The transition section between bottom dense zone and top dilute zone is important for fast gas-solids catalytic reactions

such as methanol to olefins(MTO).Solids concentration and fluctuation parameters in the transition section of a fluidized bed with 95 mm i.d.were measured by using reflective optical fiber probe(PV6D).The influences of superficial gas velocity and static bed height were investigated.An Eulerian-Eulerian two-fluid model that incorporated kinetic theory of granular flow and modified three-zone drag law was used to predict the flow structure in the turbulent fluidized bed.The axial and radial distributions of solids concentration were simulated by means of computational fluid dynamics(CFD).The experiments showed two axial profiles of solids distribution in the transition section:exponential and S-shaped.The maximum gradients of both axial and radial solids concentration profiles were located in the transition section

which meant that solids distribution in the transition section was much more non-uniform than that in the dilute and dense sections.The change of gas velocity and static bed height would cause a shift between exponential and S-shaped profiles

and significantly affect solids concentration in the dense region near the wall and at the bottom.Probability density distribution of local solids concentration revealed that when static bed height was small

the location of maximum voidage would gradually move from core region to annular region with increasing gas velocity

resulting in a bimodal distribution of voidage.A modified three-zone drag model was proposed by taking the effect of particle clustering into account for CFD simulation.The simulated solids concentration distribution agreed well with experimental data in the transition section except for the distributor zone.