Nonequilibrium flow through porous thermal protection materials, Part II: Oxidation and pyrolysis

Micro scale simulations are performed of flow through porous (pyrolyzing) thermal protection system (TPS) materials using the direct simulation Monte Carlo (DSMC) method. DSMC results for permeability are validated with computational fluid dynamics (CFD) calculations and theory, for simple porous geometries under continuum flow conditions. An artificial fiber-microstructure generation code FiberGen is used to create triangulated surface geometry representative of FiberForm® (FiberForm) material. DSMC results for permeability of FiberForm are validated for a range of pressures (transitional flow conditions) and agree with experimental measurements. Numerical uncertainty is determined to be within 2% if sufficiently large portions of the microstructure are included in the computation. However, small variations in fiber size and angle bias can combine to give +30% uncertainty when comparing with experimental permeability data. X-ray microtomography scans of FiberForm are used to create microstructure geometry for incorporation within DSMC simulations of coupled oxygen diffusion and gas-surface chemistry in the presence of a blowing pyrolysis gas. In-depth penetration of atomic oxygen is limited to 0.2–0.4 mm for the range of Knudsen number and pyrolysis gas conditions studied.