Flow and force analysis on the formation of expanded beds in gas fluidization of fine ellipsoids

A flow regime called expanded beds in gas fluidisation exists for fine powders. Some attempts have been made in the past to analyse the formation mechanism of expanded beds, indicating that the essential condition for an expanded bed to exist is that the van der Waals force balances the contact force and the particle-fluid interaction force balances the weight of the bed at bulk scale. However, how particle shape affects the expanded bed formation is still not clear. In this work, CFD-DEM approach is applied to elucidate the particle-particle and particle-fluid interaction forces on the dynamic formation of expanded beds with fine ellipsoids. The results illustrate that at the start-up stage, the bulk disconnects into two major bulks with a lowest contact force on the axial direction, especially for fine ellipsoids. Spheres have a smoother expansion process than ellipsoids, and ellipsoids need longer start-up time. A slower gas introduction process has a more moderate change in bed height. The fractures gradually disappear when the cohesive forces become smaller. At the transition stage, when the aspect ratio deviates from 1.0, the pressure drop fluctuation significantly increases. At the stable stage, when the aspect ratio decreases or increases from 1.0, the contact forces and van der Waals forces increase, and the distributions of particle-fluid interaction forces become less uniform in the beds. Ellipsoidal particles undergo an orientation rearrangement during the expansion process.