Surface Waves and Pressure Fluctuations in Fluidized Beds with an Oscillating Plate

Surface waves generated by a horizontal oscillating paddle at the upper surface of non-shallow fluidized beds were studied by Finnerty et al. (1969). The wavelength and attenuation-rate were measured from motion pictures. Measured wavelength and attenuation followed the trends of the dynamic model for pseudo-liquid with an effective viscosity, with the attenuation rate increased with increasing wave frequency. Similar experiments were performed by Kok and Benschop (1994) by measured pressure fluctuations. The wave propagation velocity and attenuation rate with frequency <1.6 Hz were found to follow the trends as predicted by the pseudo-liquid model. Beyond 1.6 Hz, the trends became opposite to those predicted from the surface wave theory, attributed to a transition from surface waves to density waves due to the off-design operation of the oscillating paddle.

Both studies used oscillating paddle surface wave generators. While Finnerty et al. measured the surface wave attenuation rate directly from recorded motion pictures of surface waves, Kok and Benschop determined the wave properties indirectly from measured local pressure fluctuations. because local pressure fluctuations in gas-solids fluidized beds are composed of multiple sources (see Bi, 2007) including global bed oscillations, local bubble passage, and propagating pressure waves originating in other locations, there is a need of further experiments to understand the difference in the two measurement techniques and the transition around 1.6 Hz in Kok and Benschop’s experiments.

A cold model fluidized bed was constructed with a variable-speed motor-driven oscillating plate and multiple pressure transducers at different locations of the bed. FCC and glass beads particles of Group A and Group B, respectively, were used as the bed material. Wave frequency, propagation velocity and attenuation rate were measured from pressure fluctuations at different locations. Key findings on the complex local pressure fluctuations and their relationship with the surface waves are to be presented.