(165b) Elastic Flow Compartments in Stirred Tanks

Elastic
flow compartments in stirred tanks

Markus
Kolano,
Matthias Kraume, TU Berlin, Chair of Chemical and Process Engineering,
Ackerstraße 76, 13355 Berlin, Germany

Mixing
processes in stirred tanks in which one or more liquids exhibit viscoelastic
flow properties are of particular importance in food, polymer and
biotechnology. It is well known that elasticity can lead to flow phenomena like
the Weissenberg effect or flow field inversion when stirring with a Rushton
turbine: While at low rotational speeds a completely axial secondary flow field
can be formed and elastic forces dominate, an increase in stirring rate and
thus in centrifugal forces finally results in a radial flow profile. Between
these extreme cases, cavern like flow compartments, that is to say almost
completely separate circulation vortices with limited convective exchange,
form, which result from mutually competing elastic and centrifugal forces. These
elastic compartments can form using any stirrer geometry and have a significant
effect on the mixing efficiency of the stirred tank (see figure 1). Knowledge
of their existence is therefore of high interest.

Figure 1: PIV measurements of secondary flow fields
showing elastic flow compartments; left: Rushton turbine in aqueous xanthan gum
solution (1 g/kg), middle and right: pitched blade turbine and 3-blade
propeller in aqueous CMC solution (10 g/kg)

In
this presentation, the influence of elastic flow properties as well as the
rotational speed on the formation of these flow compartments is investigated.
Secondary flow fields are presented for different stirrer geometries (Rushton
turbine, pitched blade turbine and propeller stirrer), which were obtained by
the PIV method for various viscoelastic fluids (aqueous xanthan gum and carboxymethyl
cellulose solutions). In addition, the pLIF method is used to investigate the
mixing behavior of these systems and to analyse the extent to which convective
mass transport takes place between the compartments as well as their sizes and
forms (see figure 2). It can be shown that by using the elasticity number a
general classification of flow regimes in a transition state between elastic or
inertia dominated flow for all analysed geometries and fluids is
possible. These results will be supported by CFD simulations using viscoelastic
constitutive equations.

Figure
2: pLIF measurements; left: detailed study of
a flow compartment caused by Rushton turbine, right: flow compartment caused by
a pitched blade turbine