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seen to increase with increasing impeller Re, (Figure 3A) and at
all suspension densities, the following relation was seen to hold:
PW ¼ C1ε1:34
where the value of C1 is 2.63 107
and would strongly depend
on the physical properties of the suspended particles (volume,
density, shape, etc.). Re was estimated using the fluid density and
viscosity at different solid mass fraction ε (%). With increasing ε
(%), Re continued to decrease, and the corresponding variation
in the estimated NP values is shown in Figure 3B. Interestingly,
on comparing the data with similar solid concentration for a
stirred tank of 1 m diameter with PBTD (filled symbols)
(Sardeshpande et al.numbers even with solid particles. Importantly, the Re range for
which the complete suspension was achieved using a FI was at
least less than 50% than that of PBTD. Here, by complete
suspension, we refer to the situation where the particles are
suspended in the entire liquid phase and there remains only a
negligible fraction at the bottom of the tank. However complete
suspension does not mean uniform spatial distribution of parti-
cles. In addition to PW, uniformity in the solid concentration in
the suspension would help to quantify the performance of this
impeller. To understand the level of suspension (particle cloud)
in the liquid, the local concentration of solid particles at various
levels from the bottom of the tank was measured at different
impeller rotation speeds. The variation in the local particle mass
fraction is shown in Figure 4. For all solid loadings, for less than
70 rpm, most of the particles were close to the bottom and were
far from being lifted. At higher impeller rotation speed, the local
solid mass fraction was well suspended with a standard deviation
of (3%. Thus, for all ε values, an impeller rotation speed of
100 rpm was sufficient to keep all the particles in suspended
condition. On achieving complete suspension, for ε = 5%, the
local solid concentration decreased slightly from bottom to top
of the stirred tank, while for ε = 7%, it was slightly higher toward
the bottomas well as at top of the tank. In general, the observations
indicated that once the solid particles are lifted from the bottom,
increasing energy input to the reactor by increasing impeller speed
primarily helps in dispersing the particles, achieving a less nonuni-
form suspension. It would be interesting to track the particle
motion throughout the tank, and such experiments are in progress.
The relatively large volume of the FI results in a better effect in
keeping the particles suspended. Importantly, the presence of
multiple blades in the section close to the bottom develops a
strong tangential flow, which helps the particles to experience lift
in the direction perpendicular to themotion of the blades.On the
other hand, the localized vortex generated due to the motion of
the blades perpendicular to the bottom helped lift the particles in
the center of the stirred tank. This vortex was seen to have a
periodic behavior and details will be studied by measuring the
local velocity field. Further, unlike the circulation cells that get
developed in the tank with conventional impellers (DT and
PBTD), in the presence of multiple blades and self-similar
behavior at different levels of the geometry of the FI, no such
circulation cycles were visible. As a result, the particles lifted from
the bottom remain mostly floating between different branches of
the impeller thereby reducing the extent of nonuniformity in the
suspension quality. At any given tank cross section, at different
impeller rotation speeds, the variation in the particle mass was