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The atmospheric reentry of space objects is the place of many physical phenomena which immerse them to extreme conditions. The strong detached shock created ahead space objects transmits very large heat fluxes to the Thermal Protection System (TPS). The huge energy to be dissipated (of the order of several mega-Joule) requires the use of such thermal protection system made of ablative materials (generally carbon-based composites) to ensure the survival of the objects structure. The multi-physical character and the extreme conditions intrinsic to ablation make it difficult to experiment those phenomena in the laboratory. Even today, thermal protection systems\' design is based on important safety factors. Numerical simulation appears to be an essential tool for reducing these safety margin, and, among others, the weight in the thermal protection systems\' design, commonly known as a heat shield. This paper is part of this process of improving computational tools related to ablative degradation. The present work is devoted to the modeling and simulation of the degradation, due to pyrolysis and surface chemical reactions, of ablative carbon composite materials used as heat shield for hypersonic reentry vehicles. Detailed models of oxidation, nitridation and sublimation of Duffa and Zhluktov are implemented in the finite volume material response solveur MoDeTheC, developed at ONERA. In order to simulate thermal degradation and surface regression of the TPS, the strategy is based onto a coupling between MoDeTheC and the multi-physics platform CEDRE using the Computational Fluid Dynamic (CFD) solver CHARME, also developed at ONERA. The chosen chemical models introduce 12 reactions as well as the notion of reactive sites on the surface of the thermal protection system. A site is a free surface carbon bond resulting from the composition of the material (carbon fibers for example) or from its degradation (by pyrolysis for example). These models are, firstly, validated on a simple test case at atmospheric pressure which consist in applying the composition and temperature of the terrestrial atmosphere at the thermodynamic equilibrium to an elementary surface of a phenolic carbon thermal protection system modeled in the MoDeTheC solver. Secondly, the material response, coupled with the hypersonic flow in the CEDRE computational fluid dynamic platform, is confronted to reference data available in the literature for the experimental flight RAM-C I carried out by National Aeronautics and Space Administration (NASA). Simulation of the trajectory point at the altitude of 40 km and Mach 23.474 has been performed using the SATOR supercomputer of ONERA. The Earths atmosphere is modeled by a Parks model of a mixture of viscous and reactive ideal gases (11 species and 23 chemicals reactions) which can be laminar or turbulent (k?? model). The coupling strategy consists in the exchange of the following quantities at the coupling surface: pressure, chemical composition, heat fluxes, pyrolysis gases fluxes, ablation mass and energy fluxes and temperature of the wall and of the gases. The results of the simulation are studied and have made it possible to determine the regression rate of the wall as well as to characterize the interaction between the flow and the thermal protection system. |
