An experimental study on the self-excited oscillations in flow of gas-solid mixture

A phenomenon of pressure oscillations within cold fluidized beds was studied by using solid particles and air introduced into uniform and variable geometry columns. These were 8 feet (2.44m) tall and had an inlet cross sectional area of 6 inches x 12 inches (0.1524m x 0.3048m).

All tests were conducted at atmospheric pressure and temperatures less than 77°F (25°c). Two particle sizes, 0.16 inch (4.064mm) and 0.30 inch (7.62mm) and specific gravities of 0.9 and 3.5 respectively, were employed with maximum bed heights of 25 inches (0.635m). A transparent test section was used to permit observation and photography. . Air was used as the fluidizing medium which entered from the bottom of the column through a multiorifice distributor plate.

A combination of pressure recordings and visual observation was used to characterize the flow regimes present. Pressure fluctuations within the bed and the plenum beneath were measured and recorded with an IC pressure transducer and a strip chart recorder.

In Chapter I, a mechanism that couples the forces acting on the fluidized particles is postulated and related to the solid-flow variations that cause pressure to oscillate.

Chapter II describes a preliminary experiment with a vertical uniform cross section column. Fluidized beds in this configuration showed a rather slugging behaviour and the frequency of oscillation was to be inversely proportional to the bed height; the amplitude dependent on particle size, particle density, gas velocity and bed depth.

In Chapter III an experiment with variable cross section columns is followed. Diffuser-type and nozzleshaped columns were employed and beds of small particles (d_P = 0.16 inch = 4.064mm) were fluidized at similar flow conditions and pressure fluctuations in the bed plenum recorded.

Results indicate that, by diverging the bed cross section area (diffuser type), the amplitude of the oscillations is decreased as the diffuser angle increases for a given bed height, superficial air velocity and distributor size. A 5 degree diffuser-shaped bed 25 inches (0.635m) deep showed an amplitude reduction of about 40 percent.

Conversely, a convergent configuration (nozzle type) increased the fluctuations in a fluidized bed at the same conditions of fluidization of those in a straight bed.

The conclusions of this work may be summarized in the following statement.

Pressure fluctuations within a fluidized bed are caused by the unbalance of the forces acting on the fluidized particles. If by varying the shape of the fluidizing column

vi these forces are altered thus changing the flow conditions of the fluidizing stream, then it is possible to control (reduce/amplify) the magnitude of the oscillations.