【作者】 温苗苗；

【导师】 高孝洪； 吕林；

【作者基本信息】 武汉理工大学 ， 动力机械及工程， 2009， 博士

【摘要】 从国内外NOx排放现状与发展趋势来看,机动车对NOx排放的贡献越来越大。在机动车持续增长和能源紧缺的形势下,发展和完善贫燃发动机NOx控制技术对减少全球NOx排放有十分重要的意义。选择性催化还原（SCR）被认为是目前最有效的柴油机NOx控制技术。由于还原剂氨存在运输风险,采用32.5%尿素水溶液代替,它可以在车辆排气系统中分解成氨气。采用尿素水溶液作为还原剂,对车用SCR系统结构设计提出了更高的要求。本文在深入理解Urea-SCR系统反应理论的基础上,建立了可以预测车用SCR系统NOx转化率的数值模型,并通过计算揭示了SCR排气后处理系统内部运动规律,包括尿素水溶液蒸发、分解、混合气形成过程及催化器内发生的选择性催化还原反应过程。主要研究内容为:（1）在了解系统工作原理和时间标尺的基础上,总结了SCR系统建模过程中涉及的相关理论和数学模型。在计算流体动力学模型基础上加入了尿素分解反应与SCR反应动力学模型,实现了较完整的车用SCR系统过程的模拟。（2）在一维定常流假设基础上建立了理想的SCR催化器反应模型,并在该模型基础上校验了所选SCR反应速率常数对试验催化剂的适用性,计算了不同温度下同物质的量的NO与NH₃完全反应所需要的时间。计算分析了不同氨过量系数及空速对催化器转化效率的影响,这些计算可以为尿素供给策略设计、催化器选型提供依据。（3）建立了Urea-SCR系统三维模型,试验NO转化率和系统压降验证了模型的可靠性。分析了尿素热解反应速率常数、喷雾初始粒子直径、排气流速对计算结果的影响,进一步验证了文中所选模型的合理性。计算结果展示了一个喷射周期内流速、各组分浓度等参数的分布,揭示了尿素水溶液到氨的转化过程及混合过程。结果表明液滴蒸发、尿素蒸气热解过程的时间尺度与选择性催化还原反应过程的时间尺度相当,对整个车用Urea-SCR系统进行三维模拟时,不能忽略液滴蒸发、尿素蒸气热解过程。在模拟研究基础上,文中提出了SCR系统结构的评价指标。（4）在三维模型基础上比较了不同排气管结构型式、混合器型式、喷嘴结构及位置对催化器转化效率的影响,得出了一些指导性的结论。结构比较计算表明尿素水溶液液滴蒸发、分解及混合过程的研究对车用Urea-SCR系统结构设计具有非常重要的意义。更多还原

【Abstract】 In the last few decades, the environmental effects of nitrogen oxides （NOx） from combustion sources have become increasingly serious, especially from motor vehicles. The diesel and lean-burn spark ignition gasoline operate with a high air-fuel ratio, thus allowing fuel-economy improvements. With the continued growth in motor vehicles and energy, the development of a DeNOx technology for vehicles equipped with lean-burn engines represents the main challenge of automotive emission control in the forthcoming years. The Selective Catalytic Reduction （SCR） of NOx by ammonia is currently considered the most effective technology for high NOx removal from lean-burn diesel engines. Ammonia in pressurized containers represents a safety risk when carried onboard a vehicle. A eutectic solution of 32.5% urea in water is used instead since they can be decomposed in the vehicle’s exhaust gas system to form ammonia.Urea is a preferred reducing agent for automotive applications. Nevertheless, the essential thermal decomposition of urea into NH3 and carbon dioxide makes the deNOx process and design of automobile SCR system complicated. An integrated computational fluid dynamics （CFD） methodology could not only assist the design of efficient and optimized SCR systems, but also reduce the number of design loops and development expenses. In this paper, a modeling approach to design optimization of SCR system with the aid of CFD coupled with chemical reaction dynamics is present including evaporation of urea-water-solution, subsequent thermal decomposition and surface catalytic reactions in the monolith. The main contents of this paper are as follows.（1） The principle and time scales on working processes of the SCR system are studied. The relevant theory and mathematical models are summarized for modeling of automobile SCR system. In order to achieve simulation of complete processes of the mobile diesel SCR system, kinetic models of urea thermal decomposition and standard SCR reaction are coupled to a two-phase flow model.（2） A one-dimensional steady model is set up to describe the performance of an ideal SCR monolithic reactor such as NO conversion efficiency and NH₃ slip. According to analysis of experimental results obtained on the diesel test stand and calculated results, kinetic data of SCR reaction are chosen for the commercial catalyst. The time needed for SCR reactions at different temperatures are calculated when NH3 is abundant relative to NO_x （n（NH₃）/n（NO_x）≥1） . Dependence of NO conversion efficiency on n（NH₃）/n（NO_x） and space velocity at different temperatures are studied on the one-dimensional steady model. The conclusions from these researches can be used to optimize design of urea supply strategy and selection of catalytic converters.（3） A detailed three-dimensional numerical modeling of Urea-SCR systems, integrating the characteristics of urea spray, the mixture of chemical species, and the complex turbulence mixing flow pattern, is discussed in this study to predict the performance of mobile diesel SCR system. The Simulation result shows a good agreement with experimental data. The effects of urea thermal decomposition rate constant, initial particle diameter and exhaust flow rate on the calculation results are examined to further verify reasonableness of the model. The results illustrate the droplet propagation in the mobile diesel SCR system, evaporation, urea decomposition and SCR reaction profile with flow and concentration distribution in an injection cycle. The time scales of evaporation of urea-water-solution droplets, subsequent thermal decomposition and surface catalytic reactions in the monolith are analyzed. The results indicate evaporation of droplets and urea thermal decomposition can’t be ignored in modeling of mobile SCR systems. A parameter to evaluate the effects of design changes on performance of SCR systems is proposed.（4） The effects of structures of exhaust pipes, mixer devices, injector location and configuration on the uniformity of the ammonia concentration distribution at the entrance of the SCR monolith as well as the concentration of reducing agent have been evaluated in the three-dimensional numerical simulation under various engine loads of a heave-duty diesel engine. The guideline for optimum design of mobile SCR systems is proposed. The calculated results show that evaporation of droplets, urea thermal decomposition and mixing process have strong impact on the performance of mobile SCR systems.更多还原