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 QMI 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 QMI varied in the interval [rom approximately 0.05 to 0.30. The magnitude QMI takes maximum values at low Reynolds number values (in laminar and transitional regions). In the turbulent region (ReM >20000) the QMI value is low and practically constant. The dependence oj the QMI 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.
Highlights
The liquid flow in mechanically stirred vessels has been studied intensively in recent decades
The liquid flow in a stirred vessel operated under steady operational conditions may be considered as a pseudo-stationary high-dimensional dynamical system constituted by hierarchically ordered unsteady flows
Nikiforaki et al [17] have explained a part of this scatter by inadequacies in the experimental data treatment and interpretation methods adopted by different authors. Howeveq it is evident from the published data that at least two different macro-instability frequencies, corresponding to different mechanisms of MI origin, exist in the mixing vessels stinedwith a Rushton turbine' Nikiforaki et al [17] observed that higher MI frequency occurs at low impeller Reynolds number values, and that the lower frequency. prevails at'high Rer
Summary
The liquid flow in mechanically stirred vessels has been studied intensively in recent decades. Nikiforaki et al [17] have explained a part of this scatter by inadequacies in the experimental data treatment and interpretation methods adopted by different authors Howeveq it is evident from the published data that at least two different macro-instability frequencies, corresponding to different mechanisms of MI origin, exist in the mixing vessels stinedwith a Rushton turbine' Nikiforaki et al [17] observed that higher MI frequency (about l0 7o of ltl) occurs at low impeller Reynolds number values, and that the lower frequency. In this paper we apply this technique to the experimental data obtained by Kratdna et al [12, 13] by measuring the tangential comPonent of the force exerted on the radial baffles in the mixing vessel with a standard Rushton turbine impeller in order to identify and quantify its macro-instability related component. The frequency of occurrence of the macro-instability related force component is analysed
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