58358 - Routes to Intermittency of Pvc Oscillations in Swirl Nozzles 

The precessing vortex core (PVC) is a self-excited flow oscillation state occurring in swirl nozzles, due to precession of the vortex breakdown bubble (VBB) around the flow axis. PVC emergence can impact emissions and thermoacoustic stability characteristics of combustors in various ways as several prior studies have shown. A recent computational study from our group showed that introducing a centerbody results in PVC suppression when the centerbody wake recirculation zone (CWRZ) merges with the VBB. This suggests that in realistic turbulent flows, intermittent VBB-CWRZ separation and merger can result in intermittent PVC oscillations.  In this paper, we show two routes to intermittent PVC dynamics in a single nozzle swirl combustor equipped with a cylindrical centerbody of nominal diameter D_C, using experimental time resolved high speed stereoscopic PIV (sPIV) measurements. The bulk flow velocity at the nozzle exit plane is kept constant as U_b = 8m/s for all cases, thereby, yielding a Reynolds number based on U_b and the nozzle exit diameter (D) of 20,000 for all the cases studied. Measurements are performed for three centerbody diameters, D_C, D_C/2, and 0 (no centerbody) and two swirl numbers, S=0.67 and 1.17. The centerbody length is chosen such that its end-face is flush with the nozzle exit plane. The position of the VBB in the S=0.67 case is downstream of the nozzle exit plane and centerbody end-face, suggesting that a distinct CWRZ is present in the flow. The S=1.17 case shows a VBB that engulfs the centerbody. Spectral proper orthogonal decomposition (SPOD) analysis of both S=0.67 and 1.17 data reveals a self-excited PVC at Strouhal number, St = fD/U_b ~ 0.5 for the cases without a centerbody and with the D_C/2 centerbody for the S=0.67 case alone. Intermittent PVC oscillations are observed with both centerbodies for S=1.17 and for S=0.67 with the nominal centerbody alone. These results suggest two possible routes for intermittent PVC oscillations. For the S=0.67 case the intermittent VBB-CWRZ separation results in intermittent PVC oscillations for the D_C case as our prior study suggests. In the S=1.17 case, the PVC oscillations appear to be caused by the flow response to stochastic forcing imposed by turbulence on the mean flow driven by the PVC mode. We will present results from LES and linear stability analysis, coupled with a new wavelet-POD (WPOD) decomposition of the sPIV data that justify these conclusions in the final paper.