Armin Wehrfritz

Large-Eddy Simulation of Subsonic Jets

The present study deals with development and validation of a fully explicit, compressible Runge-Kutta-4 (RK4) Navier-Stokes solver in the opensource CFD programming environment OpenFOAM. The background motivation is to shift towards explicit density based solution strategy and thereby avoid using the pressure based algorithms which are currently proposed in the standard OpenFOAM release for Large-Eddy Simulation (LES). This shift is considered necessary in strongly compressible flows when Ma > 0.5. Our application of interest is related to the pre-mixing stage in direct injection gas engines where high injection pressures are typically utilized. First, the developed flow solver is discussed and validated. Then, the implementation of subsonic inflow conditions using a forcing region in combination with a simplified nozzle geometry is discussed and validated. After this, LES of mixing in compressible, round jets at Ma = 0.3, 0.5 and 0.65 are carried out. Respectively, the Reynolds numbers of the jets correspond to Re = 6000, 10000 and 13000. Results for two meshes are presented. The results imply that the present solver produces turbulent structures, resolves a range of turbulent eddy frequencies and gives also mesh independent results within satisfactory limits for mean flow and turbulence statistics.

Analyzing Local Combustion Environment with a Flamelet Model and Detailed Chemistry

Measurements have been done in order to obtain information concerning the effect of EGR for the smoke and NOx emissions of a heavy-duty diesel engine. Measured smoke number and NOx emissions are explained using detailed chemical kinetic calculations and CFD simulations. The local conditions in the research engine are analyzed by creating equivalence ratio temperature (Phi-T) maps and analyzing the CFD results within these maps. The study uses different amount of EGR and the standard EN590 diesel fuel. The detailed chemical kinetic calculations take into account the different EGR rates. The CFD calculations are made with a flamelet based combustion model together with detailed chemistry. The results are compared to a previous study where a hybrid local flame area evolution model combined with an eddy breakup type model was used in the CFD simulations. It was observed that NOx emission trends can be well captured with the Phi-T maps but the situation is more difficult with the engine soot. Hence, the conclusion is the same as in the previous study with the hybrid combustion model. However, the local reaction zone is qualitatively very different with the flamelet model as compared to the hybrid model. Phi-T maps were also constructed for the total fixed nitrogen, using a detailed description of the nitrogen chemistry. EGR is typically used as an NOx abatement technique, having the purpose to lower the temperature and thus the formation of thermal-NO. However, these maps revealed a new functionality of EGR as a NOx abatement method.