EMMS-based Phase Diagram For Gas-Solid Generalized Fluidization And Its Application

Gas-solid fluidized beds operate in different ways such as concurrent upward,concurrent downward and counter-current downward modes,which can be called gas-solid generalized fluidization.No matter in which mode,both axial and radial heterogeneities are always displayed.In gas-solid concurrent upward flow,there are typical regime transitions such as bubbling,turbulent,fast fluidization and pneumatic transport with the increase of gas velocity.The transition from fast fluidization to pneumatic transport is accompanied by a sudden change in bed structure,which is called choking.In gas-so lid counter-current downward flow,the downward movement of particles can be prevented and even blown out from the reactor top if the gas velocity or solids flux is high enough,which is called flooding.The above-mentioned hydrodynamic features can be systematically described by using the so-called fluidization phase diagrams,which is of great theoretical and practical significance for the design and operation of fluidized reactors.However,current phase diagrams are generally drawn according to either experimental data or empirical correlations and mainly focused on the regime transitions in concurrent upward flow,failing to reflect the features of choking and flooding completelyGas-solid fluidization as a kind of nonlinear and non-equilibrium system exhibits typical dynamical meso-scale structures of particle clusters or gas bubbles.The Energy-Minimization Multi-Scale(EMMS)model has successed in predicting the choking as well as other regime transitions in concurrent upward flow,and has been extended to bubbling fluidization and downer systems.In this thesis,the EMMS-based counter-current model was used to predict the flooding phenomenon,which was verified further by using a computational fluid dynamics method coupled with a discrete element method(CFD-DEM).Finally,the phase diagrams of generalized fluidization for Geldart A and B particles were redrawn according to the calculation results of the EMMS model and its extensions.The main contents of this thesis are listed as follows:The EMMS model was first extended to predict the axial hydrodynamic parameters and the flooding phenomenon and clarify the effects of operating conditions,particle properties,wall-friction and entrance voidage in counter-current downward flow.It was found that with the increase of gas velocity the voidage(?full)in the fully developed region of counter-current downward flow decreases slightly at first,but then decreases significantly,and remains nearly invariable finally until the model cannot be numerically solved,showing an S-shaped profile.Recognizing this,a reasonable method was proposed to predict the flooding phenomenon.In order to validate the foregoing prediction,the regime transitions in generalized fluidization were analyzed by using the CFD-DEM simulation and the steady-state EMMS model.The two models predicted consistent results qualitatively.For counter-current downward flow,the CFD-DEM simulation validates the prediction of the EMMS-based model in the axial heterogeneity and the flooding feature.Based on the both predictions,the physical meaning of flooding was explored qualitatively.Meanwhile,both methods give good predictions for saturation carrying capacity at the choking state in concurrent upward flow.Moreover,the CFD-DEM simulation provides the details of the coexistence of bottom dense region and top dilute in the axial direction and the core-annulus structure in the radial direction,which are inaccordance with well-accepted experimental data.Finally,combining the calculation results of the original EMMS model and its extensions,the EMMS-based phase diagram of generalized fluidization was drawn to describe the variations of voidage,flooding and choking gas velocities with operating conditions under the various operation modes.However,the EMMS-based phase diagram was calculated for specific material properties,and did not capture the influence of variable material properties in actual systems.Therefore,some typical hydrodynamic parameters in the phase diagram were correlated to both operating conditions and material properties in the form of dimensionless groups,so as to broaden the application range of the EMMS-based phase diagram of generalized fluidization.This research is helpful to extending the application of the EMMS model in the steady-state simulation of gas-solid flow,and of great theoretical and practical significance for the operation and control of gas-solid fluidized systems under different operation modes.