A computational model for the evaporation of urea-water-solution droplets exposed to a hot air stream

Abstract

A simple computational model was created to predict the vaporization rates of droplets of urea-water-solution (UWS) evaporating in a hot air stream. A single-component evaporation model, which was based on Abramzon-Sirignano’s vaporization model, was adapted to handle UWS droplets, and the governing equations were solved numerically in MATLAB®. The temperature and species concentrations within the droplet were assumed to be uniform, and various methods available in literature were employed to estimate important thermophysical properties such as saturation vapor pressures, partial pressures, vaporization enthalpy, and vapor diffusivities. The suitability and limitations of the computational model were assessed by comparing the results with experimental data on UWS droplets evaporating under forced convective conditions. The temperatures considered in this study ranged from 373 K to 673 K, and the corresponding relative air velocities were between 1.5 m/s and 4.3 m/s. The model was found to be able to capture the two-stage vaporization behavior of the UWS droplet (except crystallization, puffing and micro-explosion) with reasonable accuracy. While the first-stage vaporization rates, predicted by the model, were accurate to within 1.8% to 17.7%, the accuracies of the second-stage vaporization rates were significantly dependent on the methods used to estimate fluid properties such as the vapor pressures and the vaporization enthalpy.