The rise of bubbles and slugs in gas-fluidized beds using Ultra-fast Magnetic Resonance Imaging
Christoph R. Müller, Mick D. Mantle, Andrew J. Sederman, John F. Davidson, John S. Dennis, Paul S. Fennell, Allan N. Hayhurst, Lynn F. Gladden, and Andrew C. Rees. Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA, United Kingdom
In this study Magnetic Resonance Imaging (MRI) has been used to investigate the rise velocity and coalescence of bubbles and slugs in a three-dimensional (3-D), gas-fluidized bed. MRI is a non-intrusive technique with high temporal and spatial resolution. The rise velocities of bubbles and slugs were determined by means of cross-correlation functions and the results of these measurements were compared with those described in the literature using other techniques. An important finding, for example, is that in contrast to the rise velocity of a slug, the excess gas velocity (U - Umf) should not be added to the rise velocity of an isolated bubble, Ub0, to obtain its rise velocity in a bubbling bed. Here U is the superficial velocity of the gas and U = Umf at incipient fluidization. It was also observed, for the first time in a 3-D fluidized bed, that bubbles undergo significant changes in velocity and shape when they coalesce. It has also been found that bubbles experience a significant acceleration as they approach the top of a bed. Correlations to predict the diameter of a bubble, such as that of Darton et al.  have been evaluated using MRI measurements by assuming a particular wake-angle. In this way, MRI has been shown to be an important new technique for providing novel information on important variables in 3-D gas-fluidized beds, such as the rise velocity and geometry of bubbles, with good spatial and temporal resolution.