Wednesday, 26 April 2006 - 4:30 PM

Investigation of the Formation of Jets in a Three-Dimensional Gas-Fluidized Bed Using Magnetic Resonance Imaging

Christoph R. Müller, Andrew C. Rees, Mick D. Mantle, Andrew J. Sederman, John F. Davidson, John S. Dennis, Paul S. Fennell, Lynn F. Gladden, and Allan N. Hayhurst. Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA, United Kingdom

Magnetic Resonance Imaging (MRI) has been used to examine the flow of gas just above the distributor of 3-D beds of particles, fluidized by nitrogen gas. Three types of distributors were used: (i) multiple-orifice plates containing a triangular array of 19 drilled holes, each 1.0 or 1.5 mm in diameter, (ii) a multiple-orifice plate containing a triangular array of 37 drilled holes, each of 1.0 mm diameter, (iii) a plate drilled with a triangular array of holes but provided with a separate, central nozzle. It has been possible to extract geometric information, such as the length of the jet, from MR images, each averaged over ~ 4 min. The question as to whether or not the observed voids represent permanent jets or a stream of bubbles has been considered. The evidence from ultra-fast FLASH profiles strongly suggests that the lower part of the jets are permanent, whereas bubbles detach at the top of the jet; thus the top of the jet is of a rather transient nature. It was observed that for multi-orifice distributors, the jets of gas bend towards the centre and, in some cases merge, giving a central core of bubbles rising up the bed. This central core has not, therefore, been produced solely by coalescence events between single bubbles formed at the orifices in the distributor. The length of a jet was successfully correlated with operating variables using dimensional analysis. The frequency of bubble formation, i.e. bubble detachment from a jet, was derived from ultra-fast FLASH images. The measured frequencies are significantly lower than the frequencies predicted by the correlation of Davidson and Schüler [13]. A plausible reason for the discrepancy is that the complex jet structures seen in the fluidized beds used in this work were not allowed for in Davidson and Schüler's [13] analysis, which only postulates the formation and release of a bubble from an orifice.

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