Analysis of the spray wall impingement of urea-water solution for automotive SCR De-NOx systems

Abstract

Selective catalytic reduction (SCR) is the most popular automotive exhaust after-treatment solution for nitrogen oxides (NOx) emission reduction. The mitigation of solid deposit formation is a big challenge for SCR system operation. Urea-water spray impingement is an important parameter which effects on solid deposit formation, should be investigated duly. This study presents an experimental investigation of urea-water spray impingement on the heated wall of automotive SCR systems for diesel engine exhaust gases. The investigation focused on the impingement conditions and distribution of the spray droplets, which are important parameters for system performance. This work was conducted with a commercial pressure driven SCR injector into an optically accessible test chamber at different wall temperatures and injection height. The spray impingement phenomena captured by means of Z-type shadowgraph high-speed Imaging technique. The injection quantity of the injector was measured to determine the injection rate. The high-speed imaging reveals in detail spray distributions, liquid film formation and urea crystallization. Relevant dimensions of spray distribution after the impingement have been estimated with digital image processing. The investigation reveals that high wall temperature produces swirl and bouncing that entrain rebounded droplets of the impinging spray on the wall, which leads to improved mixing. High temperature also causes longer spray front projection length which is an important factor for the mitigation of urea deposits.