Geometrical and hydrodynamical study of gas jets in packed and fluidized beds using magnetic resonance

Abstract Magnetic resonance (MR) was used to image the motion of particles and gas just above the distributor of 3D beds of particles fluidized by air. Three different distributors were used: (i) a single‐orifice distributor, with orifice diameters 1.0–4.0 mm, (ii) a plate, drilled with a triangular array of 79 holes, each of 0.35 mm diameter, with a central nozzle containing a single hole of diameter 1.0, 2.5, or 9.0 mm, (iii) distributors with two or three orifices and diameters of 1.0 or 2.5 mm. It proved possible to extract geometrical information, such as the length of a jet, from MR images, each averaged over ∼5 min. Also, light was shed on the question of why is there such a discrepancy between reported jet‐lengths. The fluidization state, the “start‐up” procedure and also the number of holes all play a significant role in determining the measured distance a jet penetrates into a bed. The question as to whether the observed voids represent permanent jets or streams of bubbles was considered. The evidence from ultra‐fast MR measurements strongly suggests that only the lower part of a jet from an orifice in a multi‐orifice distributor is permanent; bubbles form at the top of the jet. Consequently, the top of each jet is transient. However, most of the jet from a single orifice is a permanent cavity when the bed of particles is not fluidized. The length of a jet was successfully correlated with operating variables using dimensional analysis. Finally, the flow of particles around a single jet was measured with high resolution MR.