AWWA ACE95074

A study was performed to characterize the complex fluid flow and turbulence produced in the flocculation process. Mean and turbulent velocities, turbulent kinetic energy per unit mass, and energy dissipation rate per unit mass were measured at several locations in a square batch reactor. The effects of changing the vessel size and impeller type on the fluid mechanics properties were evaluated. The study was performed with a Rushton turbine and an A310 fluid foil impeller in three square tank sizes of 5L, 28L, 560L. Measurement of the fluid velocities was done using a dual-channel laser Doppler velocimeter with an enhanced burst spectrum analyzer. In order to isolate and compare the effects of tank size and impeller type on the flocculation fluid mechanics, the flocculation tanks were operated at a constant average characteristic velocity gradient, G, of 40 s-1. Results indicate that the turbulence generated by an A310 foil impeller is much different than that produced by the Rushton turbine when G=constant. The dimensionless rms fluctuating velocity, turbulent kinetic energy, and local turbulent energy dissipation rate were higher for the Rushton turbine than for the A310 foil impeller. The dimensionless rms fluctuating velocity and turbulent kinetic energy were found to increase with increasing tank size regardless of impeller type. The local turbulent energy dissipation rate was found to decrease with increasing tank size for the Rushton turbine and the energy dissipation rate was found to remain constant with increasing tank size for the A310 foil impeller. Based on the results of this study, there is clear evidence that maintaining G=constant with tank size or impeller type does not translate to constant spatial distribution of the local turbulence generated in the tank.

Product Details

Published:

01/01/1995

ISBN(s):

0898678331

Number of Pages:

22

File Size:

1 file , 560 KB