  The dynamics of granular plugs in water fluidized beds in narrow tubes is
investigated here both experimentally and numerically. The fluidized beds were
formed in a 25.4 mm-ID tube and consisted of alumina beads with 6 mm diameter
and specific gravity of 3.69; therefore, the ratio between the tube and grain
diameters was 4.23, which is considered a very narrow case. Three different
beds were arranged, consisting of 250, 400 and 500 beads, and two water flow
rates were imposed, corresponding, respectively, to superficial velocities of
0.137 and 0.164 m/s and to Reynolds numbers based on the tube diameter of 3481
and 4177. Under these conditions, it was possible to observe the formation of
granular plugs that propagate with characteristic lengths and celerities. For
the experiments, we filmed the fluidized bed with a high-speed camera, and
automatically identified and tracked the plugs along images by using numerical
scripts. For the numerical part, we performed simulations using a coupled
CFD-DEM (computational fluid dynamics - discrete element method) code, together
with numerical scripts to identify and track the granular plugs. We obtained,
for the first time, the length scales and celerities of the granular plugs in
very narrow tubes from experiments and numerical simulations, and the agreement
between them is good.
