Continuous flow stirred tank turbulence parameters in the impeller stream

Abstract Turbulence parameters in the impeller stream of a continuous flow, stirred tank have not been previously measured, although concurrent with this work, others have made measurements in batch systems. The turbulence parameters were obtained in the direction of the velocity vectors by means of a hot-film probe. The directions of the mean velocity vectors were first determined with a three-dimensional Pilot tube. The average velocities obtained with the Pilot tube were much higher than with the hot-film anemometer due to the high intensity and large scale turbulence in the flow field. The turbulance field was anisotropic and intermittent, the latter being caused by a highly turbulent and randomly moving jet generated by the impeller blades. Autocorrelations and one-dimensional spectra of the velocity fluctuations were obtained at various positions in the stream. These revealed the presence of periodic velocities close to the impeller, generated by the rotating blades. Integral velocity scales were calculated from the autocorrelation after subtracting the contribution of the periodic velocities. Velocity microscales and energy dissipation rates were calculated mostly from the derivative signals of the fluctuating velocities, although other techniques were used for comparison at selected positions. The probability density distributions of the velocity fluctuations were significantly different from the normal distribution and negatively skewed. Intermittency factors were obtained with an electronic on-off circuit and a few positions from the flatness factor. The latter method did not yield satisfactory results because of the non-normal distribution of velocities. The conclusion that one may draw from this work is that modern turbulence measuring equipment can successfully help to characterize the flow in as complex a field as that of a mixing vessel. In a later paper, we will show to what extent one can use this information to predict the mixing in such a system.