Modeling of Turbulent Nonpremixed Combustion Using a Combined Probability Density Function/Moment Method

为了提高蒙特卡罗方法的精度，提出了一种概率密度函数/矩组合方法。蒙特卡罗方法的精度在很大程度上取决于蒙特卡罗粒子的数量，并随着PDF传输概率的减小而降低。该方法同时求解模型化的PDF输运方程和均值、方差矩方程。从矩方程中得到的均值和方差值只是作为参考值与相应的pdf值进行匹配。在紊流射流扩散火焰的结构中评价了本方法的有效性，并与其他方法进行了比较；PDF方法和小火焰模型方法。为了获得组合方法的基准数据，研究了限制在圆柱炉内的射流非预混火焰的发射特性，并统计了升力射流非预混火焰的火焰基底结构。得到的更多结果如下：1)将现有的组合PDF/矩方法与小火焰模型方法进行比较，验证了PDF方法的改进。结合PDF/矩法可以再现射流非预混火焰的熄灭和重燃过程。2)密闭火焰的火焰特性可以由燃烧器和炉膛几何形状的组合强烈地支配。在本研究中，考察了圆柱炉内径、火焰边界湍流和整体等效比对NOx排放的影响。结果表明，随着这些参数的增加，NOx的排放指数大致降低。3)采用平面温度瑞利散射法对升力湍流非预混火焰火焰底部的非预混火焰统计量进行了实验研究。标量耗散率具有对数正态统计，且淬灭时的标量耗散率低于均匀应变非预混火焰预测的临界值。少

A combined probability density function (PDF)/moment method has been developed to improve the accuracy of the Monte-Carlo method. The accuracy of Monte-Carlo method depends strongly on the number of Monte-Carlo particles and declines with the decrease in the PDF transport probability. The present method solves simultaneously a modeled PDF transport equation and moment equations of mean and variance. Both the mean and variance values obtained from the moment equations are just used as the reference values to match to the corresponding values of PDFs. The usefulness of the present method is evaluated in the configuration of a turbulent jet diffusion flame, with a comparison with other methods; the PDF method and the flamelet model method. In order to obtain the benchmark data of the combined method, we investigated the emission characteristics of jet nonpremixed flames confined in cylindrical furnaces and the statistics of the flame base structure of lifted jet nonpremixed flames. The re … More sults obtained are as follows.1) The comparison between the present combined PDF/moment method and the flamelet model method have verified the improvement from the PDF method. The combined PDF/moment method can reproduce the extinction and the re-ignition in jet nonpremixed flames.2) The flame characteristics of confined flames can be strongly dominated by the combination of the burner and furnace geometries. In the present study, the effects of the inner diameter of the cylindrical furnace, the turbulence at the flame boundary, and the global equivalence ratio are examined in terms of the emission of NOx. It was found that the emission index of NOx decreases roughly with these parameters.3) Nonpremixed flamelet statistics at the flame base of lifted turbulent nonpremixed flames were investigated experimentally using a planer temperature Rayleigh scattering method. The scalar dissipation rate has log-normal statistics and the quenching scalar dissipation rate is lower than the critical value predicted using the uniformly strained counter nonpremixed flame. Less