The rise of a buoyant sphere in a gas-fluidized bed

The rise velocity at incipient fluidization, Vmf, is governed, for both types of bed, by the apparent viscosity of the incipiently fluidized bed. Therefore, Stokes's law was used to predict Vmf, but using an important modification: since each buoyant sphere appears to carry on its top a defluidized 'hood' of particles, Stokes's law was applied to the composite 'particle' consisting of the sphere plus its hood. Analysis of the measured Vmf then gave the volume of the hood, in agreement with direct measurements of it above a fixed cylinder in a two-dimensional bed. In addition, the analysis gave the apparent viscosity of the incipiently fluidized bed to be 0.66 Pa s, in excellent agreement with the estimate of Grace (Can. J. Chem. Eng. 48 (1970) 30) for similar sand.