Research On Characteristics Of Static And Flowing Liquid Film On Vertical Porous Plate

Mass transfer tower is widely used in industrial fields because of its simple structure,easy packing,corrosion resistance high efficiency and so on.As the core component,the performance of packing is of decisive significance to improve the production efficiency of mass transfer process.Therefore,the research on the performance of packing has been paid much attention by many scholars.Current studies can be divided into three scales: micro-scale,meso-scale and macro-scale,but they mainly focus on the wall-bounded film,and there are few studies on the falling film flow on the porous plate.However,in industry,many types of packings have hole structure.If the influence of the hole structure parameters on the stability of in-hole liquid film,the liquid diffusion and fluctuation can be determined,the hole structure can be used to improve the flow performance of liquid better and more effectively.At first,wet holes can accelerate the diffusion and impede the retraction of the liquid film.Therefore,it is particularly important to understand the formation and rupture of liquid films in a hole.Based on the Young-Laplace equation,the three-dimensional model of the static liquid film in the hole is established.Under certain conditions(diameter,depth and liquid volume),the finite volume method and Broyden method are used to discrete and solve the equation numerically to obtain the shape of static liquid film.Then,the critical minimum droplet volume of film formation is derived,the areas with and without solution are determined,which correspond to film formation and rupture,respectively.The influence of equilibrium contact angle is discussed.And the model is validated by experimental results.The dimensionless critical minimum droplet volume and critical hole structure parameters are obtained,which can be used to predict the formation and rupture of the in-hole film.According to the theoretical model of the static film,a hole would have two states: wet or dry.And their effects on the liquid film spreading should be inconsistent.In this paper,numerical models of liquid film flow on three-dimensional smooth and porous plates are carried out and verified by experimental results.It is found that dry holes will hinder the spreading of liquid film,while the wet holes will promote it.At the same time,the effects of holes on the wave characteristics are discussed.The results show that,comparing with the smooth plate,the liquid film on porous plate has sinuous waves,which will affect the distribution of film thickness and the wave velocity.The sinuosity of liquid film thickness and the horizontal velocity will increase with the increase of hole diameter,while the vertical velocity will decrease with the increase of hole diameter.When the hole diameter increased to a certain value,capillary wave will appear in the hole,which will greatly enhance the horizontal wave velocity and reduce the vertical velocity in in the hole.However,if the hole diameter continues to increase to a critical value,the liquid film will rupture.There are vortices on the in-hole and gas side of porous plate.The effects of hole structure parameters on the liquid film wetted area are investigated by experiments.The results show that when the holes are wet,the wetted area will decrease with the increase of hole spacing.When it increases to a certain extent,the effect of hole structure on wetted area is very weak.It is found that the hole spacing is less than 2 mm,the wetted area can be enhanced greatly.With the increase of hole diameter,the wetted area increases first and then decreases.With the increase of hole depth,the critical diameter of wet hole increases,which is beneficial to liquid film spreading.Based on the experimental results,the critical hole structure relationship in the flow in given.And according to the relationship between depth and diameter,three zones are divided: the promoting spreading zone,the transition zone and the impeding spreading zone.The strengthening factor of wetted area related to hole structure parameters is also proposed and the correlations of strengthen factor in three zones are fitted.