| With the increasing number of underground projects and the world's attention to the safety(?)exploration,nuclear waste disposal and carbon dioxide geological.storage,the study of fracture water transport in bedrock has become a very important subject in hydrogeology.In nature,the Crasture exists in the form of network system mostly.Due to strong heterogeneity and anisotropy,the mechanism of water transport in network fracture has not been completely understood up to now,among which the quantification of water flow and solute transport behavior and identification of main control factors are more challenging problems.The cross fracture,circular network fracture and granite fracture models were constructed in this paper,they were designed from simple to complex,from manual to half the state of nature by using traditional indoor test methods,the flow regime and dominant flow distribution under different conditions of network fracture,the solute non-Fickian transport characteristics were analyzed,and the behaviors were fitted by ADE,TPL and COMSOL models.The characteristic method of temperature tracing was used to monitor the temperature change,behavior of dominant flow and solute transport were reflected.The influence of aperture,roughness,cross angle,different network channel and flow velocity to water flow and solute transport in network fractured media were studied systematically,the mechanisms of different phenomena were explored,the theoretical basises were provided for pollution controlling and sustainable development and utilization of groundwater in nature real network fracture.The main conclusions were shown as follows:(1)The dominant flow phenomenon in network fracture was verified,its main influence factors were elucidated,and the mechanism of the increase of dominant flow with the decrease of flow resistance was explained.In the cross fracture:It was concluded that the water would choose the channel with less resistance when flowed through the intersection by analysing of flow resistance in the one inlet and three outlets tests,thus the dominant flow was formed.When other conditions remain unchanged,the larger the inlet and outlet fracture roughness difference,inlet angle,aperture and flow velocity,the more chaotic the state of flow,the larger the primary coefficient between the dominant flow rate ?q and the total flow rate Q,and the more obvious the dominant flow phenomenon.In the circular network fracture:The more complex the flow initiation channel,the larger the quadratic term coefficient of Forchheimer formula,the larger its non-Darcy coefficient,and the smaller the critical Re value,the easier-the appearance of non-Darcy flow,which was also the basis for the quantization of the flow.In addition,when the channels in the circular model was halved,the average flow velocity under the same hydraulic gradient condition was larger,the flow was more chaotic,and the non-Darcy coefficient was larger.(2)The behavior of dominant flow in cross fracture was captured by temperature tracing at the experimental scale,and the dominant flow channels in the network fracture were identified.Through the comparison of temperature change and flow distribution in the granite fracture,the temperature tracer had the best effect when the flow velocity was 4.24mm/s with injection volume of 20mL,which was the closest to the real flow distribution,and the minimum relative error value was 0.34%.By monitoring the temperature change of flow in the circular network fracture,temperature tracer could be used to identify the dominant flow channels and provided a basis for calculating the hydraulic gradient.(3)The behavior of non-Fickian solute transport in the network fracture dominant channel was analyzed and its mechanism was revealed.The solute transport breakthrough curves were non-normal distribution,with obvious non-Fickian transport characteristics such as peak early arrival and tailing.The dominant factors of solute transport were judged according to the distribution of Pe number and DI/Dd.The results showed that the experimental data points in the cross fracture and circular network fracture fell in the ? and ? regions of the molecular diffusion and hydrodynamic dispersion diagram.It reflected that mechanical dispersion played a leading role in solute transport,and molecular diffusion could be negligible.(4)The temperature tracer method was used to carry out the solute transport test,which proved that temperature could be used to instead of the traditional tracer for solute transport research under certain conditions.The temperature tracer breakthrough curves also exhibited non-Fickian transport characteristics,it could be obtained from the cross fracture solute transport test when the flow angle of the inlet water was 180 degree at 5.42 mm/s,and the injecting hot water was 10 mL,the breakthrough curves of temperature and brilliant blue were in high goodness of fit,and the relative error at the peak time point was only 2.13%.(5)The models for simulating the solute transport breakthrough curves were established and their simulation effects were compared.For the phenomenon of peak early arrival and tailing of the test values,the fitting effect of TPL was better than the ADE model and the simulation accuracy was higher.The parameter ?2 in the TPL model had a certain regularity,its values were between 1 and 2,and the phenomenon of non-Fickian solute transport increased with the ?2 decreased.(6)The solute concentrations of different locations in the circular network fracture model were simulated by using COMSOL model.By comparing with the measured values of the solute concentration at the outlet end,the applicability of COMSOL could be obtained.Under the conditions of same inlet and outlet,the smaller the flow velocity,the wider the transition surface of the concentration changed,the slower the concentration values decreased.Under the conditions of same the flow velocity but the different inlets and outlets,the concentrations distribution in channels were different,the concentration of solute in the dominant flow channel was higher,which was consistent with the actual situation. |
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