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The selective catalytic reduction (SCR) based on urea-water-solution is an effective technique to reduce nitrogen oxides (NOx) emitted from diesel engines. A 3D numerical computer model of the injection of urea-water-solution and their interaction with the exhaust gas flow and exhaust tubing is developed to evaluate different configurations during the development process of such a DeNOx-system. The model accounts for all relevant processes appearing from the injection point to the entrance of the SCR-catalyst: momentum interaction between gas phase and droplets evaporation and thermolysis of droplets hydrolysis of isocyanic acid in gas phase heat transfer between wall and droplets spray/wall-interaction two-component wall film including interaction with gas phase and exhaust tube The single modeling steps are verified with visualizations, patternator measurements, phase-doppler-anemometer results and temperature measurements.

Detailed experimental investigation of the instationary spray/wall interaction in an injection chamber helps to better understand the fuel injection processes in small diesel engines. They are necessary to improve spray breakup and evaporation models in CFD simulations and furthermore they are useful to validate results of numerical simulation. Experiments using Phase-Doppler-Anemometry (PDA), lightsheet visualization tools, and surface thermocouples under diesel engine conditions show that spray/wall interaction takes place only about 3 mm above the surface. In this area the impinging fuel jet is redirected parallely to the wall and forms a dense wall jet. At the tip of the wall jet vortices with a high number of drops develop. Here the liquid phase is redirected again and will finally be concentrated in a layer up to a distance of a few millimeters above the surface.