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    abstract Computational and experimental methods have been used to investigate the flow field, power and mix-
    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
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