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

• Published in: Experimental thermal and fluid science 2018-02, Vol.91, p.80-88
• Main Authors: Schmid, J., Zarikos, I., Terzis, A., Roth, N., Weigand, B.
• Format: Article
• Language: English
• Subjects: AdBlue; Aqueous solutions; Boiling; Catalysis; Crystal growth; Crystallization; Diesel engines; Emissions; Evaporation; Exhaust pipes; Exothermic reactions; Heat transfer; Internal combustion engines; Liquid-solid interfaces; Nitrogen oxides; Solidification; Sprayers; Sprays; Substrates; Surface temperature; Thermal imaging; Thermodynamics; Urea; Ureas; Wettability
• ISSN: 0894-1777; 1879-2286
• DOI: 10.1016/j.expthermflusci.2017.10.008

•Crystallization is always initiated at the triple line of the droplet indicating heterogeneous nucleation.•Two different crystallization modes are observed depending on the surface temperature and wettability.•A significant temperature rise in the form of thermal peak is observed during crystallization at the solidification front.•The crystallization speed and the obtained temperature rise are increased as the surface temperature is increased. 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.