同心双轴搅拌器气液分散性能的实验研究

针对气液搅拌操作形式单一的现状,将框式桨和Rushton桨组成的同心双轴搅拌器与黏稠体系中的气液分散操作相结合,实验考察了转动模式、内桨转速、通气量和体系黏度等因素对其气液分散性能的影响。结果表明,与单相黏稠体系中同向旋转模式下的功率与混合性能均占优不同,同心双轴搅拌器在反向旋转模式下的气液分散性能相对更好;外桨转速不变时,内桨转速从100增加到300 r·min~(-1),整体气含率提高94.3%,局部气含率也均有增大;通气量从0.4提高到1.2 m3·h~(-1),整体气含率提高了72.5%,但局部气含率和气泡尺寸的增大不显著;体系黏度增加,气泡在釜内的停留时间加长,整体气含率单调增加。作为影响搅拌釜气液分散性能的重要参数,转动模式、内桨转速和通气量的确定还必须兼顾系统的功耗与混合效率,并避免发生气泛。     

同心双轴复合式搅拌釜用于牛顿流体时的功耗及混合特性

在直径0.48 m的椭圆底搅拌槽内,液位与槽径比(H/T)为0.6,采用不同粘度的牛顿流体糖浆溶液,研究了分别以CBY, 45o四斜叶桨及Rushton涡轮桨作为快速分散桨、锚式桨作为慢速桨构成的同心双轴搅拌系统,在快、慢速轴同向和异向2种旋转方式操作时的功率特性和混合性能. 结果表明,分散桨对锚式桨的功率消耗影响较大. 两轴同向旋转时,分散桨会使锚式桨的功耗降低,转速比RN增加,降低幅度也增大,RN=14时,锚式桨功率可降至单独旋转时的约10%;异向旋转时锚式桨的功率随RN增加而增加,RN=14时,锚式桨功率可增至单独旋转时的2倍左右. 但锚式桨对分散桨的功率消耗影响很小,在±5%以内. 计算同心双轴复合搅拌系统的复合功率准数和复合雷诺数关系时考虑了RN的影响,使在实验条件下不同转向及RN的功率曲线较好吻合. 混合效果同向旋转优于异向旋转,在牛顿流体中,达到相同混合效果时,CBY桨的能量消耗仅为其他2个分散桨的20%~30%.     

不同组合桨搅拌槽内非牛顿流体的微观混合特性

在直径0.282 m的搅拌槽内,以羟乙基纤维素(HEC)水溶液为工作体系,以磷酸盐?碘化物?碘酸盐平行竞争反应为模型反应,比较了非牛顿流体体系中向心桨、Rushton桨、三斜叶桨的功率准数,考察了加料时间、桨型及双层桨组合对微观混合效果的影响。结果表明,随功率增大,功率准数基本不变,Rushton桨功率准数最大,是向心桨的两倍、斜叶桨的四倍。随加料时间增大,离集指数先减小后不变。在实验考察范围内,单位体积功耗相等的情况下,单层桨微观混合效果的顺序为Rushton桨?向心桨?斜叶桨,双层桨中高剪切的Rushton桨与强循环的斜叶桨组合的微观混合效率最高。     

双层搅拌器组合的气液分散性能研究

系统研究和比较了径流桨和径流桨组合、径流桨和斜叶桨组合以及斜叶桨和斜叶桨组合3类不同的双层搅拌器组合,在气液分散搅拌过程中的优劣。小通气量时径流桨和斜叶桨组合(DT PTD和PTU DT)在相同的单位体积搅拌功率下气含率最高,而在大通气量时,双层上翻式斜桨组合(PTU PTU)气含率最高,并发现大通气量时,下层桨不宜采用下压式斜叶桨。     

四大叶斜桨的流体动力性能

四大叶斜桨搅拌器适用于对剪切稀化流体的搅拌混合。本文对此种新型搅拌器的流体动力性能作了实验研究,测定了其Metzner常数、搅拌功率、排液量和混合时间。四大叶斜桨的密实度比为0.57,具有排液量大,混合速度快的优点,而且功耗适中。文中还给出了四大叶斜桨与常用的涡轮和螺旋桨的性能对比。     

