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

为了阐明混合气浓度不均匀的形成过程和燃烧过程，本文主要进行了以下三个方面的研究：用随机方法对高速非定常射流的流动和混合过程进行了计算流体力学分析。结果表明，甲烷和氢气燃料的射流发展时间历程与实验数据吻合较好。此外，用PDF模型成功地描述了微观混合过程的时间发展过程。利用PIV和LIF方法测量了从氦到二氧化碳不同密度的湍流射流在环境空气中的速度分布和浓度分布。为了分析射流浓度的分布及其涡流运动，直接研究了射流流体的湍流输运。结果表明，标量通量分布与雷诺应力分布沿径向具有自相似性。此外，还讨论了几种用于大涡模拟的亚网格尺度标量通量模型。根据亚格子尺度标量通量模型值与实测值的散射图，各模型在不同密度条件下的特性基本相似。基于化学动力学和随机方法的数值计算，研究了非均匀混合气的自燃过程。结果表明，当量比变化对低温氧化和着火延迟期的起点影响不大，但燃烧持续期随着当量比变化的增加而增加。燃烧持续期主要受着火不均匀性的影响，混合率对燃烧持续期影响不大。

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.