Investigations Of Near-nozzle Spray Characteristics And Its Influence Factors In Diesel Engines

Well atomization qualities of the in-cylinder fuel spray have great influences on the in-cylinder combustion process, emissions and power performance, etc. So, it is very necessary to understand the fuel injection process, atomization and evaporation process. Lots of professors had investigated the macroscopic spray characteristics in detail in real engine working conditions and atmospheric conditions. However, because of the limited available methods, it is very difficult to conduct experiments concerning near-nozzle spray. As a result, there existing a few experimental and simulated results about near-nozzle spray characteristics. Two different experimental setups were used to investigate the near-nozzle spray characteristics in diesel engines. First is the ultra-high speed imaging system, by which images of the near-nozzle spray injected from injectors with different orifice diameters were obtained under various injection and environmental conditions. Effects of the injection parameters and orifice diameters on the near-nozzle spray characteristics were investigated. Second is the high speed nanosecond flash-lamp imaging system, by which images of the near-nozzle spray injected from the nozzle hole with a micro-burr located at the nozzle outlet were obtained. Effects of the micro-burr on the near-nozzle spray structure were investigated in detail.Characteristics of the near-nozzle sprays injected from nozzle holes with different orifice diameters were investigated under various injection and ambient conditions. The results indicate that the near-nozzle spray penetration will be shorter under higher ambient pressures during the initial stage of the injection. The near-nozzle spray penetration will be longer with smaller orifice diameter. Compared with the effects of the ambient pressures and orifice diameters on near-nozzle spray penetration, effects of the injection pressures will be more considerable. Increasing the injection pressures will lengthen the near-nozzle spray penetration. During the whole injection process, the near-nozzle spray cone angle appear three different characteristics, which were defined as the front peak, stable period, and the tail peak. During the stable stage, the consequences show that the near-nozzle spray cone angle will be larger under higher ambient pressure, or with lager orifice diameter, or under higher injection pressure. The ambient pressures appear little influences on the value of the front peak and the wave length, which also means the ambient pressures have little influences on the opening performance of the injector. The value of the front peak will be larger and the wave length of the front peak will be smaller with smaller orifice diameter, indicating better opening performance of the injector. Although the injection pressures have little influences on the value of the front peak, the wave length of the front peak will be shorter under higher injection pressure, meaning better opening performance of the injector.Characteristics of the near-nozzle spray injected from the nozzle hole with a micro-burr located at the outlet were investigated by using the high speed nanosecond flash-lamp imaging system. The results indicate that the micro-burr will strengthen the turbulence of the near-nozzle spray during the initial stage. And under the effect of the micro-burr, the near-nozzle spray edge angles appear a separating point, and the appearance time of the separating point will advance with the increasing injection pressures. During the initial injection stage, the near-nozzle spray penetration increased linearly with the time and the spray penetration will be longer under high injection pressure. During the stable injection period, the micro-burr will change the near-nozzle spray structure through the viscosity force. The micro-burr will separate some fuel from the near-nozzle spray surface, which will atomized better. The near-nozzle spray cone angles change periodically along the outlet circle of the nozzle hole. And the curve of the near-field spray cone angles change with the imaging directions appear to be a ‘W’, whose cycle is 180°. The near-nozzle spray cone angles will be twice larger than that without the influence of the burr. And the injection pressures have little influence on the near-nozzle spray cone angles injected from the nozzle hole with a micro-burr located at the outlet of the orifice. The near-nozzle spray edge angles appear a joint point, after which the near-nozzle spray jet turns to be contractive. And the appearance time of the joint point will advance with the increasing injection pressures.The achievements of the researches will supply a reference file for further investigating the diesel spray characteristics and clarifying the near-nozzle spray primary breakup mechanism.