Influence of standing wave characteristics on hydrodynamic behaviours in sound-assisted fluidization of Geldart group A powder

Abstract

© 2019 Elsevier B.V. This study aimed to investigate the hydrodynamic behaviours of sound-assisted fluidization of Geldart group A powders under the effects of standing wave characteristics resulting from varying sound wave properties. The sound wave properties used in this were a sound frequency ranging from 0 to 500 Hz at a fixed sound pressure level of 80 dB, and the hydrodynamic behaviours observed included the total bed pressure drop, fixed bed pressure drop, incipient fluidization velocity, and complete fluidization velocity (minimum fluidization velocity). It was found that for full fluidization, the total bed pressure drop was always equal to the weight of the bed per cross-sectional area. However, the fixed bed pressure drop increased with sound frequency, in comparison with conventional fluidization. In addition, with increasing sound wave frequency, the minimum fluidization velocity decreased and reached a local minimum at a critical sound frequency of 50 Hz. After this point, a further increase in the sound frequency caused the minimum fluidization velocity to increase again and eventually level off. This pattern of variation was found to be consistent with the standing wave characteristics in the fluidization medium, where the average magnitude of the sound pressure level varied periodically and exponentially around the critical frequency for each harmonic number. It was also noted that the local minimum of the minimum fluidization velocity can have multiple values according to the periodic variation pattern of the standing wave in the fluidized bed. Finally, by considering the gas oscillation velocity caused by a particular pattern of the standing wave inside the fluidized bed, fixed bed pressure drop correlations were proposed based on the revised Ergun equation and Ergun equation, including their empirical closure equations.