Taylor dispersion in premixed combustion: questions from turbulent combustion answered for laminar flames
Taylor dispersion in premixed combustion: questions from turbulent combustion answered for laminar flames
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预混燃烧中的泰勒色散：层流火焰的湍流燃烧问题的答案
DOI：
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发表时间：
2018
期刊：
影响因子：
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通讯作者：
F. Al
中科院分区：
文献类型：
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作者：
J. Daou;P. Pearce;F. Al
我们提出了一个泰勒分散在预混燃烧的研究，并用它来澄清有关的火焰传播的流场的基本问题。特别是，简单的分析公式推导出变密度层流火焰与任意刘易斯数乐提供明确的答案，在湍流燃烧中出现的重要问题，当这些问题提出的情况下，一个规模的层流平行流。利用，在一个层流Poiffille流模型的上下文中，一个“厚火焰”杰出的渐近极限，其中的流量振幅是大的雷诺数Re固定，三个主要的贡献。首先，泰勒色散和Damköhler的第二个假设之间建立了联系，通过分析描述的有效传播速度UT由于小流量尺度的增强。更准确地说，它表明达姆克勒的假设是只有部分正确的单尺度平行层流。具体而言，虽然由于流动而导致的UT增加被证明与Damköhler所建议的有效扩散率的增加直接相关，但我们的结果意味着UT是Re = Re（对于Re = 1）而不是UT是Re = Re，正如Damköhler假设所暗示的那样。其次，它被证明分析和数值证实，当UT绘制与流量振幅为固定值的Re，曲线平坦的恒定值取决于Re。我们可以把这种效应称为“层流弯曲效应”，因为它模仿了湍流燃烧中已知的类似“弯曲效应”。第三，新的和有点令人惊讶的影响与UT的依赖性和有效的刘易斯数Leeff的流量报告。例如，当Re从小到大变化时，Leeff从Le变化到Le−1。此外，UT被发现是一个单调增加的函数的Re，如果Le <ε 2，和一个非单调的函数，如果Le > ε 2。电子邮件：info. joel.daou@ manchester.ac.uk
We present a study on Taylor dispersion in premixed combustion and use it to clarify fundamental issues related to flame propagation in a flow field. In particular, simple analytical formulae are derived for variable-density laminar flames with arbitrary Lewis number Le providing clear answers to important questions arising in turbulent combustion, when these questions are posed for the case of one-scale laminar parallel flows. Exploiting, in the context of a laminar Poiseuille flow model, a “thick flame” distinguished asymptotic limit for which the flow amplitude is large with the Reynolds number Re fixed, three main contributions are made. First, a link is established between Taylor dispersion and Damköhler’s second hypothesis by describing analytically the enhancement of the effective propagation speed UT due to small flow scales. More precisely, it is shown that Damköhler’s hypothesis is only partially correct for onescale parallel laminar flows. Specifically, while the increase in UT due to the flow is shown to be directly associated with the increase in the effective diffusivity as suggested by Damköhler, our results imply that UT ∼ Re (for Re ≫ 1) rather than UT ∼ √ Re, as implied by Damköhler’s hypothesis. Second, it is demonstrated analytically and confirmed numerically that, when UT is plotted versus the flow amplitude for fixed values of Re, the curve levels off to a constant value depending on Re. We may refer to this effect as the “laminar bending effect” as it mimics a similar “bending effect” known in turbulent combustion. Third, novel and somewhat surprising implications associated with the dependence of UT and of the effective Lewis number Leeff on the flow are reported. For example, Leeff is found to vary from Le to Le−1 as Re varies from small to large values. Also, UT is found to be a monotonically increasing function of Re if Le < √ 2, and a non-monotonic function if Le > √ 2. ∗ joel.daou@manchester.ac.uk