Inline Measurement,Modeling And Simulation Of Particle Hydrodynamics In Dense Multiphase Reactors

Multiphase reactors are extensively used in the industrial processes such as chemical,pharmaceutical,metallurgical and fermentation industries.In order to make full use of available space in the reactors,the concentration of a dispersed phase is usually very high(>10 vol%),which increases the complexity of interphase forces.It brings the challenges to existing measurement methods and simulation models for design and scale-up of dense multiphase reactors.Therefore,this thesis develops new measurement method based on advanced image measurement techniques and then studies particle hydrodynamics in dense multiphase reactors.Meanwhile,an interphase force model with much more precise is developed and numerical simulation tools suitable for dense reactors are obtained finally.As for the limitations of widely used laser-based velocimetry in dense systems,an inline image velocimetry(??)based on the short optical path has been developed.It determines flow field by a series of new images processing steps and coupling particle image velocimetry(PIV)and particle tracing velocimetry(PTV).The new method overcomes the upper limitation to low dispersion fraction(-4 vol%)of the laser-based methods,and the systems with particle loads up to 40 vol%could be applicable in theory.On this basis,the new method is verified in a solid-liquid stirred tank with solid loading(denser than the upper limitation of the laser-based methods)from 4.4 vol%to 8.8 vol%.The results show that the two-phase flows in the stirred tank can be accurately measured by the new method,except for the impeller discharge region.The deviation in the impeller region is caused by the limitation of camera capacity,which can be overcome by further enhancing the frame frequency of the camera.The results also reveal that the flow field appears more chaotic and the mean velocity of liquid phase decreases because of the effects of particle swarms.The particle hydrodynamics in dense solid-liquid reactors(13.2 vol%and 17.6 vol%)is studied further based on the inline image velocimetry.At the same time,the method is extended to the measurement of the turbulent kinetic energy of particles.The experimental results demonstrate that the effects of particle swarm are obvious,and the mean velocity of the liquid phase decreases significantly in dense suspensions.For the instantaneous hydrodynamic characteristics,the distance between particles becomes shorter and the collision frequency increases.The collision in this situation is not binary collision only,but more collisions occur multi-particle ones.The inter-phase slip velocity decreases,which invalidates the Fajner's formula.On this basis,a new formula suitable for the predictions of slip velocity in dense suspensions is proposed.Furthermore,the turbulent kinetic energy of particles can be measured accurately,and the results show that the local turbulent kinetic energy does not change significantly with the increase of solid loadings.Due to the defects of existing drag models having complex form and poor applicability,a new total drag model is proposed based on the measurements of particle dynamics.The algorithms for calculating particle acceleration and concentration are added to the new inline image method,in order to obtain the total drag force accurately by measuring the local characteristics at non-steady state.Then a simple constitutive model is derived by using the Newton's second law,dimensionless correlation and least squares fitting.In comparison with the experimental results,it is found that the new model could provide excellent predictions with the particle loading from 4.4 vol%to 17.6 vol%.Compared with the Gidaspow model,the new model shows similar results of flow fields in numerical simulation.However,the new model exhibits obvious advantages in predicting local phase holdup and meeting mass conservation.The research object of the new method is extended to gas-liquid reactors,and the hydrodynamic behaviors with different bubble sizes in the reactors are studied.The results show that there is tiny difference between the tangential velocities for different bubble sizes.However,the axial velocity and bubble shape for different bubble sizes show obvious diversities.In addition,the difference of bubble sizes in the spatial distribution also reflects the diversities of trajectory.Thus,it is rational to group the bubbles with different size according to the diversities of dynamics behaviors.Finally,the bubbles are reasonably divided into three groups:G1(<0.6 mm),G2(0.6-2.2 mm)and G3(>2.2 mm).The simulation results using this grouping show obvious advantages in the predictions of axial velocity,tangential velocity and gas holdup,in contrast with the models of single bubble size and average bubble velocity.