基于CFD耦合尿素分解机理的混合器性能评估

Evaluation of Mixer Performance Based on CFD Coupled Detailed Decomposition Mechanism of Urea

为评估排气后处理系统中混合器的性能,利用计算流体力学(CFD)耦合尿素分解详细机理建立柴油机排气后处理系统的三维数值模型.基于该模型计算尿素水溶液的蒸发和热解产物包括3种尿素分解副产物(缩二脲、三聚氰酸和三聚氰酸一酰胺)的形成过程,并利用尿素结晶和副产物的质量与分布、选择性催化还原(SCR)入口均匀性以及混合器前后压力差等指标评价两个不同结构混合器的性能.其中混合器1前后及底部各布置一块挡板,混合器2顶部布置旋流叶片.结果表明:湍流强度小且分布均匀的系统(如混合器1)尿素结晶质量多而液膜和副产物质量少,尿素结晶、液膜和副产物主要分布在混合器;局部湍流强度大且分布不均匀的系统(如混合器2)尿素结晶质量少而液膜和副产物质量多,尿素结晶和液膜分布在混合管和混合器,而副产物主要分布在混合管;混合器1的SCR入口NH_(3)均匀性好于混合器2,其前后压力差也小于混合器2.

In order to evaluate the performance of the mixer in the exhaust after-treatment system with selective catalytic reduction(SCR),a 3D numerical simulation model of the diesel exhaust after-treatment system was established using the computational fluid dynamics(CFD)coupled with urea decomposition detailed mechanism.Based on this model,the evaporation and thermolysis products of the urea aqueous solution were simulated,including the formation process of three urea decomposition by-products(biuret,cyanuric acid and ammelide).The mass and distribution of urea solid and by-products,the uniformity index of SCR inlet and the pressure drop of the mixer were used to evaluate the performance of the two mixers with different structures.One baffle was arranged at the bottom,front and back sides of mixer 1.The swirl blades were arranged at the top of mixer 2.The results show that in systems with small intensity of turbulence and uniform distribution(e.g.,mixer 1),which has more mass of urea solid but less mass of liquid film and by-product,its urea solid,liquid film and by-products are mainly distributed in the mixer.In systems with large local intensity of turbulence and uneven distribution(e.g.,mixer 2),which has less mass of urea solid and more mass of liquid film and by-product,its urea solid and liquid film are distributed in mixing pipe and mixer,while the by-products are mainly distributed in mixing pipe.The NH_(3) uniformity in the SCR inlet of the mixer 1 was better than that of the mixer 2.The pressure drop of the mixer 1 is less than that of the mixer 2.