LES OF TURBULENT SWIRLING JETS: STUDY OF JET PRECESSION, RECIRCULATION AND VORTEX BREAKDOWN

Large eddy simulations (LES) of turbulent isothermal swirling flows have been investigated. The Sydney swirl burner configuration has been used for all simulated test cases from a low to a high swirl and Reynolds numbers. Four test cases based on different swirl numbers have been considered and the influence of the swirl number for producing recirculation, vortex breakdown, precession vortex core and the precession frequencies have been investigated. The governing equations for the continuity and momentum are solved on a structured Cartesian grid, and a Smagorinsky eddy viscosity model with the localised dynamic procedure is used as the subgrid scale turbulence model. The results show that the LES successfully predicts both the upstream first recirculation zone generated by the bluff body and the downstream vortex breakdown bubble (VBB) induced by swirl. The plots reveal that the expansion of the upstream recirculation zone is almost similar for each test case. LES results revealed that the increasing swirl number affect to form the VBB in the downstream region, however it promotes the shear layer instability in the recirculation zones. The frequency spectrums indicate the presence of low frequency oscillations and the existence of a central jet precession. Results demonstrated distinct precession frequencies at the considered spatial jet locator and agreed well with the experimental values. The results also highlight the formation of a precessing vortex core (PVC).