Particle-level dynamics of clusters: Experiments in a gas-fluidized bed

Clustering is critical to understanding the multiscale behavior of fluidization. However, its time-resolved evolution at the particle level is seldom touched. Here, we explore both the time-averaged and time-resolved dynamics of clusters in a quasi-2D fluidized bed. Particle tracking velocimetry is adopted, and then clusters are identified by using Voronoi analysis. The time-averaged results show that the number distribution of the cluster size follows a power law (similar to n(c)(-2.2)) except for large clusters (n(c) > 100). Time-resolved analysis demonstrates that the cluster coalescence can be simplified as a collision between two inelastic clusters, during which the mean speed, kinetic energy, and total Voronoi cell area of particles decrease until the formation of the big cluster. Then, a model is proposed to predict its energy loss, which gives Delta E similar to t(3/2). Moreover, the breakup of the cluster is linked to increasing dimensionless torque on the particles.