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首页> 外文期刊>Transport in Porous Media >Magnetic Resonance Imaging of Hydrodynamic Dispersion in a Saturated Porous Medium
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Magnetic Resonance Imaging of Hydrodynamic Dispersion in a Saturated Porous Medium

机译：饱和多孔介质中水动力分散的磁共振成像

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By using nuclear magnetic resonance imaging (NMRI) we have been able to analyse dispersion at the microscopic scale during steady-state flow through water-saturated glass beads. The flow rate through the porous medium was chosen high enough in order to neglect the influence of molecular diffusion on dispersion. Velocity statistics were measured, by NMRI, within slices of increasing thickness perpendicular to the direction of flow. It took more than two bead diameters before a representative elementary volume (REV) for the mean velocity was reached. This was in a region in the middle of the column that was not influenced by the boundary conditions. There the velocity variance decreased exponentially as a function of the slice thickness, due we consider to the formation of an interconnecting streamline network. The exponential decrease in the velocity variance reflects the transition from a local pattern of stochastic-convective flow to a convective-dispersion regime at the scale of the REV. We found that the point-like preferential influx and efflux boundary condition increased velocity variances and thus enhanced longitudinal hydrodynamic dispersion. Using the transverse correlation length of longitudinal velocity variance, we derived a mean transverse dispersivity that agreed well with Saffman's (1959) model. So we have been able to provide for the first time a direct observation verification of a part of Saffman's (1959) conjectures. By NMRI we observed this value to be independent of the observation scale of the slice thickness.
机译：通过使用核磁共振成像（NMRI），我们已经能够在稳态流过水饱和玻璃珠的过程中以微观尺度分析分散。选择通过多孔介质的流速足够高，以忽略分子扩散对分散的影响。通过NMRI在垂直于流动方向的厚度增加的切片内测量速度统计数据。在达到平均速度的代表性基本体积（REV）之前，花了两个以上的磁珠直径。这是在柱中间不受边界条件影响的区域。由于我们考虑了相互连接的流线网络的形成，速度变化随切片厚度的增加呈指数下降。速度方差的指数下降反映了从随机对流的局部模式向REV规模的对流弥散状态的转变。我们发现，点状优先涌入和外流边界条件增加了速度方差，从而增强了纵向流体动力扩散。使用纵向速度方差的横向相关长度，我们得出了与Saffman（1959）模型非常吻合的平均横向弥散度。因此，我们首次能够对Saffman（1959）猜想的一部分进行直接观察验证。通过NMRI，我们观察到该值与切片厚度的观察范围无关。

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