The Tangential Force Affecting the Radial Baffles in a Stirred Vessel: Analysis of the Macro-instability Related Component

Abstract

Experimental data obtained by measuring the tangential component of the force affecting radial baffles in a flat-bottomed cylindrical mixing vessel stirred with a Rushton turbine impeller is analysed. Spectral analysis of the experimental data demonstrated the presence of its macro-instability (MI) related low-frequency component embedded in the total force. Two distinct dimensionless frequencies (both directly proportional to the impeller speed of rotation N) of the occurence of the MI component were detected: a lower frequency of approximately 0.025N and a higher frequency of about 0.085N. The relative magnitude Q_MI of the MI-related component of the total tangential force was evaluated by a combination of proper orthogonal decomposition (POD) and spectral analysis. The values of magnitude Q_MI varied in the interval [rom approximately 0.05 to 0.30. The magnitude Q_MI takes maximum values at low Reynolds number values (in laminar and transitional regions). In the turbulent region (Re_M >20000) the Q_MI value is low and practically constant. The dependence oj the Q_MI values on vertical position in the vessel is only marginal. The results suggest that the magnitude of the MI component of the force is significantly influenced by the liquid viscosity and density.