Simulation Of Circulating Fluidized Beds And Research On Transfer Coefficient-based Structural Parameters Method

A Circulating fluidized bed(CFB),which has the high flow rate of gas-solid and intensive gas-solid contacting,has been widely used in various industrial processes.Characterized by complex heterogeneous meso-scale structures in the CFB,computational fluid dynamics(CFD)has been a valuable tool to predict the fluid dynamics.The gas-solid drag model is believed to be one of the key factors to capture the characteristics of fluidized beds.Conventional drag models could not correctly simulate fluidized beds because meso-scale structures are neglected.Therefore,developing structure-based drag models has received much attention,and the key to resolve the drag model is to resolve structural parameters.Our group has developed the method where incorporates with reliable classical equations to close the insufficient solving equations.The method has been successfully applied for the bubbling fluidized bed(BFB)and turbulent fluidized bed(TFB),but not used for the CFB yet.Beisides,structure-based drag models contain many structural parameters,but the effect of various structural parameters on the drag force is unclear.This weakens the modification of drag models.Aiming at these problems,firstly,the structural parameter model of the riser is established incorporating with the equations of clusters properties.Secondly,the transfer coefficient-based structural parameters(TC-SP)method is proposed,and the sensitivity of structural parameters is analyzed.Then,this method is applied for the BFB and TFB.Besides,this thesis presents an approach where validates the drag model of the multisolid CFB by the multisolid pneumatic fluidized bed(MPTB).Lastly,the method of establishing the drag model of the riser is used for the downer reactor.The principal results and major conclusions are as follows:(1)Equations of clusters properties,including the cluster voidage and the cluster diameter,are firstly adopted to close the insufficient solving equations,and a new structural parameter model of risers is established.The model can correctly resolve the drag force coefficient of risers.The simulated solid mass flux,radial and axial voidage profiles are in reasonable agreement with the experimental data.The dilute-top/dense-bottom and the core-annular flow structure are also captured.Moreover,the spatiotemporal fluctuation of clusters can be observed from those simulations.By analyzing the averaged drag coefficient of the CFB,the transfer coefficient-based structural parameters(TC-SP)method is proposed to simplify the structural parameter model.(2)Based on the TC-SP method,a quantitative sensitivity analysis of the structural parameters is performed.It is found that since the drag force in the cluster phase is extremely larger than that in the dispersed phase,the drag coefficient is more sensitive to the parameters in the cluster phase than those in the dispersed phase.Therefore,the structural parameter model and drag coefficient based on the original TC-SP method can be further simplified which relies mainly on the cluster phase.The simplified TC-SP model and the other structure-based drag model are compared.Interestingly,despite with half number of parameters,the simplified TC-SP model can achieve same or even better simulation results than other structure-based drag model with more parameters.It indicates that the drag force in the cluster phase plays a dominant role in the total drag force.(3)The TC-SP method for the CFB is applied for the BFB and CFB.By analyzing the sensitivity of the structural parameters,it is found that the drag force in the emulsion phase for the BFB and the cluster phase for the TFB plays a dominant role in the total drag force,respectively.Then,the simplified TC-SP models are compared with other structure-based drag models,and the simulated results also show that the simplified TC-SP models can achieve same or even better simulation results than other structure-based drag models with more parameters.Therefore,for the three typical fluidized beds(BFB,TFB and CFB),the drag coefficient in the dense phase(emulsion phase or cluster phase)is the key to resolve the total drag coefficient.(4)This thesis uses a new pseudo-homogeneous method and the simplified TC-SP model of the CFB to simulate the coarse particle terminal velocity and fine particle holdup in the MPTB.These results demonstrate that the multisolid pneumatic transport system can serve as a simple binary system,with its coarse particle terminal velocity and fine particle holdup being used for the verification and validation of binary-particle CFB models.(5)A structure-based drag model for downers is developed by the similar method for establishing the model of the riser reactor.Meanwhile,the effects of clusters on the drag force are investigated.By analyzing the effects of variations of the cluster voidage and diameter on the drag force,it is found that,unlike risers,the cluster voidage and diameter have a negligible effect on the drag in downers.Previous studies are limited to simulate the radial performance in downers because of using the conventional drag model.The simulation results of the new structure-based drag model show reasonable agreements with the experimental data in the radial and axial direction.The unique trends of downers are also obtained,e.g.,the dense-ring flow structure,axial velocity flow structure and pressure distribution.