Cluster motion and particle-convective heat transfer at the wall of a circulating fluidized bed

Abstract

A new technique has been developed to observe and measure the hydrodynamic properties of solid particles flowing at the wall of a fluidized bed. This technique is called ‘thermal image velocimetry’ or TIV. The TIV technique involves heating the clusters of particles that are at the wall and using their radiant emission to distinguish them from the rest of the bed, with an infrared camera as the detection device. The current setup is in a cold scale-model circulating fluidized bed with a riser that has a 0.159 m square cross-section and is 2.44 m tall. The TIV test section is located on one wall in the upper half of the riser. The hydrodynamic properties that have been measured are the descent velocities of clusters of particles at the surface of a flat wall and the contact times of these clusters with the wall. The measured cluster velocities are between 0.9 and 1.2 m s 1, apparently accelerating to the higher value near the end of their travel. These velocities appear to be independent of the operating conditions (solid and gas flow rates) in the bed, and the apparent acceleration can be shown to affect heat transfer from the clusters. The average contact times measured are around 0.4 s, and a probabilistic analysis shows that individual cluster-wall contact times are distributed according to a gamma probability function. This analysis also highlights the concept of a thermal development region for particle-convective heat transfer.