Three-dimensional DEM-CFD simulation of a lab-scale fluidized bed to support the development of two-fluid model approach

The present work is dedicated to the numerical study of the hydrodynamics of a pressurized fluidized-bed using an Euler-Lagrange approach, with the goal to gain insight into the Two-Fluid Model (TFM) approach. The gas phase is modeled by filtered Navier-Stokes equations, and the solid particles are tracked using a Discrete Element Method (DEM). Collisions are handled using a soft-sphere model. Numerical predictions of the mean (time-averaged) vertical particle velocity are compared with experimental measurements available from the literature, obtained from a Positron Emission Particle Tracking (PEPT) technique. In addition, DEM -Computational Fluid Dynamics (CFD) results are extensively compared with predictions from TFM numerical simulations. Results accounting for inelastic frictionless particle-particle collisions show a very good agreement with the experimental data and TFM results in the central zone of the reactor. In the near wall region the numerical simulation overestimates the downward particle velocity with respect to the experimental measurements, especially when the particle-wall friction is neglected. The influence of the friction at the wall is therefore further investigated and a local analysis of the particle-wall interactions is carried out. It is demonstrated that the long sustained contacts of particle assemblies with the wall in such a dense regime play a crucial role on the overall bed behavior. Therefore, it is recommended that this effect is taken into account in the boundary conditions of a TFM approach when it is used to predict bubbling fluidized beds.