【作者】 赵治国；

【导师】 解茂昭；

【作者基本信息】 大连理工大学 ， 动力机械及工程， 2007， 博士

【摘要】 能源短缺与环境污染是当今世界首要关注的两大热点问题，作为主要的能源消耗主体和环境污染来源的内燃机受到广大研究者的关注。在众多关于发动机的技术革新中，目前仍为新生事物的多孔介质发动机以其独特的燃烧方式、低排放、低噪音等优越性必将成为日后发动机研究领域的热点。本文以多孔介质发动机的研究为背景，通过数值模拟研究了多孔介质中均匀混合气形成的机理，同时对两种类型多孔介质发动机的着火、燃烧及排放进行了模拟计算，以期对多孔介质发动机的工作特性有初步的了解，为其将来走向实用化提供理论基础。首先研究了多孔介质中均匀混合气的形成，对喷雾油束与热多孔介质的相互作用进行了详细的数值模拟：(1)针对喷雾油束碰热壁时的Leidenfrost现象，发展了油束碰撞热壁的数学模型。此模型中油滴破碎后的直径分布由概率密度函数确定，破碎油滴的数目由油滴碰撞热壁前后的质量守恒求出，用四个油滴包表示油滴碰壁后破碎生成的油滴，选用Senda的油束碰壁传热模型。用结合此碰壁模型的改进KIVA-3V，针对Senda的实验条件进行模拟计算，与原KIVA-3V的计算结果相比，改进的KIVA-3V能更好地预测油束碰撞壁面后油滴和油蒸气的分布。(2)本文中多孔介质的三种简化结构模型各有优势、缺陷和不足，在这三种多孔介质简化结构模型基础上，详细研究了喷雾油束与热多孔介质的相互作用。单股油束碰撞到多孔介质后，在多孔介质中分裂为多股油束，这为油滴的快速蒸发及油蒸气和空气充分混合创造了条件。按照油滴的分布位置，可以分为在多孔介质上方的自由喷雾区、碰到多孔介质后反弹起的油滴、多孔介质内油滴和穿越多孔介质的油滴四部分。研究了空间压力及喷雾参数对喷雾油束与热多孔介质相互作用的影响，空间压力的影响主要是通过改变油滴运动阻力而实现，喷雾锥角、油滴直径是影响喷雾油束与多孔介质相互作用的重要参数。在多孔介质第一种结构简化模型中，研究了压力旋转雾化油束与多孔介质的相互作用。在第二种和第三种多孔介质结构简化模型中，考虑了多孔介质孔隙率的改变对喷雾油束与多孔介质相互作用的影响。在KIVA-3V中实现了多孔介质固相温度的求解，考虑了多孔介质对能量传输及组分扩散的弥散作用，并把KIVA-3V与详细化学反应动力学软件Chemkin-3.0相耦合。针对两种不同形式的多孔介质发动机，分别进行了燃用气体燃料和液体燃料时多孔介质发动机运行的数值模拟，并分析了其运行的参数影响。(3)针对多孔介质与气缸永久性接触型的多孔介质发动机，对其分别燃用气体燃料(甲烷)和液体燃料(异辛烷)的着火和排放特性进行了多维数值模拟研究。采用了GRI-1.2甲烷燃烧反应机理模拟甲烷的燃烧过程，分析了此发动机燃用气体燃料时的着火特性，讨论了多孔介质初始温度、多孔介质结构、燃料喷射时刻和燃料初始温度对此多孔介质发动机着火的影响。采用GRI-3.0甲烷燃烧反应机理模拟甲烷燃烧过程，对此多孔介质发动机燃用气体燃料的排放特性进行了分析。对此发动机燃用液体燃料运行过程进行了多维数值模拟。采用异辛烷的骨架机理模拟其氧化过程，采用多孔介质单元中心实心球的假设，结合第二章中发展的喷雾油束与热壁的碰壁模型，考虑喷雾油束与多孔介质之间的相互作用。此发动机燃用液体燃料时，燃料的蒸发速度是控制燃烧快慢的主要因素。多孔介质的弥散作用可加促油蒸气与空气的混合，缩短着火条件形成的时间。燃油起喷时刻对此发动机运行有显著影响，过早或过晚喷射燃料都会引起平均压力峰值的降低，进而影响发动机的动力水平，过晚喷入燃料还会引起燃烧后期NO生成量的增加。(4)针对带有阀门的多孔介质与气缸周期性接触型的多孔介质发动机，模拟了其燃用气体燃料(甲烷)和液体燃料(异辛烷)时的运行过程。通过改变多孔介质与气缸连接处计算网格单元的属性，实现阀门的闭合与开放。采用GRI-3.0机理和异辛烷的骨架机理，分别模拟甲烷和异辛烷的氧化过程。采用多孔介质单元中心实心球的假设，结合第二章中发展的喷雾油束碰撞热壁模型，以模拟喷雾油束与多孔介质之间的相互作用。讨论了多孔介质初始温度、多孔介质结构、阀门开启时刻对此发动机着火和排放的影响。多孔介质与气缸保持周期性接触型的多孔介质发动机，其工作特性完全有别于多孔介质与气缸保持永久性接触型的发动机。气体燃料与空气的混合及液体燃料的蒸发，不再是决定多孔介质中能否着火和控制燃烧速度的关键因素。此形式发动机不论燃用何种燃料，燃烧周期短，燃烧期内燃烧放热率高。从排放角度着眼，应该使用气固换热较强的多孔介质。阀门开启时刻影响多孔介质中的燃烧温度和NO生成量，从发动机动力性和排放两方面综合考虑，阀门开启时刻不宜过早也不能过晚。更多还原

