Pressure effects and transition in the stabilization mechanism of turbulent lifted flames

This study reports novel measurements on the effects of pressure on the lift-off behavior and the stabilization mechanism of turbulent non-premixed methane jet flames. A high-pressure combustion duct (HPCD) was operated within the range of pressure P = 1-7 bar using jet velocities of 4 m.s≤ U ≤30 m.s and co-flow velocities of 0.23 m.s≤ U ≤0.60 m.s. Lift-off heights were measured from chemiluminescence pictures while joint images of hydroxyl and velocity, performed using joint PLIF-OH/PIV, were used to extract information about the stabilization mechanism. It is shown that while the lift-off height generally increases with pressure, the impact of pressure depends on the magnitude of the co-flow velocity. For U = 0.30 m.s, the flame's base remains near the nozzle over the entire pressure range and the measured flame speeds indicate that edge-flame stabilization is dominant. The slope of the lift-off height vs. jet velocity curves is positive. For U = 0.60 m.s and P > 2 bar, the flame stabilizes further downstream and a transition to turbulent premixed flame propagation appears to have occurred. At these conditions, the slope of the lift-off height vs. jet velocity curves becomes negative. This reversal at high pressure is a new result for methane. More importantly, the transition in the stabilization mechanism with increasing U is consistent with results reported earlier for ethylene and appears to be independent of the fuel.