6种常规搅拌器的剪切性能研究

不同搅拌工况下剪切性能的需求是选用搅拌器的一个重要指标,文中基于对常规的6种搅拌器流场的CFD模拟,计算了搅拌器各自的剪切性能量纲一数C_3,分析了搅拌流场中剪应力的累计概率分布及统计平均结果。二者对比发现:从宏观平均效果上看,搅拌器剪切性能满足直叶桨>斜叶桨,圆盘涡轮>开启涡轮>桨式;而剪应力极大值影响作用则是直叶圆盘涡轮>斜叶圆盘涡轮>直叶桨式>斜叶开启涡轮>直叶开启涡轮>斜叶桨式。因此,对剪切作用敏感的搅拌过程,在采用常规的搅拌器剪切性能评价的基础上,还需要充分考虑搅拌流场中剪应力极大值的影响,合理选用搅拌器。     

双轴异桨变速组合型搅拌器的轴功率与混合效率

本研究测定了齿盘(偏置)式、锚式、45°框桨式三种搅拌器和齿盘-锚式、齿盘-45°框桨式两种双轴异奖变速组合型搅拌器的功率(N~p～Re)曲线和它们在操作流域(以单桨计算,Re=63—600)内的混合效率。结果表明,在层流域至弱过渡流域,组合型各搅拌器的功率与组合前单个搅拌器的功率基本相同;但在强过渡流域至湍流域,其功率计算要进行修正。组合搅拌器的功率是各搅拌器功率的迭加。效率较低的齿盘-锚式组合搅拌器比效率较高的齿盘式节能50％。齿盘-45°框桨式组合搅拌器的混合能是60°双层斜桨式的22％;是MIG型的50％。双轴异桨变速组合型搅拌器是一种值得开发的操作弹性宽、效率高和能耗低的搅拌设备。     

最大叶片式桨在假塑性流体中的搅拌流场模拟

为研究最大叶片式桨在高黏假塑性流体中的搅拌流动行为,以黄原胶溶液为研究体系,采用计算流体力学方法重点研究了釜内流体的功耗特性、速率分布、剪切速率、表观黏度分布和总体流动状况。结果表明:最大叶片式桨具有与大多数径流桨相似的"双循环"流型结构,且预测的功耗特性与实验数据一致性良好。最大叶片式桨适用于高黏假塑性流体的混合,而对于高黏牛顿流体的混合则效果不佳。釜内的剪切速率分布较宽泛,且受转速影响较大。转速可作为该桨改善黄原胶体系混合效率的重要参数之一。     

逆流桨强化搅拌槽内流体混沌混合及流场结构失稳研究

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。     

高粘性非牛顿流体搅拌的研究

本文研究了双螺带搅拌器和另外两种新型搅拌器(收缩桨式、交叉桨式)在非牛顿流体中的搅拌特性,试验比较认为,交叉桨为比较理想的桨叶形式。利用回归组合设计,找出了交叉桨式搅拌器无因次混合时间与几个设备参数之间的关系,通过优化运算得到了最佳参数,并研究了交叉桨在粘弹性流体中的搅拌特性。     

Power consumption and flow field characteristics of a coaxial mixer with a double inner impeller

A coaxial mixer meeting the actual demand of a system with high and variable viscosity is investigated. It has an outer wal-scraping frame and a double inner impeller consisting of a four-pitched-blade turbine and Rushton turbine. The power consumption and flow field characteristics of the coaxial mixer in laminar and transitional flow are simulated numerically, and then the distribution of velocity field, shear rate and mass flow rate are analyzed. The simulation results indicate that the outer frame has little effect on the power consumption of the double inner impeller whether in laminar or transitional flow, whereas the inner combined impeller has a great effect on the power consumption of the outer frame. Compared with the single rotation mode, the power consumption of the outer frame will decrease in co-rotation mode and increase in counter-rotation mode. The velocity, shear rate and mass flow rate are relatively high near the inner impeller in all operating modes, and only under double-shaft agitation wil the mixing performance near the free surface be improved. In addition, these distributions in the co-rotation and counter-rotation modes show little difference, but the co-rotation mode is recommended for the advantage of low power consumption.     

