A new model for detailed simulation of multiple transport and conversion processes in SOFC stack repeat units

Progress in the study of local reactive transport phenomena in SOFC stacks has been achieved based on both advanced physical and numerical approaches. Specifically, the numerically unfavorable high aspect ratio of about 1'000 between a typical stack diameter and a typical cell thickness has been successful treated using the ADI (Alternating Direction Implicit) numerical scheme. Unlike conventional methods, ADI allows one to predict local gradients of chemical species and electrical charges with low computing costs and unconditional numerical stability. This is especially important in the vicinity of a current collector rib and under extreme operation conditions, e.g. when the fuel gets depleted. Furthermore, the convection-dominant transport within the gas distribution channels has been accurately calculated without using an additional (artificial) numerical diffusion. Concerning the gas flow in the channels along a porous electrode, another important feature of our model is the non-zero slip velocity at the electrode surface. In conventional approaches, this slip velocity is often estimated empirically. Alternatively, the hydrodynamic flow field is calculated from the Navier Stokes equations which are simultaneously solved in the open flow channel and the porous electrode. Within the electrode, the velocity field is then penalized artificially by adding the Karman Kozeny term. However, such a penalization suffers from inaccurate velocities in the vicinity of the electrode surface. In our approach, the compressible flow in both regions is treated in a unified manner in which the slip velocity is not known a priory, but follows from requiring continuous shear stresses at the electrode surface