Research On Liquid-vapor Phase Change Around Nanostructures Based On Density Functional Theory And Molecular Dynamics Simulation

With the advance of nano-manufacturing technology,the application of nanostructures to the occasions of liquid-vapor phase change has become a research hotspot in the area of phase change heat transfer.Due to the tiny scale of nanostructures,it is difficult for traditional experimental methods or macroscale theoretical methods to investigate the early stage of liquid-vapor phase change around nano-structures.In order to reveal the details of liquid-vapor phase change around nanostructures and promote its application,density functional theory(DFT)and molecular dynamics simulation(MD)are used here to investigate the liquid-to-vapor phase change around a nanoparticle and the vapor-to-liquid phase change on nano-pillar arrayed surfaces.Firstly,the bubble nucleation behavior around a nanoparticle under uniform temperature field is calculated by DFT.The nucleation paths and barriers of two heterogeneous bubble nucleation modes,namely spherical cap mode and core-shell mode,are obtained;effects of particle wettability and supersaturation of bulk liquid on nucleation barrier are investigated;results from classical nucleation theory(CNT)and DFT are compared.It is found that a)spherical cap mode has a lower nucleation barrier than core-shell mode in most cases,so it is easier to occur.b)nucleation barrier decreases with the decrease of particle wettability or the increase of supersaturation.c)although the results from CNT and DFT are quantitatively different,they are similar qualitatively.Then,MD is used to research on the liquid-to-vapor phase change around a nanoparticle under nonuniform temperature field.The impacts of temperature gradient on bubble configuration and stability are discussed.And the effect of particle wettability on phase change under high temperature gradient is investigated.According to the result,bubbles will transform from spherical cap shape to core-shell shape and become more stable with the increase of temperature gradient.In the case of high temperature gradient,it would require lower heat flux and higher particle temperature to generate a bubble around a nanoparticle.Finally,molecular dynamics simulation is conducted for vapor condensation on nano-pillar arrayed surfaces.The mechanism of nonuniform condensation of vapor inside nanostructures is discussed.Effects of subcooling,pillar height and the degree of closeness of array on vapor condensation are investigated.It is found that the resistance of mass transfer could make vapor tend to condense in the upper section of nanostructures,and the decrease of subcooling,the increase of pillar height and the degree of array closeness could enhance this tendency.In sum,the content of this thesis provides microscale details about liquid-vapor phase change around nanostructure,which can be beneficial to the understanding and application of these liquid-vapor phase change phenomena.