Effect of shaft eccentricity and impeller blade thickness on the vortices features in an unbaffled vessel

Different techniques, i.e. laser Doppler anemometry, flow visualisation and decolourisation, were applied in order to gain insight into the main turbulent flow features of an unbaffled vessel stirred by an eccentrically positioned Rushton turbine. Attention was paid to the effect of geometrical parameters such as eccentricity and impeller blade thickness on the flow motion. Two main vortices, one above and one below the impeller, were identified in all configurations. Decolourisation experiments showed that the two vortices behave as segregated regions. The upper vortex: dominates the flow field, driving a strong circumferential flow around it. The inclination of such vortex varies with eccentricity. Importantly, the vortex is not steady but oscillates slowly and periodically inducing a kind of flow instabilities, which may have a significant impact on macro-mixing. The characteristic frequency of flow instabilities was found to increase with reducing eccentricity or impeller blade thickness. Moreover, vortex shedding phenomena from the flow-shaft interaction were observed in different configurations. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V All rights reserved.