A digital particle image velocimetry investigation of flow field instabilities of axial-flow impellers

Digital particle image velocimetry (DPIV) has been used to examine the flow field in a vessel agitated by an axial-flow impeller in turbulent operation. Both a pitched blade turbine and a high-efficiency impeller were studied. Time series analysis indicates that the flow field is not steady; rather, it is subject to transients with frequencies well below the blade passage frequency (periods ranging from 40 to over 300 impeller revolutions have been observed). This result has important implications for computational modeling because current descriptions of agitated vessels are based upon time-averaged flow fields with superimposed turbulence. This modeling approach may not accurately capture the mixing associated with the low-frequency phenomena observed in this study.