ABLATION OF CARBON/CARBON COMPOSITES : DIRECT NUMERICAL SIMULATION AND EFFECTIVE BEHAVIOR Yvan Aspa, Michel Quintard, Institut de M´ ecanique des Fluides de Toulouse (IMFT), 1, All´ ee du Professeur Camille Soula, F 31000 Toulouse, France Fr´ ed´ eric Plazanet, C´ edric Descamps, Snecma Propulsion Solide Les Cinq Chemins – Le Haillan F 33187 Le Haillan Cedex, France Gerard L. Vignoles, Lab. des Composites ThermoStructuraux (LCTS), Universit´ e Bordeaux 1 – 3, All´ ee La Bo¨ etie, F 33600 Pessac, France ABSTRACT In this paper, we are interested in the ablation of carbon/carbon composites, as found, for instance, in rocket nozzle applications, where aggressive gases cause the composite surface recession. Global recession velocity and the appearing roughness are function of the subscale description. We model ablation in terms of surface oxidation of a heterogeneous material by a binary gas mixture. We assume the gas transport to be purely diffusive and temperature almost uniform. A finite volume code has been developed based on a VOF method using complex represen- tation of interface and elementary surfaces, and sequential solving of the coupled equations for fluid transport and surface recession. The code was used to perform many numerical experiments, which allowed us to identify different recession regimes as a function of the characteristic dimensionless numbers. A one-dimensional model was built using the concept of effective surface reaction. This ap- proach replaces the non-uniform reactivity of a geometrically complex surface by an effective reactivity of a plane homogeneous interface that gives macro-scale fluxes similar to the aver- age micro-scale fluxes. INTRODUCTION In the last decade, with the development of new solid-propellant generations, gases in rocket nozzles have reached higher and higher temperature. Carbon/Carbon composites (C/C ) are among the few materials that are able to withstand such conditions. Indeed, they combine a good ratio high temperature mechanical properties to density and a high thermal conductivity. When exposed to flow, C/C are attacked by the solid-propellant gaseous products. This attack causes a surface recession and a morphological deformation of the throat. This phe- nomenon which regroups several causes (mechanical erosion, gasification,...) is called abla- 1 paper CB-S1-110-2005. Pages 1-8 in 29th International Conference on Advanced Ceramics and Composites, Cocoa Beach, Fl, USA.