Computational Fluid Dynamics Study of Gaseous Ammonia Mixing in an Exhaust Pipe Using Static Mixers 2017-01-1018

Ever growing traffic has a detrimental effect on health and environment. In response to climate warming and health concerns, governments worldwide enforce more stringent emission standards. NO_x emissions limits are some of the most challenging to meet using fuel-efficient lean-burn engines. The Selective Catalytic Reduction (SCR) is one consolidated NO_x after-treatment technique using urea water solution (UWS) injection upstream of the catalytic converter. A recent development of SCR, using gaseous ammonia injection, reduces wall deposit formation and improves the cold-start efficiency.

The mixing of gaseous ammonia with the exhaust gases is one of the key challenges that need to be overcome, as the effectiveness of the system is strongly dependent on the mixture uniformity at the inlet of the SCR catalyst. Here a CFD modeling approach is presented to study the effects of different static mixer geometries to find the optimal trade-off between pressure-loss and ammonia/exhaust gas mixture homogeneity in a compact system, which is demonstrated on various blade mixer geometries. The effects of air and ammonia mass flow rates at different temperature conditions were investigated to identify critical operating conditions. It is demonstrated that the pressure loss over the mixer is a function of the mixer blocked area of the exhaust pipe but also of the mixer’s geometrical features.