EXPERIMENTAL STUDY ON THE DIFFERENCE BETWEEN WATER AND UREA-WATER-SOLUTION SPRAYS FROM COMMERCIAL SCR INJECTORS UNDER QUIESCENT AND HEATED CROSS-FLOW CONDITIONS

Among the many options under investigation for deNOx solutions in the automotive industry, the use of selective catalytic reduction (SCR) is one of the most reliable, efficient, and cost-effective aftertreatment systems found thus far. The operational efficiency of SCR systems is strongly linked to the spray dynamics and uniformity of urea in the exhaust flow. Development and testing of different spray configurations require the investigation of different scenarios under exhaust like conditions. Most of the studies found in the literature assume water to be a sufficient alternative to urea-water-solution (UWS), principally based on a nondimensional analysis indicating that the spray dynamics should be similar. Substituting UWS with water offers advantages regarding test setup, safety, etc., but the assumption of sufficient similarity needs further investigation, especially when moving out of normal conditions, where evaporation rates and the subsequent species production might vary. In this study, a wind tunnel capable of reaching temperatures of up to 400 degrees C at 10 m/s of cross-flow velocity was used to run a detailed comparison between water and UWS spray under different scenarios. Velocity and droplet-size distributions were obtained to characterize the sprays with a two-dimensional phase doppler anemometry instrument. Statistical quantities were constructed at key positions along the spatial development of the spray, which served to build a comparison matrix between the cases. Even though no significant differences were observed between the mean values, their distribution shapes did vary significantly, pointing toward different liquid evaporation rates in the heated cross flow cases. Although a more detailed measurement campaign might be needed to confirm this final hypothesis, it is suggested that the evaporation rate of pure water diminishes the population of smaller droplets faster than in the UWS cases. Finally, this study provides some key findings to further investigate the physical cause for these differences, pointing toward where to draw a line on the replaceability of UWS by water during experimentation for SCR systems.