Chaotic mixing performance and mass transfer enhanced by rigid-flexible impeller with pulse jet

doi:10.11949/0438-1157.20200303

Abstract

Abstract:

Conventional stirred reactors generally use rigid impeller for mechanical stirring, which leads to the easy creation of isolation mixing regions in the reactor and reduces the efficiency of fluid mixing. The use of multi-flow field coupling to induce chaos and promote more fluids into a chaotic state is one of the effective ways to improve fluid mixing efficiency. In this work, the largest Lyapunov exponent(LLE) and multi-scale entropy(MSE) are investigated with the Matlab compile pressure pulsation signals. The effects of duty ratio, paddle type, flexible paddle thickness, paddle height from the bottom and pulsed air jet flow rate on the chaotic mixing of fluids in a stirred reactor under different pulse periods are explored. In addition, the effects of different impeller types, jet types and air jet flow rate on the volume oxygen mass transfer coefficient K_La are compared and analyzed. When T=0.4 s and D=80%, the results show that the LLE of the rigid-flexible RT impeller compared with the rigid RT impeller increases 11.58% and the MSE of the rigid-flexible RT impeller is also larger than that of rigid RT impeller. It was showed that the pulsed jet rigid-flexible impeller system can better enhance fluid chaos, increase the fluid mixing efficiency and homogenize the system energy distribution. In addition, pulse jet coupling RF-RT impeller system enhances the turbulent characteristics of the fluid, promotes the reduction of the thickness of the liquid film, strengthens the mass transfer and increases the K_La value of the system. When power consumption per unit volume is 360 W/m³, the K_La of the PJ-RF-RT system compared with the PT-R-RT system increases 13.46%, and the K_La of the PJ-R-RT system compared with the SJ-R-RT system increases 11.86%.

Key words: chaos, pulse jet, rigid-flexible impeller, largest Lyapunov exponent, coefficient of mass transfer

摘要：

传统搅拌反应器普遍使用刚性桨进行机械搅拌，导致反应器内容易产生混合隔离区而降低流体混合效率。采用多流场耦合诱发混沌现象，促使更多流体进入混沌状态，是提高流体混合效率的有效途径之一。结合Matlab软件编译PJ-RF-RT体系中的压力脉动信号得到最大Lyapunov指数（LLE）与多尺度熵（MSE），探究不同脉冲周期下占空比、桨型、柔性桨叶厚度、桨离底高度及脉冲空气射流量对搅拌反应器内流体混沌混合的影响，同时，对比分析了桨型、射流类型、气流量对体积氧传质系数K_La的影响。研究结果表明，在脉冲周期T=0.4 s，占空比D=80%时，RF-RT桨体系较R-RT桨体系的LLE增大了11.58%，且RF-RT体系的MSE显著高于R-RT桨体系，表明脉冲射流刚柔组合桨体系能更好地诱发流体混沌，增大流体混合效率，均化体系能量分布。此外，脉冲射流耦合RF-RT桨体系增强了流体的湍流特性，促使液膜厚度减小，强化了传质，在单位体积功耗P_v=360 W/m³时，PJ-RF-RT体系的K_La较PJ-R-RT提高了13.46%，PJ-R-RT体系的K_La较SJ-R-RT提高了11.86%。

关键词: 混沌, 脉冲射流, 刚柔组合桨, 最大Lyapunov指数, 传质系数

CLC Number:

TQ 028.8

Zuohua LIU, Wei SUN, Xia XIONG, Changyuan TAO, Yundong WANG, Fangqin CHENG. Chaotic mixing performance and mass transfer enhanced by rigid-flexible impeller with pulse jet[J]. CIESC Journal, 2020, 71(10): 4632-4641.

刘作华, 孙伟, 熊黠, 陶长元, 王运东, 程芳琴. 脉冲射流-刚柔组合桨强化流体混沌混合及传质性能研究[J]. 化工学报, 2020, 71(10): 4632-4641.

Figures/Tables 14

Fig.1 Mixing experimental apparatus

Fig.2 Jet types

Fig.3 Impellers used in experiment

Fig.4 Pulse electric signal waveform

Fig.5 Effect of pulsed duty ratio on LLE

Fig.6 Effect of impeller types on LLE

Fig.7 Effect of thickness of flexible part on LLE

Fig.8 Effect of impeller off-bottom clearances on LLE

Fig.9 Effect of pulsed air jet flow rates on LLE

Fig.10 Effect of impeller types on MSE

Fig.11 Effect of thickness of flexible impeller on MSE

Fig.12 Effect of pulsed air jet flow rates on MSE

Fig.13 Effect of impeller types on KLa

Fig.14 Effect of pulsed air jet flow rates on KLa

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