Ph.D. (Engg): Passive control and intermittent dynamics of the precessing vortex core oscillation in swirl flows

Swirl is used in modern gas turbine combustor nozzles to achieve   reliable flame stabilization and efficient fuel-air mixing. The swirl   component in the nozzle jet flow induces an axial vortex. At high swirl   intensities, vortex breakdown occurs, creating a recirculation zone in   the flow known as the vortex breakdown bubble (VBB). VBB appearance is   typically accompanied by the emergence of a global self-excited   instability where the VBB precesses around the flow axis and causes the   axial vortex to form a co-precessing helical structure. This  instability  is referred to as the precessing vortex core (PVC). Several  prior  studies have shown that the PVC oscillation can significantly  impact  emissions and thermoacoustic stability characteristics of the  combustor.  This thesis studies the characteristics and passive control  of the PVC.  The non-reacting flow field in an axial entry swirl nozzle  combustor at  the Massachusetts Institute of Technology (MIT), USA, is  investigated.  Planar three component time resolved velocity field  measurements in the  combustor for combinations of two swirl numbers, S  = 0.67 and 1.17 and  centrebody diameters of Dc = 9.5 mm, 4.73 mm and 0  mm (i.e. no centrebody) are analysed. All cases are at a fixed bulk  Reynolds number of 20,000. A new modal decomposition method based on  wavelet  filtering and proper orthogonal decomposition (WPOD) is  developed in  this thesis to analyze the global non-stationary dynamics  of these  flows. WPOD analysis for configurations without a centrebody  for both  swirl conditions revealed a coherent PVC oscillation in the  flow. Large  eddy simulation (LES) is performed for configurations  without the  centrebody and with the Dc = 9.5 mm centrebody for both  swirl numbers.  For all four cases, LES accurately captures flow  statistics and PVC  characteristics observed in the corresponding  experimental measurements.  Linear stability analysis (LSA) on the time  averaged flow for each value  of S in the configuration without a  centrebody yields a nearly neutrally  stable global mode whose  oscillation frequency and spatial flow  oscillation amplitude  distribution characteristics match those induced  by the PVC in each  case. The wavemaker region associated with the PVC  mode is shown to be  situated at the upstream end of the VBB on the flow  centreline.  Therefore, the introduction of a centrebody disrupts the  wavemaker and  suppresses the PVC as the experiments verify. In both LES  and  experimental studies for the cases with the Dc = 9.5 mm centrebody,  low  amplitude PVC like oscillations, which are also intermittent in the   S=0.67 case, are observed. Resolvent analysis (RA) for helical forcing   on the time averaged flow field from LES for these cases is performed.  RA reveals a low rank, optimal helical mode pair at frequencies where   PVC like oscillations are observed. The output mode amplitude   distribution characteristics match those of the PVC like oscillations  at  both values of S. For the S=0.67 case, the input mode structure  suggests  that intermittent separation between the centrebody wake and  the VBB,  due to turbulence results in the startup of PVC oscillations,  which  subsequent merger then suppresses. For the S=1.17 case, the input  mode  structure shows that stochastic forcing of the flow by turbulence,  generated by vortex shedding off the upstream swirler, results in sustained PVC like oscillations due to a low-rank strongly amplified   flow response at the PVC frequency revealed by resolvent analysis.

Speaker: Saarthak Gupta

Research supervisor: Prof. Santosh Hemchandra