PARTICLE MIXING AND VOLUMETRIC EXPANSION IN A VIBRATED GRANULAR BED

The present experiments are an investigation of the expansion and mixi ng that occur in a horizontal bed of particles subjected to vibrationa l accelerations in the direction parallel to gravity. The particles ar e colored-glass balls of uniform size; three different bed heights are examined of 6, 9, and 12 particle diameters. The vibrational frequenc y and amplitude are controlled separately to cover a range of accelera tion levels from 1 to 5.5 times gravitational acceleration. The expans ion results show that above a critical frequency, the bed begins to ex pand and the bed solid fraction decreases. The result is independent o f the vibrational amplitude. Above a second critical frequency, the th ickest beds show a further decrease in solid fraction; the minimum val ue of solid fraction for all bed heights is approximately 0.21 +/- 0.0 3. The mixing results indicate that the mixing times decrease signific antly with the expansion of the bed. However, the mixing times are gre ater as the bed depth increases. Unlike the expansion results, the mix ing times depend on the amplitude of the vibration. A simple analysis of the flow is performed using a self-diffusion coefficient developed from dense-gas kinetic theory. The analysis qualitatively agrees with the experiments for the largest vibrational velocities and for the thi nnest beds.