刚-柔组合搅拌桨强化流体混沌混合

合理设计搅拌反应器的桨叶,强化流体流动与混合行为,是实现流体高效、节能混合的重要手段。柔性体与刚性体组合,可设计出具有多体运动行为的刚-柔组合搅拌桨。结合PIV流场观测和CFD模拟,对比分析了刚性桨和刚-柔组合桨对流场结构及流体混沌混合行为的影响。结果表明,与刚性搅拌桨相比,刚-柔组合桨的柔性端强化能量传递,流体流速衰减速率降低25%,有利于搅拌桨输入能量在流场结构内的有效分配。传统刚性六凹叶和六直叶涡轮桨搅拌反应器内流体形成的流线结构具有明显的周期吸引子,其时均流场的分形维数分别为1.9046和1.9138。刚-柔组合六直叶涡轮桨搅拌反应器内流体流线呈明显的准周期性吸引子性质,其流场分形维数为1.9337,而刚-柔组合六凹叶涡轮桨搅拌反应器内流体流线具有典型的混沌吸引子性质,其流场分形维数为1.9545。刚-柔组合搅拌桨可改变流体流线的吸引子来调控流场的多尺度结构,强化流体混沌混合,实现高效节能操作。     

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

运用LabView和Matlab软件分别采集和处理穿流式刚-柔组合搅拌桨扰动澄清槽中油-水两相流体内部的压力脉动信号,得出的最大Lyapunov指数（LLE）和多尺度熵（MSE）,反映流体内部的混沌程度;同时采用流场可视化技术观测流体混合状态。结果表明,相比于刚性组合桨,穿流式刚-柔组合搅拌桨通过穿流孔与柔性部分的共同作用改变流场的结构和能量耗散方式,使流体的混沌程度和混合状态都优于刚性组合桨。当转速为88 r·min^-1时,流体的混沌混合都达到最佳状态,各实验条件下的LLE均大于零,表明流场混合体系已进入混沌状态,且穿流式刚-柔组合搅拌桨体系的MSE明显高于刚性组合桨体系,说明穿流式刚-柔组合搅拌桨的混合效果优于刚性组合桨。另外,柔性片上穿流孔的数目和柔性桨叶的厚度对流场的混沌特性也有明显的影响。     

刚柔组合搅拌桨强化搅拌槽中流体混沌混合

搅拌槽内普遍存在着两种不同类型的混合区域：混沌混合区和规则区。增大混沌混合区,是提高流体混合效率、降低搅拌过程能耗的重要途径。而合理设计搅拌桨有助于流体形成适宜的流动状态,实现混沌混合。柔性体与刚性体组合,可设计出具有多体运动行为的刚柔组合搅拌桨,可强化流体混沌混合行为。结合Matlab 软件,实验研究了双层桨搅拌槽内自来水体系的最大Lyapunov指数（LLE）和多尺度熵（MSE）的变化规律,对比分析了刚性桨和刚柔组合桨两种桨叶对流体混沌混合的影响。结果表明,刚柔组合桨强化流体的运动特性,使更多流体进入混沌混合状态。在转速为210 r·min^-1时,流体的混沌混合达到最佳状态,刚性桨体系的LLE为0.041,而刚柔组合桨体系的LLE为0.048;刚柔组合桨可有效耗散能量,使整个槽体的能量分布均匀,刚柔组合桨在150 r·min^-1时的多尺度熵率与刚性桨在210 r·min^-1时基本相近;刚柔组合桨体系的混合时间均低于刚性桨体系,在转速为120 r·min^-1时,刚柔组合桨使流体的混合时间缩短了26%左右。刚柔组合桨可改变流场结构和能量耗散方式,强化流体混沌混合,实现高效节能操作。     

