【作者】 张辉亚；

【导师】 张煜盛； 魏明锐；

【作者基本信息】 华中科技大学 ， 动力机械及工程， 2007， 博士

【摘要】 二甲醚作为一种理想的柴油机替代燃料,近年来受到了众多研究者的广泛关注。针对其与柴油不同的特点,本文采用先进的激光测试手段与多维数值模拟相结合的方法,对二甲醚喷雾混合和燃烧的发展变化历程进行了深入的研究。研究内容和取得的成果主要体现在以下几个方面:为了提高喷雾模拟的精度,对O’Rourke液滴碰撞模型的网格依赖性问题进行了分析,提出了以交错网格和碰撞限制角相结合的CMC液滴碰撞改进算法。分别在定容燃烧室和发动机气缸内,从喷雾结构、索特平均半径(SMR)和喷雾贯穿距三个方面,分析了网格型式、网格精度以及喷雾粒子数对改进算法计算结果的影响。结果表明:CMC液滴模型在极坐标网格和直角坐标网格下,均具有较低的网格依赖性,和较低的网格精度和粒子数敏感性;但在喷雾贯穿距预测方面,CMC液滴算法只是略好于O’Rourke液滴碰撞算法。这主要是由于CMC算法设计用来较好的解决液滴碰撞的径向分量,而喷雾贯穿距是液滴轴向运动的测量。在综合研究了二甲醚喷雾的有关实验文献后,发现二甲醚喷射过程中易于出现闪急沸腾现象。以TSI公司的数字PIV系统为基础,构建了适用于喷雾研究的PIV喷雾测试系统。根据CCD照相机捕获的喷雾图像,分析了单孔直喷式柴油喷嘴形成的二甲醚闪急沸腾喷雾的结构及其发展过程。结果发现:按照喷雾轴线方向,闪急沸腾喷雾的结构可以分为喷雾初始核区,闪急沸腾喷雾的主体区和涡环形成区。根据涡环结构的形成与发展,其喷雾发展过程可以分为以下四个主要阶段,即涡环结构的形成阶段,涡环结构的发展与变形阶段,涡环结构的破碎阶段,以及充分发展的闪急沸腾喷雾阶段。另外,由于微爆效应的作用,二甲醚闪急沸腾喷雾与柴油喷雾在宏观特性方面也表现出较大的不同。借助于PIV喷雾测试系统,对二甲醚普通喷雾的内部特性及其相应的机理进行了研究。与柴油喷雾相似,二甲醚喷雾体内存在着不均匀的枝状结构;但喷雾体内燃油的分布特性则与柴油存在着较大的差别,二甲醚喷雾体内存在着清晰可分的高浓度燃油区和低浓度燃油区。随后,对二甲醚喷雾的速度场和涡量场等流动信息进行了测量,并将不同截面二甲醚喷雾平均轴向速度的无因次速度分布与阿勃拉莫维奇速度分布剖面进行了对比,发现在喷雾体中间部分两者吻合的较好,但在喷雾体边缘处阿勃拉莫维奇经验公式预测的值偏小;为了在准维燃烧模型中更为准确的描述二甲醚喷雾混合及其燃烧过程,给出了一个新的适用于二甲醚喷雾的速度分布公式。另外,还就撞壁对二甲醚喷雾的影响进行了研究。以ZS195单缸直喷式柴油机为原型机,通过对其燃油系统进行改装和调试,建立了直喷式二甲醚发动机的试验台架。在三种不同工况条件下,对柴油机燃用二甲醚的燃烧过程进行了试验研究。试验结果表明,对于ZS195柴油机来说,采用27°CA BTDC的供油提前角更为合适;并分析了进气温度和压缩比对二甲醚发动机燃烧与排放的影响。另外,采用类似于多点喷射式汽油机的混合气组织方式,建立了燃用DME的HCCI发动机试验台架。在三种发动机部分负荷工况条件下,对DME HCCI发动机的燃烧过程和排放特性进行了研究。在HCCI模式下二甲醚燃烧呈现出明显的两阶段放热特性,燃烧过程中存在的爆燃现象阻碍了发动机运行工况向中高负荷范围的扩展。为了深刻认识直喷式二甲醚发动机着火燃烧的机理,判明二甲醚发动机与柴油机在燃烧特性与排放方面存在较大差异的根本原因,本文采用改进的KIVA程序,对ZS195单缸直喷柴油机燃用二甲醚的着火燃烧过程进行了数值模拟与参数分析研究。结果表明:二甲醚发动机的着火燃烧过程之所以与柴油机表现出较大的不同,主要是由于二甲醚具有良好的雾化和蒸发特性。在缸内气流运动的影响下,二甲醚可燃混合气易于在燃烧室壁面附近涡旋运动的下游区域聚集而发生自燃着火;随后在缸内卷吸气流的作用下,从燃烧室壁面附近开始向气缸中心线方向移动,燃烧区域的火焰也随之向气缸中心线方向扩展,最终充满整个燃烧室空间。更多还原

