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Doctor's Theses (authored and supervised):

S. Fischer:
"Simulation of the Urea-Water-Solution Preparation and Ammonia-Homogenization with a Validated CFD-Model for the Optimization of Automotive SCR-Systems";
Supervisor, Reviewer: B. Geringer, D. Gosman; E315 Institut für Fahrzeugantriebe und Automobiltechnik, 2012; oral examination: 2012-10-19.



English abstract:
The Selective Catalytic Reduction (SCR) is a promising approach to meet future legislation regarding the nitric oxide emissions of Diesel engines. In automotive applications a liquid urea-water-solution (UWS) is injected into the hot exhaust gas that evaporates and reacts to ammonia vapor, acting as the reducing agent on a downstream SCR-catalyst. Significant criteria for an efficient SCR-system are a fast UWS preparation and a high ammonia uniformity
at the catalyst intake.
This work presents the determination, adaption and integration of relevant submodels to establish a CFD simulation method for the evaluation of UWS based SCR-systems. A systematic experimental and numerical breakdown of the UWS preparation and ammonia mixing process
is carried out to determine the relevant modelling depth for each step. A UWS decomposition model is implemented and validated with literature data. On the basis of video analysis of liquid
film formation and available literature data, a multi-regime droplet-wall-interaction model is adapted to UWS. The necessity of a conjugate heat transfer model of the exhaust system walls, to correctly capture wall temperature dependent droplet impingement and liquid film boiling is proved. Validation is performed by transient infrared thermal imaging. Buffering of ammonia by liquid film formation is analyzed with the CFD-model and correlated to transient
FTIR-concentration data. The liquid film decomposition model is validated by a comparison to a liquid film probe analysis. The resulting liquid phase model covers a wide range of operating
conditions, from massive wall wetting at low- to classical Leidenfrost phenomena at high exhaust temperatures.
The liquid phase UWS model is applied to a passenger car SCR-system, with a mixing element creating a turbulent swirling flow upstream of the SCR-catalyst. The impact of the turbulence model and the numerical differencing scheme on the prediction of the mixing process
of the gaseous ammonia is analyzed. The study proves the high impact of an advanced second order differencing scheme on the species transport. It further shows that Reynoldsaveraged k-"-models systematically underestimate the turbulence level in the swirl flow and, in consequence, the turbulent diffusion and homogenization of the ammonia vapor. In contrast, a Reynolds-Stress-model (RSM) leads to improved predictions by accounting for the
anisotropic character of turbulence in the swirl. The relevance of a detailed simulation of the liquid phase dynamics and -evaporation for precise ammonia homogeneity predictions is proved. Numerical results are validated with measurements of back pressure and the spatial ammonia distribution at the catalyst.
The presented method allows a precise prediction of the ammonia homogenization and an estimation of liquid film deposition risks for a wide range of operating conditions.

Created from the Publication Database of the Vienna University of Technology.