Velocity fields and flame structures of a partially premixed swirl flame in a gas turbine model combustor are measured in axial and transverse sections using simultaneous (Stereo-)PIV and OH-PLIF. The flame, operated under atmospheric pressure with air and methane at a thermal power of 10.3 kW and a global equivalence ratio of Φ = 0.75, features thermoacoustic oscillations at a frequency/« 295 Hz. The averaged flow field with inner and outer recirculation zones is typical of swirl-stabilized flames, and the instantaneous measurements show the presence of a helical vortex (PVC) located in the inner shear layer. The PVC, which rotates with a different frequency than the thermoacoustic oscillation, leads to an enhanced mixing of burned and unburned gas and thus to stabilization of the flame. Two distinct large-scale structures of velocity and OH are found in the transverse cross-sections. The first type is characterized by a roughly annular region of inflowing unburned gas and distinct inner and outer recirculation zones. In the second type, the region of positive axial velocity forms a spiral, and the recirculation zone consists of an inner region that is connected to the outer parts by a narrow curved zone along the spiral arm. Whereas the first type is a typical scenario of vortex breakdown with a PVC, the transient spiral recirculation zone observed here has, to our knowledge, not yet been reported. A phase-resolved analysis shows that the annular form correlates with low, and the spiral form with high rates of global heat release. © 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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