【作者】 张鹏；

【导师】 张煜盛；

【作者基本信息】 华中科技大学 ， 动力机械及工程， 2009， 博士

【摘要】 本文针对二甲醚闪急沸腾喷射雾化与蒸发混合过程进行了深入的理论与实验研究，研究内容主要集中在闪急沸腾条件下的二甲醚喷雾混合相关过程的机理分析及模型的建立上，并在此基础上进行了二甲醚闪急沸腾喷雾的多维数值模拟。为了构建闪急沸腾喷雾的非现象学模型，本文在气泡生长一维模型的基础上，编写了一个可用于模拟均匀过热液体内部气泡生长过程的计算程序。计算程序的有效性通过过热水中的气泡生长实验数据进行了验证。在准确计算二甲醚各主要热物性参数的基础上，本文利用该计算程序对二甲醚气泡生长过程进行了模拟计算，重点研究了环境压力和燃油温度对二甲醚气泡生长的影响。研究结果表明，根据气泡生长的加速度曲线，二甲醚气泡的生长过程可划分为表面张力控制阶段、过渡阶段和传热控制阶段。在不同的阶段，气泡生长表现出不同的特点，而这是抑制气泡生长的作用在气液界面上的液体动压和促进气泡生长的热边界层内的热反馈效应共同作用、相互竞争的结果。为了深入研究过热燃油喷射的雾化机理，本文将雾化问题的研究范围拓展至喷雾的上游边界—喷嘴内部流动。根据发动机燃油喷射的实际情况将气泡流确定为过热液体在喷嘴内的流动形态。鉴于目前较为常用的一维模型和多维模型都无法给出气泡在喷嘴内部生长的更多细节，本文将“内部模式”与“外部模型”有效地结合起来，在合理的假设前提下提出了一个改进的适用于过热液体的喷嘴内部流动模型。该一维模型可对气泡在喷嘴内的生长过程进行模拟计算，并为闪急沸腾喷雾的多维数值模拟提供较为准确的初始条件。模型的有效性通过初始喷雾锥角的实验结果得到了初步验证。作为分析流体运动稳定性的有效工具，线性稳定性分析法在燃油射流雾化机理的研究中得到了广泛的应用。为此，本文在已有的理论基础上，采用线性稳定性分析法推导得到了一个可描述闪急沸腾射流不稳定性的色散方程，并对影响二甲醚射流不稳定性的多个因素进行了研究和分析。研究结果表明，在普通射流条件下，提高射流速度和减小液体粘性有利于改善雾化效果，而表面张力和环境介质对射流稳定性的影响则相对比较复杂，需要具体问题具体分析。在闪急沸腾条件下，气相体积分数和射流锥角越大，射流越不稳定，射流雾化效果越好，这也被认为是除气泡的微爆效应之外，闪急沸腾喷雾能极大地改善雾化效果的另一个重要原因。大量的实验研究表明，在闪急沸腾条件下，气泡的微爆作用是促使燃油射流或者油滴发生雾化的主要原因，但是气泡为何破碎、何时破碎、如何破碎等至今仍不十分清楚。鉴于此，本文继续采用线性稳定性分析法，考虑粘性应力对气泡生长不稳定性的影响，在已有的理论基础上重新推导了气泡生长不稳定性的色散方程，建立了新的液滴破碎准则，并确定了子液滴的特性参数。研究结果表明，气泡Weber数和气泡的体积分数越大，扰动增长速率就越大。当气泡生长速度较小时，即使初始的油滴／气泡半径比不同，破碎时刻的气泡体积分数也基本保持不变，而气泡破碎时间将随着气泡生长速度的增大或者初始油滴／气泡半径比的减小而缩短。鉴于KIVA程序中的液滴蒸发子模型并不适用于闪急沸腾喷雾，本文在两区模型的基础上对其进行了相应的改进，通过质量守恒和能量守恒，得到了过热液滴的半径和温度的表达式。在将气泡破碎模型和改进后的液滴蒸发模型与KIVA程序耦合之后，本文对两组高压共轨喷雾实验进行了多维模拟，计算结果表明，喷雾贯穿距的预测值与实测值基本吻合，油滴索特平均直径(SMD)与实验结果吻合较好，这也间接验证了气泡破碎模型和液滴蒸发模型的有效性。本文采用高速摄像技术对二甲醚闪急沸腾喷雾进行了实验研究，重点考察了喷嘴启喷压力、喷孔直径和环境背压对喷雾形态和宏观特性参数的影响。实验结果表明，喷嘴启喷压力越大，喷雾贯穿距越大，而喷孔直径和环境背压的变化对喷雾贯穿距的影响比较复杂，但也基本上可认为是促进贯穿距增大的有利因素和阻碍贯穿距增大的不利因素相互竞争的结果；另外，喷嘴启喷压力越小、喷孔直径越大、环境背压越低，喷雾锥角就越大。最后，本文对基准条件下的喷雾实验进行了多维模拟，计算结果与实验结果取得了较好的一致。更多还原

