Granular behavior in gas-fluidized beds

This work investigates the behavior of granular materials driven by a uniform upward flow of gas within a vertical container. This arrangement is referred to as a gas-fluidized bed. For a bed with a large number of spherical grains, bulk properties of the sample, such as solids volume fraction (the percent of space occupied by solids) and gas pressure drop, are found to obey simple scaling relations when the superficial air velocity, the container size, or the grain size are varied. These results stand in contrast to behavior observed in other granular systems, where non-trivial interaction between individual grains leads to complex behavior for the bulk. The results suggest that there is a unique quality to the forcing provided to individual grains by gas fluidization that results in relatively simple bulk behavior. To investigate this possibility, experiments were carried out in a gas-fluidized bed with only a single grain. A large grain, a ping pong ball, was chosen for ease of visual observation. The ball's behavior is found to be exactly that of a Brownian object harmonically bound to the center of its container. Its dynamics are found to be described by a Langevin Equation, with the random forcing on related to the dissipation of energy by the Fluctuation-Dissipation Theorem. We find that the separation statistics for a two-ball system are also described by a statistical mechanics approach. These results represent the first successful application of conventional statistical mechanics to a macroscopic system.