Insight into the instability of ammonia-methane laminar diffusion flame

Ammonia is one of carbon-neutral hydrogen derivatives and is identified as a sustainable fuel for mobile applications. However, the combustion instability of pure ammonia remains a significant challenge. In this study, the combustion instability of ammonia flame with methane as a combustion promoter was investigated using high-speed photography and schlieren techniques on ammonia-methane-air co-flow laminar diffusion flames. It was found that after exceeding a threshold of fuel flow rate (Qf), the stable laminar flame turned to a regular and reproducible oscillation, accompanying by periodic bulging and separation of the flame. The addition of co-flow air increases flame flickering frequency, which can be reduced by increasing the Qf. In contrast to the pure methane diffusion flame with distinct luminous zones, the NH3/CH4 diffusion flame exhibits a reddish-orange color with no distinguishable luminous zone. Additionally, the addition of ammonia shrinks the appearance of flames, and slightly decreases the flame flickering frequency at 30 % substitution, while increases it at 50 % substitution. A spindle-shape shear layer between the flame and the surrounding air, exhibiting periodic motion during the flickering sequence. The addition of ammonia decreases the maximum shear layer diameter and increases its motion velocity. The co-flow air pushes the vortex formation location downstream, reducing fluctuation amplitude of the shear layer. Ammonia substitution further promotes this downstream shift, potentially lessening the flame flickering.