Observations on the significance of instabilities turbulence and intermittent motions on fluid mixing processes in stirred reactors

Flow instabilities of different nature have been observed inside stirred vessels and the motions associated with them may enhance mixing in a number of ways. In the present work the different kinds of flow instabilities which occur continuously for most configurations (precessional macro-instabilities, (Chem. Eng. Sci. 58 (2003) 2937) and constantly/intermittently for particular vessel/impeller configurations (Chem. Eng. Sci. 58 (2003) 2297, Chem. Eng. Sci. 58 (2003) 3859), were investigated simultaneously through two-point Laser Doppler Anemometry (LDA) measurements and their effect on mixing operations is discussed. A multi-channel LDA was used to investigate the flow instabilities in different regions of a stirred vessel for both radial and axial impellers, and velocity data were analysed with wavelet and spectral/FFT analysis. The energy associated with the different kinds of flow instabilities was estimated in many locations of the vessel. The amount of broadening of the turbulence levels that such instabilities produce through the superimposition of the instability flow variation on the mean flow pattern was quantified. The relative strength of the different instabilities in different parts of the vessel as well as the presence of intermediate frequencies that may arise from the combined effect of different instability frequencies are also discussed.