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M. Quissek, T. Lauer, O. Garcia-Afonso, S. Fowles:
"Experiments on the Wall Wetting Limit of AdBlue on Surfaces at highTemperatures";
Talk: THIESEL 2018 Conference, Valencia|Spain; 2018-09-11 - 2018-09-14; in: "THIESEL 2018 Conference", (2018), 14 pages.

Selective Catalytic Reduction (SCR) is a state of the art technology to reduce nitrogen oxide emissions in diesel propelled vehicles. In order to use the catalytic surface most efficiently and to avoid NH3 slip, a homogeneous distribution of ammonia is necessary. This uniformity depends strongly on evaporation and decomposition of the injected urea-water-solution (UWS) into gaseous ammonia. The droplet impingement plays an important role in this evaporation process and especially the critical wall temperature that separates regimes with wall wetting from those without due to the Leidenfrost effect.

Previous investigations and reports in literature indicate that this critical temperature is not a fixed value but appears to have several influence factors like the area loading of urea, the impingement angle and the impingement Weber number.

A component test bench was set up in order to investigate these influences in greater detail. An injector from a series exhaust system was used to analyse the impingement behaviour of UWS on a steel surface at different temperatures with varying injection angles and mass flow rates. To get results that are relevant for the target application, the distance from injector tip to impingement surface as well as the investigated angles and UWS mass flows were derived from a series SCR-system. Infrared thermography was used to identify the onset of wall wetting via the change of emission behaviour of the wetted surface.

The measurements revealed that the critical temperatures separating the permanent wall wetting regime from the temporary wall wetting regime and the pure Leidenfrost regime vary significantly with the area loading of the impingement zone with AdBlue. In the investigated range both critical temperatures increase to a similar extend with higher area loading values.

After having created a sufficiently detailed database with injector and single droplet experiments,an improved impingement map will be suggested, e.g. for future CFD simulations.