Experimental validation of evaporating urea-water sprays at elevated temperature and pressure

Selective Catalytic Reduction (SCR) is a standard technology for reducing nitrogen oxide (NOx) emissions of diesel engines. It is usually realised by injection of liquid urea-water-solution (UWS) into the hot exhaust gas. One major challenge of the design is to avoid deposits, especially melamine complexes, which may compromise the operation of the catalyst. Consequently, the proper understanding of the spray evaporation of UWS is key to successfully implement the SCR technology.
The main objective of this paper is to present a validation experiment of evaporating urea-water sprays under realistic operating conditions at elevated temperature and pressure. The experimental configuration is based on a twin-fluid atomizer for injecting 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 and a velocity of up to 250 m/s could be detected successfully at operating conditions of the gaseous phase of up to 773 K and 0.24 MPa.
The evaluation of droplet data at different positions downstream of the atomizer revealed details of the evaporation characteristics of urea-water solution. ... mehrIn this study, distilled water was used as an alternative liquid in the measurement campaign. This approach offers the possibility to pinpoint the effect of UWS on the evaporation process, as there is almost no difference in surface tension and viscosity between water and UWS. Hence, the atomization process is not affected and the atomizer produces the same spray for both liquids. The detailed analysis of the probability density functions revealed typical characteristics, which could be explained by the specifics of the evaporation process of UWS. As a result, the droplet size at the onset of urea thermolysis is derived as a criterion to determine the progress of spray evaporation.
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 hot gas, which is an important parameter affecting convective heat and mass transfer to the droplet. An analytical model, which serves to couple the one-dimensional droplet kinematics with a Rapid Mixing evaporation model, was developed for predicting the most important physical processes of the experimental configuration.
The results of the analytical model were found to agree very well with experimentally obtained evaporation characteristics. As a long-term objective, the experimental results may serve for validation of analytical evaporation models or complete numerical simulations of spray evaporation under realistic gas flow conditions.