垂直地下埋管外漏污染物在周期性冷热边界影响下的迁移机理

With the popularity and large-scale application of borehole and downhole heat exchangers, potential groundwater pollution risk caused by the exposed surface and leaked contaminants of underground heat exchangers is increasing. The project focuses on the migration mechanism of contaminants around the vertical heat exchangers in saturated porous media under periodic temperature boundary conditions through systematically sand tank experiments and numerical simulations. Coupled relations of fluid pressure, velocity, temperature and concentration of contaminants will be developed under various conditions including whether there is leakage in the vertical pipe, groundwater flow and temperature fluctuations on the pipe surface. On the basis of the dimensionless variables mathematical equations and physical model of above-described problems, effects of the geometric configuration of heat transfer pipe and porous medium, thermophysical properties of fluids and contaminants, as well as forces of gravity and buoyancy, fluid viscous dispersion and contaminants diffusion on its migration mechanism will be studied, through laboratory scale experiments and numerical simulations using Lattice Boltzmann Method (LBM). The obtained results are valuable for the pollution prevention and remediation of underground environment.

随着浅层地埋管换热器、井下换热器的普及和规模化应用，垂直地下埋管表面污染物、管内或井内工质随机泄露等对地下水系的潜在污染风险增加。本课题拟通过系统的砂槽实验和数值模拟研究，开展这种有周期性冷热温度边界条件影响下，地下埋管管外污染物在其周围饱和多孔介质内的迁移机理研究，探讨垂直地下埋管在有无泄露、地下水静止或流动、埋管表面温度波动等多因素影响情况下，多孔介质内流体压力、速度、温度和污染物浓度的多场耦合规律。在建立描述该问题物理模型和无因次变量数学方程的基础上，通过实验室尺度下的模拟实验和格子—Boltzmann（LBM）数值模拟方法，揭示包括换热管及多孔介质几何构造、流体与污染物热物性、重力、浮力、流体粘性耗散、以及污染物弥散等因素对污染物动态迁移的影响规律。研究结果对预防地埋管换热器应用带来的地下水污染和地下环境的有效治理及修复提供理论依据。

针对地下埋管规模化应用可能带来的地下环境污染风险，开展了地埋管运行温度工况下，埋管外泄漏污染物在饱和多孔介质内的迁移机理研究。通过实验室尺度的模拟实验和多松弛格子-玻尔兹曼（MRT-LBM）方法，重点研究了地层渗流速度，地埋管持续放热工况、室温工况、持续吸热工况和 “放热-恢复-吸热-恢复”的周期性温度工况下，泄漏污染物在多孔介质内的运移规律，分析了埋管单管和管群方式下，温度和浓度对污染物动态迁移的影响规律。结果表明：（1）与静止状态相比，当地层内有渗流时，泄漏污染物径向扩散能力增强，扩散距离增大，且在泄漏点下方产生脱壁效应；（2）地埋管放热工况下，污染物径向扩散加快，扩散范围增大，且在温差的作用下，产生了向上的自然对流，带动污染物向泄漏点上方扩散；（3）地埋管吸热工况下，污染物径向扩散速度减小，扩散范围收缩。（3）地埋管周期性温度工况下，随着 “放热-恢复-吸热-恢复”过程的进行，泄漏污染物的运移范围呈现出“扩张-收缩-扩张”的趋势。（4）渗流速度越大，弥散效应越强，污染物径向扩散越快，扩散范围越大。（5）提出了多孔介质内对流扩散问题LBM求解的统一格式。在此基础上，实现了轴对称单井及矩形空间群井几何结构条件下MRT-LBM方程的编程计算。单井外围浓度变化的模拟结果与实验结果一致。结果对实际地下埋管泄露污染物的跟踪与治理有重要的理论指导意义。