【作者】 李兵；

【导师】 颜伏伍； 胡杰；

【作者基本信息】 武汉理工大学 ， 动力机械及工程， 2012， 硕士

【摘要】 在我国汽车工业取得迅猛发展的同时,汽车尾气引发的环境问题越来越严重。综观近几年的相关统计数据,汽车排放物对环境污染的贡献度一直居高不下并有进一步升高的趋势。据统计,2010年全国柴油车排放的NOx接近汽车排放总量的60%,PM超过90%。结合我国的国情,经过多方的论证,SCR路线被公认为适合我国国情并能有效控制柴油机排放满足国Ⅳ和国Ⅴ排放标准的技术路线。目前国内满足国Ⅳ排放法规的柴油机所使用的SCR系统都是国外公司提供的产品,价格昂贵并且技术封锁,不利于国内SCR技术的普及推广以及二次开发。结合国内的制造水平及采购条件,文中提出了一种将尿素溶液的计量和雾化分开进行的新型尿素溶液供给系统,这种技术方案可以解决尿素溶液的计量和喷射雾化问题。此套尿素溶液供给系统共包括四个工作模式,分别为预注模式、计量模式、排空模式和停止模式。为解决尿素溶液在低温条件下的结冰问题,文中对尿素溶液供给系统的热管理进行了探讨,避免尿素溶液的结冰或结晶现象对管道及雾化器造成堵塞。SCR系统最理想的工作状况是在实现NOx的高转化率前提下,保证还原剂NH3的低逃逸率。要达到这两个要求,就要保证以下三个性能指标：良好的液滴蒸发及热解水平,合理的空速分布,合理的还原剂浓度分布。SCR供给系统要保证液滴进入催化器载体内部之前已经蒸发热解完全；合理的空速分布状况能够保证还原剂与反应物足够的接触及反应时间；而合理的还原剂分布则是避免出现局部氨过量系数过高或者过低的决定性条件。文中借助于流体计算软件建立了三维仿真模型,对SCR系统的喷雾过程进行了模拟,分析了尿素溶液喷射后的流场的瞬态变化,还原剂的形成以及与排气的混合过程。通过查看相关文献,并结合当前所选用催化器的特点,讨论了混合器的存在与否、喷射距离的大小、喷孔的分布形式以及喷孔直径的大小对SCR评价指标的影响,以此来判断催化器的工作性能。通过控制变量的方法,最终得到了适合当前研究状况的尿素溶液供给系统部件的布置形式,为后续的结构优化和样件制作提供一定的技术支持。更多还原

【Abstract】 The automotive industry of our country has been developing rapidly in the last few years, which has caused severe environmental problems. According to the statistical data of the last few years, the vehicle emission has contributed greatly to the environmental pollution and this phenomenon is getting worse. The amount of NOx from diesel engine accounted for60%of the overall vehicle emission, and PM90%in2010. In view of the condition of our country, SCR is deemed as the proper solution to the diesel emission problems to satisfy the emission standard of level four as well as level five in our country after a lot of argumentation.For now, all the SCR systems used on the diesel engine to meet the emission standards are provided by foreign companies. The systems are expensive. Besides, it is difficult to popularize the technology and to progress secondary development for the domestic manufacturers because of the suppliers’blockade on techniques. A new kind of Urea solution supply system is developed, considering the domestic level of manufacture and purchase. This new system can guarantee the accurate measure and good atomization of Urea solution. It has four working modes, and they are priming mode, doing mode, purging mode and off mode, respectively. In addition, the thermal management is discussed in order to solve the freezing problem, which can avoid the jam of the pipe and atomizer.The perfect working status of SCR system is achieving high NOx reduction rate along with low NH3escape rate. These two targets can be obtained if the SCR system is working with good droplet evaporation and thermolysis, sound distribution of airspeed, and sound distribution of reductant concentration. Before the droplets get into the carrier, they must have or almost have evaporated and thermolyzed completely. Good distribution of airspeed can guarantee the enough contact and reaction time between the reactant and catalyzer. The appropriate excess ammonia coefficient can be achieved, only if the concentration distribution of the ammonia before the carrier is good. In this thesis, a three-dimensional simulation model is built up to simulate the transient atomization process. The flow field changing, the reductant forming, and the mixing process of reductant with exhaust gas are analyzed. Allowing for the geometric characteristics of the catalyst in the bench test, four influencing factors are discussed in this thesis. The status of the catalyst can be judged through the influence they have over the performance criteria. Through the controlling of parameters, the simulations are carried out many times and a good layout of the supply system components, which can accord with the current research condition, is confirmed. After all these comparison and discussion, the subsequent structure optimization and sample making will have some principles to refer to.更多还原