Urea-SCR系统尿素沉积特性及混合器优化

Urea Deposition Characteristic in a Urea-SCR System and Optimization of the Mixer

搭建了快速尿素结晶试验平台,并基于Converge流体仿真软件耦合12步尿素详细分解机理搭建了目标柴油机集成式后处理装置模型,通过试验验证了模型的有效性.对现有混合器结构和喷射压力进行了优化,利用液膜和尿素分解过程中产生的3种主要副产物(缩二脲、三聚氰酸和三聚氰酸一酰胺)的分布与质量、选择性催化还原(SCR)催化器前端截面NH3的体积分数与质量、混合器背压和各均匀性系数等指标评价了优化前、后混合器的性能.结果表明:液膜和3种副产物主要分布在混合器外层套筒的球形底部和内层套筒的内侧,优化方案可以减少液膜和3种副产物的累积质量,降低尿素结晶风险;优化方案还可以增加混合区域的湍流强度从而提高SCR前端截面NH3的体积分数、质量和均匀性,但其结构会在一定程度上阻碍排气的流动从而使混合器背压稍有提高.

A test platform for rapid urea deposition was built and an integrated after-treatment device model of the target diesel engine was established based on the Converge fluid simulation software coupled with detailed urea decomposition mechanism. After the effectiveness of the model was verified,the structure and injection pressure of the existing mixer were optimized. The performance of the optimized mixer was evaluated by the distribution and mass of three main by-products of biuret,cyanuric acid and ammelide during the urea decomposition,the concentration and mass of NH3at the front-end section of selective catalytic reduction(SCR) catalyst,the back pressure and the uniformity coefficients of the mixer. The results show that the film and three by-products are mainly distributed on the spherical shell area of the outer sleeve and the inner side of the inner sleeve,and the optimized mixer decreases the mass of film and by-products. The optimized mixer also increases the turbulence intensity in the mixing area and thereby improves the concentration,mass and uniformity of NH3at the SCR inlet. But its structure hinders the flow of exhaust,thus slightly increases the back pressure of the mixer.