PIV investigation of the flow features of double and single 45° up-pumping pitched blade turbines in a square tank

The non-conventionally square stirred tank with up-pumping impellers has been recently adopted in wastewater treatment processes due to its simplicity and high efficiency. In this paper, the time-averaged flow fields in the square tank with both double and single 45° up-pumping pitched blade turbines (PBTs) are investigated using particle image velocimetry (PIV). Flow field generated by double impellers are directly compared with the flow fields by single impeller. The flow number (FI), anisotropy, total kinetic energy, and energy dissipation rate (ε_T^*) are calculated to further investigate the flow features of the double impellers and single impeller. The results show that the high velocity region generated by a single impeller is about 0.4T (T is the length of square cross-section of tank) in z direction. Flow number of the upper impeller is much larger than that of the lower impeller. In flow fields with a single impeller, the higher the impeller, the larger the flow number. The high flow anisotropy in terms of large scales occurs in the areas both above and beneath the impeller and near wall region. However, the high kinetic energy area corresponds to the high velocity region. Three numerical methods commonly used in the traditional cylindrical tank are employed to calculate ε_T^*. The maximum ε_T^* happens in areas near the impeller. Three methods give similar dissipation rate distributions but different values, which is the same as that in the cylindrical tank. It means that these methods could also be well used in square stirred tanks.