The open-channel flow of fluidized solids

Abstract

Observations of the flow properties of fluidized solids through a horizontal open channel with a porous tile distributor are reported. The shear stress at the vertical wall of the experimental section was measured directly and that across the distributor estimated from the difference between the total stress represented by the pressure drop along the experimental section and the wall shear stress. Experiments were carried out with a bed of catalyst of mean diameter 77 μm and ash of wide size distribution with a mean diameter of 380 μm over a range of fluidizing conditions between 1.75 and 3 U mf. Earlier results with a 196 μm sand have been reinterpreted on the same basis. Comparison is also made with flow property measurements using a modified Stormer viscometer. The freely fluidized and flowing fluidized beds showed pseudoplastic flow characteristics. The flowing sand and ash beds also displayed dilatant characteristics at higher fluidizing velocities. The apparent viscosity reduced with initial increase in gas flow rate, passed through a minimum and then increased. However, the minimum was not reached over the range of test with either the flowing catalyst or ash bed experiments. Whereas the distributor shear stress was consistently lower than that at the wall in the catalyst experiments, it exceeded that at the wall in both the sand and ash experiments at low shear rates and its relative value then decreased to less than that at the wall as the shear rate increased. Bed depth was a significant factor and flow behaviour was influenced by the width of the experimental channel. While there is reasonable correspondence between variation in shear stress measurements made in the modified Stormer viscometer and directly at the channel wall, the viscometer reading is sensitive to the position of the rotor within the bed. This and the varying slip at the distributor in the flowing bed system precludes direct application of small scale test data in the design of open-channel flow systems.