On the origin of bubbles and Geldart's classification

Abstract

Void propagation equations based only on conservation of mass and the Ergun equation for flow in porous media are sufficient to show that bubbles, which are continuity shocks, will form in a fluidized bed near minimum fluidization for small particles and that voids will disperse for large particles, as for Geldart's Type B and D powders. This is due to the fact that the void propagation velocity increases with the square of the porosity for Type B particles and decreases with porosity for Type D particles. A further approximate analysis involving the elastic solids stress between the particles shows the existence of a minimum bubbling velocity and hence provides a rational explanation for the boundary between Type A and B particles. This approximate analysis is further extended to the study of bubbling regimes at low and high fluidizing pressures.