Large Eddy Simulations of Solid Fuel Ramjet Combustion

Acoustic perturbations within ramjet combustion chambers affect performance and flame-holding. An improved understanding of how the wall geometry contributes to the establishment of acoustic modes is sought. The focus of this work is on the effect of wall cavities carved in the fuel grain. The combustion mechanism was developed using a counterflow burner to study the combustion and regression of solid model fuel polymethyl methacrylate (PMMA). The diffusion flame between the fuel and oxidizer was studied numerically using a solid fuel decomposition and melt layer model to simulate convection and pyrolysis of the material. This model was validated using new experimental results as well as previously published works. Experimental results were used to validate the chemical mechanism and find correlations between the independent (oxidizer mass flux and temperature) and dependent variables (fuel regression rates, flame standoff distance, flame temperature, and extinction strain rates). The numerical simulations matched the experimental results well for regression rate and the flamelet temperatures. While the flame standoff distance and extinction strain rate results showed similar trends, numerical and experimental data were offset due to several factors that will later be discussed. This mechanism was compiled into a library and implemented into the cavity simulation using a 1D flamelet code with mixture fraction and a progress variable as the independent variables. The role of near wall processes on the formation and sustainment of acoustic modes in the ramjet combustion and mixing chambers are investigated with a focus on the presence of melt layers over fuel walls and their effect on the diffusion flames and the acoustic field they support. High-order computational simulations of ramjet combustion without regressing fuel walls using a novel discontinuous Galerkin approach is performed which provides a fully conjugate solution of thermal field. The acoustic energy distribution is studied using spectral proper orthogonal decomposition of the pressure field.