利用計算流體力學探討孔隙尺度新舊流體的混合

Abstract

了解多孔隙介質中初始殘留濕潤相流體的排退行為，對於污染整治的策略至關重要。當污染物或營養鹽溶解在地表濕潤相流體並進入不飽和層中時，濕潤相流體可能滯留在孔隙介質中，或者很容易地被沖走，最終進入地下水中。原溶質或入侵溶質 (溶解的污染物或營養鹽) 的宿命取決於初始濕潤相流體和入侵濕潤相流體之間的互動，因為兩者相互作用的結果決定了溶質的去向。然而，透過傳統實驗室方法，觀察及量化孔隙介質中初始濕潤相流體和入侵濕潤相流體之間的互動是極具挑戰性的。本研究利用計算流體力學探討孔隙尺度新舊流體的混合，此法可真實地模擬不互溶之二相流的流動，並藉由體積分率精準地計算流體的飽和度。透過模擬多孔隙介質中初始殘留濕潤相流體的排退過程，評估了非濕潤相流體、界面張力、接觸角、注射速率及排水-汲取週期等因素的影響。模擬結果顯示，多孔隙介質中的非濕潤相流體會透過封存初始濕潤相流體或佔據主要流動路徑，進而限制了初始和入侵濕潤相流體的混合。界面張力或接觸角的改變，則會改變多孔隙介質中非濕潤相流體的分佈位置，從而產生不同的排退結果。在非常低的注射速率 (0.01 m/s) 下，初始濕潤相流體在孔隙率為49.73%的孔隙介質中，無法有效地被排退 (排退後的初始濕潤相流體飽和度為47.02%)。此外，連續的排水-汲取週期有助於初始濕潤相流體的排退，在孔隙率為49.73 %的孔隙介質中，初始濕潤相流體的飽和度從第一週期後的6.84-10.41%，下降到第二週期的1.17-1.46%。本研究成功的模擬，意味著藉由此方法，可從孔隙尺度探討雙水世界假說，以及預測自然界或工業製程中其它互不相溶流體間的流動現象。

Understanding the behavior of the displacement of the initial resident wetting fluid in porous media is important for the government to develop remediation strategies. When pollutants or nutrients are dissolved in the land surface wetting fluid and enter the unsaturated zone, the wetting fluid may remain in the porous system or be easily flushed out and finally arrive in the groundwater. The fate of the original or invading solute (dissolved pollutants or nutrients) is related to the interaction between the initial wetting fluid and the invading wetting fluid because the interaction result determines where the dissolved solutes go and stay. However, visualization and quantifying the interplay between the initial wetting fluid and the invading wetting fluid in porous media through traditional laboratory experiments are challenging. In this study, computational fluid dynamics (CFD) is used to study the mixing of old and new fluids at the pore-scale level. It can faithfully simulate the immiscible two-phase flow and precisely quantify the fluid saturations by volume fractions. The effects of the non-wetting fluid, interfacial tension, contact angle, injection rate, and consecutive drainage-imbibition cycles are evaluated by simulations of the initial resident wetting fluid displacements in porous media. The simulations show that the non-wetting fluid in the porous system would hinder the displacements by either trapping the initial resident wetting fluid or occupying the main flow path of the displacement, thus limiting the mixing of the initial resident and invading wetting fluids. A change in interfacial tension or contact angle would affect the distribution of the remaining non-wetting fluid in the porous system, resulting in different displacement efficiency. At a very low injection rate (0.01 m/s), the initial resident wetting fluid could not effectively be displaced (saturation is 47.02% after displacement) in the porous system with a porosity of 49.73%. Besides, the consecutive drainage-imbibition cycles could improve the displacement. In a porous system with a porosity of 49.73%, the saturation of the initial resident wetting fluid decreases from 6.84–10.41% in the first cycle to 1.17–1.46% in the second cycle. This study's successful simulation implies that this method can be applied to investigate the two water worlds hypothesis from the pore-scale study and to predict other immiscible fluid-fluid flows in natural or industrial processes.