液固流化床内固含率时空分布特性的CFD模拟

CFD simulation for spatio-temporal distribution of solid holdup in liquid-fluidized beds

采用Brandani等考虑拟平衡状态下颗粒与流体相互作用的双流体模型,通过在商业软件CFX4.4平台上增加用户自定义子程序模拟了高0.5m、宽0.1m的二维液固流化床内固含率的时空分布特性。为了保证数值模拟精度、节省计算机运行时间,首先确定了适宜的网格尺度、时间步长和收敛判据。随后,考察了液固两相物性和操作条件对流化床内固含率时空分布特性的影响,模拟结果表明:增大颗粒粒径或密度会使颗粒向下加速运动,导致床层高度下降而垂直方向上任一水平面的平均固含率呈现增大的趋势;减小液体黏度或密度则会使颗粒向下加速运动,导致床层固含率增大;突然增大液速会使颗粒向上加速运动,导致床层固含率减小;升高温度的实质是使液体的黏度和密度均呈现下降的趋势,结果使颗粒向下加速运动,床层固含率增大。上述模拟结果与颗粒受力的理论分析相一致。

Spatio-temporal evolution of solid holdup in the liquid-fluidized bed was predicted by Brandani and Zhang model,which includes additional terms in both the liquid-and solid-phase momentum equations based on the two-fluid theory by considering particle-fluid interactions under a quasi-equilibrium state.Numerical simulations were conducted in the platform of commercial Computational Fluid Dynamics（CFD）code,CFX4.4,by adding user-defined Fortran subroutines.Based on the independences of mesh,time step and convergence criterion obtained,the effects of the physical properties of liquid-solid system and the operational conditions on the solid holdup profiles were investigated numerically in the 0.5 m（long）×0.1 m（wide）2D fluidized bed.The computational results show that the solid holdup profile within the stable bed transfers to a new equilibrium state once the physical property of the system or the operational condition is changed.The bed surface decreases and the average solid holdups increase in all the vertical planes,due to the downward acceleration of particles,when particle density or size is increased.The similar phenomenon is observed with a decrease of liquid density or viscosity.When the liquid inlet velocity is abruptly increased,however,the motion of each particle accelerates upwardly and solid holdups decrease in the bed.Both liquid density and viscosity reduce with an increase of temperature,which leads to downward motion of particles and solid holdups increase.The above simulated data can be explained reasonably by the forces acting on particles.