Coherent Backscattering from Multiple Scattering Systems - KOPS ...

4.2 The samples
Figure 4.5: Fluidized bed. To fluidize the granular medium, water is pressed through
it from below. The sinter plates in the container below the fluidized bed make the water
flow smooth and laminar to establish a uniform fluidization.
and rejuvenate, which is why fluidized beds are used in oil and coal industrial processes
such as fluidized catalytic cracking and fluidized coal combustion, but also pharmaceutical
processes and bio-reactors [26].
The granular medium in a fluidized bed is fluidized only if the flow rate of the fluid exceeds
a certain minimum value. The drag force on the particles is then sufficient to balance their
net weight, and the grains become free to move. All fluidized beds tend to be unstable and
to form bubbles of fluid without particles. However, for viscous liquids or light and small
particles, a stable uniform fluidization regime can be observed over a range of flow rates just
above minimal fluidization [26].
The particles in the fluidized bed examined in this work are soda lime glass spheres from
Whitehouse Scientific, with diameters around 150 µm (fig. 4.6). We take the refractive index
of soda lime to be 1.52(1), the mass density is given by Whitehouse Scientific as 2.6 g /cm 3 [50].
As the transport mean free path is expected to be proportional to the distance between the
particles in the fluidized bed, the volume fraction was varied between 35% and 52% by using
different flow rates of the pump.
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