Molecular Scale Model And Simulation Study On Drying Of Porous Media

The drying process of porous media includes the heat transfer process and the mass transfer process.From the beginning of the last century,many scholars domestic have begun to study the moisture transfer process of porous media at drying.Due to the reasons that the pore size of the porous medium is fine,the structure of holding water is complex,the physical mechanism of the moisture transfer process is varied and the transmission process can not be observed directly,no model has been developed to describe the process of water migration preferably in the porous media.The main factor that affecting the moisture transfer rate is diffusion coefficient.However,the existing research methods are mostly based on the empirical coefficient or empirical equation to calculate the diffusion coefficient of moisture,and it is difficult to measure the diffusion coefficient accurately by the conventional experimental methods.Therefore,aiming at the problem that the mechanism of mass transfer in porous media is complex,and the diffusion coefficient is not easy to be determined,in this thesis,the molecular dynamics(MD)simulation method is used to analyze the moisture diffusion process of porous media under the condition that the pore diameter is less than 10-7m,and obtain the diffusion coefficient of moisture.(1)The construction of physical model of porous media drying at molecular scale.The drying process of porous media is analyzed,and the diffusion process of moisture in the micropores of the porous media under the two diffusion forms is considered separately.The pores contain a lot of liquid water in the early stage of drying.The diffusion form of moisture is mainly by liquid water diffusion.During the later stages of drying,most of the liquid water evaporates into water vapor in the pores,at this stage,the diffusion form of moisture is mainly based on the diffusion of the water vapor in the air.The physical model of porous media drying at molecular scale is established,including the physical model of the wetting process between liquid water and pore wall,the physical model ofliquid moisture diffusion in micropores and the physical model of water vapor diffusion into the air in micropore.(2)The mathematical model for molecular scale simulation of porous media drying.The molecular motion equation of simulation system is derived,and the Verlet algorithm is chosen as the numerical solution method of molecular motion equation.Then,the potential function model between the atoms of the system is established.Finally,the mathematical model of the contact angle in the wetting process and the microscopic mathematical model of diffusion(including the Einstein model and the Green-Kubo model)are derived.(3)The solution of the molecular scale model of porous media drying.The solving program of the model is designed.The wetting process model of liquid water and pore wall and the moisture diffusion model in micropore of porous media drying is simulated and solved by MD method.At the same time,the setting of solving parameter and the flow chart of the model are given.The program architecture is explained.(4)The simulation results and model validation.The wetting process between water droplets and the pore wall was simulated.The pore wall thickness,the pore wall atoms’ potential energy and the rough pore wall’s surface geometry of the diffusion model of liquid water in micropores are determined.The diffusion process of liquid water in micropores of porous media drying was simulated.The simulation has shown the "Kelvin effect" phenomenon of moisture diffusion process in micropores of porous media drying.The simulation results show that the molecular dynamics model of moisture diffusion in micropores of porous media established in this thesis can simulate the moisture diffusion coefficient of potato chips well.The maximum relative error of the diffusion coefficient in simulation and experimental measurements is 8.6%.In the drying process of porous media,the factors that temperature,micropore diameter,wall roughness and phase area fraction have important effects on the diffusion process of water in the micropores.With the increase of the temperature,the micro-pore diameter and the area fraction,or the decrease of wall roughness,the water diffusion coefficient of porous media will increase accordingly.The diffusion process of water vapor into air in micropores of porous media was simulated.The simulation results show that the temperaturehas little effect on the diffusion coefficient of water vapor in the air.With the increase of the amount of steam,the diffusion coefficient of water vapor in the air is gradually reduced.Considering the diffusion coefficient of moisture in the micropores under two diffusion forms,it can be seen that the main resistance to moisture migration during the whole drying process is from the diffusion of liquid water.