高雷诺数湍流预混火焰的结构表征及湍流与燃烧耦合机理研究

Turbulent premixed flames at high Reynolds number frequently occur in advanced combustion engines. Motived by the key scientific problem of “complicated turbulence-combustion interactions in confined space” in the major research plan, we propose a fundamental investigation in premixed turbulent flames at high Reynolds number and moderate Karlovitz number, which is aimed to develop numerical diagnostic tools and predictive models for turbulent combustion in the engine combustor under high turbulent intensity and a broad range of pressures. We first set up a series of data base using direct numerical simulation for hydrogen/methane/dimethyl ether-air premixed turbulent flames primarily in the thin reaction zone. The flame configurations include the propagating premixed flame in isotropic turbulence and premixed jet flames. The topology and non-local geometry in the evolution of the flame front can be characterized by Lagrangian-based, multi-scale analysis. The quantified results are then used in subsequent studies on turbulence-combustion interactions. The important issues to be addressed include the effects of the high stretch rate, differential diffusion, and low-temperature chemical reaction on the premixed flame and local extinction, and the effects of significant variations of density and viscosity cross the flame front on the statistical anisotropy and small-scale vortical structures in neighboring fluid flows. Finally, we develop the model of turbulent flame speed considering the differential diffusion, low-temperature chemical reaction and local extinction, and the subgrid scale model for the large-eddy simulation/probability density function method based on the local turbulent flame speed. In this proposed research, combustion theory on turbulent premixed flames will be extended and novel numerical diagnostic tools and predictive models will be developed for high-Reynolds number turbulent premixed combustion in the engine combustor.

高雷诺数湍流预混燃烧是先进发动机燃烧的共性问题。针对重大研究计划中“受限空间内复杂湍流和燃烧的相互作用”这一核心科学问题，本项目对发动机燃烧室中高湍流度、宽压力范围内的湍流预混火焰进行基础研究，发展高雷诺数与中等Karlovitz数下湍流预混燃烧数值诊断方法与可预测模型。本项目采用直接数值模拟建立主要处于薄反应区燃烧模式下不同燃料自由传播与射流火焰的湍流预混燃烧数据库，发展量化湍流预混火焰结构演化和几何拓扑性质的多尺度表征方法，揭示高雷诺数湍流预混火焰中高拉伸率与差异扩散影响燃烧速率和局部熄火的规律，确立火焰面前后密度与粘度变化对临近流体局部各向异性与小尺度涡结构的影响机理，发展考虑差异扩散、局部熄火及低温化学反应的湍流火焰速度模型，并借此构建大涡模拟/概率密度函数方法的亚格子模型。本项目旨在完善湍流预混燃烧理论，并为发动机内高雷诺数湍流预混燃烧提供新的数值诊断方法和预测模型。

高雷诺数湍流预混燃烧是先进发动机燃烧的共性问题。针对重大研究计划中“受限空间内复杂湍流和燃烧的相互作用”这一核心科学问题，本项目对发动机燃烧室中高湍流度的湍流预混火焰进行基础研究，发展高雷诺数与中等Karlovitz数下湍流预混燃烧数值诊断方法与可预测模型。本项目在以下四方面研究内容中取得重要结果。（1）高湍流度湍流预混火焰直接数值模拟：开发了两套分别针对低马赫数与可压缩流的湍流燃烧计算程序，并已应用于后续燃烧机理分析与燃烧模型验证工作。（2）湍流预混火焰的精细结构定量表征：发展了自传播片元模型与火焰粒子方法定量化研究预混火焰结构；拓展了涡面场方法以定量化研究湍流燃烧中的涡结构，包括其几何变形与拓扑变化。（3）湍流预混火焰中湍流与燃烧的耦合作用：设置了两类新算例，尽量解耦湍流与燃烧这两个强非线性过程。第一类算例中重点分析湍流对火焰的影响，发现火焰结构在不同雷诺数下存在自相似性。第二类算例重点分析火焰对湍流的影响，发现已燃区中涡面融合为大尺度涡结构，而在未燃侧涡面则卷并为拉伸扭曲的小尺度涡管。（4）湍流预混火焰速度及大涡模拟/概率密度函数亚格子建模：提出了基于自传播片元拉格朗日统计量的湍流预混燃烧速度模型，可在宽湍流度与压力范围内准确预测湍流燃烧速度，并考察了Lewis数和低温化学反应对湍流火焰速度的影响。全面研究了概率密度函数方法中的混合模型，包括分子输运项建模、差异扩散建模、混合模型形式和混合速率建模等混合建模策略对湍流射流火焰中温度、组分浓度等重要物理量统计预测的影响。