Comparison of Turbulence and Chemistry Closures in a Solid-Fuel Ramjet Combustor

This paper considers a direct comparison between Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulations (LES) of a model solid-fuel ramjet combustor. Numerical simulations were performed using a two-dimensional axisymmetric formulation with a coupled solid-fuel pyrolysis model. RANS simulations were conducted using a commercial software package to assess the influence of the chemical kinetics mechanism, chemistry closure, and pyrolysis boundary condition fidelity on the predicted flame structure, regression rate, and combustor efficiency. The LES were performed using an in-house code with a pressure-comprehensive skeletal mechanism to serve as a baseline for assessing the predicted flame structure. Code verification was performed to ensure grid independence and to assess the suitability of the two-dimensional axisymmetric model. It is found that the RANS and LES show overall satisfactory agreement in the prediction of temperature, characteristic velocity, and average regression rate, but all RANS models are unable to accurately model the expected LES flame brush. Increasing the chemical kinetics model fidelity greatly improves the prediction of regression rate and chemical species prediction, whereas the flame structure shows insensitivity to increasing pyrolysis boundary condition model fidelity. Viable strategies for reducing the computational complexity of the RANS model are presented with minimal effect on solution accuracy.