Numerical simulation of the evaporation process of pinned urea-water droplets in cavities

Modeling the evaporation process of pinned urea-water solution (UWS) droplets sitting on a heated surface is of large interest for reducing the deposit formation in Selective Catalytic Reduction (SCR) systems, which can lead to a significant loss in efficiency. The kinetics of diffusion-governed evaporation of sessile droplets having the shape of a spherical cap in an infinitely extended geometry has been extensively studied in the past. The present work is focused on modelling of the heat and mass transport in the gas phase during evaporation of drops with a size comparable with a capillary length. The model is valid for the wall temperatures below the saturation temperature, and the Rayleigh number is of order of 10. The influence of gravity on both the droplet shape and on the gas flow is taken into account. The droplet shape is determined by solving the Laplace-Young equation in cylindrical coordinates. The evaporation of droplets within a constrained surrounding geometry is considered. The domain geometry is close to the geometry of an experimental cell, in which validation tests have been performed. The simulations show that the influence of the droplet shape deviation from a spherical cap on the evaporation rate is negligible. The evolution of the droplet height and contact angle however are affected by the gravity. Furthermore, the evaporation rate is strongly affected by the natural convection and by the constrained geometry of the domain.