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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]

• Structure evolution and hydrocarbon generation model in artificial maturation of Nenjiang lacustrine type Ⅱ kerogen was characterized for the first time. • Four average molecular models of lacustrine type Ⅱ kerogen were constructed. • Alteration of main evolution mode of kerogen structure at oil peak was observed. • Compared the contribution of alkyl bond breaking and condensation to aromatic structures. Lacustrine type II kerogen is widely distributed in China and has important hydrocarbon potential, but the study of its structural evolution is not comprehensive enough. In this study, a closed gold tube system was used to simulate the maturation of kerogen extracted from the Nengjiang Formation of the Songliao Basin to investigate a model of hydrocarbon generation and molecular structure change in lacustrine type II kerogen. The pyrolysis products at two heating rates were collected and quantitatively analyzed, and the Easy%Ro of 20 solid kerogen residue samples (solid pyrolysis products after extraction of methylene chloride and methanol) ranged from 0.69 to 2.49. From elemental analysis and 13C nuclear magnetic resonance (13C NMR), the molecular structure evolution of kerogen was clearly determined, and four average molecular models of kerogen were established. The results show that with increasing maturity, the aliphatic fractions of kerogen decreased, and the aromatic fractions increased. Before the main oil window (Easy%Ro < 0.89), the main products were nonhydrocarbon heteroatomic functional groups (NSOs: asphaltene and resin). The oil peak occurred at an Easy%Ro of approximately 1.1 when hydrocarbon C 14+ production reached the maximum and kerogen residue was the lowest. After the oil peak, aromatic groups became dominant in the structure. C 14+ began to decrease, and the kerogen residue proportion increased, which was related to the occurrence of secondary coking reactions after the main oil window. The hydrocarbon generation capacity of kerogen was exhausted after the oil peak, and the kerogen likely became the precursor of generated gaseous products. In the gas window (Easy%Ro > 1.53), the large quantity of C 1-5 was mainly due to the secondary cracking of hydrocarbons. The number of aromatic structures caused by alkyl bond breaking and condensation was estimated. It was found that before the oil peak, the number of aromatic structures caused by alkyl bond breaking decreased and was higher than that of aromatic structures caused by condensation. After the oil peak, the latter structures were obviously dominant, which further indicates that polymerization is the main structural change mechanism of kerogen evolution. [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]

The internal nozzle flow has great influence on fuel injection and spray. This work investigates the impacts of the injection back pressure on the nozzle inner cavitation developing, especially the flow characteristic during choking process. Based on the theoretical analysis, a three-dimension numerical model is developed to investigate the details of the inner nozzle cavitation flow. The numerical model is verified by experiments on a diesel fuel system test rig under different boundary pressures, including varied injection pressure and back pressure. The investigation shows that for a given injection pressure, cavitation occurs with the drop in the back pressure. The incipient bubbles appear just at the upper corner of nozzle inlet hole, and the cavitation area continues to expand until it reaches to the nozzle outlet. The nozzle inner cavitation is divided into three periods: no cavitation period; local cavitation developing period and super cavitation period. Before the super cavitation, as the back pressure drops, bubbles at the entrance of the nozzle hole cover more area, but the average liquid velocity increases and as a result the mass flow increases. Once the liquid velocity reaches to its maximum, the cavitation of the nozzle inlet hole maintains stable, so does the vapor–liquid mixture cross section, and the effective cross section of the liquid phase is minimum; by now the super cavitation forms. Under the condition of the super cavitation, the nozzle inner flow becomes choking and the back pressure has little influence on the mass flow, but the discharge coefficient declines as the back pressure decreases. During the choking flow period, when the back pressure declines, along the nozzle hole central axis from the inlet to the outlet, the effective flow cross section maintains stable in sequence and the liquid velocity also reaches its maximum in order. Moreover, at the outlet of the nozzle hole, super cavitation induces the increase of the turbulence kinetic energy and the outlet velocity. This increased outlet velocity makes up for the loss of the contracting effective flow passage due to super cavitation, thus the total mass flow maintains the same value after choke even the back pressure decreases. [ABSTRACT FROM AUTHOR]

The fuel injectors in a heavy-duty diesel engine with a selective catalytic reduction (SCR) system operate in harsh conditions that cause them to deteriorate, resulting in changes in the fuel injection quantity. In this study, a method for determining the injection quantity of fuel injected using the output of a NOx sensor in a heavy-duty diesel engine equipped with a SCR system is developed. A mathematical model was derived to calculate the fuel injection quantity based on the oxygen concentration in the exhaust gases. Bench tests were conducted at various engine operating conditions. The results indicate that at high engine loads, the absolute error between the oxygen concentration measured by the NOx sensor and that measured by a gas analyser was less than 2%, which is acceptable, and the reactions in the SCR reactor consumed no more than 1% of the oxygen in the exhaust gases, which is negligible. An analysis of the model demonstrates that the calculated value of the injection quantity based on the oxygen concentration in the exhaust is more accurate for lower oxygen concentrations (i.e., at greater engine loads). The test data reveal that the error in the fuel injection quantity obtained from the model was less than 2% at high engine loads. This research demonstrates that the on-board calculation of the fuel injection quantity based on the oxygen concentration signals from the NOx sensor is possible at high engine loads. [ABSTRACT FROM AUTHOR]