Spectral analysis of the velocity fluctuations in a mechanically stirred tank

To characterize the flow instabilities in mechanically agitated vessels, digital particle image velocimetry (DPIV) was used to measure the flow fields within baffled, stirred tank, equipped with a Ruston turbine impeller having a large impeller and tank diameter ratio. The near-instantaneous flow fields obtained was rather complex and stochastic, so spatially averaged vorticity was introduced to identify the change of flow patterns and Fast Fourier Transform method (FFT) was used to analyze the obtained time series of the spatially averaged vorticity. Results show that the flow fields are not steady; rather, they are subject to macro instability (MI) with period ranging from 10 to 200 blade passage period. The dominant frequency of MI increases linearly with that of the impeller revolution. At lower impeller rotational speed (30-60 r·min^-1), apparent MI phenomenon can be observed. With the increase of rotational speed (120-180 r·min^-1), MI tends to weaken while random velocity fluctuations tend to strengthen.