基于大涡模拟的湍流非预混燃烧混合分数概率密度函数

混合分数概率密度函数(probability density function,PDF)反映了湍流对燃料和氧化剂混合过程的影响,在湍流非预混燃烧的理论研究和数值模拟中有非常重要的作用。该文基于大涡模拟(large eddy simulation,LES)对非预混火焰中的混合分数PDF进行了研究。利用LES预测的SandiaFlame D的速度和温度的均值和均方根分布与实验结果符合很好,瞬态温度场显示了合理的湍流火焰形态。混合分数PDF在反应区为钟形分布,在贫燃侧和富燃侧为钟形分布或单调形分布,取决于当地流场状态。对简化PDF模型的研究表明:β函数模型对钟形PDF和单调形PDF的预测效果都很好;截尾Gauss函数模型只能较好地预测钟形分布PDF;多点δ函数模型的预测能力与截尾Gauss函数模型的预测能力类似;双δ函数模型的预测结果偏差较大。

Mixture fraction PDF in turbulent nonpremixed combustion predicted using LES

The mixture fraction probability density function(PDF) reflects the influence of the turbulence on the mixing of fuel and oxygen and is very important in theoretical and numerical analyses of turbulent non-premixed combustion.The mixture fraction PDF was investigated here based on large eddy simulations(LES).The predicted distributions of the mean and root mean square(RMS) of the temperatures and velocities using LES for Sandia Flame D agree well with the experimental data,with the instantaneous temperature contours giving a reasonable flame pattern.The mixture fraction PDF given by the LES result is a bell-shape curve in the vicinity of the reaction zone,but is a bell-shape or smeared-δ-shape curve on the lean and rich sides depending on the local flow state.Of the four tested PDF models,the β-function model predicts both the bell-shape PDF and the smeared-δ-shape PDF very well while the clip-Gaussian model only gives reasonable results for the bell-shape PDF.The performance of the multi-δ PDF is similar to that of the clip-Gaussian model.The results using the double-δ function model differ greatly from the LES results.