Anisotropic enhancement of turbulence in large-scale, low-intensity turbulent premixed propane–air flames
Anisotropic enhancement of turbulence in large-scale, low-intensity turbulent premixed propane–air flames
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大范围、低强度湍流预混丙烷-空气火焰中湍流的各向异性增强
DOI：
10.1017/s0022112002008650
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发表时间：
2002
影响因子：
3.7
通讯作者：
F. Williams
中科院分区：
文献类型：
--
作者：
J. Furukawa;Y. Noguchi;T. Hirano;F. Williams
在湍流中传播的预混火焰的密度变化改变了湍流。该修改的性质取决于湍流燃烧的状态、燃烧器设计、湍流火焰的取向和火焰内的位置。本研究解决统计稳定的湍流燃烧的火焰片制度，其中层流火焰厚度小于Kolmogorov尺度，火焰稳定在一个垂直取向的圆柱形燃烧器具有充分发展的向上湍流管流上游出口。在这些条件下，快速移动的褶皱层流火焰形成轴对称湍流火焰刷，其附接到燃烧器出口。在这种情况下，已经预测了层流火焰中湍流特性的变化，但很少有测量结果来检验预测结果。本文测量了三种不同当量比下湍流火焰刷中不同位置处的单个速度变化和湍流度变化，并与理论进行了比较。测量采用三元件静电探针（EP）和两个组件的激光多普勒测速仪（LDV）。LDV测量局部气体速度的轴向和径向分量，而EP（其三个传感器位于穿过燃烧器轴线的垂直平面中，包含LDV速度分量的平面）测量火焰在该平面中的三个点处的到达时间。从到达时间，火焰面的方向和速度在平面上的投影。所有EP和LDV传感器都位于直径约为1 mm的固定体积元件内，以提供局部时间分辨信息。该技术具有响应速度快、灵敏度高的EP优点和侵入性强、解释复杂的EP缺点，但它非常适合这里所寻求的数据类型。理论预测，切向于局部平面火焰面的速度分量在通过火焰面时保持恒定。在测量的准确度范围内，数据与这一预测一致。数据还表明，垂直于小火焰的速度分量，相对于小火焰测量，往往增加通过小火焰，如预期的。因此，火焰可以在初始各向同性湍流中产生各向异性。它们还产生未燃烧或燃烧气体条件下的湍流谱的差异。实验证明了火焰对湍流的局部修正。在湍流火焰刷中的不同位置处的修改在数量上是不同的，但是在质量上是相似的，因为在这些火焰中的所有位置处，湍流在径向方向上比在轴向方向上增强得更强。
The density change across premixed flames propagating in turbulent flows modifies the turbulence. The nature of that modification depends on the regime of turbulent combustion, the burner design, the orientation of the turbulent flame and the position within the flame. The present study addresses statistically stationary turbulent combustion in the flame-sheet regime, in which the laminar-flame thickness is less than the Kolmogorov scale, for flames stabilized on a vertically oriented cylindrical burner having fully developed upward turbulent pipe flow upstream from the exit. Under these conditions, rapidly moving wrinkled laminar flamelets form the axisymmetric turbulent flame brush that is attached to the burner exit. Predictions have been made of changes in turbulence properties across laminar flamelets in such situations, but very few measurements have been performed to test the predictions. The present work measures individual velocity changes and changes in turbulence across flamelets at different positions in the turbulent flame brush for three different equivalence ratios, for comparison with theory. The measurements employ a three-element electrostatic probe (EP) and a two-component laser-Doppler velocimeter (LDV). The LDV measures axial and radial components of the local gas velocity, while the EP, whose three sensors are located in a vertical plane that passes through the burner axis, containing the plane of the LDV velocity components, measures arrival times of flamelets at three points in that plane. From the arrival times, the projection of flamelet orientation and velocity on the plane are obtained. All of the EP and LDV sensors are located within a fixed volume element of about 1 mm diameter to provide local, time-resolved information. The technique has the EP advantages of rapid response and good sensitivity and the EP disadvantages of intrusiveness and complexity of interpretation, but it is well suited to the type of data sought here. Theory predicts that the component of velocity tangent to the surface of a locally planar flamelet remains constant in passing through the flamelet. The data are consistent with this prediction, within the accuracy of the measurement. The data also indicate that the component of velocity normal to the flamelet, measured with respect to the flamelet, tends to increase in passing through the flamelet, as expected. The flamelets thereby can generate anisotropy in initially isotropic turbulence. They also produce differences in turbulent spectra conditioned on unburnt or burnt gas. Local modifications of turbulence by flamelets thus are demonstrated experimentally. The modifications are quantitatively different at different locations in the turbulent flame brush but qualitatively similar in that the turbulence is enhanced more strongly in the radial direction than in the axial direction at all positions in these flames.