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An estimation of the dominant stresses in powders with or without
interstitial gas flow yields a semiqualitative criteria for the transition
between flow regimes (plastic-fluidized, plastic-inertial, plastic-suspension,
inertial-fluidized, inertial-suspension). It will be shown that, at ambient
pressure, fine powders (particle diameter less than ~ 100 () experience a
direct transition from plastic to fuidized regime, or from plastic to
suspension. The inertial regime is absent in these fine powders. These ideas
will be applied to understand the onset of fluidization in rotating drums.
Previous measurements with xerographic toners (particle diameter around 10 (m)
in rotating drums indicated that fluidized and plastic regimes coexist in the
drum. The extent of the fluidized region grows when angular speed is
increased. Here, we present measurements on the width of the horizontal
surface of the fluidized zone as a function of rotation velocity, drum
diameter and toner properties. We find that the results depend on the product
of three fundamental parameters: (i) the ratio of the fluidized toner bulk
density to the packed toner bulk density; (ii) the ratio of centrifugal
acceleration to the acceleration of gravity and (iii) the ratio of
gravitational potential energy per unit volume (or equivalently, kinetic
energy per unit volume) to the powder tensile strength.
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