A gas-surface interaction model for the numerical study of rocket nozzle flows over pyrolyzing ablative materials

Ablative materials provide a widespread, reliable, and relatively low–cost way to manage the extremely high heat fluxes that are normally encountered in a wide variety of aerospace applications. Typically, both non–pyrolyzing carbon–based and pyrolyzing carbon– and silica–based materials are used with this intent in rocket nozzles. Unfortunately, during the rocket firing these materials undergo a consumption that modifies the nozzle internal contour increasing the nozzle throat area and causing a drop down of the chamber pressure that, ultimately, results in an overall rocket performance reduction. For this reason, it is important to advance the fundamental understanding of the nozzle erosion processes and to develop useful scientific tools in this subject area. To this aim, a comprehensive model that would allow the study of the behavior of different ablative materials in rocket nozzle environment accounting for surface ablation, pyrolysis gas in- jection and resin decomposition has been developed, tested and validated. The model relies on surface mass and energy balances and deals with the gas–surface interaction erosive phenomena, accurately solving the gas side, using a CFD ap- proach. Two different ablation models have been implemented to simulate both the erosion of carbon– and silica–based materials. The steady–state ablation approximation is used in order to estimate the solid conductive heat flux, as well as the pyrolysis gas mass flow rate, in a closed way and without requiring the accurate resolution of the material heating by means of a thermal response code. Firstly, the talk will address a thorough description of the theoretical/numerical model. Then, several simulations, from sub–scale to full–scale nozzles, will be presented and the results will be compared with the experimental results.