Flow dynamics in a multi-swirler model combustor based on LES and POD analysis

Swirling flow is often employed in gas turbine combustion chambers for the sake of improving flame stability. Swirling flow induces not only recirculation zones but also large coherent structures which show close relationship with flow dynamics and combustion instability. The flow dynamics including Precessing Vortex Core (PVC) in simple swirlers are extensively studied, while the flow instability characteristics in a multi-swirler combustor are not fully reported. In the present paper, Large Eddy Simulation (LES) of non-reacting turbulent swirling flow is conducted in a multi-swirler burner which comprises a pilot stage and a main stage. Flow dynamics in the multi-swirler combustor are analyzed based on phase-averaged evolution of instantaneous flowfield. Proper Orthogonal Decomposition (POD) is employed to identify the coherent structures in the multi-swirling flow. Results show that the main stage and pilot stage flow interact with each other generating highly turbulent swirling flow. PVC is successfully captured at the boundary of Main recirculation zone (MRZ) in the pilot stage with a dominant frequency of 1915 Hz. The PVC leads to periodic azimuthal flow instability. POD analyses for the velocity fields show dominant high-frequency modes (mode 1 and mode 2) in the pilot stage. However, the dominant energetic flow is damped rapidly downstream of the pilot stage that it has little effect on the main stage flow.