Experimental and numerical analyses of the effect of residual air bubbles in a single-hole high-pressure diesel injector nozzle are presented. Detailed information on spray structures and dynamics near nozzle exit at the Start of Injection (SOI) is described. Experimental measurements are performed using a laser-based backlit imaging technique through a long distance microscope by injecting diesel fuel into a constant volume high-pressure spray chamber. Numerical investigation of, in and near-nozzle fluid dynamics is conducted in an Eulerian framework using a Volume of Fluid (VOF) interface capturing technique integrated with Large Eddy Simulation (LES) turbulence modelling. The present flow setup includes residual air bubbles remaining from a previous injection event, in-nozzle turbulence with no-slip wall conditions. Experimental images show a toroidal starting vortex near the nozzle exit suggesting a partially filled nozzle; transparency in the emerging jet demonstrates the presence of air trapped inside the nozzle liquid from the previous injection event. The numerical model provides insight into the influence of residual air bubbles on the spray morphology and dynamics of the emerging jet at the SOI. A mathematical code is developed to replicate the backlit imaging approach with the numerical results. The virtual images demonstrate a transparent liquid jet emerging into the pressurized chamber gas showing improved agreement with experimental images. The inclusion of air bubbles in the nozzle liquid prior to injection in the numerical model also yields improved agreement in the penetration velocity profile of the jet. These results explain how inclusion of residual air bubbles inside the nozzle liquid affects the physics of the penetrating jet at the SOI. The air bubbles inclusion also provides an explanation for the transparency of the emerging jet, rough interfacial surfaces, and enhanced necking behind the jet tip captured at the SOI.

中文翻译：

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喷射开始时残留气泡对柴油喷雾结构的影响

提出了单孔高压柴油喷射器喷嘴中残留气泡影响的实验和数值分析。描述了喷射开始 (SOI) 时喷嘴出口附近的喷射结构和动力学的详细信息。实验测量是使用基于激光的背光成像技术通过长距离显微镜通过将柴油燃料注入恒定体积的高压雾化室来进行的。使用与大涡模拟 (LES) 湍流建模集成的流体体积 (VOF) 界面捕获技术，在欧拉框架中对喷嘴内和喷嘴附近的流体动力学进行数值研究。当前的流动设置包括从先前的喷射事件中残留的残余气泡、具有无滑壁条件的喷嘴内湍流。实验图像显示喷嘴出口附近的环形起始涡流表明喷嘴部分填充；出现的射流的透明度表明存在空气滞留在喷嘴液体中的先前喷射事件。数值模型提供了对残留气泡对 SOI 处出现的射流的喷雾形态和动力学影响的洞察。开发了一个数学代码来复制带有数值结果的背光成像方法。虚拟图像展示了进入加压室气体的透明液体射流，与实验图像的一致性得到改善。在数值模型中注入之前喷嘴液体中包含气泡也提高了射流穿透速度分布的一致性。这些结果解释了喷嘴液体中残留气泡的包含如何影响 SOI 处的穿透射流的物理特性。气泡内含物还解释了出现的射流的透明度、粗糙的界面表面以及在 SOI 处捕获的射流尖端后面增强的颈缩。