Nonlinear characterization of azimuthal combustion instability exhibiting flame transient phenomena

This article characterizes the nonlinear features observed in an annular combustor exhibiting self-excited azimuthal thermoacoustic instability. The annular combustor consists of 12 burners with flames stabilized by bluff bodies protruding into the combustion chamber. This burner configuration closely resembles the flame stabilization geometry and flame shape in ramjet and afterburners configurations. As a first step, two different bluff bodies stabilize the flame: 1) circular and 2) conical and associated instability dynamics are characterized. The combustor exhibits intermittent oscillation in pressure with dominant 1A-1L (first azimuthal - first longitudinal) acoustic mode at about 630 Hz. The duration of periodic oscillation in the intermittent pressure fluctuation varies non-monotonically with the airflow rate. Further, relatively large amplitude pressure fluctuations are observed for intermediate values of airflow rate in both flame stabilization. During these large-amplitude pressure fluctuations, localized flame extinction in certain burners is observed, leading to partial blow-off operation of the combustor. This localized flame extinction is subsequently followed by either successful or unsuccessful re-ignition, leading to introduction of slow time scale heat release rate modulation in the problem. Further, this slow time scale heat release rate modulation is stochastic, contributing to the appearance of intermittent oscillation in pressure.