柴油耦合喷嘴内空化流动对近场喷雾影响的模拟

Simulation of Effect of Cavitation Flow in a Diesel Injector Nozzle on Near-Field Spray

柴油机共轨压力的提高使空化现象快速发生而影响柴油近场喷雾的初次破碎,进而对后续燃烧和排放产生作用.因此搭建了单孔柴油喷油器内外流耦合喷嘴三维数值模型,并进行了验证.在此基础上,模拟研究了针阀开启初期不同喷油压力下空化流动对近场喷雾特性的影响.模拟结果表明,喷孔内空化发展经历了3个阶段:单相流区(Fv<0.3%)、空化发展区(0.3%<Fv<21%)及超空化区(Fv>21%),且空化发展中会伴随云空化的产生.喷油压力越大,空化初生时间越早,发展速度越快.进入超空化区后,空化气泡溃灭最先引起近场喷雾上游的主液柱区的破碎,破碎区逐步向喷雾下游延伸.空化引起喷雾破碎粒径减小,液滴数增加.空化使得喷雾径向速度增大,径向扩展能力增强,使得喷雾锥角增大.空化区涡值大,旋涡结构小且旋转强度大,进入喷射腔后增强了对喷雾的扰动,使得喷雾液芯变细,促进了液芯外侧的喷雾破碎.

With the increase of common rail pressure of a diesel engine,cavitation occurs rapidly,which affects the primary breakup of near-field spray and further affects combustion and emission.A coupled three-dimensional numerical model of single-hole diesel injector is established and verified.The effect of cavitation flow on near-field spray under different injection pressures at the beginning of needle valve opening is studied.The simulation results show that cavitation can be divided into three regions:single flow zone(F v<0.3%),cavitation development zone(0.3%<F v<21%)and super-cavitation zone(F v>21%).The flow of cavitation is accompanied by the occurrence of cloud cavitation.The higher the injection pressure,the earlier the cavitation initiation time and the faster the development speed.After the super-cavitation,the breakup occurs first in the main stream of the upstream of the near-field spray due to cavitation bubbles,and then the breakup area gradually extends to the downstream.Cavitation leads to decreased atomized particle size and increased droplet number.It also increases the radial velocity of spray and the tendency of radial spreading,thus increasing the spray cone.Cavitation area shows such characteristics as large vortex value,small vortex structure and large rotation intensity.When cavitation enters the injector cavity,the disturbance to the spray is enhanced,thus resulting in the thinner spray core and promoted break-up on the outside of the spray core.