Numerical Study on Urea-Water Solution Spray Evaporation and NH3 Formation Potential Under High-Pressure Transient Conditions in a Non-Firing Diesel Engine

In selective catalytic reduction (SCR) systems, urea-water solution (UWS) is injected upstream of the catalyst, where water evaporation, urea thermolysis, and isocyanic acid (HNCO) hydrolysis release the NH3 required for NOx reduction. Data at pressures of tens of bar combined with rapid temperature variation remain scarce. This study numerically evaluates the evaporation, decomposition, and NH3 formation potential of UWS sprays in an engine-like, non-combusting transient thermal environment. A motoring direct injection diesel engine model, validated against measured in-cylinder pressure, gave a peak mass-averaged temperature of about 851 K. A three-dimensional CFD model coupling a multicomponent droplet evaporation model with gas-phase urea thermolysis and HNCO hydrolysis was applied to four injection quantities of 11.5, 23.2, 28.2, and 33.1 mg/cycle. UWS injection lowered the peak pressure and temperature through the endothermicity of water evaporation and urea decomposition. The model predicted NH3 conversion, referenced to the urea content of the injected UWS, and decreased monotonically from about 43% to 17% as the injection quantity increased, while the wall film residue rose from 1% to 31% of the injected mass. Because deposit-forming side reactions are not included and high-pressure species formation was not independently validated, these values represent an upper bound of the model-predicted formation potential. The predicted NH3, therefore, represents a formation potential under non-firing conditions rather than an exhaust-gas concentration, since NH3 formed in the cylinder would be largely oxidised at firing temperatures.