Thesis for degree: Licentiate of Engineering

1.1 Research objectives
The knowledge of the effect of different processes at microscale, such as mass diffusion and
electrochemical reactions, on the unit cell in whole is expected to be clarified when the model
includes microscale phenomena within the electrodes and electrolyte. When these processes
are studied, the effect on the overall performance of the cell can provide useful information
for improvement [5].
The aim is two-fold. Firstly, it provides a description of current research on modeling of
transport processes and kinetics effect on transfer processes in the electrodes of SOFCs.
Focus is put on LBM to model the microstructural phenomena. Further, other modeling
methods and equations are briefly reviewed as well as the coupling of LBM to conventional
CFD methods. Secondly, it reveals a microscale model of an anode of an SOFC using the
LBM to carefully investigate the physical and chemical processes at smaller scales which
simulates the mass diffusion and momentum transport for a small part of the anode close to
the electrolyte. A macroscale model of an anode-supported SOFC is also developed where the
equations for mass, heat and momentum transport are solved simultaneously. Different
internal reforming reaction rates are tested to examine the effect on the cell. Also the
parameters inlet methane content and steam-to-fuel ratio are tested for different ranges. The
kinetics for the macroscale model are carefully investigated to check so no severe heat and
mass transfer limitations occur at microscale. Further, the future step is to model all processes
at microscale where microstructural effects are considered to affect the performance and also
integrate the LB model and the macroscale CFD model, i.e., coupling different physical
models at different scales, to form a multiscale model which can reproduce even more
realistic simulation results. More precisely, the objectives here are:
To identify whether the LBM can function as a method for SOFCs at microscale to
investigate the transport processes and chemical reactions, in terms of capabilities and
limitations.
To capture and study the microscale effect of mass diffusion and momentum
transport within the anode close to the active area.
To identify whether diffusion and chemical reactions will significantly affect the cell
performance by modeling them at both macro- and microscale.
1.2 Methodology
In order to analyze the transport processes and chemical reactions in SOFCs in detail, an LB
model for the porous region close to the three phase boundary (TPB) is developed in
MATLAB. The LB model uses the single relaxation time BGK (Bhatnagar-Gross-Krook)
method and is developed for a D2Q9 (two dimensional domain with eight interconnected
directions and nine interconnected speeds) [3]. The model focuses solely on the mass
diffusion and momentum transport at microscopic level in this region. The simulation
procedure is divided into stepwise cases, from a simple channel to a more complex porous
media. The macroscale model of an SOFC is developed in COMSOL where the equations for
mass, heat and momentum transport are solved simultaneously. Finally, a study of limiting
effects on the heat and mass transfer by the kinetics is also performed.
1.3 Thesis outline
Chapter 1 contains a short presentation of the thesis. Chapter 2 gives a general description of
SOFCs, and an overview of the relevant literature of the LB methodology. A detailed
description of the mathematical model of the LBM is presented in Chapter 3 with governing
equations and boundary conditions. The results are presented in Chapter 4 while Chapter 5
provides conclusions drawn from the results. Finally, Chapter 6 gives reflections over future
work.
2