| Groundwater pollution is worsening in recent years.Because of the good connectivity of the porous media constructed by packing granular solid grains,pollutants can more easily enter it and lead to groundwater pollution.For a long time,the traditional advection dispersion model(traditional ADE model)has been used to simulate the solute transport process in the porous media.The traditional ADE model describes the solute transport process in porous media as the mass center of the solute plume moving uniformly at the mean velocity of fluid along the mainstream direction,and the dispersion behavior of the solute plume around its mass center can be accurately characterized by Fick's law.According to the description of traditional ADE model,the solute diffusion coefficient is constant,and the longitudinal first and second spatial moments of solute plume increase linearly with time.However,with the development of research,more and more experimental and simulation results show that the solute transport process in porous media exhibits many systematic deviations from the prediction of traditional ADE model.For example,compared with the BTC predicted by the traditional ADE model,the actual breakthrough curve BTC often exhibits early breakthrough and long tailing at the later time.In addition,the solute dispersion coefficient often increases with time rather than is a constant as predicted by the traditional ADE model,and the first and second moments nonlinearly increase with time.These phenomena are called scale effect.The solute transport process that exhibits early breakthrough,long tailing at later time,and scale effects is called non-Fick migration or anomalous migration.In many practical problems,it is necessary to accurately predict the early and late breakthrough behavior of solute transport in porous media.In addition,deeply understanding the scale effect contribute to improve the accuracy in predicting solute transport in porous media.Therefore,the research on anomalous solute transport behavior is of great importance in both theory and application.The anomalous solute transport behavior varies signigicantly with the change in the microstructure of porous media.The changes in cementation degree and sorting degree of solid grains and the presence of dead-end pore can significantly affect the geometric characteristics of pore structure.However,there are seldom research es on the changes in anomalous solute transport behavior in porous media with different cementation degrees or sorting degrees.The study on the influences of dead end pore on anomalous solute transport is also still limited.This paper performed pore-scale numerical simulations to investigate the characteristics of anomalous dispersion in porous media with different cementation degrees and sorting degrees.In addition,this paper also probed into the mechanism of the changes in the anomaly degree and time behavior of solute transport.Meanwhile,the probability distribution of waiting time of the solute particles in the dead-end pores and its influence factors were also explored.Based on that,the influences of dead-end pore on solute transport were investigated.The conclusions are made as follows:(1)The simulation results of the solute transport at the pore scale in both two-dimensional and three-dimensional porous media with different cementation degrees indicate that,with the cementation degree increases,both early breakthrough of the solute and long tailing at late times are enhanced simultaneously in both two-dimensional and three-dimensional porous media.The enhancement of the long trailing is more significant.Accordingly,the breakthrough curve BTC increasingly deviates from the BTC predicted by traditional ADE model.The solute transport becomes more anomalous.The simulation results of spatial moments of solute plume and propagator of solute transport in three-dimensional porous media show that the evolution behavior of the spatial distribution of solute plum with time varies significantly when the cementation degree is different.In the porous media with larger cementation degree,the second spatial moment no longer increases linearly with time but in the form of power law.Correspondingly,the dispersion coefficient is not a constant but rather increases with time.The scale effect is enhanced.The immobile peak concentration appears in propagator and becomes increasingly notable and persistent with the increase of cementation degree.(2)This paper has also analyzed the characteristics of flow field in porous media with different cementation degrees.The results show that,when the cementation degree increases,the flow field characteristics in two-dimensional and three-dimensional porous media show the similar change tendency.Specifically,with the cementation degree increases,the flow field heterogeneity is enhanced,the porous media have wider spreads of velocities.At the same time,there are more preferential flow region and the stagnant region in the flow field.The transport connectivity indicator(CT)and propagator indicate that it is the changes of flow field characteristics give rise to the different transport behavior.(3)In the porous media by mingling and packing two kinds of solid grains with different radii,when the volume fractions of larger grain Rb increases from 0.0 to 1.0,there is a maximum and a minimum in the degree of anomalous transport..Moreover,the value of Rb corresponding to the anomalous transport maximum and minimum i.e.Rbmax and Rbmin vary little with the radii of solid grains,the flow field and diffusion coefficient of the solute.(4)The distributions of waiting time in different dead-end pores show similar power-law decline at early time and transit to an exponential decline in the end.The transition time between these two decline modes is proportional to the square of the length of dead-end pore and inversely proportion to diffusion coefficient of solute.It is well known that power-law distributions of waiting time may lead to anomalous(non-Fickian)dispersion.Therefore,anomalous dispersion is highly dependent on the sizes of dead-end pores. |
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