Shear flow-driven combustion instability: Evidence, simulation, and modeling

Combustion instability arises mostly due to the coupling between heat-release dynamics and system acoustics. In these cases, the acoustic field acts as the resonator, while the heat-release dynamics, induced due to the impact of pressure or flow velocity perturbations on the equivalence ratio, flame length, residence time, shear layer, etc., supplies energy to support pressure oscillations. In this paper, we discuss another mechanism in which the resonator may not be the acoustic field; instead, it is an absolutely unstable shear layer mode acting as the source of sustained oscillations, which morph as large-scale eddies at a frequency different from acoustic modes. These perturb the flame motion and provide energy to the acoustic field, acting here as an amplifier to support pressure oscillations. We present evidence for the existence of this mechanism from several experiments, in which a pressure spectral peak can not be explained using acoustic analysis of the system, but instead matches the predicted absolute instability mode of the separated shear layer, and from numerical simulations. We also examine the impact of the mean velocity and temperature distribution on the frequency and growth rate to determine the dynamics leading to an absolute instability, and show that absolutely unstable modes are likely to arise at low-equivalence ratios. In some cases, they can also be present at near-stoichiometric conditions, that is, only low- and near-unity stoichiometry can support an absolutely unstable mode. We use numerical simulation to distinguish between different unstable modes in the separating shear layer, the layer mode and the wake mode, and derive a reduced-order model for the shear-driven instability using proper orthogonal decomposition analysis. Estimates of pressure amplitudes of fluid dynamic modes, when applied as forcing functions to the acoustic field, using this analysis compare favorably with experimental data.