针阀运动对断油阶段空化和倒吸的影响

基于流体体积函数(volume of fluid,VOF)模型和动态重叠网格技术对柴油机喷油嘴断油过程中燃油流动特性进行了三维动态模拟。并通过可视化试验对透明喷嘴断油前后的流动进行了试验研究,着重分析了针阀运动对空化和倒吸的影响,其数值模拟与试验一致。结果表明:在针阀落座过程中,喷孔先于压力室发生空化现象,但二者最大空化量表现为压力室大于喷孔;通过对空化和倒吸进行量化分析证实空化溃灭后导致喷嘴内的压力降低是引起空气倒吸的原因;同时对3种不同针阀落座速度喷嘴内的空化和倒吸展开研究,结果显示3种喷嘴压力室内的空化量随针阀运动速度增大而增大,而喷孔内空化量基本相等;最后,通过理论推导得出了反映断油过程中喷嘴内空化的新空化数,解释了喷嘴内产生空化的机理,及针阀运动对喷嘴内空化和倒吸影响的原因。

Influence of Needle Valve Movement on Cavitation and Ingestion in Nozzle at the End of Injection

Based on the volume of fluid method and overlapped grid technology, the fuel flow characteristics in a diesel engine injector nozzle during the ending of injection were studied through the three-dimensional simulation. The cavitation and ingestion in a transparent nozzle before and after the end of injection were investigated by visual experiment, and the effect of needle valve movement on the cavitation and ingestion was analyzed. The experimental results were consistent with those of the simulation. The results showed that during the seating process of the needle valve, the cavitation at the orifice was earlier than that at the pressure chamber, but the maximum cavitation at the pressure chamber was larger than that at the orifice. A quantitative analysis of the cavitation and ingestion indicated that the collapse of the cavitation was the direct cause of the ingestion. At the same time, the cavitation and ingestion in three nozzles of different needle valve seating speeds were also simulated. The results show that the cavitation at the pressure chambers of the three nozzles is different, showing that the cavitation increases with the increase of the needle valve moving speed; and the cavitation at the orifices of the three nozzles is basically equal. Finally, a new value reflecting the cavitation in a nozzle during the ending of injection was derived by a theoretical derivation, which can be used to explain the mechanism of cavitation generation in a nozzle and the influence of the needle valve movement on the cavitation and ingestion in a nozzle.