of the domain used in the ... DE denote the extension of the computational domain. ... to the injection edge and vary significantly depending on the domain.
lattice boltzmann simulations of mass transfer properties in partially-saturated pem fuel cell gas diffusion layers P. A. Garc´ıa-Salaberria, J. T. Gostickb, M. Veraa, A. Z. Weberc, G. Hwangc a
Dept. de Ingenier´ıa T´ermica y de Fluidos, Universidad Carlos III de Madrid, Legan´es, Madrid, Spain b Dept. of Chemical Engineering, McGill University, Montreal, Quebec, Canada c Lawrence Berkeley National Laboratory, Berkeley, California, USA
Experimental
Lattice Boltzmann Method
Micron resolution X-ray Computed Tomography (xCT) was used to image GDL samples at different applied capillary pressures during water injection experiments [1]. The images were obtained using the Advanced Light Source (ALS) synchrotron at Lawrence Berkeley National Lab. A delrin holder as shown in Fig. 1 was used with a center bore hole equal to the sample diameter and a narrowed waist section through which images were taken. Water was injected through a hydrophobic pipe and a hydrophobic membrane was inserted above the sample to act as a capillary barrier.
The segmented images are combined with the Lattice Boltzmann Method (LBM) to determine the effective mass transfer properties of GDLs, i.e., diffusivity and permeability, based on the simulated diffusion and convection processes at the pore-scale level. The LB models were implemented using the built-in capabilities of the opensource CFD solver Palabos. To exploit the parallel features inherent to the LBM, simulations were run on McGill’s supercomputing cluster CLUMEQ.
Global vs. Local Computations
Through-plane effective properties are largely influenced by the local blockage of the region adjacent to the injection edge and vary significantly depending on the domain considered for calculation. Correlations based on local computations across the GDL thickness lead to higher effective values as compared with correlations obtained by considering the whole thickness of the GDL. The information extracted from these local measurements will give a more realistic approach for continuum models, thus improving their predictive capabilities. 0.4
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Figure 1: Schematic diagram
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PTFE Membrane Outside = 4.5 mm D l i Holder Delrin H ld
Figure 5: Effective diffusivity, Deff , as a function of void fraction, ε(1 − s); the results obtained by considering the whole thickness of the GDL and different domain extensions, DE, are compared with the results obtained based on local computations across the GDL thickness.
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of the sample holder used for water injection experiments by applying fixed capillary pressures to GDLs while obtaining X-ray radiographs.
Figure 3: (up) Schematic of the domain used in the LB simulations, and, (down) velocity field corresponding to permeability calculations along one in-plane direction.
PTFE Pipe
Image Segmentation
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LB TGP-H-120 10% PTFE (DE = 0.6 mm2 , local TP) LB TGP-H-120 10% PTFE (DE = 0.6 mm2 ) LB TGP-H-120 10% PTFE (DE = 3.8 mm2 )
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Effective Properties Conclusions First, fibers/PTFE are segmented applying a global thresholding (Otsu’s method) and a 3D closing op. to the dry sample stack.
The results obtained by the LBM are in good agreement with other experimental and numerical data previously presented in the literature [3–5]. I Relative Diffusivity 1 0.9
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Then, 3D bilateral filtering [2] is applied to remove noise and smooth the images while preserving the shape of water blobs.
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Ros´en et al. [3] TGP-H-060 20% PTFE LB TGP-H-120 0% PTFE (DE = 0.6 mm2 ) LB TGP-H-120 0% PTFE (DE = 1.9 mm2 ) LB TGP-H-120 10% PTFE (DE = 0.6 mm2 ) LB TGP-H-120 10% PTFE (DE = 3.8 mm2 )
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[4] G. S. Hwang, A. Z. Weber, JES (2012) 159(11): F683–F692.
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[5] I. S. Hussaini, C. Y. Wang, JPS (2010) 195: 3830–3840.
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Figure 4: (up) Gas-phase relative diffusivity, g(s), and, (down) relative permeability, Figure 2: Main steps followed for the segmentation of the 3D reconstructed stacks.
[3] T. Ros´en, J. Eller, J. Kang, N. I. Prasianakis, J. Mantzaras, F. B. B¨uchi, JES (2012) 159(9): F536–F544.
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Finally, global thld. is used to differentiate water from air. The final segmented stack is obtained by combining all the partial results.
[1] J. T. Gostick, H. Gunterman, B. Kienitz, J. Newman, A. MacDowell, A. Weber, ECS Trans. (2010) 33(1): 1407–1412. [2] S. Paris, F. Durand, MIT tech. report (MIT-CSAIL-TR-2006-073) (2006).
Ros´en et al. [3] TGP-H-060 20% PTFE Hussaini et al. [5] TGP-H-120 0% PTFE LB TGP-H-120 0% PTFE (DE = 0.6 mm2 ) LB TGP-H-120 0% PTFE (DE = 1.9 mm2 ) LB TGP-H-120 10% PTFE (DE = 0.6 mm2 ) LB TGP-H-120 10% PTFE (DE = 3.8 mm2 )
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A methodology has been developed to simulate mass transport phenomena at the pore-scale level by combining the LBM and 3D images of partially-saturated GDLs acquired by xCT. Preliminary results of the work-in-progress show good agreement with previous reported data [3–5]. However, two-phase properties in the throughplane direction are very sensitive to the extension of the domain considered for calculation. Effective properties based on local computations across the thickness of the GDL will provide more useful information for future continuum modeling studies.
k rg, as a function of liquid saturation, s; (left) in-plane, and, (right) through-plane direction. DE denote the extension of the computational domain.
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