Magnetic resonance studies of jets in a gas–solid fluidised bed

Abstract

Magnetic resonance imaging (MRI) has been used to study the behaviour of jets at the distributor of a 50mm diameter fluidised bed of 0.5mm diameter poppy seeds. Two perforated-plate distributors were examined, containing either 10 or 14 holes, each 1mm diameter. Ultra-fast MR imaging was able to show the transient nature of the upper parts of the jets, where discrete bubbles are formed. Imaging in 3D showed that the central jets were the longest for flow rates below minimum fluidisation. Above minimum fluidisation, the outer jets, nearest the wall of the fluidised bed, arched inward towards the central axis. In this latter case, interpretation of the time-averaged 3D image required the use of ultra-fast MR imaging to identify the approximate height above the distributor at which discrete bubbles were formed. The apparently continuous void extending along the central axis above this height in the time-averaged 3D image was thus identified, using ultra-fast MR imaging, as representing the averaged paths of released bubbles. Time-averaged MR velocity mapping was also used to identify dead zones of stationary particles resting on the distributor between the jets. The dead zones could be observed when the superficial velocity of the gas approached minimum fluidisation, but they were smaller than those observed at lower gas superficial velocity. Comparable images of a single jet through 1.2mm diameter poppy seeds from MRI and electrical capacitance volume tomography (ECVT) are also demonstrated.