(80b) Fully-3D DEM fluidization simulation of a shallow fine powder bed

A fully-3D Lagrangian-Eulerian fluidised bed model has been developed and applied to the simulation of fluidisation in a fine powder bed. The Lagrangian direct numerical integration of the particle trajectories is based on the Discrete Element Method (DEM), Cundall and Strack (1970) and uses theoretical contact mechanics laws to model particle interactions, an approach which facilitates the use of real physical properties for the particles. The DEM solution of the particle motion is then coupled to a continuum (Eulerian) integration of the Navier-Stokes equations of fluid motion through a fluid-particle interaction term.

Three powder beds each consisting of 100,000 spherical particles of average diameter 50 ?Ým and different interparticle surface energy values equivalent to pull-off forces of zero, one and two particle weights were employed. Air at a temperature of 293K, standard atmospheric pressure and superficial velocity of 14.3 mm/s (~ 3.5 umf) was used as the fluidising gas.

Visual characterisation of the fluidisation behaviour of the three beds were built from external and sectional views of video images for particle flow, particle velocities, fluid velocities and particle-particle collisions.

The external views of the simulations show particle motion and fluid flow consistent with bubbling for all three beds. Sectional views indicated that bubble formation was initiated by the coalescence of air pockets originating from the base of the bed. Bubble rupture in the freeboard was also observed. Faster particle mixing occurred in the cohesionless particle bed while the frequency of interparticle collision was significantly higher in the cohesive particle beds.

The simulation results shed some light on the long-standing controversy over the mechanism of bubble splitting in fluidised beds. Visual evidence from the simulations suggested that bubble splitting occurs as a result of particle influx into the base of a bubble meeting up with particles from the roof of the bubble as illustrated by the sequence in figure 1.

An estimate of bubble speed from the simulations was found to agree well with that calculated from an empirical correlation.

References

Cundall, P. and Strack (1979). A discrete numerical model for granular assemblies. Geotechnique, 29, 47-65.

Figure 1 Particle configuration and fluid flow field illustrating bubble splitting in cohesionless particle bed.