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In a bed packed with particles, the introduction of an upward-flowing fluid stream into the column through a nozzle or opening in a flat or conical base creates a drag force and a buoyancy force on the particles. A cavity forms when the fluid velocity is high enough to push particles aside from the opening, as illustrated in Figure 2.1(a). A permanent and stable vertical jet is established if the nozzle-to-particle-diameter ratio is less than 25 to 30. Further increase in fluid flow rate expands the jet, and an “internal spout” is established, as in Figure 2.1(b). Eventually, the internal spout breaks through the upper bed surface, leading to the formation of an external spout or fountain, as in Figure 2.1(c). Such an evolution process to a spouted bed, involving cavity formation, internal spout/jet expansion, and formation of an external spout, was well documented in the 1960s.
Figure 2.2 shows the evolution of measured total pressure drop in a half-circular conical bed for 1.16-mm-diameter glass beads and ambient air, starting with a loosely packed bed, Run 1. It is seen that the total pressure drop increases with increasing gas velocity in the gas flow ascending process (shown by closed squares), and reaches a peak value before decreasing to approximately a constant value with a further increase in the gas velocity.
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