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, paying attention to the effect of geometrical configurations such as eccentricity and impeller blade thickness. Two main vortices, one above and one below the impeller, were identified in all configurations. The former vortex dominates all flow motion, 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 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.