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• The transient jet characteristics of gaseous ammonia fuel was investigated with LES simulation. • Sonic structures of under-expansion and instantaneous vortices was analyzed qualitatively and quantitively. • The transient propagation features of ammonia jets were compared with hydrogen jets. • Numerical results by LES and RANS were compared to exhibit jet characteristics. This study investigates the characteristics of gaseous ammonia jets under an injection pressure of 0.8 MPa and an ambient pressure of 0.1 MPa, using LES and RANS simulation approaches to inform applications of ammonia in internal combustion engines. A three-dimensional constant volume chamber model was established, and the Dynamic Structure model was applied in LES modeling with AMR techniques. The near-nozzle transient structures were successfully captured. Furthermore, a direct comparison between LES and RANS simulation was made, and main jet characteristics were investigated. LES outperforms RANS in predicting the instantaneous structures of ammonia jets and provides a more distinct visualization of oblique shock waves and diamond-like shock cells. RANS excels at displaying averaged features with a smoother distribution curve that represents self-similar characteristics. Additionally, the transient characteristics of ammonia and hydrogen jets were compared. The vortex formation and mixing process in ammonia jets occur at a much slower rate compared to hydrogen jets. Meanwhile, transient vortex ring behaviors were analyzed in detail, including their development tendencies regarding initial, secondary, and broken vortex formation. It is revealed that petal-like swirling movements are strongly correlated with the air–fuel mixing process. [ABSTRACT FROM AUTHOR]

• An optical test rig is built to observe the high-pressure methane gas direct injection in a CVB. • A 3D numerical model of high-pressure methane injection in a CVB is verified by the test data. • The methane jet penetration has a two-stage behavior induced by the dynamic inner flow inside the nozzle. • The methane jet tip penetration and the penetration speed become saturated as the injection pressure rises high enough. • This jet saturation behavior is caused by inner flow saturation of Mach number and gas velocity inside the nozzle. For a direct injection natural gas engine, the fuel jets into the cylinder in cycles, and this gas pulsed injection process causes crucial effects on the combustion. This study presents an experimental investigation on the methane direct injection. An optical test rig is designed to observe the high-pressure methane jet into a constant-volume bomb (CVB), and a numerical model is built to analyze the gas flow. The methane jet has a two-stage feature. The gas jet characteristic parameters (the tip penetration, penetration speed, jet cone cover area) change rapidly in dynamic stage I, and become stable in stable stage II. The dynamic process inside the nozzle induces this two-stage gas jet flow. The inner methane flow experiences a time delay from the inlet to the outlet, and the gas jet undergoes the same time delay to maintain stable. Furthermore, the injection pressure has great effects on the gas jet. As the injection pressure rises high enough, the gas jet saturation behavior occurs, and it is caused by inner flow saturation of Mach number, Reynolds number and gas velocity inside the nozzle. It is not necessary to increase injection pressure as high as possible for improving the gas jet characteristics. [ABSTRACT FROM AUTHOR]

• Numerical modeling of methane injection and combustion is developed and validated in CONVERGE software. • A k -value was defined by the ratio of jet velocity to laminar combustion velocity. • The relationship between k and the change of flame area and turbulent kinetic energy in the flow field was analyzed. • The laminar flame appears to be partially extinguished and intensified after the injection in the constant-volume bomb. Because of stringent emissions regulations, internal combustion engines using natural gas have become an important direction for the low-carbon development of the internal combustion engine industry. The combustion process of the internal combustion engines using natural gas involves the interaction between the gas jet and the flame. In this study, the process of a methane jet impinging on a methane premixed ignition flame inside a constant volume bomb was simulated by CONVERGE software, and the changes in combustion and flow within the jet impinging flame flow field were investigated. A k -value was defined by the ratio of jet velocity to laminar combustion velocity, and k is found to be inversely proportional to the maximum value of the rate of change of flame surface area A , and k is proportional to the peak value of the rate of change of turbulent kinetic energy. It was also found that the jet intensifies the initial premixed flame under three k -value operating conditions, and the OH-based change rate within the flow field increases abruptly by a factor of 4–9 after the jet is added. The process of jet impingement on the flame is divided into three stages according to the mutual position of the jet and the premixed flame surface, and the evolution of the flame structure and the combustion behavior in the flow field under different stages are analyzed in the article. It was found that the initial premixed flame induced by the jet became unstable in the second stage and a stable turbulent combustion flame was formed in the third stage. Analysis of the parameter changes at three points on the axial direction of the jet nozzle revealed that the jet disturbance at the near-nozzle end was too large to form stable combustion. Meanwhile, at the far-nozzle end, the jet disturbance formed a vortex volume suitable for combustion, and turbulent combustion was produced by the heat-induced by the flame moving toward the unburned area and the vortex volume disturbance. [ABSTRACT FROM AUTHOR]