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 turbulen t operation. Both a pitched blade turbine and a high-efficiency impell er 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 30 0 impeller revolutions have been observed). This result has important implications for computational modeling because current descriptions o f agitated vessels are based upon time-averaged flow fields with super imposed turbulence. This modeling approach may not accurately capture the mixing associated with the low-frequency phenomena observed in thi s study.