Study On Rayleigh-B(?)nard-Marangoni Characteristics In Liquid-Liquid Mass Transfer Between Two Immiscible Phases

Liquid-liquid mass transfer between two immiscible phases is a typical heat and mass transfer process in liquid phase extraction,sewage treatment,and other practical engineering applications,and it is an important research topic in the field of engineering thermo-physics.Notably,in these liquid-liquid mass transfer processes,local concentration difference is induced when the mass transfer is uneven,which results in the density gradient along the gravity direction and the interface（surface）tension gradient along the interface direction,and respectively induces the Rayleigh-B（?）nard effect and Marangoni effect.When these two effects are strong enough to overcome viscous resistance and other external constraints,the convection phenomena,namely Rayleigh-B（?）nard convection and Marangoni convection,will be induced in the system.These two convection often interact and interact with each other,and are then coupled to form unique Rayleigh-B（?）nard-Marangoni convection（RBM convection）.The results show that the RBM convection induced by the mass transfer system can greatly promote the renewal of interface fluid and enhance the mass transfer rate.Moreover,RBM convection induced by mass transfer is a good research object for nonlinear hydrodynamics.Therefore,the in-depth researches on the RBM convection in the liquid-liquid mass transfer process can not only develop the control methods of liquid-liquid mass transfer intensity in the actual production process,but also further improve the theory of mass transfer and interface science.However,the researches on the RBM convection characteristics in the liquid-liquid mass transfer process is still limited,and many deep-seated mechanism problems have not been well revealed.For example,compared with gas-liquid mass transfer,the researches on liquid-liquid mass transfer are still scarce.Moreover,lacking an in-depth understanding of the whole basin evolution law and the internal mechanism of RBM convection in the liquid-liquid mass transfer process.Furthermore,the quantitative analysis of RBM convection,especially the quantitative analysis of concentration field,flow field,and mass transfer rate are few.More importantly,the Marangoni effect or the Rayleigh-B（?）nard effect are mostly studied in isolation,and little attention has been paid to the coupling mechanism of the Marangoni effect and the Rayleigh-B（?）nard effect,which leads to an insufficient comprehensive and profound understanding of RBM convection characteristics and mechanism in the liquid-liquid mass transfer process.In this paper,visualization experiments and numerical simulations are carried out to study the inter-phase mass transfer characteristics and internal mechanism of two typical liquid-liquid two-phase systems（i.e.liquid-liquid mass transfer system and single drop liquid-liquid mass transfer system）.The real-time visualization of concentration distribution and quantification of related parameters in the system during mass transfer is realized through the visualization observation platform of the Shadowgraph method or the Schlieren method.The typical RBM convection structures are captured.The convection modes and the law of the generation and evolution of convection structures are summarized,and the internal generation mechanisms are analyzed.In addition,based on the DPIV method,a flow field observation platform is built for parallel experiments to quantify the fluid motion parameters caused by mass transfer,such as velocity and vorticity.Combined with the observation results of the Shadowgraph method or Schlieren method,the hydrodynamic mechanism of RBM convection structures generation and evolution process is further discussed.Moreover,a finite element model of liquid-liquid mass transfer between two vertical liquid layers is established.The coupling relationship between the Marangoni effect and the Rayleigh-B（?）nard is studied in detail.The influence of the Rayleigh-B（?）nard with different intensities on the RBM convection and mass transfer characteristics is analyzed.The main important contents and conclusions are as follows:（1）Based on the principle of the Shadowgraph method,a visual experimental research platform of liquid-liquid mass transfer system between two liquid layers is designed and built.The whole process of RBM convection developing from the interface to the lower mainstream region is observed from the vertical direction.The characteristic convection structures of RBM convection and its mutual evolution law are systematically summarized.At the same time,the parallel observation is carried out by using the DPIV method,and the hydrodynamic mechanism behind the development of RBM convection is deeply analyzed based on quantifying the flow field velocity information of the mass transfer process.In addition,the number of convective structures,the extension velocity of convective structures,and the roughness of the water-toluene interface under different working conditions are calculated quantitatively.Based on these parameters,the effects of initial acetone concentration in the water layer,initial acetone concentration in the toluene layer,and the thickness of the toluene layer on RBM convection strength and water-toluene interface morphology are analyzed quantitatively.The main conclusions are as follows:due to the lag of mass transfer,there is obvious density stratification and disturbance near the interface of the lower layer.Due to the Rayleigh-B（?）nard effect on the"interface"of density stratification,the unstable state of density stratification is rapidly unbalanced,and the"interface"gradually becomes concave and convex,forming a mound-like"interface",and due to the further imbalance of density and pressure,a columnar plume is formed.There is a vortex zone on both sides of the plume.Under the coupling effect of the pressure difference in the vortex zone and the obstruction of the surrounding main fluid,the neck of the columnar plume shrinks,and the end expands and flattens.When the end can evolve into an inverted mushroom shape,it evolves into a strong plume.Due to the mass transfer behavior between the plume region and the body region,the end of the plume disappears,and then it can evolve into a feather-like weak plume.Under the condition of high concentration,a large number of RBM convective structures formed in a short period agglomerate into convective clusters under the influence of each other,and eventually develop into independent strong plumes with the passage of time and the weakening of mass transfer.Increasing the initial concentration gradient of acetone in the aqueous phase can enhance the RBM convection,while increasing the initial concentration gradient of acetone in the toluene phase will make the RBM convection weakened.When the thickness of the toluene layer is larger,the number of RBM convective plumes increases,the roughness of the water-toluene interface is larger,and its numerical fluctuation is more obvious,which means that the RBM convection intensity becomes stronger.