Department of Mechanical Engineering, Tokyo Institute of Technology, 2-12-1-I6-33 Ookayama, Meguro-ku, Tokyo 152-8550, Japan.
Phys Rev E. 2019 Dec;100(6-1):063110. doi: 10.1103/PhysRevE.100.063110.
With the aid of nondestructive microfocus x-ray computed tomography (CT), we performed three-dimensional (3D) tracer dispersion experiments on randomly unconsolidated packed beds. Plumes of nonreactive sodium iodide solution were point injected into a sodium chloride solvent as a tracer for the evaluation of the dispersion process. The asymptotic dispersion coefficient was obtainable within the experimental scale and was summarized over Péclet numbers from 11.7 to ∼860. Then, the lattice Boltzmann method and moment propagation method were used to elucidate the mechanisms embedded in the dispersion phenomenon. The methods were rigorously verified against the classical Taylor dispersion problem and extended to simulate fluid flow and tracer dispersion in high-resolution 3D digital porous structures from CT. The method of moments, Lagrangian velocity correction function, and dilution index were thoroughly analyzed to evaluate the dispersion behaviors. Numerical simulations revealed ballistic and superdiffusive regimes at the transient times, whereas asymptotic dispersion behaviors appear at longer characteristic times. Besides, the observed transient times unanimously persist over convective length scales of around 12 particles transversely and 16 particles longitudinally. The estimated dispersion coefficients from simulation are in consistence with the experimental result. Furthermore, the simulation also enabled the identification of regimes, including diffusive, power law, and mechanical dispersion. Thus, the proposed experimental and computational schemes are of practical means to study dispersion behaviors by direct pore scale imaging and modeling.
借助无损微焦点 X 射线计算机断层扫描 (CT)，我们对随机未固结的填充床进行了三维 (3D) 示踪剂弥散实验。将非反应性碘化钠溶液的液柱作为示踪剂点注射到氯化钠溶剂中，以评估弥散过程。可在实验尺度内获得渐近弥散系数，并在 11.7 到约 860 的 Peclet 数范围内对其进行总结。然后，格子玻尔兹曼方法和矩传播方法被用来阐明弥散现象中所蕴含的机制。这些方法经过严格验证，可与经典的泰勒弥散问题相对比，并扩展到使用 CT 从高分辨率 3D 数字多孔结构中模拟流体流动和示踪剂弥散。对矩方法、拉格朗日速度修正函数和稀释指数进行了深入分析，以评估弥散行为。数值模拟揭示了在瞬态时存在弹道和超扩散阶段，而在更长的特征时间出现渐近弥散行为。此外，观察到的瞬态时间一致地持续在约 12 个横向颗粒和 16 个纵向颗粒的对流长度尺度上。从模拟中得出的弥散系数与实验结果一致。此外，模拟还能够识别包括扩散、幂律和机械弥散在内的不同阶段。因此，所提出的实验和计算方案通过直接孔隙尺度成像和建模来研究弥散行为是一种实用的手段。
