【作者】 王楚翘；

【导师】 罗福强；

【作者基本信息】 江苏大学 ， 动力工程及工程热物理， 2020， 博士

【摘要】 随着排放与油耗法规日益严苛,内燃机向着清洁、高效、节能的方向继续前进。而严格的排放法规对燃油供给系统提出了更高的要求,喷油压力可达250MPa左右甚至更高。高压共轨因其独特的技术优势已在柴油机领域成为主流,而喷油器作为连接上游高压共轨系统与下游燃油喷射的终端执行器,其关键性不言而喻。针阀偶件不仅承载着燃油润滑、密封与流通作用,更是作为燃油喷射的直接执行部件,决定喷油与喷雾特性,进而影响后续的油气混合质量与排放特性。针阀偶件由针阀与针阀体组成,随着高压共轨喷油压力的持续升高,高压燃油在喷嘴内（由针阀偶件、压力室等组成）的流动不仅存在高雷诺数的瞬时湍动,还伴随着由缩脉、流体涡旋运动等引起的空化现象,形成复杂的气液两相流动,进而影响喷孔出口的燃油雾化质量。基于此,本文从针阀瞬态运动的角度出发,分析了针阀规则运动（不考虑偏心）与不规则运动（考虑偏心）对各孔内部流动及喷射特性的影响,发展了考虑燃油可压缩性的喷嘴内气液两相跨声速流动模型,探究了针阀弹性变形与偏心运动的影响因素,为优化喷油器设计与制造提供了理论基础。论文主要研究内容如下:（1）基于动量守恒定律以及伯努利定理,设计并搭建了基于喷雾动量流测试原理的各孔瞬态喷油特性测试系统。该测试系统可同时采集多孔喷油器的各孔瞬时喷油速率,并实现多个喷油持续期的循环采集,对测试系统做了精度和可靠性分析。（2）基于开源C++类库Open FOAM^?平台,在rho Central Foam求解可压缩跨声速流动求解器的基础上,开发并植入了求解喷嘴内部可压缩气液两相流动的代码。该求解器采用均相流模型求解喷嘴内的多相燃油流动,并通过正压状态方程求解两相相变。由于标准RNG k-ε湍流模型对可压缩流动的求解尤其是低压区空化现象的捕捉不够精确,通过引入一个与空化状态相关的密度函数来代替恒定的密度,修正湍流模型。此外,在求解可压缩流体的欧拉方程时,由于方程具有强双曲的数学特征,通常存在间断解,即黎曼问题。本文采用近似黎曼求解器对网格数值通量做处理,以捕捉间断解,即可压流体的激波现象。利用该模型分析了针阀运动对某双层喷孔结构喷油器喷嘴内部空穴流动的影响,并利用实测各孔喷油规律验证了建立的CFD模型。模拟结果表明该模型可以准确的捕获喷嘴内周期性发展的空化与湍流拟序结构,且上层喷孔的空化发展较下层喷孔更剧烈。（3）喷孔内空穴流动对喷孔外的射流雾化有重要影响,建立了多孔喷油器喷嘴内部空穴流动的欧拉-欧拉双流体模拟方法与喷孔出口的拉格朗日燃油离散液滴喷雾模拟方法的耦合模型。该模型将求解连续相与求解离散相的两种截然不同的数值解法间接耦合起来,有效地分析了内部流动与燃油雾化的相关性。通过搭建的各孔喷油特性测试台架与基于纹影法的喷雾试验台架,分别获得了各孔的喷油速率与循环喷油量等喷油特性,以及各孔喷雾贯穿距等宏观喷雾特性,并对所建立的数值计算模型进行了验证。结果表明,模拟与试验值吻合良好,下层喷孔的喷油速率与喷雾贯穿距均比上层喷孔大,且下层喷孔空穴发展较弱。（4）基于材料力学、弹性力学与线弹性理论,将针阀视作悬臂梁,利用梁的弯曲变形理论计算针阀头部的变形量,并基于针阀的受力分析建立针阀有弹性变形时偏心运动的数学模型,探究针阀偶件上游处导向配合间隙变形量与下游处针阀头部偏心量之间相差一个数量级的原因。借助于环形缝隙流动理论与流固耦合模拟方法,分析了针阀偏心运动的影响因素。研究发现,针阀的偏心量随着针阀长度与针阀导向长度之比线性变化。此外,当喷油压力升高时（尤其大于200MPa时）,针阀的偏心量也随之迅速增加。（5）基于建立的针阀偏心数学模型,通过计算得出的偏心量数值建立了针阀有弹性变形时的偏心运动对各孔流动与喷油特性影响的三维CFD模型。模拟结果表明,针阀偏心运动导致各孔流动与喷油特性产生显著差异,且针阀偏向侧喷孔的喷油速率与循环喷油量在同层喷孔中均为最大。此外,针阀偏向侧喷孔的空化均在喷孔下表面处产生,且随着针阀的抬起逐渐转移到上表面。更多还原

