Wake volume of injected bubbles in fluidized beds: A magnetic resonance imaging velocimetry study

Rapid magnetic resonance imaging (MRI) was used to determine the volume of wakes of bubbles injected into incipiently fluidized beds. MRI was used to generate 2D maps of particle velocity surrounding bubbles with a temporal resolution of 18 ms. The bubble rise velocity, u(b), was determined from the change in bubble position in successive maps, and pixels of particles which had an upward velocity greater than or equal to 0.8 u(b) were considered to be in the wake if they were below the bubble and in the crown if they were above the bubble. Results show that (i) the wake volume is much larger than the crown volume and (ii) the ratio of the wake volume to bubble volume increases as the bubble rises in the bed but decreases with increasing bubble volume. The influence of two bubbles interacting on bubble wake volume is also investigated. Comparison with geometric theory for wake volume exposes flaws in this theory for fully developed bubbles, bubbles near the distributor and interacting bubbles. (C) 2019 Published by Elsevier B.V.