Internal Gas Flow Effects on Porous Thermal Protection System Thermal Conductivity: Stardust Case

Porous carbon fiber preforms, such as Calcarb and FiberForm, are used as reinforcements in thermal protection system (TPS) materials. In the state-of-the-art material response models, the thermal conductivity of ablative materials is typically tabulated as a function of temperature and pressure. The theory of homogenization in porous media predicts that the apparent conductivity also depends on the internal gas flow velocity—in this case caused by the flow of pyrolysis or boundary-layer gases within the material. This study aims to quantify this effect, known as thermal dispersion. Pore-scale numerical simulations are used to evaluate dispersion as a function of the Péclet number (the convection-to-diffusion ratio) for ideal structures and a 3D tomography of Calcarb, the carbon fiber preform of ASTERM. This is achieved by deriving and implementing a closure problem in the Porous Material Analysis Toolbox based on OpenFOAM (PATO). The results obtained on Calcarb are introduced to quantify the impact of dispersion during Stardust’s atmospheric reentry at the stagnation point, demonstrating a negligible effect. This work shall be extended to other materials and conditions to inform design margin policies.