Observation of Reverse Core-Annulus Behavior in Risers and Its Relation to Stokes Number

         This work is part of a larger collaborative effort to develop first-principles, continuum models for polydisperse, gas-solid risers using a combination of theory, simulations, and experiments. The focus of this portion of the effort is to generate a comprehensive experimental dataset for purposes of model validation. Experiments involving monodisperse Geldart Group B particles have been carried out in the pneumatic conveying regime of a riser with a rounded elbow exit.  Several combinations of superficial gas velocity (U_s), solids flux (G_s), average particle diameter (d_ave), and material density (ρ_p) were investigated. "Core-annulus" profiles (i.e., a dilute core with a dense annulus) have been widely reported in literature to date, mostly for FCC particles (Geldart Group A). Surprisingly, the experiments performed here reveal the presence of a "reverse" core-annulus profile (i.e., a dense core with a dilute annulus) under some conditions.  Three monodisperse materials were investigated, namely large glass beads (d_ave = 650 μm, ρ_p = 2500 kg/m³), large high-density polyethylene, HDPE (d_ave = 650 μm, ρ_p = 900 kg/m³), and small glass beads (d_ave = 170 μm, ρ_p = 2500 kg/m³). Specifically, for larger glass beads the reverse core-annulus profile was observed near the top of the riser for all U_s and G_s combinations examined. For HDPE, reverse core-annulus was observed at the top of the riser only at relatively low G_s (i.e., G_s = 120 kg/m²s). Finally, for the smaller glass beads, the traditional core-annulus profile was not observed under any condition. The unexpected reverse core-annulus profile noted above appears correlated with the Stokes number (St) of the material, i.e., the bigger the St, the greater the likelihood and extent of reverse core-annulus behavior.