Effect of acoustic excitation on the combustion and emission characteristics of methane-ammonia-air swirling flame

The blending characteristics of ammonia with conventional fuels have recently received much attention from researchers. This paper investigated the effect of acoustic excitation on the combustion and emission characteristics of methane-ammonia-air swirling flame on a laboratory-scale swirl burner. The high-speed schlieren imaging system was used to visualize the flame flow under acoustic excitation. The emission characteristics of CO and NOx at different excitation frequencies (70–270 Hz) and sound pressure levels (SPL, 117–128 dB) were determined. Results show that adding ammonia causes a significant increase in NOx emission and an increase in CO emission under fuel-rich conditions. The flame shows a layered V-shape with periodic development under weak low-frequency (70 Hz) acoustic excitation and an inverted cone with step distribution under high-frequency (270 Hz) acoustic excitation. Low-frequency acoustic excitation can reduce CO and NOx emissions under fuel-lean conditions. Especially for the excitation condition of 120 Hz (Prel = 0.3), NOx and CO emissions are reduced by 203 ppmvd and 170 ppmvd, respectively. However, when the excitation is stronger, the CO emission will rise sharply due to the destruction of the flame structure. Besides, the more intense acoustic excitation will cause a decrease in CO emission and an increase in NOx emission under fuel-rich conditions.