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A conceptual design and numerical analysis of the mixerless urea-SCR system

In order to reduce nitrogen oxides (NOx) in urea-selective catalytic reduction (SCR) systems more efficiently, emphasis is placed on improving the mixing of the urea-water solution (UWS) with exhaust gases. Nowadays, this is mainly achieved by implementation of static mixers. However, some designs can lead to a build-up of a wall film and solid deposit formation. In this study, an innovative design of the aftertreatment system without conventional mixing elements was developed. The aim was to obtain a high degree of UWS decomposition, and uniform ammonia distribution at the inlet to the catalyst, while minimising the liquid film deposition and keeping the compact design. The concept of the design was based on creating high turbulences and elongating the flow paths of the droplets. The design was verified through a series of numerical simulations based on the Reynolds-averaged Navier–Stokes (RANS) approach and a discrete droplet model (DDM) spray representation. The analysis included various operating conditions as well as subcooled and superheated sprays. A uniform ammonia distribution was achieved regardless of the operating points and spray properties. Additionally, in the case of the flash-boiling injection, a further reduction of the wall film was observed. The research shows that the application of strong turbulence and rapid changes of the flow paths as a method of mixing UWS with exhaust gases enables the achievement of uniform ammonia distribution and low film formation, while simplifying the system’s design, increasing its reliability and lowering manufacturing cost.
Topic: Exhaust emissions and aftertreatment
Author: Bartosz Kaźmierski
Co-authors: Krzysztof Górka, Łukasz Jan Kapusta