Mixing Enhancement by Variable Rotational Speed Coupling Eccentric Structure and Its Flow Field Visualization by PLIF

doi:10.3901/JME.2021.08.240

Abstract

Abstract: Mechanical mixing has been widely used in chemical engineering, food industry, metallurgy and environmental protections. In laminar mixing, doughnut-shaped toroidal regions, which are also termed as isolated mixing regions(IMRs), appear above and below the each impeller in the vessel due to the periodic flow structure. Fluids can neither penetrate nor leave the islands. Therefore, IMRs are barriers to mixing. In this work, it is employed a planar laser induced fluorescence(PLIF) technology to visualize the dynamic structure of the flow field. The post-image processing has been done with custom-made functions which could identify the IMRs and calculates the mixing efficiency. The results show that the mixing efficiency of traditional center mixing stays at about 63%. The mixing efficiency could be improved to 78% approximately by applying time-periodic RPM fluctuations. The eccentric arrangement of the stirring shaft destroys the periodic symmetry of the flow field and thus reduces the IMRs. When the eccentricity E=0.1, 0.3, 0.4, 0.5, 0.6, the mixing efficiency could achieve 79%, 85%, 89%, 90%, 86% in 200 s, respectively. The results demonstrate that eccentricity and mixing efficiency are not linearly related. There is an optimal range of eccentricity (E=0.4-0.5) for the best mixing efficiency. However, the IMRs cannot be completely eliminated only by arranging the stirring shaft eccentrically. Thus, it proposed a strategy to combine the eccentric arrangement of stirring shafts with variable rotational speeds, which might improve the mixing quality dramatically by imposing unsteady asymmetric disturbances. The PLIF images exhibit that the flow structures have been remarkable alternated by eccentric stirring with variable rotational speeds. Under the eccentricities E=0.3, 0.4, 0.5, the IMRs has been greatly reduced by variable rotational speeds subjected to time-periodic fluctuations. Specifically, the mixing efficiency can reach more than 97%. This research could provide technical and theoretical guidance for the design of laminar flow agitators.

Key words: planar laser induced fluorescence(PLIF), mixing enhancement, eccentric, variable rotational speeds, laminar mixing

CLC Number:

TQ27

LIU Hailong, WU Honglei, CAO Yu, SUN Jingchen, MAO Baodong, WANG Junfeng. Mixing Enhancement by Variable Rotational Speed Coupling Eccentric Structure and Its Flow Field Visualization by PLIF[J]. Journal of Mechanical Engineering, 2021, 57(8): 240-246,254.

