Modeling Gasification of Carbon Fiber Preform in Oxygen Rich Environments

摘要

In order to better understand the role of surface reactions in carbon fibers based Thermal Protection System materials, the oxidation of a simple geometry was studied. A porous carbon fiber-based plug was subjected to a flow of oxygen at various temperatures and pressures. Although the microscopic geometry of these materials is extremely complicated, a homogenized model was used. In this model, the properties of the material only vary only along the direction of the flow. It can be shown that the governing equations for the unidirectional flow along the porous plug depend only on two parameters, which characterize the materials microscopic structure: the porosity and the interfacial area of solid carbon exposed to the gaseous phase. Furthermore, the flow evolution can be considered quasi-steady based on comparison between the much shorter residence time through the porous material and the total gasification time. The characteristic time of the surface reactions relative to the residence time through the plug is quantified with Damkohler numbers. The regime of small Damkohler numbers compared to unity, usually called volumetric ablation in the literature, is characterized by slow surface reactions distributed through the whole volume of the plug. At the opposite limit, the regime of Damkohler numbers large compared to unity, usually called surface ablation, is characterized by fast surface reaction that exhausted the oxygen right at the front face of the plug.