【作者】 许健；

【导师】 张欣；

【作者基本信息】 北京交通大学 ， 载运工具运用工程， 2014， 博士

【摘要】 近年来随着日益严峻的能源与环境危机,低热值气体燃料以其清洁性与可持续性给气体发动机的推广应用带来了较大的发展空间。由于燃用低热值气体时发动机容易发生燃烧过程不稳定现象,因此如何提高低热值气体发动机的燃烧稳定性成为研究代用燃料发动机领域的热点问题。为了进一步深化气体燃料发动机缸内湍流燃烧的理论研究,推进缸内混合气组织与燃烧过程控制的技术发展,本文以低热值气体发动机缸内着火与燃烧过程中火焰面结构的微观演化过程为研究重点,开展了缸内预混燃烧的湍流涡团与火焰面的相互作用过程以及火焰内在不稳定性效应等的多维数值模拟的基础研究。研究工作阐明了低热值气体发动机缸内燃烧过程中火焰面形态与结构的演化机理,为清洁高效气体发动机燃烧系统的优化和设计提供了理论支持,具有较高的学术意义和工程应用价值。本文研究了湍流扰动下平面火焰传播过程中Darrieus-Landau不稳定性(D-L不稳定性)的发展过程,求解了湍流场作用下的Michelson-Sivashinsky方程(T-M-S方程),在此基础上得出了平面火焰传播速度增量的修正公式；在湍流燃烧三维模型中采用桥函数的方法将D-L不稳定性的函数表达式引入组分方程的化学反应源项中,建立了包含D-L不稳定性效应的PaSR-LES燃烧模型,并研究了发动机缸内流场的湍流分形维数、涡团周转时间与粘性截止尺度等特征参数的内在联系,提出了湍流微混合时间尺度和湍流分形维数的函数表达式；基于电弧与火核跟踪-欧拉(AKTIM-Euler)方法,建立了适用于大涡模拟的火花点火模型,描述了以燃烧反应进程变量为权重的点火能量分配方式；搭建了低热值气体燃料发动机缸内燃烧的三维数值模拟仿真平台,提出了多面体顶点运动和分裂重构的动网格耦合算法,此方法解决了网格单元结构出现的偏斜度较大与负体积等问题；开展了燃用低热值气体燃料的定容弹湍流燃烧试验和发动机缸压测定试验研究,分别验证了本文的湍流燃烧模型和发动机缸内燃烧的数值模拟仿真平台。本文分析了湍流强度和无量纲马克斯坦长度特征参数对平面火焰锋面的位置和形态随时间变化的影响规律；通过低热值气体发动机工作过程的模拟计算,研究了进气与压缩过程中各阶段缸内大尺度拟序结构的演变规律,比较了不同发动机转速下缸内拟序结构的生成、发展以及耗散等过程；研究了从点火到初始火核形成的过程中火核半径等参数的变化历程和各发动机转速下涡对与火核相互作用的特征区域范围；分析了缸内涡团运动对各火焰面结构形态的作用；研究了D-L不稳定性效应作用下湍流火焰面结构的演化历程,分析了斜压扭矩对增加火焰面皱褶的作用等。由计算结果的分析可知：1.高强度涡团容易出现在远离燃烧室壁面的火焰自由发展区域,火焰锋面处的涡团有助于增大火焰面皱褶度,提高湍流火焰传播速率；涡对运动会对火焰面产生卷吸与拉伸的作用,从而促使火焰面上皱褶的产生。2.当低热值气体中惰性气体组分体积比增大时,火焰面皱褶度减小；低热值气体中掺混一定量氢气将有利于提高火焰传播速率,促进涡团运动对火焰面的作用,增大火焰面皱褶程度。3.D-L不稳定性会导致火焰锋面处产生斜压扭矩,此斜压扭矩将会增加火焰面皱褶程度；火焰面穿过湍流涡对,应变率随之被D-L不稳定性效应影响,其正负符号与曲率相同,火焰锋面的焰后已燃区逐渐出现与焰前未燃区中方向相反的涡团。图140幅,表14个,参考文献184篇。更多还原

【Abstract】 With the increasing prominence of energy and environment problems recently, the gas fuel has become the research focus around the world because of its cleanliness and sustainability. Therefore, the research and development of the gas engine is necessary. When the gas engine is fueled with the low calorific value gas (LCV gas), the combustion instability phenomenon become more and more serious. Therefore, the systematic study on the micro-evolution of the flame structure during the ignition and combustion process of a single engine cycle is necessary for the stability of the gas engine. Through the modeling of the vortex-flame interactions and flame intrinsic instability during the in-cylinder gas fuel’s premixed turbulent combustion, the unstable propagation mechanism of the flame can be elucidated and the theoretical basis for the development of the clean and efficient engine combustion system can be provided, which has high academic and engineering application value.Darrieus-Landau instability (D-L instability) phenomenon existed in turbulence disturbed plane flame propagation is studied. The Michelson-Sivashinsky equation forced with external turbulence (T-M-S equation) is solved. The fitting equation for the velocity increment of the plane flame propagation is formulated. Instability function is introduced into the chemical reaction source term of the species equation. The PaSR-LES combustion model with D-L instability is established. The relationships among the characteristic parameters such as the turbulence fractal dimension, the eddy turn over time, and the viscous cutoff scale are investigated. An improved expression for the turbulent micro-mixing time scale is proposed. Based on the arc and flame kernel tracking-Euler (AKTIM-Euler) method, the large eddy simulation model of the spark ignition is set up. The ignition energy distribution of the weight of the combustion process variable values is described. The three-dimensional numerical simulation platform for the low calorific value (LCV) gas fueled engine combustion is established. In order to deal with the engine dynamic mesh, a solution method is proposed to couple polyhedron vertex movement algorithm and Mesquite algorithm. This method can avoid the large skewness and the negative volume of the mesh cells. The experimental research on the constant volume vessel and the engine fueled with the LCV gas is conduced. The turbulent combustion model and the numerical simulation platform for the engine combustion in this paper are validated.The change rule of the plane flame front position and shape with respect to different turbulent intensity and dimensionless Makstein length is studied for the weak turbulent flame. Furthermore, the factors which influence the flame propagation increment are considered. Through the large eddy simulation on the working process of LCV gas fueled engine, the evolution of the large-scale coherent structures during the intake and compression stages is investigated. In addition, the generation, development, and dissipation of the coherent structure under different engine speed are compared. The development process of the flame kernel formation and the characteristic regime area of the flame kernel-vortex pair interactions under different engine speed are analyzed. The effect of the vortex-pair movement on the flame surface morphology is researched. The in-cylinder evolution of the turbulent flame surface with the D-L instability effect is studied. The effect of baroclinic torque on increasing the flame wrinkles is analyzed. It is shown from the simulation results that:1. The higher strength vortex mostly appears in the flame free propagation area. The vortex on the flame front is helpful to the increase of flame wrinkles and turbulent flame speed. The vortex pair produces entrainment and stretching effects on the flame surface, resulting in the formation of the wrinkles.2. With the increase of the volume fraction of inert species in LCV gases, the flame wrinkles decrease. A certain amount of hydrogen addition in LCV gases improves the flame propagation speed, promotes the flame-vortex pair interactions, increases the flame wrinkles.3. D-L instability generates the baroclinic torque which incrases the flame wrinkles. The flame strain rate which has the same symbol with the flame surface curvature is influenced by the D-L instability as the flame front passes through the vortex pairs. The vortex in the burnt area has the opposite symbols with the one in the unburnt area.更多还原