| Liquid-liquid systems are used extensively in industrial processes.Rotor-stator mixing devices are well-known for the ability to generate very high shear stress with highly-focused delivery of energy and to control the droplet size and product quality.The devices are thereby widely used in chemical,pharmaceutical,biochemical,agricultural and food processing industries for dispersion,mixing,mass transfer and chemical reactions.Although there are a large number of experimental studies for RS devices and rotating disk contactor(RDC),due to the limitations of measurement techniques,the studies hardly obtain sufficient details of flow field,dispersion,mixing and mass transfer in the reactor.In recent decades,CFD coupled with PBM and mass transfer model have become a new method,and has been widely used to simulate RS devices and RDC.However,the multiphase flow in RS devices and RDC systems is complex,including molecules/atoms,single droplet and breakup and coalescence,posing greatly challenges on numerical simulation.In recent decades,the Energy Minimization Multi-Scale(EMMS)method has been used to study how to couple the physical constraints and mechanisms contained in mesoscales of gas-solid,gas-liquid,liquid-solid,gas-liquid-solid and liquid-liquid systems into CFD models,thus improving the accuracy of CFD simulation.However,the application in liquid-liquid systems is relatively less,especially for RS devices multiphase flow system.This work focused on analyzing the physical constraints and mechanism involved in the mesoscale behavior of the RS devices,exploring the coupling method between the two mesoscales(adsorption of emulsifier on droplet interface and droplet breakup due to turbulent stress and hydrodynamic interactions),and modify the original CFD model on this basis to explore the interaction between the two mesoscales.For RDC,numerical simulation and experimental research are carried out.Chapter 2 firstly applied the Energy-Minimization Multi-scale(EMMS)approach for the surfactant-free liquid-liquid flow in RS devices.The so-called mesoscale energy dissipation for droplet breakage was derived to close the population balance equations through a breakage rate corrector.A correction factor was then integrated into the fully-coupled CFD-PBM simulation for a surfactant-free MCT-oil/water system.Compared to the original Alopaeus breakage model or the combination of Alopaeus breakage model and Prince and Blanch coalescence model,this new model could greatly improve the prediction of droplet size distribution,Sauter mean diameter,median diameter and span of size distribution for both the dilute and the dense system of dispersed oil phaseChapter 3 attempts to connect the two mesoscales of two different levels,viz.,emulsifier adsorption at interfacial level(Mesoscale 1)and the droplet breakage and coalescence in turbulence at rotor-stator device level(Mesoscale 2).While the first mesoscale can be simulated by coarse-grained molecular dynamics(CGMD),the second has been investigated in computational fluid dynamics and population balance model(CFD-PBM)simulation through the EMMS approach.Then we developed a model framework,coupling CGMD and CFD-PBM simulation through surfactant transport equations in bulk phase and interface,with source terms taking account of emulsifier adsorption parameters.The parameters including maximal adsorption amount,diffusion coefficient and adsorption/desorption kinetic constants are acquired from CGMD.The coalescence efficiency is then corrected by the interfacial area fraction not occupied by surfactant and fed into the coalescence kernel functions in PBM.Compared to traditional CFD-PBM simulation,the coupled model can greatly improve the simulation of DSD,Sauter mean diameter,median diameter and span for high dispersed phase amount(DPA),and correctly reflect the influence of DPA,surfactant concentration and rotational speed of RS devices.While the simulation cases validate and demonstrate the advantage of this new model frameworkChapters 4 verified the new CFD-MD coupling model(mesoscale 1 and mesoscale 2 coupling model)for a low-viscosity Solvesso 200 ND-water emulsification system The adsorption capacity of surfactant at droplet interface was obtained by CFD-PBM simulation in the new CFD-MD coupling model,and then the coalescence efficiency corrector was calculated by the relationship between adsorption capacity at droplet interface and coalescence efficiency corrector.Finally,coalescence efficiency of two droplets was corrected by the coalescence efficiency corrector,this method can effectively describe the inhibition effect of surfactant on droplet coalescence,so that CFD-PBM simulation can accurately predict the droplet size distribution,median diameter,span of droplet size distribution and d32 on the outlet of RS emulsification devices.Chapter 5 implemented a simulation and an experimental study for RDC.Different turbulence models were evaluated by single-phase CFD simulation.The SST k-? model can also predict the flow field in RDC more accurately.The influence of kerosene feed rate and rotor speed on kerosene hold-up in RDC was studied by experiments.Kerosene feed rate and rotor speed were the main influencing factors of averaged kerosene hold-up in the RDC.The axial and radial inhomogeneity of dispersed phase hold-up in RDC was studied by ERT.The variation of dispersed phase hold-up with axial height in the plane below rotors was quite different from that in the plane below stators.With the increase of axial height(rotating speeds are more than 500 rpm),the dispersed phase hold-up in the plane below rotors decreases monotonously,whereas the dispersed phase hold-up in the plane below stators first increases and then decreases.The effect of rotational speed on the radial distribution of dispersed phase hold-up in the plane with different axial heights was investigated.At lower speeds(less than 500 rpm),the radial distribution of dispersed phase hold-up in the plane near the center of RDC is higher than that of the plane near wall.For the plane in the upper half of RDC,the radial distribution of dispersed phase hold-up in the same plane is opposite,that is,the radial distribution of dispersed phase hold-up in the same plane near the center of RDC is lower than that of the plane near wall.At higher rotating speeds(more than 500 rpm),for the radial distribution of dispersed phase hold-up in all planes below rotors and stators,dispersed phase hold-up near the center of RDC is higher than that of the plane near wall.In summary,this work analyzes the multi-level and multi-scale characteristics of the liquid-liquid flow in RS emulsification systems.The physical constraints and mechanism involved in the mesoscale behavior are obtained,and the coupling method between the two mesoscales(the adsorption of emulsifier on the droplet interface and the breaking effect of turbulent stress on the droplet caused by hydrodynamic interaction)is proposed,A new model framework of coupling CGMD and CFD-PBM-EMMS is developed.Based on this framework,the original CFD model is corrected,and a mesoscale simulation method more suitable for RS emulsification devices is established. |
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