Experimentation and numerical modeling of SCR spray droplets pre and post impingement on a mixer plate

Selective Catalytic Reduction (SCR) systems are one of the leading techniques used in the after treatment systems of the diesel engines for the abatement of nitrogen oxides (NOx) emissions. Ammonia is used as reducing agent in the catalyst which is supplied in the mixing chamber in the form of Urea Water Solution (UWS - 32.5% urea solution) spray. After evaporation of water, urea droplets undergo a series of secondary reactions eventually leading to the formation of solid deposits on the walls of the system. This not only hinders the deNOx efficiency but also interferes with the overall after treatment system by creating back pressure. Therefore, a deep understanding of the factors and mechanisms of spray–wall interaction is a basic requirement to prevent any undesired decomposition phenomena that could lead to failure of the whole system. In the present paper, droplet diameter distributions are analyzed pre and post impingement events on a hot mixer plate surface, both experimentally and numerically. The work was performed using a commercial six-hole pressure driven injector in a test section of heated cross flow wind tunnel. Phase Doppler Anemometry (PDA) was used to measure the droplet kinetics prior and after the wall impingement on the mixer plate. Based on the data, numerical simulation was setup using a commercial CFD code, AVL FIRE and droplet characteristics as well as thermal response of the mixer plate analyzed against the experimental data.