An experimental study of double-to-single-loop transition in stirred vessels

The velocity characteristics of the flows in a fully baffled vessel of diameter T = 290 mm stirred by a Rushton impeller of diameter D = T/3 were investigated by means of laser-Doppler anemometry measurements. The effects of clearance and rotational speed on the flow patterns in the vessel were studied. It was found that at impeller clearances from the bottom of the vessel (C) around 0.2 T the characteristic double-loop flow pattern undergoes a transition to a single-loop one with the impeller stream direction becoming partly axial and being inclined at around 25 to 30° to the horizontal. The impeller stream inclination varied with radial distance from the impeller, as well as with angular position between blades (blade angle). Impeller speed was found to have no effect on the flow pattern or the mean velocities and turbulence levels normalized by Vtip for C/T > 0.20 or C/T < 0.15. The flow structure measured with C = 0.15T is described in detail and the implications of the data for fluid mixing in stirred vessels are discussed.