Abstract:

Mixing power were characterized by torque sensor. The largest Lyapunov exponents were computed by Matlab software. The fluid mixing performances were observed by the fluid field's visualization technology. A comprehensive experimental study on the effects of impeller type, the distance between impeller and bottom, the length of flexible wire, and the diameter of flexible wire on the number of mixing efficiency (C_e), and the largest Lyapunov exponents (LE_max) was carried out. Results showed that, single-wire flexible impeller can regulate and control the flow field structure and the way of energy dissipation, and effectively enhance the chaotic mixing. At rotation speed 120 r·min⁻¹, compared with the traditional rigid impeller, C_e was decreased by 87.4% and LE_max was increased by 53.2% with single-wire flexible impeller, C_e was decreased by 43.8% and LE_max was increased by 10.8% compared with single-wire rigid impeller. In addition, at the same agitation speed, the longer the flexible wire, the more conducive to the chaotic mixing, but its power consumption would also increase obviously. The diameter of flexible wire was measured at 0.8 mm, while C_e was less than the others and LE_max was larger than the others, which indicated that the degree of chaotic mixing was the largest in the case. Increasing the diameter of flexible wire led to the increase of fluid motion irregularity and the degree of chaotic mixing. If the diameter of flexible wire is more than 0.8 mm, the disturbance of flexible wire was reduced, which was close to the traditional rigid impeller, resulting in the decrease of chaotic mixing characteristics. It was not conducive to fluid mixing while the distance between impeller and bottom was too high or too low, the optimal distance between impeller and bottom was at 0.25T (T was used to express the inner diameter of stirred vessel).

Key words: wire, flexible impeller, mixing time, number of mixing efficiency, largest Lyapunov exponents, chaotic mixing

摘要：

实验运用扭矩传感器测量搅拌功率特性，Matlab软件编程计算最大Lyapunov指数（LE_max），流场可视化技术观测流体混合状态。研究了桨叶类型、桨叶离底距离、柔性钢丝长度、柔性钢丝直径对混合效率数（C_e）、LE_max的影响。结果表明：单层钢丝柔性桨通过刚-柔-流耦合作用，改变流场结构和能量耗散方式，提高了流体混沌混合程度，实现了流体的高效节能混合；当转速为120 r·min⁻¹时，与传统刚性桨相比，单层钢丝柔性桨使流体C_e减小了87.4%，LE_max增大了53.2%，与单层钢丝刚性桨相比，单层钢丝柔性桨使流体C_e减小了43.8%，LE_max增大了10.8%。另外，当搅拌转速相同时，柔性钢丝越长，越有利于流体混沌混合，但其功耗也会随之明显增加；当柔性钢丝直径为0.8 mm，桨叶离底距离为0.25T（T为搅拌槽内径）时，各个转速对应的C_e小于其他情况、LE_max大于其他情况，流体达到相对最佳混沌混合状态。

关键词: 钢丝, 柔性桨, 混合时间, 混合效率数, 最大Lyapunov指数, 混沌混合