Enhanced premixed ammonia combustion by hydrogen-oxygen enriched pre-chamber jet ignition

Pre-chamber turbulent jet ignition holds significant potential for improving ammonia combustion. Hydrogen- and oxygen-enriched combustion in the pre-chamber can further enhance ammonia combustion. However, fundamental research on this enhanced combustion mechanism remains limited. This study systematically examines the individual impacts of oxygen-enriched combustion in a hydrogen-enriched multi-orifice pre-chamber and nozzle geometry specifications on jet behavior in a nonreactive environment, as well as the impact of the jet behavior on ammonia ignition behavior, combustion processes, and emissions in a reactive environment. The results show that oxygen enrichment significantly enhances ignition characteristics, with three ignition modes observed as the oxygen concentration increases from 30 % to 70 % by volume: localized re-ignition, jet-induced secondary ignition, and jet flame ignition. With increasing oxygen concentration, combustion duration decreases initially and then increases. Unburned NH₃ and N₂O emissions decrease, while NO_x emissions slightly increase with oxygen enrichment. In the pre-chamber nozzle design, two nozzles with identical cumulative orifice areas, i.e., six 1.50 mm orifices and three 2.12 mm orifices, demonstrate that larger orifices improve ammonia ignition. On the other hand, when both nozzles have an identical orifice diameter, the nozzle featuring a smaller cumulative orifice area enhances ignition behavior as it generates higher-velocity hot jets. For two nozzles featuring the same number of orifices and similar hot jet velocities, the nozzle featuring the larger orifice demonstrates superior ignition behavior. These findings could offer valuable insights into enhancing ammonia combustion and optimizing nozzle design.