Characterisation of oscillations in multi-phase unbaffled mixing vessels

Abstract

Unbaffled mixing systems have several benefits over conventional baffled systems, including significantly lower power required for solids suspension. However, one drawback of unbaffled systems is the possibility of oscillations occurring within the tank. Experimental laboratory-scale experiments have revealed oscillations in settling solids and large variations in the input power, despite the impeller speed remaining constant. These oscillations have been found to be linked through having the same frequency. The frequency and amplitude of these oscillations has been determined across a range of agitator and tank configurations, including variations in impeller size and type, impeller clearance and submerged depth, and the surface boundary conditions. The effect of solids concentration has also been investigated. The oscillations were found to be inherently multi-phase and had a strong concentration dependence. Impeller size and type influenced the amplitude and frequency of the oscillations, as did the impeller clearance. Liquid surface condition did not influence the oscillations. The oscillation frequency appears to be related to the circulation time in the tank. Although these oscillations fall into the definition of previously observed macro-instabilities, several distinguishing features are noted.