Large eddy simulation of impacted obstacles' effects on premixed flame's characteristics
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摘要: 障碍物在预混气体火焰传播过程中对其结构及传播特性造成较大影响，对火焰的加速和爆燃转爆轰过程(deflagration-to-detonation transition, DDT)起到直接的促进作用。通过障碍物条件下可视管道中甲烷/空气预混火焰传播实验，捕获其火焰微观结构变化。采用三维物理模型，采用壁面自适应局部涡黏模型(wall-adapting local eddy-viscosity, WALE)的大涡模拟(large eddy simulation, LES)，并用火焰增厚化学反应模型(thickened flame model, TFM)对实验过程进行重现。分析开口管道中预混火焰翻越障碍物后的复杂流场变化，并分析层流向湍流转变过程的特点。揭示了在障碍物影响下预混火焰扰动失稳现象的直接原因，是由障碍物引发的3个气流涡团同时作用而形成Kelvin-Helmholtz不稳定及Rayleigh-Taylor不稳定现象耦合作用所导致。Abstract: In the process of the premixed gas flame propagation, obstacles have vital influence on the flame's structure and propagation, and will enhance the flame acceleration and the DDT process. Through the methane/air premixed flame propagation experiment with obstacles in the visual pipe, the flame microstructure changes and the propagation characteristics were captured. By means of the three-dimensional physical model, a large eddy simulation mode (LES) with the WALE sub-grid scale models, and the thickened flame model (TFM) were used to repeat experiment process. The complex changes of the flow field were obtained when the premixed flame climbed over the obstacle, and the characteristics of the flow turbulence transition were analyzed. Finally, the direct cause of the premixed flame disturbance instability under the influence of the obstacles was revealed. It was induced by the coupling effect of the Kelvin-Helmholtz instability and the Rayleigh-Taylor instability, which in turn was affected by three vortexes as a result of the obstacle.
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Key words:
- mechanics of explosion /
- thickened flame model /
- large eddy simulation /
- premixed flame /
- obstacles
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图 2 甲烷/空气预混火焰传播过程纹影图片
Figure 2. High-speed schlieren images of premixed methane/air flame propagation
图 3 LES模型计算三维预混火焰结构时间序列图
Figure 3. Sequence diagram three-dimensional premixed flame structure by LES model
图 4 火焰翻越障碍物时管道内流场结构示意图
Figure 4. The pipe flow field structure diagram when the flame climb over the obstacle
图 5 特征点处压力时程曲线实验与LES模型计算值对比
Figure 5. Comparison of pressure histories at characteristic point between experiment and LES
图 6 预混火焰翻越障碍物过程中管道内的速度矢量及流场变化图
Figure 6. Velocity vector and the flow field when premixed flame climbed over obstacle
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