Periodic ignition phenomena in flames approaching lean blow off

A periodic ignition phenomena in a premixed bluff-body stabilized flame approaching lean blow-off was investigated experimentally. Simultaneous high-speed PIV and OH-PLIF or OH*-chemiluminescence have been employed to obtain time-averaged and time-resolved flame images and flow fields, in which OH* intensities, velocity profiles and other turbulent flow characteristics can be obtained. The lean blow-off (LBO) limits are measured and compared using both methane and decomposed ammonia fuel blends with different ammonia volume fractions in both a long and short enclosure geometry. Flames in the long enclosures have wider LBO limits which may be related to changes in heat loss. Furthermore, for NH₃/H₂/N₂ fuel blends, the higher the percentage of H₂, the wider the LBO limits. The near lean blow-off oscillation behavior only happens in the long enclosure fueled with both CH₄ and NH₃/H₂/N₂ blends. The oscillation frequency has a linear relationship with the bulk velocity and the Strouhal number range in this study is 0.04 to 0.07, which indicates the oscillation might be related to the jet precession behavior near lean blow-off. The global oscillation behavior will be presented, with a focus on the jet flow deflection, which can lead to the occurrence of alternative asynchronous flame pockets on the left and right hand sides of the enclosure.