Introduction
Agitator is one of the core components of the stirred tank reactor, with
which efficient homogeneous mixing of fluid or uniform dispersion of
multiphase fluid is always expected. Although many kinds of agitators
have been proposed, there is still no universal agitator for any working
condition. Therefore, it is still an important task to explore new high
efficiency agitator in the field of chemical engineering.
Traditional
agitator, e.g. propelling agitator or Rushton turbine (RT) impeller, has
the characteristics of simple structure, easy operation and relatively
mature design principles. Nevertheless, these agitators are only
suitable for a specific process, e.g. the propelling agitator is mainly
used for heat exchange due to its large amount of axial flow, the RT
impeller is mainly used for the process that requires high values of
turbulent kinetic energy (TKE). However, the TKE generated by the RT
impeller is usually larger only around the impeller. In addition, the
mass exchange between two loops generated by the radial pumping of the
blades is very poor due to the existence of the disk. To improve the
distribution uniformity of the TKE and its dissipation rate, as well as
to intensify the mixing performance in the case of a wide range of fluid
viscosity, three typical large
cross-section
impellers, i.e. the Maxblend impeller, Fullzone impeller and Sanmeler
impeller, have been proposed. These agitators have shown excellent
mixing performance and flexible adaptability in the practical
applications1,2,
in particular, in the case of medium viscosity system. However, to our
knowledge, no significant increase in the magnitude order of the TKE has
been reported. Recently, our group has proposed a new design of the
multi-blade combined (MBC) agitator3, with which the mixing
can be intensified significantly with higher TKE and narrower
distribution of the TKE. Thus, it is necessary to carry out a
fundamental study of the flow characteristics in the stirred tank
equipped with the MBC agitator.
Velocity distribution is the basic flow characteristics of the stirred
tank. Different velocimetry measurement techniques like laser-doppler
velocimetry (LDV)4,5and particle image velocimetry (PIV)6,7have been widely adopted to study the flow patterns and turbulent
characteristics. Traditional PIV technique uses two cameras to capture
the consecutive image pair illustrated by the double pulse laser. Thus,
an external trigger is required to match the image capture to the shot
with the pulsed laser, resulting in a comprehensive measurement system.
In recently, with the development of high-speed CMOS cameras and
continues laser, time-resolved PIV (TR-PIV), which is a simple
velocimetry system, has been gradually used for the flow field
measurement 8. The image
capture frequency can reach thousands of frames per second, allowing a
very short interval time between two adjacent images for calculating the
velocities directly. For example, a capture frequency of 500 fps
corresponds to 2000 μs interval time, which is similar to that of
traditional PIV 9.
Therefore, the TR-PIV system provides a simpler and easier to operate
experimental tool for the dynamic measurement of unsteady flow without
using comprehensive external trigger system.
In addition to the measurement techniques, computational fluid dynamics
(CFD) technology has been flourishing in predicting flow characteristics
in different types of the reactors, mainly using Reynolds Averaged
Navier–Stokes (RANS) approach, large eddy simulation (LES) or direct
numerical simulation (DNS). RANS approach solves Reynolds Averaged
Navier–Stokes (RANS) equations with a closure turbulent model, e.g. the
standard k-ε model, however, the predicted turbulent kinetic energy is
generally under-predicted because the flow is extremely anisotropic
especially near the impeller region10. DNS can give the
most accurate predictions of the flow fields. However, it needs very
high-quality grids and a lot of computing resources. LES is a
filter-based approach in which larger eddies are solved directly and
smaller eddies are modeled by the sub-grid scale (SGS) model. Murthy and
Joshi 11 have
summarized the published literatures on the predictions of the flow
pattern using LES approach. Several works have proved that the LES
approach lies between RANS and DNS in terms of accuracy and
computational cost.
The objective of this work is
to
study the flow characteristics in the stirred tank equipped with our
newly developed MBC agitator by using
TR-PIV technique and LES prediction. The CFD predictions are firstly
assessed by the TR-PIV experiments. Then the turbulent characteristic,
time-averaged flow patterns, flow rates and power number are analyzed by
LES approach in detail, thus, providing a fundamental understanding of
the flow characteristics and guiding its practical applications.
Experimental