Influence of co- and counter-swirl on flow instability inside axial/radial dual-stage swirlers

To better understand the effect of swirl direction on the turbulent structures, vortex breakdown, and pressure fluctuations inside the axial/radial dual-stage swirler, co-swirl and counter-swirl swirler configurations were designed and numerically studied using the large eddy simulation (LES) method. The numerical data were verified by the particle image velocimetry technique. LES analysis of turbulent structures demonstrates that the counter-swirl configuration exhibits higher turbulent kinetic energy density and a more compact central recirculation zone (CTRZ) compared to the co-swirl configuration, with significantly elevated strain rate and vorticity amplitudes, which enhance turbulent mixing. Precessing vortex core (PVC) and dynamic pressure characteristics, which depend on swirl direction, show that the co-swirling flow has a dominant frequency of 686 Hz (PVC frequency) due to shear between the main flow and CTRZ. The counter-swirling flow, however, displays an additional 842 Hz frequency associated with Kelvin–Helmholtz instability in the tangential shear layer (SL), indicating more complex turbulent evolution. Small-scale vortices densely distributed in the counter-swirl SL are conducive to promoting flame perturbations. Q-criterion analysis reveals PVC near the swirler outlet and fragmented vortices in the CTRZ for both configurations. Proper orthogonal decomposition shows that the co-swirling flow retains higher energy in the first six modes, while counter-swirl energy concentrates in SL with rapid downstream decay, reflecting stronger Kelvin–Helmholtz instability effects. Spectral proper orthogonal decomposition modal analysis at PVC frequencies confirms that the counter-swirl SL forms more stable helical structures.