Abstract

Swirled jets are technologically important flows that have wide ranging applications in gas turbine propulsion and power generation systems. Combustors in these devices use nozzles that impart swirl to the flow as it passes through. The intensity of swirl is quantified by the swirl number, S, which is the ratio of the streamwise fluxes of azimuthal and streamwise flow momentum. Sufficiently high values of S cause the axial vortex in the flow to break down and create an axi-symmetric recirculation zone in the flow – commonly referred to as the ‘vortex breakdown bubble’ (VBB). The precessing vortex core (PVC), is a self-excited instability in this type of flow that occurs due to the precession of the VBB , resulting in a helical rollup of the surrounding shear layer.

I will present results from a weakly non-linear asymptotic analysis of a swirled axi-symmetric jet at a Reynolds number, Re=59,000. This jet has been the subject of several studies in Jacqueline O’Connor’s group at Penn. State. The results show that the onset of vortex breakdown at a critical swirl number, S_c=0.61 results in the emergence of a linearly marginally unstable hydrodynamic mode that induces VBB precession. This in turn, results in the emergence of a stable limit-cycle flow oscillation, as verified by the coefficients in the Stuart-Landau equation governing the oscillation amplitude.

The PVC limit cycle oscillation arises due to intrinsic flow feedback at the leading edge of the vortex breakdown bubble. More recent experiments performed in collaboration with other groups have shown that introducing a centrebody upstream of the breakdown bubble, does result in the stabilization of the PVC . This provides further evidence that supports conclusions of our non-linear asymptotic theory for the PVC .