Impact of the Spray-Wall-Interaction-Model on the Prediction of the Ammonia Homogenization in Automotive SCR Systems

摘要

Nitrogen oxide emissions of diesel engine powered vehicles have to be significantlyreduced in order to meet future international legislative restrictions. The Selective CatalyticReduction (SCR) has meanwhile been established as a promising technology to cope withthe challenging limits in the sector of mid-sized and large passenger cars. Despite itssuccessful market launch, high optimization potentials remain for the automotive SCRsystem with respect to the fast preparation of the injected urea-water solution (UWS) and asufficient ammonia homogenization at the SCR catalyst.In cooperation with BMW Motoren GmbH Steyr, the Institute for Powertrains andAutomotive Technology from Vienna University of Technology implemented a CFDsimulationmodel of the UWS preparation and mixing processes upstream of the SCRcatalyst.A series type passenger car SCR system with a swirl mixing element has beeninvestigated, to analyze the impact of the droplet-wall-interaction and liquid-film formationon the ammonia homogenization for a wide range of exhaust gas conditions. An optimizednumerical description of the UWS-droplet-interaction with the hot surfaces of the exhaustsystem has been established and validated with engine test bed measurements of theammonia homogenization. A remarkable correlation between a fast formation of ammoniaand a high degree of uniformity at the catalyst was proven by the CFD. Especially for highexhaust temperatures an early UWS decomposition was hindered by the Leidenfrosteffect, which leads to a high amount of droplets that reach the catalyst and a relatively lowlevel of homogenization.