Experimental Study On Macro-instabilities In Single- And Dual -impeller Stirred Tanks

Stirred-tank reactors are widely used in the environmental project, and the knowledge of their complex hydrodynamic behavior is an important goal not only for academic purposes but also especially for industrial aims. Numerous experimental and theoretical studies have been carried out to characterize the time-averaged flow fields in stirred-tanks. In reality, the fluid flow is unstable in stirred tank, and the fluxion changing from time to time, as well as space. Macro-instabilities (MI) are the most apparent. The experimental method for macro-instabilities studies, as well as the analytical methods for experimental data has crucial influence to the results.To characterize the Macro-instabilities (MI) and deepen the understanding of mixing mechanism in agitated vessels, pressure sensor technology has been used to measure the flow fields in the stirred tank, equipped with a Rushton turbine impeller with a large impeller and tank diameter ratio. The near-instantaneous flow fields obtained was rather complex and stochastic. In this paper, the digital-signal processing approaches for macro-instabilities was employed and the combined approach of power spectral density and wavelet packet analysis was proved to be suitable for analyzing one-dimensional low-frequency signal with noise and peak value. Experiments were carried out in a flat-bottom cylindrical stirred tank with 0.48m in diameter. A Rushton turbine was used in the experiments. The pressure sensor technology was used to measure the flow fields and analyzed by using the combined approach. Results show that there were macro-instabilities in the stirred vessel. The dimensionless macro-instability frequency (f/N) obtained by the combine approach is in good agreement with the literatures. The combine approach of power spectral density and wavelet packet analysis was proved to be the proper tool for the investigation of MI. For all the selected conditions, single and dual impellers, the pressure sensor of the measured pressure-time traces evidence the presence of the MI recognized previously in single-impeller systems. It was proved that the combined approach of power spectral density and wavelet analysis is an effective method for the research of MI in stirred tank.A new method for the studies of MI was proved in this paper, and the influences of the operating parameters on the macro-instabilities were also discussed. The results deepen the understanding of the flow characteristics in stirred tank.