Evaluation of mixture-fraction-based turbulent-reaction-rate model assumptions for high-pressure reactive flows
Evaluation of mixture-fraction-based turbulent-reaction-rate model assumptions for high-pressure reactive flows
复制标题
基于混合分数的高压反应流湍流反应速率模型假设的评估
DOI：
10.1016/j.combustflame.2017.02.004
复制
发表时间：
2017
影响因子：
4.4
通讯作者：
J. Bellan
中科院分区：
文献类型：
--
作者：
J. Bellan
本文对具有湍流特性的高压单相湍流反应速率模型的几个假设进行了检验。该研究使用了其他地方描述的直接数值模拟(DNS)数据库(Bellan，2017)。该数据库是由高p条件下多组分混合、真实气体状态方程和单步化学反应相结合的模型得到的。该数据库建立在时间混合层的构型中，探测初始雷诺数Re 0、初始压力p0和两个混合层气流的初始组成的影响。反应是在湍流中启动的，在每次模拟中，计算都超过了达到最大平均体积p的时间tp*。在反应开始前和在tp p*时对涡度和涡度方程的检验突出了高密度梯度等量区(Bellan，2017)在产生湍流中的关键作用，以及在tp*代表热力学变量梯度错位的斜压效应的关键作用。结果表明，与经典混合分数方程相比，混合分数服从一个附加扩散项，该扩散项的均方根值大于混合分数典型扩散项的均方根值。对条件源项估计模型假设的评估发现，使用精确的混合分数概率密度函数可以很好地表示最强烈反应区的湍流反应率，但是当使用用于条件源项估计的通用模型β-PDF时，预测质量显著恶化，尽管β-PDF是用从dns中提取的准确矩构造的。在最小反应区，CSE模型不能准确地表示湍流反应速率，这是由于反应速率和混合分数的波动之间缺乏相关性，以及在较冷和较稠密的流体区域通过真实气体状态方程的热力学变量之间的强相关性的综合影响。
Several assumptions of atmospheric-pressure (atmospheric-p) single-phase turbulent reaction rate models are examined for high-p reactive flows having turbulent characteristics. The study uses a Direct Numerical Simulation (DNS) database described elsewhere (Bellan, 2017). This database was obtained with a model combining multi-species mixing under high-p conditions, a real-gas equation of state (EOS) and a single-step chemical reaction. The database, created in the configuration of a temporal mixing layer, probes the effect of the initial Reynolds number, Re 0, of the initial pressure, p 0, and of the initial composition of the two mixing-layer streams. The reaction is initiated in a turbulent flow and in each simulation the computations are pursued past a time when a maximum average-volumetric p is attained, t p p*. The examination of the vorticity and enstrophy equations at a time before reaction initiation and also at t p p* highlights the pivotal role of the high density-gradient magnitude regions (Bellan, 2017) in producing turbulence and the crucial role at t p p* of the baroclinic effect representing the misalignments of gradients of thermodynamic variables. Compared to the classical mixture fraction equation, the results show that the mixture fraction obeys an equation in which there is an additional diffusion term having a larger rms magnitude than that of the mixture-fraction typical diffusion term. An assessment of the Conditional Source-term Estimation (CSE) model assumptions found that, using an accurate mixture fraction probability density function (PDF), one can obtain a very good representation of the turbulent reaction rate in the most intense reaction regions, however the quality of the predictions of CSE deteriorated significantly when a common model for the PDF, the β-PDF, was used despite the β-PDF being constructed with the accurate moments extracted from the DNS. In the regions of minimal reaction, the CSE model using the exact PDF extracted from the DNS does not provide an accurate representation of the turbulent reaction rate, a fact which is attributed to the combined effect of lack of correlation between fluctuations of the reaction rate and of the mixture fraction, and to the strong correlation of the thermodynamic variables through the real-gas EOS in regions of colder and denser fluid.