| Presently the application of geothermal direct use in China ranks first in the world,and the ground source heat pump system,which is mainly based on buried pipes,accounts for 66% in the geothermal use in China.However,as the heat source for building heating or cooling,the large-scale application of buried pipe has also brought some problems,such as the leakage of antifreeze working fluid in the pipe in severe cold areas,the aging and cracking of the polyethylene pipe materials and the life expiration of the buried pipe,which will cause underground pollution,however,there are not much literature available so far.This paper performed a numerical simulation of the underground leaked contaminants migration under the buried-pipe operating conditions,and the results can theoretically guide the control of such environmental pollution.In this paper,a three-dimensional numerical model of flow and heat transfer in porous media is established at the scale of Representation Element(REV),based on the real temperature conditions of the buried pipe,the D3Q15 model was constructed using the lattice-Boltzmann method with external forces.The migration of leaked contaminants for the single buried pipe and group buried pipes was programmed and calculated,respectively.The temperature and concentration distribution in the formation(Rat/Rac),geological parameters of porous media(porosity and permeability)and fluid physical properties parameters(Le number)on the migration of leaked contaminants were analyzed,the results indicate that:(1)When Rat/Rac<1,the fluid flow is mainly affected by the gravity caused by the concentration difference,which drives the contaminants to migrate downward from the leaked point.When Rat/Rac=1,the gravity caused by the concentration difference is equivalent to the buoyancy force caused by the temperature difference,and their directions are opposite,the contaminants are accumulated at the leak point,the diffusion range is reduced.During the heat reinjection of the buried pipe,the surrounding high-temperature fluid flows upward under the action of buoyancy,and the contaminants diffuse downward under the action of gravity.During the heat extraction stage,the fluid near the leak point flows downward,and the contaminants diffuse downward under the action of gravity.In the restoration stage,the formation temperature gradually recovers to the initial temperature.(2)When the porosity is larger,the contaminants diffuse faster.However,the peak concentration under different porosity are almost the same.When Da=5E-4,the contaminants diffused down faster to the bottom,and the contaminants concentration at the bottom is 1.5 times of the case of Da=5E-5.It can be seen that the effect of permeability on the contaminant migration is greater than that of porosity.(3)The simulation results with Lewis numbers of 0.01,0.1 and 1 show that the contaminant diffusion range at Le=0.01 is twice as large as the case of Le=1.Therefore,the fluid with a large Lewis number diffuse slowly.(4)Under the same operating conditions,the migration range of leaked contaminants under the condition of group buried pipes is smaller than that of single-pipe conditions,and also the difference of concentration and temperature distribution is smaller.This is because the group buried pipes increase the resistance of fluid flow,and the diffusion of contaminants concentration and temperature is slowed down,therefore the group buried pipes can restrain the large-scale diffusion of leaked contaminants. |
