喷雾碰壁模型及柴油机冷起动的数值研究

发布时间：2017-12-31 02:32

本文关键词：喷雾碰壁模型及柴油机冷起动的数值研究　出处：《湖南大学》2016年博士论文　论文类型：学位论文

【摘要】：内燃机缸内过程的多维数值模拟技术已经成为当前汽车发动机产品先进设计与开发的重要手段。与一维模拟相比,多维模拟包含流场的空间信息,通过这些结果可以洞察燃油的喷射、混合气的制备、燃烧的过程以及排放物生成的更多细节,其对于先进发动机技术开发的重要性不言而喻。可靠的多维模拟需要准确地模拟发动机工作的各个物理化学过程,比如燃油的喷射、雾化和蒸发,因此需要建立不同的数值模型来完成这些任务,燃油的喷雾碰壁模型就是其中之一。近年来,随着石油资源的日益枯竭以及全世界对环保问题的更加关注,内燃机的节能与环保成为技术发展的新要求,缸内直喷技术得到了越来越广泛的应用。无论是柴油机还是直喷汽油机,冷起动工况都是有害排放最严重的工况之一。冷起动时由于缸内热环境差,燃油雾化蒸发都不好,为了顺利起动需要增加喷油,因此会有大量喷雾油滴碰壁形成壁面油膜,这是其排放恶劣的主要原因之一。为了准确地模拟内燃机中的喷雾碰壁现象并探讨它对直喷发动机冷起动工况下排放的影响,需要开发更加精确的喷雾碰壁模型,这就就构成了本文研究的核心内容。本文首先对内燃机多维模拟中广泛应用的开源程序KIVA-3V Release2的框架进行了简单的介绍,并重点介绍了喷雾模拟的相关子模型。接着对现在广泛使用的一些喷雾碰壁模型进行了详细分析,对比了它们之间的差异和优缺点。结果表明:现有模型开发较早,建立它们的试验来源多为单个液滴或液滴串的碰壁试验,与现代高压共轨直喷发动机中的实际现象有较大出入,因此在某些工况下这些模型对高压喷雾碰壁的模拟效果并不能令人满意,需要开发新的数值模型。根据上面的分析提供的思路和方向,本文根据高压喷雾碰壁的特点,提出了三个方面的改进意见:一是针对CFD软件中喷雾计算的特点,提出了壁面射流子模型来修正碰壁液滴的气液相对速度,从而能更加准确地计算液滴的阻力,而且极大地降低了计算对网格的依赖性;二是针对高压喷雾碰壁后运动形态的特点,针对碰壁后液滴增加了升力的计算,从而能再现试验中观测到的壁面喷雾前端的涡旋;三是针对单个液滴碰壁和喷雾碰壁的差异,提出了计算高压喷雾碰壁后粘附油膜比例的公式。通过与柴油喷雾试验结果对比,新模型能明显地改进预测效果,预测的壁面喷雾运动形态、贯穿度和厚度等与试验值吻合良好。此外,在喷雾碰壁过程中同时发生的还有热力学过程,即喷雾和壁面之间的传热传质过程,这一过程虽然短暂,但也十分重要。在现有的数值模型中,液膜和壁面之间的传热基本都是按照热传导计算的,没有考虑喷雾碰壁过程对传热的巨大影响。为此,本文基于柴油喷雾碰壁的传热学试验开发了一个新的子模型来考虑这一过程。新模型考虑了不同喷雾碰壁情况下,碰壁液滴的动能对传热的影响,而对于气相和液膜之间的热交换则采用了气体射流碰壁下的传热关系式来计算。模拟结果表明,新模型与试验数据吻合良好,是现有液膜传热模型的有益补充。本文最后将开发的高压喷雾碰壁的动力学和热力学模型应用于某款小型直喷柴油机冷起动工况的模拟,并探讨了排放物的生成机理和不同喷油规律对排放的影响。结果表明:冷起动时会有大量的壁面油膜产生,这些油膜是HC和CO排放的直接来源;燃烧之后壁面上剩余的油膜最多仍有25%,和试验值比较接近;相对于单次喷射,采用两次喷射有助于改善喷雾蒸发,从而减少排放。
[Abstract]:The simulation technology has become an important means of advanced design and development of automobile engine products the multi-dimensional numerical processes of internal combustion engines. Compared with the one-dimensional simulation, simulation of multi-dimensional spatial information including flow field, through these results can insight into the jet fuel, gas mixture preparation, combustion process and more details of emission, the the importance of advanced engine technology development is self-evident. Reliable multidimensional simulation of each physical and chemical processes need to accurately simulate the engine work, such as fuel injection, atomization and evaporation, because of the need to establish different numerical models to accomplish these tasks, the fuel spray impingement model is one of them. In recent years, with the increasing depletion of oil resources and all over the world pay more attention on environmental issues, energy saving and environmental protection has become the new engine technology development for cylinder Direct injection technology has been more and more widely used. Either diesel or gasoline direct injection engine, cold start condition is one of the most serious harmful emissions. Due to the cold start cylinder thermal environment, fuel atomization and evaporation are not good, in order to successfully start to increase fuel injection, so there will be a lot of spray droplets the formation of wall wall film, which is one of the main reasons for its bad emissions. In order to accurately simulate the engine spray impingement phenomenon and to explore it in a direct injection engine cold start effect under the condition of emissions, need to develop a more accurate model which was spray impingement, constitute the core content of this paper. The framework of this the simulation of the internal combustion engine is widely used in the multi-dimensional open source KIVA-3V Release2 are