Change in macroscopic concentration at the interface between different materials: Continuous or discontinuous
Change in macroscopic concentration at the interface between different materials: Continuous or discontinuous
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DOI：
10.1029/2009wr008853
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发表时间：
2010-10
影响因子：
5.4
通讯作者：
Xiaoxian Zhang;Xue-bin Qi;D. Qiao
中科院分区：
文献类型：
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作者：
Xiaoxian Zhang;Xue-bin Qi;D. Qiao
有人推测，空间平均可能导致在不同材料之间的尖锐界面上不再连续的体积-平均浓度。然而，由于在多孔介质中测量溶质浓度的困难，没有令人信服的实验证据表明这种不连续的存在。在本文中，我们使用孔隙尺度模拟来探索当溶质从一种材料移动到另一种材料时宏观浓度的变化。假设水在空隙中的流动为层流，溶质输运包括分子扩散和平流；用晶格玻尔兹曼方程方法对两者进行了模拟。为了准确表征流固界面，采用多重弛豫时间晶格玻尔兹曼方程方法模拟流体流动。我们首先模拟了溶质在3D柱中的运移，其中一半填充了细玻璃珠，另一半填充了粗玻璃珠。然后将模拟的溶质浓度和孔隙尺度上的溶质通量进行空间平均，分别得到体积-平均和通量-平均浓度曲线，试图了解溶质在从一种介质进入另一种介质时是否在介质界面积聚。结果表明：当溶质从粗质介质向细质介质迁移时，溶质确实在介质界面处积累；我们还发现在储层-柱界面处有质量聚集。这样的堆积使得溶质从粗质介质向细质介质的流动比从细质介质向粗质介质的流动需要更多的时间来突破柱体。我们还模拟了溶质在一个理想的二维柱中的运动，柱中填充了不同的矩形固体和高孔隙率；结果表明，虽然两种介质的弥散特性存在较大差异，但在介质界面处不存在质量聚集现象，且宏观浓度是连续的。这些模拟结果表明，中等孔隙率的材料性质的急剧变化可能导致质量积累，但仅知道两种材料的输运性质不足以确定是否会发生质量积累。导致质量积累的似乎是界面附近的一些微观结构，这不能用两种介质的宏观输运参数来解释。
There have been conjectures that a spatial average could result in a volume‐average concentration which is no longer continuous at sharp interfaces between different materials. However, convincing experimental evidence showing the existence of such a discontinuity is not available because of the difficulty associated with measuring solute concentration in the void space within a porous medium. In this paper we used pore‐scale simulations to explore the change in macroscopic concentration when solute moves from one material into another. Water flow through the void space was assumed to be laminar, and solute transport consisted of molecular diffusion and advection; both were simulated using the lattice Boltzmann equation methods. To accurately represent the fluid‐solid interface, the multiple‐relaxation‐time lattice Boltzmann equation method was used to simulate fluid flow. We first simulated solute transport in a 3D column with one half packed with fine glass beads and the other half with coarse glass beads. The simulated solute concentration and solute flux at pore scale were then spatially averaged to produce volume‐average and flux‐average concentration profiles, respectively, in attempts to understand if solute accumulates at the media interface when moving from one medium into another. The results revealed that, when solute migrated from the coarse medium into the fine medium, it did accumulate at the media interface; we also found mass accumulation at the reservoir‐column interface. Such accumulations made solute take more time to break through the column when flowing from the coarse medium to the fine medium than from the fine medium to the coarse medium. We also simulated solute movement in an idealized 2D column packed with different rectangular solids and with high porosity; the results indicated that, although the dispersive properties of the two media differed considerably, there was no mass accumulation and the macroscopic concentration was found to be continuous at the media interface. These simulated results suggest that a sharp change in material properties with moderate porosity will likely lead to a mass accumulation, but knowing the transport properties of the two materials alone is not sufficient to determine if a mass accumulation could develop. What causes mass accumulations appears to be some microstructures in the vicinity of the interface, which cannot be accounted for by the macroscopic transport parameters of each of the two media.