Coupled CFD analysis of thermochemical erosion and unsteady heat conduction in solid rocket nozzles

The interaction between the high-temperature, high-velocity combustion gas flow and the ablative protection material in SRM nozzles is studied by an approach which relies on a validated full Navier-Stokes flow solver coupled with a material thermal response code. The flow solver is integrated with a surface ablation thermochemistry model which takes into account heterogeneous chemical reactions at the nozzle surface, rate of diffusion of the species through the boundary-layer, ablation species injection in the boundary layer, transient heat conduction inside the nozzle material, and variable thermophysical properties for both the gas and solid phase. Application to a graphite nozzle test case permits to evaluate the transient nature of the coupled heating/erosion problem for different aluminized propellants. Results show that the steady-state condition is approached in the throat region while for regions far from the throat, characterized by much lower heating rates, results can be far off the steady-state. The erosion rate build-up is characterized by a time delay during which the erosion rate is only a fraction of its steady-state value. The propellant composition is shown to have a significant influence on the erosion build-up and delay. © 2012 by D. Bianchi, A. Turchi, F. Nasuti, and M. Onofri.