Study On The Law Of Water,Heat And Salt Transport And Multi-Field Coupling Simulation In Saline Soil During Freezing Process

Soil salinization is the main limiting factor of ecological environment and agricultural development in western Jilin Province,which is a typical seasonal frozen soil area in China,and the important cause of soil salinization is water and salt migration caused by freezing.At present,the study on the mechanism of water,heat and salt transport in saline soil during freezing process has become a hot spot and difficult point in academic circles.On the one hand,salt crystallization will occupy soil pores during freezing,which not only changes soil hydraulic characteristics,but also directly affects the water,heat and salt transport fluxes caused by phase change and latent heat release.On the other hand,the presence of salt will also reduce the initial freezing temperature of saline soil,slow down the freezing process of soil,and promote the redistribution of water and salt by changing the soil hydrothermal condition,which makes the water,heat and salt transport mechanism of saline soil more complicated during freezing process.Therefore,it is very important to explore the law of water,heat and salt transport of saline soil and multi-field coupling model under the combined the reduction effect of freezing temperature and salt crystallization,which can not only improve the water-salt transport mechanism of saline soil in western Jilin,enrich the technical methods of water-thermo-salt coupling simulation of saline soil,but also provide a scientific basis for effective control and improvement of soil salinization in seasonal freezing areas.In view of the salt crystallization phenomenon and the reduction effect of freezing temperature of saline soil when the ambient temperature decreases,this study takes saline soil in western Jilin province as the research object,and carries out the research according to the idea of"field investigation and sampling—internal driving mechanism of water and salt migration—representation of hydrothermal parameters—improvement of multi-field coupled migration model—simulation solution".Based on the physical and chemical properties of field soil samples,through laboratory experiments,mathematical statistics and numerical models,the rules and driving forces of water and salt transport in saline soil during freezing process were analyzed,and the interaction mechanism and prediction model of initial freezing temperature of saline soil under the comprehensive action of multiple factors were determined.On this basis,a mathematical model of multi-field coupling of water,heat and salt transport of saline soil was established,which considered the reduction effect of freezing temperature and salt crystallization.Based on laboratory experiments,the validity of the model was verified,and the law of water and salt phase transition,redistribution process and migration mechanism of saline soil were revealed.The main research contents and achievements of this paper are as follows:（1）Transport law and driving mechanism of water and salt in saline soil during freezingThe law and driving force of water and salt transport in saline soil are analyzed comprehensively through the experiment of water and salt transport under non-freezing and freezing condition.During the freezing process,the gravity potential has a weak influence on the vertical migration of water and salt.Temperature potential is the main driving force of water and salt migration in closed system.In the open system,temperature potential and matrix potential are the dominant driving forces of water and salt migration in the sensitive region affected by low temperature,and the dominant effect of matrix potential will be weakened with the distance from the recharge end,and the dominant effect of temperature potential will be enhanced.The dominant factor of water and salt migration near the water source recharge region is matrix potential.（2）Study on the initial freezing temperature of saline soil under the influence of different factors and their interactive effectsA soil freezing experiment with 3 factors and 5 levels was designed and carried out.The independent influence and interaction of multiple factors on the initial freezing temperature of saline soil were judged by the multivariate analysis of variance,and the initial freezing temperature of saline soil under the comprehensive action of multiple factors was predicted by the multivariate nonlinear regression model.The results showed that the main effects of each factor were in the order of salt content（X₂）>water content（X₁）>freeze-thaw cycles（X₃）.The interaction between the factors is verified by the response surface method.The determination coefficient R² of the nonlinear regression model for predicting the initial freezing temperature of saline soil under the interaction of multiple factors is 0.997,which can be used for the dynamic characterization of the initial freezing temperature of saline soil.（3）The improved mathematical model of multi-field coupling transport of water,heat and salt in saline soil is constructedBased on the existing coupled water,heat and salt transport model,the mass equation of salt crystallization and the concentration equation of soil solution were established respectively,aiming at the effects of salt crystallization on temperature field,water field and salt field during the cooling process.The traditional water and heat transport equation and convection-dispersion equation were improved by combining the effects of salt crystallization on unfrozen water consumption and latent heat release on temperature.The important variables such as ice content and permeability coefficient in the soil hydrothermal salt motion equation and their related equations are deduced.Combined with the prediction model of initial freezing temperature of saline soil under the comprehensive action of multiple factors,a mathematical model of coupled water,heat and salt transport of saline soil was established considering the reduction effect of freezing temperature and salt crystallization,which can achieve a more accurate calculation of soil water,heat and salt balance.（4）Numerical simulation of multi-field coupling transport of water,heat and salt in saline soil during freezing processBased on the improved mathematical model of multi-field coupled water,heat and salt transport of saline soil,the indoor soil column experiment was simulated by using the partial differential equation interface in COMSOL Multiphysics.The established model can quantitatively simulate the law of water,heat and salt transport during the freezing process of saline soil.The variation of temperature field with time during freezing process can be divided into fast decreasing stage,slow decreasing stage and steady stage.The change of unfrozen water content is mainly influenced by the combined action of water migration and outwash phase transformation,and its change trend is consistent with temperature.The salt content at the bottom of the soil decreases gradually with time,and the salt content near the cold end decreases first and then increases.The chemical crystallization of salt accounts for a relatively large proportion,which significantly affects the accumulation of salt at the surface.The simulation results can well reflect the influence of the crystallization phenomenon of saline soil on the migration law of water field,temperature field and salt field during freezing,improve the application value and accuracy of multi-field coupled migration mathematical model,and provide reference for the prevention and control of saline soil in seasonal freezing area.