The precessing vortex core (PVC) phenomenon in swirling jets is a helical instability in the flow driven by the coherent precession of the vortex breakdown bubble (VBB) around the flow axis, resulting in the helical rollup of the shear layer. This instability is driven mainly by flow processes in the region upstream of the VBB. Centrebodies, commonly employed in combustor nozzles create recirculation zones in their wake (CWRZ) that can interfere with VBB precession and hence suppress the PVC. We study this phenomenon in a swirl nozzle with a centerbody whose end face is flush with the nozzle exit plane, using large eddy simulations (LES) and linear hydrodynamic stability analysis for a nominal Reynolds numbers Re=48,767 and 82,751. For one of the Re=82,751 cases the centrebody end face diameter halved. This case shows the presence of coherent VBB precession in the flow. For the other two cases, linear stability analysis shows the presence of a stable VBB precession mode whose stability characteristics are sensitive to the time averaged flow structure in the region where the VBB and CWRZ merge. This shows that the reduction in the CWRZ length, due to halving the centrebody end face diameter, causes the VBB precession mode to become unstable. The oscillation frequency and the spatial distribution of oscillation amplitude predicted by linear stability analysis for this case, agree very closely with equivalent results determined from the LES using spectral proper orthogonal decomposition (SPOD). These results suggest that in general, spatial separation between the CWRZ and VBB can result in the onset of VBB precession and the emergence of PVC oscillations in swirl flows


The Role of the Centerbody Wake on the Precessing Vortex Core Dynamics of a Swirl Nozzle

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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