Spray evaporation of urea–water solution: Experiments and modelling

Abstract

Selective catalytic reduction is a key technology for reducing nitrogen oxide (NOx) emissions of diesel engines, which requires the injection of urea–water solution (UWS) into the hot exhaust gas. The main objective of this paper is the development of a methodology for predicting the evaporation of urea-water sprays. For this reason a generic experiment was conducted in order to provide validation data under realistic operating conditions. The configuration is based on a twin-fluid atomiser, which is used to inject UWS into a hot pipe flow. Microscopic imaging was applied in conjunction with particle tracking velocimetry to record droplet size and velocity distribution profiles. Using this approach, droplets of a diameter as small as 4 μm were successfully detected at operating conditions of the gaseous phase of up to 773 K and 0.24 MPa. The analysis of droplet data at different positions downstream of the atomiser revealed details of the evaporation characteristics of UWS. In particular, a characteristic droplet size is proposed, which may serve to characterise the overall progress of UWS spray evaporation. Using this criterion, the comparison of experimental results to numerical predictions is straightforward. A numerical model, which is based on one-dimensional droplet kinematics as well as a rapid mixing evaporation model, was developed to describe the most important physical processes of the experimental configuration. A distinct advantage of the experimental methodology is that the velocity of the gas phase can be approximated by the velocity of the smallest detectable droplets with sufficient accuracy. This information was used to calculate the relative velocity between larger droplets and the hot gas, which is an important parameter considering convective heat and mass transfer in the evaporation model. The results obtained via the numerical model were found to agree very well with experimentally obtained evaporation characteristics.