Abstract

The study of flow dynamics in porous media or randomly packed beds is essential because these systems are commonly applied in a wide array of engineering applications, such as thermal energy storage, catalytic reactors for processing and distillation, oil recovery, fuel cells, and pebble bed nuclear reactor cores. The flow mixing and turbulence characteristics in pore-scale regions of an experimental facility composed of randomly packed spheres with an aspect ratio of 4.4 were experimentally investigated in this study. Velocity measurements at several pores in the test facility were conducted by combining the matched-index-of-refraction and time-resolved particle image velocimetry techniques, and these measurements were performed at Reynolds numbers of 490, 1023, and 1555. The results obtained from the velocity measurements in the pore regions revealed different and complex flow patterns depending on the pore geometries. In pore region 1, it was found that a strong axial flow entered the pore and impinged into the sphere’s surface when it encountered another flow entering laterally. These dynamic interactions created a high level of turbulent mixing, a recirculation flow region, and a shear layer, as observed in the computed statistical results for \(u^{\prime }_\mathrm{{rms}}\), \(v^{\prime }_\mathrm{{rms}}\), and \(u^{\prime }v^{\prime }\). In other pore regions, the flow field results exhibited complex interactions among jet flows discharging into the pores from various flow gaps created by neighboring spheres. These mutual interactions between the entering jet flows were found to create highly turbulent mixing regions at the pore’ centers and low-velocity or stagnation-flow regions in the vicinity of the sphere surfaces. Flapping shear layers were also observed when two jet flows with unequal strengths entered the pores. The characteristics of turbulent flow mixing in different pore regions were investigated via the spatiotemporal two-point correlation of fluctuating velocities along the velocity streamlines. This approach mitigated the constraints of the random geometries of pore regions on the extent of spatial separation lengths in the two-point velocity correlations. Using this approach, spatial distributions of integral length scales and convection velocities were estimated along the streamlines within the random geometries of pore regions. It was found that the estimated values of the integral length scales and convection velocities were spatially dependent on the localized flow characteristics within the pores. The integral length scales were found to be less than \(0.4D_\mathrm{{sp}}\), experimentally confirming the pore-scale prevalence hypothesis that turbulent structures in porous media are restricted by the pore sizes. In addition, the ratio of the convection velocity to the interstitial velocity (or local velocity magnitude) decreased as the Reynolds number increased.

GraphicAbstract

中文翻译：

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多孔介质孔隙尺度区域湍流特性的实验研究

摘要

研究多孔介质或无规则填充床中的流动动力学至关重要，因为这些系统通常广泛应用于各种工程应用中，例如热能存储，用于加工和蒸馏的催化反应器，采油，燃料电池和卵石床核反应堆堆芯。本研究通过实验研究了纵横比为4.4的由随机堆积的球体组成的实验装置的孔尺度区域内的流动混合和湍流特性。通过结合匹配折射率和时间分辨粒子图像测速技术在测试设备中的几个孔处进行速度测量，这些测量是在490、1023和1555的雷诺数下进行的。从孔隙区域的速度测量获得的结果显示出取决于孔隙几何形状的不同且复杂的流动模式。在孔隙区域1中，发现有强大的轴向流进入孔隙，并在遇到另一条横向流入的流体时撞击到球体的表面。这些动态相互作用产生了高水平的湍流混合，再循环流动区域和剪切层，如计算得出的\（u ^ {\ prime} _ \ mathrm {{rms}} \），\（v ^ {\ prime} _ \ mathrm {{rms}} \\）和\（u ^ {\ prime} v ^ { \主要 }\）。在其他孔隙区域中，流场结果显示出从相邻球体形成的各种流隙排放到孔隙中的射流之间的复杂相互作用。发现进入的射流之间的这些相互作用会在孔的中心产生高湍流的混合区域，在球体表面附近产生低速或停滞的流动区域。当两个强度不等的射流进入孔隙时，还观察到了拍打剪切层。通过沿速度流线的脉动速度的时空两点相关性，研究了在不同孔隙区域的湍流混合特性。这种方法减轻了两点速度相关中孔隙区域随机几何形状对空间间隔长度的限制。使用这种方法，沿孔隙区域的随机几何形状内的流线估计了整体长度尺度和对流速度的空间分布。发现积分长度尺度和对流速度的估计值在空间上取决于孔内的局部流动特性。发现积分长度标尺小于\（0.4D_ \ mathrm {{sp}} \），通过实验证实了孔隙率普遍性假设，即多孔介质中的湍流结构受孔径限制。此外，对流速度与间隙速度之比（或局部速度大小）随雷诺数的增加而减小。

图形摘要