Quantitative Analysis of Low Pressure-Driven Spray Mass Distribution and Liquid Entrainment for SCR Application through a Mechanical Patternator

The application of liquid aqueous Urea Solution (AUS) as reductant in SCR exhaust after-treatment systems is now a commonly accepted industry standard. Unfortunately, less acceptable are the associated difficulties caused by incomplete decomposition of the liquid, resulting in solid deposits which accumulate in the exhaust pipe downstream of the dosing components. The correct prediction of the spray pattern and, therefore, the spray impact on the walls is a key feature for the system optimization. A mechanical patternator, designed on the basis of CFD performance assessment, involving a Lagrangian representation of the dispersed liquid fully coupled with a 3D Eulerian description of the carrier phase, has been built and used to measure the spray mass distribution. The effect of the droplets generated from the interaction among spray, probes and contingent liquid film is taken into account in the geometrical definition of the shape and the position of the patternator, as well as the flow field modification. The whole simulation process is carried out with the open source finite volume platform OpenFOAM®. The investigation of the effect of a wide range of thermal and kinematic cross flow conditions on the behavior of a multi-hole pressure-driven injector for SCR applications has been performed in an engineless test bench at the Empa laboratories. Major focus is put on the evaluation of the complete spray footprint, which is collected with high spatial resolution to generate a reliable map of the conditions, aiming at avoiding the formation of permanent liquid film, which is undeniably the basis of solid deposit formation. A detailed parametric analysis of the liquid entrainment and of the impact location is reported to provide a concise description of the spray behavior.