(633g) 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.