CFD Ablation Predictions with Coupled GSI Modeling for Charring and non-Charring Materials

To this day, a major objective of TPS design is to reduce empiricism, and to increase fundamental modeling capability through increased understanding. One of the most challenging aspect is the proper coupling between the material response and the external flow field. With this regard, the goal of this research activity is the improvement of the numerical modeling capabilities through the development of advanced CFD tools integrated with Gas-Surface Interaction (GSI) modeling.

Numerical prediction of ablation is ambitious and cpu-time demanding due to the complex multiphase physical and chemical processes that occur. With improvements in computational algorithms and advances in computer hardware, Navier- Stokes based approaches have become the norm in recent years for coupling to material thermal response predictions. The present state of the art in fluid-material coupling is represented by loose coupling of a high-fidelity CFD flow solver with a material thermal response code. In that respect, some major restrictions are still present in these state of the art coupled solutions:

• surface chemical equilibrium assumption
• non-ablating flow field prediction
• simplified diffusion modeling based on transfer coefficient

Chemical equilibrium is a special condition of the general chemical nonequilibrium condition and surface recession rate predicted by the chemical equilibrium surface chemistry is usually reasonably conservative and is considered to be a best alternative when the nonequilibrium computation is too expensive or unlikely to be achieved. The ablation models are currently largely based on the surface equilibrium assumption and the effects and importance of non-equilibrium ablation models coupled with CFD tools are only beginning to be explored. Moreover, the coupling between CFD solver and material response code is often made considering non-ablating flow field solutions assuming a fully/super-catalytic, radiative equilibrium wall. This means that the effect on the flow field solution of the ablation and pyrolysis gas injection and of variable surface temperature are treated only approximately relying on the use of mass and energy transfer coefficients and semi-empirical blowing correction equations. Finally, the ablation rate is generally computed by the material response code using thermochemical tables and extremely simplified diffusion models based on transfer coefficients and semi-empirical relations relating mass and energy transfer.

The objective of this research activity is to remove these major limiting assumptions developing suitable finite-rate GSI models and integrating CFD technology with Computational Surface Thermochemistry (CST) to take into account the effect of surface ablation and pyrolysis gas injection on the flow field and to allow surface ablation and surface temperature distributions to be determined as part of the CFD solution. Because the entire flow field is to be solved with ablative boundary conditions, the film-transfer theory assumption is no longer needed; this will permit to avoid all of the classical approximations such as transfer coefficients, equilibrium thermochemical tables, and blowing correction equations which needs to be used when ablative boundary conditions are not accounted for in the CFD solution. The ablative boundary conditions, based on finite-rate chemistry, species mass conservation and surface energy balance, is discretized and integrated with the CFD code to predict aerothermal heating, surface temperature, gas-phase surface composition, and surface ablation rate. The concentrations of chemical species at wall are determined from finite-rate gas-surface chemical reactions balanced by mass transfer rate. The surface temperature is determined from the surface energy balance assuming steady-state ablation or coupling with a thermal response code. The surface recession rate and the surface temperature are thus obtained as part of the flow field solution. The computational tool developed in this work is used to simulate two sets of experimental data for nozzle material ablation: sub-scale motor tests carried out for the Space Shuttle Reusable Solid Rocket Motor and the static firing tests of the second and third stage solid rocket motors of the European VEGA launcher which use carbon-carbon for the throat insert and carbon-phenolic for the region downstream of the throat.