VELOCITY-FIELD MACRO-INSTABILITIES IN AN AXIALLY AGITATED MIXING VESSEL

The how pattern produced by a 45 degrees pitched-blade turbine in a fu lly baffled cylindrical flat-bottomed tank is investigated by a qualit ative approach using a laser sheet flow visualisation and a quantitati ve approach using laser Doppler velocimetry measurements in the turbul ent and/or transition regime, with water and glycerine solution as the working liquids. The flow visualisation shows the turbulent macro-ins tabilities as vortices structures appearing between the turbine and th e liquid surface. These macro-structures characterise an organized mod ification of the flow pattern, creating a transient violent flow activ ity in the upper part of the vessel which is in contrast with the conc ept of turbulence as being completely random. The time series and the spectral analysis show a non-stationary, pseudo-periodic phenomenon at a frequency much lower than the blade passage frequency, about 6%, an d linearly coupled with the frequency of impeller revolution. This phe nomenon is analysed in a volume surrounding the impeller and correspon ds to a sharp increase in the intensity of the fluctuating motions whe n the Reynolds number is increased around 600. information on the time organization is obtained by a time frequency analysis. We show that t he phenomenon is present about 20% of the total time in water and more than 40% in a glycerine solution, for a same impeller rotation speed. (C) 1997 Elsevier Science S.A.