A Study of Spontaneous Transition in Swirl-Stabilized Flames

The goal of this study was to select and test some analysis tools to identify and characterize precursors to the onset of thermo-acoustic pulsation in a gas turbine combustor. The target flame was a turbulent, swirl-stability ethylene-air flame operated at ϕ = 0.91, and 5 bars pressure. 3-component stereo-particle image velocimetry (PIV) measurements, acquired at 9.3 kHz over periods of approximately 4 seconds were used to characterize the flow-field near the exit plane of the combustor. Acoustic measurements and OH*-chemiluminescence images were acquired synchronously, with OH* images acquired at every third cycle of the PIV measurement system. Analysis revealed the presence of two characteristic frequencies in the data for the stable flame; a 475 Hz oscillation associated with the shear-layer dynamics and a weak thermo-acoustic oscillation 610 Hz. In the excited state, a 720 Hz self-excited thermo-acoustic oscillation dominated combustor dynamics. A possible precursor event was identified in the form of a 635 Hz oscillation that appeared in the data 0.15s prior to transition. Bandpass filtering of the velocity data at this frequency showed this oscillation originates in the outer recirculation zone of the combustor.