Experiments of evaporating urea-water sprays for the validation of SCR-systems

Selective Catalytic Reduction (SCR) is an established technology for reducing nitrogen oxide emissions of diesel engines. It is usually realised by the injection of urea-water-solution (UWS) into the hot exhaust gas. One major challenge of the design is to avoid deposits, which may compromise the operation of the catalyst. Consequently, the proper understanding of spray evaporation of UWS is the key to successfully implement the SCR technology. The main objective of the present study is the development of a validation methodology for the prediction of evaporating urea-water sprays under realistic operating conditions. 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 UWS. ... mehrAs a result, the droplet size at the onset of urea thermolysis is introduced as a novel quantity, which may serve as a distinct criterion for quantifying the overall progress of spray evaporation. An analytical model, which serves to couple the one-dimensional droplet kinematics with a Rapid Mixing evaporation model, was developed for predicting the most relevant 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.