Modelling Axial Mixing of Fuel Particles in the Dense Region of a Fluidised Bed

A semi-empirical model for the axial mixing of fuel particles in the dense region of a fluidised bed is presented and validated against experimental magnetic particle tracking in a fluid-dynamical, down-scaled fluidised bed that resembles hot, large-scale conditions. The model divides the bottom region into three mixing zones: a rising bubble wake solids zone; a zone with sinking emulsion solids; and the splash zone above the dense bed. In the emulsion zone, which is crucial for the mixing, the axial motion of the fuel particle is shown to be satisfactorily described by a force balance that applies experimental values from the literature and an apparent emulsion viscosity of Newtonian character. In contrast, the values derived from the literature for key model parameters related to the bubble wake zone (such as the upwards velocity of the tracer), which are derived from measurements carried out under cold laboratory-scale conditions, are known to underestimate systematically the measurements relevant to h...