Velocity Field Macro-instabilities in an Axially Agitated Mixing Vessel

Abstract The flow pattern produced by a 45° pitched-blade turbine in a fully baffled cylindrical flat-bottomed tank is investigated by a qualitative approach using a laser sheet flow visualisation and a quantitative approach using laser Doppler velocimetry measurements in the turbulent and/or transition regime, with water and glycerine solution as the working liquids. The flow visualisation shows the turbulent macroinstabilities as vortices structures appearing between the turbine and the liquid surface. These macro-structures characterise an organized modification of the flow pattern, creating a transient violent flow activity in the upper part of the vessel which is in contrast with the concept 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%, and linearly coupled with the frequency of impeller revolution. This phenomenon is analysed in a volume surrounding the impeller and corresponds to a sharp increase in the intensity of the fluctuating motions when the Reynolds number is increased around 600. Information on the time organization is obtained by a time frequency analysis. We show that the 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.