Chaotic mixing performance enhanced by rigid-flexible impeller with long-short blades in stirred tank

doi:10.11949/0438-1157.20200225

Abstract

Abstract:

The presence of a mixing isolation regions in a stirred reactor is a major obstacle to enhancing fluid mixing. Breaking the symmetrical flow field structure in the stirred tank and destroying the mixing isolation area can improve the fluid mixing efficiency. The Matlab software was used to calculate the maximum Lyapunov exponent (LLE) and multi-scale entropy (MSE). The effects of different blade types, flexible blade length, flexible blade number, blade height from bottom and rotation speed on fluid mixing were compared. The results show that the rigid-flexible impeller with long-short blades (RF-LSB) can enhance the flow field structure more unstable and asymmetric with deformation and random vibration of flexible pieces, destroy the symmetry flow in the process of fluid mixing, induce the asymmetric flow field, and make more fluid into the chaotic state. When at 90 r/min and three pieces of flexible, the LLE of the RF-LSB is larger than that of rigid impeller and rigid-flexible impeller RF-LSB with increase of 20.22% and 7.98% respectively. The mixing time (θ_m) of the three systems [RF-LSB (three pieces), rigid impeller, rigid-flexible impeller] has an exponential relationship with the power consumption per unit volume (P_v). When P_v is constant, θ_m of the RF-LSB system is the smallest. Results showed that the RF-LSB (three pieces) is superior to rigid impeller and rigid-flexible impeller, which is more conducive to fluid chaotic mixing.

Key words: rigid-flexible impeller with long-short blades, largest Lyapunov exponents, chaotic mixing, multi-scale entropy

摘要：

搅拌反应器中混合隔离区的存在是强化流体混合的主要障碍。打破搅拌槽中的对称性流场结构，破坏混合隔离区，可以提高流体混合效率。采用Matlab软件编程计算最大Lyapunov指数（LLE）和多尺度熵（MSE），比较了不同桨叶类型、柔性片长度、柔性片数量和桨叶离底高度以及转速对流体混合的影响。结果表明，长短叶片复合型刚柔桨（RF-LSB）桨叶通过刚柔耦合错位连接，柔性片的形变与随机振动对流体的非稳态扰动，使流场结构不稳定性和不对称性增强，强化了流体混合效果。当柔性片数量为3，搅拌转速为90 r/min时，RF-LSB体系比刚性桨和刚柔桨体系的LLE值分别提高了20.22%和7.98%；三种体系[RF-LSB（柔性片数量为3）、刚性桨和刚柔桨体系]的混合时间（θ_m）与单位体积功耗（P_v）呈指数型关系，当P_v相同时，RF-LSB（柔性片数量为3）的θ_m最小，表明RF-LSB（柔性片数量为3）更有利于流体混沌混合。

关键词: 长短叶片复合型刚柔桨, 最大Lyapunov指数, 混沌混合, 多尺度熵

CLC Number:

TQ 027.2

Zuohua LIU,Hongjun WEI,Xia XIONG,Changyuan TAO,Yundong WANG,Fangqin CHENG. Chaotic mixing performance enhanced by rigid-flexible impeller with long-short blades in stirred tank[J]. CIESC Journal, 2020, 71(11): 5080-5089.

刘作华,魏红军,熊黠,陶长元,王运东,程芳琴. 长短叶片复合型刚柔桨强化搅拌槽内流体混沌混合行为[J]. 化工学报, 2020, 71(11): 5080-5089.

Figures/Tables 10

Fig.1 Mixing experimental apparatus

Fig.2 Impellers used in experiment

Fig.3 Analysis of power consumption per unit volume

Fig.4 Effect of impeller types on θm

Fig.5 Effect of flexible piece number on θm

Fig.6 θmof three systems at different Pv

Fig.7 Effect of impeller types on LLE

Fig.8 Analysis of LLE under constant speed

Fig.9 Effect of impeller types on MSE

Fig.10 Analysis of MSE under constant speed

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