带搅拌器的机械密封容器英文文献和翻译(5)
process originating from large eddies. The presence of uniform
structure helps to maintain similarity in the flow throughout thus
making the energy distribution uniform. Moreover, since there
are no fluid sweeping blades, the formation of large eddies is
avoided, and the impeller basically cuts the fluid continuously.
This practically eliminates the typical energy dissipation and
shear zones observed in conventional impellers. Also, the ab-
sence of any wakes behind the blades helps further reduce the
drag and hence the energy consumption. Reduction in the
contact area of the impeller also helps to decrease the extent of
form and skin drag. The design yields stream lines that would
follow the flow separation over the blades and interaction with
other streamlines in the compartments formed due to self-similar
feature. This specifically reduces the value of form drag to a great
extent, and also the possibility of any wake formation behind the
blade is almost zero. However, continuous passage of blades in
the same plane helps develop local circulation zones restricted to
the blade dimensions thereby creating several similar local
circulating zones that interact with each others. More detailed
work on flow visualization of the zones and the interacting
circulation zones is in progress. Different design alternatives
with varied blade angles, etc. may yield better flow, and more
efforts on understanding the effect of design of the FI on the
performance are under investigation. To further characterize the7670 7667–7676
performance of FI, we studied the liquidsolid suspension,
gasliquid dispersion, and the mixing time.
3.2. Suspension. Usually, the flow pattern from an axial flow
impeller is conducive to easier suspension than that of by a radial
flow impeller, while the mixed flow impellers show an inter-
mediate performance. Suspension of solids in liquid in a stirred
tank reactor has been studied over many decades, and certain
guidelines on the selection of suitable impeller is known.
68
Typically weak recirculation induced loops occur just below the
impeller and also at the junction of the tank base and the wall. For
the case of the impeller operating close to the tank base, the
efficiency of energy transfer from impeller to particles is max-
imum. The particulate mass trapped in the stagnant zone below
the impeller is, therefore, easily driven to the corners with enough
velocity to get suspended. If the off-bottom clearance of the
impeller is increased, then the stagnant zone below the impeller
also increases and more particles get accumulated there. In such
cases, higher impeller rotation speed would be needed to lift the
particles from the bottom and then get completely suspended at
further greater impeller rotation speeds.
The flow generated by the FI is largely a tangential flow as all
the blades simply cut the fluid in different planes thereby
avoiding any possibility of sweeping or pushing the fluid in its
path. Thus, the flow separation over the blades is a prominent
phenomena, and the fluid interacting with different rotating
zones mix with each other. This results in a strong tangential
flow at the bottom of the impeller, and thus helps to lift the
particles while pushing them toward the wall; however, once
these particles are lifted, they are trapped in the rotating structure
which keeps the particle floating between different zones. Also,
the velocity gradients in the vicinity of the blade were seen to help
get the particles lifted in the direction perpendicular to the
motion of the blade. For different suspension densities of the
resin particles (as mentioned in section 2), the value of PW was
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