Chapter 45 - Analysis of Macro-Instabilities (MI) of the Flow Field in a Stirred Tank Agitated with Axial Impellers

Abstract

This chapter focuses on the measurement of frequency distribution of velocity macro-instabilities (MI) in a stirred tank. A fully baffled fiat-bottomed cylindrical stirred tank with diameter T=240mm and liquid height, H=T, is agitated with three different axial impellers (PBT, A310 and HE3). All of the velocity time series records are measured with a one-component Laser Doppler Velocimeter (LDV) on a rectangular grid of evenly spaced points upstream of the baffle at vertical distances z/T=0.3-0.75 from the bottom of the tank. The off-bottom clearance (C) and impeller diameter (D) are varied. The frequency content of the velocity time series is analyzed numerically using the Lomb periodogram algorithm. The statistical space distributions of the frequencies show that in the case of a 45 ° PBT with diameter D=T/2 and Re=48000 there is a resonant frequency that dominates, fMI=0.62Hz. The frequency distribution analysis shows that for the A310 and HE3 there is no dominant frequency. The observed frequencies are scattered and broad banded. Once the resonant geometry was determined (45° PBT, D=T/2, C/D=0.5), the setup tracks the dependence of the dominant MI frequency on the impeller Reynolds number Rei, with variations in rotational speed N and fluid viscosity v.