超临界低氧氛围高压喷雾射流减速瞬时火焰回撤现象研究

Combustion recession usually occurs during the end-of-injection transients of high-pressure burning spray jet. It can be used as an important marker of whether the combustion is complete near the injector holes region. However, information on the combustion recession phenomena is rarely reported. The objective of this study is to clarify the mechanism behind the combustion recession phenomena, and illustrate the boundary conditions of combustion recession during the end-of-injection transients of a burning spray jet, providing necessary information for construction for highly efficient clean low-temperature diesel combustion theory. Firstly, we will employ the high-speed microscopic imaging technique and the Linear Gradient Theory to reconstruct the detailed gas-liquid interfacial structure for the mixture near the injector holes during the end-of-injection transients under the supercritical non-reactive conditions. Then we use the modified Benedict-Webb-Rubin (BWR) equation of state in conjunction with the extended corresponding states principle to build the real-fluid vapor-liquid equilibrium model. Coupling with the simplified gas jet model, we can describe the detailed history of phase transition of liquid droplets. Next, we will derive the first-order nonlinear wave equation by analytically solving the one-dimensional partial equation for the axial momentum flux during the deceleration phase of transient incompressible jets. With the high-speed Mie-scattered imagine and the high-speed Particle Image Velocimetry (PIV), we observe the interaction between the liquid behavior and the entrainment wave during the end-of-injection transients, clarifying the mechanism of fuel-air over-mixing near the injector holes region. After we calibrate the CFD models coupled with detailed chemical reactions by using the high-speed Schlieren imaging and OH/HCHO imaging techniques, we solidify the method for the analysis of combustion recession during the end-of-injection transient of a burning jet. Finally, with the parametric study of injection rate, ambient oxygen concentration and fuel auto-ignitability, we achieve the boundary conditions for combustion recession during the end-of-injection transient of the burning jet.

火焰回撤现象通常发生于高压喷雾射流减速瞬时，可作为判断近喷孔区燃烧是否完全的重要标志，但相关研究信息极少。本研究旨在探究火焰回撤机理，解明影响火焰回撤的边界条件，为构建高效清洁低温燃烧理论提供必要信息。首先，针对超临界无氧氛围高压喷雾射流减速瞬时近喷孔区域，利用高速显微摄影观测与线性梯度理论分析重建其气液界面结构，基于修正的BWR状态方程构建真实流体气液平衡模型，耦合简化射流理论模型得出液雾相变历程。采用离散控制体方法求解射流减速时一维轴向动量流偏微分方程，利用高速背景散射光成像技术结合高速PIV观测喷射结束瞬时燃油射流变迁与卷吸波的相互作用，揭示近喷孔区过度混合机制。利用高速纹影摄影及OH成像技术观测喷雾射流减速瞬时火焰浮起长度变化，检证耦合简化化学反应的计算流体动力学模型，揭示火焰回撤现象内在理化机制。最后，进行氛围氧浓度，喷射率及燃料自着火特性等参数学习，明确影响火焰回撤的边界条件。

柴油机中高压共轨技术，低温燃烧技术以及废气再循环等技术的应用在提升效率的同时，还可以大幅降低柴油机氮氧化物与碳烟等有害物排放。然而与之相对应的高压喷雾射流减速瞬时火焰回撤现象的相关基础研究较少，特别是关于喷油结束瞬时射流减速对近喷孔区域混合气形成机理方面还存在诸多疑点亟待阐明。本项目研究旨在利用多种光学测试技术以及CFD仿真计算研究高压射流混合气形成特性，阐明瞬态油气混合控制机理。首先，课题组自主设计研制了预混合燃烧式高温高压定容燃烧容器并搭建了高压共轨燃油喷射系统；利用高速阴影摄影技术对柴油在不同喷射压力、不同环境密度、温度等惰性条件下非蒸发喷雾与蒸发喷雾锥角与贯穿距离等宏观特性进行了观测与分析；基于大涡模拟框架，对柴油喷雾混合气形成过程进行三维仿真。以高压定容弹内的喷油实验测试结果为基准，引入Box-Behnken实验设计和响应曲面法，对各模型参数进行数值试验，并进行子模型间的相关性分析；利用高速显微摄影系统，研究高温高压下的单液滴的蒸发过程；搭建高速Micro-PTV测速系统并开发相应算法，测试高压射流喷雾在不同喷射压力和环境密度下的近喷孔区域空气卷吸特性；最后采用一维控制体积模型对非蒸发喷雾的射流头部和尾部进行建模与实验验证并进行高压柴油喷雾火焰回撤过程研究。研究结果表明，在蒸发喷雾仿真计算过程中，破碎时间常数是影响KH-RT模型效果的主要因素，而湍动能系数是动态结构模型中的主要影响因素，优化了柴油蒸发喷雾的仿真模型并验证了其预测性；喷雾锥角与卷吸速度随时间的变化曲线呈现出双峰的结构，即在喷雾开始和喷雾结束瞬间的卷吸速度大于准稳态期间的卷吸速度。喷射压力的增加，空气卷吸速度加快，但其对空气卷吸量的影响有限。环境密度的增加，近喷孔区域的空气卷吸速度几乎没有变化，而空气卷吸量显着增加。通过分析同时测得的空气卷吸速度和喷雾角度，并分析它们之间的相关性，表明射流减速瞬时空气卷吸增强受喷雾锥角控制。