柴油机孔式喷嘴内气液两相流场的数值模拟研究

论文摘要

柴油发动机已在世界各地被广泛用作移动装置的动力源,同时,它也是大气环境、特别是城市大气环境的主要污染源。随着各国排放法规日趋严格和石油供求矛盾日趋尖锐,人们发现,柴油机喷嘴在高压和脉动喷射下的喷射特性,对柴油机喷雾的形成,及至喷雾燃烧和排放特性均具有十分显著的影响。柴油机的喷雾特性主要受喷油压力、小尺度喷孔内部结构和湍流气液两相流态的影响。由于细小喷孔内部的结构和流动极为复杂,加上喷油压力和燃油流速很高,因此,关于孔式喷嘴内部流动特性分析的台架试验难度极大,迄今为止国内外对此研究甚少。在此情况下,开展高压燃油喷嘴喷孔内气液两相湍流场三维数值模拟研究就显得十分必要。本文的主要工作内容和取得的研究成果主要有:1)以柴油机孔式喷嘴内部伴有空穴现象的复杂流动为研究对象,构筑了描述柴油机喷嘴内部流动的数学模型,采用分块耦合方法和局部加密处理生成了三维结构化网格,建立了基于Fluent软件包、旨在分析柴油机小尺度喷孔内气液两相流态的计算平台。2)在对计算方法和计算网格的适用性进行验证的基础上,应用CFD分析软件,通过对喷孔内单相和多相流质量守恒、动量守恒和能量守恒方程的求解,预测分析了不同发动机工况下的孔式喷嘴内部流场特性,喷嘴几何参数对流场速度分布、压力分布和出口速度的影响,以及稳定喷射时嘴内的空穴流动现象。3)借助于对孔式喷嘴内部空穴现象和两相湍流流动的多维数值模拟,分析了喷孔内空穴形成的机理及其分布情况,以及各流动参数与空穴形成之间的关系。在此基础上,进一步探讨了燃油喷射压力、背压、发动机气缸温度和喷孔长径比等几何结构参数对喷嘴内的流动形态与空穴分布的影响。4)研究表明,孔式喷嘴内存在的“气泡”(空穴)现象与喷嘴结构设计参数和流动参数密切相关,同样地,空穴考虑与否及其准确的描述反过来又又会显著影响整个喷嘴内复杂流态的预测值大小。将本文计算得到的喷孔内流场分布及空穴分布的结果与国外相关试验结果对比表明,二者基本吻合。这也进一步说明,借助于对喷孔内部空穴现象和湍流流动的多维数值模拟,可以帮助人们进一步理解喷孔内部的复杂流动现象及其变化规律,为优化孔式喷嘴结构、强化燃油雾化和混合过程提供有效的研究手段及基础数据资料。

论文目录

• 摘要
• Acknowledgment
• Abstract
• Content
• List of symbols and abbreviations
• List of figures
• List of tables
• 1 Introduction
• 1.1 Motivation
• 1.2 Objective and outline of the study
• 2 Literature Review
• 2.1 Introduction
• 2.2 Diesel spray atomization
• 2.3 Spray behavior and the influence of the nozzle
• 2.4 The study of internal nozzle flow
• 2.4.1 Experiments with cavitating nozzles
• 2.4.2 One-Dimensional modeling of the cavitating nozzle
• 2.4.3 Multi-Dimensional modeling of the cavitating nozzle
• 2.5 Cavitation bubble dynamics and multiphase flow
• 2.6 Previous investigations of the links between the inner nozzle flow and its spray
• 2.7 Summary and aim of present work
• 3 Mathematical modeling
• 3.1 Introduction
• 3.2 Overview of physical models
• 3.2.1 Continuity and momentum equations
• 3.2.1.1 The mass conservation equation
• 3.2.1.2 The momentum conservation equation
• 3.2.2 Turbulence modeling
• 3.2.2.1 Turbulence k-ε models theory
• 3.2.2.2 Modeling turbulent production in the k-εmodels
• 3.2.2.3 Effects of buoyancy on turbulence in the k-εmodels
• 3.2.2.4 Effects of compressibility on turbulence in the k-εmodels
• 3.2.2.5 Convective heat and mass transfer modeling in the k-ε models
• 3.3 Modeling of multiphase flows
• 3.3.1 Multiphase flow regimes
• 3.3.2 Choosing a general multiphase model
• 3.3.2.1 Approaches to multiphase modeling
• 3.3.2.2 Mixture model theory
• 3.3.3 Cavitation models
• 3.3.3.1 Basic cavitation model
• 3.3.3.2 Extended cavitation model capability
• 4 Simulation results and discussion
• 4.1 Introduction
• 4.2 Results of generating real geometry nozzle computational Grid
• 4.2.1 Diesel injector nozzle microscopic geometry
• 4.2.2 Procedure to generate real diesel nozzle computational grid
• 4.2.2.1 Overview
• 4.2.2.2 Creating geometry
• 4.3 Numerical investigation of flow inside diesel injector nozzles
• 4.3.1 Effect of nozzle geometry on the flow inside the diesel injector nozzle
• 4.3.2 Effect of injection pressure on flow inside the diesel injector nozzle
• 4.3.3 Effect of engine cylinder pressure on flow inside the diesel injector nozzle
• 4.4 Cavitation inside Diesel Injector Nozzles
• 4.4.1 Effect of injection pressure on cavitation
• 4.4.2 Influence of nozzle L/D on cavitation in side the diesel injector nozzle
• 4.4.3 Thermodynamic/thermal effect of cavitation in diesel injector nozzle
• 4.4.4 Phenomenological comparison between simulation and experimental results
• 5 Conclusions and recommendations
• 5.1 Conclusions
• 5.2 Suggestions and recommendation for future work
• References
• Appendix