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abstract Computational and experimental methods have been used to investigate the flow field, power and mix-
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ing time in a fully baffled stirred vessel with two six-blade Rushton turbines. Flow in a stirred tank
involves interactions between flow around rotating impeller blades and stationary baffles. In computa-
tional fluid dynamics (CFD), the flow field was developed using the sliding mesh (SM) approach. The
large eddy simulation (LES) was used to model the turbulence. For validation of simulation results two
series of experimentswere performed: (i) velocitymeasurements of the liquid phase using particle image
velocimetry (PIV) and (ii) concentrationmeasurements of the determining tracer in the liquid phase using
the planar laser-induced fluorescence (PLIF) technique. In each series three different rotational speeds
of impellers: 225, 300 and 400 rpm were employed. The stirring power input was also calculated based
on the PIV results. A considerable reduction in mixing time was achieved and stirring power input was
increased by increasing the impeller speed. The satisfactory comparisons indicate the potential usefulness
of this CFD approach as a computational tool for designing stirred reactors.9570
© 2009 Elsevier Ltd. All rights reserved.1. Introduction
Stirred tanks are widely used for mixing two miscible fluids in
chemical, food and processing industries.Normally, in a stirred tank
with the impellers positioned centrally, a rotating motion with a
pair of vortices behind each blade, one above and one below the
disk, is generated. The fluid in the vicinity of an eddy is highly
sheared, resulting in the local reduction of a property, e.g. the con-
centration of a tracer. The swirling motion of the fluid causes a
complex recirculating turbulent flow in the tank, where the sta-
tionary baffles interact with the flow, improving the agitation. The
flow discharged by rotating impeller forms a jet towards the tank
wall. After flowing vertically along the wall, the fluid will have a
recirculation flow pattern towards the axis of the tank.
Whenmore than one impeller in amixing tank is used, the flow
complexity is greatly increased. Experimental investigations have
contributed significantly to the better understanding of the com-
plex hydrodynamics of stirred vessels. Experimental investigation
of the flow generated by two Rushton impellers has been reported
by Rutherford, Lee, Mahmoudi, and Yianneskis (1996) using laser
Doppler anemometry (LDA), with the main focus on the trailing
vortex structure behind the Rushton impeller. The various values of
impeller clearance from the vessel bottom and spacing between
the impellers were also studied. Bonvillani, Ferrari, Ducrós, andOrejas (2006) experimentally determined the mixing times for a
tank equipped with a stirrer propelled by two Rushton turbines.
In their work the mixing time was experimentally determined
using the pH-response techniquewithout any validation. Chunmei,
Jian, Xinhong, and Zhengming (2008) used the two-dimension PIV
method for measuring velocity, and also studied the flow patterns
and the effects of impeller clearance.
In the experimental investigation of the flow generated by two
Rushton impellers, awide variety of impellerswith different shapes
and sizes, andwith varying impeller clearance and such, are used in
practice for different applications. Therefore, a computational tool
which can predict the flow around an impeller of any shape and its
interaction with another impeller mounted on the same shaft will
have enormous applications in mixing technology.
For a dual impeller case there is a relatively fewer numeri-
cal studies of mixing in the literature. Vrabel et al. (2000) used
the compartment model approach (CMA) to develop a flow model