【Abstract】 In this paper, the mechanism of atomization and vaporization process as well as mixture formation of dimethyl ether (DME) flash boiling spray are investigated numerically and experimentally in detail. The research contents focus on the analyses of mechanism of spray mixing process and the construction of spray models under flash boiling condition, and the multi-dimensional simulation of DME flash boiling spray is then performed.In order to establish the non-phenomenological model of flash boling spray, a computational program is written to simulate the process of bubble growth in uniformly superheated liquid. The program is validated by the experimental results of superheated water. On the base of calculating the main thermophysical properties of DME accurately, the computational program is used to simulate the process of DME vapor bubble growth under flash boiling condition and the study of influences of ambient pressure and fuel temperature on the DME bubble growth is carried out. The results show that according to the curve of time varying interface acceleration, the process of DME bubble gorwth can be divided into three domains: surface tension controlled growth, transition domain and heat transfer controlled growth. The bubble growth behaves differently in the three domains, which results from the interaction and competition between the hydrodynamic pressure and thermal feedback effect.To understand the atomization mechanism of superheated fuel further, the researching range is extended to the spray upstream boundary, namely the internal flow of injector nozzle. Accoding to actual situation of fuel injection of engine, the bubbly flow is confirmed as the flow pattern in the injector nozzle. Considering the known one-dimensional and multi-dimensional flow models can not reveal more details of bubble growth in the injector nozzle, the "internal flashing mode" and "external flashing mode" are combined effectively in this paper and an improved flow model for the superheated liquid in the injector nozzle is developed under reasonable hypotheses. The established one-dimensional model can not only simulate the process of bubble growth in the injector nozzel, but also supply the more accurate initial conditions for the multi-dimensional simulation of DME flash boiling spray. The validity of model is testified by the comparison between predicted and experimental results of initial spray cone angle.As a useful tool to analyse the stability of fluid flow, linear stability analysis method is adopted widely in the studies of atomization mechanism of fuel. Based on the linear stability analysis method, a dispersion equation which can describe the instability of flash boiling jet is derived in this paper and the instability of DME jet under normal and superheated conditions are analyzed in detail. Conclusions can be made that under normal condition, increasing the relative velocity and decreasing the liquid viscosity are able to improve the atomization of DME jet. The influences of surface tension and surrounding air density on the instability of DME jet are relatively complicated, which needs to be treated differently under different conditions. Under superheated condition, the more obvious the effect of flash boiling is, the more unstable the DME jet is. This is another important point for flash boiling spray in obtaining better atomization compared with common spray.A lot of experimental studies reveal that the spray is atomized primarily by bubble micro-explosion under flash boiling condition, but the exact atomization mechanism concerning when and how the bubble breakup operates is not very clearly identified up to now. Considering the influence of viscous stress additionally, the linear stability analysis method is adopted to deduce the more complete dispersion equation to represent the disturbance development during the bubble growth, and a new criterion for bubble breakup is established. The results show the bubble becomes more unstable with the increase of bubble Weber number and void fraction. The breakup void fraction is nearly constant at the lower bubble growth rate with different initial radius ratios of droplet to bubble, while the breakup time tends to be shorter with the increase of bubble growth rate or the decrease of initial radius ratio of droplet to bubble.For the original droplet vaporizaiton model in KIVA is not valid for the simulation of flash boiling spray, it is improved on the base of two-zone model. The formulations of superheated droplet’s radius and temperature varying with time are obtained by conservation of mass and energy. Through coupling the bubble breakup model and the improved droplet vaporizaiton model with KIVA, two groups of DME flash boiling spray within a common-rail injection system are simulated numerically. The study indicates that the predicted spray tip penetration agrees with the experimental result basically, while the calculated sauter mean diameter (SMD) of droplet agrees with that of experiment well, which indicates indirectly that the bubble breakup model and the improved droplet vaporizaiton model are valid.The ultra high-speed video system is applied to conducted the experimental research on DME flash boiling spray, and the investigations of spray figure and macroscopical characteristic parameters of spray under different nozzle opening pressure, nozzle hole diameter and ambient pressure are carried out in this paper. The results show that the spray tip penetration increases with the enhancement of nozzle opening pressure,while the influences of nozzle hole diameter and ambient pressure on the spray tip penetration are relatively complicated, which result from the competition between the positive factors and passive ones. In addtion, decreasing the nozzle opening pressure and ambient pressure, or increasing the nozzle hole diameter, will increase the spray cone angle. Finally, the multi-dimensional simulation of DME flash boiling spray is performed and the comparison between calculation and experiment has a good agreement.更多还原