Experimental Study of Auto-Ignition Phenomena in Swirl-Stabilized LPP Flames in Gas Turbine Model Combustors using kHz Framerate OH-PLIF and Stereo- PIV

摘要

Long-duration, kHz-framerate OH-PLIF measurements were analyzed to determine the physical mechanism responsible for a previously observed phenomenon wherein isolated pockets of high OH-concentration fluid suddenly appear in regions of unburned gas away from the contiguous flame zone in a gas turbine model combustor. Prior research on the burners indicate these isolated flame-kernels may result either from auto-ignition of hot, unburned reactants upstream of the primary flame or from transport of reacting fluid into the field of view from beyond the imaging plane. An image-processing routine was developed to autonomously identify and statistically characterize these flame- kernels. Phase sorting of the kernel centroids with respect to the dominant fluid-dynamic structure of the combustors (a helical precessing vortex core or thermo-acoustically forced shear-layer vortices) indicate through-plane transport of reacting fluid best explains their sudden appearance the PLIF images. The concentration of flame-kernel events around the periphery of the mean location of the precessing vortex core (PVC) indicates they are likely the result of wrinkling and break-up of the primary flame sheet associated with the passage of the PVC as it circumscribes the burner centerline. The prevailing through-plane velocity of the swirling flow-field transports these fragments into the imaging plane of the OH-PLIF system. The lack of flame-kernel events near the center of the PVC, where conditions are more conducive to auto- ignition (lower strain, longer fluid dynamic residence time), indicates auto- ignition is not a viable explanation for these flame-kernels in a majority of cases. The lack of flame-kernel centroid variation in the case of the TM-Burner 'Quiet flame' (which has no PVC) further supports this explanation.