Mesoscale Model Of Gas-Liquid-Solid Three-Phase Flow System

Gas-liquid-solid fluidized beds are widely used in process industry.However,the complexity of the internal multiphase flow structures restricts its scientific design,amplification,optimal operation and effective control.Thus,introducing a new theory or method to model such a reactor is necessary.In this dissertation,based on the Energy-Minimization Multiscale(EMMS),the mechanism models describing the global,axial and the radial of multiphase flow behaviors in the gas-liquid-solid fluidized bed have been established,and experimental verification and prediction studies have been carried out.Research contents and conclusions are as follows:1)An improved global mathematical model on multi-phase flow behavior has been established by introducing the acceleration of bubbles and solid particles in the gas-liquid-solid fluidized bed.Considerations of the accelerations of bubbles and solid particles allow this model to predict the slip velocity between the phase interfaces and the bed shrinkage phenomena more accurately.Under the conditions of pseudo-steady state,the acceleration of bubbles varies with operating conditions and that of solid particles is 0 m/s~2 at lower gas velocity;whereas the opposite is true at the higher gas velocity.In the non-steady state,small bubbles with large velocity and acceleration,and large bubbles of lower velocity and acceleration will axially aggregate.This model is applicable to the dispersed and coalesced bubble flow regimes in the gas-liquid-solid fluidized beds with small(or light)solid particles.2)Based on the entrainment phenomenon of solid particles by the bubble wake in the gas-liquid-solid expanded bed,an axial meso-scale multi-phase model has been developed.Model demonstrates that the entrainment of the particles by the bubble wake is the formation mechanism of axial non-uniform flow structures.The wake shedding length,the volume ratio of the bubble-wake to the bubble,and the ratio of solid holdups of the bubble-wake phase to the liquid-solid phase vary with the flow conditions.In the operation range of gas and liquid velocities that promote gas bubble growth,length of the transition section increases as these two velocities increase.Also,as the liquid velocity increases,starting point of the transition section moves upward.3)Considering the flow behavior of particle-cluster,an axial meso-scale flow model of gas-liquid-solid circulating fluidized bed has been built.For the first time,the model successfully predicts the transition points between expanding fluidized mode,circulating fluidized mode,and liquid transporting bed mode.Acceleration of the particle-cluster phase depends on the operating conditions,and its maximum value is an indicator to a change in the length of the transition section.At the point of the saturated entrainment of particles,the axial distribution of solid holdup presents in‘S'shape with a dense bottom-dilute top structure;and the starting point of the dilute phase depends on the operating conditions.The particle-clusters gradually form as the bubbles grow.In the pseudo-steady state,acceleration of particles in the liquid-solid suspension tends to be-9.8 m/s~2,and that of the bubbles is 0 m/s~2.4)Based on the radial distribution of liquid shear stress,a radial meso-scale flow model of gas-liquid-solid fluidized bed has been established,which can predict the radial annulus-nucleus structure.Operating conditions affect the radial position of annulus-nucleus interface,maximum velocity and diameter of bubbles.The acceleration of the solid particles is 0 m/s~2.The suspending and transporting energy consumed per unit mass of solid particles can be used as an indicator of radial position of the annulus-nucleus interface.