Chaotic mixing intensification and flow field structure instability in stirred reactor by counter-flow pitched-blade turbine

doi:10.11949/0438-1157.20210596

Abstract

Abstract:

On the basis of the traditional three-pitched-blade propeller, combined with the structure of the counter-current blade, a three-pitched-blade countercurrent blade is proposed to destroy or eliminate the stable symmetrical flow field structure in the stirring tank, and improve the fluid transfer efficiency and the degree of chaotic mixing. Power consumption, mixing time, chaotic characteristic parameters, flow field structure and fluid velocity distribution of up-flow pitched-blade turbine (PBTU), up-down counter-flow pitched-blade turbine (PBTC-U), down-up counter-flow pitched-blade turbine (PBTC-D) systems and the PBTC-U blade systems with different outer blades lengths were investigated through experiment and simulation research. The results showed that, the mixing time of the PBTC-U impeller was reduced from 22.0 s, 37.5 s to 16.5 s, and the power consumption was reduced by 5.6% and 12.8% at 130 r/min, compared with PBTU and PBTC-D impellers system, respectively. Largest Lyapunov exponent (LLE) value increased by 13.69% and 37.01%, respectively. Meanwhile, the results showed that the minimum power consumption and the minimum mixing time occurred with the outer blade length of 0.375D of the PBTC-U impeller. The PBTC-U impeller strengthened the random movement of the fluid through the counter-flow action of the inner and outer blades, enhanced the instability of the flow field and destroyed the symmetry fluid structure, resulting in much more unstable flow field and higher mixing efficiency. In addition, the PBTC-U impeller enhanced the fluctuation of axis velocity and radial velocity distribution, which was beneficial to improve the mixing efficiency of the system.

Key words: counter-flow impeller, largest Lyapunov exponents, mixing power consumption, numerical simulation

摘要：

在传统三斜叶桨的基础上，结合逆流桨结构，提出三斜叶逆流桨，以破坏或者消除搅拌槽内稳定的对称性流场结构，提高流体传递效率及混沌混合程度。结合实验和模拟两种方法,主要研究了上推式三斜叶桨(PBTU)、外推内压式三斜叶逆流桨(PBTC-U)、外压内推式三斜叶逆流桨(PBTC-D)三种桨叶体系以及不同外层桨叶长度的PBTC-U桨体系内搅拌功耗、混合时间、混沌特性参数、流场结构以及流体速度分布。实验结果表明，N=130 r/min时，PBTC-U桨相对于PBTU桨和PBTC-D桨，体系混合时间分别从22.0、37.5 s缩短到16.5 s，功耗分别降低了5.6%和12.8%，LLE值分别提高了13.69%和37.01%。在确定PBTC-U桨适宜外层桨叶长度的研究中发现当外层桨叶长度D₂=0.375D时，搅拌功耗最低且混合时间最短。PBTC-U型逆流桨通过内外层桨叶的逆流作用，强化体系内流体的随机运动，使得流场的不稳定性得到增强，对称性被破坏，进而流场结构失稳，流体混合效率得到提高。另外，PBTC-U桨可以增强流体轴、径向速度分布的波动性，有利于提高体系的混合效率。

关键词: 逆流桨, 最大Lyapunov指数, 搅拌功耗, 数值模拟

CLC Number:

TQ 027.2

Zuohua LIU, Yilin ZHOU, Xia XIONG, Changyuan TAO, Yundong WANG. Chaotic mixing intensification and flow field structure instability in stirred reactor by counter-flow pitched-blade turbine[J]. CIESC Journal, 2022, 73(1): 222-231.

刘作华, 周毅林, 熊黠, 陶长元, 王运东. 逆流桨强化搅拌槽内流体混沌混合及流场结构失稳研究[J]. 化工学报, 2022, 73(1): 222-231.

Figures/Tables 13

Fig.1 Mixing experimental apparatus and mesh model

Fig.2 Impellers used in experiment and impeller size

Fig.4 Effect of impeller types on P

Fig.5 Effect of outer blade length on P

Fig.3 Grid independence (PBTU)

Fig.6 Different outer blade length of PBTC-U impeller

Fig.7 Effect of impeller types on θm

Fig.8 Effect of outer blade length on θm

Fig.9 Effect of impeller types on LLE

Fig.10 Effect of outer blade length on LLE

Fig.11 Effect of impeller types on contour plots of velocity magnitude

Fig.12 Effect of impeller types on streamline of flow distribution

Fig.13 Effect of impeller types on axial velocity and radial velocity

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