基于详细尿素沉积物形成机理的柴油机SCR系统优化研究

发布时间：2018-02-09 03:42

本文关键词： SCR系统尿素沉积物过渡态混合器混合算法优化设计　出处：《广西大学》2016年硕士论文　论文类型：学位论文

【摘要】：为了应对柴油机国家排放法规中对氮氧化物排放量的严格要求,机外净化选择性催化还原(selective catalystic reduction,SCR)成为了必不可少的关键技术,然而此系统也带来了新的问题-尿素沉积物风险。尿素沉积物的存在会影响催化器进口处NH3摩尔浓度的分布均匀性,从而影响SCR系统的NOx转化效率。本文运用CFD仿真手段建立了详细的沉积物机理模型,研究沉积物生成规律,寻求影响沉积物产生的因素。基于这些因素,运用先进的混合算法对SCR系统进行多参数结构优化,实现减少甚至避免SCR系统中沉积物生成的目的,以此来降低发动机背压和提高SCR的NOx的转化效率。构建了尿素分解化学反应动力学模型包含缩二脲、三聚氰酸和三聚氰酸一酰胺的生成机理,涉及15种组分和11步反应,弥补了液膜模型现象级分析的不足。同时采用分子贡献基团的方法,建立三聚氰酸一酰胺的比热容与温度的关系式,获取了机理模型所需要的热力学参数。提出了基于量子化学计算方法、利用过渡态求解化学反应速率的思路,解决了在无法通过文献和实验获取反应式C3H3N3O3+ NH3→C3H4N4O2+H2O的活化能和指前因子的难题。开展了几何结构和喷射参数对尿素沉积物和NH3均匀性的影响研究。研究表明:管路长度和管路半径的增加,都会增加尿素液滴在排气管路中的停留时间,提升催化器入口处NH3均匀系数,虽然管路长度对NH3均匀系数的影响低于管路半径的,但对尿素分解效率的影响会比管路半径的显著。增加混合器后,增加了管路内的湍流和涡流强度,能够大幅度提升NH3均匀系数和降低尿素沉积物的生成。喷雾锥角和喷射角度的改变对NH3均匀系数和尿素沉积物的浓度的影响并不明显。运用先进的SHERPA算法,设立管路长度、管路半径、混合器安装位置、混合器叶片数目、叶片长度、叶片倾斜角度、喷雾锥角以及喷射角度这8个设计变量,并以此8个变量构建优化搜索空间,对整个SCR模型进行优化研究。结果表明:当管路长为1.2m、直径为0.15m、混合器安装位置为0.6m、叶片数为3、叶片长为0.02m、叶片倾斜角为30度、喷射角度为45度以及喷雾锥角为80度时NH3均匀系数最好,为0.816,提高了 39%。
[Abstract]:In order to meet the strict requirements of the national emission regulations for NOx emissions from diesel engines, selective catalytic reduction of selective catalystic reduction (scr) has become an indispensable and key technology. However, this system also brings about a new problem-urea sediment risk, the presence of urea sediment will affect the distribution uniformity of NH3 molar concentration at the catalyst inlet. In order to affect the efficiency of NOx transformation in SCR system, a detailed sediment mechanism model is established by CFD simulation method, and the rules of sediment generation are studied, and the factors affecting sediment generation are sought. The advanced hybrid algorithm is used to optimize the multi-parameter structure of SCR system, so as to reduce or even avoid sediment generation in SCR system. In order to reduce the back pressure of the engine and improve the conversion efficiency of NOx of SCR, a kinetic model of urea decomposition chemical reaction including biuret, cyanuric acid and monoamide tripolycyanate was constructed, which involves 15 components and 11 steps. The method of molecular contribution group was used to establish the relationship between specific heat capacity and temperature. The thermodynamic parameters needed for the mechanism model are obtained. Based on the quantum chemistry calculation method, the idea of solving the chemical reaction rate by using the transition state is presented, which solves the problem of obtaining the reactive C _ 3H _ 3N _ 3O _ 3 NH3 in the paper and in the experiment. 鈫扵he effects of geometric structure and jet parameters on the uniformity of urea sediment and NH3 were studied. The results showed that the length and radius of the pipeline increased. Both increase the residence time of urea droplets in the exhaust pipe and increase the NH3 uniformity coefficient at the entrance of the catalytic converter, although the influence of the length of the pipeline on the uniformity coefficient of the NH3 is lower than that of the radius of the pipeline. However, the effect on urea decomposition efficiency is more significant than the radius of the pipe. When the mixer is added, the turbulence and eddy current intensity in the pipe are increased. The uniform coefficient of NH3 can be greatly increased and the formation of urea sediment can be reduced. The change of spray coning angle and injection angle has no obvious effect on the uniformity coefficient of NH3 and the concentration of urea sediment. Using advanced SHERPA algorithm, the length of pipeline is established. The eight design variables, such as pipe radius, mixer installation position, mixer blade number, blade length, blade tilt angle, spray cone angle and spray angle, are used to construct the optimal search space. The optimization of the whole SCR model shows that when the pipe length is 1.2 m, the diameter is 0.15 m, the installation position of the mixer is 0.6 m, the number of blades is 3, the blade length is 0.02 m, the blade inclination angle is 30 degrees. When the injection angle is 45 degrees and the spray cone angle is 80 degrees, the NH3 uniformity coefficient is the best (0.816), which increases 39%.
【学位授予单位】：广西大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TK423

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