【作者】 周磊；

【导师】 解茂昭；

【作者基本信息】 大连理工大学 ， 工程热物理， 2010， 博士

【摘要】 精确预测燃料在发动机中的喷射雾化过程和喷雾结构是预测发动机燃烧过程、工作性能和排放特性的一个重要前提。作为一种先进的湍流模拟方法,大涡模拟(LES)近年来在许多学科领域和工程部门都得到了应用。但国内关于内燃机缸内流动及燃烧的大涡模拟迄今尚鲜有报道。作为该领域初步的探索,本文将大涡模拟湍流模型应用到定容弹和柴油发动机中的燃油喷射与雾化过程,同时对多孔介质发动机的湍流流动和燃烧过程进行多维模型的计算与分析。本文主要完成了以下工作。(1)通过在发动机通用CFD软件KIVA-3V中加入大涡模拟计算程序模块,对固定容器及发动机内的燃油喷雾和流场特性进行模拟研究,同时以实验结果作为参照,与RANS(RNG k-ε)模型进行了对比分析。分析了喷射压力对液相贯穿度,油滴平均直径、气相速度场和湍能分布,燃油蒸发率等参数的影响。研究表明采用大涡模拟方法可以捕捉到喷雾场中湍流涡团的随机性的三维复杂结构。同时提高喷射压力,可以显著增大喷雾贯穿度,减小液滴的平均索特直径,同时压力的提高也增大了气相湍动能,促进燃油的蒸发。(2)亚网格尺度模型是影响大涡模拟结果的一个关键因素。本文在K-方程亚网格湍动能模型的基础上,对其它比较常用的三种代数模型(Smagoringsky模型,动态Smagoringsky模型和WALE模型)在燃油喷雾中的适用性做了探讨。在定容弹中采用三个不同的燃油喷雾实验作为数值模拟的验证对象,分析了喷雾的演变过程。从本文的结果中可以看出,三个代数亚网格模型在蒸发和非蒸发过程中喷雾的演变过程基本与实验相符,液相贯穿距也与实验接近。(3)基于一方程亚网格湍动能模型的大涡模拟程序,对一台Capelliar 3401柴油发动机缸内燃油喷雾、混合及燃烧进行了数值模拟。计算结果与RANS(RNG k-ε)模型结果及实验进行了比较分析,同时讨论了LES结果对网格的敏感性。LES计算得到的液滴和蒸汽相贯穿长度大于RANS结果,亚网格湍动能和湍流粘性则明显小于RANS结果。并发现初始涡流比明显影响喷雾贯穿距和燃烧室中的蒸汽相分布。(4)应用大涡模拟方法和不同的雾化模型研究了亚网格尺度湍动能,特别是燃油喷雾所诱导的亚网格湍能源项对液滴运动和喷雾特性的影响,并进一步探讨了亚网格气相湍动能对液滴弥散速度乃至喷雾场的影响。计算结果表明,KH-RT破碎模型要优于MTAB模型,湍流弥散效应和亚网格湍动能喷雾源项对燃油喷雾有着重要的影响,其作用均是减少喷雾贯穿距,使模拟结果更接近于实验,其中湍流弥散效应的影响更为显著。(5)鉴于多孔介质燃烧技术是一项很有发展潜力和应用前景的新技术,本文采用多维CFD方法对多孔介质(PM)发动机的燃烧和工作过程机理进行了初步的探索,并对不同布局方案和参数下PM发动机的工作过程做了循环模拟。针对Ferrenberg所提出的回热式发动机模型,用二维模型探讨了在不同当量比和孔隙率条件下的燃烧和排放特性。结果表明,回热式多孔介质发动机在燃烧和排放方面较之常规发动机都有明显的优势。采用低当量比能有效地降低污染物的排放,而孔隙率的影响则与当量比有关。本文的分析证明,采用多孔介质回热器是发动机实现稀薄燃烧的一条可供选择的途径。对活塞顶置式(活塞顶凹坑燃烧室内嵌入多孔介质的)发动机的燃烧与排放特性进行了二维数值模拟,并与未加多孔介质的常规发动机进行了对比。着重讨论了多孔介质固体温度的变化规律及其影响。同时分析了发动机燃烧与排放的主要影响因素,如孔隙率,多孔介质热物性,发动机的转速和当量比等。对多孔介质发动机的燃烧特性采用大涡模拟进行了初步分析。首先计算了考虑多孔介质随机结构特性的定容燃烧室内气体燃料喷射过程,并与自由空间中的喷射过程进行了对比。多孔介质的存在增强了湍流涡团的小尺度结构,明显改变了燃料的空间分布,而采用LES方法得到的气体喷射流场较之RANS具有更强的不规则度。最后采用大涡模型对两种结构形式的多孔介质发动机的燃烧过程进行了初步的计算分析。更多还原

