Closed-flow column experiment results (zip-archive 67 kB)

Transport studies that employ column experiments in closed-flow mode complement classical approaches by providing new characteristic features observed in the breakthrough behavior and an equilibrium between liquid and solid phase. Specific to the closed-flow mode is the recirculation of the column effluent to the inflow via a mixing vessel. Depending on the ratio of volume of the water-filled pore space to the volume of the mixing vessel, a damped oscillating solute concentration emerges in effluent and mixing vessel. Oscillation frequency, extent of damping and amplitude are thereby governed by the transport properties of the porous medium. These characteristics allow for the analysis of transport processes in soils in a similar fashion as known for classical open-flow column experiments. However, the experimental design considers feedbacks of liquid solid interactions by connecting the effluent solution with the inflow. In this way, solute and porous medium can equilibrate with respect to all physicochemical parameters, thereby permitting a convenient consideration of mass balances. With this paper, the features emerging in the breakthrough of column experiments run in closed-flow mode and methods of evaluation are illustrated under experimental boundary conditions forcing the appearance of these oscillations. Additionally, the effect of flow velocity and mixing vessel volume on the breakthrough is investigated. We demonstrate that the water content of the porous medium and the pumping rate can be determined from a conservative tracer breakthrough curve. In this way, external preconditioning of the soil material, e.g., drying, can be avoided. This renders the closed-flow column approach especially interesting for the study of porous media with diverse mineral content and bacterial community that react strongly on changes in the water content. Furthermore, the basis for the modeling of closed-flow experiments is given by the derivation of constitutive equations and numerical implementation, validated by the presented experiments.

采用闭流模式（closed-flow mode）的柱实验（column experiments）输运研究，可作为经典输运研究方法的有益补充，能够获取经典实验范式下难以观测到的穿透行为特征，以及液固两相（liquid and solid phase）间的平衡状态。闭流模式的核心特征为：柱实验流出液经混合容器（mixing vessel）回流至进液端。根据充水孔隙体积（water-filled pore space）与混合容器体积的比值，流出液与混合容器内的溶质浓度（solute concentration）会呈现阻尼振荡现象。振荡频率、阻尼程度与振幅均由多孔介质（porous medium）的输运特性所支配。此类特征使得我们能够采用与经典开流柱实验（open-flow column experiments）相似的思路，分析土壤中的输运过程。不过，该实验设计通过将流出液与进液端相连，纳入了液固相互作用的反馈效应。借此，溶质与多孔介质可在所有物理化学参数（physicochemical parameters）上达到平衡，从而可便捷地开展质量平衡分析。本文阐述了闭流模式柱实验穿透过程中出现的特征与评估方法，并通过设置实验边界条件来促使这类振荡现象显现。此外，本文还研究了流速与混合容器体积对穿透曲线的影响。我们证实，可通过保守性示踪剂（conservative tracer）穿透曲线测定多孔介质的含水率与抽提速率（pumping rate）。如此一来，便可避免对土壤材料进行诸如干燥处理之类的外部预处理（preconditioning）。这一特性使得闭流柱实验方法尤其适用于研究那些对含水率变化响应强烈、具备多样矿物组成（mineral content）与细菌群落（bacterial community）的多孔介质。此外，本文通过推导本构方程（constitutive equations）与数值实现（numerical implementation）方法，构建了闭流实验的建模基础，并通过文中展示的实验对该模型进行了验证。