Laser-Doppler measurements of turbulent-flow parameters in a stirred mixer

Abstract

It is recognized that a detailed knowledge of turbulence parameters, as well as velocities, can aid in understanding and modelling mixing rate dominated phenomena in stirred vessels. Turbulent-flow parameters were measured in a baffled, Rushton turbine agitated vessel with a laser-Doppler velocimeter. The necessary corrections for the periodic, nondissipative velocity fluctuations in the near impeller region were made by an autocorrelation method. Two components of periodic fluctuation, one corresponding to impeller blade frequency, the other corresponding to twice that frequency, were found to be significant. With the periodicity removed, meaningful turbulence intensities, autocorrelation functions, turbulence scales, energy spectra, and turbulence energy dissipation reates were obtained. Integral scales and turbulence energy dissipation rates were a particular objective in this work because of their usefulness in modelling local mixing rates in turbulence flows. From an energy balance around the impeller and impeller stream, it was found that 60% of the energy transmitted into the tank via impeller was dissipated in that region, and 40% was dissipated in the bulk of the tank. An equation for calculating local energy dissipation rates from resultant fluctuation velocities and resultant turbulence macroscales, ▪, appeared adequate. Constant A was found to be 0.85.