A reduced-order model for the onset of combustion instability: Physical mechanisms for intermittency and precursors

In combustors, the transition from low-amplitude, aperiodic fluctuations termed combustion noise to large-amplitude, periodic oscillations termed combustion instability is presaged by an intermediate regime in flow conditions characterized by bursts of intermittent, high-amplitude, periodic oscillations that appear in a near-random fashion amidst aperiodic fluctuations. In this study, we show that, a reduced-order model from first principles that incorporates the hydrodynamic-acoustic coupling can capture these intermittent burst oscillations and the subsequent flow-acoustic lock-in observed in combustors. The physical mechanism that leads to intermittency in pressure fluctuations is described using the model. The paper concludes by illustrating through the model, ideas that use intermittency in the signal as an early warning signal - a precursor - to an impending combustion instability.