【作者】 东明；

【导师】 解茂昭；

【作者基本信息】 大连理工大学 ， 动力机械及工程， 2009， 博士

【摘要】 随着人类迈入21世纪,资源和环境的问题愈加突出。基于气体或液体燃料在大孔隙率多孔介质中燃烧的过滤燃烧技术在高效、节能和净化诸方面都显示出诱人的前景,加之多孔介质材料和技术的进展,而日益引起人们的研究兴趣,并已在工程实际中获得越来越多的应用。本文以多孔介质燃烧器和多孔介质发动机为研究背景,通过数值模拟研究了大孔隙率多孔介质内湍流流动与组分输运过程,主要目的是更系统深入地理解多孔介质内湍流流动和混合气的形成过程的特点及规律,并进一步推动该领域的理论研究及其实际应用。本文主要在下列几方面进行了研究与探索:1、在众多的多孔介质湍流模型中,对具有代表性的三种模型进行了较全面的介绍与分析,并针对内部置入多孔介质的长方形管道内的湍流流场,利用主要的两种宏观湍流模型进行了数值研究。与实验结果进行比较和分析后发现,A-L湍流模型对湍流的抑制作用略大一些。2、为了深入研究大孔隙率多孔介质内的湍流特性对多孔介质内输运和燃烧过程的重要影响,建立了多孔介质的一种二维孔隙网络模型。此模型由一系列的圆形孔隙体和长方形喉道互相连接而成。使用标准k-ε模型模拟孔隙网络内的微观流场,利用体积平均方法将微观流场计算结果转换为宏观流场的信息,从而确定了宏观湍流模型中修正项系数的值。与相关文献比较和分析显示吻合良好。3、为考察气缸内加入多孔介质蓄热体后对燃烧室内湍流流场及混合气形成的影响,对模拟内燃机气缸的一个圆柱状多孔介质燃烧室进行了数值研究。建立了四种多孔介质简化结构模型,详细研究了多孔介质内湍流流动和喷雾油束与多孔介质的相互作用,对能量方程和油滴碰壁模型进行了修正。着重分析了多孔介质结构、多孔介质孔隙率、喷射压力和喷雾锥角对燃烧室内湍流流场、混合气形成和均匀化过程的影响。研究表明多孔介质结构和孔隙率对多孔介质内湍流流动的影响很大,从而影响整个区域的湍流流场。喷雾锥角的大小直接影响到自由流体区域湍动能水平和喷雾油束与多孔介质相互作用过程,以及多孔介质内湍动能水平和流场。而喷射压力对喷雾油束与多孔介质相互作用和湍流流场的影响,主要是通过改变油滴运动速度而得以实现。喷射压力增大,油滴运动速度得以增加,湍能水平也相应增加。油蒸汽的分布更加均匀、更加广阔。4、大孔隙率多孔介质内流动和燃烧问题的一大难点是同时存在时间上的湍流脉动和空间上的扰动即弥散。弥散是由于介质中数量巨大而形状又极不规则的微孔对流体流动产生强烈扰动而引起的一种输运现象。利用由圆柱形单元的周期性阵列作为多孔介质结构简化模型,对甲烷和空气在多孔介质中扩散过程进行了数值研究,利用微观流场的计算结果和体积平均方法求出了甲烷在多孔介质内的纵向质量和横向质量弥散系数,并与相关实验数据和理论公式进行了比较和分析。对总的质量扩散率的各分量,即分子扩散、湍流扩散、时均流弥散和湍流弥散的相对重要性做了评价,为深入研究多孔介质中的弥散效应和湍流扩散效应,提供了一种可借鉴的模拟方法。更多还原

【Abstract】 As the 21st century,the resources and the environment have become an increasingly primary and prominent problem.Because of its high combustion efficiency and low pollution, the filtation combustion technique of the gas or liquid fuel in high porosity porous media has become an international research hotspot and has gained more and more practical applications. To understand the working mechanism of the porous media burner and of the porous media internal combustion engine,in this thesis,turbulent flow behavior and species transport processes in high porosity porous media are studied by numerical simulation.The main objective is to gain insights into the characteristics of turbulent flow and fuel-air mixture formation in porous media more systematically,and furthermore to promote theoretical research and practical applications in this field.In this thesis,the following aspects have been studied and explored.1.Three representative macroscopic turbulent model for the flow in porous media are introduced and comprehensively analyzed.The characteristics of turbulent flow in open channels with a porous bed are numerically studied by using two leading turbulent models. Numerical results were compared with the experimental data from thr literature and found that the calculated results from the P-dL macroscopic turbulent model were closer to the experimental data than those from the A-L model.2.To understand essential influences of turbulence on transport and combustion processes in porous media with high porosity,a two-dimensional pore network model for random porous media structure is presented,which consists of a number of cylindrical pores and parallelepiped connecting throats.A standard k-εmodel is employed to simulate the turbulence effect in the microscopic pore-level flow field.Computational results obtained from the pore-network model are transformed into information of the macro flow field through a volume average approach over the entire computation domain.The results are then used to deduce the value for the unknown coefficient in the macroscopic model.Comparisons with available data in the open literature seem to indicate good agreement.3.To understand the working mechanism of the porous medium(PM) internal combustion engine,effects of a porous medium heat regenerator inserted into a combustion chamber on the turbulent flow characteristics and fuel-air mixture formation are studied by numerical simulations.A simplified model for the random structure of the porous media is presented,to simulate turbulent flows in the porous media and spray/wall interactions,energy equation and spray model are modified.Effects of PM structure and porosity and spray injection and spray cone angle on the turbulent flow and fuel air mixture formation and homogenization process are studied with emphasis.Calculation results show great influences of PM structure and porosity on the turbulent flow in the porous media,as well as influence of the flow field inside the PM on the flows in the entire combustion chamber.The spray cone angle affets directly the level of turbulent kinetic energy in the clear fluid region and the spray/PM interaction process as well as the level of turbulent kinetic energy in PM.The variation in spray injection pressure can modify the velocity of fuel droplets.With increasing spray injection pressure,the velocity of fuel droplets to increases and the level of turbulent kinetic energy also increases accordingly, consequently,the fuel vapor becomes more homogeneous and distributed over a wider area.4.One of the main difficulties in the flow and combustion PM with high porosity is the co-existence of the turbulent fluctuation in time and the disturbance in space.The disturbance in space is namely dispersion.The dispersion is a transport phenomenon that is caused by the numerous and irregular pores in the medium.A simplified model for the cylindrical periodical structure of the PM is presented,the diffusion process of methane and air in the PM is studies numerically.The computational results obtained from the microscopic computation domain are employed to derive the longitudinal mass dispersion coefficient and transverse mass dispersion coefficient through a volume average approach.The results are analyzed and compared with the correlative experimental data and empirical formulas in the literature.The results are then used to deduce the value for the unknown coefficient in the macroscopic model.Comparisons with available data in the open literature show good agreement.This provides a useful approach for studying dispersion and turbulent diffusion effects in porous media.更多还原