Development of a nitridation gas-surface boundary condition for high-fidelity hypersonic simulations

Gas-surface interaction phenomena have a strong impact on the heat flux experienced by atmospheric entry bodies in the hypersonic regime. Numerically, they can be expressed as a boundary condition to be imposed to the Navier-Stokes equations to achieve predictive engineering simulations. The mass and energy conservation can be abstracted in a thin layer containing both the solid and the gas phases. Such a balance was implemented in the open source MUTATION++ library. It is convenient to easily plug verified models in any type of CFD solver to model the response of material surfaces. We have extended the library to accommodate a state-of-the-art nitridation and nitrogen recombination mechanisms derived from beam experiments. MUTATION++ was coupled to US3D, a high-fidelity finite-volume flow solver, to simulate an experimental campaign conducted in the VKI Plasmatron facility. The experiment consists in applying a subsonic high-enthalpy nitrogen flow over an axi-symmetric ablative material sample. The simulation results on the stagnation line were compared to those obtained using a one-dimensional solver. Both results showed good agreement, verifying the implementation of the boundary condition. The computational model predicts a lower mass blowing rate than the experimental value. The catalytic behaviour of the mechanism, in agreement with the beam experiment predictions, induces higher heat flux values than those expected for the testing conditions of the Plasmatron facility.