  Fluidization of solid particles by an ascending fluid is frequent in industry
because of the high rates of mass and heat transfers achieved. However, in some
cases blockages occur and hinder the correct functioning of the fluidized bed.
In this paper, we investigate the crystallization (defluidization) and
refluidization that take place in very-narrow solid-liquid fluidized beds under
steady flow conditions. For that, we carried out experiments where either
monodisperse or bidisperse beds were immersed in water flows whose velocities
were above those necessary for fluidization, and the ratio between the tube and
grain diameters was smaller than 6. For monodisperse beds consisting of regular
spheres, we observed that crystallization and refluidization alternate
successively along time, which we quantify in terms of macroscopic structures
and agitation of individual grains. We found the characteristic times for
crystallization, and propose a new macroscopic parameter quantifying the degree
of bed agitation. The bidisperse beds consisted of less-regular spheres placed
on the bottom of a layer of regular spheres (the latter was identical to the
monodisperse beds tested). We measured the changes that macroscopic structures
and agitation of grains undergo, and show that the higher agitation in the
bottom layer hinders crystallization of the top layer. Our results bring new
insights into the dynamics of very-narrow beds, in addition to proposing a way
of mitigating defluidization.