Effect of Impeller Spacing on the Flow Field of Yield-Pseudoplastic Fluids Generated by a Coaxial Mixing System Composed of Two Central Impellers and an Anchor

The three-dimensional flow field generated by a coaxial mixer composed of double Scaba impellers and an anchor in the mixing of the xanthan gum solution, a non-Newtonian yield-pseudoplastic fluid was investigated using the computational fluid dynamics (CFD) technique. The mixing time measurements were performed by a non-intrusive flow visualization technique called electrical resistance tomography (ERT). To evaluate the influence of the impeller spacing on the hydrodynamics of the double Scaba-anchor coaxial mixer, the upper impeller submergence was set to 0.140?m while the lower impeller clearance and the spacing between two central impellers were changed within a wide range. The experiments and simulations were conducted for both co-rotating and counter-rotating regimes at different impeller spacing. The analysis of the collected data with respect to the power number, flow number, mixing time, and pumping effectiveness proved that the co-rotating mode had superiority over the counter-rotating regime. Furthermore, the impact of the impeller spacing in the co-rotating mode was assessed with respect to the mixing time, power number, and mixing energy. The results demonstrated that a coaxial mixer with the impeller spacing of almost equal to the central impeller diameter (C₂?=?0.175?m) and the impeller clearance of C₃?=?0.185?m was the most efficient configuration compared to the other cases. Additionally, the influence of the impeller spacing on the flow pattern was assessed in terms of the radial velocity, tangential velocity, axial velocity, shear rate, and apparent viscosity profiles. When the impeller spacing (C₂) was varied, the merging flow and parallel flow patterns were observed.     

Experimental investigation on micromixing characteristics of coaxial mixers in viscous system

An experimental investigation into the micromixing performance of coaxial mixers in a viscous system is reported, in which the iodide-iodate reaction system is chosen to quantitatively characterize the product distributions. The effects of feeding time, feeding position, impeller speed, inner impeller configuration, and operation mode on the segregation index, Xs, are examined. It is revealed that the feeding near the inner impeller benefits micromixing and should be regarded as the preferred position. The presence of the rotating outer impeller causes the micromixing performance of the coaxial mixer to be significantly better than the single-shaft mixer. While an increase in the outer impeller speed has a limited influence on micromixing, the inner impeller speed is the dominant influencing factor, that is, the Xs decreases obviously when the inner impeller speed is increased. On the other hand, the coaxial mixers with multiple and axial inner impellers have a better micromixing performance at the same specific power consumption, P_V, than that with single and radial inner impellers. Among the configurations consisting of a Rushton impeller (RT), six-straight-blade turbine impeller (SBT), and six-pitched (45°)-blade turbine impeller (PBT), the Xs of the coaxial mixer is always the smallest at the same P_V when the PBT + RT configuration is used as the inner impeller. In addition, it is found that the difference in Xs that results from various operation modes is small in terms of power consumption; however, the co-rotation mode is still recommended for the micromixing of the coaxial mixer due to its excellent performance in general.     

Mixing analysis in a coaxial mixer

The performance of a coaxial mixer in the laminar-transitional flow regime was numerically investigated with Newtonian and non-Newtonian fluids. These mixers comprised two shafts: a central fast speed shaft mounted with an open turbine, and a slow speed shaft fitted with a wall scraping anchor arm. To model the complex hydrodynamics inside the vessel, the virtual finite element method (POLY3D^TM software) coupled with a Lagrange multiplier approach to cope with the non-linearity coming from the rheological model was employed. Co-rotation and counter-rotation mode were compared, based on several numerical criteria, namely, mixing time, power consumption and pumping rate. It was found that co-rotating mode is more efficient than counter-rotating mode in terms of energy, pumping rate and homogenization time.     

