Study On The Coupling Mechanism Of Adsorption Phase Change And Energy Transport At Vapor-solid Interface

Interfaical adsorption phase change and heat transfer is fundamental and significant in nature and in various applications of industry engineering.For example,electronic device heat management,adsorption refrigeration,and phase change energy storage are closely related to the adsorption phase change phenomenon.When the vapor is adsorbed on the cooled surface,the temperature difference will affect the adsorption amount.Simultaneously,the variation of adsorption amount will release the adsorption heat and change interfacial thermal resistance,making the whole process very complex.Thus,the heat and mass transfer at the solid-vapor interface is coupled,while a series of interfacial physical processes such as the evolution of interface adsorption clusters,wetting transition,vapor-liquid phase transition,liquid film instability,and the evolution of surface tension is involved.Besides,these physical processes are limited by additional variables such as the solid structure,morphology,environmental conditions,and coupling relationship at the interface.Therefore,the study of the whole evolution process from adsorption phase change to wetting at the vapor-solid interface is a comprehensive problem,which contains the fields of heat and mass transfer,nonequilibrium thermodynamics,interface dynamics,and so on.In this work,the molecular dynamics simulation and theoretical analysis are hybrid to study the coupling mechanism of adsorption phase change and energy transfer at vapor-solid interface.The molecular insight into the structures of the adsorbate is presented as clusters with different number of particles,while the formation and evolution of clusters are clarified.The occurrence conditions of adsorption phase transition are determined,and the phase transition model from adsorption to condensation is obtained.Then,the underlying mechanism of energy and mass transfer at the phase change surface is revealed.Firstly,the kinetic and thermodynamic characteristics of adsorbate in an equilibrium state are investigated.From the microscopic point of view,the molecular insight into the structures of the adsorbates are presented as clusters with different number of particles,and adsorption sites are formed in the hexagonal center of the silicon(111)surface.For a relative small pressure ratio,most of the clusters are composed of a single molecule.As the increase of pressure ratio,larger-sized clusters appear,forming various cluster-types.The molecular insights into the formation and evolution of clusters in the adsorbate are presented.The increasing rate of adsorbed clusters and the declining rate of empty adsorption sites are dependent on pressure ratio.For a large pressure ratio,the single-molecule-clusters are aggregated to incubate large clusters,and the fraction of single-molecule-cluster decreases with time.The simulated adsorption rate is consistent with the theoretical analysis of the statistical rate theory.For the adsorption of water,the structure of the adsorption layer will change due to the existence of hydrogen bond,and the hydrogen bond is distributed along the diagonal direction on the gold surface,and the density of hydrogen bond near the wall is the highest.Secondly,the critical occurrence condition for adsorption phase change is determined based on the adsorption cluster model.Once the pressure ratio exceeds a certain value,the adsorption sites are occupied by homogeneous clusters,which initiates the liquid phase to wet the surface.At this time,the gas-solid surface tension is equal to the gas-liquid surface tension.The influence of solid-vapor interaction strength and temperature difference is discussed,and the phase change model from adsorption to condensation is obtained.A spatial-temporal evolution model from adsorption to the phase transition is constructed.Vary with the adsorption entropy and mass flux,three regions are distinguished during the phase transition process,they are stable adsorption region,metastable adsorption,and condensation regions.Initially,the solid-vapor interfacial effect is dominated and the argon atoms are adsorbed as clusters and aggregate on the surface.So the time-dependent adsorption rate can be well described by the theoretical predictions based on the concept of cluster evolutions.When the adsorption layer begins to transform into the liquid-like film,the liquid phase temperature is enlarged due to the release of condensation latent heat,the solid-liquid interface,temperature jump and the mass flux enhance.When the liquid-like film is formed,the adsorption sites are fully occupied by clusters,and the adsorption behavior almost vanishes.As a result,the mass flux keeps constant.Finally,the underlying mechanisms of interfacial heat transfer across the solid-vapor interface are thoroughly revealed through cluster adsorption and phase change models.It is found that the adsorption cluster evolutions and vibrational behaviors have notable roles in the variation of interfacial thermal resistance.With the increase of adsorption,the empty adsorption sites disappear sharply.As a result,the spatial distribution in the first peak density becomes denser,and the interfacial thermal resistance decreases.Then,the small-sized clusters are transformed into larger-sized clusters,and the matching degree of interface phonons is dominant.The vibrational main frequency of the argon adsorption phase shift to 0.7THz,and the interface thermal resistance decreases further.The variations of interfacial thermal resistance for argon-silicon and water-silicon systems are further studied.For the argon-silicon system,argon is adsorbed on the surface as small-sized clusters,so the formation of clusters cannot enhance the matching of the phonon frequencies at the solid surface.Due to the influence of additional thermal resistance,the interfacial thermal resistance increases with the increase of pressure ratio.However,for the water-silicon system,with the increase of pressure ratio,the vibration frequency type of interfacial adsorbed clusters increases,which enhances the matching of the phonon frequencies at the surface and improves the interfacial heat flux.