Experimental Investigation of Novel Ammonia Mixer Designs for SCR Systems

Meeting Euro 6d NOx emission regulations lower than 80 mg/km for light duty diesel (60 mg/km gasoline) vehicles remains a challenge, especially during cold-start tests at which the selective catalyst reduction (SCR) system does not work because of low exhaust gas temperatures (light-off temperature around 200 °C). While several exhaust aftertreatment system (EATS) designs are suggested in literature, solutions with gaseous ammonia injections seem to be an efficient and cost-effective way to enhance the NOx abatement at low temperature. Compared to standard SCR systems using urea water solution (UWS) injection, gaseous NH₃ systems allow an earlier injection, prevent deposit formation and increase the NH₃ content density. However non-uniform ammonia mixture distribution upstream of the SCR catalyst remains an issue. These exhaust gas/ NH₃ inhomogeneities lead to a non-optimal NOx reduction performance, resulting in higher than expected NOx emissions and/or ammonia slip. Thus, efficient mixers upstream of the SCR are crucial for the overall EATS performance. In the experimental study reported in this article, planar laser induced fluorescence (PLIF) is used to quantify mixing performance of four novel CFD optimized static mixers in an optically accessible flow bench. The variation of boundary conditions and the change of exhaust line configurations (e.g. w/wo DOC upstream, w/wo DPF downstream) show a major effect on the mixing process and subsequently the homogeneity of the ammonia-exhaust gas mixture (for example: drop in uniformity index from UI = 0.95 to UI = 0.60 for a blade mixer design). This points out the need to purposefully design and optimize static mixers for a specific exhaust line configuration.