Direct Numerical Simulation Study Of Flow Characteristcs And Drag Force In Gas-Solid Fluidized Beds

The flow in a gas-solid fluidized bed is a typical dense gas-solid two-phase flow.It's a key of numerical simulations of this kind of flow to accurately solve the four-way coupling interactions between the fluid and particles.Direct numerical simulations directly solve the governing equations on a fine mesh whose length scale is smaller than the particle sizes,and the no slip boundary conditions on the particle surfaces are well satisfied in this kind of motheds.Therefore,it has great calculation precision and can be well-suited for investigating the micro-scale fow mechanisms in fluidized beds.In addition,the database obtained from DNS can be used to verify and improve the traditional methods applied in the two-fluid method and the CFD-DEM method.The original immersed boundary method combined with multi-direct forcing scheme overestimates the effective hydrodynamic diameters of the particles.In the present thesis,this problem has been solved by retracing inward the Lagrangian grids.The new method can dynamically correct the effective hydrodynamic diameter of each particle in the flow based on its calculated particle Reynolds number and the computational grid size.It not only improves the calculation accuracy of the particle drag force but also captures the flow file more accurately.What is more,the modified method is found to be less dependent on the grid resolution,which set a foundation for simulations of large-scale particle-fluid systems.Subsequently,the new immersed boundary method has been combined with the soft-sphere collision model to deal with particle collision processes,and the scheme for simulating polydisperse particles has been also implemented.All these methods have been validated by a series of classical problem.The flow characteristics in fluidized beds have been studied by DNS.The particle motions,flow structures,particle mixing and bubble dynamics in both shallow bubbling bed and deep bubbling bed are investigated.Some obtained macroscopic fluidization phenomena agree well with the experimental measurements,and some microscopic flow characteristics which are hard to observe in experiments are found.Besides,we have performed direct numerical simulation of a laboratory-scale fluidized bed using the immersed boundary method and the solid velocity distributions are checked.The results are compared with experiments as well as CFD-DEM simulations.It suggests that the DNS is quite precise and performs better than CFD-DEM.The drag force in fluidized beds has been analyzied by using DNS database.The results indicate that the classical drag models capture the overall trend for the dependence of the drag force on particle Reynolds number and void.However,these models underestimate the drag force in most situations.The particle fluctuating Reynolds number and void fluctuation in fluidized beds are found to result in higher drag force than in fixed bed,but they are not the only extra factors.