Finite element modeling of the laminar and transition flow of the Superblend dual shaft coaxial mixer on parallel computers

The macro-mixing mechanisms of the Superblend coaxial mixer consisting of a Maxblend impeller and a double helical ribbon agitator mounted on two independent coaxial shafts rotating at different speeds are numerically investigated. The simulations are based on the resolution of the Navier-Stokes equations with help of a parallel three-dimensional finite element solver exploiting the capabilities of high performance computers. To model the rotation of agitators a hybrid approach based on a novel finite element sliding mesh and fictitious domain method is used. The power consumption, the flow patterns, the shear rate distribution, the pumping capacity and the mixing time of the Superblend mixer are calculated from the simulated hydrodynamics. The simulations allow observing the flow as it evolves from deep laminar (Re=0.1) to transition (Re=520) regime. As Reynolds number increases, several recirculation zones above and below the middle of the tank are formed. It is found that operating the agitators in co-rotation mode requires less power consumption and exhibits equal or shorter mixing time than counter-rotation mode. The larger power consumption in counter-rotating mode is caused by the presence of high shear vortices generated between the two coaxial agitators. Furthermore it is shown that the shear distribution throughout the Superblend coaxial mixer operating in co-rotation mode is almost homogenous, which is highly desirable for shear sensitive products. In view of the results obtained in this work, the Superblend coaxial mixer is found as a good alternative for tough mixing applications.     

Power Consumption in the Turbulent Regime for a Coaxial Mixer

The power consumption of a new coaxial mixer composed of an anchor impeller and a pitched‐blade turbine impeller, and a series of rods operated in a contra‐rotating mode has been characterized experimentally in the turbulent regime. It is shown that both the power curve and the turbulent power number vary significantly with the speed ratio between the impellers. Likewise with single impeller mixers, the transition regime starts at a Reynolds number above 100 and the turbulent regime between 10³ and 10⁴ irrespective of the definition of the Reynolds number used.     

Coaxial Mixer Hydrodynamics with Newtonian and non‐Newtonian Fluids

The performance of several combinations of a wall scraping impeller and dispersing impellers in a coaxial mixer operated in counter‐ and co‐rotating mode were assessed with Newtonian and non‐Newtonian fluids. Using the power consumption and the mixing time as the efficiency criteria, impellers in co‐rotating mode were found to be a better choice for Newtonian and non‐Newtonian fluids. The hybrid impeller‐anchor combination was found to be the most efficient for mixing in counter‐rotating or co‐rotating mode regardless of the fluid rheology. For both rotating modes, it was shown that the anchor speed does not have any effect on the power draw of the dispersing turbines. However, the impeller speed was shown to affect the anchor power consumption. The determination of the minimum agitation conditions to achieve the just suspended state of solid particles (N_js) was also determined. It was found that N_js had lower values with the impellers having the best axial pumping capabilities.     

Power demand and mixing performance of coaxial mixers in a stirred tank with CMC solution

Experimental investigation was carried out in an el iptical based stirred tank with a diameter of 0.48 m to explore the power demand and mixing performance of coaxial mixers. Syrup and CMC solution (sodium carboxy methyl cellulose) were used as the Newtonian and non-Newtonian fluids, respectively. Four different coaxial mixers were combined with either CBY or Pfaudler impeller as the inner one, and anchor or helical ribbon (HR) as the outer one. Results show that Pfaudler-HR is the optimized combination among four coaxial mixers in this work, which provides the shortest mixing time given the same power consumption. Compared with the syrup solution, the increase of power input can make the mixing time decreasing more obviously in the CMC solution. The quantitative correlations for both syrup and CMC solutions were established to calculate the power draw and the mixing time of four coaxial mixers.     

剪切变稀体系同心双轴搅拌釜内的气液分散模拟

气液搅拌设备因其良好的适用性被广泛应用于过程工业中。为更好地比较不同工况下剪切变稀体系中的气液分散情况,通过实验研究整体气含率和相对功耗确定适宜的转动模式,进而模拟研究表观气速、体系黏度、搅拌转速对气含率和气泡尺寸的影响。结果表明,相同功率下内外双桨反向旋转模式在理想气液分散条件下,相较于单轴内桨和内外双桨同向旋转模式具有更高的气含率和更好的气体泵送能力;表观气速的增加有利于气泡的均匀分散,但气泡尺寸也会随之增大;有效黏度的增加使得搅拌桨的影响区域变小,不利于气泡的均匀分散,气泡尺寸也随之增大;搅拌转速的增加使得循环涡流的影响区域变大,高气含率区不断扩大。