Salvador Izquierdo

An open-source library for the numerical modeling of mass-transfer in solid oxide fuel cells ☆

The generation of direct current electricity using Solid Oxidize Fuel Cells (SOFCs) involves several interplaying transport phenomena. Their simulation is crucial for the design and optimization of reliable and competitive equipment, and for the eventual market deployment of this technology. An open-source library for the computational modeling of mass-transport phenomena in SOFCs is presented in this article. It includes several multicomponent mass-transport models (ie Fickian, Stefan-Maxwell and Dusty Gas Model), which can be applied both within porous media and in porosity-free domains, and several diffusivity models for gases. The library has been developed for its use with OpenFOAM(R), a widespread open-source code for fluid and continuum mechanics. The library can be used to model any fluid flow configuration involving multi-component transport phenomena and it is validated in this paper against the analytical solution of one-dimensional test cases. In addition, it is applied for the simulation of a real SOFC and further validated using experimental data

Analysis of open boundary effects in unsteady lattice Boltzmann simulations

The effects of the interaction between the open boundaries (inflow and outflow) and the fluid domain are studied in unsteady lattice Boltzmann (LB) simulations of fluid flow. The confined unsteady laminar flow past a square cylinder is used as test case due to the continuous vortex shedding generated in the wake of the cylinder. Three different approaches to treat open boundaries are considered as they are expected to be representative of the most common ones typically used in LB. We conclude that lattice Boltzmann methods suffer from the same problems with open boundaries as other compressible or pseudo-compressible approximations for the numerical solution of fluid flows: these boundaries reflect pressure waves and these have a relevant influence on the solution and convergence rate of both steady and unsteady flows, even at very low Reynolds numbers. However, practical solutions considering zero derivatives at the outflow or non-reflecting boundaries are possible.

Momentum transfer correction for macroscopic-gradient boundary conditions in lattice Boltzmann methods

The boundary conditions used to represent macroscopic-gradient-related effects in arbitrary geometries with the lattice Boltzmann methods need a trade-off between the complexity of the scheme, due to the loss of localness and the difficulties for directly applying link-based approaches, and the accuracy obtained. A generalization of the momentum transfer boundary condition is presented, in which the arbitrary location of the boundary is addressed with link-wise interpolation (used for Dirichlet conditions) and the macroscopic gradient is taken into account with a finite-difference scheme. This leads to a stable approach for arbitrary geometries that can be used to impose Neumann and Robin boundary conditions. The proposal is validated for stress boundary conditions at walls. Two-dimensional steady and unsteady configurations are used as test case: partial-slip flow between two infinite plates and the slip flow past a circular cylinder.

Optimal preconditioning of lattice Boltzmann methods

A preconditioning technique to accelerate the simulation of steady-state problems using the single-relaxation-time (SRT) lattice Boltzmann (LB) method was first proposed by Guo et al. [Z. Guo, T. Zhao, Y. Shi, Preconditioned lattice-Boltzmann method for steady flows, Phys. Rev. E 70 (2004) 066706-1]. The key idea in this preconditioner is to modify the equilibrium distribution function in such a way that, by means of a Chapman-Enskog expansion, a time-derivative preconditioner of the Navier-Stokes (NS) equations is obtained. In the present contribution, the optimal values for the free parameter @c of this preconditioner are searched both numerically and theoretically; the later with the aid of linear-stability analysis and with the condition number of the system of NS equations. The influence of the collision operator, single- versus multiple-relaxation-times (MRT), is also studied. Three steady-state laminar test cases are used for validation, namely: the two-dimensional lid-driven cavity, a two-dimensional microchannel and the three-dimensional backward-facing step. Finally, guidelines are suggested for an a priori definition of optimal preconditioning parameters as a function of the Reynolds and Mach numbers. The new optimally preconditioned MRT method derived is shown to improve, simultaneously, the rate of convergence, the stability and the accuracy of the lattice Boltzmann simulations, when compared to the non-preconditioned methods and to the optimally preconditioned SRT one. Additionally, direct time-derivative preconditioning of the LB equation is also studied.

Quadrature-based moment closures for non-equilibrium flows

Recently the Quadrature Method of Moments (QMOM) has been extended to solve several kinetic equations, in particular for gas-particle flows and rarefied gases in which the non-equilibrium effects can be important. In this work QMOM is tested as a closure for the dynamics of the Homogeneous Isotropic Boltzmann Equation (HIBE) with a realistic description for particle collisions, namely the hard-sphere model. The behaviour of QMOM far away and approaching the equilibrium is studied. Results are compared to other techniques such as the Grads moment method (GM) and the off-Lattice Boltzmann Method (oLBM). Comparison with a more accurate and computationally expensive approach, based on the Discrete Velocity Method (DVM), is also carried out. Our results show that QMOM describes very well the evolution when it is far away from equilibrium, without the drawbacks of the GM and oLBM or the computational costs of DVM, but it is not able to accurately reproduce equilibrium and the dynamics close to it. Static and dynamic corrections to cure this behaviour are here proposed and tested.

Preconditioned Navier-Stokes schemes from the generalised lattice Boltzmann equation

Preconditioning of Navier-Stokes equations is a widely used technique to speed up Computational Fluid Dynamics simulations of steady flows. In this work a systematic study is performed of time-derivative preconditioners of Navier-Stokes equations that can be derived from the generalised lattice Boltzmann equation. In this way, lattice Boltzmann models equivalent to preconditioned Navier-Stokes systems are constructed, and it becomes possible to take advantage of the knowledge generated in this field to improve the convergence to steady state of lattice-Boltzmann flow-calculations. Two different preconditioners are obtained, which are analysed according to their condition numbers and compared with typical Navier-Stokes preconditioners.