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首页> 外文期刊>Combustion Science and Technology >The Physics of Scalar Gradients in Turbulent Premixed Combustion and Its Relevance to Modeling
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The Physics of Scalar Gradients in Turbulent Premixed Combustion and Its Relevance to Modeling

机译：湍流预混燃烧中标量梯度的物理性质及其与建模的关系

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Equations for the absolute value of the scalar gradient and for the infinitesimal distance, Delta x(N); between two adjacent iso-scalar non-material surfaces are obtained. 'Effective' strain rate normal to the iso-surfaces, which includes flow and physicochemically 'added' contributions, is shown to be an essential variable, causing either enhancement or destruction of scalar gradients and reduction or growth of distances between surfaces. Two DNS datasets for turbulent premixed flames, one simulating a statistically planar propagating front in an inflow-outflow configuration and the other computing a jet of a methane-air mixture, surrounded by a coflow of hot products, have been examined. DNS are used to estimate the relative importance of different processes determining the gradients fate. The flow normal strain rate apparently scales with the inverse of the Kolmogorov time microscale. Using as characteristic time, delta(L)/S-L, and length, delta(L), where delta(L) and S-L are the laminar flame thickness and propagating velocity, a dimensionless equation for the time rate of change of Delta x(N)/delta(L) depends on five dimensionless parameters, among them the Karlovitz number, Ka; the contribution of every term in the rate equation depends on the magnitude of Ka compared to the other dimensionless groups. The chemically 'added' normal strain rate dominates the time evolution of Delta x(N)/delta(L) in the burning and a good share of the preheat regions of the statistically planar flame, whereas 'added' and flow normal strain rates are comparable in the turbulent jet flame. Large values of some of these dimensionless parameters hint at the likely importance of accounting for Reynolds and/or Damhohler numbers dependencies in future work. A consistent definition of an average premixed turbulent flame thickness is presented and its computed values are compared to a previous proposal. Some suggestions to model the molecular mixing term in the context of the scalar PDF transport methodology are discussed. It is hypothesized that the characteristic mixing time should be proportional to the inverse of the 'effective' normal strain rate.
机译：标量梯度的绝对值和无穷小距离的方程Delta x（N）;得到两个相邻的等量非材料表面之间的距离。垂直于等值面的“有效”应变率，包括流动和物理化学“增加”的贡献，被证明是一个重要变量，它会导致标量梯度的增加或破坏，以及两面之间距离的减小或增长。已检查了两个用于湍流预混火焰的DNS数据集，一个数据集模拟了流入-流出配置中的统计平面传播前沿，另一个数据集计算了由热产品同流包围的甲烷-空气混合物射流。 DNS用于估计确定梯度命运的不同过程的相对重要性。流动法向应变率显然与Kolmogorov时间微尺度成反比。使用时间delta（L）/ SL和长度delta（L）作为特征时间，其中delta（L）和SL是层流火焰厚度和传播速度，这是Delta x（N）的时间变化率的无量纲方程）/ delta（L）取决于五个无量纲参数，其中的Karlovitz数Ka；与其他无量纲组相比，速率方程中每一项的贡献取决于Ka的大小。化学“加”法向应变率主导着燃烧过程中的Δx（N）/ delta（L）的时间演化，并且统计平面火焰的预热区域占很大比例，而“加”法和流法向应变率是在湍流喷射火焰中具有可比性。这些无量纲参数中的一些较大的值暗示着在将来的工作中考虑雷诺和/或达姆霍勒数依赖性的重要性。给出了平均预混湍流火焰厚度的一致定义，并将其计算值与先前的建议进行了比较。讨论了在标量PDF传输方法中模拟分子混合项的一些建议。假设特征混合时间应与“有效”法向应变率的倒数成正比。

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