The effect of pellet shape on turbulent hydrodynamics in narrow packed beds: A magnetic resonance velocity imaging study

Magnetic resonance (MR) velocity imaging has been used to investigate the hydrodynamics of turbulent gas flow through narrow packed beds (tube to pellet diameter ratio of N≈ 5) for four different pellet shapes: spheres, hollow cylinders with two different hole sizes and quadrilobes with an internal hole. Images of the time-averaged velocity were acquired in 3D at a spatial resolution of 0.70 mm (x) × 0.70 mm (y) × 1.0 mm (z) for particle Reynolds numbers, Rep, of 500, 2500 and 6500. The results facilitate a direct comparison of the flow fields within beds of different pellet shapes, at commercially relevant (turbulent) flow conditions. The flow fields show that the pellet shape influences the near-wall flow behaviour, with spheres having the slowest velocity at the wall, and quadrilobes having the highest. The velocity distribution within the bed was also influenced by the pellet shape. The bed comprised of spheres had a more uniform velocity distribution at Rep≥ 2500, showing less high-speed (u¯/Uint > 2.5) fluid and less backflow (u¯z/Uint≤ 0), as compared to pellets with holes. The average fluid speed within the pellet holes was found to be dependent on pellet shape with the average fluid speed in holes ranging from 42 % to 79 % of the average fluid speed in the bed. The measurements reported here provide clear evidence that pellet shape influences the hydrodynamics at both the pellet scale and the bed scale, and demonstrate the ability of magnetic resonance imaging methods to directly investigate the hydrodynamics in packed beds using pellets and flow conditions of true commercial relevance.