No abstract

Numerical implementation schemes of drag force effects on Lagrangian particles can lead to instabilities or inefficiencies if static particle time stepping is used. Despite well known disadvantages, the programming structure of the underlying, C++ based, Lagrangian particle solver led to the choice of an explicit EULER, temporal discretization scheme. To optimize the functionality of the EULER scheme, this paper proposes a method of adaptive time stepping, which adjusts the particle sub time step to the need of the individual particle. A user definable adjustment between numerical stability and calculation efficiency is sought and a simple time stepping rule is presented. Furthermore a method to quantify numerical instability is devised and the importance of the characteristic particle relaxation time as numerical parameter is underlined. All derivations are being conducted for (non-)spherical particles and finally for a generalized drag force implementation. Important differences in spherical and non-spherical particle behaviour are pointed out.

In filtration processes it is necessary to consider both, the interaction of the fluid with the solid parts, as well as the effect of particles carried in the fluid and accumulated on the solid. In this first part a closer look is taken on the influence of the fluid on the solid regions. The required algorithm to couple the governing differential equations is derived on the basis of the equations, governing the solid and fluid regions. For discretization, only one single computational mesh is used and this is adjusted to the deformation at each time-step. The simulation of the fluid-structure interaction is realised in a single finite volume flow solver on the basis of the OpenSource software OpenFoam.

No abstract

Fundamental aad phenomenological models for cells, stacks, and complete systems of PEFC and SOFC are reviewed and their predictive power is assessed by comparing model simulations against expedmetrts. Computationally efficient models suited for eDgineering design include the (1+1) dimensionality approach, which decouples the membrane in-plane and through-plane processes, and the volume-averaged-method CVAM) that considers only the lùmped effect of pre-selected system comporents. The former model was shown to capture the measured lateral current density inhomogeneities in a PEFC ald the latter was used for the optimization of commercial SOFC systems. State Space Modeting (SSM) was used 10 identiry the main reaction pathways in SOFC and, in conjunction with the implemertatior of geometrically well-defined electrodes, has opened a new direction for the understarding of electrochemical reactions. Furthermore, SSM has advanced the understanding of the CO-poisoning-induced aûode impedance in PEFC.Detâiled nùmerical models such as the Lattice Boltzmarn (lB) method for tuaûsport in porous media and the fulI 3-D Computational Fluid Dynamics (CFD) Navier-Stokes simulations are addressed. These models contain all components of the relevant physics and they can implove the understanding of the rçlated phenomena, a necessary condition for the developmert of both appropriate simplifred models as well as reliable technologies.Within the LB ïiamework, a technique for the charactedzation and computer-recoDstmction of the porous electrode sfuùcture was developed using advanced pattem recognition algorithms. In CFD modeling, 3-D simulations were used to investigate SOFC with intemal mgthare steam reformitrg and have exemplified the significance of porous and novel fractal channel distributors for the fuel and oxidant delivery, as well as for the cooling of PEFC. As importantly, the novel concept has been pùt forth of functionally designed, fractal-shaped fuel cells, showing promise of significant performance improvements over the conventional reclangular shaped units. Themo-economic modeling for the optimization of PEFC is finally addressed Kelvords: multidimensional simulations of fuel cells; porous electrode stucture chaÉcterization; state-spâce modeling of electrochemical reactions; thermo-economic o?timization