Mixing in a Model Bioreactor Using Agitators with a High Solidity Ratio and Deep Blades

The mixing behaviour of three up-pumping agitators (D/T = 0.45) have been measured in a T = 0.15 m diameter model bioreactor in the turbulent flow regime. The agitators used were a six-blade PBT, a Hayward Tyler B2 hydrofoil and an Applikon ‘elephant ear’ (EE). Fill heights ranging from 1/2T to 2T were investigated. Measured power numbers, Po, were independent of fill height unless surface aeration occurred; values of turbulent Po were 1.5,1.1 and 1.7 for the PBT, B2 and EE agitators, respectively. Global flow fields produced using PIV were similar for all agitators, with a high energy recirculation loop pumping upwards from the agitator and returning to it from below and a low energy recirculation loop in the upper half of the vessel. Velocity magnitudes in the upper recirculation loop decreased with increasing fill height. Axial flow numbers were 0.72, 0.71 and 0.87 for the PBT, B2 and EE agitators, respectively. Measurements of τ95 using PLIF showed good agreement with the Ruszkowski correlation. Despite EE agitators being extensively marketed as ‘low shear’ devices, the measurements gave no indication that they were different from the more conventional PBT and relatively-new B2 designs in this respect.