Spatial resolution effects on the measurement of scalar variance and scalar gradient in turbulent nonpremixed jet flames
Spatial resolution effects on the measurement of scalar variance and scalar gradient in turbulent nonpremixed jet flames
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空间分辨率对湍流非预混射流火焰中标量方差和标量梯度测量的影响
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
R. Barlow
中科院分区：
文献类型：
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作者：
Guanghua Wang;R. Barlow
空间平均效应对于湍流非预混火焰的标量梯度测量非常重要，特别是在局部耗散长度尺度较小的情况下。利用拉曼线成像、瑞利线成像和CO-LIF线成像研究了实验分辨率对雷诺数为15,200和22,800 （DLR-A和B）和雷诺数为13,400、22400和33,600 （Sandia火焰C/D/E）的非预混CH4/H2/N2湍流射流火焰近场的量方差和径向梯度的影响。采用变宽的盒形滤波器，研究了有限空间分辨率效应。所得的标量方差和均方梯度的分辨率曲线与由Pope模型湍流动能谱计算的理论曲线具有几乎相同的趋势。在几乎所有的研究情况下，均方梯度分解曲线的崩塌表明耗散谱的形状是近似普遍的。流体输运特性对耗散分辨率曲线没有影响，这意味着从平方梯度推断的耗散长度尺度相当于标量耗散率的长度尺度，其中包括扩散系数。使用Box滤波器，解析98%的平均耗散率所需的空间分辨率约为耗散截止长度尺度的1 - 2倍（类似于湍流等温流中的Batchelor尺度）。还比较了分辨率对混合分数、温度和倒瑞利信号方差的影响。滤波方差与真实方差的比值与探头分辨率呈近似线性关系。即使在瑞利散射截面不恒定的情况下，倒置瑞利散射信号也可以用于研究温度变化的分辨率效应。实验结果还表明，这些实验室尺度湍流射流火焰具有较小的有效雷诺数，因此存在大（含能）和小（耗散）标量长度尺度的直接相互作用，特别是在x/d = 7.5的近场情况下，桑迪亚火焰C/ d /E。
The effect of spatial averaging is important for scalar gradient measurements in turbulent nonpremixed flames, especially when the local dissipation length scale is small. Line imaging of Raman, Rayleigh and CO-LIF is used to investigate the effects of experimental resolution on the scalar variance and radial gradient in the near field of turbulent nonpremixed CH4/H2/N2 jet flames at Reynolds numbers of 15,200 and 22,800 (DLR-A and B) and in piloted CH4/air jet flames at Reynolds numbers of 13,400, 22,400 and 33,600 (Sandia flames C/D/E). The finite spatial resolution effects are studied by applying the Box filter with varying filter widths. The resulting resolution curves for both scalar variance and mean squared-gradient follow nearly the same trends as theoretical curves calculated from the model turbulence kinetic energy spectrum of Pope. The observed collapse of resolution curves of mean squared-gradient for nearly all studied cases implies the shape of the dissipation spectrum is approximately universal. Fluid transport properties are shown to have no effect on the dissipation resolution curve, which implies that the dissipation length scale inferred from the square gradient is equivalent to the length scale for the scalar dissipation rate, which includes the diffusion coefficient. With the Box filter, the required spatial resolution to resolve 98% of the mean dissipation rate is about one−two times of the dissipation cutoff length scale (analogous to the Batchelor scale in turbulent isothermal flows). The effects of resolution on the variances of mixture fraction, temperature, and the inverted Rayleigh signal are also compared. The ratio of the filtered variance to the true variance is shown to depend nearly linearly on the probe resolution. The inverted Rayleigh scattering signal can be used to study the resolution effect on temperature variance even when the Rayleigh scattering cross section is not constant. The experimental results also indicate that these laboratory scale turbulent jet flames have small effective Reynolds numbers, such that there is some direct interaction of the large (energy containing) and small (dissipative) scalar length scales, especially for the near field case at x/d = 7.5 of the piloted Sandia flames C/D/E.