【Abstract】 Recently, as an ideal alternative fuel for diesel engine, dimethyl ether(DME) has been attracting much attention from many researchers. Because the properties of DME are large different from diesel fuel, the spray mixture and combustion processes of DME are investigated in detail using advanced laser technique and multi-dimensional simulation in this paper. The main contents and achievements of this research are as following.In order to improve accuracy of spray simulation, the grid dependency of O’Rourke droplet collision model is analyzed, and proposed a cross mesh droplet collision(CMC) algorithm which is a combination of cross mesh method and collision limit angle. In constant volume apparatus and D.I. diesel engine, the effects of mesh schemes, mesh resolutions and parcel numbers to the calculation results using modified algorithm are checked from spray structure, Sauter mean radius(SMR) and spray penetration, respectively. The results show that: CMC droplet model is less mesh-dependent and less sensitive to mesh resolution and parcel number in both polar mesh and Cartesian mesh. However, the predicted spray penetration using CMC droplet model is only slightly better than that of O’Rourke algorithm. The main reason is that the CMC method is designed to better resolve the radial component of the droplet collision, the spray penetration is a measure of axial motion.Through completely analyzing the papers about DME spray, found that flash boiling phenomena are easily occurred in DME spray. Based on the digital particle image velocimetry (PIV) system of TSI Company, a PIV test bench for DME spray investigation is set up. According to the spray images captured by CCD camera, the spray structure and developing process of DME flash boiling spray formed by a single-hole D.I. diesel injector are analyzed. The results show that: according to the axial direction, the spray structure of flash boiling spray can be classified into follows: spray initial core region; main body region of flash boiling spray; vortex ring formation region. Based on the process of vortex ring structure formation and development, the flash boiling spray development can be classified into four stages: vortex ring structure forming stage; vortex ring structure developing and distorting stage; vortex ring structure breaking up stage; full development stage of flash boiling spray. In addition, due to the effect of micro-explosion the spray macro-characteristics of DME flash boiling spray are large different from diesel fuel. The internal characteristics and forming mechanisms of DME conventional spray are investigated using the above PIV test bench. Similar with diesel fuel spray, there are non-homogeneous“branch-like structures”in DME spray. However, the fuel distribution characteristics of DME spray are large different from diesel fuel, and there are apparent high concentration fuel region and low concentration fuel region in DME spray body. Subsequently, the velocity and vorticity of DME spray are measured, and compared the dimensionless velocity distributions at each cross section with Abramovich distribution. It is found that the dimensionless velocity of DME spray is better consistent with the predicted value using Abramovich distribution in the middle of spray, but the predicted value using Abramovich function is less than the experimental results at the edge of spray. In order to improve the simulation accuracy of mixture and combustion process of DME spray using quasi- dimensional combustion model, a new velocimetry distribution function which is suitable for DME spray is proposed. In addition, the influence of spray impingement against the wall on DME spray is investigated.Based on a single cylinder diesel engine ZS195, the test bench of D.I. DME engine is set up through the modification of fuel system. In three different conditions, the combustion processes of diesel engine fuelled with DME are analyzed. It is found that the advance angle of oil supply of 27°CA BTDC is better for ZS195 diesel engine fuelled with DME, and investigated the effects of intake air temperature and compression ratio on the combustion and emission of DME engine. In addition, the test bench of DME HCCI engine is set up using the mixture formation method similar with M.P.I. gasoline engine. In three partly engine loads, the combustion process and emission characteristic of DME HCCI engine are investigated. In HCCI condition the DME combustion shows clear“two stages”heat release, and the detonation phenomena in combustion process impede the expansion of engine performance to medium or high load.In order to profoundly understand the ignition and combustion mechanics of D.I. DME engine, and clear the essential reason that there are large difference in combustion and emission characteristics between DME engine and diesel engine, this work analyzes the ignition and combustion processes of single cylinder diesel engine ZS-195 fuelled with DME using modified KIVA code. The results show that: The main reason that the ignition and combustion processes of D.I. DME engine are large different from diesel engine is that DME has better atomization or evaporation characteristics. In the influence of flow in cylinder, the combustible mixtures of DME easily accumulate at the wall of combustion chamber and downstream of vortex, and complete auto-ignition. Subsequently, the combustible mixtures are moved from the wall of combustion chamber to the axis of cylinder by the action of entrainment flow in cylinder, and the flames of combustion region are also expanded to the axis of cylinder and finally overflow the whole combustion chamber.更多还原