A comprehensive experimental and numerical study on turbulent jet ignition mechanisms of lean hydrogen mixture using a super-rich pre-chamber combustion

In this study, the combustion mechanism of turbulent jet ignition (TJI) with a super-rich hydrogen pre-chamber under globally lean conditions to achieve efficient combustion with low NOx emissions is investigated. Experiments were conducted in a rapid compression machine using a diaphragm-isolated pre-chamber filled with uniformly rich mixtures. Time-resolved measurements of OH* chemiluminescence images, near-infrared emission images from water molecules, and pressure in both chambers were performed to analyze the ignition and subsequent flame propagation. Furthermore, large-eddy simulations (LES) were conducted to investigate the ignition characteristics and flame structures in more detail. Results indicated that peak pressures for TJI were slightly lower than for conventional spark ignition (SI) at the same global equivalence ratio, likely due to additional heat losses. However, TJI significantly increased the pressure rise rate, enabling more efficient combustion in ultra-lean conditions by enhancing the degree of constant-volume heat release. Distinct flame structures were observed experimentally, consisting of a bright core plume surrounded by lower-intensity zones. During the TJI process, a large initial pressure difference between the pre-chamber and main chamber resulted in flame lift-off, characterizing the jet-ignition mode. As the pressure gap decreased, the flame transitioned to the flame-ignition mode, characterized by the attachment of the flame to the nozzle. Statistical analysis concluded that the ignition mode transition duration and lift-off height level during the transition exhibited an inverse correlation with pre-chamber richness and global equivalence ratio. The 2 mm orifice reduced pressure gaps and facilitated a faster mode transition compared to the 1 mm orifice. LES results showed good agreement with the experiments and exhibited the flame mechanism consisting of an outer lean premixed zone and an inner non-premixed core. Although stoichiometric combustion in the non-premixed region was observed H${}_2$O from pre-chamber products likely contributed to dilution, which may have helped limit the temperature rise and reduce NOx formation.

Novelty and Significance Statement

The novelty of this study lies in the investigation of lean combustion mechanisms using turbulent jet ignition (TJI) with a super-rich hydrogen pre-chamber as a staged combustion technology with low NOx emissions. A unique combustion mechanism consisting of a non-premixed core diluted by H${}_2$O and surrounding lean flame front was identified. The significance of this work is its potential to stabilize ignition under globally lean conditions while effectively controlling combustion temperatures to reduce NOx emissions. These findings offer valuable insights for the practical application of TJI in hydrogen internal combustion engines, paving the way for high-efficiency, low-emission designs in future hydrogen-powered systems.