On the settling and clustering behavior of polydisperse gas-solid flows with application to pyroclastic density currents*

Sedimenting flows occur in a wide range of industrial and natural systems, such as circulating fluidized bed reactors and pyroclastic density currents (PDCs). In systems with sufficiently high mass loading, momentum coupling between the phases gives rise to mesoscale behavior, such as clustering. These structures have been demonstrated to generate and sustain turbulence in the carrier phase and directly impact large-scale quantities of interest, such as settling time. As an added complexity, all real-world flows consist of a polydisperse particulate phase, in which parameters, such as particle diameter, vary widely across the ensemble. In this study, we characterize the sedimentation behavior of a range of polydisperse gas-solid flows, sampled from a parameter space typically associated with PDCs. Highly resolved data for two polydisperse distributions of particles at two volume fractions is collected using an Euler-Lagrange framework and compared with analogous monodisperse configurations. We propose a new metric for predicting the degree of clustering, termed 'surface loading' and quantify the effect of polydispersity and volume fraction on both clustering and settling behavior.