EXPERIMENTAL AND NUMERICAL STUDY TO EVALUATE THE EFFECT OF FLASH BOILING ON UREA-WATER SOLUTION SPRAYS AND SELECTIVE CATALYTIC REDUCTION SYSTEM PERFORMANCE

In this study, we apply flash boiling to low-pressure injection of a urea-water solution (UWS) and investigate its influence on the selective catalytic reduction (SCR) system performance using numerical simulations. The research was done in two steps: first, the spray models for both subcooled and flash-boiling conditions were calibrated in a stationary condition based on experimental measurements. Second, the calibrated models of flashing and nonflashing sprays were used to simulate the whole SCR system to evaluate the influence of flash boiling on its performance. The aim of this evaluation was to include all important effects of flash boiling that were previously reported. Therefore, much emphasis has been put on the first part of the study, where different approaches to flash boiling spray modeling were validated. From the result of the study, a simple approach was proposed to model a low-pressure flashing spray based on a conventional evaporation model, with modified input parameters to include the flash-boiling effects on droplet size, spray penetration and angle, and the spatial droplet distribution. The full exhaust system simulations, in turn, have shown that the flash-boiling effect can be considered as an effective way to promote smaller droplets' formation while avoiding air assistance and a pressure increase in low-pressure UWS injection systems; furthermore, it will improve the performance of SCR systems. However, a direct increase of the UWS's temperature, without any modifications to the SCR system initially designed for nonflashing sprays, may lead to the opposite effect.