Abstract Turbulent non-premixed flames with local extinction and re-ignition exhibit multiple combustion modes including ignition waves, diffusion flames, partially premixed flames, and ignition-assisted partially premixed flames. The mechanisms of local extinction and re-ignition are not well understood and numerical modeling of multi-mode combustion is a challenging task. In this work, a specially designed swirl-burner was used to study local extinction and re-ignition of non-premixed turbulent methane/air flames. High speed Particle Image Velocimetry (PIV) and laser induced fluorescence of OH radicals (OH-PLIF) measurements along with Large Eddy Simulation (LES) were carried out to investigate the mechanisms of extinction and re-ignition processes in the burner. LES is based on a transported probability density function model within the framework of Eulerian Stochastic Fields (PDF-ESF). It is found that local extinction occurs when the scalar dissipation rate around the stoichiometric mixture fraction is high. The characteristic time scale for local extinction and re-ignition in the present flames is an order of magnitude longer than the characteristic time scale of diffusion/extinction of laminar flamelets. There are two mechanisms for flame hole re-ignition in the present flames. First, under low degree of local extinction conditions (i.e., for small flame holes surrounded by flames) the flame hole re-ignition is due to the mechanism of turbulent flame folding. Second, under high degree of extinction conditions (i.e., with large regions of extinction and lifted flames), re-ignition of the locally extinguished flame is due to the mechanism of ignition assisted partially premixed flame propagation. The results show that the PDF-ESF model is capable of simulating the quenching and re-ignition process found in the experiments.

中文翻译：

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LES/PDF 模拟涡流稳定非预混甲烷/空气火焰局部熄灭和重新点燃

摘要 具有局部熄灭和重燃的湍流非预混火焰表现出多种燃烧模式，包括点火波、扩散火焰、部分预混火焰和助燃部分预混火焰。局部熄灭和重新点燃的机制尚不清楚，多模式燃烧的数值建模是一项具有挑战性的任务。在这项工作中，一个专门设计的旋流燃烧器被用来研究非预混湍流甲烷/空气火焰的局部熄灭和重新点燃。进行了高速粒子图像测速 (PIV) 和 OH 自由基的激光诱导荧光 (OH-PLIF) 测量以及大涡模拟 (LES)，以研究燃烧器中的消光和重燃过程的机制。LES 基于欧拉随机场 (PDF-ESF) 框架内的传输概率密度函数模型。发现当化学计量混合分数周围的标量耗散率高时会发生局部消光。当前火焰中局部熄灭和重新点燃的特征时间尺度比层流火焰扩散/熄灭的特征时间尺度长一个数量级。目前的火焰中有两种火焰孔重新点燃的机制。首先，在低程度的局部熄灭条件下（即对于火焰包围的小火焰孔），火焰孔重新点燃是由于湍流火焰折叠的机制。其次，在高度灭绝的条件下（即有大面积的灭绝和火焰），局部熄灭的火焰的重新点燃是由于点燃辅助部分预混火焰传播的机制。结果表明，PDF-ESF 模型能够模拟实验中发现的熄火和重燃过程。