulation tools in constructing simplified chemical kinetic models, which has wide implications for TPS modeling, is also discussed.