Crystallization of urea from an evaporative aqueous solution sessile droplet at sub-boiling temperatures and surfaces with different wettability

The injection of urea-water-solution sprays in the exhaust pipe of modern diesel engines eliminates NOx emissions in a very great extent. However, as water evaporates from the solution, urea is crystallized and causes wall-deposit formations hindering the performance of selective-catalytic-reaction. In this study, the crystallization of urea from an evaporative aqueous solution droplet placed on a heated wall is experimentally investigated, aiming to understand macroscopically the morphology of crystal growth at various conditions. Using optical and thermal imaging, urea crystallization patterns are examined at sub-boiling temperatures and substrates with different wettability. In all cases, the macroscopic initiation of crystal growth starts at the solid-liquid interface when urea concentration has reached supersaturated conditions. The experiments indicate two different crystallization modes depending on surface temperature and wettability as well as a significant heat release at the solidification front due the exothermic character of the process.