柴油机SCR催化转化器的数值模拟

Numerical Simulation of Diesel SCR System

在实际应用中,选择性催化还原(SCR)系统中尿素水溶液的不完全分解容易导致在喷嘴、混合器和壁面等处形成沉积物,不仅降低NO_x转化效率,还会增加排气背压,影响发动机动力性能.采用数值模拟的方法对SCR系统内化学反应和流动过程进行研究,建立了SCR反应动力学模型,该模型能够较为准确地预测转化器出口各气体组分随时间的变化规律.将验证后的动力学模型和三维流动模型相耦合,考察SCR反应器内部化学反应和流动过程,并模拟了混合器对温度场、浓度场和壁膜的影响.结果表明:混合器能够显著提高催化器前流动及气体浓度均匀性,并有助于降低尿素沉积物生成风险.

Incomplete decomposition of urea water solution may lead to the formation of urea deposits on injector, mixer and pipe wall in the selective catalytic reduction （SCR） systems, resulting in lower NOx conversion efficiencyand increasing back pressure, and affecting engine performance. Chemical kinetics and fluid dynamics of an SCRsystem was studied numerically. A 1-D kinetic model for SCR was developed and validated against the experimentaldata. The validated model can predict the time dependence of outlet gas concentration. Then the kinetic model was coupled to 3-D fluid model to study the chemical reactions and gas flow in SCR. The effects of mixer on temperaturedistribution, species concentration distribution and formation of wall-film were investigated. Results show that themixer can improve the uniformity of flow and gas concentration at the entrance of SCR system significantly and reducethe risk of urea deposit formation.