  We study the effects of fluid-particle and particle-particle interactions in
a three-dimensional monodispersed reactor with unstable fluidization.
Simulations were conducted using the Immersed Boundary Method (IBM) for
particle Reynolds numbers of 20-70 with an Archimedes number of 23600. Two
different flow regimes were identified as a function of the particle Reynolds
number. For low particle Reynolds numbers ($20 < Re_p < 40$), the porosity is
relatively low and the particle dynamics are dominated by interparticle
collisions that produce anisotropic particle velocity fluctuations. The
relative importance of hydrodynamic effects increases with increasing particle
Reynolds number, leading to a minimized anisotropy in the particle velocity
fluctuations at an intermediate particle Reynolds number. For high particle
Reynolds numbers ($Re_p > 40$), the particle dynamics are dominated by
hydrodynamic effects, leading to decreasing and more anisotropic particle
velocity fluctuations. A sharp increase in the anisotropy occurs when the
particle Reynolds number increases from 40 to 50, corresponding to a transition
from a regime in which collision and hydrodynamic effects are equally important
(Regime 1) to a hydrodynamic-dominated regime (Regime 2). The results imply an
optimum particle Reynolds number of roughly 40 for the investigated Archimedes
number of 23600 at which mixing in the reactor is expected to peak, which is
consistent with reactor studies showing peak performance at a similar particle
Reynolds number and with a similar Archimedes number. Results also show that
maximum effective collisions are attained at intermediate particle Reynolds
number. Future work is required to relate optimum particle Reynolds number to
Archimedes number.
