The development of low-carbon or zero-carbon fuel-driven engines has become a key technology to help reduce emissions under the background of carbon neutrality. Ammonia fuel has the advantages of low boiling point and high octane rating, but it also has the disadvantages of high self-ignition temperature and slow combustion speed. Direct injection of liquid ammonia has the potential to reduce greenhouse-gas emissions while maintaining engine power output, especially under high replacement ratio conditions. Meanwhile, liquid ammonia has a lower boiling point than traditional fossil fuels and is more likely to enter superheat state with plenty of bubbles nucleated inside the nozzle. After injection, bubble puffing/micro-explosion occurs inside the jet and thus accelerates the atomization and evaporation process of the spray. In this study, a micro-imaging technique was adopted to investigate the near-field characteristics and the gasification process inside the nozzle. Besides, both Schlieren and diffused backlit imaging techniques were applied to characterize the two-phase distribution evolution. By scanning a wide range of flash-boiling conditions, it is found that 3.5% of gasification ratio and 200.0% of near-field volume increment were achieved respectively inside and outside the nozzle at high fuel temperature conditions. Moreover, the linear increment of the gasification ratio accelerates the two-phase distribution area of ammonia spray exponentially. These results would provide insightful information for two-phase evolution and developing simulation models of ammonia sprays.

Spray simulations involve much effort in calibrating the empirical constants in the break-up model case-by-case to obtain good predictions for the target experiments. Most calibrations are based on the ideal-gas evaporation model for the evaporating sprays even under high-pressure conditions. The effect of droplet evaporation on the calibration of the break-up model is first evaluated, and an accurate multi-component evaporation model is suggested to reduce the dependency of the break-up constants on the ambient conditions and fuel types. First, a multi-component evaporation model with the real vapor-liquid equilibrium is introduced and implemented into the OpenFOAM-v6. Then, extensive sprays under a wide range of conditions are simulated by the typical ideal- and suggested real-gas frameworks, respectively. Since the ideal-gas evaporation model underestimates the droplet evaporation rate, especially for the multi-component sprays under high pressures, calibrating the breakup constants is needed to compensate the errors induced by the ideal-gas evaporation model. Even though the spray liquid penetration could be well reproduced through calibrating the constants in the KH-RT model, the droplet size is seriously underestimated. In contrast, the real-gas evaporation model can accurately predict the droplet evaporation rate, and the dependency of break-up constants on the ambient conditions is reduced. Finally, the comparison of n-dodecane and multi-component diesel sprays confirmed that the break-up constants for the multi-component spray simulation are more sensitive to the ambient conditions, and an accurate multi-component evaporation model can reduce this sensitivity.

The possibility of lube oil droplets’ existence in cylinders for two-stroke low-speed gas engines is higher because of the much higher lube oil consumption rate. Some droplets are directly injected into cylinders by lube oil injectors, and some are blown into cylinders through the scavenging ports. Autoignition of cylinder oil droplets is the main cause of preignition. This research study indicates that under in-cylinder conditions, overlarge single droplets cannot autoignite due to the long evaporation time, and overly small single droplets cannot autoignite because of the low vapor concentration. To find out what kinds of oil droplet groups could autoignite and cause preignition, 3-D computational fluid dynamics simulation in OpenFOAM was carried out. The model predictions were validated against the experimental results, including the evaporation rate of n-heptane droplets and the ignition delay of lube oil droplets. Also, the simulation was used to investigate the characteristics of multiple droplets under different ambient temperatures and pressures. The evaporation lifetime and the ignition delay of a large single droplet are dozens of times longer than that of multiple droplets, which confirms that the droplet group is more dangerous than a large single droplet. The evaporation rate and ignition delay are affected by the distance and number of droplets. A larger number causes a lower average evaporation rate. A smaller distance causes a shorter ignition delay. The local vapor concentration and temperature could be greatly reduced due to the existence of multiple evaporation and ignition cores. Additionally, these findings of multiple droplets were confirmed under nonconstant cylinder conditions of a natural gas engine. This research provides a guide to design the lube oil injectors and scavenging ports so as to reduce the preignition caused by lube oil autoignition.