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analyses, but no power draw analysis is shown. Heiser et al.
(2004) described the performance of a co-axial mixer consist-
ing of a helical ribbon and a central screw.
An important consideration in the design of an agitated
system is the power required to drive the impeller. A reliable
design and scale-up of important mixing tasks such as
blending and heat transfer is generally achieved by invoking
geometric and hydrodynamic similarity based on selected
dimensionless parameters. For single impellers in standard
baffled tanks the utilization of dimensionless numbers, such
as Reynolds, Newton, Prandtl, Nusselt is very well estab-
lished. For complex mixing systems such as co-axial
mixers still there is a lack of understanding of the fluid
mechanics characteristics required for design guidelines
and scale-up of co-axial impeller systems. The design of
co-axial mixers today is essentially based on the vendor’s
experience and/or trial-and-error approach. Consequently,
the utilization of such non-conventional mixing technology
in industrial processes today is not common.
An experimental and numerical programme was started
by the authors to explore and determine design and mixing
performance characteristics of co-axial agitation systems.
The aim of the present work is the analysis and characteriza-
tion of the power consumption of a suitable co-axialmixer con-
figuration for chemical processes. The mixing system studied
consists of an anchor and a dual set of pitched blade turbines.
METHODS AND MATERIALS
Experimental Set-Up
Apparatus
The experiments were carried out in a lab-scale tank
equipped with a fully instrumented pilot-scale co-axial mixing
system, which is shown in Figure 1. It combines a proximity
impeller with a dual set of centered open impellers. The proxi-
mity impeller used in the experiments is the classical anchor
impeller. The open impeller was a standard four-bladed,
pitched turbine with 458 blade angle (A200 by Lightnin,
USA). The cylindrical tank of 490 mm inner diameter is madeof Plexiglas with a dished bottom in torispherical shape. The
mixers are powered by two independent electric drive-motors
of 3 kW and 1.5 kW; one drives the inner impellers and the
other the outer proximity impeller, respectively. The mixing
system was instrumented to measure continuously the
torque, and rotational speed of the inner impellers and power
and rotational speed of the proximity impeller. The measured
total power consumption for the outer impeller was corrected
by subtracting the measured power from a calibration curve,
which includes the motor and bearing friction losses.
Test fluids and rheology
To achieve a wide range of viscosity and Reynolds number
in the laminar regime, two different glucose syrups were used
as Newtonian fluids, C
Sweet Syrup (Cerestar GmbH) and
Glucomalt (Tate&Lyle Europe). At 258C, the viscosity is
19.4 Pa s and the density is 1415 kg m23
for CSweet
(SWE) and 111 Pa s and 1400 kg m23
for Glucomalt at
96.2 wt% (GLU). The glucose syrup solutions are very sensi-
tive to temperature changes. Thus corrections in the viscosity
had to be made to account for the temperature increase
during the measurement of the power curves. Fluid tempera-
ture was not measured continuously, but only after a set of
power measurements.
Aqueous solutions of hydroxyl-ethyl-cellulose
(CELLOSIZETM HEC QP 300, The Dow Chemical Company)
at concentrations from 5 to 8 wt% were employed as non-
Newtonian fluids. The rheological behavior of HEC solution
was described by a power law. The shear-thinning index
varies from 0.42 to 0.51, the consistency index varies between