Abstract :

Combustion instability in gas turbine combustors is driven by the interaction between unsteady heat release and combustor acoustic modes. Heat release unsteadiness is caused by unsteady wrinkling and distortion of the flame surface due to unsteady flow structures that are in general, generated by the combined effect of acoustic velocity oscillations and hydrodynamic instabilities of the combustor flow-field.The talk will focus on recent work on a Re=59,000 swirling jet performed in collaboration with Jacqueline O’Connor’s group at Penn State. These experiments have been performed by varying the intensity of swirl while keeping the mass flow rate fixed. The onset of coherent oscillations is shown using a combination of Spectral proper orthogonal decomposition (SPOD) and spatial cross-coherence analysis of the data. The oscillation frequency and amplitude squared of the coherent are found to vary linearly with increasing swirl intensity. This suggests that the PVC instability is a stable limit cycle oscillation that emerges with increasing swirl intensity. Interestingly, the onset of vortex breakdown, i.e. the formation of the central recirculation zone is found to coincide with the onset of PVC oscillations. Results from a weakly-nonlinear analysis of the above study using time averaged base flow fields determined from the experimental measurements show a linearly marginally unstable mode for the swirl intensity case beyond which PVC oscillations are observed. This happens due to the onset of vortex breakdown at this swirl intensity. The Stuart-Landau coefficients associated with the limit cycle generated by this mode, are determined from the leading order non-linear analysis which confirm the stable limit-cycle nature of the PVC. Interestingly, the mode shape associated with the PVC does not show shear layer oscillations that are seen in the experimental SPOD mode. This suggests that the helical shear layer rollup commonly associated with the PVC is not generated by the linear instability but rather due to a non-linear coupling between globally stable, shear layer instability modes that are “forced” by the PVC mode. The talk will end with a discussion on how PVCs can cause thermoacoustic oscillations in combustors, even though they do not by themselves generate global heat release rate fluctuations.