3. Gas velocity at fluidization threshold. Bed expansion correlations

Consider a thick stack of divided solids at rest, resting on a horizontal, porous plate. This forms a fixed bed of interstitial porosity ε ₀ , cross-section A and height H ₀ .

If, after crossing the support plate, an upward gas flow is imposed through the bed, the vertical aeraulic force imposed increases with the gas flow rate:

Above a certain flow rate, this force can compensate for the apparent weight of the particle bed, the inter-particle distance increases and the bed relaxes, its thickness becoming H > H _0...