Mixing of viscous Newtonian fluids in a vessel equipped with steady and unsteady rotating dual-turbine impellers

•Toroidal IMR structure in vicinity of the impeller in case of unidirectional mixing.•Spiral-shape IMR structure for unsteady forward–reverse mixing.•The forward–reverse mixing is much more efficient than standard mixing.•Power requirements of forward–reverse and unidirectional mixing are comparable.

This paper presents investigation on the possibility to improve the efficiency of mixing of the highly viscous Newtonian fluids in a vessel with utilization of unsteady rotating dual turbine impellers. Flow visualization experiments were used to examine the size, positions and structure of the IMR (Isolated Mixing Regions) regions as a function of Reynolds number as well as mixing time. Additionally, the effect of frequency of impellers’ oscillation on mixing efficiency was examined. It was found that the use of unsteady forward–reverse mixing mode enhance the mixing efficiency in comparison to standard mixing (up to about eight times). The structure of IMR in the forward–reverse mixing is much more complicated than in standard mixing, because of liquid division into spiral-shape filaments. This had caused that dimensionless mixing time was up to about eight times shorter in comparison to standard mode.

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