Lattice-Boltzmann-Simulation in faserbasierten Mikrostrukturen

Despite its simple underlying principle to gain electrical current from hydrogen and oxygen, a real fuel cell application is a complex system studied in various areas of science. An essential way to raise the efficiency of an H2-O2 fuel cell is to ensure an optimal flow of inand outgoing gases. This can be done through a gas diffusion layer that supports an equal distribution along the electrode in polymer electrolyte membrane-based fuel cells. In simulations, the detailed structure of the gas diffusion layer is commonly not taken into account and is integrated in fuel cell models as a single parameter. In this thesis, concepts from fluid dynamics are used to gain a fundamental understanding of the gas diffusion layer’s structure and its influence on the fuel cell. The virtual structure used for the gas diffusion layer is based either on data generated from real 3D-tomograms or on data generated from a statistical model developed by the Universität Ulm. The fluid simulation is done with the Lattice-Boltzmann algorithm. After parallelization of code, the simulation can be done on a macroscopic scale. This leads to conclusions well beyond mere statistical effects. Moreover, the microscopic structure is resolved to the level of single fibers, whose influence on results can thus be considered in the analysis. Relying on the validation of the algorithm on experimental and analytical references, an ideal set of input parameters can be determined. Within this study, the single and the multi-relaxation-time method are compared. Thus, a tool is established for an application in fluid dynamics in general as well as in gas diffusion layer-like structures. The now optimized Lattice-Boltzmann algorithm allows a comprehensive fluid dynamical analysis of different gas diffusion layer materials. Based on characteristic numbers, the statistically modeled structures as well as the real tomogram-data are studied in comparison with experimental and analytical data. Subsequently, the analysis of minimal examples is used to identify the structure’s properties that influence the gas flow essentially. From here, conclusions about different realizations of the virtual material can be drawn to find the similarities between the 3D-tomograms and the virtual model. In this way, an almost one to one virtual copy of the underlying gas diffusion layer can be constructed and comes along with clear advantages: The virtual construction of a sample of the statistical model on the computer can be realized quickly at low costs. This is necessary to facilitate the virtual construction of gas diffusion materials in order to take the statistical properties of the material into account. Throughout the underlying work, the operating conditions of the high temperature polymer electrolyte fuel cell enter the model through the boundary conditions. Being of superior importance for the efficiency of this fuel cell, the gas distribution along the electrode is analyzed in a channel-land scenario on the basis of the tomographic data. A one-component anode scenario as well as a two-component system on the cathode side is studied. For the one-component case, it is shown how the gas distribution along the electrode depends on the compression of the fuel cell and that the limit for the volume compression should not exceed 30 %. For the two-component case the gas distribution is checked on plausibility with regard to the earlier results. This shows in principle that the underlying two-component model can be used for the cathode simulation.