Numerical investigation on turbulent flame jet characteristics of ultra-lean H2/air ignited by pre-chamber enrichment strategy

Abstract

Pre-chamber enrichment for igniting ultra-lean premixed H₂/air is an effective strategy for achieving ultra-low emission and improved fuel economy. This study employed a three-dimensional numerical simulation based on a constant volume combustion chamber to investigate jet ignition characteristics under varying main and pre-chamber equivalence ratios (ERs), and compared the results with those of the spark ignition (SI) mode. The results showed that with an increase in ER in the main chamber, the SI mode primarily accelerates the late combustion stage, whereas the turbulent jet ignition (TJI) mode focuses on accelerating the early and middle combustion stages. In the TJI mode, when the ER in the main chamber is 0.25 (TJI-M0.25), and the pre-chamber ERs are 0.5 and 0.9, which yields combustion rates comparable to those of SI modes with ERs of 0.4 and 0.5, respectively. Because of the weaker reaction intensity of TJI-M0.25, flame propagation is primarily driven by the rebound effect from the wall, causing the flame to spread downward from the top of the main chamber. In contrast, TJI-M0.35 relies primarily on flame diffusion, leading the flame to spread upward from the bottom of the main chamber. For TJI-M0.25, the fastest combustion rate is achieved at an ER of 1.7 in the pre-chamber, while for TJI-M0.35, it is achieved at an ER of 0.9 in the pre-chamber. The combustion duration difference between the two is 8.8 ms, indicating that the pre-chamber enrichment strategy plays a more significant role in improving combustion performance under ultra-lean conditions.