Self-Diffusion in a Gas-Fluidized Bed of Fine Powder

Abstract

We have investigated the self-diffusion in a stable gas-fluidized bed of fine powder. Two regimes have been observed: for gas velocities $v_g$ above the minimum fluidization velocity $v_m$ and below a critical gas velocity $v_c$ smaller than the minimum bubbling velocity $v_b$ the powder does not mix. Experimental measurements show the existence of yield stresses in this regime which are responsible for the static behavior of the bed. For $v_g>v_c$ the yield stress vanishes; the bed behaves like a fluid and displays a diffusive dynamics. In this region we have found that the diffusion coefficient $D$ increases with gas velocity until the bed expansion approaches its maximum value.