Macro-instabilities of velocity field in stirred vessel: detection and analysis

Results of the analysis of velocity data obtained by the laser Doppler velo cimetry (LDV) in a flat-bottomed cylindrical stirred tank (300 mm diameter, filled with the liquid to the height equal to its diameter) equipped with four radial baffles and stirred with a pitch-blade turbine impeller are pre sented. Water and 85% aqueous glycerine solution were used as working fluid s at three values of Reynolds number of the impeller: 75000, 1200 and 750. The fluid velocity was recorded in a rectangular region of evenly spaced po ints close to the stirrer region. The macro-instability (MI) of the velocit y field was extracted from the experimental data using the proper orthogona l decomposition (POD) technique and spectral analysis. This approach enable d not only detection of the macro-instability in measured velocity data but also to quantify its relative magnitude (a ratio of kinetic energy capture d by the macro-instability to total kinetic energy of the flow). The depend ence of chaotic invariants of the MI on the Reynolds number and position of the measuring point in the vessel is constructed. The time evolution of th e MI is reconstructed from experimental records. (C) 1999 Elsevier Science Ltd. All rights reserved.