Thesis for degree: Licentiate of Engineering

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g i j LB P r s w gas phase molecule i molecule j discrete system, lattice Boltzmann physical system steam reforming reaction solid phase or water-gas shift reaction wall Abbreviations AFL anode functional layer CFD computational fluid dynamics CFL cathode functional layer FDM finite difference method FEM finite element method FVM finite volume method IEA International Energy Agency IT intermediate temperature LBM lattice Boltzmann method LTNE local temperature non-equilibrium MC Monte Carlo method MD molecular dynamics SEM scanning electron microscopy SF steam-to-fuel ratio SMM Stefan-Maxwell model SOFC solid oxide fuel cell TPB three-phase boundary YSZ yttria-stabilized zirconia XCT x-ray computer tomography Chemical CH 4 methane CO carbon monoxide CO 2 carbon dioxide H 2 hydrogen H 2 O water Ni nickel O 2 oxygen vii
1 Introduction After some time of decreasing interest, fuel cell research is now receiving a lot of attention. Fuel cells are considered a promising future resource for both stationary and distributed electric power stations because of their high performance and high reliability [1, 2]. In order to explore these aspects in greater depth, there is a need for both multiphysics and multiscale modeling. The approach is by solving the equations for momentum, charge, heat and mass transport and chemical reactions at the same time at corresponding scales. At this point, simulation of mass diffusion and convection as well as chemical and electrochemical reactions at microscale will offer crucial insight to improve the performance of the fuel cell [3]. Furthermore, future prospects for fuel cell research are to connect microscale to macroscale and obtain stable solutions for multiscale modeling. Solid oxide fuel cells (SOFCs) are particularly interesting because it operates at high temperature and can therefore handle the reforming of hydrocarbon fuels directly within the cell. SOFCs have a number of advantages, e.g., high conversion efficiency, high quality exhaust heat and flexibility of fuel input. However, as expected, SOFCs also have disadvantages. For instance, because they operate at very high temperatures (800–1100C [1]), material performance and manufacturing costs are currently of concern. Recently, attempts have been made to lower the operating temperature of SOFCs by adopting a porous, anode-supported structure to reduce the thickness of the electrolyte. It is important to model all physical processes and chemical reactions simultaneously since the mass and charge transport depends on the multifunctional material structure in the porous electrodes, the chemical reactions, the temperature distribution and the species concentrations. The fluid flow depends on the chemical reactions, temperature and fluid characteristics. The heat transport depends on the polarization losses, the chemical reactions, the fluid flow and the material structure. The reforming reactions depend on temperature, concentration and amount of catalyst available. The interaction issues are numerous and here the microscopic contributions are important to include. Also it is necessary to model SOFCs into detail and explore them at a microscopic level, in order to fully understand how different parameters affect the performance, by connecting different physical phenomena at different scales [2, 4]. The purpose of this study is to develop a microscale model of an anode of an SOFC for the transport processes and the chemical reactions to get a deeper understanding of the effect on the different physical processes at multiple scales. The lattice Boltzmann method (LBM) is used to model the mass transport at microscale for a limited part of the porous anode. Also an evaluation of a macroscale model for the whole unit cell is carried out. 1
Page 1 and 2: SOFC modeling from micro to macrosc
Page 3 and 4: Abstract The purpose of this work i
Page 5 and 6: katalytisk omvandling av naturgas o
Page 7 and 8: List of publications Journal public
Page 9 and 10: 3.2 Governing equations for the mac
Page 11: Q source term (heat), W/m3 q heat f
Page 15 and 16: 2 SOFC modeling at smaller scales T
Page 17 and 18: etween 0.3 and 1.5 mm. In this conf
Page 19 and 20: Volume Method (FVM) which adopt the
Page 21 and 22: 2.3 Lattice Boltzmann method LBM ha
Page 23 and 24: streaming [3]. The concentration ρ
Page 25 and 26: ease the correspondence between the
Page 27 and 28: applicable for a mixture of two spe
Page 29 and 30: Figure 2.6: Effective boundary arra
Page 31 and 32: Table 2.1: Comparison of previous w
Page 33 and 34: Figure 2.7: Schematic illustration
Page 35 and 36: model is presented after the data c
Page 37 and 38: 3.2.1 Mass transport In the electro
Page 39 and 40: where h v is the volume heat transf
Page 41 and 42: Achenbach [43], it was found that t
Page 43 and 44: cathode are assumed to be similar t
Page 45 and 46: (3.48) where, besides the parameter
Page 47 and 48: for an isothermal spherical particl
Page 49 and 50: Velocity carried out to check wheth
Page 51 and 52: Figure 4.4: Velocity field [lu/ts]
Page 53 and 54: Table 4.2: Parameters in the SOFC a
Page 55 and 56: eactions are safely fulfilled. This
Page 57 and 58: Figure 4.8: Mole fraction distribut
Page 59 and 60: Table 4.5: Inlet mole fractions for
Page 61 and 62: Figure 4.12: Mole fraction distribu
Page 63 and 64:
5 Conclusions The physics and the t
Page 65 and 66:
7 References [1] Andersson M., Yuan
Page 67 and 68:
[25] Latt J., Hydrodynamic Limit of
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Thesis for degree: Licentiate of Engineering

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