Effective Permeability of Carbon Composites Under Reentry Conditions

Effective permeability of a porous carbon composite used as thermal protection system on space capsules is computed using the direct simulation Monte Carlo technique. The microstructure of the carbon composite is synthetically generated using an in-house code. Permeabilities obtained using synthetic microstructures of the precursor (carbon fibers) are compared with two independent experimental dataset, and good agreement is observed. It is conclusively shown that the difference in permeability values between the two experimental datasets arose from the variations in the sample density. An approach to digitally infuse resin into the precursor is proposed. Comparison of the permeability for the full composite (carbon fibers with resin) with experimental data resulted in errors within 7.5%, indicating that representative microstructures generated digitally can be used to compute and predict effective permeability of porous carbon composites. Simulations of Ar and air permeating through the full composite are performed, and an intrinsic material permeability (K_o ) of 1.866 × 10⁻¹¹ m² and a Klinkenberg constant (b ) of 5.655 × 10⁻⁸ that is only dependent on the temperature and the molecular weight of the gaseous species are obtained. It is shown that this new generalized relation could be used for other gases such as CO and CO₂ .