Direct Numerical Simulation Of Liquid-Vapor Phase Transition With A Diffused-Interface Multiphase SPH Model

Vapor-liquid phase transition exists widely in natural phenomena and practical applications.It is of great practical significance and scientific value to study its mechanism and phenomena in depth.The treatment of gas-liquid interface is one of the difficulties.Especially when the research scale reduces to micro and nano scales,the traditional sharp interface method will fail.Therefore,a diffuse interface method which can describe the continuous density gradient is needed.In addition,the wettability of the solid wall also has an important influence on the liquid movement and phase transition at micro-scale.In this dissertation,a SPH multiphase flow model based on diffuse interface and solid wall boundary treatment method will be established to realize the direct numerical simulation of gas-liquid phase transition and to study the boiling process of three-dimensional water film on hydrophilic and hydrophobic walls.Here,the motion of gas-liquid two-phase is described by the one-component fluid governing equation.Van der Waals equation is used as the fluid state equation to realize gas-liquid separation,and the diffuse interface between gas and liquid is realized by coupling the Korteweg tensor in the momentum equation.The Lagrangian fluid governing equation is discretized by SPH fluid particles.At the same time,for the second kind of boundary conditions and solid walls with different wettability,the corresponding boundary treatment methods are proposed.The accuracy of SPH numerical model in describing gas-liquid phase transition system,second boundary conditions,different wettability surfaces and classical flow problems is verified by several benchmark cases.Based on the above multiphase SPH model,the spontaneous gas-liquid separation process of droplets heated instantaneously in microgravity system is studied without considering the boundary effects.Four different boiling modes,surface evaporation,internal bubbling,breakup and flash evaporation,were found.The phase transition mechanism of different boiling modes is explained from the thermodynamic point of view,mainly is the spinodal decomposition.The main phenomena and characteristics of various boiling modes are qualitatively and quantitatively analyzed.The phase diagrams of these four boiling modes were drawn under different fluid temperature and density.The influence of initial shape,size and thermal conductivity on droplet boiling was analyzed.Then we use the wall wettability model to study the spontaneous movement of droplets on the wettability gradient surface.Firstly,the influence of wettability on the moving speed of droplets is quantitatively analyzed,and the relationship between the surface tension on the liquid-solid interface and the movement of droplets is clarified.Then the recombination process of droplets on the surface with periodic variation of wettability is studied.The main principles of droplet movement under variable wettability are summarized.The influence of temperature change and arrangement of variable wettability on droplet fusion process is analyzed.Combining the above-mentioned SPH models of diffuse interface and wall wettability,we directly simulated the heating and boiling process of three-dimensional water film on hydrophobic and hydrophobic walls.The problem was studied by partial heating and overall heating.The trajectories of fluid states on density-temperature phase diagrams are plotted,and the different mechanisms of normal boiling and explosive boiling are explained,and the boiling modes are distinguished.The critical heat flux(CHF)is predicted by numerical simulation,and the conclusions are consistent with the experimental results in the literature.Then the influence of fluid thickness and heating mode on the critical heat flux is analyzed,and the influence of wettability on boiling is discussed.It is found that the explosive boiling of liquids occurs more easily on relatively hydrophilic surfaces.On the hydrophobic surface,the vapor phase is more unstable,and jumping boiling may occur under some operating conditions,thus enhancing the heat transfer effect.