Laser-based and optical measurements of a gas turbine (GT) model combustor undergoing transitions between a thermo-acoustically stable and unstable state are presented. Planar laser-induced fluorescence of the OH radical, OH chemiluminescence and the planar three-component velocity field were simultaneously measured at a sustained repetition rate of 5 kHz. The combustor was operated with a lean, technically premixed CH4/air flame at ambient pressure that transitioned unpredictably between a thermo-acoustically unstable (‘noisy’) state and a state without pulsations (‘quiet’ state). The transition from the noisy to the quiet state was correlated with the lift-off of the flame from the burner nozzle and a subsequent stabilization of the flame above the nozzle. During the transition from the quiet to the noisy state, the flame reattached to the nozzle. It was observed that the transitions occurred consistently at a particular phase of the thermo-acoustic cycle. The axial velocity fields indicated that the reattachment of the flame was assisted by an increase of the backflow velocity in the inner recirculation zone.

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