introduced, and mainly introduces the related simulation model. Then the spray is now widely used Some spray impingement model in detail, compares the differences between them and their advantages and disadvantages. The results show that the existing model developed earlier, establish their test source for a single droplet on the wall or drop test, and the modern high pressure common rail direct injection engine in the actual phenomenon are quite different, so in in some cases these models to the effect of high pressure spray impingement is not satisfactory, need to develop a new numerical model. According to the above analysis to provide ideas and direction, according to the characteristics of high pressure spray impingement, puts forward suggestions from three aspects: first, according to the characteristics of CFD spray calculation software, put forward the wall jet model to correct the liquid droplet impingement velocity, which can more accurately calculate the droplet resistance, and greatly reduces the computation of the mesh dependence; two is to High pressure spray impingement after the movement characteristics of droplets increased after hitting the wall for lift calculation, which can reproduce the experimental vortex observed in the wall of the front end of the spray; three is the difference of single droplet impingement and spray impingement, the calculation of high pressure spray impingement after the adhesion of oil film formula. The proportion of compared with diesel spray test results, the new model can significantly improve the prediction effect, prediction of wall spray movement patterns, penetration and thickness agree well with experimental data. In addition, also occurred in the process of spray impingement and thermodynamic process, namely heat transfer process between spray and wall, although this process short, but is also very important. In the existing numerical model, heat transfer between the liquid and wall are calculated according to the heat conduction, without considering the great influence on the process of spray impingement on heat transfer for this ring, In this paper, the experimental study on heat transfer of diesel spray impinging on a new model based on the consideration of the development process. The new model takes into account the different spray impingement case, impingement droplet on the heat transfer effect of kinetic energy, and the heat exchange between the gas and liquid by gas injection flow under the impingement heat transfer formula to calculate. The simulation results show that the new model is in good agreement with the experimental data, is a useful complement to the existing heat transfer model. Finally, high pressure spray will develop into the kinetic and thermodynamic model is applied to a small direct injection diesel engine cold starting condition simulation, and discusses the influence mechanism of emissions and fuel the law of discharge. The results showed that the cold start there will be a lot of wall film, the film is a direct source of HC and CO emissions after combustion; remaining on the surface of the oil film is still up 2 5%, it is close to the test value; compared with the single injection, the two injection helps to improve the spray evaporation and thus reduce the emission.

【学位授予单位】：湖南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：U464.172

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