Modeling of Chemical Nonequilibrium Effects in a Charring Ablator

Charring ablators remain the premium choice for space exploration missions that involve atmospheric re-entry. These type of ablative material are composed of a carbon matrix, usually made of fibers, which is then impregnated with a resin. During re-entry, the high heat flux produced by convective heating causes the material to chemically react. First, the resin pyrolyzes, and is vaporized into a gas that travels through the material, and is eventually ejected at the surface. Since the composition of the gas at the surface greatly affects the heat flux, and therefore the surface temperature, it is thus important to be able to accurately predict its composition. When the temperature becomes high enough, the surface of the ablator also vaporizes. That phenomenon is of much concern when sizing Thermal Protection Systems (TPS): for instance, if the recession is severe, the shape of the vehicle could be altered, as would be the aerodynamics properties. The research presented here demonstrates two physical models that have been integrated into a material response code that aims at predicting surface recession more accurately. First, a non-equilibrium homogeneous chemistry approach for the pyrolysis gas is presented, and results obtained using a legacy finite-rate chemistry model is reported. Although it is clear that such an approach is necessary, the lack of an appropriate chemistry model prevents that feature from giving meaningful results. Then, a volume-averaged fiber-scale oxidation model is presented, based on the one previously developed by Lachaud et al. The present model, however, solves the momentum equation as well as the energy equation. Results based on a series of experimental test cases are presented