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This paper demonstrates a method for calculating thermoacoustic energy transfer (viz. Rayleigh Index) fields in complex swirl-stabilized flames having asymmetric 3D flow structures using high-repetition-rate OH* chemiluminescence measurements. Measurements were acquired in a variety of perfectly premixed methane-air flames, each of which contained a helical velocity disturbance that was coupled with a precessing vortex core (PVC). The azimuthal position of the PVC and helical disturbance relative to the viewing angle was determined by tracking the position of the chemiluminescence centoid. Tomographic reconstruction of multiply-phase-conditioned mean chemiluminescence fields then was performed to determine the mean 3D shape of the helically-perturbed heat release field at different phases over the thermoacoustic cycle. These fields, in combination with measured pressured signals, allowed

Lift-off limits and mechanism of biogas swirl flames were investigated in a gas turbine model combustor using high-repetition-rate OH* chemiluminescence and simultaneous particle image velocimetry (PIV) and OH planar laser induced fluorescence (PLIF). The biogas fuel was represented by 60% methane and 40% carbon dioxide, volumetrically. The test matrix consisted of three preheat temperatures and three target adiabatic flame temperatures, with a total of 9 test cases. Total lift-off was defined as a distinct and complete flame detachment from the burner nozzle, which was approached by increasing both air and fuel flow rates at a fixed equivalence ratio. With the increase of bulk velocity, more intermittent lift-off events were observed, with the flame temporarily detaching from the nozzle before reattaching. By analyzing flow-flame interactions during these events from the temporally-resolved PIV and OH PLIF measurements, the lift-off mechanism was observed. As a precursor of lift-off, a local flame extinction event occurred near the flame base due to a pulse of high strain-rate on the flame. Noticeable flow and flame asymmetry subsequently developed, which led to further thinning and wrapping-up of the flame base. Further local extinction subsequently occurred due to the high strain-rate associated with the asymmetric flow. This eventually caused the entire flame base to quench and the flame to detach. Analysis of the vorticity field indicated that the flow asymmetries were due to the formation of a helical precessing vortex core (PVC) after the first local extinction event as a result of density field change near the nozzle exit.

This paper demonstrates a method for calculating thermoacoustic energy transfer (viz. Rayleigh Index) fields in complex swirl-stabilized flames having asymmetric 3D flow structures using high-repetition-rate OH* chemiluminescence measurements. Measurements were acquired in a variety of perfectly premixed methane-air flames, each of which contained a helical velocity disturbance that was coupled with a precessing vortex core (PVC). The azimuthal position of the PVC and helical disturbance relative to the OH* chemiluminescence viewing angle was determined by tracking the position of the chemiluminescence centroid. Tomographic reconstruction of multiply-phase-conditioned mean chemiluminescence fields then was performed to determine the mean 3D shape of the helically-perturbed heat release field at different phases of the thermoacoustic cycle. These fields, in combination with measured pressured signals, allowed calculation of the thermoacoustic energy transfer distribution. Complex patterns were found, which generally involved considerable energy transfer in the periphery of the burner (i.e. towards the outer recirculation zone). The method described provides a relatively simple and robust diagnostic for determining combustor regions driving thermoacoustic oscillations.