The Research Of Fluid Migration In Soil Using The Pore-scale Modelling Method

The transport of water, salt and gas in soil is directly affect agricultural production and ecological environment, animal and plant and microbial life activities, and so there is a significant importance to study the fluid migration in soil. Due to technical limitations, it was improbable to get the geometric features of soil, so the study of fluid movement in the soil was usually based on experiments and to extract some empirical or semi-empirical models. However whether these models or the parameters of these models are the average of all the fluid, not reflect the heterogeneous of soil structure, much less describe the details of fluid motions. So it is very necessary to study the fluid migration from the micro pore scales and to reveal the macro level phenomena from the microscopic mechanism. Soil contains lots of pores with different diameters. These pores are the main channel for migration of water, salt and gas. Studying the fluid migration in soils from the pore scale can improve our understanding of the transport patterns, and it is of a very important guiding significance in practice on improving the soil ecological environment, advancing agriculture and forestry production, preventing and controlling pollutant dispersion.In this paper, we directly simulated fluid movement process in the aggregated soil using tomography and pore scale modelling method, and to get the distribution and motion state of the fluids in soil and to analyze the characteristics. We calculated the mass transfer rate between the mobile and immobile water by simulating the process of solute diffuse from the inter-aggregates pores into the intra-aggregates pores under different saturation conditions. Based on the relationship between mass transfer rate and the memory function, we used the memory function to describe the mass transfer rate and proposed a memory function model which can be applied to reactive solutes. Simulating the gas diffusion process in the soils of different average pore diameters, we directly calculated the gas effective diffusion coefficient, and a new method to obtain the gas effective diffusion coefficient has been proposed.The main research results are as follows,1. The pore-scale modelling method to simulate the mass transfer rate between the waters of inter-aggregate pores and intra-aggregate pores has been put forward and verified. The process of solute diffusing from the water between the aggregates into the aggregates under different saturations conditions has been simulated using this method, and the effect of saturation on the mass transfer rate has been discussed.2. Using the memory function to describe the mass transfer rate, we proposed a memory function model which can be applied to reactive solutes, and proved that the contact area between aggregates surface and water play the essential role on the effect of saturation on the mass transfer rate.3. A method which can be used to directly simulate the process of gas diffusion in soil on the pore scale has been proposed. Using this method, we calculated the gas effective diffusion coefficient in soils with different average pore diameters under three conditions respectively,(1) just Knudsen diffusion,(2) just continue diffusion,(3) both Knudsen diffusion and continue diffusion, and discussed the impact of the average pore diameter on the effective diffusion coefficient and the relationships between the effective diffusion coefficients under different conditions.4. We proposed a general dusty gas model which can be applied to complex porous mediums, and obtained the formula of effective Knudsen diffusion coefficient with average pore diameter. Based on this, we presented a new method to gain the gas effective diffusion coefficient in porous medium precisely in a very simple way.The memory function obtained in this paper can be applied to simulate the solute mass transfer rate in actual unsaturated soil, and the general dusty gas model can be used to calculate the gas effective diffusion coefficient in any porous medium. Moreover, the research in this paper reveals the patterns and the influence factors of the solute mass transfer and gas diffusion process in soil, and lays a foundation for describe and predict fluid migration in soil better.