Numerical investigation on turbulent flame jet characteristics of a pre-chamber fueled with CH4/O2 and CH4/NH3/O2 mixture under different initial pressures

Abstract

Ammonia is a very potential carbon-free alternative fuel with extremely low reactivity. The application of ammonia in internal combustion engines faces the risks of ignition failure, low burning rate, and poor combustion stability. Pre-chamber turbulent jet ignition (TJI), which can provide much more ignition energy, is a promising method to ignite ammonia fuel. However, the transient propagation process of pre-chamber jet flame has not been fully investigated yet. In this paper, the propagation characteristic of methane jet flame was numerically studied based on a constant volume combustion system. The supersonic jet flame propagation and Mach disk phenomenon were further analyzed with numerical methods. And the propagation characteristic of methane/ammonia mixed jet flame was investigated. According to the results, the formation and propagation process of methane jet flame has different characteristics at different stages. In High-speed development stage (Stage II) and Low-speed oscillation development stage (Stage III), the penetration distance of flame jet tip is almost proportional to the penetration time. However, the penetration speed of the two stages is significantly different. The change trend of the jet velocity at the outlet of orifice hole can also be divided into four stages: slow increasing, rapid increasing, gradual decreasing, and periodic oscillation. The periodic oscillation of the jet tip velocity and the jet velocity at the outlet of orifice hole is caused by the effect of forward and reverse shock waves. The calculation results of CH4/NH3/O2 mixture jet flame show that the addition of ammonia has significant effects on the flame propagation process in PC, the thermodynamic state of PC, and the propagation characteristics of jet flame. With the increase of ammonia proportion, the maximum pressure of PC gradually decreases and the pressure rise rate slows down. Meanwhile, the two-stage (the high-speed development stage and the low-speed development stage) characteristic of jet propagation process gradually disappears. And the addition of ammonia is more likely to destroy the two-stage characteristic under low initial pressure.