We present pore-scale simulations of two-phase flows in a reconstructed fibrous porous layer. The three-dimensional microstructure of the material, a fuel cell gas diffusion layer, is acquired via X-ray computed tomography and used as input for lattice Boltzmann simulations. We perform a quantitative analysis of the multiphase pore-scale dynamics, and we identify the dominant fluid structures governing mass transport. The results show the existence of three different regimes of transport: a fast inertial dynamics at short times, characterised by a compact uniform front, a viscous-capillary regime at intermediate times, where liquid is transported along a gradually increasing number of preferential flow paths of the size of one–two pores, and a third regime at longer times, where liquid, after having reached the outlet, is exclusively flowing along such flow paths and the two-phase fluid structures are stabilised. We observe that the fibrous layer presents significant variations in its microscopic morphology, which have an important effect on the pore invasion dynamics, and counteract the stabilising viscous force. Liquid transport is indeed affected by the presence of microstructure-induced capillary pressures acting adversely to the flow, leading to capillary fingering transport mechanism and unstable front displacement, even in the absence of hydrophobic treatments of the porous material. We propose a macroscopic model based on an effective contact angle that mimics the effects of the such a dynamic capillary pressure. Finally, we underline the significance of the results for the optimal design of face masks in an effort to mitigate the current COVID-19 pandemic.

中文翻译：

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纤维多孔层中的孔隙尺度传输和两相流体结构：在燃料电池及其他领域的应用

我们提出了重建纤维多孔层中两相流的孔隙尺度模拟。该材料的三维微观结构（燃料电池气体扩散层）通过 X 射线计算机断层扫描获得，并用作晶格玻尔兹曼模拟的输入。我们对多相孔隙尺度动力学进行了定量分析，并确定了控制质量传输的主要流体结构。结果表明存在三种不同的传输方式：短时间内的快速惯性动力学，其特征是紧凑的均匀前沿，中间时间的粘性毛细管状态，液体沿着逐渐增加的优先流动路径传输一到两个孔的大小，以及较长时间的第三种状态，液体到达出口后，仅沿着这样的流动路径流动，并且两相流体结构稳定。我们观察到纤维层在其微观形态上表现出显着的变化，这对孔隙侵入动力学具有重要影响，并抵消了稳定粘性力。即使在没有对多孔材料进行疏水处理的情况下，液体传输确实会受到对流动产生不利影响的微结构诱导的毛细管压力的存在，从而导致毛细管指进传输机制和不稳定的前沿位移。我们提出了一个基于有效接触角的宏观模型，该模型模拟了这种动态毛细压力的影响。最后，