Characterization of the hydrodynamic structure of a 3D acoustic fluidized bed
Multiphase flow systems are used widely in various industrial processes. For example fluidized bed reactors are attractive because they provide uniform temperature distributions, low pressure drops, and high heat/mass transfer rates. Due to the complexity of this gas-solid system, characterizing the hydrodynamics of a fluidized bed has become critical in understanding system behavior.
Minimum fluidization velocity and local gas holdup are important parameters used to characterize the hydrodynamic behavior of a fluidized bed. These characteristics may be modified through the inclusion of sound vibrations.
The hydrodynamic behavior in a 3D fluidized bed filled with glass beads or ground walnut shell, using different particle size ranges, different initial bed heights, and various flow conditions, with and without acoustic intervention, is investigated in this research. X-ray computed tomography (CT) imaging is used to determine time-average local gas holdup. Using the local time-average gas holdup images, qualitative and quantitative information regarding the jetting phenomena near the distributor plate is obtained.
Results show the minimum fluidization velocity is influenced by sound frequency. As the frequency increases, the minimum fluidization velocity decreases until a specific frequency is reached, beyond which the minimum fluidization velocity increases. With increasing sound pressure level, the minimum fluidization velocity decreases because the additional vibration forces imparted to the bed particles helps to loosen the bed, reducing the interparticle forces, which reduces the required energy for particle fluidization. Thus, acoustic fields provide an improvement in the ease of particle fluidization.
Local time-average gas holdup results show that the fluidized bed under the presence of an acoustic field provides a more uniform fluidization; material exhibit less channeling, the acoustic fluidized beds had fewer amounts of active jets than the no acoustic fluidized bed conditions, which allow for a more homogeneous gas holdup region deep into the bed. Thus, an acoustic field affects the hydrodynamic behavior of a fluidized bed.
The acoustic field also influenced the jetting phenomena present near the aeration plate, where it was observed that the jets in the acoustic fluidized bed merged higher in the bed compared to the no acoustic condition.
Finally, the inclusion of acoustic vibrations on the fluidized bed allows larger bubbles to break into smaller ones, increasing the presence of solid particles which decreases the jet momentum dissipation loss allowing the jets to have a higher average jet length compared to the fluidized bed without acoustics vibrations. Moreover, the addition of acoustic vibrations also produced an increase in the expansion angle of the jets.