Development and transition process of jet ignition and jet flame ignition in hot turbulent jets by methane active prechamber system

• The effect of prechamber volume and hole diameter on ignition characteristic is investigated. • The jet ignition and combustion process in the prechamber system is elucidated. • The transition process from jet ignition to flame ignition is revealed. • The turbulent jet ignition coefficient is proposed to quantify prechamber ignition process. Prechamber structure has significant influence on the jet ignition phenomenon, thereby directly affecting the overall performance and efficiency of natural gas engines. However, the correlation between the prechamber structural features and ignition phenomenon has not yet been clearly elucidated. This work employs a combined experimental and computational approach in a constant volume combustion chamber (CVCC) with single-hole prechamber system to investigate the ignition phenomena and transition process of jet ignition and jet flame ignition. The investigation reveals the prechamber structural control mechanisms governing the ignition phenomenon in lean methane/air ambience, where both the orifice diameter and prechamber volume were varied. The results show that the smaller hole diameter produces a turbulent jet with lower temperature and active radical concentration, promoting a transition from flame to jet ignition and consequently delaying combustion. However, the volume has less effect on the ignition phenomenon. Moreover, the investigation of both ignition phenomena reveals that the initial flame formation consistently occurs in the upstream region of the jet, with subsequent propagation toward the downstream region driven by the jet development. Finally, the turbulent jet ignition coefficient (k) is proposed to characterize the transition of the jet ignition to jet flame ignition. The k represents the ratio of the characteristic reaction time t to the ignition delay τ T , which is related to initial jet velocity, ambient temperature of main chamber and adiabatic flame temperature of prechamber mixture. In the initial stage of the jet, the k increases faster as the prechamber hole diameter increases, in favour of forming the jet flame ignition. This work provides guidance for the design of prechamber structure, which contributes to the development of more efficient and reliable prechamber engines.