Experimental And Modeling Study On Particle Cluster Behavior In The Fast Fluidized Bed

Fast fluidized bed(FFB)is widely used in coal combustion and gasification,fluid catalytic cracking and Fischer-Tropsch synthesis due to its high efficiency in gas-solid reaction,heat and mass transfer.Researchers have conducted a series studies on basic theory and gas-solid flow characteristics of fast fluidized bed.However,the lack of understanding on meso-scale structure,e.g.cluster behavior in fast fluidized bed,especially for handling Geldart B particles,results in difficulties in design and optimization of fast fluidized bed systems.Therefore,the cluster behavior in fast fluidized bed is studied by experimental and numerical simulation method in this paper.A three-dimensional visual fast fluidized bed with riser height H=3 m and diameter D=0.1 m was established.The influences of operating conditions,including static bed height and superficial gas velocity on flow structure in fast fluidized bed were comprehensively studied.Five flow patterns in fast fluidized bed were carefully identified,and the criterion and typical flow structures were proposed for these flow patterns.The influences of fluidized air velocity and solid mass flux on solid volume fraction and gas-solid slip characteristics were also studied,revealing the relationship between the flow characteristics and operating conditions.Moreover,based on the image and pressure analysis,typical flow patterns in high speed fluidization were observed,and the fitting transition velocities for these patterns were developed.The cluster measurement system consists of cluster visualization system and fast fluidized bed system was established.A self-developed MATLAB program was developed to identify and measure particle clusters.Four kinds of particle cluster in the riser were observed;the characteristics,the formation and evolution process for different particle clusters were revealed.Meanwhile,the effects of operating conditions and particle properties on particle clusters properties were studied.Based on the multi-phase particle-in-cell(MP-PIC)methodology,three-dimensional Eulerian-Lagrangian numerical model,boundary conditions and numerical method were developed for fast fluidized bed handling Geldart B particles.The sensitivity of some crucial modeling parameters,including grid resolution,the number of particles per parcel,the drag force model and the particle close pack volume fraction have been systematically analyzed,and optimized.The full-loop numerical simulation of three-dimensional fast fluidized bed,including riser,two-level cyclones,donwer and J-type back-feeder was acheived.The gas-solid flow characteristics of fast fluidized bed under different operating conditions were obtained by numerical simulation method.Three typical patterns of particle cluster in three-dimensional fast fluidized bed riser were captured by simulation method,and the cluster properties,i.e.solid volume fraction and velocity of particle cluster in the riser were revealed,further illustrated the evolution process of particle cluster in the fast fluidized bed.