Discharge of a fluidised bed of particles through an orifice

This paper describes measurements and modelling of the discharge of a pressurised bed of fluidised particles through a circular orifice. A simple one dimensional flow model is used to describe three stages of flow; radial convergence of flow towards the orifice, axial acceleration through the orifice, and continued acceleration of the free particle jet beyond the orifice. This model correctly predicts previously observed trends in the final velocity and voidage of particle jets discharging from low pressure beds. Experimental results reported here for discharge from higher pressure beds are also well predicted. Experimental results are described for discharge of particles with a mean diameter of 150 μm to atmosphere through 3 mm and 4 mm diameter orifices, from a fluidised bed at pressures up to 5 bar. For the larger orifice diameter, the solids mass flow rate increased with the square root of the pressure drop across the orifice, consistent with theory and observation described by other authors for lower pressure systems. For the smaller diameter orifice the solids flow rate had a dependency on the pressure drop to a power of 0.4. This is explained in terms of changes in the pressure profile through the orifice that occurs when the orifice diameter is small compared to the thickness of the orifice plate. Gas volumetric flow rates increased approximately linearly with pressure.