Mixing performance experiments in impeller stirred tanks subjected to unsteady rotational speeds

Two unsteady stirring approaches, co-reverse periodic rotation and time-periodic fluctuation of rotational speed, are adopted to enhance global mixing in a stirred tank with high viscosity materials. The relations of complete decolorization time with Re and with mean energy dissipation per unit volumetric weight are used as quantitative methods to compare the mixing performance under an unsteady stirring with that under a steady one. The experimental results show that mixing time can be significantly reduced in stirred tanks when unsteady stirring approaches are used. However, it is also found experimentally that for the method of co-reverse periodic rotation, only when the Re is larger than a critical value, can significant enhancements in mixing be obtained. In contrast to this, for the case of time-periodic RPM fluctuations, such a critical value is not as easy to define. The higher the frequency of periodic co-reverse rotation and the larger the amplitude of time-periodic fluctuation, the shorter is the mixing time. In both cases, after Re becomes larger than a certain value, further increases in Re yields relatively small returns. The experimental results are presented and discussed with the help of photographs taken during the mixing process. (C) 1998 Elsevier Science Ltd. All rights reserved.