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Combustion instability arises mostly due to the coupling between heat release dynamics and system acoustics; a condition elegantly stated by the Rayleigh criterion. In these cases, the acoustic field acts as the resonator, or pace maker, 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, or pace maker, 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 the energy to the acoustic field, acting here as an amplifier, to support pressure oscillations. We present evidence to the existence of this mechanism from several experiments where 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, i.e. 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 POD 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.

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Thermoacoustic instability in premixed c ombustors occurs occasionally at multiple frequencies, especially in configurations where flames are stabilized on separating shear layers that form downstream of sudden expansions or bluff bodies. While some of these frequencies are related to the acoustic field, others appear to be related to shear flow instability phenomena. It is shown in this paper that shear flows can support self-sustained instabilities if they possess absolutely unstable modes. The associated frequencies are predicted using mean velocity profiles that resemble those observed in separating flows and for profiles obtained from numerical simulations, and are shown to match those derived from experimental and numerical investigations. It is also shown that the presence of density profiles compatible with premixed combustion can affect this frequency and can change the absolute instability mode into a convectively unstable mode thereby reducing the possibility of the generation of self-sustained oscillations. A qualitative prediction of the pressure amplitudes resulting from these shear layer modes is shown to be consistent with experimental measurements. The results from the stability analysis are combined with those using the Proper Orthogonal Decomposition (POD) method to yield a reduced-order model.

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