Numerical Study on Evaporation Characteristics of Single Urea-Water Solution (UWS) Droplet and Variation of Evaporation and Wall-Interaction Characteristics of UWS Spray with Cell Density in SCR Mixing Chamber

Selective Catalytic Reduction (SCR) is a most promising technique for reduction of nitrogen oxides (NOx) emitted from the exhaust of diesel engines. Urea Water Solution (UWS) is injected to hot exhaust gas stream to generate reducing agent ammonia. The droplet evaporation of Urea Water Solution (UWS) is investigated for single droplet in heated environment ranging temperatures 373K-873K theoretically. The theoretical methods which are implemented into CFD code Fire 8.3 from AVL Corp. involve Rapid Mixing model and Diffusion Limit model which consider stationary droplet and variable properties of the UWS. The UWS droplet revealed different evaporation characteristics depending on its ambient temperatures which are numerically predicted by simulated results. The simulated results are validated with experimental values of Wang et al. [9] which are helpful in predicting the evaporation and UWS dosing strategy at different exhaust gas temperatures in real SCR system. The comparative study of droplet diminishment from transient evaporation histories for various ambient temperatures with simulated results helps in quantitative evaluation of evaporation characteristics of UWS droplet as well as provides valuable empirical data required for modeling or simulation works on urea-SCR system. The two different stages of UWS evaporation and decomposition found at lower temperatures but as the temperature increases it occurs in single stage as urea also start melting above 406K. Effect of cell density on evaporation and thermolysis and wall interaction behavior is extensively studied. There are no much variations are found in droplet vaporization characteristics whereas significant changes are observed in wall interaction.