内燃机循环间性能波动机理的大涡模拟研究

In the multidimensional simulation of internal combustion (IC) engines, large eddy simulation (LES) has the potential to improve the accuracy of simulation and perform more extensive study compared to Reynolds Averaged Navier Stokes (RANS) modeling approaches. Since complex phenomena such as spray and combustion exist in the cylinder, the theory of engine simulation using LES should be in the context of very large eddy simulation (VLES) to reduce the requirement on grid resolution. In this case, complex subgrid scale models for turbulence, spray, and combustion are required. For the subgrid turbulence effects, subgrid scale stress models based on the transport equation of subgrid kinetic energy need to be used. The combustion simulation in LES mostly applies the models that have been successfully used in RANS, but an adaptation of the models is usually required for such application. Specifically, for the combustion models, a subgrid turbulence timescale is usually needed from the turbulent flow field to properly consider the interaction between turbulence and combustion. In this project, the ways to obtain various forms of subgrid turbulent timescales based on the subgrid turbulent kinetic energy will be first studied. The liquid spray also can have strong interactions with the small turbulence structures at the subgrid scale. For the Eulerian-Lagrangian approach of liquid spray simulation, this project tries to find a correct expression for subgrid velocity at the drop location to evaluate the momentum and energy exchange between the drops and gas phase. Besides that, methods to evaluate the effect of the subgrid turbulence intensity on the vaporization, breakup, and collision of liquid drops will also be investigated to reduce the dependency of results on grid resolution. Since the combustion in IC engines does not easilly fall into the category of premixed or diffusion mode and may occur in multiple modes, the idea of multimode combustion simulation will be used in the simulation of the combustion processes. The key of multimode combustion simulation is to designite appropriate parameters to identify the mixing state of fuel-air mixture and predict the swiching between the modes. Values of the parameters, at which the swiching happens, are also of great interest. The subgrid scale models obtained in this project will be used to perform the large eddy simulation of IC engines in multiple consecutive cycles. Since open boundary conditions are involved in the multicycle simulation and the boundary flow perturbation can significantly affect the combustion, methods to correctly evaluate the perturbations on the inflow and outflow boundaries will be developed. The goal of the multicycle engine simulation is to study the mechanism of several factors, for example, the change in the flow field near the injector nozzle or spark plug, causing the cycle-to-cycle variabilities of the combustion processes and overall performance of IC engines.

本项目拟在极大涡模拟的理论框架内，通过研究更为细致和复杂的亚网格模拟模型，提高气缸内的流动、喷雾和燃烧的数值模拟的准确度并减少大涡模拟计算对于网格密度的依赖，最终完善内燃机的大涡模拟理论。针对燃烧的模拟着重讨论根据亚网格湍动能为燃烧模型给出亚网格湍流时间尺度的方法，以准确考虑湍流与燃烧化学反应在亚网格尺度的相互作用。对于喷雾的模拟则重点研究气相在液滴当地位置的亚网格速度的具有统一物理意义的表达方法，用于衡量液滴和气相在亚网格尺度的动量和能量交换，同时探讨亚网格湍流对于液滴蒸发、碰撞和破裂的影响机理及相关计算方法。尝试利用多燃烧模式的思想模拟内燃机的燃烧，即对燃料-空气混合情况所决定的具体燃烧模式采用对应的燃烧模型，并重点研究计算燃烧模式之间转换的方法。在此基础上，对直喷汽油机或柴油机进行多循环的数值模拟，研究非稳态开放边界条件的给出方法以及多种因素造成内燃机循环间燃烧过程和性能波动的机理。

柴油机气缸内流动及喷雾燃烧的数值模拟是研究其性能的重要技术手段。由于大涡模拟较雷诺平均能够有效改进湍流数值模拟的精度，本项目组在极大涡模拟的理论框架内，围绕柴油机气缸内液态燃料的喷雾和燃烧的大涡模拟，开展了一系列研究工作。首先，针对喷雾的模拟，研究了液滴所在位置气相亚网格速度的表述方法，用于计算液滴和周围气体在亚网格尺度上的动量和能量交换，并将其体现在亚网格湍动能输运方程的源项中。在KIVA-4程序中植入了常系数、动态结构等亚网格模型，研究了柴油喷雾过程的不同模型的大涡模拟结果对于网格密度的依赖，分析了模拟结果较雷诺平均方法的改进。在此基础上，项目组在程序中进一步植入基于正庚烷的复杂燃烧反应机理的燃烧模型，并与上述多种亚网格模型实现了耦合，对恒定容积燃烧室内的柴油喷雾燃烧进行模拟。结果表明，所得碳烟和温度的分布相对于雷诺平均结果呈现明显的随机性和不规则性，其中动态结构模型的结果较好地与实验数据吻合。其次，探讨了高精度数值格式应用于KIVA的大涡模拟。采用ROE通量差分法结合MUSCL的高阶迎风格式，对简单几何形状计算域内的气体流动进行了模拟和分析，讨论了高阶计算格式对模拟结果的改进。再次，考虑到柴油由200~300种碳氢化合物混合而成，项目组基于连续热力学方法对其建立多组分蒸发模型，在液滴表面采用拉乌尔定律计算气-液相平衡，所得模型应用于单个液滴蒸发的模拟，结果与实验数据很好地吻合。另外，考虑到柴油机实际工作条件下较高的缸压，进一步探索基于高压条件下的相平衡的多组分蒸发模型。最后，针对柴油机建立了包含气缸盖和活塞的流-固耦合计算网格，并进行了多循环的数值模拟研究。为提高计算效率，利用信息传递接口（MPI）对程序进行并行化处理，所得并行KIVA-4程序可以有效地减少柴油机数值模拟的计算时间。项目资助发表SCI论文1篇，中文核心1篇，会议论文1篇，此外还有一篇SCI论文在审。培养博士生1名，硕士生1名。