农用柴油机喷油器各孔喷油规律验证及流动特性模拟

Building 3-D model of diesel injector used in agriculture verified by injection rate of each hole and simulation on internal flow characteristics

针对农用柴油机喷油器各孔内部流动特性存在差异的现象,该文以某两气门用非均匀布置的5孔无压力室喷油器为研究对象建立三维模型,运用双流体模型及空穴模型计算了模拟各孔喷油规律,与实测各孔喷油规律吻合较好。通过该模型分析了喷嘴各孔瞬态流动特性及喷孔轴线与针阀轴线夹角对喷孔内部流动的影响。模拟结果如下:在针阀全开阶段,随着凸轮轴转角的增加,喷油压力不断变化,各孔内部出现不稳定空化,影响喷孔出口喷油速率;在喷射初期,喷孔内部未形成完全空穴,各孔喷油速率的差异不明显。另外,喷嘴喷孔轴线与针阀轴线夹角从67°增至80°时,各孔内部空穴区增加且延伸的空穴逐渐向喷孔中心轴线移动,喷油速率逐渐减小。该研究可为农用柴油机喷油器各孔的分布设计提供参考。

Agriculture-used diesel engine is widely used because of its simple structure, low engine cost and popularity. Because of the offset of the injector position to the cylinders＇ center axis in a two-valve diesel engine, the angles between the nozzle hole axis and the needle axis are different, which hence enhances efficient distribution of the mixture. It was observed from investigations that the injection rate of each nozzle hole of the injector was different but smaller for the nozzle hole with the higher angle（between the nozzle hole axis and the needle axis）. The injection process is important to the spray process, mixture formation and combustion. The internal flow characteristics of the injector hole is the boundary condition for the spray, combustion and so on, which play a crucial role in improving the spray quality, optimizing the combustion process and decreasing the pollutant emissions. In the present study, a three-dimension model of valve covered orifice（VCO） injector with 5 holes used in a two-valve diesel was established. The simulation of internal cavitation and velocity distributions of each hole in the VCO injector was based on the two-fluid model and the cavitation model. Because of the needle movement and the fluctuations of the injection pressure, the internal flows in the nozzle holes were unsteady. The internal transient flow could be technically reflected by the moving mesh. The simulated and measured fuel injection rates and cyclical fuel injection quantity of each nozzle hole were compared and analyzed. Experimental validation showed that their differences were under limits, and the relative error of the cyclical fuel injection quantity per cycle of each hole between the simulated and experimental value was less than 5%, which proved that such model could be used to study the transient flow characteristics and the influences on angle between each nozzle hole axis and needle axis of the nozzle. Comparison and analysis were done, and the results showed that there were significant differences in fuel flow characteristics and cavitation among nozzle holes, which were variable during the injection process. Firstly, the continuous changing of injection pressure destabilized the internal cavitation of each hole, which influenced the injection rate at the nozzle outlet in cam angle at maximum needle lift. The increasing injection pressure resulted in the bubble＇s collapse, which made the effective flow area increase and the injection rate decrease, and vice versa. During the initial part of injection, the internal cavitations of the 5 holes were different and did not progress to the outlet of the nozzle holes. This extension in length of the internal cavitation did not affect the injection rate of each hole. Secondly, the bigger internal cavitation zone of the holes moved to the center with the increase in the angle between each nozzle hole axis and needle axis of the nozzles. This increased the flow velocity at the center of the holes, which enhanced the spray characteristics. The results obtained indicate that the spray characteristics and the injection rate should be comprehensively considered when designing and installing the two-valve multi-hole nozzle to ensure the optimum mixture formations, the combustion optimization and the reduction of emissions.