Physical Mechanisms in Combustion for Ablating Carbon-fiber Composite Thermal Protection*

Thermal protection systems based on light-weight carbon-fiber composites are attractive in aerospace propulsion applications, though their use depend on several physical mechanisms, which their extreme conditions make difficult to diagnose. To assess these mechanisms, a model system is considered in which candidate materials are placed above a flat-flame McKenna burner. A corresponding slug calorimeter provides a measure of the heat flux. Point-to-point comparisons between detailed simulations and the experiments highlight important components of the physics coupling. Heat conduction and species diffusion back to the burner are shown to be an unexpectedly relevant factor. The relative importance of additional physics is assessed by studying chemical kinetics mechanisms and transport models, from unitary Lewis number to multicomponent diffusion including Soret effects. Their effect on flow dynamics, flame structure, products temperature and heat flux into the calorimeter is discussed. Physics model integration for ablation of a phenolic permeated carbon fiber material is also presented.