Various mechanisms explaining the transition from bubbling to turbulent fluidisation have been proposed. These are typically based on the changing behaviour of bubbles/voids as the gas velocity is increased, e.g. through coalescence. However, no definitive mechanism governing this transition has yet been established due to a lack of experimental measurements with sufficient spatial and temporal resolution.

In this study magnetic resonance (MR) was used to track the transition to turbulent fluidisation. Three types of measurement were acquired: (1) snap-shot (26 ms acquisition time) images of the void distribution, (2) time-averaged images of the velocity, and (3) maps of the velocity fluctuations in the bed. These measurements were performed for gas velocities increasing from bubbling through to turbulent fluidisation and are compared with measurements of the pressure fluctuations. The measurements provide high-resolution imaging of the voids during the transition to turbulent flow providing evidence of the coalescence of bubbles and formation of tongues. These results indicate that the transition to turbulent flow in this system was a gradual process. The velocity of the particles was also tracked providing a quantitative measurement of the changing particle dynamics as the gas flow rate increases. These measurements reveal a transition from a “Gulf Streaming” flow profile at low gas velocities through to a core-annular profile at high gas velocities. These are the first reported measurements of turbulent fluidisation using MR and indicate that MR can provide new insight into the mechanism for the transition to turbulent fluidisation.