Multi-scale Modeling Of Hydrodynamics,Mixing And Reactions In Fluidized Beds

As an important industrial reactor,the fluidized bed has been widely used in the fields of energy,chemical engineering,metallurgy,etc.The fluidized bed is a typical application of dense gas-solid reactive flows,in which various flow regimes and multi-scale flow structures exist.In the fluidized bed process,there are many parameters(e.g.,operating parameters,solid properties,geometrical factors)to be handled,accompanying by the coupling of multi-field(e.g.,flow field,temperature field,reaction field and electric field),making it a non-linear,transient and multivariate coupling system.Compared with experimental measurements,numerical simulations are relatively inexpensive and can be used to efficiently assess the design variations and produce detailed information of gas-solid flows.Therefore,the numerical simulation has been increasingly used to investigate fluidized beds.Nevertheless,the reliability of modeling methods still needs improvement and more comprehensive investigations are required to better understand the gassolid flows in fluidized beds.To this end,this thesis aims to develop multi-scale modeling methods for dense gas-solid reactive flows.Specifically,the CFD-DEM,coarse-grained CFD-DEM,MP-PIC and TFM are extended to handle fluidized bed reactor with heat transfer and chemical reactions.These extended methods are validated against experimental measurements of various fluidized beds.Based on these methods,investigations on the gas-solid hydrodynamics,heat transfer,chemical reactions,mixing,meso-scale structures are performed:(Ⅰ)Literature review.A systematic evaluation is conducted on the accuracy and efficiency of various modeling methods and sub-models under different fluidization conditions(flow regime,solid properties and geometrical configuration).(Ⅱ)Cold-flow fluidized beds.Numerical investigations on bubbling and circulating fluidized beds are performed,with emphasize on the characteristics and influences of meso-scale structures(gas bubbles and particle clusters).For the bubbling bed,the effects of pressure on the tube erosion are demonstrated.For the circulating bed riser,the mechanisms of temporal evolution of the cluster are revealed.The results underscore the important role of meso-scale structures in fluidized beds.(Ⅲ)Lab-scale fluidized beds with heat transfer and/or chemical reactions.(a)The effects of particle collisional parameters on gas-solid hydrodynamics and heat transfer in a spouted bed are studies and the underlying controlling mechanism is revealed.(b)The influence of particle shrinkage on the biomass fast pyrolysis process in a bubbling bed is investigated.Also,the differences of predictions by Eulerian-Eulerian/Eulerian-Lagrangian methods are studied.(c)Simulations are conducted to investigate the effects of operating parameters(particle size and bed temperature)on the coal gasification in a bubbling fluidized bed.(d)The influences of secondary gas injection on the coal combustion in a bubbling fluidized bed are examined,and new insight on the mechanism of hot spot formation is provided.The results of above studies highlight the strong interactions among gas-solid hydrodynamics,heat transfer and chemical reactions.(Ⅳ)Commercial-scale fluidized bed reactor.A 300 MW circulating fluidized bed combustor is simulated.The interactions of gas-solid hydrodynamics,heat transfer and combustion are studied and the influence of coal feeding pattern is investigated.