On the origin, frequency and magnitude of macro-instabilities of the flows in stirred vessels

The mean flow and turbulence fields in a fully baffled vessel stirred by a Rushton impeller at three clearances and a pitched-blade turbine at one clearance have been investigated with laser-Doppler anemometry (LDA) to characterise the macro-instabilities (MIs) present in such flows. Time-resolved velocity measurements were made and the frequency content of the velocity recordings was analysed with FFT techniques. The study aims to throw light into the frequency, magnitude, nature and origin of such flow variations, especially in view of the different findings that have been reported in the published literature. The frequency of the MIs was found to be linearly related to the rotational speed of the impeller and to be essentially independent of impeller design. A single fundamental frequency, around 0.015-0.02N (Hz), where N is the impeller rotational speed, was present for all configurations, together with harmonic frequencies, the prominence of which depended on impeller geometry and/or clearance. The LDA data and direct observations made with laser-sheet flow visualisation indicated clearly that the macro-instability stems from a precessional motion about the vessel axis, similar to the precession encountered in most swirling flows. The results show that MIs, as a mean flow motion superimposed on the flow pattern in the vessel, can result in a broadening of the measured turbulence levels by up to 25%, and a mean velocity variation of up to 0.3Vtip. The findings indicate therefore that it might be necessary and indeed it could be advantageous for improved process prediction to take MI into account in models of the flows in stirred vessels. © 2003 Elsevier Science Ltd. All rights reserved.