A comparative study of hydrogen and methane enrichment on laminar flame propagation of ammonia

Abstract

Active pre-chamber jet ignition technologies can mitigate the challenges of poor ignition and slow combustion in spark-ignition ammonia engines. The fuel injection in pre-chamber can largely affect mixture reactivity and thereby the jet ignition performance. However, the mechanisms for the impacts of active mixture preparation on flame propagation within the active pre-chamber remain not fully understood. In this work, we conducted a comparative study on the effect of hydrogen and methane enrichment on ammonia flame propagation in both planar and spherical configurations. A non-monotonic behavior in the laminar flame propagation of ammonia with hydrogen enrichment is identified. Specifically, as hydrogen enrichment is raised, the laminar flame speed and burning flux of ammonia initially increase and then decrease, peaking at $X_A=$ 40% and $X_A=$ 30%, respectively. In contrast, methane enrichment results in a continuous reduction in both flame speed and burning flux. Thermal and kinetic analysis indicates that the initial decrease in activation temperatures primarily enhances burning flux, but thermal effects start to dominate and suppress burning flux when hydrogen concentration $X_A>$ 30%. Conversely, methane enrichment primarily reduces burning flux through thermal inhibition. Regarding stretched spherical laminar flames, two distinct stages are involved during ammonia combustion, i.e., ignition assisted flame kernel propagation and normal laminar flame propagation. The normal laminar flame propagation is dominantly controlled by stretching effects. Compared to methane, hydrogen tends to undergo a weakened stretching flame, manifesting pronounced flame stabilities. This work provides useful insights into the optimization and control of the active pre-chamber jet ignition in ammonia-hydrogen engines.