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The Large Eddy Simulation / Filtered Mass Density Function Approach Applied to High Pressure Turbulent Non-Premixed Flames

机译：大型涡模拟/滤波质量密度函数法在高压湍流非预混火焰中的应用

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摘要

Large Eddy Simulation (LES) coupled with the transported Filtered Mass Density Function (FMDF) approach has shown great potential in simulating turbulent flames. In the beginning of this work, we conduct a priori analyses of the conditional diffusion in LES/FMDF that focus on the velocity scalar mixing time scale ratio (CO,alpha) which is a widely used parameter in LES combustion modelling. The analysis is based on an existing database of direct numerical simulations (DNS) of high pressure temporally developing shear flames. The DNS includes generalized heat and mass diffusion, a real fluid equation of state (EOS), pressure and temperature dependent properties and detailed H 2/O2 and H2/Air kinetics. Results show CO,alpha for the high pressure hydrogen flames in this work is approximately 3. However, the optimal value is found to vary both for different species and for different locations within the flame. This is particularly evident for the light species (e.g. H and OH), for which CO,alpha is observed to be typically twice that of major species such as oxygen.;Then we extend LES/FMDF to simulate the DNS temporally developing reacting shear flames at high pressure (up to 100atm), and Reynolds numbers (up to 4500). Our LES/FMDF solver includes all of the high pressure physics of the DNS. The standard LES equations are solved on an Eulerian mesh while the FMDF equation is solved with the notional particle technique and is used to close the filtered chemical source terms for the LES. Reasonable agreement is found for the majority of first and second order flow statistics for both the velocity and scalar fields. We also conduct a comparative study of several conditional diffusion models used in current FMDF simulations, including the Interaction and Exchange with the Mean (IEM), Euclidean Mean Spanning Tree (EMST), and Modified Curl's model. The most recent and relatively untested Shadow Positioning model is also assessed at high pressure. All four models predict temperature and scalar fields fairly well; however, the EMST model provides the best overall results; particularly so for the H2/Air flames with their associated large levels of local extinction. The EMST model is also shown to be the least sensitive to the choice of the modeling constant relating the velocity scalar mixing timescale.;To further incorporate generalized diffusion in FMDF, an effective diffusion coefficient model is derived. The model is found to be able to accurately reproduce the exact generalized diffusion vector. With this effective diffusion coefficient, the counter-gradient diffusion (caused by different directions among the pressure gradient, temperature gradient, and other species gradients) is able to be captured. In addition, the consistency between FMDF and LES Eulerian transport equations is better obtained. A posteriori analyses show this model has significant improvement in minor species prediction in high pressure reacting flows.;In addition, the subgrid pressure is studied in a priori analyses. Results show the subgrid pressure and its gradient are significant in reacting flows at high pressure; especially in specific local regions (e.g. regions of large subgrid temperature fluctuation). Therefore, the subgrid pressure may need to be modelled. To close the subgrid pressure, a dynamic similarity subgrid pressure model is proposed. The model is tested by both a priori and a posteriori analyses. Results show the model can improve the estimation of the flow fields in LES.
机译：大型涡流模拟（LES）与运输的过滤质量密度函数（FMDF）方法相结合，在模拟湍流火焰方面显示出巨大潜力。在这项工作的开始，我们对LES / FMDF中的条件扩散进行先验分析，重点是速度标量混合时间标度比（CO，alpha），它是LES燃烧建模中广泛使用的参数。该分析基于高压瞬时发展的剪切火焰的直接数值模拟（DNS）的现有数据库。 DNS包括广义的热和质量扩散，真实的流体状态方程（EOS），与压力和温度有关的特性以及详细的H 2 / O2和H2 /空气动力学。结果表明，在这项工作中，高压氢火焰的CO，α约为3。但是，对于火焰中的不同物种和不同位置，最佳值均发生了变化。这对于轻质物质（例如H和OH）尤其明显，对于这些轻质物质来说，观察到的CO，α通常是主要物种（例如氧气）的两倍;然后，我们扩展LES / FMDF以模拟DNS暂时发展的反应性剪切火焰高压（最高100atm）和雷诺数（最高4500）。我们的LES / FMDF求解器包括DNS的所有高压物理。标准LES方程在欧拉网格上求解，而FMDF方程通过概念粒子技术求解，并用于关闭LES的过滤后的化学源项。对于速度场和标量场，大多数一阶和二阶流统计信息都可以找到合理的一致性。我们还对当前FMDF模拟中使用的几种条件扩散模型进行了比较研究，包括与均值的交互和交换（IEM），欧氏平均生成树（EMST）和Modified Curl模型。还在高压下评估了最新且未经测试的阴影定位模型。所有四个模型都很好地预测了温度场和标量场。但是，EMST模型提供了最佳的总体结果；特别是对于H2 / Air火焰及其相关的大量局部熄灭。还显示了EMST模型对与速度标量混合时间尺度有关的建模常数的选择最不敏感。；为了将广义扩散进一步包含在FMDF中，得出了一个有效的扩散系数模型。发现该模型能够准确地再现精确的广义扩散向量。利用此有效扩散系数，可以捕获反梯度扩散（由压力梯度，温度梯度和其他物种梯度之间的不同方向引起）。此外，FMDF和LES欧拉输运方程之间的一致性更好。后验分析表明，该模型在高压反应流中对次要物种的预测有显着改进。此外，对先网格分析也对亚网格压力进行了研究。结果表明，亚网格压力及其梯度在高压下的反应流中是显着的。尤其是在特定的局部区域（例如子电网温度波动较大的区域）。因此，可能需要对子网格压力进行建模。为了关闭子网格压力，提出了一种动态相似的子网格压力模型。通过先验和后验分析对模型进行测试。结果表明该模型可以改善LES中流场的估计。

著录项

  • 作者

    Ma, Zhiyuan.;

  • 作者单位

    Clemson University.;

  • 授予单位 Clemson University.;
  • 学科 Mechanical engineering.
  • 学位 Ph.D.
  • 年度 2015
  • 页码 214 p.
  • 总页数 214
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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