穿流-柔性组合桨强化搅拌槽中流体混沌混合特性

刚性搅拌桨在搅拌混合过程中得到广泛的应用,在搅拌容器内容易形成两种不同的混合区域:混沌混合区和混合隔离区。强化流体混合的有效途径是合理设计搅拌桨,从而调控流体混沌混合行为。实验运用Labview和Matlab软件采集和处理流体内部压力脉动信号,并获取流体混沌特性参数Kolmogorov熵,对穿流-柔性组合桨体系的Kolmogorov熵随转速的变化规律进行了研究。结果表明,相比传统刚性桨,穿流桨对Kolmogorov熵影响不大。穿流-柔性组合桨通过穿流孔与柔性部分的共同作用调控流场结构,使流体混沌混合的效果最好,在转速为180 r·min^-1时流体的混沌混合达到最佳状态,穿流-柔性组合桨体系的Kolmogorov熵为0.285,而传统刚性桨体系的Kolmogorov熵只为0.125;穿流-柔性组合桨体系的混合时间明显低于传统刚性桨体系,当转速为120 r·min^-1时穿流-柔性组合桨体系的混合时间比传统刚性桨体系缩短了17%。     

刚柔组合搅拌桨与刚性桨调控流场结构的对比

高黏度流体处于层流状态时,普遍存在的混合隔离区,降低了流体的混合效率。减小或消除隔离区,是实现流体高效混合的基本途径。采用实验研究与数值模拟相结合的方法,对刚性六直叶涡轮桨(刚性桨)和刚柔组合六直叶涡轮桨(组合桨)的流场结构进行研究,对比分析了两种桨叶在相同功耗(3 kW·m-3)时的轴向、径向和切向的速度矢量图、速度云图以及速度分布散点图。结果表明,刚性桨的能量集中在桨叶尖端部分,远离桨叶区域的流体速度很小甚至为0 m·s-1;而组合桨可将能量从桨叶尖端扩散至全槽,使槽内流体均具有一定的流速,提高了混合效率,且显色实验与数值模拟结果一致,组合桨体系的混合隔离区在短时间内就可消除,混合良好,而刚性桨体系的混合隔离区始终存在,混合效果不佳。     

刚柔组合搅拌桨与刚性桨调控流场结构的对比

高黏度流体处于层流状态时,普遍存在的混合隔离区,降低了流体的混合效率。减小或消除隔离区,是实现流体高效混合的基本途径。采用实验研究与数值模拟相结合的方法,对刚性六直叶涡轮桨（刚性桨）和刚柔组合六直叶涡轮桨（组合桨）的流场结构进行研究,对比分析了两种桨叶在相同功耗（3 kW·m^-3）时的轴向、径向和切向的速度矢量图、速度云图以及速度分布散点图。结果表明,刚性桨的能量集中在桨叶尖端部分,远离桨叶区域的流体速度很小甚至为0 m·s^-1;而组合桨可将能量从桨叶尖端扩散至全槽,使槽内流体均具有一定的流速,提高了混合效率,且显色实验与数值模拟结果一致,组合桨体系的混合隔离区在短时间内就可消除,混合良好,而刚性桨体系的混合隔离区始终存在,混合效果不佳。     

多层翼形桨搅拌槽内混合过程的CFD模拟

The mixing process in a stirred tank of 0.476m diameter with single, dual and triple 3-narrow blade hydrofoil CBY impellers was numerically simulated by using computational fluid dynamics （CFD） package FLU-ENT6.1. The multi-reference frame （MRF） and standard k-ε turbulent model were used in the simulation. The shaft power and the mixing time predicted by CFD were in good agreement with the experiment. The effects of tracer feeding and detecting positions on mixing time were investigated. The results are of importance to the optimum design of industrial stirred tank/reactors.     

