Impact of the High Hydrogen Enrichment on the Flame Structure of Confined Lean Premixed CH₄/H₂/air Swirl Flames

Hydrogen is a promising clean energy, and hydrogen-enriched fuel is a feasible approach to reduce carbon emissions. The hydrogen enrichment affects the flame structures, and further affects the pollutant emissions and the flame instability. However, the mechanism of the H₂-induced transition of confined swirl flames is not fully addressed, especially when a high H₂ fraction is enriched. The present study was focused on the lean premixed CH₄/H₂/air flames with H₂ fractions up to 80%. The flame structure was obtained with Planar Laser-Induced Fluorescence (PLIF) of the OH radical. Flow fields were measured with particle image velocimetry (PIV). It was observed that the flame tip in the outer shear layer (OSL) gradually propagated upstream and finally anchored to the injector with the hydrogen fractions increase, yielding the transition from the "V" to "M" flame. The enlarged outer recirculation zone was expected to be crucial for forming the "M" flame since the burned gases could reside within and helped ignite unburned gases around the root of the OSL flame.