Numerical And Experimental Study On The Characteristic Of Heat And Moisture Transfer Process In Frozen Soil During Freeze-Thaw Process

Frozen soil is a kind of soil which is recognized as unstable and sensitive.The frozen earth region covers 70%of the total land area of the earth.Furthermore,China owns the third largest frozen earth area in the world.Such properties of frozen soil have brought negative impacts to industrial and agricultural production activities,environment change,construction and maintenance of traffic infrastructures in frozen earth area.In the recent years,with the development of national economic construction in China,the need for new tools for frozen soil research has become increasingly urgent.The proposed research focused on the characteristic of heat and moisture transfer of frozen soil during freeze-thaw process,improved the forecast model of water content in frozen soil during freeze-thaw process.The results are significantly important not only for the development of theoretical explanation to heat and moisture transfer phenomenon in frozen soil during freeze and thaw process but also for diminishing the difficulties of the construction in frozen earth area.Base on the basic theory of lattice Boltzmann method,the freeze and thaw process of frozen soil was separated into the interaction between the displacement process of two-phase fluid,convective heat transfer process,the evolution of interface surface of soil-liquid transition,and the complex microscopic structure of the soil.By using modularized lattice Boltzmann model,a mesoscopic model for heat and moisture transfer process of frozen soil during freeze and thaw process was proposed.The accuracy,eff-iciency,and stability of the model were tested by solving the Neumann-Stefan problem.The error analysis in model validation implied that the total dimensionless time tmaxα/l2 should be firstly calculated,then the thermal diffusivity should be chosen based on the need of computational accuracy in order to reach the required precision.St（Stefan number）had a high impact on the accuracy of the model,especially in the initial period of calculation.However,for the simulation of freeze-thaw process of frozen soil,usually tmaxα/l2>1.Therefore,although the proposed model showed significant error during the first steps of simulation,considering the total evolution time,the proposed still showed good accuracy.Base on percolation theory,a numerical model for calculating the porosity and estimating the percolation threshold of soil during freeze-thaw process was proposed.By using the proposed model,the evolution of porosity nf and effective porosity n_eff of soil with three kinds of particle distribution（uniform distribution,normal distribution,exponential distribution）,various minimum diameter dmin,maximum diameter dmax,variation of diameter dvari during freeze-thaw process was simulated.During the freezing process,due to the isolating effect of pore ice,the deviation of nf and n_eff was observed,n_eff decreased faster comparing with nf.When nf = ncritical,non-linear variation occurred to n_eff,n_eff=0 and no longer changed along with the temperature drop.While calculating the thermal properties and permeability of the frozen soil,different correction factor should be chosen.During the freezing process of soil,the critical porosity ncritical was slightly affected by dmin and dvari,but was strongly affected by the form of distribution of particles.For unsaturated soil,the thickness of water film was significantly lower than saturated soil with the same nf.ncritical in unsaturated soil decreased along with the increase of saturability,and reached the same ncritical with saturated soil once it reached saturated state.Based on the fundamental theory of macroscopic lattice Boltzmann method,a macroscopic lattice Boltzmann model for heat and moisture transfer process of frozen soil during freeze and thaw process was proposed.The accuracy and stability of the model were validated by comparing the experimental results and numerical results.In compare with the mesoscopic model,the proposed macroscopic model not only showed significantly higher computational efficiency,but also fully considered the permeability drop at freezing front due to the moisture transfer process and freezing process.The first step of simulation with the proposed macroscopic model should be the calculation of four dimensionless numbers,which are Fo,St,Da,and Re,respectively.The computational efficiency and accuracy of the macroscopic model were better than the mesoscopic model.The theoretical analysis implied that the maximum moisture transfer rate is a result of the interaction between a constant cryosuction force and variable permeability and phase-transition temperature.The full-scale frozen soil freeze-thaw test bed showed good stability during the online monitoring research on the freeze and thaw process of frozen soil.When external load or deformation exists,for the application of electromagnetic soil moisture sensor,the stiffness of electrode rod should be thoroughly evaluated,or it will lead to the sensor damage.During the state transforamation,sudden change of water content occurred at the frozen front.The intensity was propotional to the rate of temperature change.The proposed macroscopic lattice Boltzmann model showed its accuracy in the simulation of long-term freeze and thaw cycle with complex temperature boundary conditions.The main technical indicator and the most challenging problem in the design work of "Special region full-scale accelerated loading testing facility" were introduced.TDR and FDR sensors are both suitable for the measurement of the evolution of soil water content during the freeze-thaw process of frozen soil.However,the massive usage of TDR sensors will lead to significant high costs.What’s more,under the condition that deformation exists in the test,the application of TDR sensors should firstly begin with the evaluation of the stiffness of electrode rod and make the reinforcement when necessary,or alter the method of installation.The soil water content after freezing process was strongly affected by the freezing rate,while the temperature of top surface was higher than-10℃,freezing rate,initial water content,and porosity of soil all had strong affections,among which the temperature of top surface showed the strongest influence.While the temperature of top surface was below-20℃,the above three factors all showed slight influence,porosity showed higher influence than initial water content regarding the soil water content after freezing process.It was concluded from the research that,for a specific soil water content after freezing process,it can be approached by adjusting the freezing rate,porosity of soil and initial water content.In the accerlated loading test,the response surface of soil water content after freezing process was firstly carried out with the numerical simulation by uing the proposed numerical model,then the controlling strategy curve could be obtained by computing the intersection curve of two surfaces.