Investigation Of Fluid Flow In A Multi-Impeller Stirred Tank Using PIV

Stirred tank with multi-impeller is widely used in many kinds of industry fields. The flow fields in a triple-impeller stirred tank with diameter of 0.476m were measured by using Particle Image Velocimetry (PIV). A deep hollow blade (semi-ellipse) disk turbine (HEDT) was used as bottom impeller and two upward-pumping 4-wide-blade hydrofoil impellers (WHU) were used as middle and upper impellers. The effect of the locations of middle impeller on the Phase-resolved velocity, Phase-averaged velocity and turbulent kinetic energy distributions was investigated while keeping the rotational speed and the submergence of the upper impeller constant. The results show that the position of the middle impeller has more influence on the flow characteristics of the upper region than that of the lower region in the stirred tank. As the middle impeller moves down, the reverse flow near the liquid surface is gradually disappeared, and the axial flow between middle and upper impellers is finally separated each other, and the maximum phase-averaged radial velocity of HEDT impeller is declined, and the maximum phase-resolved velocity of middle and upper WHU impellers moves separated, while the upper circulation region of the lower impeller is became to smaller. For phase-resolved plot of velocity, there is no distinct change on trailing vortex generated by middle impeller, but the trailing vortex of upper impeller becomes more and more clear, and that of lower impeller appears earlier at a phase angle of 10°; For the distribution of turbulent kinetic energy, the interactions between the upper and middle impellers decrease, but the values of middle and lower impeller increase. The results are of importance to the design and optimization of industrial multiple impeller stirred tanks.On the other hand, flow pattern and ensemble-averaged mean velocity at different spacing between the impellers are studied by using PIV. The results show that when the combination is HEDT+2WHu,the lower axial flow of HEDT has no varies, the upper axial flow of HEDT is shrunk by the middle impeller, but there is little influence when C3≤0.48T; The integrate axial flow between middle and upper impellers can separated each other when C2≥0.48T; The reverse flow near the liquid surface is against for the mixture in the stirred tank, the field of it is small when C1≤0.36T; So there is a ideal flow pattern when C1=0.36T, C2=0.4T, C3=0.48T. The fluid between middle and upper impellers owns a high velocity distribution, and the same to the fluid of upper region in the upper-impeller and the radial jet flow region of HEDT. When the combination is HEDT+2WHD,there is two flows in the stirred tank totally, The one is the flow integrated with the axial flow generated by the middle and upper impellers and the upper radial flow generated by the HEDT impellers, the other is the lower radial flow generated by the HEDT impellers. The flow pattern has no distinct change in different stations, but the fluid velocity between two impellers and the radial jet flow direction is influenced strongly. It is right for industry when C1=0.48T, C2=0.48T, C3=0.48T.