Limitations on fluid grid sizing for using volume-averaged fluid equations in discrete element models of fluidized beds

Bubbling and slugging fluidization was simulated in 3D cylindrical fluidized beds using a discrete element model with computational fluid dynamics (DEM-CFD). A specialized CFD grid was used in which the volume of all fluid cells in the grid was kept equal. Ninety total simulations were conducted with different fluid grid cell lengths in the vertical (dz) and radial (dr) directions in order to discern at what fluid grid sizes, as compared to the particle diameter (d_p), the volume-averaged fluid equations broke down and the predictions became physically inaccurate. Simulations were compared with experimental results for time-averaged particle velocities as well as frequencies of pressure oscillations and bubble eruptions. The theoretical predictions matched experimental results most accurately when dz = 3-4 d_p, with physically unrealistic predictions produced from grids with lower dz. Within the valid range of dz, variations of dr did not have a significant effect on the results.