Advanced Spray Impingement Modelling for an Improved Prediction Accuracy of the Ammonia Homogenisation in SCR Systems 2015-01-1054

A fast preparation of the liquid urea water solution (UWS) is necessary to ensure high conversion rates in exhaust aftertreatment systems based on Selective Catalytic Reduction (SCR). Droplet wall interaction is of major importance during this process, in particular droplet breakup and the Leidenfrost effect. A deeper understanding of the underlying mechanisms is a basic requirement to calibrate CFD models in order to improve their prediction accuracy.

This paper presents a detailed literature study and discussion about the major impact factors on droplet wall interaction. Measurements of the Leidenfrost temperature were conducted and the corresponding regimes classified based on optical observations. The pre- and post-impingement spray was analysed using the laser diffraction method. Further, the validity of spray initialisation based on measurements at room temperature was verified. The Leidenfrost effect, partial droplet evaporation, droplet breakup and evaporation at the catalyst are the crucial aspects of impingement modelling. Their implementation and calibration against measurement data is presented. Sensitivity studies of single impact factors were carried out based on two exhaust systems that are representative for the spectrum of typical automotive applications. A set of operating points was used that covers the whole range of typical operating conditions. Finally, a best practice setup was derived and validated with measured ammonia distributions.

With the help of the presented numerical and experimental investigations the prediction accuracy of the simulated ammonia homogenisation could be significantly increased for inherently different exhaust pipe geometries.