An experimental investigation on hydrogen jet ignition of ammonia: Emphasis on reactivity stratification

Hydrogen jet ignition can effectively address the challenges associated with the poor ignition and slow flame propagation of ammonia. The ignition of ammonia initiated by hydrogen jet occurs within a reactivity stratification, which heavily relies on the injection parameters of active hydrogen fuel. However, the role of injection parameters in flame dynamics and turbulence/flame interactions is not fully investigated. Meanwhile, few studies are focusing on hydrogen-ammonia reactivity stratification through optical experiments. In this work, the ignition and combustion characteristics of hydrogen jet igniting ammonia were experimentally investigated using a rapid compression machine equipped with an active pre-chamber (PC), emphasizing hydrogen injection timing and hydrogen quantity. Instantaneous pressure acquisition combined with simultaneously high-speed imaging was employed for experimental analysis. The results demonstrate that under lean burning conditions, four types of ignition modes are identified in the main chamber (MC), i.e., jet ignition, post-jet quenching ignition, jet wake ignition, and flame ignition. Delaying hydrogen injection reduces hydrogen overflow into the MC and shortens the MC ignition delay effectively. Increasing hydrogen quantity enhances the reactivity of mixtures in both the MC and PC, with the PC achieving the shortest ignition delay at a hydrogen/ammonia energy ratio of 40 %. Delaying hydrogen injection or increasing hydrogen quantity leads to a switch in ignition modes from "jet ignition" to "flame ignition", which corresponds to the advance of the MC ignition timing from the moment after jet termination to the moment during jet processes. "Jet ignition" is prone to misfire events, whereas "flame ignition" manifesting stable ignition is notably characterized by fast early heat release and slow later heat release in the MC. Furthermore, "flame ignition" tends to occur under a high hydrogen/ammonia energy ratio in the PC or stoichiometric conditions in the MC, while late injection of hydrogen can effectively extend the boundary for low hydrogen requirements.