（2）The finite element model of liquid-liquid mass transfer between two vertical liquid layers is established and verified by visual experiments.The coupling relationship between the Marangoni effect and the Rayleigh-B（?）nard effect in the RBM convection is studied through mass transfer models under three effects,namely the pure Marangoni effect（PME）,pure Rayleigh-B（?）nard effect（PRE）,Marangoni and Rayleigh-B（?）nard coupling effect（CMRE）.The convection characteristics and mass transfer intensity of RBM in the lower liquid layer with different initial acetone concentrations are studied.The influence of the Marangoni effect and the Rayleigh-B（?）nard effect on RBM convection characteristics and mass transfer characteristics of the system are studied.The simulation results show that the typical convection structures of PME mode,PRE model,and CMRE mode are Marangoni convection cell,needle-plume,and toe-plume.In CMRE mode,the shape of the Marangoni convection cell formed in the initial stage changes due to the Rayleigh-B（?）nard effect,Marangoni convection cell deforms vertically under the action of gravity,and finally forms a toe-plume.The Marangoni effect mainly acts on the interface and its surrounding areas,while the Rayleigh-B（?）nard effect is more on the main area of the liquid layer.In CMRE mode,the Marangoni effect promotes the merging of RBM convection structures.Although the Rayleigh-B（?）nard effect delays the emergence of RBM convection,it can promote the downward penetration of RBM convection structures.The Marangoni effect and Rayleigh-B（?）nard effect can promote mass transfer,but the effect of the pure Marangoni effect on mass transfer is about 100 times that of the pure Rayleigh-B（?）nard effect.In CMRE mode,the Rayleigh-B（?）nard effect inhibits the mass transfer in a short time at the initial stage of mass transfer,but it can further strengthen mass transfer by coupling with the Marangoni effect at a larger time scale.With the increase of initial acetone concentration for the lower liquid layer,the stronger Rayleigh-B（?）nard effect is induced in the system,and the influence of the Rayleigh-B（?）nard effect on RBM convection or mass transfer increases gradually,which results in the faster permeation rate of convection structures,the more convection structures,and the higher total mass transfer rate.（3）A visual observation platform for a liquid-liquid mass transfer system of moving a single droplet is built.The whole process of droplet formation and free-floating in the continuous phase（deionized water）is observed experimentally.The oscillation phenomenon of the droplet formation process and droplet floating trajectory are studied qualitatively and quantitatively.The influence of initial solute concentration in the droplet phase,initial solute concentration in the continuous phase,and initial droplet size on the RBM convection and mass transfer characteristics is summarized.The experimental results show that increasing the initial concentration of the droplet phase can enhance the RBM convection intensity and mass transfer intensity.For example,when w_d0≤10wt%,the droplet will not oscillate in the formation process,but when w_d0≥15wt%,the droplet begins to oscillate.The greater initial concentration of the droplet phase can conduct the stronger RBM convection,the greater difference in the resultant force of interfacial tension between two sides of the droplet.Therefore,the lateral displacement gets larger and the total mass transfer rate becomes greater.Increasing the initial concentration of the continuous phase can inhibit the RBM convection intensity and the mass transfer intensity,the increase of the initial concentration of the continuous phase leads to the decrease of the RBM convection intensity,which makes the oscillating droplets nonoscillatory,the floating trajectory of droplets approaches to the central axis,the lateral displacement decreases,and the total mass transfer rate decreases.Moreover,when the initial size of the droplet is larger,the specific surface area of mass transfer is larger and the internal circulation is more likely to occur,so the initial mass transfer rate is stronger,and the stronger RBM convection can be induced in the system,leading to the droplet changing from non-oscillation to oscillation,and the lateral displacement and total mass transfer rate increase when the droplet floats.（4）DPIV method and Schlieren method are used to study the mass transfer process of acetic acid（solute）from droplet（deionized water）to butyl acetate in a vertical slit.The RBM convection evolution process and the flow field evolution process during the mass transfer process are observed and analyzed qualitatively.The hydrodynamic mechanism behind various typical convection structures and their generation and evolution laws are mainly clarified.In addition,the effects of initial solute concentration（i.e.initial Marangoni number（Ma₀）and initial Rayleigh number（Ra₀））on the RBM convection characteristics,mass transfer performance,and droplet morphology are also discussed.The experimental results show that the RBM convection in the droplet is determined by the competition between the buoyancy/gravity and interface gradient during the mass transfer process,which includes three main modes,i.e.,Marangoni effect dominant mode（MEDM）characterized by some counter-rotating small vortices,transition mode（TM）characterized by several small co-rotating vortices,and Rayleigh-B（?）nard effect dominant mode（REDM）characterized by two large counter-rotating vortices.When Ma₀≥0.816×10⁷,MEDM,TM,and REDM appear successively during the mass transfer process and MEDM turns to disappear when Ma₀≤0.719×10⁷.Especially,only REDM occurs at Ma₀≤0.627×10⁷.The REDM convection tends to stabilize fluid convection in the droplet while the MEDM convection makes the fluid flow to be turbulent.Owing to the downward fluid flow caused by gravity and the interfacial tension gradient caused by the local high solute concentration for the REDM and TM convection,the shrinking of the droplet top is faster than the other parts,which induces the faster droplet shrinking in the vertical direction than that in the horizontal direction during the mass transfer process.Increasing the initial diameter of the droplet phase and the initial concentration of acetic acid in the droplet phase can enhance the RBM convection intensity,while reducing the initial concentration of acetic acid in the external phase can weaken the RBM convection intensity.