| Microfiltration,a most widely utilized technology,has played a significant role in various fields like preparation of pure water,industrial wastewater treatment,medicine,food,etc.However,the membrane pollution makes the permeation flux decline dramatically with the extension of running time,increase energy consumption,maintenance and operation cost and hinder the large-scale application of microfiltration technology accordingly.In order to inhibit membrane fouling and increase its flux,the turbulence promoter is utilized to strengthen the flow field and then its structure design and condition optimization are conducted.Meanwhile,the research mechanism is studied via CFD method,which is an useful way to enhance microfiltration process and realize effective application.Nevertheless,the numerical simulation,which is used to study the mechanism of flux attenuation in the microfiltration process currently,was not considered the combined influence of permeate boundary conditions and cake forming process simultaneously,and hard to predict the permeation flux and cake deposition accurately by established models.Furthermore,for lack of the suitable module in common software to describe the surface flow and deposit growth of suspended particles in the porous medium,it is difficult to do simulation and design works with consideration of the microfiltration process directly.Thus the secondary development for the existing solvers is needed to be done.In the light of problems above,this work revolves around problems occurred in CFD study in current flux enhancement process of tube microfiltration,independently develops three dimensional particleFoam model with permeate boundary and particle deposition,numerically simulates suction pipe and the same one with disturbed flow baffles,predicts the permeation flux and distribution of filter cake layer in different operation conditions and baffle structural parameters and finally analyzes mass transfer mechanism,proposes a prediction and evaluation index about flow field enhancement.The model of suspension liquid deposited on the surface of the microfiltration membrane was constructed at first,and the 3D numerical module particleFoam was developed which could calculate the instantaneous incompressible flow and the suspended particles flow simultaneously.Volfrac,the volume fraction calculation method of the particles deposited on the wall surface was realized by calculate the average height of particles on the underlying grid,and the problem in numerical simulation of suspended particles adhesion and further formation process of filtration resistance on the membrane surface was solved in this model.Adopted groovyBC as the wall boundary condition type,and combined with volfrac method,the particleFoam module makes the real-time calculation of cake thickness(the key parameters in the calculation formula of permeate flux),and the calculation of permeate flux with the change of cake resistance became true finally.As a result,the relative error of permeate flux between calculated results and experimental ones is less than 15%.It is proved that the particleFoam module is effective and reliable during the simulation of the membrane fouling of the actual microfiltration process.Two kinds of deposition model of particles on membrane surface was proposed for the tube microfiltration membrane:the all-stick model which judge the deposited particles as all-stick when contact with membrane surface;and the deposition-judgment model which could accurately predict the adhesion process of suspended particles in tube microfiltration membrane.Considered with the influence of stochastic force field near the permeate boundary layer on particle deposition motion,the second model used the Bernoulli random function to randomize the nomal velocity ur of the particles on the membrane surface,then the criterion of the deposition behavior of particles on the membrane surface is the ratio of the random velocity ur and the maxium normal permeate velocity umax on the wall in the pure water condition.Meanwhile,the effects of factors like transmembrane pressure,entrance velocity and solution concentration on permeation flux and filter cake deposition are analyzed in detail.By embedding random functions about turbulent flow and particle mobile,the random adhesion model realizes the accurate calculation of permeation flux which results are highly consistent with real conditions.The numerical error has been declined from40%to nearly 20%and the relative error of average thickness of cake is less than 15%.Through analysis the CFD results of the flow field,the dynamic and morphology detail of filter cake formation is firstly gained and the changing process of membrane flux is thus predicted,which can provide forecasting and inhibiting ways for cake deposition.The tangential velocity of particles at the boundary of tube membrane is remarkably increased after the implantation of baffles,and the force distribution is more complicated.The single criterion(ur/umax)is no longer applicable to the simulated prediction of particle deposition in present of baffles.Therefore,in order to uncover the mechanism about adding baffle into the tubular microfiltration process and get the statistics about filter cake deposit which are hardly supervised in experiments,this work redesigns modules about the permeation boundary and particle deposition on the basis of tube model,puts forward the way of using the normal penetration rate uf and the tangential cross flow velocity uc to simply express the effects of seepage drag force and cross flow drag force on particle deposition from the judgment of random deposition of suspended particles on membrane surface.The simulation design is conducted on the purpose of optimizing permeation flux and filter cake thickness.Moreover,the effects of baffle structure parameters on flux enhancement process are revealed in various operating conditions and the error between prediction value and experimental results in literature is under 10%,proving the model's reliability and effectiveness.The simulation results reflect that when shrinking percentage of baffle ? is higher,the permeation flux is larger.The resistance of mass transfer is reduced after the optimization of baffle structure(?=0.64,L/D=1.5).Therein,the average thickness of filter cake is reduced from 0.318nm to 0.026nm in the tube while permeation flux increases 60%accordingly.This model provides a rapid prediction and accurate design of microfiltration process.In order to reveal the mechanism of flow field vortex structure on the enhancement of microfiltration process,this work introduces flow field vortex intensity and chooses the criterion Q of vortex core as the identification method of vortex structure of turbulent field.The key problem of the difference between the particles deposition and the permeate flux in the cases of turbulence approximation,which was difficult to be explained using single wall shear force or turbulent kinetic energy was solved by this method.The constriction ratio ? of baffle can obviously enhance the flow strength.When ? is enhanced from 0.28 to 0.64,the maixmum average Q value reaches 332%,which means the membrane fouling is reduced significantly and strengthening effect is clear.The variation of spacing ratio among baffles L/D changes from 3 to 1,which is less obvious than ?,can make the maximum average of Q value reach 92%.The index of Q contour surface can distinguish the differences of turbulence strength in the flow field clearly and visually,accurately evaluate the effect of turbulence intensity in tube,and further reveal the mechanism of flux enhancement of microfiltration process resulted from the difference of vortex structure of flow field,which is also an effective way to find the optimal operating condition in the rapid screening during microfiltration process. |
PDF Full Text Request