Large Eddy Simulation of Fuel Spray Combustion

Combustion is the main source for energy today and in the foreseeable future. It is used in a wide range of applications for power generation and transportation systems, such as internal combustion engines. In order to improve the efficiency of such systems and to reduce the emission of pollutants, a better understanding of the physical and chemical processes related to combustion is required. Fuel sprays play a major role in combustion systems like diesel engines, which is the topic of this thesis. Large Eddy Simulation (LES) constitutes an advanced computational method that has gained more attention in the recent years, as it allows for detailed investigations of the unsteady flow and combustion phenomena that occur in internal combustion engines. The present dissertation belongs to the field of computational physics, more precisely to computational fluid dynamics and combustion. For the research this thesis comprises, the LES method is employed to investigate turbulent spray combustion in the context of diesel engines. A challenge arises from the description of the complex chemical reactions that take place during the oxidation of fuels used in such engines. The approach to address this in the present work is based on the Flamelet Generated Manifold (FGM) method, which allows to take a detailed description of these reactions into account while the computational cost remains feasible. The objectives of the dissertation are to explore and implement modeling approaches which allow to investigate high-velocity fuel sprays, and specifically their ignition and combustion characteristics, in LES. The investigated spray combustion cases correspond to Spray A, a reference case defined within the Engine Combustion Network (ECN). The resolution of the computational mesh and the implications of modeling fuel sprays in LES were first studied in non-reacting simulations. The results indicate that an adequate mesh resolution is crucial to the simulation approach. Investigations of canonical combustion systems with two chemical mechanisms show that the mechanism significantly affects the prediction of the ignition timing. The results of the turbulent spray combustion simulations give insight into the ignition process and flame stabilization. A comparison of the results from LES and novel experimental data of hydroxyl and formaldehyde show a good agreement. The results further show that the chosen approach towards the simulation of turbulent spray flames is suitable and allows for a detailed analysis of the unsteady processes. An important achievement of this work is the implementation of the FGM method in the open-source flow solver OpenFOAM.