Experiment And Study Of Bimolecular Reactive Transport And Scale Dependence In Porous Media

Quantitative research of reactive material transport in porous media is one of the key and difficult issues in environmental geology.Its research results have important guiding significance for groundwater pollution prevention,water environment management and protection,and underground engineering construction.In this paper,Brilliant Blue was used as the inert solute,copper sulfate?Cu SO₄?and disodium ethylenediaminetetraacetic acid?EDTA-2Na?as the reactive solutes.Large-scale laboratory solute transport experiments and numerical simulation studies were carried out in the laboratory;Combined with digital imaging and image processing technology,dynamic monitoring and recognition of the migration process.By using advection-diffusion equation?ADE?and the advection-diffusion-reactive equation?IM-ADRE?considering incomplete mixing,the migration process of inert and reactive materials was simulated numerically to explore the scale dependence of solute transport in porous media.The main conclusions are as follows:?1?Image analysis can effectively monitor and identify the concentration change process of chromogenic solute transport in porous media.The linear correlation coefficient R² of gray value and corresponding concentration is greater than 0.989,indicating that image analysis has a high recognition accuracy and can effectively avoid the impact of traditional sampling and sensor detection methods on the migration process.?2?Under different experimental conditions,there is an obvious linear relationship between hydraulic gradient J and velocity v in porous media,which conforms to Darcy's law,indicating that pore water flow is Darcy flow,and the permeability coefficient K increases with the increase of porosity.?3?The ADE model was used to fit the penetration curve of the inert solute bright blue,and the results showed that the overall accuracy was good,which could effectively reflect the concentration change trend during the migration process.At the same time,non-Fickian phenomena such as early arrival and towing were found,which were obvious with the increase of migration velocity,medium particle size and migration scale.?4?The IM-ADRE model was used to fit the penetration curves of the migration of bimolecular reactive substances.Under different experimental conditions,the peak errors of the experiment and the fitting results were less than 3.71%,indicating that the fitting accuracy is very high.The new model improves the problem of"over-prediction"in the traditional ADRE model due to the incomplete reaction between solutes effectively simulates the concentration change of substances during the reaction.The parameters D,m and?₀ of the IM-ADRE model are affected by different experimental conditions.As the migration scale increases,the parameters D,m,and?₀ all increase;as the migration speed increases,the parameters D and?₀ increase,and the m value decreases;as the medium particle size increases,the dispersion coefficient D increases,the parameters?₀and m decrease.Based on the sensitivity analysis,it can be seen that the model is most affected by the parameter m and least affected by the parameter D.?5?The relationship between the Peclet?Pe?number and the ratio of the longitudinal dispersion coefficient to the molecular diffusion coefficient?D_L/D_d?in the porous medium satisfies the law of DL/Dd=a Peⁿ,which is consistent with the relevant results of the hydrodynamic dispersion study of the porous medium.?6?The experiment proves that the dispersion coefficient D in the porous medium has scale dependence in the migration,and it is in accordance with the fractal dimension empirical formula of lg?a lg L with the migration distance.The difference results in a low fractal dimension calculated in the laboratory experiment.The scale dependence of the dispersion coefficient is also affected by the medium particle size and migration velocity.With the increase of the medium particle diameter and flow velocity,the dispersion coefficient keeps increasing.