Characteristics of acoustic standing waves in fluidized beds

Fine particles are typically difficult to fluidize with gas, due to interparticle forces that cause the bed material to be cohesive. The use of acoustic waves, generated by a loudspeaker positioned above the bed, to agitate the bed material and enhance fluidization, has been investigated. The bed was fluidized with air at conditions near minimum bubbling. Sound-pressure measurements within the bed showed the presence of acoustic standing waves throughout the bed. Acoustic standing-wave theory, which assumes that the bed behaves as a 1-D, quasi fluid with constant speed of sound, was used to model the sound-pressure-level data. It was found that the parameter kh, where k is the wave number and h is the bed depth, determines the sound-pressure amplitude throughout the bed. The system reaches its highest sound pressure at resonance conditions for kh = (2n - 1)pi/2. A comparison indicates good agreement between the theory and the experimental data.