The effects of turbulent mixing on the correlation between two species and on concentration fluctuations in non-premixed reacting flows
The effects of turbulent mixing on the correlation between two species and on concentration fluctuations in non-premixed reacting flows
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湍流混合对两种物质之间的相关性以及对非预混合反应流中浓度波动的影响
DOI：
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发表时间：
1991
影响因子：
3.7
通讯作者：
Y. Murakami
中科院分区：
文献类型：
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作者：
S. Komori;J. Hunt;T. Kanzaki;Y. Murakami
当两种物质 A 和 B 通过边界表面的不同部分引入湍流区域时，A 和 B 的分子在湍流速度场和分子扩散的共同作用下聚集在一起。基于 Durbin (1980) 和 Sawford & Hunt (1986) 的模型，开发了随机飞行模型来模拟流体元件对的相对运动和分子的随机运动。该模型用于估计在具有中等快或慢二阶化学反应的非预混均匀湍流中，A 和 B 的波动浓度 $overline{c_Ac_B}$ 在某一点之间的互相关性。该相关性表明湍流和分子混合对平均化学反应速率的影响，当通过平均浓度 $overline{C}_A$ 和 $overline{C}_B$ 标准化时，通常表示为“分离”或“未混合”参数 $alpha(=overline{c_Ac_B}/overline{C}_Aoverline{C}_B) $。研究发现，随着高雷诺数流中两种物质释放时刻（或位置）的时间（或距离）的增加，α从接近-1增加到零。此外，模型（和实验）与 Danckwerts (1952) 的确切结果一致，即对于无反应的混合，$overline{c_Ac_B}/(overline{c^2_A}overline{c^2_B})^{frac{1}{2}} = -1$。然后扩展模型以考虑 A 和 B 之间化学反应对偏析参数 α 的影响。这导致 α 最终减小，具体取决于湍流混合和化学反应的相对时间尺度（即 Damköhler 数）。该模型还表明了许多其他参数（例如湍流尺度、施密特数、两种反应物的初始浓度比和平均剪切力）如何影响偏析参数 α。该模型解释了我们和其他作者之前发表的研究中对有反应和无反应混合的 α 的测量，前提是反应速率不是很快。此外，该模型仅严格适用于有限的混合时间 t，例如 t [lsim ] TL，其中 TL 是拉格朗日时间尺度，因为该模型要求 A 和 B 之间的界面有效连续且薄，即使是高度复杂的。给出了流动可视化结果，这与双粒子模型的物理思想一致。
When two species A and B are introduced through different parts of the bounding surface into a region of turbulent flow, molecules of A and B are brought together by the combined actions of the turbulent velocity field and molecular diffusion. A random flight model is developed to simulate the relative motion of pairs of fluid elements and random motions of the molecules, based on the models of Durbin (1980) and Sawford & Hunt (1986). The model is used to estimate the cross-correlation between fluctuating concentrations of A and B, $overline{c_Ac_B}$, at a point, in non-premixed homogeneous turbulence with a moderately fast or slow second-order chemical reaction. The correlation indicates the effects of turbulent and molecular mixing on the mean chemical reaction rate, and it is commonly expressed as the ‘segregation’ or ‘unmixedness’ parameter $alpha(=overline{c_Ac_B}/overline{C}_Aoverline{C}_B) $ when normalized by the mean concentrations $overline{C}_A$ and $overline{C}_B$. It is found that α increases from near −1 to zero with the time (or distance) from the moment (or location) of release of two species in high-Reynolds-number flow. Also, the model (and experiments) agrees with the exact results of Danckwerts (1952) that $overline{c_Ac_B}/(overline{c^2_A}overline{c^2_B})^{frac{1}{2}} = -1$ for mixing without reaction. The model is then extended to account for the effects on the segregation parameter α of chemical reactions between A and B. This leads to α eventually decreasing, depending on the relative timescales for turbulent mixing and for chemical reaction (i.e. the Damköhler number). The model also indicates how a number of other parameters such as the turbulent scales, the Schmidt number, the ratio of initial concentrations of two reactants and the mean shear affect the segregation parameter α. The model explains the measurements of α in previously published studies by ourselves and other authors, for mixing with and without reactions, provided that the reaction rate is not very fast. Also the model is only strictly applicable for a limited mixing time t, such that t [lsim ] TL where TL is the Lagrangian timescale, because the model requires that the interface between A and B is effectively continuous and thin, even if highly convoluted. Flow visualization results are presented, which are consistent with the physical idea underlying the two-particle model.