Urea-Water Droplet Phase Change and Reaction Modelling: Multi-Component Evaporation Approach

Abstract

Urea-water solution droplet evaporation is modelled using multi-component droplet evaporation approach. The heat and mass transfer process of a multi-component droplet is implemented in the Langrangian framework through a custom code in ANSYS-Fluent R15. The evaporation process is defined by a convection-diffusion controlled model which includes the effect of Stefan flow. A rapid mixing model assumption is used for the droplet internal physics. The code is tested on a single multi-component droplet and the predicted evaporation rates at different ambient temperatures are compared with the experimental data in the literature. The approach is used to model the injection of urea-water solution spray in a duct carrying hot air to predict the urea to ammonia conversion efficiency. Thermolysis reaction of the evaporated urea and the hydrolysis of the byproduct iso-cyanic acid are solved as volumetric reactions in the Eulerian framework using laminar finite rate approach. The spray simulation results are compared with the experimental data and the numerical results of surface reaction based direct thermolysis approach available in the literature.

Modeling the Effect of Nonuniformities from Urea Injection on SCR Performance Using CFD

“…Therefore, spray-wall interaction was simplified by specifying a reflection boundary condition for droplet impingement along the walls of the spray chamber. As suggested by Shirodkar, this is a reasonable assumption since experimental results of Kim et al did not reveal any wall film formation due to droplet impact. In fact, our simulation results indicate that the number of droplets hitting the wall is negligible.…”

“…Saturation pressure of water is calculated using the Tetens equation, and the urea saturation pressure is calculated using the expression of Birkhold et al The saturation pressure expressions for urea and water are as given below The binary diffusivity expressions for the fluid are defined as a function of film-averaged temperature which is computed using the 1/3 rule. The expressions for binary diffusivities of urea and water are taken from Shirodkar as given below The specific heats of the individual components are calculated through piecewise polynomial fits, and the mixture specific heat is calculated as the mass-weighted average The conversion of evaporated urea to ammonia occurs through a series of reactions, viz., thermolysis of urea to produce ammonia and isocyanic acid (HNCO) and hydrolysis of isocyanic acid to produce ammonia and carbon dioxide. A two-step power-law kinetic model is used to capture the chemistry of urea decomposition and conversion by Yim et al The reactions involved and kinetic parameters used are summarized in Table .…”