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Keywords:

  • mixing;
  • macroinstability;
  • Pitched blade impeller;
  • stirred vessels

Abstract

The flow variations or macroinstabilities (MIs) occurring in a vessel stirred by a pitched blade turbine (PBT) are studied through particle image velocimetry (PIV) experiments. Proper orthogonal decomposition and fast Fourier transform techniques are applied to the PIV velocity data at one vertical and nine horizontal planes below the impeller, to identify and characterize the flow structures present in the vessel. It is shown that the PBT MI is manifested as a precessional movement around the impeller axis and an oscillation in the direction of the axial mean stream around the shaft axis. The identified flow structures are similar to those previously observed in vessels stirred by Rushton impellers and are characterized by two dominant frequencies, equal to one-tenth and one-fifth of the impeller rotational speed. The nature and extent of these structures and their interaction with the trailing vortices emanating from the turbine blades are discussed. © 2011 American Institute of Chemical Engineers AIChE J, 2011