(7b) Exceptional Intruder Sphere Sinking Due to Local Fluidization of Apparently Fixed Powder Bed

It is well known that a powder bed can be fluidized when the velocity of an airflow through the bottom of the bed is higher than the minimum fluidization velocity u_mf, and that the powder bed is a fixed bed when the air velocity is smaller than u_mf. The fluidized bed has liquid like properties such as density and viscosity. When an intruder sphere is put into the fluidized bed, the sphere floats or sinks based on the difference between the fluidized bed density and the sphere density. The float-sink can be utilized for density separation; the continuous separators are commercialized for waste treatments in Japan. Fundamentals of the fluidized bed, such as properties of air bubbles and convective flow of particles, have been widely investigated by many researchers. Generally, of course, the air velocity u₀ > u_mf is employed for the fluidized bed research. We have been focusing on the spheres’ float-sink in the fluidized bed. During the float-sink experiments, we accidentally found that spheres can sink even if u₀ is smaller than u_mf. Therefore, we investigated the sphere sinking in the powder bed for u₀ < u_mf. We varied the air velocity ratio u₀/u_mf and the density ration ρ_sphere/ρ_bed (sphere density/powder bed density). The spheres sinking was observed in the range 0.83 < u₀/u_mf < 1.0, indicating that the powder bed behaves as a solid for u₀/u_mf ≤ 0.83 and as an apparently fixed bed for 0.83 < u₀/u_mf < 1.0. The final depth of sphere sinking increases with increasing the sphere density in the range 1.3 < ρ_sphere/ρ_bed < 2.6. However, surprisingly, smaller density spheres sink “deeper” than larger density spheres in the range 1.0 < ρ_sphere/ρ_bed < 1.3. We recorded the state of the top of the power bed by a video camera. When ρ_sphere/ρ_bed < 1.3, air bubbles are observed to percolate from the powder bed surface at the location of the sinking sphere, whereas for ρ_sphere/ρ_bed ≥ 1.3, such percolation bubbles are not present. The rising air stream can cause local fluidization around the sphere when the sphere density is close to the powder bed density. As a result, air voids can then form underneath the sphere and migrate to the powder bed surface where they are observed as emerging percolation bubbles. As the sphere density ρ_sphere/ρ_bed decreases from 1.3 to 1.0, the decreasing weight of the sphere allows larger regions of local fluidization to develop. This creates larger voids that allow the sphere to sink deeper in spite of the decreasing sphere density.