【作者】 魏明锐；

【导师】 刘永长；

【作者基本信息】 华中科技大学 ， 动力机械及工程， 2004， 博士

【摘要】 二甲基醚(DME)作为柴油的代用燃料,与柴油相比,其理化特性有着明显的差异。而燃油喷雾混合过程是发动机缸内热力过程的基础,对后续的燃烧和排放性能具有决定性的作用。在阅读和分析大量国内外相关文献的基础上,针对迄今对雾化机理认识不足和燃油碰壁现象较为复杂的现状,并考虑到二甲基醚的理化特性,本文以理论研究为主,结合数值模拟计算与实验研究,较为深入地对二甲基醚喷雾特性进行了研究。根据油嘴内燃油空穴产生的机理,建立了油嘴流动模型,以准确地描述具有髙饱和蒸气压的二甲基醚在油嘴内的流动过程,为模拟研究缸内燃油雾化过程提供初始条件(如射流速度和直径),从而将油嘴内的流动过程与缸内的雾化混合过程联系起来。为燃油喷雾模拟研究奠定了基础。将燃油在油嘴内与缸内流动、雾化联系起来对DME进行的模拟研究,迄今在国内处于领先地位。首次考虑蒸发对射流表面非轴对称不稳定扰动的影响,并推导了色散方程。以色散方程为基础,详细分析了二甲基醚的表面张力、粘性系数、射流速度和环境气体密度等对二甲基醚射流表面不稳定性的影响,从中认识了二甲基醚的雾化特性,加深了对表面不稳定气动雾化机理的理解。在现有单液滴零维蒸发模型的基础上,考虑液滴运动、物性参数、高压以及液滴群等对蒸发过程的影响。本文建立了液滴蒸发的一维数学模型(又称有限热传导蒸发模型),并给出了TDMA追赶法的数值计算过程和移动边界的处理方法。研究表明:本文所建立的单液滴一维蒸发模型可以更准确地模拟喷雾液滴的蒸发过程。分析和归纳了前人对喷雾碰壁的研究成果,考虑液滴、壁面、油膜上方气体层之间的相互作用,即动量和热量的交换过程,以离散液滴油膜铺展和撞壁飞溅为主建立了喷雾碰壁数学模型。利用该模型对二甲基醚碰壁过程进行了模拟计算,并与柴油作了对比分析。研究表明:二甲基醚蒸发较快,与柴油相比,油膜厚度较薄,说明,二甲基醚碰壁后在壁面的沉积会大为减弱,这在燃烧过程中是非常有利的一面。基于KIVA3程序,以张量的形式叙述了数值解法中的交错网格系统、求解步骤、控制方程及其离散、状态方程、气液耦合、计算步长的控制以及边界条件等。在方程离散过程中,给出了时间瞬态项的差分格式以及求解边值的中心和上风差<WP=5>分格式。为模拟研究缸内的湍流流动,本文以RNG双方程模型为基础对湍流模型进行了论述。为整个喷雾过程的数值模拟计算奠定了基础。利用高速摄影技术,本文在高压定容室内对二甲基醚及其他燃料进行了喷雾实验研究,其中二甲基醚的碰壁实验研究在国内尚属首次。实验中考察了喷射压力、环境背压、油嘴半径以及碰壁入射角度和碰壁距离等因素对喷雾特性如贯穿距、喷雾锥角、壁面扩展度、壁面喷雾体和油膜厚度、空气卷吸等的影响。归纳和分析了实验测得的大量原始数据,获得了许多有价值的研究结论,为国内在该领域的研究提供了可靠的第一手实验资料和经验。同时,依据实验条件进行了数值模拟计算,并与实验结果进行了比较,深化了对二甲基醚喷雾混合特性的理解,为二甲基醚最终满足工程应用提供理论依据和技术措施。更多还原

【Abstract】 The property of dimethyl ether (DME), which is used as a substitute for diesel, is obviously different from diesel. And fuel spray, which is as the source of in-cylinder working process, has the most important influence on the subsequent combustion and exhaust of engines. Unfortunately, the mechanism of liquid jet breakup is not understood completely, and the process of fuel wallinteraction is complicated. So, after reading a great number of literature on relevant research field, and basing on the theory, the author take a thorough study on the spray characteristics of DME by combining experiments with multi-dimensional numerical simulation.According to the formation mechanism of cavitations inside a nozzle, a nozzle flow model is established and first applied to DME that possesses higher saturation vapor pressure in this thesis, which can simulation more truly the flow status of DME and provides accurately initial parameters such as the velocity and diameter of a liquid jet for in-cylinder atomization. It is obvious that the nozzle flow model links the flow process inside a nozzle with the in-cylinder spray process.A dispersion equation of three-dimension unstable disturb wave on the surface of a viscous liquid jet with evaporating is first derived. And based on the dispersion equation which has been simplified, analysis analyses in detail the effects of surface tension, liquid viscosity, gas density and liquid jet velocity upon the surface instability of DME liquid jet. Form the analysis, the deeply understand about the spray characteristics of DME liquid jet and the aerodynamic atomization mechanism of surface unstable disturb wave is obtained.Based on the existing zero-dimensional evaporation model, a one-dimensional one (it is also called limited heat conductivity evaporation model) is constructed, which takes into account the effects of droplet movement, liquid property, high pressure and droplets group upon the evaporating process. And a solving method of TDMA(Tri-diagonal Matrix Algorithm) and the treatment method of moving boundary for the model are given. Droplets evaporation during spray process can be simulated more accurately by using the present model.Based on the detailed comparisons and analysis to many kinds of spray im<WP=7>pingement models, two representative spray impingement models, wall spread and splashing, are introduced in this thesis. In the model, the exchange of heat and momentum between the wall, droplets and the gas above liquid film are modeled. The spray impingement of DME is simulated and compared with that of diesel. Due to DME quickly evaporating, its film thickness is much smaller than the one of diesel, which show that the accumulation of DME on the wall is not serious to be of advantage to combustion process.The stagger grid system, solving process, governing equations and numerical approaches, gas state equations, the coupling of gas-liquid, the calculating of time step, and boundary conditions and so on are emphatically introduced in the form of tensor, according to the KIVA3. During the process of numerical approximations, the difference formula of time transient terms, and the central/upwind difference expressions for acquiring boundary value are chosen. And the turbulence models, especially the RNG model, are depicted here. Through the above mentioned, a ground for in-cylinder spray numerical simulation is made.Using the high speed camera, the liquid spray experiments of DME and other fuels in pressure constant volume chamber are carried out, of which the spray wall impingement tests of DME is the first time in domestic researches. The effects are tested of inject pressures, ambient pressures, the diameters of nozzle hole, impinging angles and distances, etc. upon the spray characteristics such as penetration, spray cone angle, wall spread, the thickness of spray body and liquid film on the wall, air entrainment, and so on. A great deal of data is obtained from experiments and is treated, and many valuable conclusions are derived, which provide reliable origin更多还原