Modal decomposition of the unsteady non-reactive flow field in a swirl-stabilized combustor operated by a Lean Premixed injection system

Abstract

This work is focused on the numerical study of low-order coherent structures of a high-swirled laboratory-scaled combustor operated by a Lean Premixed (LP) injection system in non-reacting conditions through different flow modal decomposition techniques. This will provide valuable insight into the time-spatial modal structure detecting coherent spatial patterns. Experiments suggest the appearance of a self-excited hydrodynamic instability characterized by a single dominant frequency. On the one hand, the dominant pulsating energy components associated with the Precessing Vortex Core (PVC) are identified through the application of Proper Orthogonal Decomposition (POD) to the instantaneous velocity field. On the other hand, Dynamic Mode Decomposition (DMD) is proven to effectively highlight the relation between the frequency of the most dominant unsteady vortex structures and their spatial distribution within the combustor. Since DMD analysis generates a global frequency spectrum in which each mode corresponds to a specific discrete frequency, its application has been demonstrated to be more efficient than POD when dealing with temporally coherent problems. In this way, the DMD technique has proved to be a robust and systematic method that can give accurate and consistent interpretations of the periodic physics underlying hydrodynamic instabilities in the combustor studied in the present investigation.