Force effects of the macro-instability of flow pattern on radial baffles in a stirred vessel with pitched-blade and Rushton turbine impellers

Experimental data obtained in a flat-bottomed cylindrical mixing vessel stirred with pitched blade impellers (four or six blades) or with a Rushton turbine impeller by measuring the tangential force affecting radial baffles are analysed. The presence of the macro-instability (MI) related component of the force was detected by spectral analysis of measured data. A single MI-related component occurring with a frequency of about 0.074N (N is the impeller speed) was detected for the pitched blade impellers. Two distinct frequencies, both directly proportional to the impeller speed, were detected for the Rushton turbine impeller: a lower frequency of approximately 0.025N and a higher frequency of about 0.085N. The upper frequency occurred only at low Re-M values. The lower frequency component occurred over the entire Rem range. The dimensionless frequencies were independent of the stirring speed, the vertical position along the baffle, the number of impeller blades, and liquid viscosity. The relative magnitude (k(MI)) of the MI-related component of the total force was evaluated by a combination of proper orthogonal decomposition (POD) and spectral analysis of experimental time series. The magnitude kMI varied in the interval from approximately 0.2 to 0.5. The dependence of the k(MI) on the impeller Reynolds number, the vertical position in the vessel, the number of impeller blades, the impeller off-bottom clearance and liquid properties is discussed.