Simulation And Experimental Study On Network Fracture Flow And Solute Non-Fickian Transport

Bedrock fractures are often important storage sites and migration channels of groundwater resources.However,with the increasing of underground engineering activities,such as tunnel excavation,mining and landfill,groundwater has been more and more seriously damaged.Most natural fractures exist in the form of network.Due to the strong heterogeneity and multi-scale of bedrock fractures and the complex chemical reaction of fracture water,it is difficult to quantitatively calculate the migration process of bedrock fracture water.As a result,the migration mechanism of water flow and pollutants in network fracture media has not been fully understood.In order to quantify the migration process of bedrock fissure water and explore its migration mechanism,this paper designs vertical cross fracture and network fracture models.The numerical simulation of water flow and solute transport in cross fracture and network fracture under different flow regimes was carried out.The redistribution of water flow and solute in cross fracture and the non-Darcian and non-Fickian transport mechanism of water flow and solute in network fracture were explored.The classical advection-dispersion equation(ADE model)and random walk model(CTRW-TPL model)were used to simulate solute transport curve fitting analysis was carried out.Finally,the numerical simulation results are compared and verified by indoor experiments.The main conclusions are as follows(1)There is a highly linear relationship between the recirculation zones volume and the distribution ratio of cross fissure discharge.It is found that the development of recirculation zones will seriously hinder the flow of fracture water,resulting in a significantly smaller ratio of fracture flow to total inflow;(2)The mixing behavior of solutes at fracture intersections is closely related to the Pe number.The results show that with the increase of Pe number,the solute mixing ratio of cross fractures tends to 0 from 0.5,and the mixing mode changes from complete mixing mode to streamline mode;(3)There is an obvious negative linear relationship between the recirculation zones volume and the network fracture permeability.With the increase of flow velocity,the recirculation zones in the network fracture develops and expands,and the nonlinear flow is the result of the development and growth of the recirculation zones.With the increase of Re number,the recirculation zones volume increases,which results in the decrease of the main channel and the permeability of the fracture;(4)There is a positive correlation between the nonlinear degree of flow and the non-Fickian degree of solute.In the process of flow regime changing from Darcy to non-Darcian in complex network fractures,solute also gradually changes from Fick transport to non-Fickian transport.It is found that there is a negative correlation between forchheimer coefficient F₀ and TPL model parameter?,and a positive correlation between forchheimer coefficient F₀ and breakthrough curve power-law fitting parameter b;(5)For the experimental conditions of continuous injection,the lagged mass exchange process can be captured by the time evolution of dilution index,which indicates the severity of breakthrough curve tailing.