3-D Numerical Study of Flow Mixing in Front of SCR for Different Injection Systems

The urea Selective Catalytic Reduction (SCR) exhaust system has been proved to be the reliable aftertreatment device with the capability of reducing tail pipe NOx emission by 75% to 90%, HC by 50% and Particulate Matter (PM) by 30%. Constrained by increasingly stringent packaging envelope, flow mixing in front of substrate is becoming one of the major concerns to achieve ideal performance of higher NOx conversion and lower ammonia (NH3) slip. Three dimensional CFD simulations are performed in current study to investigate flow mixing phenomenon in a SCR system. First, for a traditional tube injector with single or multiple nozzles, the effects of mass flow rates of injected NH3 and exhaust gas on flow mixing and pressure loss are investigated. Then, a concept of ring shape injector with multiple nozzles are initiated and built for 3-D CFD simulations. The comparisons of flow mixing index and injection pressure are made between two type injectors. Next, the locations of nozzles in ring injector are studied and the effect on flow mixing index and NH3 injection pressure are addressed. The optimal nozzle locations are found. Finally, the effect of a swirling flow, which can be generated by a span element or flow mixer in front of substrate, is studied and the enhancement on flow mixing index is discussed.