Droplet-Size Distribution Effects on Urea–Water Spray Evaporation and Decomposition: Knowledge of Sauter Mean Diameter (SMD) is not Sufficient

Urea water solution is widely used in selective catalytic reduction systems to reduce nitrogen oxide emissions. Urea decomposes via thermolysis and hydrolysis to produce ammonia, which reacts on the catalyst to form non-toxic gases, while solid residues formed during decomposition may affect system performance. A stand-alone reduced model is used to investigate spray decomposition, enabling systematic analysis of droplet size distribution effects independent of full flow-field complexity. The model is based on a progress-variable formulation derived from detailed one-dimensional droplet simulations with comprehensive chemistry over a wide range of temperatures. A parcel-based approach represents arbitrary droplet size distributions. The influence of droplet size, temperature, and distribution shape on ammonia formation and residue production is quantified. Distributions with identical Sauter mean diameter are compared to assess its validity. The results show significant differences in ammonia release and residue formation. These differences are strongly temperature dependent and demonstrate that the Sauter mean diameter alone is insufficient to characterize sprays for accurate decomposition modeling. A strong positive correlation between ammonia production and residue formation is observed. The findings show that representing the full droplet size distribution is essential for accurate prediction of ammonia availability and deposit formation.