Effect of Inflow Turbulence on Premixed Combustion in a Cavity Flameholder

Abstract

A discontinuous Galerkin finite element method code, JENRE, was used to perform highly resolved simulations of ramjet-mode combustion in the University of Virginia Supersonic Combustion Facility cavity flameholder at a flight enthalpy of Mach 5. Prior experiments measured a freestream turbulence intensity at the inflow to the cavity ranging from 10 - 15%. A synthetic turbulence inflow generator was implemented for the simulations in this work to reproduce the turbulence at the inflow to the cavity. Velocity perturbations and turbulence intensity generated by the turbulent inflow boundary condition are shown to match those values measured in the facility using particle induced velocimetry. Simulations were performed both with and without inflow turbulence to study the effect of turbulence on flame stability and structure. In both cases, a cavity-stabilized flame was achieved. The inflow turbulence promoted more robust combustion, causing the flame to propagate further from the cavity into the core flow, broadening the flame angle with respect to the axial flow direction. The flame angle captured in the simulation agrees with experimental results and theoretical prediction. The cavity's vortex shedding frequency was identified. Flame strain rate and pressure fluctuations in the cavity shear layer were also measured and found to vary periodically at a frequency equal to that of the vortex shedding. High flow strain rate in the cavity shear layer, driven by the vortex shedding process, is identified as a cause of flame stretching and low OH concentrations as the flame traverses the cavity ramp. The effect of spatial resolution on the simulations is discussed through a comparison of cases using second-order and third-order accurate discontinuous Galerkin finite elements.