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No external suction was used to capture the particles as that
would affect the local flow.
2.3.4. GasLiquid Dispersion. To study the performance of
the FI for dispersion of gas into liquid, experiments were carried
out by sparging compressed air in the stirred tank using a ring
sparger located at the bottom of the reactor. The sparger had 16
holes of 1 mm diameter spaced at equal distance. The superficial
gas velocity was monitored and controlled using precalibrated
rotameter. The power consumption during the stirring at differ-
ent impeller rotation speeds and over a range of superficial
velocities was measured as mentioned before. The fractional gas
hold-up was estimated from the difference in the height of
dispersed liquid and clear liquid. The bubble size was estimated
from the images obtained from a high speed camera (Red lake).
The observations from these experiments are discussed in
section 3.4.
3. RESULTS AND DISCUSSIONS
3.1. Power Consumption. The actual power consumption
(P) by the impeller was estimated using the measured torque
data (τ) at different impeller rotation speeds as P =2πτN, where
N is the impeller rotation speed (per second). Subsequently
the volumetric power draw (P/V) and the power consumption
per unit mass PW (W/kg) were calculated. The impeller power
number NP was estimated as NP = P/(FLN3
D5
), where FL is the
bulk fluid density (estimated by taking into account the dispersed
phase properties). Typical variation in PW with increasing
impeller Reynolds number (Re= ND2
F/μ) showed power law
relations (Figure 2A). Since the energy dissipation per unit mass
or the energy draw scales as N3
D2
in the turbulent regime, the
plot of PW vs N3
D2
showed positive relationship for all the three
impellers. Interestingly, while the linear relation exists for the DT
and PBTD in the turbulent regime, for the FI, a linear variation
was noticed for the entire range of impeller rotation. While the
values of the intercept for the linear straight line for DT and
PBTD were very close (Figure 2B), the slope of the relation for
DT was higher than that for PBTD. For the FI, the intercept as
well as the slope were very small as compared to the other two
impellers. This indicates that the overall energy draw with the FI
is much lesser than that for the other two impellers. In other
words, at identical Re, PW for the FI is lower than the conven-
tional impellers. Typical plot of power versus speed (not shown
here) also shows a change in the slope. The plots showed an early
transition for the FI than the conventional impellers. However,
with the varying distance of blades and branches, it may need
some other way to define the Re to characterize the conventional
laminar and turbulent regimes. For the present calculations, we
have used the actual lateral distance between the farthest blades
as the impeller diameter (D). On estimating the power number
for these cases (Figure 2C), the NP value for a single DT
independent of Re was 6.014 (which is close to the value known
for standard DT: 6), while for the PBTD of the same dimensions
and standard geometry, NP independent of Re was 1.84. How-
ever, for identical rotation speed of the FI, the Re was much
higher due to larger diameter, and the value ofNP independent of
Re was 0.38, which ismuch lower than the NP for DT and PBTD.
Thus, beyond the critical Re, PW,FI is lower by many times that of
the conventional impellers. One of the reasons for such lower
power consumption is the lack of significant energy dissipation
gradients in the tank as they exist for the conventional impellers
and where the energy dissipation occurs due to the cascading