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Subgrid-Scale Modeling of Reaction-Diffusion and Scalar Transport in Turbulent Premixed Flames

机译：湍流预混火焰中反应扩散和标量传输的亚网格规模建模

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A numerical study of premixed flame-turbulence interaction is performed to investigate the effects of turbulence on the structural features of the flame and the subgrid-scale (SGS) effects on vorticity dynamics, energy transfer mechanism, and turbulent transport across the flame. We consider a freely propagating methane-air turbulent premixed flame interacting with a decaying isotropic turbulence under three different initial conditions corresponding to the corrugated flamelet (CF), the thin reaction zone (TRZ), and the broken/distributed reaction zone (B/DRZ) regimes. We employ the well-established linear eddy mixing (LEM) model in large-eddy simulation (LEMLES), a new subgrid closure for reaction-diffusion occurring in the small-scales based on LEM (RRLES), and a quasi-laminar chemistry based closure in large-eddy simulation (QLLES) to simulate flame-turbulence interactions and to compare predictions with direct numerical simulation (DNS). We assess the accuracy and robustness of the closures by comparing statistical features to highlight their abilities and limitations. The newly proposed RRLES subgrid closure uses a dual-resolution grid for solving the species transport equations. Such an approach is shown to improve the existing LEMLES subgrid model especially at low Reynolds numbers. All SGS closures reveals good agreement, although there are some differences due to the closure used for convective transport of the scalar field and the reaction rate. Further analysis of the DNS dataset shows that there is a significant contribution by dilatation and baroclinic torque terms across the flame. In particular, at higher Karlovitz number, there is an abrupt change in the sign of the dilatation term, which is related to the competing effects of thermal expansion due to heat release and enhanced molecular mixing by the turbulence across the flame brush region. The enhanced mixing leads to localized pockets of cold reactants surrounded by hot products, which is only partly captured by the employed closures. The analysis of SGS kinetic energy and scalar dissipation rates indicates the presence of back-scatter of turbulent kinetic energy, and we also observe counter-gradient transport across the flame. The results suggest that further improvement of the traditional closures is needed to accurately capture the dynamics of flame-turbulence interaction.
机译：进行了预混火焰与湍流相互作用的数值研究，以研究湍流对火焰结构特征的影响以及亚网格尺度（SGS）对涡旋动力学，能量传递机制和湍流在火焰中传输的影响。我们考虑自由传播的甲烷-空气湍流预混火焰与衰减的各向同性湍流在三种不同的初始条件下相互作用，分别对应于波纹小火焰（CF），稀薄反应区（TRZ）和破碎/分布的反应区（B / DRZ） ）制度。我们在大涡模拟（LEMLES）中采用了公认的线性涡旋混合（LEM）模型，基于LEM（RRLES）在小规模中进行了新的反应扩散的子网格封闭，并采用了基于层流化学的方法大涡模拟（QLLES）中的闭环模拟火焰湍流相互作用，并将预测结果与直接数值模拟（DNS）进行比较。我们通过比较统计特征以突出其功能和局限性来评估密封件的准确性和坚固性。新提出的RRLES子网格闭合使用双分辨率网格来求解物种迁移方程。事实证明，这种方法可以改善现有的LEMLES子网格模型，尤其是在雷诺数较低的情况下。尽管由于用于标量场的对流传输和反应速率的封闭而存在一些差异，但所有SGS封闭均显示出良好的一致性。对DNS数据集的进一步分析显示，整个火焰的膨胀和斜压扭矩项具有显着贡献。特别是，在较高的卡洛维兹数下，膨胀项的符号会发生突然变化，这与热释放的竞争效应有关，这是由于热量释放以及火焰刷区域上的湍流引起的分子混合增强所致。增强的混合导致冷的反应物的局部袋被热产物包围，其仅部分地被所采用的封闭物捕获。对SGS动能和标量耗散率的分析表明存在湍流动能的反向散射，并且我们还观察到了穿越火焰的反梯度传输。结果表明，需要进一步改进传统的密封件，以准确捕获火焰-湍流相互作用的动力学。

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