Lift-off characteristics of non-premixed jet flames of CH4 and cracked NH3 mixtures in laminar/turbulent transition

With the increasing interest in ammonia combustion technologies, using cracked ammonia gas is becoming an essential strategy. This study examined the stabilization characteristics of non-premixed lifted flames using mixtures of methane and cracked ammonia gas. A surrogate fuel of cracked ammonia consisting of 75 % H₂ and 25 % N₂ was used. Significant variations in the lift-off characteristics were observed, including the transition from laminar to turbulent regime. Flame structures at the base were investigated by capturing simultaneous Schlieren and OH-PLIF images, and the flame stabilization mechanisms were investigated based on the interaction between the fuel jet flow and the flame structures. Stable flames were formed below the mixing core in the laminar flame regime. Their behavior can be explained in terms of flame quenching. Flame stabilization below the turbulent core regime was also investigated in detail. In addition, the experimental results with cracked ammonia gas were significantly different than the previous relationship between lift-off height and fuel jet velocity in the fully turbulent regime. Thus, an improved relationship is proposed to estimate reasonable lift-off heights for all mixtures of methane, propane, hydrogen, and cracked ammonia gas. Finally, the blowout limits were explained by the decrease in flame propagation velocity due to the reduced turbulent intensity along the jet stream.