Determination of Critical Speed for Complete Solid Suspension Using Acoustic Emission Method Based on Multiscale Analysis in Stirred Tank

Abstract

According to the mechanism that acoustic emission (AE) signals with different frequency ranges were emitted when solid particles impacted the wall, a characteristic scale (S1) of the AE signals representing the movement of solid particles was obtained by using wavelet transform and Hurst analysis. A regular evolution behavior of energy fraction on each frequency with the increasing impeller speeds was observed. A new method to determine critical impeller speed was presented by analyzing the relationship between energy fraction in S1 scale and the impeller speed. The critical impeller speed was associated with the emergence of steady state in energy fractions in the S1 scale. The method was further verified with four groups of experiments with the particle sizes of 0.5 mm and 0.7 mm by comparing experimental results with that from the conventional visual method for a water and glass beads system. The average relative errors were no more than 4.28%. It is a fast, nonintrusive, and accurate method to investigate the Njs by multiscale analysis of acoustic energy.