Insights on the interaction of serpentine channels and gas diffusion layer in an operating polymer electrolyte fuel cell: Numerical modeling across scales
Meysam KhatoonabadiMohammad Amin SafiNikolaos I. PrasianakisJörg RothJohn MantzarasН. КировFélix N. Büchi
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transport in gas diffusion layers (GDLs) Polymer electrolyte fuel cells (PEFCs) Channel cross-flows in PEFCs Impact of dry and wet GDLs on cross-flows X-ray tomographic microscopy a b s t r a c t Three-dimensional direct numerical simulations were performed for investigating the flow in a serpentine channel and the under-laying porous gas diffusion layer (GDL) of a micro polymer electrolyte fuel cell (PEFC).The flow field comprised three straight sections and two U-turns.The geometry was acquired with high-resolution (2.9 μm) in situ X-ray tomographic measurements on an operating cell.Simulations considered the GDL under dry and partially saturated conditions, whereby saturation was established via electrochemically produced water.A lattice Boltzmann (LB) methodology was adopted for simulating the single-phase (gas) transport in the actual 3D channel and porous GDL geometry.The global pressure drop in the dry GDL was dominated by the turns in the gas channel, while the pressure drops were quite small along the straight channel sections.In the wet GDL case, however, the pressure drop was mainly dictated by the neck-shaped passages created by the large water clusters inside the channel.Owing to the water blockage, the local accumulated cross-flows along the serpentine channel length, when normalized by the inlet channel flow, were substantially higher in the wet GDL, reaching local values up to 45% compared to 18% for the dry GDL.The implications are that in an electrochemically operating cell, the GDL under the rib would receive more gas (and thus O 2 ).The creation of cross-flows through the porous GDL would enhance cell performance under the ribs since diffusion will not be the main driving mechanism for oxygen transport and water evaporation.The analysis indicated that the flow field, although designed as serpentine, behaved like half-interdigitated (with a rib of 1.5 mm, half-serpentine flow field, depending on the state of channel flooding).Keywords:
Gaseous diffusion
Diffusion is the primary migration form of coal seam gas. This paper establishes a conceptual diffusion model to describe microdiffusion based on the diffusion form and the pore scale. First, the researchers obtained the pore structure characteristics of 10 coal samples through low-pressure carbon dioxide and nitrogen adsorption experiments. Through quantitative calculations and literature collection, it determines the main diffusion space, that is, a network of pores ranging from 1.5 to 100 nm. Second, the main diffusion space is further divided into three parts: pores ranging from 1.5 to 50 nm, from 50 to 70 nm, and from 70 to 100 nm, where Knudsen diffusion, transition diffusion, and Fick diffusion, respectively, control the gas transport. The three parts are connected in series with parallel pores inside each part. The average pore diameters of the parallel pores in the three parts are calculated as 26, 60, and 85 nm, and the equivalent total pore volumes are 6.52 × 10–4 to 1.71 × 10–2 cm3/g, 1.72 × 10–4 to 1.77 × 10–3 cm3/g, and 1.49 × 10–4 to 1.69 × 10–3 cm3/g, respectively. It shows that the part where Knudsen diffusion occurs has more and smaller pores than the other two parts. Finally, the relation between the cumulative methane diffusion volume in 30 s, and the pore volume in the diffusion zone of 10 coal samples is analyzed using a linear regression method. The results showed that the pore volume in the Knudsen zone has the highest influence on the diffusion flow, with a correlation of 0.914–0.925. This research provides a theoretical basis for gas migration under in situ conditions.
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Gas diffusion characteristics inside shell-type sorbents were investigated in some aspects,such as microstructural feature,gas diffusion mechanism and diffusion coefficient,which were compared with that of the limestone.Scanning electron microscope(SEM)photos indicate that the shells' microstructure is more beneficial to SO2 and other reaction gases diffusing inside them than the limestone.The research results on gas diffusion mechanism show that gas diffusion mode of the raw shells is transition diffusion,those of the calcined and sulfated shells are both molecular diffusion mainly,and those of the raw,calcined and sulfated limestones are all Knudsen diffusion.The effective diffusion coefficients of the shells are all in the same order of magnitude approximately,which are bigger 2 or 3 orders of magnitude than those of the limestone respectively.In conclusion,gas diffusion characteristics of the shell are similar,and they are better than that of the limestone.The conclusion derived from a thermogravimetric experiment is that the relation between desulfurization performance and gas diffusion characteristic is very close.
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Knudsen diffusion
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This paper has evaluated the diffusion amount of the sourcerock of K_1q_n and K_1n_(1+2) in Binbei region, established a method for estimating gas diffusion amount through mathematical model of gas diffusion loss, and the method for establishing the diffusion losing amount in the condition of multi-gas sourcerock. Using the method this paper estimates the gas diffusion loss amount of gas sourcerock of K_1qn and K_1n_(1+2) in Binbei region. The result indicates that the total gas diffusion loss amount of K_1qn in Binbei region is about 5.3×10~(11) m~3, which is 31.2% of the total generation gas amount. The total gas diffusion loss amount of K_1n_(1+2) in Binbei region is about 2.5×10~(11) m~3, which is 32.5% of total generation gas amount. It indicates that the diffusion is the important factor of gas loss in Binbei region.
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Concentration distribution expression of gas diffusion, reaction process at limited distance in small diameter pipe containing solids, the relation of gas concentration,diffusion time and diffusion distance have been derived by the second Fick's low.In the certain diffusion time the relation of gas concentration and diffusion depth is parabolic.The ratio of the square of certain concentration gas diffusion depth and diffusion time is constant when it diffuses and reacts through the solids bed.The theoretical analysis is in accordance with experimental results.
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In this work, an in-house made Loschmidt diffusion cell is used to measure the effective O2–N2 diffusion coefficients through four porous samples of different simple pore structures. One-dimensional through-plane mass diffusion theory is applied to process the experimental data. It is found that both bulk diffusion coefficient and the effective gas diffusion coefficients of the samples can then be precisely determined, and the measured bulk one is in good agreement with the literature value. Numerical computation of three-dimensional mass diffusion through the samples is performed to calculate the effective gas diffusion coefficients. The comparison between the measured and calculated coefficient values shows that if the gas diffusion through a sample is dominated by one-dimensional diffusion, which is determined by the pore structure of the sample, these two values are consistent, and the sample can be used as a standard sample to test a gas diffusion measurement system.
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