The effect of swirling injector geometry on the flow dynamics of precessing vortex core

Precessing vortex core (PVC), a typical vortex structure, universally exists in swirling flow fields and originates from the Kelvin–Helmholtz instability of the swirling shear layer. In the combustion chamber, pressure pulsations from PVC might resonantly couple with acoustic modes, leading to thermoacoustic oscillations which is a significant challenge in the development of rocket engines and gas turbines. The present work focuses on the time-averaged and unsteady flow characteristics of different swirling flow fields in the premixed swirling combustor. The PVC observed in the swirlers adopted presents a single helical structure and the PVC frequency is proportional to the incoming flow velocity. Moreover, another low-frequency helical mode located in the downstream region of the flow fields has been identified and categorized as PVC type II to be distinguished with common PVC mode in this study, which originates from the precession of the central recirculation zone. The experimental results further emphasize the effects of the swirling injector geometry on PVC and the low-frequency helical modes. This indicates that as the diameter of the bluff body or the divergence cup angle increases, the energy of the PVC mode decreases while that of the helical mode increases significantly. In addition, the PVC mode would be gradually suppressed as the tail size of the bluff body increases. These results, especially the coexistence of the PVC mode and the low-frequency helical mode, are rarely reported in previous studies, and they may provide an effective guidance for investigating the mechanism and control of thermoacoustic oscillations.