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通过大涡模拟得到三维等密度平面射流中的混合分数的动态演化过程并统计其概率密度（PDF）的分布，对非预混燃烧雷诺时均模拟中常用的简化模型，包括β函数和截尾高斯函数，进行了定性和定量分析。混合分数的PDF分布在流场的不同区域上定性存在较大的区别，可以分为四类：追随型单峰分布、非追随型单峰分布、平板型分布和双峰分布。单峰追随型主要分布在湍流充分发展区，对不同的简化模型都有较好的适应性；单峰非追随型PDF通过β函数模拟可以得到很好的结果；平板型分布PDF，定性上难以模拟但定量模拟效果好；双峰分布主要分布于湍流外间歇性较大的区域，现有简化PDF模型难以得到满意的结果。

The distribution of probability density function （PDF） of the mixture fraction in a constant density planar jet is investigated by using large eddy simulation. Based on the LES results, the common-used presumed PDF models in Reynolds Averaged Numerical Simulations （RANS） in non-premixed combustion, such as β-function model and clip-Gaussian model and, have been analyzed. The results show that shapes of the mixture fraction PDFs are various significantly in the flow field. The mixture fraction PDFs can be divided to four category, those are single-peak marching behavior PDF, single-point non-marching behavior PDF, plane-like PDF and bimodal PDF. The single-peak marching behavior PDFs exist mainly at the fully-developed turbulent zones and can be well represented by most of the presumed PDF models. The sing-peak non-marching behavior PDF can only be reproduced by the β-function PDF. The plane-like PDF can＇t be well simulated qualitatively, while can well predict the mean and variance of thermodynamics variables. For the bimodal PDFs, all the presumed PDF models can＇t predict them very well both qualitatively and quantitatively. The results imply that the intermittency in the flow field is a main obstacle in using conserved scalar method in RANS of non-premixed turbulent combustion.