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.