An improved reduced model for the evaporation and decomposition of urea-water solution (UWS) droplets

Urea-water-solution (UWS) is often used as a source for the reduction agent ammonia for the selective cat-alytic reduction of nitric oxides in combustion after-treatments systems. In applications, however, some tech-nical problems, for example the formation of solid residuals or incomplete decomposition of urea, appear. To solve these problems complex phenomena must be taken into account which involve turbulent flow, mul-tiple phases, chemical reactions of products of urea decomposition with exhaust gas etc. Hence, in order to understand and optimize the process of nitric oxides reduction, these very complex processes with different temporal and spatial scales need to be addressed with a reasonable computational effort. In this study the influence of the urea decomposition in the liquid phase onto the evaporation and decomposition of UWS droplets is investigated. It is shown that the liquid chemistry model (13 species and 13 reactions) appropriately describes the decomposition. A detailed parametric and comparative study for typical system parameters of the NOx reduction conditions is performed. Moreover, a simple but robust model reduction concept is sug-gested and employed to describe the process with only 3 parameters. A self-similar behavior of the droplets during the evaporation process is used for the reduced description and applied to the entire droplet evapora-tion process. The diameter and the species source terms can be represented with a simple polynomial model for the boundary and initial conditions ambient gas temperature, water concentration and initial droplet diameter, by using a fixed number of tabulation parameters.& COPY; 2022 Published by Elsevier Inc. on behalf of The Combustion Institute.