Effect of Fuel stratification on OH and CH₂O PLIF Multiplication of Turbulent Hydrogen-Enriched Flames

Multiplication of hydroxyl and formaldehyde planar laser-induced fluorescence signals for turbulent hydrogen-enriched methane–air flames with compositionally inhomogeneous mixtures is investigated experimentally. Hydrogen-enriched methane–air flames with a global fuel–air equivalence ratio of 0.8 and hydrogen-enrichment percentage of 60% are examined. Two nozzles, each containing 4 fuel/air injection lobes are used in the experiments. The lobes of the first nozzle are straight, while those of the second nozzle are not, generating a swirling motion. The fuel is injected through several small diameter holes into the lobes. The amount of injected fuel flow rate varies between the lobes, generating stratified conditions. For each nozzle, two mean bulk flow velocities of 5 and 25 m/s are tested. Simultaneous hydroxyl and formaldehyde planar laser-induced fluorescence as well as separate stereoscopic particle image velocimetry are performed for the tested reacting conditions. For non-reacting flow tests, separate particle image velocimetry and acetone planar laser-induced fluorescence experiments are conducted to study the background turbulent flow characteristics and fuel/air mixing, respectively. The results show that stratification can lead to fragmentation of the flames and generation of islands with noticeable multiplication of hydroxyl and formaldehyde planar laser-induced fluorescence signals. Due to their significantly large number of occurrences, such flame structure can generate relatively large integral of the PLIF signals multiplication.