(85f) Magnetic Resonance Velocimetry of Particle Hydrodynamics in a Three-Dimensional Draft Tube Spout-Fluid Bed

Draft tube spout-fluid beds (DTSFB) are widely used industrial reactors for processes requiring intensive mixing of solids and high heat and mass transfer rates between contacting gas and solid phases, such as particle drying¹ and coating². Due to the presence of a draft tube, the solid flow in a DTSFB is compartmentalized in a slow annular region and a fast spout in the draft tube, whereby the gap height of the draft tube regulates the particle circulation rate and the gas contacting time³.

However, the inherent opacity of granular materials impedes the visual examination of the complex, internal hydrodynamics found in DTSFBs. Hence, semicircular beds⁴ or visual probes⁵ are often used to inspect the dynamics, which, in turn, perturb the particle flow due to the presence of additional walls or the insertion of probes.

The present work employs magnetic resonance imaging (MRI) to non-invasively study the hydrodynamics of the solid phase in a fully three-dimensional DTSFB⁶. To this end, a lab-scale spout-fluid bed of oil-filled particles is equipped with a draft tube and measured by a 16-channel radio-frequency detector array in a 3T MRI scanner^7,8. Solid distribution maps and particle velocity maps are obtained for various operational settings of the DTSFB, allowing to assess the entrainment of solid material from the annular region to the draft tube and, additionally, revealing flow phenomena such as a vena contracta flow at the entrance of the draft tube and the suppression of gas bubbling in the annular region of the bed. Complementary simulations using the discrete element method coupled with computational fluid dynamics confirm the experimental observations and provide further insight into the gas flow field of the DTSFB which is currently inaccessible by magnetic resonance imaging.

References

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