The permeability of ablative materials under rarefied gas conditions

Numerical meshes of both cork and carbon fibre ablative materials in their virgin and pyrolised states, with realistic porosity and tortuosity, have been created from micro-computed tomography (μCT) scans. The porosity of each material has been calculated from the μCT scans and used to extract smaller representative sample volumes to perform numerical simulations on. Direct simulation Monte Carlo simulations of rarefied gas flow through these materials have been performed to find the permeability of each material to argon gas and to a gas mixture. The method has been validated by comparing the measured permeability for a Berea sandstone material to previously published experimental values. For the specific pressure conditions investigated here, the cork-phenolic material becomes around ten mores permeable after being pyrolised, while the carbon-phenolic material only becomes five times more permeable than its virgin form. The permeability to the gas mixture is found to be greater than to argon for most of the samples, showing the importance of choosing the correct gas for rarefied permeability studies. The form of the pressure and Mach number profiles through the materials is indepedent of the applied pressure gradient.