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particles at different solid concentrations for PBTD and for FI are
exactly opposite. The values of PW for PBTD were estimated from
the values of Re, D reported in Sardeshpande et al.
10
and NP for
PBTD in turbulent region. These observations indicate that this
novel impeller design is useful in efficiently suspending particles at
higher solid loadings, which is not very easy with the conventional
impellers. Importantly, the solid concentration for glass particles
along the height (measured in similar manner as for low density
particles) (not shown here) when the dimensionless cloud height is
1 was very much uniform with a standard deviation of 6%.diameter that can inhibit the secondary flow. Also, since gen-
erating a relatively larger radial component of the mean velocity
results into large energy dissipation at the wall and thus lowers
the mixing efficiency, it is always preferred to avoid such a
situation. Further, a larger impeller diameter demands higher
torque and hence higher capital cost. Hence, the selection should
be made on the basis of capital and operating costs. While these
observations are valid for the conventional impellers, it does not
necessarily apply for the FI. Hence experiments were carried out
to understand the characteristic mixing time for a FI.
The mixing time was measured as described in the section 2.
The conductivity signal was smoothened to eliminate the
spurious effects due to data acquisition noise, and the smooth-
ened signal was analyzed tomeasure themixing time. Themixing
time at identical N for PBTD was 2 to 3.5 times higher than the
FI. This particular situation can again be explained on the basis of
the existence of fractal structure which develops self-similar flow
structures in the entire vessel and hence a uniform randomness.
As a result, the tracer gets continuously distributed in several
mixing zones existing in the reactor due to the fractal structure of
the impeller, and it helps achieve better mixing. However a
comparison of the θmix variation as a function of the PW shows
that both impellers have similar performance (Figure 6). Experi-
ments were also carried out to measure the mixing of a tracer
liquid in the bulk viscous liquid (50% glycerol solution). In this
case, the mixing time was 40% higher than that of water, and this
observation was consistent over the entire range of impeller
rotation speed for FI.Mixing time was alsomeasured for different
solid loadings for the suspensions and the dimensionless mixing
time showed a positive dependence on the solid loading. This
observation is consistent with an increase in the solid loading; the
bulk viscosity and density increase thereby leading to enhanced
viscous forces, higher drag, and hence a longer mixing time.
The fact that the presence of number of blades and mixing
zones would create a uniform randomness was verified by taking
the fast Fourier transforms of the acquired time series of the
torque data. The resulting power spectra for one such experiment
with N = 100 rpm is shown in Figure 7. It can be clearly seen thatunlike the literature information on variety of conventional
impellers, where the impeller rotation frequency, blade passage
frequency are prominently seen in the power spectra, in the case
of a FI no specific dominance was seen. The power distribution
over a range of frequencies showed similar features and thus
support the notion that with the help of such a self-similar
structure for mixing of fluids, one can attain a uniform random-
ness in the flow at different scales, and no specific instabilities
(associated with certain frequency)
12
exist that are usually
considered to promote spatial mixing. Thus, the scaling effects