CFD modelling and simulation of multiphase flow in a PEM fuel cell using OpenFOAM

A proton exchange membrane (PEM) fuel cell is an electrolytic cell that converts chemical energy of hydrogen reacting with oxygen into electrical energy, without producing any greenhouse gases. To meet increasingly stringent application needs, improved performance and increased efficiency are paramount. Computational fluid dynamics (CFD) is an ideal means for achieving these improvements. In this thesis, a comprehensive CFD-based numerical toolbox that can accurately simulate the major transport phenomena which take place within a PEM fuel cell is presented. The tool is developed using the Open Source Field Operation and Manipulation (OpenFOAM) software (a free open-source CFD code) which makes it very flexible and well-suited for use by fuel cell manufacturers and researchers to rapidly gain important insights into the cell working processes which are crucial to the cell optimization. The toolbox includes a three-dimensional (3D) non-isothermal model for both single-phase flow and multiphase flow simulations. Case studies in steady-state operating conditions were conducted with both models. The results for the distribution of velocity, pressure, chemical species mass fractions, Nernst potential, local current density and temperature, cathode exchange current density, activation overpotential, membrane ionic conductivity, ohmic overpotential, cathode limiting current density, and concentration overpotential are as expected, for both flow models. The results of mesh independence studies indicate that in terms of the cell performance, the case study mesh provides adequate resolution. Qualitative comparisons were made between numerical and experimental results taken from the literature, and the results obtained with the multiphase flow model. Good agreement was obtained over the entire range of the cell operation. Furthermore, a parametric study with the multiphase flow model revealed the effects of operating temperature and pressure, charge transfer coefficient, gas diffusion layer thickness and porosity, catalyst layer porosity, stoichiometric flow ratio, flow configuration, and concentration constant on cell performance. Interestingly, it was found that: the counter-flow flow configuration results in reduced cell potential at high current densities compared to the co-flow flow configuration; and the value of the concentration constant (c) greatly affects the proportions between the activation, ohmic and concentration losses. A c value between 0.2 and 0.3 can ensure realistic proportions between the three overpotentials in the fuel cell which are crucial in shaping the cell polarization curve. The PEM fuel cell numerical toolbox presented in this thesis has many novel features that enhance the physical realism of the simulations. The models that are provided within the toolbox can serve as a basis to develop other features such as improved catalyst and membrane models. More importantly, since the toolbox has been developed using OpenFOAM, it can form a framework for research and development which is not possible with commercial software. The work therefore contributes to achieving the objectives outlined in the International Energy Agency (IEA, France) Advanced Fuel Cell Annex 37 which promotes open-source code modelling of fuel cells. The source code for the single-phase flow model and the multiphase flow model are available at http://dx.doi.org/10.17632/3gz7pxznzn.1 and http://dx.doi.org/10.17632/c743sh73j8.1, respectively