Effect of vibration on the stability of a gas-fluidized bed of fine powder

Abstract

We have investigated the effect of vibrations on the stability of gas-fluidized beds of fine powders (particle size ~10 mm). The powder is uniformly fluidized by an adjustable gas flow that enables us to control the average solid volume fraction f0. The fluidized bed is then subjected to a vertical oscillatory motion of controlled amplitude and frequency. The response of the fluidized bed depends essentially on the value of f0. For f0.0.28 the fluidized bed is in a weak solidlike regime, it has a mechanical strength, and particles are static. In this regime vibration causes compaction of the loosely packed bed. For f0,0.28 the mechanical strength vanishes and stresses are carried by interstitial gas and collisions. In this fluidlike regime the fluidized bed displays a diffusive dynamics and particles aggregate due to the strong interparticle van der Waals forces. When vibration is applied the powder expands due to the partial disruption of aggregates. However at a critical value of the vibration amplitude A5Ac either surface ~sloshing! or flow ~bubbling! instabilities develop. The nucleation of gas bubbles has been correlated to the saturation in particle diffusivity measured elsewhere. The size of the bubbles increases as A is further increased above Ac and as the vibration frequency is reduced. Moreover, as it should be expected from the predictions of hydrodynamic models, Ac is independent of cohesivity for particles of the same size and density.