1. INTRODUCTION A multiphysics across-the-channel model is presented for the anode of a liquid-feed Direct Methanol Fuel Cell (DMFC). The model considers both two-dimensional (2D) single-phase anisotropic transport of methanol and anisotropic electron transport, coupled to a one- dimensional (1D) model for the membrane and the cathode, which describes the electrochemical reactions kinetics, water and methanol crossover, and oxygen transport [1]. As new contribution, the 2D model takes into account the effects of the inhomogeneous compression of the GDL associated to the repetitive rib-channel pattern, including non- uniform porosity, diffusivity and bulk electrical conductivity, as well as non-uniform contact resistances over the rib/GDL and membrane/GDL interfaces. Figure 1. Operating sketch of a DMFC showing the physical domains of the 2D/1D model. FLUID MECHANICS RESEARCH GROUP http://fluidos.uc3m.es / NON-UNIFORM ANISOTROPIC PRO- PERTIES & CONTACT RESISTANCES contact resistances REFERENCES The mechanical properties of the (graphite) bipolar plate are assumed isotropic (10 GPa Young’s modulus and 0.25 Poisson’s ratio), while for the GDL we consider the nonlinear orthotropic properties of carbon paper (Toray ® TGP-H series) [2]. MECHANICAL PROPERTIES ParameterValueGDLReference E y (ε y )Numerical fittingTGP-H-060/090[3, 4, 5, 6] E x = E z 7 GPaTGP-H-060[5] G xy 18.5 MPaTGP-H-060[5] ν xz = ν zx 0.25TGP-H series[7] Table 1. Toray ® carbon paper TGP-H series mechanical properties. Figure 2. Nonlinear mechanical behavior in the through-plane direction. Figure 3. Left: Porosity field ϕ (x,y) and GDL intrusion obtained for different imposed rib displacements (expressed both in μm and as a percentage of the initial GDL thickness) and for an initial porosity of 0.8. Right: Effective diffusivity estimated from Bruggemann correction and effective in-plane and through-plane diffusivities predicted by the empirical correlations given in Eqs. (6,7) for an imposed displacement of 60 μm. Effective diffusivity (TGP-H-060) [8,9] (6,7) Due to the in-plane arrangement of the fibers in conventional carbon paper (and cloth) the pores are preferentially oriented in this direction, which results in lower tortuosity and thus higher effective diffudivity of species. It is worth noting that the isotropic spherical agglomerate model of Bruggemann oversimplifies the complex geometry of real GDLs, resulting in an overestimation of the effective diffusivity. Bulk electrical conductivity (TGP-H-060) [10] (8,9) Electrical contact resistances (TGP-H/Graphite) [4,11,12,13] [S/m] [mΩcm 2 ] (p c in MPa) The electrical contact resistance at the membrane/GDL interface is strongly influenced by the presence of a microporous layer (MPL). If a MPL is coated to the GDL the contact resistance can be similar to that observed at the rib/GDL interface [5], but if a MPL is not coated the values can be one order of magnitude higher [14]. (10) RESULTS AND DISCUSSION (5) 2D model (1,2,3,4) [1] Vera, J. Power Sources 171 (2007) 763-777. [2] García-Salaberri et al., Int. J. Hydrogen Energy (2011), In press. [3] Escribano et al., J. Power Sources 156 (2006). [4] Mathias et al., in Handbook of Fuel Cells Fundamentals, Technology and Applications, Vol 3: Fuel Cell Technology and Applications, Chapter 46, 2003 John Wiley & Sons Ltd. [5] Kleemann et al., J. Power Sources 190 (2009) 92-102. [6] Matsuura et al., J. Power Sources 161 (2006) 74-78. [7] Serincan et al., J. Power Sources 196 (2011). [8] Flückiger, Doctoral dissertation, ETH Zurich (Switzerland), 2009. [9] Möst et al., J. Power Sources 191 (2009). [10] Reum, Doctoral Dissertation, ETH Zurich (Switzerland), 2008. [11] Lai et al., J. Power Sources 182 (2008). [12] Zhang et al., J. Power Sources 162 (2006). [13] Zhou et al., J. Power Sources 163 (2007). [14] Nitta, Doctoral dissertation, Helsinki (Finland), 2008. A multiphysics across-the-channel model is presented for the anode of a liquid-feed Direct Methanol Fuel Cell. Multiphase flow effects and experimentally determined membrane/GDL contact resistances should be included to improve the predictive capabilities of the model. Figure 5. Polarization curve for different compression ratios of the GDL (expressed as a percentage of the initial thickness). t GDL =190 μm, w rib =0.5 mm and C m = 0.3 M. Figure 7. Power density curve for different up: GDL thickness down: channel/rib width ratios. The membrane/GDL contact resistance is ten times that at the rib/GDL interface. C m = 0.3 M. Figure 6. Local current density profile for different compression ratios at different cell voltages. The membrane/GDL contact resistance is ten times that at the rib/GDL interface. t GDL =190 μm, w rib =0.5 mm and C m = 0.3 M. CONCLUSIONS & FUTURE WORK Figure 4. Contact pressure distributions for different imposed rib displacements (expressed both in μm and as a percentage of the initial GDL thickness). GDL Constitutive equation (Plane strain)