Phase distributions and hydrodynamic flow field at the pore scale can greatly affect the tracer mass transfer process in a two-phase co-flow system, yet the underlining mechanism is not well understood. In this work, dispersion of non-reactive tracers in the aqueous-phase flow field in porous structures (reconstructed from high-resolution Micro-CT images) is numerically investigated under an immiscible two-phase co-flow condition. The spatial-temporal evolution of tracer fronts is observed over a range of from 0.45 to 0.95, and Péclet numbers from 130 to 650. The topology of the two-phase flow field is evaluated in terms of the fluid-phase morphology, rescaled Eulerian velocity distributions, and the WP fractional flow rate ()-saturation () curve, etc. Meanwhile, solute dispersion and mixing are comprehensively studied through tracer breakthrough curves, residence time distribution, dispersion coefficient, mean concentration gradient, and dilution index. A well-defined Fickian transport regime was achieved for ≥ 0.75, where uniform invading fronts with finer fingers were discovered, strengthening the dispersion process through merging of fingertips and transverse mixing. In contrast, multipeak and strong tails were discovered for lower saturations of = 0.45 and = 0.55, where ramified dispersive finger structures were observed, regulating solute transport into oriented channels while weakening its transport into diffusive barriers of disconnected/isolated clusters. In addition, transverse mixing contributes to 25 %-50 % of the overall mixing, which increases with due to finger merging. The majority of mixing takes place in the well-connected branches, while ganglia and singlet may act as sources and sinks that weakens tracer transport especially for low . The dilution index is 2 or 3 orders larger in the branches than that in the isolated clusters of ganglia or singlet. This research provides insight into the relationship between solute transport properties and two-phase hydrodynamics at the pore scale.

中文翻译：

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两相共流条件下多孔介质中溶质分散行为的孔隙尺度研究

孔隙尺度的相分布和流体动力流场可以极大地影响两相同流系统中的示踪传质过程，但其背后的机制尚不清楚。在这项工作中，在不混溶的两相共流条件下，对多孔结构（从高分辨率显微 CT 图像重建）中水相流场中非反应示踪剂的分散进行了数值研究。示踪剂前沿的时空演化在 0.45 到 0.95 的范围内观察到，佩克莱特数在 130 到 650 的范围内。两相流场的拓扑结构根据流体相形态、重新标度的欧拉速度进行评估同时通过示踪剂突破曲线、停留时间分布、分散系数、平均浓度梯度、稀释指数等综合研究溶质的分散和混合情况。达到≥ 0.75 的明确定义的菲克输运机制，其中发现了具有更细指的均匀入侵前沿，通过指尖的合并和横向混合加强了分散过程。相比之下，在 = 0.45 和 = 0.55 的较低饱和度下发现了多峰和强尾部，其中观察到分枝的分散指状结构，调节溶质向定向通道的运输，同时削弱其向不相连/孤立簇的扩散势垒的运输。此外，横向混合占总混合的25%-50%，并且由于手指合并而增加。大部分混合发生在连接良好的分支中，而神经节和单线态可能充当源和汇，削弱示踪剂运输，特别是对于低浓度的示踪剂运输。分支中的稀释指数比孤立的神经节或单线簇中的稀释指数大 2 或 3 个数量级。这项研究深入了解了孔隙尺度上溶质输运特性与两相流体动力学之间的关系。