Factors influencing the dissolution rate of residual DNAPL in unconnected pores based on PIV technology
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摘要:
密度大于水的重非水相液体(dense non-aqueous phase liquids, 简称DNAPL)泄露进入地下环境成为长期的污染源。前人基于土柱、砂箱实验及数值模拟等手段研究了冲洗液流速、助溶剂浓度及介质性质对DNAPL清除效率的影响, 但是孔隙流速如何影响孔隙尺度残余DNAPL的溶解速率还不清楚。在微孔隙模型中注入乙醇冲洗液模拟孔隙中残余PCE的清除过程, 利用微尺度粒子图像测速技术(PIV)获取孔隙通道中的水相流速场分布, 分析不同孔隙结构中的残余PCE溶解清除速率的影响因素。实验结果表明: 影响不连通孔隙残余PCE溶解速率
R 的因素不仅仅是水相流速, 还有孔隙断面通量q 、孔隙开口方向和孔隙通道水相流速方向的夹角α 以及通量梯度I 等; 基于实验数据拟合得出溶解速率与影响因素之间的定量关系为R =3 876.79q (-0.016α+2.28/I )2;q 越大, 不连通孔隙附近的孔隙通道水相更新速率就越快;α 越大(α >90°), 有更多的冲洗液进入不连通孔隙内, 从而增加残余PCE溶解速率;I 越大, 垂直进入不连通孔隙内部的水相通量的分量衰减越快, 界面附近的流速就会越小, 残余PCE的溶解速率减小。基于微孔隙PIV技术定量揭示了孔隙流速及介质孔隙结构等多因素共同影响孔隙中DNAPL的溶解速率, 为深入理解孔隙中残余DNAPL的溶解机理、定量评估实际场地条件下残余DNAPL清除效率提供新的手段。Abstract:Objective The leakage of dense nonaqueous phase liquids (DNAPL) with a density greater than that of water into the underground environment becomes a long-term pollution source. Previous researchers have studied the effects of flushing fluid flow rate, cosolvent concentration, and media properties on DNAPL removal efficiency through methods such as column experiments, sandboxes, and numerical simulations.
Methods However, the effect of pore-scale flow rate on the dissolution rate of residual DNAPL in the pores remains unclear. In this study, ethanol flushing solution was injected into a micropore model to simulate the removal process of residual PCE from pores. Microscale particle image velocimetry (PIV) was used to obtain the distribution of the water phase velocity field in the pore channel, and the factors affecting the dissolution and removal rate of residual PCE in different pore structures were analysed.
Results The experimental results indicate that the dissolution rate (
R ) of residual PCE in unconnected pores is influenced by several factors, including the water phase flow rate, the cross-sectional flux (q ) within the pore, the angle (α ) between the direction of pore opening and the direction of water phase flow, and the flux gradient (I ). Based on fitting the experimental data, the quantitative relationship between dissolution rate and influencing factors is obtained as follows:R =3 876.79q (-0.016a+2.28/I )2. A larger value ofq corresponds to a higher renewal rate of the water phase in pore channels near unconnected pores, leading to an increased dissolution rate of residual PCE. Whenα is larger (α >90°), more flushing fluid enters the unconnected pores, further enhancing the dissolution rate of residual PCE. On the other hand, a larger value ofI leads to a faster attenuation of the vertical component of the water phase flux entering the unconnected pores. This results in lower flow velocities near the interface, causing a decrease in the dissolution rate of residual PCE.Conclusion Based on micropore PIV technology, multiple factors, such as pore flow rate and medium pore structure, have been quantitatively revealed to jointly affect the dissolution rate of DNAPL in pores. This finding offers a novel approach to enhance our the understanding of the dissolution mechanism of residual DNAPL in pores and to quantitatively assess the removal efficiency of residual DNAPL under real-site conditions.
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Key words:
- pore velocity /
- micropore-model /
- dissolution rate /
- unconnected pore
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图 2 微孔隙模型结构图
(灰色为固体颗粒，白色为孔隙通道，蓝色字符表示不连通孔隙的位置)
Figure 2. Structure diagram of the micropore model
图 3 微孔隙模型中捕获有PCE的不连通孔隙附近冲洗液流速分布
Figure 3. Velocity distribution of flushing fluid near unconnected pores with PCE captured in the micropore model
图 4 不连通孔隙周围孔隙断面的选取
Figure 4. Selection of pore cross-section around the unconnected pore
图 5 乙醇溶液冲洗期间微孔隙模型内残余PCE的变化
(红色为残余PCE，灰色为固体颗粒)
Figure 5. Change in residual PCE in the micropore model during the ethanol solution flushing period
图 6 乙醇溶液冲洗期间不同孔隙处残余PCE的变化
红色为残余PCE，灰色为固体颗粒，A1至A7表示不连通孔隙中的PCE，B2、B5表示孔隙通道中的PCE
Figure 6. Change in residual PCE at different pores during ethanol solution flushing period
图 7 不连通孔隙中残余PCE面积随时间变化图
Figure 7. Residual PCE area at unconnected pores changes with time
图 8 孔隙开口方向与水相流速方向(a)和孔隙截面的选取(b)
Figure 8. Pore opening direction and water phase flow velocity direction(a) and selection of pore cross-section(b)
图 9 残余PCE溶解速率与孔隙断面通量的关系(a)和残余PCE溶解速率实验值与拟合值的关系(b)
Figure 9. Relationship between residual PCE dissolution rate and pore section flux (a), and relationship between experimental and fitting values of residual PCE dissolution rate(b)
表 1 微孔隙模型参数
Table 1. Micropore model parameters
参数 数值 长度/mm 20.089 宽度/mm 15.866 厚度/mm 0.931 孔隙度/% 63 
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表 2 不连通孔隙残余PCE溶解速率的影响因素
Table 2. Influencing factors of the dissolution rate of residual PCE in unconnected pores
点位 孔隙断面通量/(10-7m2·s-1) 角度/(°) 通量梯度/(10-4m·s-1) 溶解速率/(μm2·min-1) A1 1.32 130.4 0.58 9 425.9 A2 1.11 105.4 1.48 4 102.5 A3 0.92 133 0.94 5 477.1 A4 1.41 72.7 1.74 2 909.0 A5 1.26 142.1 2.22 5 728.6 A6 1.07 90.7 1.43 3 176.2 A7 0.43 142.1 0.75 2 421.0
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