Study on the Effect of CH 4 /C 2 H 6 /C 3 H 8 Addition on Laminar Flame Velocity and Flame Stability for Ammonia Composite Combustion

Currently, research on improving the laminar flame speed and flame stability of ammonia by using carbon-neutral oxidizing fuel is emphasized by researchers, but research on the combustion characteristics and reaction mechanism of ammonia during the initial exothermic phase of small‑molecule reaction is still insufficient. To investigate the effect of small‑molecule reactions on combustion characteristics of ammonia, in this study, the effect of small molecule groups CH4, C2H6 and C3H8 on the velocity and instability of ammonia laminar flame has been investigated using high-speed schlieren technology in a constant-volume combustion bomb and chemical kinetics. The results show that the laminar flame velocity of ammonia assisted by CH4, C2H6, and C3H8 can reach a rapid flame development level at addition ratio of 20.6%, 25.4%, and 65.0%, respectively. During pure ammonia combustion and ammonia-small molecule hydrocarbon composite combustion, the free radicals H, O and OH have a significant influence on the laminar flame velocity. Small‑molecule hydrocarbons primarily affect the laminar flame velocity through the CH3 and C2H4 groups, and the differences between different small‑molecule hydrocarbons in their influence on the laminar flame velocity are originated from the CH3 group. Small hydrocarbons can maximize the Markstein length of ammonia flame when added in small proportion. The addition of CH4 is more effective than C2H6/C3H8 in alleviating the increase in hydrodynamic instability of ammonia laminar flame. The addition of C2H6 and C3H8 can effectively improve the thermal-mass diffusion stability of the ammonia laminar flame.