Numerical Simulation Of Flow Hydrodynamics In The Bottom Zone Of A Circulating Fluidized Bed

Circulating fluidized bed (CFB) technology has been widely used in many industrial fields, such as coal gasification, fluidized catalytic cracking, etc. The hydrodynamics of gas-solid phase in CFB are complicated. Many researches pay attention to the hydrodynamics in the top section of CFB. There is little knowledge about hydrodynamics in the bottom section of CFB. Based on Eulerian two-fluid model, hydrodynamics in circulating fluidized bed of bottom section is numerically simulated by means of Fluent CFD software.The experiments were carried out in a cold model CFB. The fluidized bed has an inner diameter of 0.4 m and a height of 8.5 m. Quartz sand (mean diameter 170μm, particle density 2600 kg/m3) was used as the bed material. The fluidizing agent is ambient air. A fiber-optic probe was used to measure the local particle concentration of different cross-sections. While, the pressure signals were sampled and analyzed using the advanced measuring equipment consisting of differential pressure transmitter.Both simulation and experiments were performed at low gas velocity (1.0～2.5 m/s) and low solids mass flux (5.2～34.5 kg/(m3·s)). Laminar model was suitable for simulation of local particle concentration. Local solid concentrations of three cross-sections (H=0.27 m, H=0.33 m, H=0.42 m) in the bottom region were measured by a fiber-optic probe. The predicted radial concentration distributions by numerical simulation are in agreement with the experimental data measured from our circulating fluidized bed. It has been demonstrated that both riser section and bottom section have the core-annual structure. Furthermore, we find the dimensionless radium of r/R=0.8 is the cut-off point of wall zone and core zone.Meanwhile, different size CFB reactors were simulated in this paper. In the numerical simulations, it is hard to converge for the smallest diameter bed. Due to wall effect, for the smallest bed, the H/D is too large and the wall effect is so obvious which can always lead to slugging. When the diameter becomes larger the wall effect has weak influence on the hydrodynamics in the CFB.