Abstract
In compression ignition engines, the urea selective catalytic reduction (SCR) system is used to reduce NOX in the exhaust gas. However, under low exhaust gas temperature conditions, the urea is not decomposed completely, leading to a decreased NOx purification rate. In this study, the temperature of the urea water solution (UWS) was controlled to promote atomization, evaporation, and the chemical reaction of the UWS spray. To understand the effect of the degree of superheat on UWS spray characteristics, the urea SCR nozzle tip was electrically heated, and the UWS spray was optically measured at different temperatures. The Super High Spatial Resolution Photography (SHSRP) method, which allows for whole spray imaging while maintaining the spatial resolution of small droplets, was applied, and the UWS droplet size distribution in quiescent conditions was analyzed. By applying a phase change model of flash boiling with energy balance, the vapor mass fraction at the nozzle outlet of the heated UWS spray was analyzed. Assuming a low-temperature exhaust gas flow field, the mass fraction of UWS spray droplets following the gas flow, the adhesion mass on the wall, and the location of water evaporation were estimated. The efficiency of UWS heating in urea SCR systems was discussed.
