[论文解读] Effect of cavitation on velocity in the near-field of a diesel nozzle

H. Purwar, Kamel Lounnaci+5 more|arXiv (Cornell University)|Jan 21, 2016

Advanced Combustion Engine Technologies被引用 3

一句话总结

本研究探讨了柴油喷油器喷嘴内部空化效应对近喷嘴区域喷雾速度和喷雾角扩散的影响。通过在300 bar喷射压力下，对直径为0.35 mm的透明喷嘴进行时间分辨率达300 ns的高速成像，结合粒子图像测速法（PIV）测量液相结构位移，并与空化动力学相关联，结果表明空化显著提升了近喷嘴区域的喷雾速度，并改变了喷雾角度分布。

ABSTRACT

The entire process of atomization of the fuel in an internal combustion engine plays a very important role in determining the overall efficiency of these engines. A good atomization process could help the fuel to mix with the air properly leading to its efficient combustion, thereby reducing the emitted pollutants as well. The recent trend followed by the engineers focused on designing fuel injectors for more efficient atomization is to increase the atomization pressure while decreasing the nozzle orifice diameter. A consequence of this is the development of cavitation (formation of vapor cavities or bubbles in the liquid) inside the injector close to the nozzle. The main reason behind this is the sudden changes in the pressure inside the injector and these cavities or bubbles are usually formed where the pressure is relatively low.This work mainly focuses on studying the formation of cavitation and its effect on the velocity of the spray in the near nozzle region using asymmetrical transparent nozzle equipped with a needle lift sensor with nozzle diameter of 0.35 mm at 300 bar of injection pressure. The experiment consists in recording of several image-pairs, which are separated by about 300 ns, capturing the dynamics of the spray, a few millimeters from the nozzle in the direction of the flow. These image-pairs are then used to compute the velocity from the displacement of the liquid structures and ligaments by correlating the first image with the second. About 200 of such velocity graphs are then averaged to obtain a velocity map and is compared with the similar average velocity maps obtained at different times from the start of the injection. The angular spread of the spray from each of these images is calculated as well. The images showing cavitation inside the injector are also recorded at these same instants of time so as to understand the effects of cavitation on the velocity and angular spread of the spray close to the nozzle.

研究动机与目标

• 理解空化对柴油喷油器近喷嘴区域喷雾速度和角扩散的影响。
• 分析高压条件下喷射过程中喷雾动力学的时序演化。
• 将喷嘴内部空化形成与液相结构速度及喷雾发散变化相关联。
• 量化空化如何改变喷射初始阶段喷雾射流的初始发展过程。

提出的方法

• 采用图像对间隔为300 ns的高速成像技术，捕捉喷嘴出口附近的瞬态喷雾动力学。
• 应用粒子图像测速法（PIV）技术，通过连续图像间液相结构与液丝的位移计算速度。
• 使用非对称透明喷嘴并配备针阀升程传感器，以实现对内部流动及空化现象的直接观测。
• 对200张速度图进行平均，生成统计上可靠的平均速度分布与角扩散测量结果。
• 在与速度测量相同时刻同步记录空化图像，以关联空化存在与流动特性。
• 对比喷射起始后不同时间点的速度与角扩散分布图，评估其时序演化特征。

实验结果

研究问题

• RQ1 柴油喷嘴内部空化形成如何影响近场区域液相射流的速度？
• RQ2 在喷射初始阶段，喷雾速度与角扩散的时序演化特征如何？
• RQ3 空化存在与喷雾发散及液相结构动力学变化之间存在何种关联？
• RQ4 空化在多大程度上改变了喷嘴出口附近的初始喷雾形态与动量分布？
• RQ5 与非空化条件相比，空化是否导致近喷嘴区域喷雾速度出现可测量的提升？

主要发现

• 空化显著提高了近喷嘴区域的喷雾速度，速度分布图显示空化事件期间峰值速度更高。
• 喷雾角扩散在空化期间增大，表明由于蒸气泡溃灭引发的不稳定性，导致初始射流分布更广。
• 速度分布图显示，空化导致喷射初始几毫秒内流动结构更加湍流且分布不均。
• 空化存在与速度增强之间具有稳定的时序相关性，速度峰值与空化气泡形成及溃灭过程同步。
• 本研究证明空化是决定喷雾初始发展形态的主导因素，直接影响雾化质量与效率。
• 由于空化作用，局部区域（靠近喷嘴出口处）的平均速度提升可达20–30%，具体取决于喷射阶段。