Insights into the Bending Effect in Premixed Turbulent Combustion Using the Flame Surface Density Transport
Insights into the Bending Effect in Premixed Turbulent Combustion Using the Flame Surface Density Transport
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使用火焰表面密度传输深入了解预混湍流燃烧中的弯曲效应
DOI：
10.1080/00102202.2019.1577241
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发表时间：
2019
影响因子：
1.9
通讯作者：
Ahmed U
中科院分区：
文献类型：
--
作者：
Ahmed U
湍流火焰速度变化的弯曲效应（即，火焰速度随湍流均方根速度波动增加而线性增加的偏差）已经基于统计平面湍流预混火焰传播到强制未燃气体湍流的直接数值模拟数据库进行了研究。Damköhler的第一个假设的有效性已被用来分析广义火焰表面密度（FSD）的演变方面的弯曲效果。FSD输运方程的切向应变率项的体积积分值保持为正值，而曲率项的体积积分值假定为负值。在统计平稳状态下，体积积分切向应变率项的正值与体积积分曲率项的负值保持平衡。已经发现，对于小湍流强度，法向应变率对火焰表面积的贡献仍然为负，而对于大湍流强度，法向应变率对火焰表面积的贡献最终变为正。这是共线排列的反应进展变量梯度的变化的结果，从最广泛的主应变率方向的特征向量的方向与最压缩的主应变率与湍流强度增加。湍流强度的增加增加了火焰曲率的概率密度函数的宽度，从而增加了表面平均曲率平方值。这最终使得FSD曲率项由于位移速度的切向扩散分量而成为FSD输运方程中体积积分曲率项对于大湍流强度的负贡献的主要贡献者。然而，体积积分FSD曲率项的负贡献并不随着湍流强度的增加而无限增加，并且内部截止尺度也限制了统计稳定状态下体积积分FSD应变率项的最大可能值，控制着火焰表面积的最大可能破坏。有人认为，火焰表面积的产生和破坏的上限是负责弯曲效应的湍流火焰速度和火焰表面积的变化。
The bending effect of turbulent flame speed variation (i.e., the deviation from the linear increase of flame speed with increasing root-mean-square turbulent velocity fluctuation) has been investigated based on a Direct Numerical Simulation database of statistically planar turbulent premixed flames propagating into forced unburned gas turbulence. The validity of Damköhler’s first hypothesis has been utilized to analyze the bending effect in terms of generalized Flame Surface Density (FSD) evolution. The volume-integrated value of the tangential strain rate term of the FSD transport equation remains positive, whereas the volume-integrated value of the curvature term assumes negative values. Under statistically stationary state, the positive value of the volume-integrated tangential strain rate term remains in equilibrium with the negative value of the volume-integrated curvature term. It has been found that the contribution of the normal strain rate to the flame surface area remains negative for small turbulence intensities, which eventually become positive for large turbulence intensities. This is a consequence of the change of collinear alignment of the reaction progress variable gradient from the most extensive principal strain rate direction to the direction of the eigenvector associated with the most compressive principal strain rate with increasing turbulence intensity. An increase in turbulence intensity increases the width of the probability density functions of flame curvature, and thereby increases the surface-averaged curvature squared values. This eventually makes the FSD curvature term due to the tangential diffusion component of displacement speed as the major contributor to the negative contribution of the volume-integrated curvature term in the FSD transport equation for large turbulence intensities. However, the negative contribution of the volume-integrated FSD curvature term does not increase indefinitely with increasing turbulence intensity and the inner cut-off scale, which also limits the maximum possible value of the volume-integrated FSD strain rate term under statistically stationary state, governs the maximum possible destruction of flame surface area. It has been argued that the upper limits of the flame surface area generation and destruction are responsible for the bending effects in the variations of turbulent flame speed and flame surface area.
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