Direct determination of energy dissipation in stirred vessels with two-point LDA

The ensemble-averaged kinetic energy viscous dissipation rate was determined with a multipoint LDA technique in a vessel stirred by a Rushton turbine. Nine out of the total of 12 mean squared velocity gradients constituting the dissipation rate were directly measured in the impeller stream, and the deviation of the flow from local isotropy was investigated. The dissipation rate normalized with (ND2)-D-3, where N and D are the impeller rotational speed and the impeller diameter, respectively, was found to be approximately constant for Reynolds numbers of 20,000-40,000, whereas the fluctuating gradients were found to vary significantly, by up to three times the ensemble-averaged values, with blade angular position. The ensemble-averaged dissipation rate values were also assessed through a local turbulence kinetic energy balance. (c) 2005 American Institute of Chemical Engineers