Vortex breakdown and its topologies in turbulent flows within a typical swirl combustor geometry

Abstract

We investigate vortex breakdown (VB) and its dominant topologies in turbulent, non-reacting flows within a canonical swirl combustor using large-eddy simulations (LES). A baseline configuration and operating conditions are first used to validate the LES solver, then five additional cases differing only by swirler vane-angle are simulated. The onset of VB is quantified using the generic swirl-number formulation, SNg, by detecting an internal recirculation zone (IRZ) in the mean flow, excluding highly-intermittent VB cases. Analysis of the mean flow shows that SNg measured within 40 mm downstream of swirler best represents the flow&#39;s swirl-strength compared with commonly used alternatives. A stable-VB first appears in the flow with 25° vane-angle, SNg=0.35. Q-criterion iso-surfaces and velocity time-series at VC footprints show a single-helix VC to prevail across all investigated vane-angles; up to 60°, SNg=0.98. Weaker double-helix signatures also appear, resulting from quadratic self-interaction of single-helix tone for vane-angles <= 50°. They are quadratically independent in 60° case, indicating association with a distinct helical hydrodynamic mode. Axisymmetric IRZ oscillations interact with helical-VC dynamics in 25° and 60° cases. VC precesses as stable limit-cycle oscillation via a marginally stable mode in 40° to 50° cases, while it waxes and wanes strongly in 25° and 60° cases driven by stochastic forcing of a slightly stable mode. Alongside the coherent VC-strand, a weakly-coherent strand originates from swirler. Its precession frequency matches with the lowest coherent precession frequency, corroborating its precession-based origin. Overall, we establish critical-values, evaluation locations and a topology-map for predicting and interpreting VB-states in isothermal swirl combustor flows.