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

为了弄清油气混合气不均匀性的形成过程以及这类混合气的燃烧过程，本文主要进行了以下三方面的研究：采用随机方法，考虑混合气浓度的不均匀性，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分析。结果表明，对于甲烷和氢燃料，射流发展的时间历程成功地与实验数据相一致。此外，PDF模型成功地描述了微观混合过程的时间发展。利用PIV和LIF方法测量了不同密度的氦气和二氧化碳气体湍流射流与周围空气的速度和浓度分布。直接研究了射流的湍流输运过程，分析了射流浓度分布及其旋涡运动。结果表明，标量通量和雷诺应力沿径向沿着分布具有自相似性。此外，还讨论了几种适用于大涡模拟的亚网格尺度标量通量模型。基于亚网格尺度标量通量模型值与实测值的散射图，在各种密度条件下，各模型的特性基本相似。结果表明，当量比变化对低温氧化起始点和着火延迟期影响不大，但燃烧持续期随当量比方差的增大而增大。此外，燃烧持续期主要受点火时的不均匀性影响，而受混合率的影响不大。

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.