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steel ball (6 mm diameter) was sandwiched between the bottom
of the shaft and the center of the vessel bottom. The volume of
the liquid inside the reactor was 21.2 L for all experiments. The
solidity of all the impellers was maintained in a close range.
2.3. Measurements. The experiments were carried out to
compare the performance of the FI with the conventional
impellers, vis DT and PBTD. To facilitate such a comparison,
the power consumption by the impeller, the mixing character-
istics, and the efficacy of suspending the particles andmaking gas
liquid dispersion were considered as the measurable parameters.
2.3.1. Power Consumption. Power draw can be measured
using various methods.
5
In our experiments we used a rotary
torque transducer (CTime Sync, UK). These transducers are
noncontact optical devices, which function using the displace-
ment principle causing a variation of volume of light. Depending
upon the extent of torsion experienced by the impeller shaft
during itsmotion, a proportional volume of light is generated by a
low power demand solid state laser. This volume of light is
captured by the optical components attached to the transducer
torsion shaft, and the value helps us to know the torque
experienced for a given impeller rotation speed. An in-built shaftencoder 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.