Intermittent Burst Oscillations: Signature Prior to Flame Blowout in a Turbulent Swirl-Stabilized Combustor

This experimental study focuses on characterizing the nonlinear combustion dynamics prior to blowout in a partially premixed combustor with a swirl-stabilized burner. Experiments were performed by varying the global equivalence ratio from 0.92 to 0.33. The acquired time-series data of pressure fluctuations were analyzed using nonlinear time-series analysis. When the combustor was operated at an equivalence ratio of 0.92, broadband low-amplitude pressure oscillations called combustion noise were observed. As the equivalence ratio is decreased to 0.62, the oscillations change their characteristics from low-amplitude broadband oscillations to high-amplitude discrete tones, and the combustor undergoes limit-cycle oscillations. A sharp peak is observed in the amplitude spectra of the acoustic pressure oscillations at this stage. On reducing the equivalence ratio to 0.42 and lower, the periodic motion is interrupted by occasional irregular bursts and the system switches back and forth between the noisy (periodic) and quiet (aperiodic) zones. In the dynamical systems theory parlance, this irregular switching between periodic and aperiodic behaviors is termed intermittency. On reducing the equivalence ratio further, the system reaches blowout condition. Intermittency can be used as the signature prior to flame blowout. The dynamical patterns and inherent nonlinearity during the transition from chaotic behavior to intermittent burst oscillations prior to blowout are shown qualitatively by drawing phase portraits, return maps, and recurrence plots.