单层钢丝柔性桨强化搅拌槽中流体混沌混合行为

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。     

刚柔组合搅拌桨强化流体混合的流固耦合行为

传统刚性搅拌桨通过对流体的剪切作用实现能量的传递,而刚柔组合搅拌桨可通过其多体运动行为强化能量传递。基于搅拌桨桨叶与流体之间的耦合运动作用,结合ANSYS Workbench仿真平台,采用双向流固耦合方法,模拟计算了刚性搅拌桨与刚柔组合搅拌桨桨叶的等效应力和总变形量,研究了流场的宏观结构;并通过测定混合时间和计算搅拌桨功耗对比分析了两种不同搅拌体系的混合行为。结果表明:刚柔组合搅拌桨使体系的混合时间缩短了近32%,搅拌桨功耗下降了7%,其桨叶尖端的变形量是刚性搅拌桨的10⁵倍,其应力比刚性搅拌桨增加了83%;与刚性搅拌桨相比,刚柔组合搅拌桨在流固耦合作用下对流体的作用力更大,能够更好地传递能量,增强流体运动,强化流体混合。     

刚柔组合搅拌桨增强混合澄清槽内流体宏观不稳定性

流体宏观不稳定性可以反映流体轴向能量和质量的传递行为。为揭示刚柔组合搅拌桨（简称柔性桨）作用下混合澄清槽中油水液-液两相非稳态流动规律,采用频谱分析和小波分析组合法研究混合澄清槽内宏观不稳定性,并进行模拟验证。研究表明,柔性桨在转速低于250 r·min^-1时,流体宏观不稳定频率与转速呈线性关系,而转速超过250 r·min^-1,流体因界面卷吸行为吸入空气,宏观不稳定频率谱图呈现功率谱带,流场结构呈多尺度结构特征,流体宏观不稳定频率消失,液-液混合体系出现明显的乳化现象。与刚性桨相比柔性桨能增强宏观不稳定性,提高流体混合效率,强化能量传递行为。计算模拟发现,柔性桨能明显提高桨叶的抽吸能力,增强流体轴向运动的行为,避免流体过度搅拌,有利于流体澄清。     

多层刚柔组合桨诱发流场界面失稳强化非牛顿流体混沌混合行为

传统多层刚性桨用于假塑性非牛顿流体混合搅拌死区较大,流场界面稳定,混合效率低。提出多层刚柔组合桨诱发流场界面失稳强化非牛顿流体混沌混合的方法。实验以羧甲基纤维素钠为非牛顿流体体系,通过扭矩传感器测量功率特性,酸碱中和脱色法测定混合时间,并利用Matlab软件编程计算最大Lyapunov指数,分析了非牛顿流体混合过程中的混沌特性及其混合性能。结果表明,组合方式为RF-(PBTD+PBTD+DT)、桨叶排列方式θ=60°、柔性片长度安装比例r=0.8、1.2时,混沌程度较高,混合性能较好。多层刚柔组合桨可以产生多股螺旋流,并在层间柔性片扰动频率差下实现流场界面失稳,搅拌死区减小,在较低转速下使体系进入混沌状态(多层刚柔组合桨体系N>88 r/min时LLE>0,多层刚性桨体系N>125 r/min时LLE>0);在相同转速下,多层刚柔组合桨混合速率、单位体积功率高于多层刚性桨,而单位体积混合能大致相同。     

Chaotic mixing behavior of non-Newtonian fluid intensified by multilayer rigid-flexible impeller induced flow field interface instability

