Numerical Simulation Of Multiphase Fluid Dynamic In Air Dense Medium Fluidized Bed Based On Euler-Euler Model

Air Dense Medium Fluidized Bed (ADMFB) is a highly effective separation technique fordry coal separation, which provides a novel method and pattern for the development andapplication of the coal classification technique in arid and hydropenic regions of the world. It isfound that the dense gas-solid fluidized bed is a complicated gas-solid multiphase fluid systemin the research process of the air dense medium separation theory. Therefore, it is significantlynecessary to conduct the further research on the multiphase fluid dynamical behavior of thefluidized bed from the macroscopical and microcosmic view respectively. Based on theEuler-Euler multiphase fluid model, the combination methods of theoretical analysis,experimental measurement and numerical simulation were applied to carry out the studying offorce measurement of the separation particles, moving bubbles behavior, dynamical behaviorsof the medium solids and coupling interaction mechanism between gas and solid phases. It aimsto provide the theoretical foundation for the classification mechanism of the separation particlesby the density, synergistic mechanism of interaction between the medium solids and separationparticles to complete the fluidization classification theory, and to develop and improve thefluidized separator.The Euler-Euler multiphase fluid model was proposed to conduct the numerical simulationon the complex gas-solid multiphase dynamical behaviors in the fluidized bed. On the basis of alarge quantity of empirical models and theoretical inferences, the constitutive equations of thegas-solid drag coefficient, particle-particle interaction coefficient and solid stress were raisedrespectively to approach the governing equations, which laid the solid foundation for thefollowing numerical simulation.The methods of experimental measurement, numerical simulation and theoretical modelverifying were combined to investigate the bubble dynamical behaviors in the fluidized bed.The results show that the mean diameter of35mm<Db<49mm along the direction of bed heightsand the mean diameter of40mm<Db<61mm along the axial direction were obtained with thesuperficial gas velocity of1.5Umf≤U≤2.2Umf. The Rowe (1976) model was raised to predict themean diameter of the generated bubbles. Then the rising velocity calculation model of thesingle bubble was also proposed. The models provide the theoretical foundation for the researchon the movement rules of the separation particles in the fluidized bed and separation efficiencycaused by the bubble dynamical behaviors.The force property of the separation particle in the fluidized bed is the basis to achieve theefficient separation of the feeding materials by the classification density of the fluidized bed.Based on the analysis of the force on the spheroidal particle moving in the fluidized bed, the load balance equation and basic dynamical formula of the separation particle in the Air DenseMedium Fluidized Bed were established finally. The force measurement device of theseparation particle was self-designed. The experimental measurements of the forces on twospheroidal particles with different particle sizes were conducted in the fluidized beds withdifferent types of medium solids respectively. The results indicate that the forces on theseparation particles moving in the fluidized bed follow the steady variation rules. The variationrelationship between the mean values and standard deviations of the forces and gas velocitycould be represented by the certain standard functions or corrected functions. The remarkablequasi-periodicity laws of the successive forces on the separation particles were further analyzed.The fluctuant ranges of the force frequency and amplitude of the separation particle areobtained finally.The force fluctuation data of the separation particle in the fluidized bed was carried out thepiecewise fitting. The dynamical fitting equations of the instantaneous forces were established.On the basis of experimental measurement, numerical simulation and theoretical calculation,the force results of different separation particles were detailedly compared. The results showthat the mean values and fluctuations of the forces on the separation particle are nearly closewith the application with above three approaches. The achievements provide the necessaryprerequisite for seeking for the complex dynamic behavior of the separation particles withdifferent densities in the fluidized bed.The3D model of the fluidized bed in accordance with the experimental device wasestablished with the software of ICEM CFD and the meshes were divided accurately. Based onthe numerical calculation model established in the study, the joint methods of experimentalmeasurement and numerical simulation were adopted to carry out the qualitative andquantitative investigation on the bed pressure drop and density distribution. The results showthe variation law of the bed pressure drop and its standard deviation with the gas velocity hasno relationship with the variety of medium solids, which only depends on the value of U/Umf.The study results of the bed density distribution show that1.5Umf≤U≤2.2Umfis the optimaloperation range, which could provide the steady and uniform bed density distributionenvironment. At this moment, the time-average bed densities of different bed heights anddifferent axial directions mainly distribute the range of1.95~2.10g/cm~3, and the standarddeviation of bed density is less than0.20g/cm~3. The achievements have great promotion tocomplete the regulation mechanism of the pressure drops and bed density.Based on the experimental and simulation results of the pressure drop and bed density of thefluidized bed, the further numerical simulation of the gas-solid multiphase fluid dynamicalbehaviors in the fluidized bed was carried out. The gas phase and the dense phase of medium solids both keep uniform and steady distribution condition after the stable fluidization. From themacroscopical view, it appears the favorable bed activity, stable sectional pressure drops anduniform bed density distribution of the fluidized bed. From the microcosmic view, it appears thefavorable connectivity of the gas phase in the fluidized bed, and the solid phase mainly presentsthe steady circumfluence, and elevation and subsidence movement trace and velocity variationrules. It not only ensures the steady flow of the fluidized bed, but also provides the stable anduniform bed density environment for the classification of the separation particles. The studyingresults have certain reference and significance on the distributor design of the fluidized bed andthe selection of medium solids.Based on the numerical calculation model and the self-designed acquisition system of thebubble movements and force measurement device of the separation particle, the bubbledynamical behavior, the force property of the separation particle, the fluctuation of pressuredrop and bed density distribution, gas phase distribution and movement laws of medium solids,synergistic mechanism of interaction between the medium solids and separation particles werefurther studied in the research. It aims to play the important effort on the basic separationtheories of the fluidization and the improvement and regulation mechanism of the fluidizedclassification equipments.