【Abstract】 An important precondition for accurate simulating the combustion process as well as working and emissions characteristics of the internal combustion (IC) engine is correctly predicting the fuel-air mixture formation and spray structure in the engine. As an advanced method for turbulence modeling, the large eddy simulation (LES) has been applied to various disciplinary and engineering fields recently. However, little has been reported on the application of LES to the in-cylinder flow and combustion of IC engines, especially rarely in China. As a preliminary study on the LES approach, in this paper a LES model is applied to the numerical computation of fuel sprays in a constant volume vessel and a diesel engine. Meanwhile, computations and analysis of the turbulent flow and combustion process in porous media (PM) engines are performed with a multi-dimensional model. Research work completed in this thesis is as the following.(1) The LES approach was implemented in the general engine CFD software KIVA-3V, which was employed for numerical computation of in-cylinder flows and fuel sprays in a constant volume vessel and in a Caterpillar 3400 series diesel engine. Computational results are compared with those obtained by a RANS (RNG k-ε) model as well as with experimental data. The influence of injection pressure was researched on relevant parameters, i.e. the spray penetration, the droplet SMD, the velocity and turbulent kinetic energy of the gas phase and the fuel evaporation rate. Computational results show that three dimensional, complex and stochastic turbulent eddy structures in the spray field can be captured by using the LES method, and that higher injection pressure can enhance prominently the spray penetration, reduce droplet sizes and accelerate fuel evaporation.(2) Sub-grid turbulence model is a key factor affecting LES computation results. Besides the one equation subgrid model (K-equation model), the applicability of three common algebraic subgrid models (Smagorinsky model, dynamic Smagorinsky model and WALE model) to fuel spray in IC engines were analyzed. Three different experiments on the evolution process of fuel sprays in constant volume vessels were used to validate the numerical simulations. The results show that the predicted evolution of the liquid and vapor phases in the spray by the three sub-grid models are generally in agreement with the experiment data, and the predicted penetrations are close to the measurement. (3) Based on the K-equation sub-grid turbulent kinetic energy model, the LES approach was employed for numerical computation of fuel sprays and combustion in a Caterpillar 3400 series diesel engine. Computational results are compared with those obtained by a RANS (RNG k-ε) model as well as with experimental data. The sensitivity of the LES results to mesh resolution is also discussed. The results show that LES generally provides flow and spray characteristics in better agreement with experimental data than RANS; and that small-scale random vortical structures of the in-cylinder turbulent spray field can be captured by LES. Furthermore, the penetrations of fuel droplets and vapors calculated by LES are larger than the RANS result, and the sub-grid turbulent kinetic energy and sub-grid turbulent viscosity provided by the LES model are evidently less than those calculated by the RANS model. Furthermore, it is found that the initial swirl significantly affects the spray penetration and the distribution of fuel vapor within the combustion chamber.(4) The LES approach and different breakup models were used to analyze sub-grid turbulent kinetic energy, especially the source term induced by the fuel spray, on the droplet movement and spray characteristics. Furthermore, the effects of the subgrid kinetic energy on the turbulent dispersion of droplets as well as on the fuel spray are examined. Computational results show that the KH-RT breakup model is superior to the MTAB model, and both the subgrid turbulent energy source term induced by the spray and the turbulent dispersion of the droplets have important effects on the fuel spray, resulting in decreased spray penetration. The effect of the turbulent dispersion is more prominent.(5) Since the porous media (PM) combustion technology is a promising and advanced combustion technique, in this thesis, a preliminary exploration of the combustion mechanism and working process in PM engine was carried out for different configurations and work parameters by a multi-dimensional simulation method.The characteristics of combustion and emission of the regenerative PM engine proposed by Ferrenberg were computed and analyzed using two dimension models under different equivalence ratios and porosities. Results show that the regenerative engine has advantages in both combustion efficiency and pollution emissions over conventional engines, and that using lower equivalence ratios can reduce emissions significantly, while the effect of the porosity is dependent on the equivalence ratio used. The regenerative PM engine is an effective alternative for realizing lean homogeneous combustion.A two-dimensional model for combustion and emission characteristics in the PM engine with a PM insert mounted onto the piston head is presented. Comparisons with the traditional engine without the PM insert were conducted. Temperature evolution of the PM and its effects are discussed with emphasis. Relevant influencing factors on combustion and emission of the PM engine, such as porosity, the initial PM temperature, engine speed and equivalence ratio, are analyzed.A preliminary LES Analysis of the the combustion characteristics of the PM engine was conducted. First, the gas fuel injection process in a constant volume vessel was computed by a PM model taking into account the random structure of porous media, and compared with that in the same vessel but without PM. It is demonstrated that the small-scale structure of turbulent eddies is enhanced and the fuel distribution is modified due to the presence of the porous media. Compared to the RANS model, the flow field due to gas injection obtained by LES is more irregular. Finally, a preliminary calculation and analysis of the combustion process in two PM engines with different configurations was performed by using the LES model.更多还原