Periodic behavior in the reaction zone of a multiple nozzle combustor undergoing self-sustaining combustion oscillations is examined. This combustor has three stages: a high-swirl pilot stage, a lower-swirl intermediate stage, and a low-swirl outer stage. The high-power conditions reviewed in this paper have fuel supplied to all three stages. Four conditions are examined in which thermoacoustic coupling is observed at a well-defined frequency. The highest overall equivalence ratio case displays a significantly higher oscillation amplitude than the other cases. Changes to the fuel distribution (with constant overall equivalence ratio) results in smaller effects on the oscillation strength. Phase averaged images of the OH* chemiluminescence emission show dramatic changes in the OH* distribution and intensity over an average period of the oscillation. This variation in chemiluminescence is dominated by recurrent quenching and reignition downstream of the intermediate and outer fuel stages. The pilot stage reaction zone also displays periodic variation in intensity which is out-of-phase and precedes the intermediate and outer fuel stages. Proper orthogonal decomposition is used to extract the most energetic spatial components which form the periodic behavior in the OH* distribution. The POD modes allow direct field visualization of fluctuation location and magnitude. For all four cases, the phase and location of the OH* emission variations are generally similar with small differences in the location and rates of periodic changes in the reaction oscillation.

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