On the Mechanism of Anomalous Sphere Sinking in Apparently Fixed Particle Beds (Discrete Particle Simulation and Ultra-Fast MRI Measurement)

It is well known that a particle bed behaves as a fluid in the case it is aerated with gas velocity higher than the minimum fluidization velocity Umf. This is called as a fluidized bed and widely used in a large number of industrial processes. Under Umf, on the contrary, gravity force working on a particle overcomes fluid force and a particle bed behaves as a solid. Oshitani et al. (Phys. Rev. Lett., 116, 068001, 2016) reported anomalous sinking of intruder spheres in granular beds aerated under but close to Umf. They observed deep and slow sinking of spheres in the case density of sphere is close to the bulk density of the bed. They attributed this phenomena to local fluidization around the spheres while its detailed mechanism was not clear due to opaque nature of granular beds. In the present study, anomalous sinking and stopping mechanism of intruder spheres in an apparently fixed bed aerated just below Umf were explored and revealed in detail by means of discrete particle simulation and ultra-fast MRI measurement. Our numerical simulation and MRI measurement enable us to observe detailed flow dynamics and forces working on the spheres with sufficient spatial and temporal resolutions. Fluid force, contact force from surrounding particles and the gravity force working of the sphere are almost balanced in the case sphere density matches with the bulk density of a bed and small deviations from the equilibrium make slow sinking. The deviation is due to the detachment of small bubbles from the sphere surface. Bubble detachment from the sphere surface loosens local gas pressure gradient that supports the sphere weight and the sphere falls at the time.