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Abstract: An experimental and numerical programme has been carried out to explore and
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determine design and mixing performance characteristics of co-axial agitation systems. Power
consumption analyses in Newtonian and non-Newtonian fluids in laminar regime for a co-axial
mixer configuration for chemical processes are discussed. An anchor impeller was used in com-
bination with a dual set of pitched blade turbines in co-rotating mode. It was demonstrated that
the power consumption of the proximity impeller is affected by the tip speed ratio, but no influ-
ence of the proximity impeller speed on the power drawn by the pitched blade turbines was
observed. Computational fluid dynamics (CFD) was employed to calculate the flow field created
by the co-axial mixer. CFD was able to predict well the power consumption of the co-axial mixing
system. Power and Reynolds number were adapted to obtain the power characteristics of
co-axial mixers. The approach employed to obtain a single master power curve succeeded
for the investigated co-axial mixer configuration and is a useful engineering tool to predict
the power consumption of co-axial mixing systems.8012
Keywords: co-axial mixer; non-Newtonian fluids; CFD; power consumption; anchor; A200.
INTRODUCTION
Stirred tanks are the most commonly used
fluid mixing devices in the chemical proces-
sing industries. Efficient mixing is crucial for
product quality, reduction of by-product for-
mation, suspension of solids, heat and mass
transfer. The design of mixers is particularly
challenging in industrial applications when
the fluid viscosity increases during the manu-
facturing process. Co-axial impeller systems
are a very promising alternative for such
processes because of the synergistic fluid
dynamic effects of two independently rotating
impellers on the same reactor axis. A
common co-axial configuration consists of a
combination of high speed impellers and
close-clearance impellers. In conventional
mixing systems, close-clearance or proximity
impellers are primarily used for mixing of vis-
cous fluids and to increase heat transfer
rates by frequent exchange of the material
close to the wall. In low viscosity fluids, their
mixing efficiency is usually very poor,
because primarily tangential motion is
induced. This results in a solid body rotation
due to the lack of baffles that deflect the tan-
gential flow. Solid body rotation also occurs
with open impellers at low viscosities in
unbaffled tanks. In a co-axial mixing system,
at low viscosities where there is a need for
baffling, the proximity impeller can serve as
a baffle for the open impeller that would
have the primary mixing task. At high viscos-
ities, the inner impeller would gradually loose
its efficiency and the outer impeller would do
the major mixing work.
Co-axial mixers are used in industry but
detailed analysis of their performance charac-
teristics have only recently appeared in the
open literature. Intensive studies on the
subject have been conducted by Tanguy and
co-workers (Espinosa-Solares et al.,1997;
Foucault et al., 2004, 2005, 2006; Thibault
and Tanguy, 2002). The investigated co-axial
mixers consisted of a dispersing turbine (e.g.,
rushton or sawtooth impellers) combined with
a proximity impeller (e.g., anchor or helical
ribbon). Ko ¨hler and Hemmerle (2003) studiedthe power characteristic of a co-axial mixer operating in coun-
ter-rotating mode in transitional and turbulent regime
(Re . 100). Todtenhaupt and Schneider (1990) discussed
the performance of a co-axial mixer with anchor and a dual
setofVISCOPROPw in terms of blend time and heat transfer