Ignition of ultra-lean premixed H₂/air using multiple hot turbulent jets generated by pre-chamber combustion

A detailed investigation on the ignition characteristics of ultra-lean premixed H₂/air mixtures by multiple hot turbulent jets in a dual combustion chamber system was carried out. Simultaneous high-speed Schlieren and OH^∗ chemiluminescence imaging were applied to visualize the jet penetration and ignition processes inside the main combustion chamber. The focus was on the effects of the spark location and fuel/air equivalence ratio within the pre-chamber on the ignition pattern of the main-chamber mixture. The results show that multiple jets resulted in similar lean flammability limit of H₂/air as the single jets, given that the total nozzle areas are the same. However, the ignition probability improved significantly near the lean flammability limit using multi-jets compared to single jets. Additionally, depending on the spark location and pre-chamber equivalence ratio, either the side jet, the middle jet, or all the jets can initiate ignition in the main chamber. The ignition characteristics of straight and angled multi-jets were compared, and angled jets produced shorter main chamber burn time. Numerical modeling of the flame propagation process within the pre-chamber was carried out to explain the behavior of individual jets in a multi-jet system. It was found that depending on the pre-chamber spark position, the flame shape inside the pre-chamber changes drastically. The effective L/D ratio governs the flame dynamics in the pre-chamber and subsequently determines which jet (side, middle, or all) will first ignite the main chamber.