Lift-off of non-premixed turbulent CH₄ jet flames at elevated pressures

In this study, the lift-off behavior of non-premixed turbulent jet flames (120 kPa to 500 kPa) is explored at elevated pressures, extending the current knowledge limited to sub-atmospheric and atmospheric pressure conditions. Experiments are conducted in the high-pressure combustion duct (HPCD) available at CCRC-KAUST. Flames are issued from a 4.58 mm inner diameter nozzle using methane as fuel and are maintained within an air coflow featuring a 0.6 m.s^-1 velocity. Results show that, regardless of pressure, if the fuel jet velocity U exceeds a critical value, detachment occurs and flames are lifted and stabilized at a height h above the nozzle. The velocity leading to detachment decreases with pressure. Regardless of pressure, if the fuel jet velocity U is further increased after detachment the lift-off height h increases linearly. However, it is observed that the slope of the h(U) curve is insensitive to pressure if p ≤ 300 kPa but decreases rapidly with pressure if p > 300 kPa. One-dimensional laminar flames calculations are conducted to understand the influence of pressure on important properties of methane/air flames such as their laminar flame speed and extinction strain rate. These results are used to explain the experimental observations.