【Abstract】 The problems of energy crisis and environmental pollution become more and more serious, renovations in the traditional energy technology are being focused. As a main energy consumer and the main emission source of pollution, the automotive engine faces double stresses as energy saving and environmental protection. Porous medium(PM) engine, as a newborn thing, is characterized with the advantages, such as flammability, low-pollution, high-efficiency and extended ignition limits, which will be received more attentions in future. With the background of PM engine, this dissertation is concerned the numerical study of the interactions between fuel spray and hot PM for understanding the mechanism of mixture formation in the PM. Numerical simulation about the properties of compression ignition, combustion and emission of two types of PM engine is another subject in the dissertation. The principle ai,m of this work underling the thesis has been to have a primary recognition about the working characteristics of PM engine, with the purpose of providing some theoretical foundations for its application.Firstly, in order to study the formation of homogenous mixture in PM, numerical simulations about the interaction between fuel spray and hot PM were performed.(1) An impingement model was improved for simulating the process of fuel droplets collision with hot wall, where Leidenfrost phenomena occurred. The velocity of splashed droplets was calculated by the method of probability density function, and the number of the splashed droplets was derived based on the mass conservation. In the model, four parcels are used to represent all of the splashed droplet and heat transfer between droplets and hot wall was described with the formula proposed by Senda.. Under the experimental conditions of Senda, the process that fuel spray vertically impinges on a high temperature wall was simulated by using the modified KIVA-3V. Compared with the results by original KIVA-3V, the results by the modified KIVA-3V made more well agreement with experimental results.(2) Numerical simulations were conducted for investigating the interactions between fuel spray and three simplified models of PM with the modified KIVA-3V, which was implemented with the new impingement model. In all of computational results, the single fuel spray was separated into several small fuel streams in the PM after colliding with the PM, which provided the conditions for the fast evaporation of droplets and the formation of homogenous mixture. There were four characteristic phases of fuel spray interaction with the PM, the first represented Outlet from the nozzle and free jet formation, the second represented jet interaction with PM-interface, and the third represented liquid distribution throughout the PM-volume and the fourth represented liquid leaving the PM-volume.Each of the three simplified models of the PM deserved advantages and defects. The influences of ambient pressure and spray parameters on the interactions between fuel spray and PM were studied computationally. The variation in ambient pressure can modify the resistant force to fuel droplets. Spray cone angel and droplet diameter are the most influential factors in affecting the interactions. The interaction between pressure swirl spray and the first simplified model of PM was simulated. The effects of porosity on the interactions were also evaluated in the second and third simplified model of PM.KIVA-3V was improved by incorporating with the chemical kinetics software Chemkin-3.0. An additional energy conservation equation for describing the temperature of the solid phase in the PM was included in KIVA-3V. The dispersion effects of the PM on energy and species diffusion were also considered. The working processes of two types of PM engine, fueled with gas fuel and liquid fuel, were simulated respectively, and the effects of some important factors on the working of PM engine were also analyzed.(3) The characteristics of combustion and emission of PM engine fueled with gas fuel and liquid fuel, with permanent contact between PM and working gas in engine cylinder, were investigated numerically. The GRI 1.2 chemical kinetic mechanism was used for modeling the oxidation of CH4 in the PM engine. We investigated the effects of various parameters that were expected to control the onset of the compression ignition of the PM engine, such as the initial PM temperature, the PM structure and fuel injection timing. The emission properties of the PM engine were investigated by using the GRI 3.0 chemical kinetic mechanism.In the simulation cases about the working processes of the PM engine fueled with liquid fuel, a skeleton mechanism for iso-octane oxidation was used and the interactions between fuel spray and the PM were considered by applying the second simplified model of PM and the impingement model developed in chapter two. The effects of different operating conditions on the characteristics of ignition and emission of the PM engine fueled with iso-octane were evaluated.The evaporation rate of iso-octane is the most important factor in controlling the combustion velocity of the PM engine fueled with iso-octane. The dispersion effects of the PM can accelerate the mixing process between the fuel vapor and air, and decrease the time needed to achieve the ignition conditions. The fuel injection timing has outstanding influences on the working of the-PM engine fueled with iso-octane. Over-early or over-late fuel injection timing induce the reduction of the maximum value of average pressure, and over-late fuel injection timing also make the increase of the NO amount. (4) The characteristics of combustion and emission of PM engine fueled with gas fuel and liquid fuel(iso-octane), with periodic contact between PM and working gas in engine cylinder, were investigated numerically. The close and open processes of the valve between the PM and engine cylinder were simulated by changing the properties of the computation cells at the interface. The GRI 3.0 chemical kinetic mechanism and the skeleton mechanism were used for simulating the oxidation of CH4 and iso-octane, respectively. The interactions between fuel spray and the PM were considered by applying the second simplified model of PM and the impingement model developed in chapter two. The effects of the initial PM temperature, the PM structure and valve opening timing on the characteristics of ignition and emission of the PM were discussed.The characteristics of the PM engine, with periodic contact between the PM and working gas in engine cylinder, are completely different from those of PM engine with permanent contact between the PM and working gas. The mixture formation is not important factor in determining the onset of ignition and controlling combustion velocity. Regardless of the type of fuel, the combustion period is very short and the heat release rate is also very high in the PM engine. From the point of view of the emission, the PM with enhanced intensity of heat exchange between the solid and gas phase should be adopted. The timing of valve opening has influences on the temperature in the PM and the NO amount. For obtaining power and low emission, the timing of valve opening should be an optimal value.更多还原