PARTICLE MOTION AT THE WALL OF A CIRCULATING FLUIDIZED-BED

The motion of alumina particles of mean size 74.9-mu-m in the region near to the wall of the 305 mm diameter riser of a cold model circulating fluidized bed has been studied using a high-speed video camera employing normal and magnifying lenses. Particles in this region were found to move predominantly downwards, against the main gas flow. High density groups or swarms of particles typically arch-shaped were observed to descend in contact with the wall at velocities in the range 0.3-0.4 m s-1. The distribution of swarm descent velocities was shown to be little affected by changes in superficial gas velocity over the range 3-5 m s-1 and imposed mean solids mass flux over the range 2 to 80 kg m-2 s-1. A region of steady bulk downflow of solids with a velocity of approximately 1.0 m s-1 was observed to appear a few millimetres from the wall at mean suspension densities greater than 5.6 kg m-3. Motion of particles in contact with the riser was analysed by identification of three flow forms; dilute, dense and swarm flow. Results of the analysis are linked with the observations of other workers concerning the onset of the so-called 'similar profiles' regime. The relationship between the measured effective particle swarm length and the cross-sectional mean suspension density was established and the implications for modelling suspension-to-wall heat transfer discussed.