Fine cohesive powders in rotating drums: Transition from rigid-plastic flow to gas-fluidized regime

We investigate the dynamics of fine cohesive powders inside rotating drums. We show that these powders may be fluidized due to entrapment of ambient gas, and we determine the onset of fluidization. Experimental measurements on the bed expansion as a function of the rotation velocity have been performed. Drums of different diameters and fine powders of varying cohesiveness have been tested. We show that (i) fine powders transit directly from a rigid-plastic state to a gas-fluidized state in accordance with the flow regime boundaries predicted elsewhere [A. Castellanos et al., Phys. Rev. Lett. 82, 1156 (1999)], (ii) the onset of fluidization in the rotating drum is determined by the ratio of the powder kinetic energy per unit volume to its tensile strength, and (iii) once the powder is completely fluidized the average interstitial gas velocity increases proportionally to the rotation velocity. The last two results imply that the required velocity to fluidize a powder, omegaR (omega angular velocity, R radius of the drum), must increase as the square root of its tensile strength, and this has been confirmed by independent measurements and estimations.