References

[1] PAUL E L，ATIEMO-OBENG V A，KRESTA S M. Handbook of industrial mixing：Science and practice[M]. Hoboken：John Wiley &; Sons，Inc.，2004．
[2] 陈志平，章序文，林兴华．搅拌与混合设备设计选用手册[M]．北京：化学工业出版社，2004． CHEN Zhiping，ZHANG Xuwen，LIN Xinghua. Handbook of stirring and mixing equipment design selection[M]. Beijing：Chemical Industry Press，2004.
[3] HWANG W R，JUN H S，KWON T H．Experiments on chaotic mixing in a screw channel flow[J]. AIChE Journal，2002，48(8)：1621-1630.
[4] 杨敏官，来永斌. 搅拌槽内固液悬浮特性的试验研究[J]. 机械工程学报，2011，47(6)：186-192. YANG Minguan，LAI Yongbin. Experimental investigation on solid-liquid suspension characteristics in stirred tanks[J]. Journal of Mechanical Engineering，2006，47(6)：186-192.
[5] ANDRÉ C，DEMEYRE J F，GATUMEL C，et al. Dimensional analysis of a planetary mixer for homogenizing of free flowing powders：Mixing time and power consumption[J]. Chemical Engineering Journal，2012，198-199：371-378.
[6] JUNG S Y，AHN K H，KANG T G，et al. Chaotic mixing in a barrier embedded partitioned pipe mixer[J]. AIChE Journal，2017，64(1)：717-729.
[7] LAMBERTO D J，LVAREZ M M，MUZZIO F J. Experimental and computational investigation of the laminar flow structure in a stirred tank[J]. Chemical Engineering Science，1999，54(7)：919-942.
[8] ARRATIA P E，LACOMBE J P，SHINBROT T，et al. Segregated regions in continuous laminar stirred tank reactors[J]. Chemical Engineering Science，2004，59(7)：1481-1490.
[9] ARRATIA P E，KUKURA J，LACOMBE J，et al. Mixing of shear-thinning fluids with yield stress in stirred tanks[J]. AIChE Journal，2006，52(7)：2310-2322.
[10] MASIUK S，RAKOCZY R. Power consumption，mixing time，heat and mass transfer measurements for liquid vessels that are mixed using reciprocating multiplates agitators[J]. Chemical Engineering and Processing，2007，46(2)：89-98.
[11] JANA S. Chaotic mixing of viscous fluids by periodic changes in geometry：Baffled cavity flow[J]. AIChE Journal，2010，40(11)：1769-1781.
[12] 刘海龙，曹宇，丁学翀，等. 搅拌槽内液相层流荧光可视化及高效混合技术[J]. 化工学报，2018，69(12)：5042-5048. LIU Hailong，CAO Yu，DING Xuechong，et al. Mixing enhancement technique for laminar flow in stirred tank[J]. Journal of Chemical Industry and Engineering，2018，69(12)：5042-5048.
[13] YAO W G，SATO H，TAKAHASHI K，et al. Mixing performance experiments in impeller stirred tanks subjected to unsteady rotational speeds[J]. Chemical Engineering Science，1998，53(17)：3031-3040.
[14] LAMBERTO D J，MUZZIO F J，SWANSON P D，et al. Using time-dependent RPM to enhance mixing in stirred vessels[J]. Chemical Engineering Science，1996，51(5)：733-741.
[15] 董敏，夏晨亮，李想. 组合桨搅拌槽内部流场及混合时间数值模拟[J]. 排灌机械工程学报，2019，37(1)：43-48. DONG Min，XIA Chenliang，LI Xiang. Numerical simulation of internal flow field and mixing time in stirred tank with combined impellers[J]. Journal of Drainage and Irrigation Machinery Engineering，2019，37(1)：43-48.
[16] 高殿荣，郭明杰，李岩，等. 变速搅拌混沌混合的PIV试验研究[J]. 机械工程学报，2006，42(8)：44-49. GAO Dianrong，GUO Mingjie，LI Yan，et al． PIV research on unsteady rotational speed chaoticmixing[J]. Journal of Mechanical Engineering，2006，42(8)：44-48．
[17] 梁瑛娜，高殿荣. 双层直斜叶及其组合桨搅拌槽三维流场数值模拟[J]. 机械工程学报，2008，44(11)：290-297. LIANG Yingna，GAO Dianrong. Numerical simulation of the three-dimensional flow field in stirred tank with double straight and inclined impeller and its combination[J]. Journal of Mechanical Engineering，2008，44(11)：290-297.
[18] 梁瑛娜，高殿荣，拜亮. 双层桨搅拌槽内层流流场与混合时间的数值模拟[J]. 机械工程学报，2015，51(16)：185-195. LIANG Yingna，GAO Dianrong，BAI Liang. Numerical simulation of the laminar flow field and mixing time in stirred tank with double layer impeller[J]. Journal of Mechanical Engineering，2015，51(16)：185-195.
[19] 刘作华，孙瑞祥，王运东，等. 刚-柔组合搅拌桨强化流体混沌混合[J]. 化工学报，2014，65(9)：3340-3349. LIU Zuohua，SUN Ruixiang，WANG Yundong，et al. Chaotic mixing intensified by rigid-flexible coupling impeller[J]. Journal of Chemical Industry and Engineering，2014，65(9)：3340-3349.
[20] 刘作华，陈超，刘仁龙，等. 刚柔组合搅拌桨强化搅拌槽中流体混沌混合[J]. 化工学报，2014，65(1)：61-70. LIU Zuohua，CHEN Chao，LIU Renlong，et al. Chaotic mixing enhanced by rigid-flexible impeller in stirred vessel[J]. Journal of Chemical Industry and Engineering，2014，65(1)：61-70.
[21] 刘作华，曾启琴，王运东，等. 柔性桨强化高黏度流体混合的能效分析[J]. 化工学报，2013，64(10)：3620-3625. LIU Zuohua，ZENG Qiqing，WANG Yundong，et al. Energy efficiency analysis for high-viscosity fluid mixing enhanced by flexible impeller[J]. CIESC Journal，2013，64(10)：3620-3625.
[22] 刘作华，杨鲜艳，谢昭明，等. 柔性桨与自浮颗粒协同强化高黏度流体混沌混合[J]. 化工学报，2013，64(8)：2794-2800. LIU Zuohua，YANG Xianyan，XIE Zhaoming，et al． Chaotic mixing performance of high-viscosity fluid synergistically intensified by flexible impeller and floating particles[J]. CIESC Journal，2013，64(8)：2794-2800.
[23] 刘作华，许传林，何木川，等. 穿流式刚-柔组合搅拌桨强化混合澄清槽内油-水两相混沌混合[J]. 化工学报，2017，68(2)：637-642. LIU Zuohua，XU Chuanlin，HE Muchuan，et al．Oil-water biphase chaotic mixing enhanced by punched rigid-flexible combination impeller in mixer-settler[J]. CIESC Journal，2017，68(2)：637-642.
[24] KARCZ J，CUDAK M，SZOPLIK J. Stirring of a liquid in a stirred tank with an eccentrically located impeller[J]. Chemical Engineering Science，2005，60(8-9)：2369-2380.
[25] ALVAREZ M M，ZALC J M，SHINBROT T，et al. Mechanisms of mixing and creation of structure in laminar stirred tanks[J]. AIChE Journal，2002，48(10)：2135-2148．
[26] PARVIZI S，ALAMDARI E K，HASHEMABADI S H，et al. Investigating factors affecting on the efficiency of dynamic mixers[J]. Mineral Processing and Extractive Metallurgy Review，2016，37(5)：27-39．
[27] 杨锋苓，周慎杰，王贵超，等．非标准挡板搅拌槽内湍流流场的数值模拟[J]. 高校化学工程学报，2012，26(6)：952-958. YANG Fengling，ZHOU Shenjie，WANG Guichao，et al. Study on the hydrodynamics of a stirred tank equipped with non-standard baffles[J]. Journal of Chemical Engineering of Chinese Universities，2012，26(6)：952-958．
[28] 王文坦，张梦雪，赵述芳，等. 激光诱导荧光技术在液体混合可视化研究中的应用[J]. 化工学报，2013，64(3)：771-778. WANG Wentan，ZHANG Mengxue，ZHAO Shufang，
et al. Application of laser-induced fluorescence technology in visualization of liquid mixing process[J]. Journal of Chemical Industry and Engineering，2013，64(3)：771-778．
[29] GONZALEZ R C，WOODS R E，EDDINS S L. Digital image processing using matlab[M]. Beijing：Publishing House of Electronics Industry，2009．