Model Of Solute Redistribution During Freezing Process In Saturated Porous Media

Seasonal frozen soils are widely distributed in China,and there are engineering problems such as melting collapse,salt expansion,frost expansion,and environmental problems such as migration of pollutants.Frozen soil is a typical multi-phase system,there is interaction of soil,water and heat.Under the temperature gradient,water will migrate and affect the changes in soil structure,causing salt swelling and forming a pollution migration channel.The self-purification effect caused by the soil freezing process causes the migration of soluble salts in the groundwater,causing problems such as secondary salinization and pollution diffusion.Therefore,it is very necessary to carry out simulation study of solute migration in the freezing process.In this paper,based on the theory of Ginzburg-Landau,using Karma's model as the basis,coupled with the solute transport equation and temperature field equation,a phase transition model at the pore scale and soil column scale is constructed to study the law of solute transport.Acquire the following preliminary understanding:(1)An aqueous solution phase transition model was constructed to simulate the effects of initial solute concentration and solution temperature on ice crystal growth and solid-liquid solute partition coefficient.The simulation results show that the lower the temperature,the faster the growth rate of ice crystals and the more salt is discharged from the ice phase.In addition,an increase in the initial concentration of the solution decreases the migration rate of the frozen front;(2)A two-dimensional pore-scale freezing model for saturated porous media was constructed,and the effects of initial solute concentration and solution temperature on the freezing front and solute distribution in the pores were simulated,and the formation process of solute inclusions in porous media was characterized and the parameters were passed.Sensitivity analysis studies the formation of solute inclusions.The simulation results show that,because the heat transfer effect of porous media particles is better than that of solution,the migration rate of the freezing front in the local area will be greater than the solute peak migration rate,resulting in the formation of solute inclusions.In addition,the initial solution concentration has a greater influence on the inclusion solute concentration,while the temperature has less influence on the inclusion solute concentration;(3)A one-dimensional freezing model of soil column scale saturated porous media is established by means of pore scale rise and coupled with non-thermodynamic equilibrium equation.The effects of surface temperature constant and periodic change of surface temperature on freezing depth,skeleton temperature,solution temperature and solute distribution in pores are simulated and studied.Different porosity,freezing temperature and distribution coefficient are analyzed by parameter sensitivity And the law of change under the initial concentration.The results show that the energy transfer process in porous media has hysteresis and attenuation with the increase of distance.The smaller the porosity and the higher the freezing temperature is,the higher the concentration of solute peak and the more prone to salt swelling is.When the surface temperature changes regularly,the freezing depth of 47 cm is less than 58 cm at constant temperature,but the maximum concentration of solute peak is greater than that at constant temperature.