Abstract: Large eddy simulation (LES) was applied to the non-reactive cases with medium swirl numbers (N29S054 and N29S045) of the Sydney swirl burner, and the instability mode of the vortex breakdown bubble (the downstream second recirculation zone) was investigated. The statistical moment obtained from LES results shows good overall agreement with experimental data. The vortex breakdown bubble (VBB) did not exhibit the spiral-type, and was not surrounded by the spiral precessing vortex core (PVC), which indicates that the VBB shows no distinct precession motion. The instantaneous velocity contours suggest the existence of cyclic collapse/contraction and expansion of VBB. The distinct peak of power spectrum density gives further evidence for periodic motion.

Key words: unconfined swirling flow, large eddy simulation, vortex breakdown bubble, precessing vortex core