No abstract

No abstract

Low-order coherent structures of non-reacting and combusting low- and high-swirl (swirl rates S = 0.41 and S = 1.0) jet flows in an open-ended cylindrical model combustor have been studied using the proper orthogonal decomposition (POD) and the dynamic mode decomposition (DMD) of high-repetition stereoscopic particle image velocimetry (PIV) velocity measurements. Lean methane-air mixture with the equivalence ratio of 0.6 was chosen for the reacting case as lean flames are generally less stable and more receptive to active control. The nonreacting and reacting flows at the same swirl rates exhibit qualitatively similar overall features characterized by highly turbulent annular jets enveloping the central retarding zone (for S = 0.41) or a recirculating bubble (for S = 1.0). At low swirl the dominant coherent structures appeared in the form of tilted ring vortices, evolving into helices as the swirl rate increases. The high-swirl flows are characterized by a pair of well-organized counter-rotating co-winding helical vortices originating in the inner and outer jet shear layers. Vortex cores were detected in both cases, but at low swirl it was less energetic and not associated with a distinct precession frequency. The core region of the low-swirl flame shows intermittent mild flow reversal without permanent central recirculation zone. The thermal expansion enhances the spreading of the reacting flow, especially at the high swirl number. Here the helices pitch is smaller and some neighbouring vortices merge into vortical sheets that act as shear shield confining the flame within the inner shear layer. Despite the significant effects of combustion on the time-averaged characteristics for the high-swirl, the dynamics of both flows were dominated by the global inviscid helical instability mode with a DMD detected conspicuous frequencies of 223 Hz for the nonreacting jet and 257 Hz for the flame.

The present work reports stereoscopic particle image velocimetry (PIV) measurements in a strongly swirling nonreacting jet and partially premixed lifted flame. The spatial distributions of the average velocity and components of turbulent kinetic energy were calculated from the measured ensembles of the instantaneous velocity fields. A pronounced bubble-type vortex breakdown was observed for the studied flows. Based on proper orthogonal decomposition (POD) of the PIV data and on estimates of velocity fluctuation spectra by a laser Doppler velocimetry (LDV) probe, it was concluded that the combustion did not fundamentally affect the type of coherent structures in the strongly swirling flow: a pair of secondary helical vortices was induced by a precessing vortex core in both cases. Because a strongly swirling jet flow is usually insensitive to weak forcing, strong perturbations were superimposed on the flow bulk velocity to force the formation of ring-like vortices in the flow and to investigate the possible outcomes on the turbulent combustion process. The forcing frequency was below that of the precession. Based on the CH Ã chemiluminescence signal, it was observed that the forcing provided an increase in the turbulent combustion rate near the flame onset, as the entrainment of ambient air to the rich mixture must have increased. Moreover, for a forcing amplitude typically above the magnitude of reverse flow inside the bubble-type recirculation zone, a dramatic suppression of the vortex core precession took place in the reacting case. This effect was accompanied by a quasi-periodical vanishing of the recirculation zone due to interaction of the forced ring-like vortices with the lifted flame.