Simulation Of Underground Contaminants Migration Under Periodic Temperature Boundaries Using Lattice Boltzmann Method

The risk of underground environmental pollution caused by the surface contaminants of buried tubes and the leakage of its internal working fluid is increasing with the large-scale application of buried tubes.Previous studies have shown that periodic freezing and melting processes can cause redistribution of contaminants concentration.Similarly,the periodic heat injection and heat extraction of buried tubes can also cause redistribution of contaminants concentration.However,fewer related studies can be found presently.In this paper,the Multiple-relaxation–time lattice Boltzmann model(MRT-LBM)is used to simulate the contaminants migration in porous media under periodic temperature boundary.By regarding the contaminants leakage point as the continuous injection point source and the accumulative flow rate of contaminants as an additional item of source term,the mathematical model of solute migration and the Darcy velocity model are established;the unsteady heat conduction and convection equations are established by regarding the heat injection and heat extraction in summer and winter as a various cylindrical heat source.Coupling the above equations and combined with the D3Q7 model of lattice Boltzmann method(LBM),the relationship between the contaminants concentration front and the periodic temperature boundary is obtained,thus the accumulation position of the contaminants can be determined.Based on the REV(Representative Elementary Volume)scale,this paper simulates the double-diffusion natural convection in the square cavity filled with porous medium under periodic temperature boundary,the results indicate 1)The temperature gradient causes natural convection in the porous media.The heat source strengthens natural convection and the concentration diffusion of contaminants accelerates.On the contrary,the cold source increases the flow resistance,so the contaminants accumulate near the cold source;2)Influenced by the temperature and concentration,the concentration diffusion of contaminants at the top and bottom layers of the square cavity is different.The natural convection at the top layer is enhanced by the buoyancy lift,thus the concentration diffusion is accelerated.Contaminants at the bottom layer of the cavity are affected by the gravity,which leads to the concentration accumulation near the cold source;3)After the heat source stops,the temperature contour has a significant shift near the concentration point source,while the temperature field tends to be uniform after the cold source stops;4)Effect on the contaminants concentration due to the sequence of hot and cold boundary has slight different for the concentration diffusion of the underground contaminants.Compared with the temperature boundary of the heating-heat recovery-cooling-heat recovery periods,the concentration of pollutants under the cooling-heat recovery-heating-heat recovery process is more accumulated near the heat source;5)When there is a positive temperature gradient between the left and right walls,the movement of the fluid molecules will move forward near the point source of the contaminants concentration as the direction of the percolation velocity.When there is a negative temperature gradient on the left and right walls,the fluid molecules are affected by the natural convection caused by the temperature gradient and the driving force caused by the seepage velocity.The distribution of the flow field is more influenced by the cooling boundary.The periodic operating conditions of the buried tubes make the migration trajectory of the contaminants have certain characteristics,which at first gradually expands and then gradually shrinks,so that the diffusion range of the contaminants is concentrated in an annular region.Besides,the shrinkage range of the contaminants concentration in winter is greater than the expansion range in summer,and the contaminants accumulate more in the lower part of the leakage position.Related environmental prevention departments can take professional measures to monitor the location of the contaminants and extract or dissolve them.The simulation results in this paper can provide theoretical basis for the removal of groundwater contaminants and the restoration of underground environment,and bring good social and environmental benefits.