The influence of particle size on the flow of fluidised powders

Abstract

Fluidised particles can form thin coherent flows that move swiftly over horizontal surfaces. When the particles are cohesionless, Eames and Gilbertson [J. Fluid Mech. 424 (2000) 169] demonstrated that the flow may be understood through vertically integrated momentum and mass balances, and good agreement was found between the model predictions and experimental observations. The behaviour of gas fluidised fine particles (diameter of the order of tens of micrometres) differs significantly from that of larger particles. This is true with respect to their flow which takes place through a series of episodic avalanches and waves rather than as a continuous flow. This behaviour can be explained in terms of a mild cohesive force between the particles. When cohesion is included in the vertically averaged momentum equation, the dynamics of fine particle flows can be reasonably predicted for low and high gas flow rates, when the cohesive forces are respectively weaker than internal frictional forces or drag forces generated by the gas flow. A quasi-static analysis of the combined effect of gas flow and cohesion shows that where cohesive sediments are thin they are not necessarily mobilised, even when fluidised, and this is supported by new experimental observations. A flow of cohesive particles thus has two distinct regions: a thin nose region where the material is not mobilised and a pile of mobile particles behind it. At intermediate gas flow rates comparable to the point of minimum fluidisation, the internal structure of the particle flow dominates how the pile flows along a horizontal surface, and continuum models fail to predict the pile dynamics.