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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 consumptionencoder helps to monitor the impeller rotation speed. Signal
processing is done within the transducer, and the transducer can
be fixed either by base flange or in-line, between suitable
couplings. The torque data were acquired online on a PC using
a data acquisition system and were later subjected to Fourier
analysis to identify the possible dominant frequencies that would
affect the flow and which may be characteristics to the impeller.
The FI impeller structure was given a support at the bottom. It
was seated on a steel ball and was seen to have a very smooth
motion without offering any significant friction due to the
contact between the impeller bottom and the steel ball, and thus
the measured torque was entirely due to the friction experienced
by the impeller.
2.3.2. Mixing Time. The mixing time was measured by giving a
tracer (of 0.3% of the total reactor volume) in the form of
concentrated salt solution (1 M, NaCl in the form of pulse of) at
the liquid surface. The tracer concentration wasmeasured in time
using the conductivity probe (connected to a standard conduc-
tivity electrode with cell constant of 1.0 along with a digital
conductivity meter) fixed at a given location in the tank. The
mixing time is considered as the time at which the measured
concentration of the tracer reaches to within 9598% of the final
concentration. The transient variation in the concentration was
used for the estimation of θmix. In general, under turbulent flow
conditions, θmix is inversely proportional to the impeller speed,
and the product N3
θmix known as dimensionless mixing time is
used as a performance parameter.
2.3.3. SolidLiquid Suspension. The FI was also used for
checking its ability to suspend solid particles. Two different types
of particles were used: (i) resin particles (Fs = 1080 kg/m3
) of the
particle size in the range of 350500 μm and (ii) glass bead
particles (Fs = 2500 kg/m3
) of diameter 250 μm((6 μm) in tap
water (FW≈1000 kg/m3
). For the case of resin particles the local
particle concentration at different distances from the bottom of
the tank was measured, and for the suspension of glass particles,
cloud height was measured. A SS316 straight tube (4.5 mm o.d.
and 3 mm i.d.) was used to collect the resin particles locally, and
their mass wasmeasured to estimate the local solid mass fraction.
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