Selective catalytic reduction (SCR) is an efficient and widely used method for NOₓ emission control of diesel engines. However, maintaining stringent legislations for NOₓ tailpipe emissions of light and heavy duty vehicles brings challenges which have lead to considerable public attention and discussions. In all common mobile SCR systems, an aqueous urea solution is injected into the tailpipe in front of the SCR catalyst providing the reducing agent ammonia for NOₓ removal by thermal decomposition. Due to highly transient conditions and a limited mixing length, incomplete spray evaporation may lead to droplet impingement on mixing elements and the tailpipe wall. Accumulated liquid films induce evaporative cooling and result in solid formation on the tailpipe surfaces due to urea crystallization and by-product formation. Liquid and solid deposits impede system efficiency by modifying surface properties, reducing ammonia homogeneity and increasing back pressure. This work deals with the interaction of urea water solution with heated targets and the resulting film and deposit formation. Spray/wall interaction and deposit formation are studied experimentally on multiple scales. Impingement characteristics of single droplets and sprays are investigated by high-speed imaging and infrared thermography. A hot gas test rig is used for investigations on spray/wall interaction and deposit formation on application scale. Resulting deposits are analyzed in terms of topology and chemical composition. Based on these data, a kinetic model for urea decomposition is enhanced and integrated to a 3D CFD simulation of the test rig. A comprehensive CFD modelling approach for urea deposit formation is proposed including interaction of all relevant physical and chemical phenomena.weiterlesen