Department of Earth Sciences, Uppsala University, Villavägen 16, 75236 Uppsala, Sweden.
J Contam Hydrol. 2013 Jun;149:88-99. doi: 10.1016/j.jconhyd.2013.03.006. Epub 2013 Mar 29.
Understanding the dissolution behavior of dense non-aqueous phase liquids (DNAPLs) in rock fractures under different entrapment conditions is important for remediation activities and any related predictive modeling. This study investigates DNAPL dissolution in variable aperture fractures under two important entrapment configurations, namely, entrapped residual blobs from gravity fingering and pooling in a dead-end fracture. We performed a physical dissolution experiment of residual DNAPL blobs in a vertical analog fracture using light transmission techniques. A high-resolution mechanistic (physically-based) numerical model has been developed which is shown to excellently reproduce the experimentally observed DNAPL dissolution. We subsequently applied the model to simulate dissolution of the residual blobs under different water flushing velocities. The simulated relationship between the Sherwood number Sh and Peclet number Pe could be well fitted with a simple power-law function (Sh=1.43Pe⁰·⁴³). To investigate mass transfer from dead-end pools, another type of trapping in rock fractures, entrapment and dissolution of DNAPL in a vertical dead-end fracture was simulated. As the entrapped pool dissolves, the depth of the interface between the DNAPL and the flowing water increases linearly with decreasing DNAPL saturation. The interfacial area remains more or less constant as DNAPL saturation decreases, unlike in the case of residual DNAPL blobs. The decreasing depth of the contact interface changes the flow field and causes decreasing water flow velocity above the top of the DNAPL pool, suggesting the dependence of the mass transfer rate on the depth of the interface, or alternatively, the remaining mass percentage in the fracture. Simulation results show that the resultant Sherwood number Sh is significantly smaller than in the case of residual blobs for any given Peclet number, indicating slower mass transfer. The results also show that the Sh can be well fitted with a power-law function of Pe and remaining mass percentage. The obtained relationships of dimensionless groups concerning the mass transfer characteristics at the level of individual fractures can be further used in predictive modeling of dissolution at a larger (fracture network) scale.
了解不同捕获条件下致密非水相液体（DNAPL）在岩石裂缝中的溶解行为对于修复活动和任何相关的预测建模都很重要。本研究调查了两种重要捕获构型下，即重力指进和死端裂缝中聚积捕获的残余 DNAPL 液滴在变孔径裂缝中的溶解情况。我们使用透光技术在垂直模拟裂缝中进行了残余 DNAPL 液滴的物理溶解实验。开发了一种高分辨率的机制（基于物理的）数值模型，该模型能够很好地再现实验观察到的 DNAPL 溶解。随后，我们应用该模型模拟了不同水冲洗速度下残余液滴的溶解情况。实验观察到的 Sherwood 数 Sh 与 Peclet 数 Pe 的关系可以很好地用简单的幂律函数拟合（Sh=1.43Pe⁰·⁴³）。为了研究死端池（岩石裂缝中的另一种捕获类型）中的传质，模拟了垂直死端裂缝中 DNAPL 的捕获和溶解。随着捕获的液池溶解，DNAPL 与流动水之间的界面深度与 DNAPL 饱和度的降低呈线性关系。与残余 DNAPL 液滴的情况不同，随着 DNAPL 饱和度的降低，界面面积几乎保持不变。接触界面的减小深度改变了流场，导致 DNAPL 池顶部上方的水流速度降低，表明传质速率取决于界面深度或裂缝中剩余的质量百分比。模拟结果表明，对于任何给定的 Peclet 数，残余液滴的 Sh 都显著小于残余液滴，这表明传质较慢。结果还表明，Sh 可以很好地用 Pe 和剩余质量百分比的幂律函数拟合。在单个裂缝层面上获得的关于传质特性的无量纲组的关系可以进一步用于较大（裂缝网络）尺度的溶解预测建模。
