Stefano Ubertini

Modeling Solid Oxide Fuel Cells: Methods, Procedures and Techniques

This book fills the need for a practical reference for all scientists and graduate students who are seeking to define a mathematical model for Solid Oxide Fuel Cell (SOFC) simulation. Structured in two parts, part one presents the basic theory, and the general equations describing SOFC operation phenomena. Part two deals with the application of the theory to practical examples, where different SOFC geometries, configurations, and different phenomena are analyzed in detail.

Recent advances of Lattice Boltzmann techniques on unstructured grids

Further research on both theoretical and practical aspects of a previously defined finite-volume formulation to integrate the Lattice Boltzmann equation (LBE) on unstructured grids is presented. Moreover, additional explicit schemes for time discretisation are proposed. The method is tested on an impulsively started channel flow with the aim of exactly locating solid boundaries. Cavity flow simulations are used to measure the inaccuracies due to compressibility, grid and time discretisation effects. Finally, the proposed method has been tested against a laminar flow over a backward facing step in order to assess the effectiveness of open outlet boundary treatment.

Transverse harmonic oscillations of laminae in viscous fluids: a lattice Boltzmann study

In this paper, we use the lattice Boltzmann method with the Bhatnagar–Gross–Krook linear collision operator to study the flow physics induced by a rigid lamina undergoing moderately large harmonic oscillations in a viscous fluid. We propose a refill procedure for the hydrodynamic quantities in the lattice sites that are in the vicinity of the oscillating lamina. The numerically estimated flow field is used to compute the complex hydrodynamic function that describes the added mass and hydrodynamic damping experienced by the lamina. Results of the numerical simulations are validated against theoretical predictions for small amplitude vibrations and experimental and numerical findings for moderately large oscillations.

Performance estimation and experimental measurements of a photovoltaic roof

The market for photovoltaic systems is rapidly expanding. Currently, there are a few large utility photovoltaic power plants, thousands of residential systems, and tens of thousands of remote power systems in use. Even if photovoltaics is a technology that has already demonstrated its effectiveness and holds great promise in electrical generation, the costs are still too high to guarantee a commercial competitivity.

A Comparison Between the Interpolated Bounce-Back Scheme and the Immersed Boundary Method to Treat Solid Boundary Conditions for Laminar Flows in the Lattice Boltzmann Framework

In this paper, the interpolated bounce-back scheme and the immersed boundary method are compared in order to handle solid boundary conditions in the lattice Boltzmann method. These two approaches are numerically investigated in two test cases: a rigid fixed cylinder invested by an incoming viscous fluid and an oscillating cylinder in a calm viscous fluid. Findings in terms of velocity profiles in several cross sections are shown. Differences and similarities between the two methods are discussed, by emphasizing pros and cons in terms of stability and computational effort of the numerical algorithm.

Lattice Boltzmann simulations of phase-separating flows at large density ratios: the case of doubly-attractive pseudo-potentials

It is shown that the cooperation between short and mid range attraction in Lattice Boltzmann (LB) models with multi-range pseudo-potentials, permits the achievement of phase-separation at liquid/vapor density ratios in excess of 1 : 500, nearly an order of magnitude above the current limits of standard single-range LB models. Inspection of the density configurations reveals that this favourable behaviour results from a sizeable reduction of spurious currents near the liquid/vapour interface. The aforementioned enhancement of the density ratio is obtained at virtually the same computational cost of the standard single-range models. As a consequence, the present results may open the way to a broader spectrum of LB applications, involving the dynamics of complex phase-separating flows with large density ratios.

Lattice Boltzmann schemes without coordinates.

We discuss recent developments extending the scope of the lattice Boltzmann method to unstructured (coordinateless) grids with arbitrary connectivity. Besides their intrinsic interest as examples of discrete kinetic systems living in irregular phase–space, the above extensions bear a direct relevance as computational tools for multi–scale applications.

Numerical stability of entropic versus positivity-enforcing lattice Boltzmann schemes

A preliminary study of the non-linear stability properties of entropic schemes versus positivity-enforcing (FIX-UP) schemes is presented for the case of two-dimensional cavity flow. It is shown that, although they operate on fairly distinct schedules, both methods achieve substantial stability enhancements over the standard single-time relaxation Lattice Boltzmann scheme.

Modeling Carbon Monoxide Direct Oxidation in Solid Oxide Fuel Cells

In the present study, the results of the numerical implementation of a mathematical model of a planar anode-supported SOFC are reported. In particular, model results are validated and discussed when the fuel is a mixture of hydrogen and carbon monoxide, focusing on the importance of simulating direct oxidation of carbon monoxide. The mathematical model is solved in a 3D environment and the key issue is the validation comparing with experimental data, which is made in different operating conditions to establish the reliability of the presented model. The results show the importance of simulating direct oxidation of carbon monoxide and its effect on the fuel cell performance.

The unstructured lattice Boltzmann method for non-Newtonian flows

Non-Newtonian models with shear-thinning viscosity are commonly used to solve a variety of complex flow problems. A new finite-volume discretization based upon an unstructured grid is used to integrate the differential form of the lattice Boltzmann equation with a shear-dependent viscosity, using a cell-vertex finite-volume technique. The unknown fields are placed at the nodes of the mesh and evolve on the basis of the fluxes crossing the surfaces of the corresponding control volumes. Numerical results show a satisfactory accuracy also in the case of relatively complex geometries and demonstrate the ability of the method to predict the main features of non-Newtonian flows in straight and stenosed channels.