Characterization of helical impellers by circulation times

Measurements of the circulation time during batch mixing of Newtonian and non‐Newtonian fluids were used to analyse and compare the performance of six different helical‐ribbon impellers and a screw impeller inside a draft coil. The distribution of circulation times obtained from two hundred repeated measurements is shown to depend markedly on the impeller geometry. Mixing time data with the helical‐ribbon impellers are correlated with the mean value and the reduced standard deviation of the circulation times. Effective (rapid) mixing corresponds to a large circulation capacity and a wide distribution of the circulation times. The mixing mechanism of Newtonian fluids with the helical‐ribbon impellers is qualitatively described by the Voncken model. The helical impellers' circulation capacities are not affected by the shear‐thinning properties of non‐Newtonian fluids. However, in highly shear‐thinning fluids, the presence of important stagnant zones is responsible for much longer mixing times which consequently do not correlate with the circulation parameters. The relative efficiencies of the different impellers in Newtonian fluids are compared using a criterion based on the the total energy required to achieve a certain degree of mixing. The wide blade impeller is the most efficient of the helical‐ribbon impellers but is considerably less efficient than the screw impeller in a draft coil.