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to avoid high shear zones (mainly for shear sensitive media), the
conventional impellers may not be applicable.
In the present work, we propose a new impeller design that
occupies less than 0.4% of the volume of the reactor, which is
similar to the conventional impeller system, but the design allows
it to spread over almost the entire vessel, yielding a structure with
relatively large voids. The objectives are (i) to achieve uniformity
throughout the stirred tank and (ii) to develop an innovative and
efficient impeller that can yield better mixing and low shear at
relatively low power consumption. Both of these objectives can
be achieved using the principle of self-similarity, and hence a
fractal design will be more appropriate for an impeller. In the
present work, we report on the comparative performance of such
fractal impellers (referred hereafter as FI) vis-a-vis the conven-
tional ones. While several fractal geometries, configurations and
resolutions can be used for such a concept, in thismanuscript our
emphasis is on proving this concept and hence only one impeller
design has been used for this study. In section 2 we have
discussed the experiments, details of the fractal impeller, and
measurement techniques followed by observations and discus-
sion in section 3.
2. EXPERIMENTAL SECTION
2.1. Fractal Impeller (FI). Conceptually, the self-similarity in
the geometry of an impeller at different scales can be expected to
replicate in the self-similar distribution of energy to achieve
uniformity in the flow properties in a STR. It is known that for
mixing at small scale, generation of local chaotic advection by
different mechanisms including the mechanical movements
Received: February 12, 2011
Accepted: April 18, 2011
Revised: April 7, 2011helps to achieve better mixing.
4
Here we attempt to generate
such chaotic advection by using a novel impeller which has self-
similar geometrical features at different scales. The schematic of
the impeller, the fabricated unit, and the setup are shown in
Figure 1. The impeller has four main branches, each of which
further gets split in three sub-branches. On each of such sub-
branch we have four blades. Of which 2 blades are horizontal and
the remaining two are vertical. Importantly, in the entire design,
the orientation of the blades is kept such that none of the blades
actually sweep any liquid with thembut simply fragment the fluid
as they pass through it. An additional sub-branch at the bottomof
the impeller helps to generate the necessary flow in the region
close to the tank bottom. Also, for a given impeller rotation
speed, the angular distances covered by the blades vary and yield
variation in the local blade passage velocity. However with the
confined nature of the entire system, such variations do notmake
significant effects on the flow uniformity. Since the design of
impeller is expected to distribute energy in a more uniform
manner throughout the tank, it was thought desirable to char-
acterize the performance of this concept. More details on the
fractals are given in Appendix 1.
2.2. Experimental Setup. The experiments were carried out
in an acrylic stirred tank (T = H = 0.3 m) with a single impeller
system. The vessel was fitted with four baffles (width,W= T/10).
The impeller shaft was connected to a DC motor via a shaft
mounted torque transducer. Experiments were carried out with
three different impellers: 6 blade-disk turbine, 6 blade-PBTD,
and the FI. For DT and PBTD, the impeller diameter was D =
T/3 = 100 mm, and the off-bottom clearance (C) was equal to
T/3. The FI was supported fromthe bottombymaking a counter
groove on the shaft (Figure 1D) and for the FI, DFI = T/1.58. A