Thesis for degree: Licentiate of Engineering

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Copyright © Eva Hedvig Charlotta Paradis, 2011 Division of Heat Transfer Department of Energy Sciences Faculty of Engineering (LTH) Lund University Box 118, SE-221 00, Lund, Sweden ISRN LUTMDN/TMHP-11/7075 – SE ISSN 0282-1990
Abstract The purpose of this work is to investigate the interaction between transport processes and chemical reactions, with special emphasis on modeling mass transport by the Lattice Boltzmann method (LBM) at microscale of the anode of a solid oxide fuel cell (SOFC). In order to improve the performance of an SOFC, it is important to determine the microstructural effect embedded within the physical and chemical processes, which usually are modeled macroscopically. Without detailed knowledge of the transport processes and the chemical reactions at microscale it can be difficult to capture their effect and to justify assumptions for the macroscopic models with regard to the source terms and various properties in the porous electrodes. The advantage of an anode-supported SOFC structure is that the thickness of the electrolyte can be reduced, while still providing an internal reforming environment. For this configuration with an enlarged anode, more detailed knowledge of the porous domain in terms of the physical processes at microscale is called for. In the first part of this study, the current literature on the modeling of transport processes and chemical reactions mechanisms at microstructural scales is reviewed with special focus on the LBM followed by a report on the emphasis to couple conventional CFD to LBM. In the second part, two models are described. The first model is developed at microscale by LBM for the anode of an SOFC in MATLAB. In the LB approach, the main point is to carefully model the diffusion and convection at microscale in the porous region close to the threephase-boundary (TPB). The porous structure is reconstructed from digital images, and processed by Python. The second model is developed at macroscale for the whole unit cell. For the macroscale model the kinetic model is evaluated at smaller scales to investigate if any severe limiting effects on the heat and mass transfer occur. LBM has been found to be an alternative method for modeling at microscale and can handle complex geometries easily. However, there is still a need for a supercomputer to solve models with several physical processes and components for a larger domain. The result of the macroscale model shows that the three reaction rate models are fast and vary in magnitude. The pre-exponential values, in relation to the partial pressures, and the activation energy affect the reaction rate. The variation in amount of methane content and steam-to-fuel ratio reveals that the composition needs a high inlet temperature to enable the reforming process and to keep a constant current-density distribution. As experiments with the same chemical compositions can be conducted on a cell or a reformer, the effect of the chosen kinetic model on the heat and mass transfer was checked so that no severe limitation are caused on the processes at microscale for an SOFC. For future work, macroscale and microscale models will be connected for the design of a multiscale model. Multiscale modeling will increase the understanding of detailed transport phenomena and it will optimize the specific design and control of operating conditions. This can offer crucial knowledge for SOFCs and the potential for a breakthrough in their commercialization. Keywords: mass transport, diffusion, microscale, porous media, kinetics, LBM, CFD, anode multicomponent, MATLAB. 2
Page 1: SOFC modeling from micro to macrosc
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 and 12: Q source term (heat), W/m3 q heat f
Page 13 and 14: 1 Introduction After some time of d
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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