Investigation on Uniformity Indices Used for Diesel Exhaust Aftertreatment Systems

When developing diesel exhaust aftertreatment systems the often constrained packaging envelope is a challenge. In addition, the stringent emission legislation can necessitate the injection of additives, for example urea and/or hydrocarbons, to achieve performance targets. In such cases, a good distribution of reactants over the catalyst surface is beneficial but might be difficult to achieve. The uniformity of these distributions is often studied separately using a non-dimensional measure denoted as the Uniformity Index (UI). However, a combination of the exhaust gas UI and the UI for the additive is also interesting to study.

In this work the origin, applicability and advantages of the uniformity index is discussed as a guide for developing diesel exhaust aftertreatment systems. Moreover, a matching index (UI_α) is proposed for the analysis of systems where additives are injected. It is found that two skew distributions can give a good conversion if the profiles match.

A sensitivity study for some critical parameters is also performed. Trade-off relations between conversion and space velocity as a function of the discussed uniformity indices are given. Most of this work is carried out using a validated modelling tool developed for catalyst systems. This tool allows for calculation of two dimensional distributions of reactants and products. An application example is also given where the theoretical discussion is compared with experimental data.