Image-based simulations at pore scale provide direct insight into the impact of the microstructure on flow and transport processes in porous media. Diffusion is an important mechanism of mass transfer in gas or liquid phases, confined in porous media. Similar to fluid flow, the diffusive transport in porous media is a strong function of pore size and structure. Although the effect of porosity, pore connectivity and constrictivity of homogeneous porous media on macroscopic properties is clear, this is not well understood for heterogeneous porous media. This study uses a dual-structural-scale medium to analyze the effect of topological and morphological parameters on effective properties such as permeability and the effective diffusivity. A synthetic porous medium was created by two sizes of small and large glass beads, and 3D pore structure image of the sample was captured by X-ray computed tomography technique. The Stokes and diffusion equations were directly solved on the extracted pore geometry of sample, using a finite element method. The results show a strong nonlinear relationship between constrictivity, as a morphological parameter, with permeability and effective diffusivity. Based on the results obtained from pore-scale imaging and modeling, the connectivity of the pore space is increased by decreasing the Euler number and consequently the permeability and effective diffusivity is increased. Good agreement between image-based computed effective diffusivity with that estimated by van Brakel and Heertjes empirical relation confirms the reliability of this relation for heterogeneous porous medium, which includes constrictivity in addition to porosity and tortuosity, as three important morphological properties. The capability of X-ray CT technique was used to better understand the microstructural properties of porous media. Porosity and constrictivity of a heterogeneous porous medium was correlated to its permeability and effective diffusivity. The reliability of Van Brakel and Heertjes relation for estimating effective diffusivity in porous media was confirmed.

中文翻译：

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形态和拓扑特征对双结构尺度合成多孔介质有效扩散率和渗透率的影响

孔隙尺度的基于图像的模拟可以直接洞察微观结构对多孔介质中流动和传输过程的影响。扩散是气相或液相中传质的重要机制，限制在多孔介质中。与流体流动类似，多孔介质中的扩散传输是孔径和结构的强函数。尽管均质多孔介质的孔隙率、孔隙连通性和收缩性对宏观性质的影响是明确的，但对于非均质多孔介质，这一点尚不清楚。本研究使用双结构尺度介质来分析拓扑和形态参数对渗透率和有效扩散率等有效特性的影响。合成多孔介质由两种尺寸的小玻璃珠和大玻璃珠制成，样品的 3D 孔结构图像由 X 射线计算机断层扫描技术捕获。斯托克斯方程和扩散方程直接在提取的样品孔隙几何结构上求解，使用有限元方法。结果表明，作为形态参数的收缩率与渗透率和有效扩散率之间存在很强的非线性关系。根据孔隙尺度成像和建模的结果，通过减小欧拉数来增加孔隙空间的连通性，从而增加渗透率和有效扩散率。基于图像的计算有效扩散率与由 van Brakel 和 Heertjes 经验关系估计的有效扩散率之间的良好一致性证实了这种关系对于非均质多孔介质的可靠性，除了孔隙度和曲折度之外，它还包括收缩性，作为三个重要的形态特性。X射线CT技术的能力被用来更好地了解多孔介质的微观结构特性。非均质多孔介质的孔隙率和收缩率与其渗透率和有效扩散率相关。Van Brakel 和 Heertjes 关系用于估计多孔介质中的有效扩散率的可靠性得到证实。