Performance Of Flow And Mixing Characteristics In Cascade Stirred Tanks

In this paper, the performances of fluid flow and mixing were studied in the mockup of both square and circular stirred tanks. The flow characteristics, power input, mixing performance and back-mixing behavior were systematically investigated in agitated tanks with six blade disc turbine,45°pitched blade turbine or45°A315impeller.Low frame rate of high speed camera was used to visualizing the fluid flow pattern and vortex structure in square and circular tanks with various impellers. The results show that the non-continuous smooth transition structure can easily lead to boundary layer separation sighting as secondary flow, forming large scale vortexes behind the baffles under the impeller area and near the surface. Furthermore, the scale and stability of the vortex in mixing system were affected by tank structure and impeller types.By measuring the power input and mixing characteristics of various impellers, it is suggested that virtual baffle effect of square tank improves the ability of power input, and various impellers have different sensitivity to the virtual effect; under the same power input per unit volume, the mixing time: tm,RT>tm,45°PBTU> tm,45°A315, and the dimensionless mixing time: NtRT<Nt45°PBTU<Nt45°A3i5.Quantitative results of inter-stage back-mixing between cascade stirred tanks reveal that the degree of back-mixing corresponding to various agitators under the same condition is presented as following:LRT> L45°PBTU>L45°A315, increases with the increasing of power input per unit volume. Optimizing the structure of flow channel, such as location, orientation, flow channel size, etc., can significantly reduce the back-mixing. It is hopeful that the quantitative back-mixing would be instrumental for the commercial stirred tanks.