Mixing enhancement of solid-liquid two-phase by coupling double-layer rigid impeller and non-constant chaotic rotating speed

Efficient mixing in stirred tanks could be achieved in the presence of isolated mixing regions using a controlled fluctuation of the impeller rotation rate. In this paper, nine rigid impellers with double-layer were used under two various stirring modes including constant and chaotic speeds to compare and investigate the solid–liquid two-phase (i.e., polyformaldehyde–glycerol) mixing systems. Results show that the chaos degree of the mixing system was the highest with the Kolmogorov entropy of 0.1818, the largest Lyapunov exponent of 0.2263, and the 0–1 test indicator of 0.8134 when the upper and lower double-layer impellers with circular three-blade were used for unidirectional chaotic speed stirring at speed peak of 200 r/min and speed change interval of 5 s. The mixing performance of solid–liquid two-phase mixing system was the best with the mixing time of 14 s and the mixing energy consumption of 99.04 J while the upper and lower impellers were both with round four-blade. Chaotic rotational speed stirring consumes less power per unit time under the same working condition, but its energy saving was still not enough to eliminate the influence caused by the long mixing time. It is found that the disturbances below a certain amount have a weak effect on the mixing performance although the chaotic motor could generate stronger dynamic disturbances and is more likely to cause chaotic mixing. Furthermore, the speed change times decreased, and the velocity gradient in the flow field decrease accordingly while the speed change time of chaotic speed stirring was too long. To sum up, it maybe that the mixing performance of the solid–liquid mixing system under unidirectional chaotic speed stirring is not as good as that under constant speed stirring on this condition, which is the main merit of this work.