Modal dynamics and instability characteristics of a swirl combustor under varying inlet conditions

This study investigates the unsteady flow characteristics in a multi-stage axial-swirl combustor under various inlet mass flow rates and pressure conditions using Large Eddy Simulation (LES) combined with Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). The double-helical Precessing Vortex Core structure (|m|=2) is identified as the energetically dominant global mode under all investigated inlet conditions. The frequency of this mode increases linearly with the inlet mass flow rate, while the corresponding Strouhal number (St) rises significantly with increasing mass flow rate, revealing a nonlinear scaling relationship between the PVC precession frequency and flow energy levels. Modal analysis further demonstrates that inlet conditions critically regulate the modal evolution: high flow rates suppress nonlinear interactions, concentrating energy in lower-order modes and simplifying the flow structure, whereas high-pressure conditions enhance feedback coupling within the shear layer through density effects, promoting energy transfer to higher-order modes and exciting more complex multi-frequency unsteady dynamics. By introducing a local swirl intensity parameter, this work quantitatively elucidates the coupled modulation mechanism of inlet parameters on vortex breakdown and PVC generation, establishing a dynamic connection between the swirl structure, pressure conditions, and global instability modes. These findings provide a theoretical basis for flow control and stable combustion design in combustors.