2026/07/11 by František Vondráček, Frantisek Vondracek, Tereza Trávníčková +8
Engineering · #Granular flow and fluidized beds #Heat and Mass Transfer in Porous Media #Particle Dynamics in Fluid Flows
paper · doi:10.1016/j.powtec.2026.122923
Friction-controlled size segregation is investigated in a vertically stirred bidisperse granular bed (2:1 size ratio) using discrete element method simulations and experiments. The sliding friction coefficient is varied over μ = 0.1 –0.9 at fixed impeller speed for two initial configurations. Increasing friction redistributes motion from azimuthal co-rotation toward radial–axial (meridional) transport, thereby intensifying secondary flow. Two trapping regions govern large-particle localization: a lower-bed toroidal band at intermediate radii and an upper near-wall annulus beneath the free surface. Both structures are present in the flow topology across the investigated friction range; friction controls which structure becomes accessible and populated, rather than creating or eliminating the structures themselves. Three regimes emerge: at low friction ( μ ≈ 0.1 ), meridional circulation is too weak to populate either trap and large particles remain nearly homogeneously distributed in the lower bed; at intermediate friction ( μ ≈ 0.2 –0.3), the lower trap fills while the upper remains inaccessible; at high friction ( μ ≳ 0.4 ), the upper trap dominates. Secondary flow governs transport to both traps. The lower trap forms because access to the dilated wake behind the blade is size selective, excluding large particles. Transport to the upper trap follows an indirect route: shaft-adjacent upflow delivers particles to the free surface, where centrifugal force and surface slopes carry them toward the wall. The transition between regimes reflects both the intensified secondary circulation and the change in free-surface topology from a funnel to localized heaps. Initial placement affects only transient kinetics; both configurations converge to the same friction-controlled attractor.