Figure 3(b): Chronological sequence (i to vi) of video images
of bubbles in a two-dimensional fluidized bed of a Group B powder,
chosen to show coalescence of bubbles and bubble eruption
at the bed surface.
Video 2 [6.6 min to download at 28.8kbaud]:
Bubbles in a two-dimensional fluidized bed of a Group B powder
at low gas velocity.
The equipment shown is a so-called "two-dimensional fluidized bed".
This is a favourite tool of researchers looking at bubble behaviour.
It consists of a vessel of a rectangular cross-section, whose shortest
dimension (in the direction being viewed) is usually only 1 cm or so.
At superficial velocities above the minimum fluidization velocity,
fluidization may in general be either bubbling or non-bubbling.
Some combinations of fluid and particles give rise to only bubbling
fluidization and some combinations give only non-bubbling fluidization.
Most liquid-fluidized systems, except those involving very dense particles,
do not give rise to bubbling. Figure 4 and Video 3 show a bed of glass
spheres fluidized by water and exhibiting non-bubbling fluidized bed behaviour.
Figure 4: Expansion of a liquid-fluidized bed.
(i) just above Umf,
(ii) liquid velocity several times Umf.
Note the uniform increase in void fraction.
(Sketches are based on the video).
Video 3 [5.6 min to download at 28.8kbaud]:
Expansion of a liquid-fluidized bed.
Gas-fluidized systems, however, give either only bubbling fluidization
or non-bubbling fluidization beginning at Umf , followed by bubbling
fluidization as fluidizing velocity increases. Non-bubbling fluidization
is also known as particulate or homogeneous fluidization and bubbling
fluidization is often referred to as aggregative or heterogeneous
fluidization.
