Turbulence Intensity of Mixing in Relation to Flocculation

An assessment has been made of fluid turbulence intensities and local velocities throughout a 1‐L square reactor in relation to flocculation efficiency. Measurements have been taken using laser‐Doppler anemometry for three types of stirrer, and aluminum sulphate has been used as the coagulant for a synthetic raw water based on kaolin as suspension with controlled alkalinity. Stirrer speeds ranging from 16 to 100 rpm were adopted, and the torque applied to the fluid was measured directly. Turbulence intensity is shown to be an alternative measure of flocculation efficiency to velocity gradient or power input. The power input required for minimum turbidities is much the same for all stirrers, but where optimum conditions cannot be predetermined, a stirrer that extends throughout the volume of the reactor and has sharp‐edged blades is shown to be more versatile than the other types.