Fluidization of a collection of granular particles immersed in an upward gas stream is a physical system which reveals many interesting phenomena such as segregation of particles, density waves, convection cells, and some others. These phenomena appear as a consequence of unusual, and often complex, dynamical response of the system. As the mean velocity of the gas stream increases above a minimum value U{sub}(mf), at which the drag exceeds the weight of the particles, a uniform fluidized bed exhibits a kind of dynamical phase transition to "bubbling" and "slugging" regimes. In the slugging regime, each rising bubble occupies nearly the entire cross section of the bed and provokes the displacement of large particles, such as those classified by Geldart [1] as Groups B or D. In these powder groups the formation of bubbles starts at, or slightly above, the minimum fluidization velocity, U{sub}(mf), and along their upward motion the bubbles coalesce intensively.
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