EAGER: Experimental investigation of physical-space scalar structure and unresolved mixing to improve large-eddy simulation of turbulent combustion

本项目研究了燃烧过程中的一个关键过程——湍流混合。湍流燃烧在喷气发动机、燃气涡轮发电机、内燃机和化学加工厂等许多工程应用中都有发生，并发挥着重要作用。这种装置的现代设计过程需要采用数值模拟来评估设计选择和优化设计参数。然而，目前最先进的建模方法，大涡模拟（LES），尽管几十年的研究努力，仍然没有达到先进工程设计所需的精度，主要是因为未解决的混合过程没有很好地预测。以前理解混合的努力在一定程度上受到主流LES方法的阻碍。本研究将使用s.b. Pope开发的新颖的LES方法作为框架来研究混合，该方法克服了传统方法的局限性。然而，新方法在实验技术和数据分析方法上也提出了挑战。研究人员将采用一种新的激光诊断技术和新的统计分析方法。该研究将极大地促进对湍流燃烧基础物理的理解，这将有可能使燃烧模型的发展具有足够的精度，从而设计高性能和低排放的工程设备，这将对社会和环境产生积极的影响。对物理学的理解也将有利于其他涉及湍流和强非线性现象之间相互作用的研究领域，如化学工程过程、大气物理、大气化学和环境系统。该项目利用最近开发的自条件场LES方法（Pope 2010）作为框架，研究了物理空间结构和未解决的混合以及它们与化学的相互作用，该方法克服了过滤LES方法的局限性。研究人员将在湍流非反应性同轴射流和射流火焰中进行实验。二维图像将在同轴射流中获得。将采用一种新的基于光解离的诊断技术和新的统计分析方法。悉尼火焰的直接数值模拟（Thorsten et al. 2019）包含3D图像，将补充实验。研究人员将分析物理空间标量结构及其在自条件联合PDF中的表示及其输运方程中的不闭合混合项。研究结果为研究导湍流部分预混（Sandia）火焰中混合分数和温度的混合提供了依据。研究人员将分析物理空间标量结构对自条件标量JMDF和不闭合混合项的影响。该项目是第一个使用新颖的自条件LES方法作为框架的项目。这也是第一次在没有重大妥协假设的情况下获得湍流碳氢化合物火焰中具有可分辨耗散尺度的混合分数和温度的真实二维图像。这些结果有望显著促进对未解决的物理问题的理解，这对于开发能够准确预测多标量混合和湍流-化学相互作用的改进混合模型至关重要。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This project investigates a key process, turbulent mixing, in combustion. Turbulent combustion occurs and plays an important role in many engineering applications, such as jet engines, gas turbine generators, internal combustion engines, and chemical processing plants. The modern design process of such devices needs to employ numerical modeling to evaluate design choices and to optimize design parameters. However, the current state-of-the art modeling approach, large-eddy simulation (LES), despite decades of research efforts, still does not have the accuracy needed for advanced engineering design, primarily because the unresolved mixing process is not well predicted. Previous efforts to understand mixing had been partially hampered by the predominant LES approach. This research will investigate mixing using the novel LES approach developed by S. B. Pope as the framework, which overcomes the limitations of the traditional approach. The new approach, however, also poses challenges in experimental techniques and data analysis methods. The researchers will employ a new laser diagnostic technique and new statistical analysis method. The research will significantly advance the understanding of the fundamental physics in turbulent combustion, which will potentially enable development of combustion models with sufficient accuracy for designing high-performance and low emission engineering devices, which will have a positive impact on society and the environment. The understanding of the physics gained will also benefit other research areas involving interactions between turbulence and strongly nonlinear phenomena, such as chemical engineering processes, atmospheric physics, atmospheric chemistry, and environment systems.This project investigates the physical-space structure and the unresolved mixing as well as their interaction with chemistry using the recently developed self-conditioned fields LES approach (Pope 2010) as the framework, which overcomes the limitations of the filtering LES approach. The researchers will conduct experiments in turbulent non-reactive coaxial jets and in jet flames. Two-dimensional images will be obtained in coaxial jets. A new photo-dissociation-based diagnostic technique and new statistical analysis method will be used. Direct numerical simulation of a Sydney flame (Thorsten et al. 2019), which contains 3D images, will complement the experiments. Researchers will analyze the physical-space scalar structure and its representation in the self-conditioned joint PDF and the unclosed mixing terms in its transport equation. The results provide a basis for investigating the mixing of mixture fraction and temperature in piloted turbulent partially premixed (Sandia) flames. The researchers will analyze the effects of the physical-space scalar structure on the self-conditioned scalar JMDF and the unclosed mixing terms. The project is the first to use the novel self-conditioned LES approach as the framework. It is also the first time true two-dimensional images of mixture fraction and temperature with resolved dissipation scales are obtained in turbulent hydrocarbon flames without major compromising assumptions. The results are expected to significantly advance the understanding of the unresolved physics essential for developing improved mixing models capable of accurately predicting multiscalar mixing and turbulence-chemistry interaction.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.