Turbulence in flocculators: Effects of tank size and impeller type



Past research shows that for the same power per unit volume, flocculation performance varies with tank size and impeller type. This study was performed to characterize effects of scale and impeller design on turbulence produced in the flocculation process. The study was performed with a Rushton turbine and an A310 foil impeller in three square tanks of 5-, 28- and 560-L volume. Fluid velocities were measured using a dual-channel laser Doppler velocimeter with an enhanced burst spectrum analyzer. Flocculation tanks were operated at a constant average unit-mass energy-dissipation rate of 0.0016 m2/s3. The results show that the turbulence intensity and local turbulent energy-dissipation rate were higher for the Rushton turbine than for the A310 impeller. The turbulence intensity was found to increase with increasing tank size regardless of impeller type. The local turbulent energy dissipation rate decreased for the Rushton turbine and remained constant for the A310 impeller with increasing tank size.