A novel concept of pre-chamber turbulent jet ignition-induced liquid ammonia spray flame

Ammonia is one of the most promising alternative fuels owing to its nature of zero-carbon emission. However, the weak ignition characteristic of ammonia imposes a great hindrance on its application on engines. In the present work, a novel concept of pre-chamber turbulent jet ignition (TJI) induced liquid ammonia spray flame based on a high-pressure injection in a constant-volume chamber is proposed. The ignition modes associated with flame development of the transient ammonia spray flame are investigated. The effects of mixture reactivity in the pre-chamber and turbulent jet velocity are investigated. During the TJI-induced ammonia spray flame process, the three distinguished stages, including pre-chamber combustion, partially premixed combustion, and mixing-controlled combustion, are observed. The ignition takes place in the jet-liquid ammonia spray interaction region, but the flame fails to stabilize and propagates downstream along the ammonia spray. However, a spray flame is ignited when the hot combustion products are re-entrained by the ammonia spray, with the liftoff length gradually decreasing. The effects of mixture reactivity are further explored by enriching the pre-chamber mixture to φp = 3. The results indicate that a richer mixture can extend the ignition ability, as the critical oxygen concentration necessary for successful ignition decreases. In addition, a combustion mode transition of extinction-re-ignition phenomenon of spray flame is observed. The fundamental mechanism for the extinction-re-ignition phenomenon is attributed to the cooling effect of flash boiling and re-entrainment of hot combustion products. Furthermore, different nozzle diameters are employed to study the effects of turbulent jet velocity on the ignition tolerance. It can be concluded that a lower turbulent jet velocity can extend the ignition ability in two aspects. On the one hand, the slower hot jet increases the ignition energy by enabling a sufficient mixing between the hot jet and the ammonia spray. On the other hand, the reduced turbulence intensity weakens the turbulent heat dissipation that suppresses ignition. This work provides insightful views for ammonia compression engines.