【作者】 汪翔；

【导师】 苏万华；

【作者基本信息】 天津大学 ， 动力机械与工程， 2010， 博士

【摘要】 高压的液相柴油经由微细喷孔而形成的射流雾化对柴油机缸内的油气混合速率以及燃烧发展的路径有着决定性的影响。最近,一系列的实验研究和现象学分析已经证实喷孔中的空化现象是造成柴油射流雾化的主导因素之一。出于深入探讨柴油在高压真实喷射条件下的雾化机理及特性并为柴油机最优低温燃烧路径的开发提供理论基础的目的,本课题借助于改进的模型实验和三维数值模拟方法对柴油高压喷射过程中的压力波动、空化、湍流、射流雾化等典型的物理现象以及它们相互之间的作用机理进行细致的分析和研究。首先设计一套合理的实验方案对柴油喷嘴内部的压力波动进行测量。其结果显示,随着喷射压力的提高喷嘴内部压力波动的幅度逐步增加,并且高频的子波动逐渐增多,最大的波幅可达到平均压力的10%左右,最大的频率可达到40 khz左右。基于流动相似原理的分析显示,由空化气泡初始数密度决定的空化尺度必须与流动条件决定的流场特征尺度匹配起来,以前的空化流模型(包括单流体模型和双流体模型)将空化气泡初始数密度设为定值的做法是不合理的。本研究结合空化流发生的机理与喷嘴内流的特征提出一套对空化气泡初始数密度进行修正的方法,并推导出一个新的气泡初始数密度的计算式。实验验证的结果显示,气泡初始数密度的新计算式与双流体模型的结合能够较好的预报高喷射压力条件下的喷孔空化流特性。在压力波动存在的条件下,喷孔内部的空化过程是不稳定的,进而证明前人针对喷孔空化的稳态分析很难反映真实的喷孔空化特性。数值分析的结果还显示,无论压力波的形态及喷孔的几何形态如何,喷孔内部空化过程的演变与压力波对应的压力变化率( Dpin / Dt)密切关联着。Dpin /Dt的波动幅度越大,喷孔内的空化过程越不稳定。对于位于喷孔入口回流区附近的空化气泡而言,当地流场的液相张力条件能够保证它们的稳定成长,其平均动力特性几乎不受上游压力波动的影响。但是对于位于回流区尾流中的空化气泡而言,它们越靠近喷孔出口时,其平均动力特性对上游压力的变动越敏感。另外,由于几何影响的存在,相同量级的上游压力波动对非对称喷孔中的空化过程的影响程度明显低于其对对称喷孔中的空化过程的影响程度。不稳定的的空化过程对应着瞬变的喷孔出流流态。对于喷孔出口截面上的液相湍动能分布而言,最大的液相湍动能分布在近壁区,且当地流场的空化程度越高其值也越大。对于喷孔出口处的液相质量流量而言,其最大值和最小值的量级随着喷孔内部的总空化程度而改变,这是因为喷孔内部的总空化程度越高液相流体的有效流通面积也越小。为了考察压力波动和不稳定空化共存条件下的喷孔内流对柴油高压射流雾化的影响,本研究采用“两阶段方案”来进行数值分析,并在雾化模型中计入了空化气泡在喷孔近场的溃灭所造成的扰动。模型方法的验证结果显示,空化现象对喷孔近场的雾化过程起促进作用,原始的雾化模型由于没有考虑空化的影响而严重低估喷孔近场的射流破碎率。“两阶段方案”能够在喷雾初始条件和一次雾化模型中考虑空化的影响,因而基于该方案的数值结果与实验结果吻合得较好。另外,在压力波动和空化共存的条件下,喷孔出流对喷雾场有着较大的扰动,并且喷孔出流越不稳定就越有利于喷孔近场的雾化。更多还原

【Abstract】 It is well known that the atomization process of high-pressure diesel jets issuing from micro-hole nozzles has a decisive impact on air-fuel mixing rates and combustion processes in diesel engines. More recently, a series of experiments and phenomenological analyses have also verified that cavitation phenomenon inside the nozzle holes is one of dominant factors causing diesel jet atomization. In order to further clarify the nature of diesel jet atomization under realistic fuel injection conditions and provide a solid theoretical basis for control strategy optimization of in-cylinder low temperature combustion (LTC), some key phenomena (such as injection pressure fluctuaiton, cavitation, turbulence and jet atomization) related to diesel fuel injections and the interaction mechanisms among these phenomena have been systematically investigated in this study by using improved experimental and 3D computational approaches.A reasonable experimental schmeme has been firstly designed to measure pressure fluctuations close to the inlet of diesel nozzle hole. The measured results show that the pressure close to the inlet of diesel nozzle hole fluctuates more dramatically as fuel injection pressure increases. Moreover, statistics results indicate that the maximum amplitude and frequency of pressure fluctuation can even be around 10% of the average pressure and 40 kHz, respectively.The result of cavitating flow analysis by similarity theory based method reveals that the cavitation bubble length scale determined by bubble number density must match well with the hydrodynamic length scale determined by different flow conditions in modeling of cavitating flows, and accordingly cavitation bubble number density being assumed to be a constant for different cavitating flows is unreasonable. A modeling idea for initial cavitation bubble number density, which is originated from combination of cavitation bubble dynamics and internal flow characteristics of nozzle hole, has been brought forward in this study. Based on this modeling idea, a novel formula of initial cavitation bubble number density has also been developed. Model validation results verify that this formula together with a two-fluid model can predict well cavitating flow behavior under high-pressure injection conditions.Some important conclusions can be drawn from numerical analysis of cavitating flows inside diesel nozzle holes. Firstly, cavitation processes within the nozzle hole are very unsteady due to the inet pressure fluctuations, and traditional steady analysis of nozzle cavitation is hard to reveal its realistic behavior. Secondly, Amplitude of the Dpin /Dt (time derivative of inlet pressure) wave can be regarded as an index to evaluate the unsteadiness of cavitation process inside a diesel nozzle hole. Thirdly, cavitation content in the recirculation flow region does not show an obvious change as inlet pressure fluctuates due to enough liquid tension in this region. But cavitation content within the wake of recirculation flow is very sensitive to the variation of inlet pressure because of interactions between shedding vortices and cavitation bubbles. In addition, compared to cavitation content in a symmetrical nozzle, cavitation content in an asymmetrical nozzle shows less change under the same inlet pressure fluctuation because of geometry effect.Unsteady cavition processes induce transient flow conditions at nozzle exit. On the exit section, the maximum turbulence kinetic energy of liquid phase appears at“boundary zone”, and its value increases as local cavitation content increases. The mass flow rate of liquid phase mainly depends on the total cavitation content in the nozzle hole, and it usually decreases as total cavitation content increases.An unique“two-stage simulation scheme”, together with an improved primary breakup model that can take into account cavitation effect, has been applied to numerically evaluate the impact of nozzle hole internal flow corresponding to unsteady cavitation and inlet pressure fluctuation on diesel high-pressure jet atomization. Analysis results indicate that cavitation phenomenon enhances the atomization process close to nozzle exit, and original primary breakup model seriously underestimates the breakup rate of near-field spray. Furthermore, the internal flow of nozzle hole corresponding to unsteady cavitation and inlet pressure fluctuation is a big disturbance source both for gas phase and liquid phase in the spray field, and the level of breakup rate of near-field spray closely depends on the unsteadiness of nozzle hole internal flow.更多还原