The traditional multilayer rigid impeller has large dead zone for the mixing of pseudoplastic non-Newtonian fluid, stable flow field interface and low mixing efficiency. A method for enhancing the chaotic mixing of non-Newtonian fluid by multilayer rigid-flexible impeller induced flow field interface instability was proposed. In the experiment, sodium carboxymethylcellulose was used as the non-Newtonian fluid system. The power characteristics were measured by the torque sensor. The mixing time was determined by the acid-base neutralization and decolorization method. The largest Lyapunov exponents were calculated by using Matlab software programming. The chaotic characteristics and mixing performance in the mixing process are analyzed. The results show that when the combination mode was RF-(PBTD+PBTD+DT), the impeller arrangement mode θ=60°, and the flexible sheet length installation ratio r=0.8, 1.2, the degree of chaos was higher and the mixing performance was better. Multilayer rigid-flexible impeller can generate multiple spiral flows, and realize the flow field interface instability under the disturbance frequency difference of the flexible sheet between the layers, the stirring dead zone was reduced, and the system enters a chaotic state at a lower speed (when the multilayer rigid-flexible impeller system N>88 r/min, LLE>0; when the multilayer rigid impeller system N>125 r/min, LLE>0). At the same speed, the mixing rate and power per unit volume of the multilayer rigid-flexible combined impeller are higher than that of the multilayer rigid impeller, but the mixing energy per unit volume is approximately the same.     

刚柔组合桨强化粉煤灰酸浸搅拌槽内固液混沌混合

传统粉煤灰提铝工艺中酸浸搅拌槽均采用刚性搅拌桨。因刚性桨卷吸能力有限，导致固体颗粒易沉槽、流体混沌混合效率低。提出刚柔组合桨强化酸浸搅拌槽中固液混沌混合行为。实验基于固含率为30%的粉煤灰-自来水体系，研究了刚柔组合酸浸搅拌槽内混沌混合特性及能量耗散规律。采用扭矩传感器采集扭矩时间序列信号，借助Matlab软件编译计算混合过程中最大Lyapunov指数和多尺度熵等混沌特性参数，以单位体积功耗表征搅拌反应器的功率特性。实验考察了搅拌桨安装离底高度、柔性片长度、柔性片宽度等因素对酸浸槽内粉煤灰混沌混合的影响，对比了刚性桨与刚柔组合桨体系的能耗差异。研究结果表明：刚柔组合桨通过柔性片的作用，能增大搅拌桨的卷吸力，进而减少固体颗粒沉槽现象，促进全槽混沌混合；在最优化条件（120 r/min，搅拌桨安装离底高度为T/4，柔性片长度为1.2H ₁、柔性片宽度为D/8）下，体系最大Lyapunov指数达到最大值0.0645，各尺度下的MSE均比其他条件更大，表明刚柔组合桨能够通过柔性片的多体运动，强化体系混沌混合，均化体系能量分布；刚性桨与刚柔组合桨的单位体积功耗随着转速的增加呈现指数规律增长。     

双层刚柔组合搅拌桨调控流体宏观不稳定性行为

流体宏观不稳定性是搅拌槽内流体流动存在大尺度低频非稳态准周期现象,可以影响流体的能量﹑质量的传递行为。为揭示在双层组合桨作用下搅拌槽内流体的非稳态流动规律,实验采用频谱分析和流场可视化技术研究双层组合桨搅拌槽内自来水体系的宏观不稳定性,对比分析了双层刚性桨和双层组合桨对流体混合的影响。结果表明:直径为T的搅拌槽内流体宏观不稳定频率与转速呈线性增大趋势,在转速为180 r·min^-1时离底距离 0.25T刚柔组合桨体系的宏观不稳定性频率消失,出现谱带现象,流场呈现多尺度结构特征,而离底距离为0.33T和0.5T的刚柔组合桨体系的宏观不稳定性频率分别为0.5096 Hz和0.3459 Hz。双层组合桨体系分别使流体的混合时间缩短了22.5%和35%左右,减小离底距离,可使流场的规则区减小。双层刚柔组合桨调控流体宏观不稳定性,强化流体的能量传递行为,从而缩短混合时间,提高了流体的混合效率。     

长短叶片复合型刚柔桨强化搅拌槽内流体混沌混合行为

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(θm)与单位体积功耗(Pv)呈指数型关系,当Pv相同时,RF-LSB(柔性片数量为3)的θm最小,表明RF-LSB(柔性片数量为3)更有利于流体混沌混合。     

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

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.     

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

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%.