Flame dynamics during intermittency and secondary bifurcation to longitudinal thermoacoustic instability in a swirl-stabilized annular combustor

In this experimental study on a laboratory-scale turbulent annular combustor with sixteen swirl-stabilized burners, we study the flame-flame and flame-acoustic interactions during different dynamical states associated with the longitudinal mode of the combustor. We simultaneously measure the acoustic pressure and CH* chemiluminescence emission of the flame using a high-speed camera. Upon increasing the equivalence ratio, the combustor undergoes the following sequence of transition: combustion noise (CN) to low-amplitude longitudinal thermoacoustic instability (TAI) through the state of intermittency, and from low-amplitude to high-amplitude longitudinal TAI through a secondary bifurcation. We report the first evidence of secondary bifurcation from low-amplitude TAI to high-amplitude TAI for a turbulent thermoacoustic system which allows us to test the flame response at two different amplitude of perturbation in a natural setting. TAI arises at the first longitudinal mode of the combined burner tube and combustion chamber. The frequency of oscillations remain the same during the above transition. We find a significant difference in the dynamics of the flame interactions during the periodic part of intermittency and low- and high-amplitude TAI. Specifically, during the periodic part of intermittency, the phase difference between the local heat release rate (HRR) measured from various burners show significant phase slips in time. During low-amplitude TAI, there are fewer phase slips among the HRR response of the burners, which result in a state of weak synchronization among the flames. During high-amplitude TAI, we find that the flames are in perfect synchrony amongst themselves and with the pressure fluctuations. We then quantify the degree of temporal and spatial synchronization between different flames, and flames and pressure fluctuations using the Kuramoto order parameter and the phase-locking value. We show that synchronization theory can be conveniently used to characterize and quantify flame-acoustic interactions in an annular combustor.