Experimental study on the dynamics of gas-fluidized beds

Abstract

Gas-fluidized fine powders display three regimes of fluidization: solidlike, fluidlike, and bubbling. We investigate, from both macroscopic and local measurements, the transition between these regimes. We show that the transition between the solidlike and the fluidlike regimes takes place along an interval of gas velocities in which transient active regions alternate with transient solid networks. Although in the apparently homogeneous fluidlike regime large amplitude bubbles are not perceived and the bed expands continuously with increasing gas flow, optical probe local measurements show the existence of mesoscale pseudoturbulent structures and short-lived voids, reminiscent of liquid-fluidized beds behavior, and whose characteristic temporal frequency increases with gas velocity. These mesostructures might be responsible for the fast diffusion measured in gas-fluidized beds.

The Sevilla Powder Tester: A Tool for Characterizing the Physical Properties of Fine Cohesive Powders at Very Small Consolidations

“…To homogenize the sample, the bed is driven into the freely bubbling regime by imposing a large gas f low. Once in the bubbling regime, the bed loses memory of its previous history [31]. In the case of highly cohesive powders, such as toners with very low additive concentration, it is necessary to shake the sample in order to break up channels that conduct f luidizing gas away from the bulk of the powder and prevent f luidization.…”

“…This Powder Tester enables us to investigate fluidization properties such as bed expansion, sedimentation, aggregation of fine particles within the fluidized bed, onset of bubbling for sufficiently large gas f low, transition to a solid-like regime for gas flows smaller than a critical one, etc [5]. For the solid state, in which particle contacts are permanent, the tester measures the uniaxial tensile yield stress of the powder and its average particle volume fraction as a function of the consolidation stress, which can be controlled in a range spanning from a few to thousands of Pascal by means of forward and reverse controlled gas f low.…”

Jamming Threshold of Dry Fine Powders

“…2 of Ref. [21]). A similar behavior has been reported for the gelation transition of colloids, the colloidal glass transition [4], and the melting transition of foams [9].…”

“…The average is obtained from the bed height measured by an ultrasonic pulser receiver at the top of the vessel. As a previous step we impose a gas flow sufficiently large to drive the bed into the bubbling regime, in which the powder loses memory of its previous history [15]. Then the gas flow is decreased and p and measured.…”

“…In this fluidlike regime primary particles are clustered due to attractive forces and clusters undergo an intense hydrodynamic interaction with the gas that prevents the development of macroscopic gas bubbles [18]. The number of particles per cluster (N) and the ratio of cluster size to d p (k) are obtained by means of sedimentation experiments (see Ref.…”