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largely due to the local regions generated by the interaction of
flow domains from different blades in different branches. With
increasing particle loading, the extent of uniformity in the
suspended particle concentration also increased. In the case of
a PBTD, two different circulation loops, one below and another
above the impeller are established. Unlike this, in the FI, higher
solidity practically breaks such loops and develops several net-
works of zones which interact with each other. This also helps to
keep the particles suspended while being transferred from one
zone to another self-similar zone. More detailed experiments on
tracking of particles between zones would help to quantify the
residence time of particles in different zones and thereby
characterize the energy distribution in the stirred reactor with
such an impeller. Such details are being investigated and will be
reported separately.
In another set of experiments, the performance of FI for
suspending solid glass particles was studied. The Pw variation for
different glass particle suspension densities is shown in
Figure 5A.While the nature of plots is similar to that of Figure 3,
the value of Pw at similar impeller rotation speed is almost twice
that of the suspension of particles with density 1069 kg/m3
. This
extent of difference is almost equivalent to the settling velocity of
these particles in water, which is proportional to their densities.
Onmeasuring the cloud height for suspension of glass particles, it
was seen that the extent of lifting of the particles in the bulk
increased with increasing suspension density. This implies that
the increase in power consumption at higher suspension den-
sities was indeed utilized in suspending particles. Visual
), the FI showed much lower powerobservations showed that at very low suspension density, the
particles always remained in the lower half of the impeller. This
was largely because the particles were seen to remain entrapped
in the smaller mixing zones formed by the blades of fractal
impeller. This observation was also seen for higher suspension
density, but at lower impeller rotation speed (<100 rpm) the
particles were seen to get aligned in a peculiar manner at the tank
bottom, in two lines each connecting the diagonally opposite
baffles. On increasing impeller speed, they eventually get sus-
pended. This indicates that at lower N, the confluence of radial
flow, tangential flow, and the presence of baffles makes the
particles to assemble in a specific manner. At higher suspension
density (5%), the particles were seen to get easily suspended,
which is not very common largely because of the variation in the
bulk property which helped the particles get suspended easily.
Equation 1 was seen as valid even for the suspension of glass
particles with the value of C1 = 5.5 107
.
The performance of suspending identical glass particles in a
stirred tank using FI and PBTD(in a large tank having identicalT/H
ratio10
) is shown in Figure 5B. The observations for three different
solid concentrations can be summarized as follows: (i) at 1% solid
concentration, PBTD (filled symbols) performs much better than
FI (open symbols) in suspending particles even at very low impeller
speed; (ii) at 3% and 5% solid concentration, the power required for
lifting of particles with PBTDis relatively lower than that of FI.With
glass particles, the terminal velocity being higher, while achieving
complete suspension was possible at lower Pw, achieving uniform
suspension needed relativelymuch higher power. (iii)With 5%solid
fraction, the FI is efficient in suspending particles at higher
concentrations; (iv) the trend in the efficiently suspending the