柴油机喷孔内空化过程与流动特性研究

Cavitation Process and Flow Characteristics inside Diesel Injector Nozzle

为研究柴油机喷孔内燃油空化过程及其对流动能力的影响,基于试验和CFD三维仿真,保持入口压力不变,通过改变出口压力改变空化数,研究了随空化数变化,喷孔内空化过程及空化流动特性。研究结果表明:随空化数降低,喷孔内流动经历3个阶段:无空化阶段、空化发展阶段和空化饱和阶段。在不同的空化阶段,随空化数降低,空化过程和燃油的流通能力不同:无空化阶段,喷孔轴向各截面气液混合相体积分数不变,燃油流量增加,流量系数不变;空化发展阶段,空化从喷孔入口端向出口端发展,沿流向各截面气液混合相体积分数均增加,燃油流量增加,但流量系数下降;空化饱和阶段,喷孔入口端面的气液混合相体积分数首先达到稳定,沿流动方向,其他截面依次达到稳定,燃油流量达到最大并保持稳定,流量系数不断减小。

To study the cavitation occurring in diesel nozzle and its effects on the flow capacity, the process of cavitation and the flow characteristics in diesel injector orifice were analyzed based on optical testing and three-dimensional CFD simulation. In the experiment, the visualization of cavitation was recorded by a high-speed camera, the frame rate was set to 6 688 frames per second. The high-frequency Kistler pressure transducers were assembled before and after the nozzle to acquire pressure. Ultimately, the data of pictures and pressure were stored by a computer. The 3D CFD diesel cavitation simulations were carried out under conditions of different inlet pressures and back pressures based on the commercial code Fluent. The CFD simulations were validated by the experimental data. The results showed that： when K 〉 2.49, there were no bubbles in the nozzle, which was named no-cavitation period; during nocavitation period, both the gas-liquid mixed phase volume fraction and the fuel mass flow in each section did not increase and the flow coefficient was constant. When K≤2.49, it is the cavitation developing period when the gas-liquid mixed phase volume fraction of the entrance of the nozzle （plane A） started to increase which represented cavitation inception. At the same time, the growth rate of fuel mass flow reduced, and the flow coefficient began to decreased. When K≤1. 91, the gas-liquid mixed phase volume at plane A became stable, which meant the cavitation saturation period. During the saturation period, the fuel mass flow rised to its maximum and maintained stable while the flow coefficient decreased. It can be seen that the cavitation occurred in diesel injector nozzle had large impact on the flow characteristics. Meanwhile, the cavitation had impact on the fuel mass flow, the flow coefficient and the gas-liquid mixing phase volume fraction at different nozzle cross sections.