Background - The exhaust gases of engines contain carbon monoxide (CO) which is toxic for humans and the environment. With a catalyst system the toxic gases can be converted. For this purpose, an aqueous urea solution is added to the combustion products in the exhaust system of the engine. In the catalyst, this mixture reacts to carbon dioxide and nitrogen.
Important for the efficient action of the catalyst is a homogeneous mixing of the exhaust gases with ammonia and a uniform utilization of the available catalyst area.

Application - In the investigated catalyst system of a ship's engine, ammonia is added to the exhaust gases through a slanted disc which forms a downstream vortex system. Assuming an optimum geometry, the long residence time of the flow in this vortex region ensures a homogeneous mixing of the exhaust gases with ammonia.

Simulation - By means of a numerical flow simulation, the mixing behavior of the two fluids was reproduced in a realistic manner. The influence of the flow direction onto the disk on the formation of the vortex structure was investigated. The final design was subsequently determined by means of purpose-oriented variant calculations for the optimum combination of the disk's angle of attack as well as the direction and rate of the ammonia addition.

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