Experimental Investigation of Forced Intermittency in Swirl-Stabilised Turbulent Methane-Air Flames

Thermoacoustic oscillations continue to be a major problem affecting combustor performance and operation in gas turbines. Acoustic forcing is often employed to simulate dynamical states in a combustor, however reproducing intermittent-like behaviour can be difficult. This paper, for the first time, reports a forced intermittent behaviour in a laboratory-scale, swirl-stabilised combustor burning methane-air mixtures that mimics the configuration of an industrial gas turbine. A frequency sweep using loudspeakers to generate velocity fluctuations identified 90, 220 and 320 Hz as the dominant frequencies of the burner. Amplitude sweeps were performed at these same frequencies and the flame response was obtained. At low forcing amplitude the acoustic flame response was in the linear range and the dominant frequency of the response matched the forcing frequency. Intermittency was observed beyond an amplitude threshold at 320 Hz, consisting of bursts of high-amplitude oscillations corresponding to broadband excitation around the forcing frequency. Further amplification resulted in excitation of a low frequency band. Wavelet analysis showed intermittent bursts to have a frequency signature matching the forcing frequency, and pressure and heat release oscillations were found to be coupled. In low amplitude regions between bursts, signals were found to be decoupled and response at forcing frequency was weak. Phase-space reconstruction showed acoustic modes to be consistent with those of self-excited flames found in literature. The outcomes of this work could provide further understanding on the causes of intermittency and its relationship with limit cycle oscillations