Numerical Research On Separation Mechanism Of Vapor-Liquid Mixed Flow Impacting On Solid Wall In Porous Media

The wire mesh demister is an efficient gas-liquid separation device,which is widely used in the field of desalination to remove small droplets of seawater entrained in the steam flow and improve the quality of freshwater obtained.At present,domestic and foreign research on wire mesh demisters is mainly based on experiments.Semi-empirical formulas about demister separation efficiency and pressure drop are obtained through experiments.A few scholars have simplified the complex structure of demisters by numerical simulation and established effective model.In the actual working process of the wire mesh demister,re-entrainment often occurs,which seriously affects the demister efficiency of the demister.Therefore,in-depth study of the influencing factors of the performance of the wire mesh demister is very important.This paper uses the CFD software FLUENT to calculate and analyze the working performance of the wire mesh demister and the mechanism of re-entrainment.The main research contents are as follows :(1)First,the wire mesh demister is simplified into a porous medium,and the single-phase flow field in the demister is simulated.The effects of steam velocity,viscous resistance coefficient,inertial resistance coefficient and packing density on the dry pressure drop are discussed.Comparison of experimental data shows a good agreement.(2)A staggered array model is established to simplify the complex structure of the demister.Without considering the re-entrainment,the two-phase flow field in the wire mesh demister is simulated to discuss the effect of the flow velocity,ayer spacing,droplet diameter and mesh diameter on pressure drop and efficiency.The simulation results show that the pressure drop and efficiency of the demister increase significantly with the increase of speed;in addition,the reduction of the mesh diameter and the mesh layer spacing will also increase the efficiency and pressure drop of the demister;It also increases with the diameter of the droplet,but the pressure drop decreases with the diameter of the droplet.(3)Drawing on the modeling method of droplet impact surface,a two-dimensional model of droplet impact on the drying screen and the liquid film on the surface of the mesh is established.The VOF model is used to track the phase interface and the generation mechanism of re-entrainment is discussed.Analyze the mechanism and method of impact speed,droplet diameter and liquid film thickness affecting re-entrainment.The numerical results show that when the droplets impact on the dry wire mesh under the disturbance of the steam flow,as the droplet diameter increases and the velocity increases,the secondary droplets produced gradually increase,and the time for splashing occurs gradually.And the larger the diameter of the droplet,the smaller the speed required for the droplet to splash.When the droplets impact the liquid film on the surface of the wire mesh under the disturbance of the steam flow,the thickness of the liquid film will affect the number of secondary droplets and the diameter of the secondary droplets.Within a certain diameter and speed range,the number of secondary droplets will decrease as the thickness of the liquid film increases.And the thicker the liquid film on the surface of the wire mesh,the stronger the buffering effect on the impact of the droplet,and the droplet needs more kinetic energy to overcome the viscous resistance and surface tension and splash.