Combustion Control Using Non-homogeneity in Fuel of Mixture and Its Modeling

为了弄清燃料-空气混合气中不均匀性的形成过程以及这类混合气的燃烧过程，主要进行了以下三项研究。采用计算流体力学（CFD）方法对高速非定常射流的流动和混合过程进行了随机分析。结果表明，射流发展的时间历程与甲烷和氢燃料的实验数据很好地吻合。此外，用PDF模型成功地描述了微混合过程的时间发展。讨论了甲烷射流和氢气射流局部非均质性的差异。用PIV和LIF方法测量了不同密度的氦-二氧化碳紊流射流在周围空气中的速度和浓度分布。直接研究了射流流体的湍流输运，分析了射流流体浓度分布及其涡旋运动。结果表明，在径向方向上，标量通量和雷诺数应力分布具有自相似性。此外，还讨论了几种用于大涡模拟的亚网格尺度尺度尺度通量模型。从亚网格尺度标量通量模型值与实测值的散射图可以看出，各密度条件下各模型的特征基本相似。采用基于化学动力学和随机方法的数值计算方法研究了非均匀混合物的自燃过程。结果表明：低温氧化起始点和滞燃时间不受当量比变化的影响，但燃烧时间随当量比变化的增大而增大；此外，燃烧持续时间主要受点火时的非均匀性影响，受混合率影响不大。

In order to clarify formation process of the non-homogeneity in fuel-air mixture, and combustion process of this type of mixture, following three studies were mainly performed.The flow and mixing process for high-speed unsteady jets are analyzed using computational fluid dynamics (CFD) included in heterogeneity of mixture concentration with stochastic approach. The results show that the time history of jet development is successfully agreed with experimental data for methane and hydrogen fuels. Furthermore, the time-development of micro-mixing process is successfully described with PDF model. Also, the difference of local heterogeneity between methane and hydrogen jets is discussed.Distributions of velocity and concentration of turbulent gas jets with various densities from helium to carbon dioxide against ambient air are measured by using PIV and LIF methods. The turbulent transport of the jet fluid is directly investigated to analyzing the distributions of jet-fluid concentration and its vortex motion. The result shows that the self-similarity exists between profiles of scalar flux and Reynolds stress along the radial direction. Furthermore, several sub-grid scale scalar-flux models for large eddy simulation are discussed. Based on the scattering plots of model values of sub-grid scale scalar flux against the measured values, the characteristics of each model are basically similar for every density conditions.An auto-ignition process of the non-homogeneous mixture was investigated using a numerical calculation based on the chemical kinetics and stochastic approach. The results show that the starting points of the low temperature oxidation and ignition delay period are hardly affected by the equivalence-ratio variation, however, the combustion duration increases with increasing variance of equivalence ratio. Furthermore, the combustion duration is mainly affected by the non-homogeneity at the ignition and not very much affected by the mixing rate.