Suppression and Intermittency of Precessing Vortex Core Oscillations in a Swirl Nozzle

Abstract

The precessing vortex core (PVC) is a self-excited flow oscillation in swirl nozzle combustors that can influence combustor operation by impacting unsteady flame dynamics, fuel-air mixing and emissions. The flow configuration studied is a swirl nozzle mounted in a dump combustor with an axial swirler. The bulk flow velocity at the nozzle exit plane is kept fixed at Ub = 8 m/s (Re ~ 20,000) and the swirl number, S=0.67. Our prior experimental study has shown that PVC oscillations can be rendered intermittent when centrebodies are introduced. Time averaged flow states needed for linear stability and resolvent analysis are determined from LES for two cases, with and without a centrebody respectively. For the case without a centrebody, linear stability analysis reveals a marginally stable PVC mode with a flow feedback region("wavemaker") located at the upstream end of the vortex breakdown bubble(VBB). This shows that the wake behind the centrebody interferes with flow feedback causing PVC suppression. Resolvent analysis on the case with a centrebody shows peak gain and low rank behaviour close to the intermittent PVC frequency determined from experiments. The spatial flow oscillation amplitude structure of the output mode confirms that the response to the input is a PVC. The input mode structure shows stochastic turbulent fluctuations whose helical component imposes a radial deflection of the VBB away from the centreline at its upstream end and causes the nominally merged centrebody wake and the VBB to separate. This results in the emergence of intermittent PVC oscillations.