Flame transfer function measurements in an annular combustor: Comparison with single flame response under hydrogen enrichment

The full heat-release-rate (HRR) field was imaged in an annular combustor to assess the flame response at each injector simultaneously. The combustor geometry was modified to remove optical occlusions from the imaging setup. It was demonstrated that even very minor optical occlusions could lead to significant differences in the measured flame HRR response. When subject to carefully forced azimuthal modes, the sector-integrated HRR response could be decomposed into azimuthal clockwise and anticlockwise components. When decomposed, the HRR response amplitude was shown to be nearly independent of the azimuthal mode direction in the studied configuration. A comparison was made between the response of the same flame in annular and single flame confinement. For some operating conditions, the sector-wise annular flame transfer functions (FTFs) were found to deviate significantly in amplitude from those for a corresponding single flame combustor. This was attributed to inter-flame interactions and differences in confinement. These effects were modulated by studying flames with different hydrogen content and thus different laminar flame speeds. The flames became more compact with increasing hydrogen content, leading to reduced inter-flame and confinement effects. This lead to more similar FTFs in both amplitude and phase, with the phase for shorter flames being partially explained by a convective time delay based on the bulk velocity and flame height. Under azimuthal forcing, preferences to azimuthal mode direction were not significant and were largely unaffected by inter-flame interactions or confinement effects. However, with increasing hydrogen content, there was a larger variability in the thermoacoustic response between different nominally identical injectors, suggesting a sensitivity to small details in the flow. Novelty statement The symmetry of the azimuthal thermoacoustic response in an annular combustor was assessed in a full view of the annulus, compared to the blocked view from previous studies. It was shown that even a slight blocking of the view leads to a substantially different measurement. The full view revealed that the acoustic wave direction does not affect the flame transfer function (FTF) as much as what was reported in the past, meaning that parameters in models based on direction-dependent FTFs potentially need adjustment. For the first time, the FTF response was compared in an annular combustor and corresponding single flame under a wide range of carefully forced azimuthal modes, and for a wide variety of flame shapes. Though past studies found differences between the annular and single flame combustors, it was shown here that these differences are reduced when inter-flame and confinement interactions are reduced via hydrogen enrichment.