关键词:

流化床, 颗粒混合, 气泡运动, 颗粒横向扩散系数

Abstract:

The dynamic processes of solids mixing were obtained from the CFD-DEM simulations for two cases at different fluidizing velocities.The mechanisms of solids mixing in view of the particle and bubble scales were examined, and it was established that bubble movement through the bed, bubble burst at the bed surface and emulsion flowing downward induced by bubble rise played crucial roles in solids mixing.The kinetic theory based two-fluid model was used to simulate 2-dimensional fluidized beds with the width of 0.2 m, 0.4 m and 0.8 m,respectively, covering regimes from bubbling to turbulent fluidization.Based on the simulation results from the two-fluid model, ideal tracing particles were used to track the solids phase movement, then according to these trajectories the average solids dispersion coefficients were computed.The results indicated that the lateral dispersion coefficient (D_x) increased with the increase of gas velocity, but it was greatly limited by the bed size.The axial dispersion coefficient also increased with the increase of gas velocity, yet was weakly limited by the bed size.The computational results agreed in the same order of magnitude with the values of D_x reported by different authors in the range of 10^-4—10^-1 m²·s^-1 orders.The prediction established that the values of D_x in larger fluidized beds at a higher fluidizing velocity was much larger than that in small bubbling beds, which explained the discrepancy between the experimental values of D_x in the literature and the methodology for scaling-up solids dispersion rate in industrial fluidized beds.

Key words:

流化床, 颗粒混合, 气泡运动, 颗粒横向扩散系数