Global and Local Hydrodynamic Stability Analysis as a Tool for Combustor Dynamics Modeling

Abstract

Coherent flow structures in shear flows are generated by instabilities intrinsic to the hydrodynamic field. In a combustion environment, these structures may interact with the flame and cause unsteady heat release rate fluctuations. Prediction and modeling of these structures are thereby highly wanted for thermo-acoustic prediction models. In this work, we apply hydrodynamic linear stability analysis to the time-averaged flow field of swirl-stabilized combustors obtained from experiments. Recent fundamental investigations have shown that the linear eigenmodes of the mean flow accurately represent the growth and saturation of the coherent structures. In this work, biglobal and local stability analyses are applied to the reacting flow in an industry-relevant combustion system. Both the local and the biglobal analyses accurately predict the onset and structure of a self-excited global instability that is known in the combustion community as a precessing vortex core (PVC). However, only the global analysis accurately predicts a globally stable flow field for the case without the oscillation, while the local analysis wrongly predicts an unstable global growth rate. The predicted spatial distribution of the amplitude functions using both analyses agrees very well to the experimentally identified global mode. The presented tools are considered as very promising for the understanding of the PVC and physics based flow control.