Particle Image Velocimetry Study of Flow Patterns and Mixing Characteristics in Multiple Impeller Stirred Tank

The flow generated in a stirred tank of 0.48 m diameter, agitated by three impellers of 0.19 m diameter, including one half-elliptical blade disc turbine (HEDT) below and two up-pumping wide-blade hydrofoils (WH(U)), were investigated by using 360 degrees ensemble-averaged particle image velocimetry (PIV) technique. The flow characteristics for seven impeller combinations with different impeller spacings were investigated. Results show that global circulation patterns in the stirred tank are strongly dependent on the impeller spacing. Three types of flow patterns were observed, including five, four, and three circulation loops in the flow field, by changing the spacing between the bottom and the middle impellers (C(1)), the spacing between the middle and the top impellers (C(2)), and the submersion depth of the top impeller (C(3)). While the clearance (Delta C) of the bottom impeller is equal to 0.4T, the five circulation loops flow pattern occurs when C(1) = 0.48T, C(2) = 0.48T, and C(3) = 0.4T, and the four circulation loops flow pattern occurs when C(1) = 0.48T, C(2) = 0.4T, and C(3) = 0.4T, and the three circulation loops flow pattern occurs when C(1) = 0.48T, C(2) = 0.4T, C(3) = 0.32T. The mixing efficiency defined based on the mixing time and the power consumption were also discussed and the results show that less circulation loops could lead to higher mixing efficiency, but the mixing efficiency does not solely increase with the decreasing of the circulation loops.