3-D Numerical Study of Mixing Characteristics of NH3 in Front of SCR

The Urea Selective Catalytic Reduction (SCR) technology is one of the major mature exhaust aftertreatment technologies which are demonstrated to be able to lower tail pipe NOx emission by 90%. The system consists of a urea injection at upstream pipe and a downstream SCR converter. A well mixed flow (exhaust gas and NH3) in front of SCR substrate, which is usually constrained by tight design packaging, is very critical to ensure the desired performance. Current paper addresses the geometrical effects on flow mixing by using three dimensional Computational Fluid Dynamics (CFD) tool. The mixing enhancement is achieved by adding flow mixer. The shapes and locations of flow mixers, as well as the number of blades inside mixer are investigated to show the effect on fluid mixing in downstream along the flow direction. Results show great improvement of flow mixing by adding a delta wing mixer. The turbulent flow generated by delta wings is proved to be very useful for a significant mixing enhancement within a short distance. It is found that a flow mixer in front of injector results in higher flow mixing index right behind the mixer. Higher flow mixing indexes are found 200 mm downstream and beyond when mixer is located right behind injector. Besides turbulent flow, Swirling flow is another major contributor for higher flow mixing index. A simple flow mixer with twisted blades is constructed based on original delta wing mixer for the purpose of creating swirling flow as well as turbulent flow. It is found turbulent flow has dominant effect on flow mixing index in the short distance right behind the flow mixer while the swirling flow dominates contribution through a longer distance. It is surprise to see that flow mixing index is higher when the number of twisted blades is reduced from six to three.