CFD Simulation of Turbulent Combustion for Description of Non-Uniform Process by a Stochastic Approach

将随机方法应用于计算流体动力学（CFD）模拟，建立了有效的湍流燃烧预测程序。该程序特别强调了由于局部热释放而引起的浓度和密度波动的描述，这对于高湍流燃烧（如发动机燃烧）中污染物的形成至关重要。现将本研究得到的结果总结如下。(1)在有限差分CFD程序的每个计算单元中考虑燃料质量分数的概率密度函数（pdf），以描述混合物浓度的微观非均质性。采用隐式混合格式求解对流和扩散引起的pdf变化。此外，湍流混合过程用碰撞-再分散模型表示。根据k-epsilon模型的湍流特性估计碰撞率。(2)以一维和二维湍流为例，研究了上述预测程序的准确性。结果表明，只要在每个细胞中选择20个或更多的流体元素，平均浓度的时间演变与理论一致。此外，还对更为实际的缸内流动进行了模拟程序，以进行进一步的试验。(3)为了研究混合气非均质性对发动机燃烧过程中氮氧化物生成的影响，采用基于实测热释放率的热力学模型，计算了不同非均质性混合气条件下的温度分布历史和NO浓度历史。结果表明，较高的NO浓度来源于初始点燃的热燃烧气体。因此，火花点火发动机的稀薄燃烧与更均匀的混合物应该用于减少氮氧化物。还建议，一旦燃烧后的气体被搅拌，可以实现大量的减少NO。少

A stochastic approach was applied to computational fluid dynamic (CFD) simulation to establish efficient program valid for predicting turbulent combustion. The program placed a special emphasis on description of concentration and density fluctuations due to local heat release which are essential to the pollutant formation in highly turbulent combustion, such as an engine combustion. Obtained results in the present study are summarized as follows.(1)The probability density function (pdf) of the mass fraction of fuel was taken into consideration in each computational cell of a finite difference CFD program to allow description of the microscopic heterogeneity of mixture concentration. Changes of pdf due to convection and diffusion were solved using hybrid scheme in an implicit method. Furthermore, the process of turbulent mixing was expressed by a collision-redispersion model. The collision rate was estimated from the turbulence characteristics based on the k-epsilon model.(2)The accurac … More y of the above prediction program was investigated for a simple case of one- and two-dimensional turbulent flows. It was shown that the time evolution of the mean concentration was consistent with theoretical one as long as twenty or more fluid elements in each cell are chosen. In addition, the simulation program for a more practical in-cylinder flow was performed for a further test.(3)To investigate the effect of mixture heterogeneity on the formation of oxides of nitrogen in the engine combustion, the histories of temperature distribution and NO concentration were calculated using a thermodynamic model based on the measured rates of heat release for the mixture condition having different degrees of heterogeneity. It was shown that a higher NO concentration originates from the hot burnt gas initially ignited. Thus, a lean burning of spark-ignition engines with a more homogeneous mixture should be used for NOx reduction. It was also suggested that much reduction of NO may be achieved once the burnt gas is made stirred. Less