喷孔空化特性和近孔初始射流结构研究

设计了透明的有机玻璃喷嘴头部代替原喷油器的压力室和喷孔,在高压共轨试验台架上搭建了喷油器孔内流动和近孔喷雾可视化试验装置,采用高速可控闪光摄影与长距离显微成像技术相结合方法,获得了喷孔内部空化流动和近孔区域初始射流结构形态的发展过程图像。结果表明,所有试验喷孔内均呈现空化流动,空化强度和空化类型与针阀升程和喷油压力有关,喷油压力越高,对应空化初生的喷油时刻越早,并且空化类型在喷孔内出现的频率和时间也不同;喷油前喷孔内存在初始气泡,初始气泡大小不同导致了近孔区域初始射流结构不同。基于大涡模拟(Large eddy simulation,LES)和界面追踪法(Volume of fluid,VOF)多相流模型,根据喷嘴内部几何形状和试验条件,模拟计算喷孔内初始气泡的演变过程以及初始射流结构形态的形成发展过程,试验结果与模拟结果相符。

Cavitation Characteristic Inside Nozzle and Near-nozzle Initial Spray Structure

A transparent nozzle was designed to replace the sac and orifice of the original injector. Visualization experimental apparatus with a long distance microscope together with an ultrahigh speed CCD camera was employed to study the cavitation characteristic in the nozzle and the initial spray structure in the near nozzle region by photography technique with the help of backlighting. High spatial and temporal resolutions allowed a detailed observation of the very emergence of fuel from the nozzle orifice to the outside of nozzle. The experimental results showed that the residual fuel and initial bubbles were trapped in the injector hole after the end of injection, which had an important effect on the initial spray structure of the next injection. The cavitation was related to the needle lift and the injection pressure, and it was found that higher injection pressure led to earlier cavitation inception. Moreover, the types of free cavitation, cloud cavitation and string cavitation were also observed. In addition, based on the large eddy simulation (LES) and volume of fluid (VOF) multiphase flow model, the evolution of the initial bubble in the injection hole and the formation of the initial spray structure were simulated. The computational domain and settings mimicked the experimental injector internal geometry and experimental operating conditions. The initial bubbles inside the injector influenced the spray structure and could be a source of the observed deviation between experimental and numerical results. But the experimental results were basically consistent with the simulation results.