Characterization of gas–solid fluidization: A comparative study of acoustic and pressure signals

The hydrodynamics of fluidized beds strongly influence their operation, but are complicated and chaotic. There are many measurement techniques, but none fully characterizes gas–solid fluidized beds. Acoustic signals from fluidized beds cover a wide frequency spectrum and can be correlated to bed characteristics. Experiments were conducted to study the acoustic signals from ultrasonic transducers mounted on the outer wall of a two-dimensional fluidization column. The acoustic signals were related to bubble behavior in 550 μm glass beads. Simultaneous acoustic and pressure measurements allowed direct comparison of these signals for single bubbles, pairs and chains of bubbles. The envelope of acoustic signals, generated by particle collisions and particle–wall impacts, provided information on the behavior of bubbles. Significant peaks appeared as the top portions of the bubble wakes approached the acoustic sensor. Pressure waves propagated considerably in the horizontal direction, whereas acoustic signals propagated little in the lateral direction, but transmitted forward in the wall in the direction of bubble motion, maintaining the wave profile invariant during transmission. The strong lateral localization of acoustic signals is promising for determining the lateral bubble position in the bed. Acoustic signals provide a potential means of determining such bubble properties as velocity, frequency and volume, with some advantages relative to pressure signals.

Peaks in acoustic signals essentially correspond to wake tops of bubbles. Pressure waves propagate considerably horizontally, whereas acoustic signals propagate little in the wall transversely, but well vertically forward, maintaining the wave profile invariant. Acoustic signals determine bubble velocity, frequency and volume with advantages relative to pressure signals.Figure optionsDownload as PowerPoint slideHighlights
► Acoustic signals reflect collision intensities of particles on the column wall.
► A peak occurs in acoustic signal when the top of a bubble wake reaches the sensor.
► Acoustic signals can provide information on bubble velocity, frequency, and volume.
► Acoustic signals propagate little horizontally, but much better in the direction of bubble motion.