【Abstract】 As the regulations of emissions and fuel consumption are becoming more strigent,internal combustion engines continue self-improvement,moving towards more clean,efficient and energy-saving.The fuel supply and injection system,which injection pressure could reach about 250 MPa or even higher is required to fulfill the increasingly strict emission regulations.Consequently,the common rail technique has become the mainstream in diesel engines because of its unique technical advantages,where the injector,as the final actuator connecting upstream high pressure common rail system and downstream fuel injection,is critical.The needle valve couple,being composed of needle valve and needle valve body,not only bears the role of fuel lubrication,sealing and circulation,but also acts as the direct execution components of fuel injection,determining the fuel injection and spray patterns.Hence,the subsequent combustion process,power performance and emissions will be affected.As the injection pressure going higher,the fuel flowing within the nozzle（is consist of needle valve pair,SAC,et al.）is not only accompanied by the instantaneous turbulence with high Reynolds number,but also the cavitation caused by vena-contraction and vortex flow.As a result,the high pressure fuel within the nozzle becomes the complex vapour-liquid two-phase flow,affects the atomization of the spray jet at the nozzle outlet.Based on the above,the impact of needle transient moving in terms of the regular motion（non-eccentric）and irregular motion（eccentric）on the internal flow and injection characteristics of each nozzle hole were the main focus of the paper.Besides,a vapour-liquid mixture model considering the compressibility of the fuel was developed to predict the cavitation within the nozzle.Last but not the least,the influence factors of needle valve elastic deformation and eccentric motion was explored,which provides a theoretical basis for optimizing the design and manufacture of the injector.The main research contents of the thesis are as follows:（1）A measurement system for transient injection characteristics of each nozzle hole was designed and built based on the momentum conservation and Bernoulli’s theorem.The measurement system could acquire the instantaneous injection rates of each nozzle holes in the multi-hole injector simultaneously,and realize the cyclic acquisition of multiple injection durations in the meantime.Finally,the accuracy and reliability of the test system are validated.（2）The code solving the compressible vapour-liquid two-phase flow in the nozzle was developed and implemented based on the rho Central Foam in Open FOAM^?,an open source C++platform.The solver utilizes the homogeneous equilibrium model to solve the multiphase flow within the nozzle,while the phase change is predicted by the barotropic equation of state.Since the standard RNG k-εturbulence model is not accurate enough for solving the compressible flow,especially for capturing the vortex shedding,a density function related to cavitation state is introduced to replace the constant density,and thus the turbulence model is modified.In addition,due to the strong hyperbolic mathematical characteristics of Euler equation for compressible flow,there is the Riemann problem which usually has discontinuous solutions.In this thesis,the approximate Riemann solver is used to deal with the numerical flux of the grid to capture the shock wave in the compressible flow.The established CFD model was validated by the measured injection rates of each nozzle holes and used to analyze the influence of transient needle movement on the internal cavitating flow with a double-layered hole nozzle.The numerical results indicated that the model could capture the periodic vortex shedding and turbulent coherent structure within the nozzle accurately,and the cavitation development in the upper layered hole is more acute than that of the lower layered hole.（3）The cavitation flow within the nozzle has a critical impact on the atomization of spray jet at the exit of the nozzle hole.The coupling model of the Euler-Euler two-fluid simulation method for the cavitation flow in the nozzle and the Lagrangian discrete droplet spray simulation method at the nozzle outlet of the multi-hole injector is established.This model indirectly coupled the two completely different numerical methods for solving continuous phase and discrete phase respectively,which could analyze the correlation between internal flow and fuel atomization effectively.The established CFD model was validated through the measurement system of the injection characteristics for each hole and the spray test bench based on the schlieren method.To be specific,the injection characteristics such as injection rates,cycle fuel injection quantities and et al.,were measured together with the macroscopic spray patterns in terms of spray penetrations of each orifices.Afterwards the results between the experiment and simulation were compared to validate the model.The results show the good consistency between the experimental and numerical results.In addition,the injection rates,cycle fuel injection quantities and spray tip penetration of the lower layer nozzle holes were larger than those of the upper layer nozzle holes.The acuteness of the upper layer nozzle holes contributed to the formation of a higher degree of cavitation development in them as compared to the less acute lower layer nozzle holes.（4）The mathematical model of elastic deformation and eccentric movement of the needle within the nozzle is established based on the theory of material mechanics,elastic mechanics and linear elasticity.The model successfully built the relationship between the deformation of the needle valve couple in the upstream and the eccentricity of the needle tip in the downstream.By means of the theory of flow in the annular clearance and fluid-structure interaction（FSI）numerical method,the impact factors of needle eccentric motion were analyzed.The study found that the eccentricity of needle valve changed linearly with the ratio of needle valve length to needle valve guide length.In addition,as the fuel pressure increases to 100MPa,especially more than 200MPa,the total eccentricity increases remarkably,and the changes rate of the eccentricity increases with the growth of fuel pressure.（5）Based on the mathematical model of the needle eccentric moving mentioned above,the calculated eccentric magnitude was used as a boundary condition to analyze the effects of needle eccentricity together with elastic deformation on the internal flow as well as injection characteristics through the three-dimensional CFD model.The numerical results showed that the eccentric motion of the needle valve is one of the most important reasons for the discrepancies in the flow and injection characteristics among each nozzle hole.When needle is oriented at one orifice,the injection rate and cycle fuel injection quantity are the largest among the same layer compared with other orifices.In addition,when needle orients at one nozzle hole,the cavitation is primary developed at the lower surface of the orifice,and gradually moved to the upper surface as the needle valve lifts.更多还原