Conclusions
In this work, time-resolved PIV and LES approach are used to investigate
the flow characteristics in a tank of H /T =1.5 equipped
with our newly developed multi-blade composite (MBC) agitator, which
consists of four composite blades. Each composite blade includes a
connecting blade in the middle, a long blade, and two upper and lower
short blades. The LES prediction accuracy using the dynamic kinetic
energy sub-grid scale is first assessed by the analysis ofY+ values and the time-dependent power
spectrum, as well as the validations of the velocity and its
fluctuations obtained by the TR-PIV technique.
Then, the turbulent characteristics including turbulent vorticity
analysis and turbulent kinetic energy (TKE) evaluation, the flow
patterns, and the power consumption are predicted and discussed. TR-PIV
results reveal that many anisotropic eddies of about 0.25R in
size are uniformly distributed along the axial direction. Through LES
analysis, the vortexes visualized by the Q -criterion are fully
distributed in the whole stirred tank, resulting in an obvious
improvement of the TKE distribution uniformity, as well as the increase
of the TKE magnitude, compared with that of the Rushton turbine
impeller. This excellent TKE performance is attributed to the effective
mass exchange in both axial and radial directions. Velocities
distribution analysis demonstrates that there exist two regions in the
tank. Upper and lower short blades produce a strong axial down-flow in
the top half and an axial up-flow in the bottom half of the vessel,
respectively. Part of the down-flow (or up-flow) becomes radial flows
during going down (or going up) along the axial direction continuously,
which should be attributed to the radial pumping of the long blades.
Strong impingement of the down-flow and the up-flow in the middle
produce a strong radial flow, resulting in effective axial mass exchange
between the two upper and lower regions. Power consumption analysis
indicates that the power number is much larger than that of the
traditional impellers, i.e. the energy can be loaded into the system
effectively, consequently, both radial mixing and axial mixing are
intensified by the combination of the short blades and long blades,
thus, leading to an obvious improvement in the TKE distribution
uniformity with high intensity.