Microscopic Mechanism Of Gas Nuclei Evolution In Water And Nucleation On Metal Surface

In order to study the microscopic mechanism of bubble nucleation in liquids,avoiding the damage caused by cavitation to hydraulic mechanical systems,molecular dynamics technology is used as the main method in this paper,and the physical properties of water are calculated by using a variety of water molecular models.The stability of air nucleus in water,the development of microscopic cavitation phenomena and the nucleation on metal surface are studied by using suitable water molecular models.The main contents are as follows:1.On the basis of previous studies on phase diagrams of water molecules,the advantages and application scope of different water molecules are calculated and compared.By comparing the calculation accuracy of equilibrium molecular dynamics with that of non-equilibrium molecular dynamics by liquid argon,it is concluded that the calculation accuracy of equilibrium molecular dynamics is higher than that of non-equilibrium molecular dynamics in the calculation of physical parameters.The viscosity,thermal conductivity and diffusion coefficient of SPC/E,SWM4-NDP,TIP3 P,TIP4P,COMB3 molecular model at different temperatures are simulated by molecular dynamics.The results show that the simulation error of the polarized water molecules model SWM4-NDP,COMB3 is generally smaller than that of the traditional non-polarized rigid water molecule model.In rigid water molecular models,the TIP4 P model has relatively high simulation accuracy without considering factors such as phase change and heat transfer.2.Based on TIP4 P rigid water molecular model,the stability air nuclei in water are studied.The computational domain of water molecules with air nuclei is studied by using molecular dynamics simulation method.The transient characteristics of air nuclei in liquid water are obtained and the key factors affecting their stability are analyzed.The results show that air nucleus with certain mass maintains dynamic equilibrium in liquid water.Under the combined action of external liquid pressure,air nuclei pressure and liquid surface tension,the air nuclei gradually reaches dynamic equilibrium in water.There is a critical density of air nuclei with certain volume which makes it stable in water.Air nuclei mass is positively correlated with its equilibrium radius.When the air nucleus is in equilibrium in water,the density in the nuclei is negatively correlated with its radius.Liquid surface tension is an important factor affecting the air nuclei stability.The surface tension of liquid water decreases with the air nuclei radius.The larger the initial radius of the air nuclei,the smaller the water pressure and the easier cavitation occurs.3.Based on the polarized water molecular model SWM4-NDP and using molecular dynamics simulation method,the density distribution,radial distribution function and molecular dipole moment of the water molecule calculation domain with gas cavity during nucleation development are studied.The results show that: in the primary stage of nucleation,the gas cavity gradually increases to become gas nuclei;in the fusion stage,new gas nucleus are generated,nucleus merges into larger cavitation nucleus;in the diffusion stage,gas nucleus collapse,gaseous water molecules are distributed throughout the system,and liquid water molecules become clusters.In the primary stage of nucleation,the hyperbolic function can be used to fit the density distribution from the center of the gas nuclei.The temperature affects the liquid water phase change during nucleation,and the system with high temperature has higher phase change degree,so the system also has a larger diffusion coefficient.4.Based on the COMB3 polarized water molecular model and the molecular dynamics simulation method,the nucleation process of liquid water on the copper metal wall is studied.It is found that when the liquid water is subjected to tensile stress and the internal pressure is reduced to a negative pressure,nucleation gradually occurs on the moving metal surface.When the nuclei grows to a certain volume,gaseous water molecules appear in it.The gas nuclei shape is relatively regular in lower temperature system,and the higher temperature system has a faster nucleation rate.During the nucleation process,the diffusion coefficient of the liquid water system gradually increases,and the local pressure fluctuates regularly.The molecular arrangement in the nucleation process of different temperature systems also differs.