Validation of a volume-averaged fiber-scale model for the oxidation of a carbon-fiber preform

Abstract

The oxidation of Fiberform, an industrial carbon-fiber preform, has been studied in an oxidation reactor. The microscopic oxidation behavior of the fibers has been analyzed by scanning electron microscopy. The carbon fibers ablate showing progressive reduction of their diameter. The overall material recession occurs when the fibers are consumed. A reaction/diffusion-convection competition is shown to drive the oxidation process and control the depth of oxidation. A fiber-scale model is proposed for the prediction of carbon-fiber preform oxidation. A macroscopic model is derived by volume-averaging the micro-scopic model and a porous-medium formulation is used to model mass transport in the preform. The proposed model has been implemented in a Carbon Oxidation Analysis Code based on OpenFOAM (COACO). Using inverse analysis, it was possible to estimate the intrinsic fiber reactivity and then validate the model. The reactivity obtained is sur-prisingly high compared to literature data. This is explained by the fact that the carbon fibers contain some traces of calcium and potassium, which are known to be catalysts for oxidation. They progressively accumulate at the surface in the form of combustion residues. Nomenclature D Diffusion coefficient, m2 · s−1 dp Mean pore diameter f Rarefaction function J Molar oxidation flux, mol ·m−2 · s−1 kf Reactivity constant, m · s−1 n Surface normal, m Pe Péclet number