MODELING OF AIR-ASSISTED SPRAY BREAKUP OF UREA-WATER SOLUTION USING A VOLUME-OF-FLUID METHOD.

摘要

Spray breakup is an important phenomenon for the injection of a urea-water solution in selective catalytic reduction (SCR) systems for efficient reduction of NOx (nitrogen oxides) from diesel engine exhaust gas. The resulting spray distribution of the urea-water solution has a high impact on the efficiency of the SCR system and possibly determines the formation of urea deposits. For air-assisted liquid breakup in a hot gas, the volume-of-fluid (VOF) simulation model is suitable to capture the interface between the liquid and gaseous fluids. In the current work, a three-dimensional computational fluid dynamics (CFD) modeling for air-assisted spray breakup of the urea-water solution is done using a volume-of-fluid method. A second order high-resolution interface capturing (HRIC) convection discretization scheme is used to capture the interface between phases. A sharpening factor is used to reduce the numerical diffusion of the ligaments. An algorithm is written, which scans the ligaments from the running simulation at the predetermined time interval and export their properties. The simulation facilitates detailed investigation of spray characteristics such as spray penetration, spray angle, droplet spectrum, and the possibility of deposit formation on the injector. The spray characteristics from VOF simulations are in good agreement with the high-speed shadowgraphy measurements carried out at a hot gas test-rig. Consequently, our results demonstrate the benefits of using VOF method to understand the mechanism involved in air-assisted spray break of the urea-water solution in SCR applications and can be useful in determining the optimum operating conditions of the air-assisted urea-water solution injector.