自吸射流柔性搅拌桨振动特性

为了研究工况对新型自吸射流柔性搅拌桨的影响，运用实验和单向流固耦合数值模拟分析了装置的搅拌功耗、搅拌桨应力分布和振动特性。结果表明，相同工况下柔性Rushton搅拌桨的功率准数比自吸射流柔性搅拌桨高121.93%，并随Reynolds数的提高而减小。自吸射流柔性搅拌桨在支架与桨叶连接处应力最高，在本文研究范围内最大应力低于材料的许用应力。对自吸射流柔性搅拌桨在静模态和预应力模态下的固有频率和振型进行对比分析，两种模态前8阶为弯曲型，后4阶为扭变型；与静模态相比，预应力模态下固有频率和振型最大值的最大偏差分别为0.25%和27.56%。随着搅拌桨的转速和介质运动黏度提高，搅拌桨预应力模态固有频率增大，但同阶固有频率增长比变化较小。自吸射流柔性搅拌桨搅拌效率优势明显，本研究为多种工况下搅拌桨的强度和稳定性提供基础数据。     

柔性Rushton搅拌桨的功耗与流场特性研究

基于传统的Rushton桨,开发了一种柔性叶片Rushton搅拌桨。采用数值模拟方法研究了柔性桨的功耗及层流和湍流流场特性,并分别采用扭矩测量法和粒子图像测速法进行了实验验证。结果表明,对于实验规模的搅拌容器,当介质黏度与甘油接近时,可用橡胶作为柔性桨叶制作材料。Reynolds数≤100时,柔性桨的功耗大于刚性桨;Reynolds数大于该值后,柔性桨的功耗小于刚性桨。柔性桨叶对被搅拌流体具有自适应特性,流固耦合作用下产生的变形增加了流体的径向流动能力。搅拌低黏度流体时,柔性桨能提升近桨区流体的速度,增加桨叶远端流体的循环流动能力;搅拌高黏度流体时,近桨区和桨叶远端流体的速度均大于刚性桨。就尾涡而言,柔性桨产生的涡量较小,耗能少。     

Rushton桨搅拌槽内轴弯矩的数值模拟

针对搅拌槽内流体流动、柔性结构振动和流动流体与柔性结构相互作用(流固耦合)的特征,分别采用计算流体动力学(不考虑结构振动)、计算结构动力学(不考虑流体作用)和两种计算动力学相互瞬态耦合模拟(同时考虑结构振动和流体作用)研究Rushton桨搅拌轴的弯矩幅值平均和波动特性。研究结果表明:搅拌槽内流体充当了振动的阻尼作用,抑制了搅拌桨轴侧向振动的幅度,但主体流动的低频宏观不稳定性显著地增加了搅拌桨轴旋转的不稳定性,同时搅拌桨轴侧向振动增加了搅拌桨叶片上的流体载荷不稳定性,但对不均衡性影响很小;弯矩流体成分(来源于流体压力和粘性力)与结构成分(来源于结构重力和惯性力)之间夹角是随机的,但平均夹角接近于90°;耦合模拟结果与实验数据吻合较好,且明显优于计算流体动力学和计算结构动力学分离模拟计算结果。研究结果有助于深入理解搅拌槽内流固耦合对搅拌轴弯矩的影响,对搅拌设备的机械设计具有指导意义。     

柔性Rushton搅拌桨混合性能的实验研究

提高混合效率是流体搅拌混合领域的重点研究内容之一。几十年来,人们在开发新型搅拌桨及研发流体混合新技术方面做了大量工作。基于刚性Rushton桨,开发了一种柔性叶片Rushton搅拌桨,并以罗丹明6G为荧光剂,采用平面激光诱导荧光法对介质为水时该桨在湍流状态下的混合性能进行了实验测试研究。结果发现,标定实验结果表明,荧光剂强度与浓度呈线性关系,可以此为基准衡量同等实验条件下的宏观混合时间。荧光剂的扩散情况表明,与刚性桨相比,柔性桨具有更好的混合性能,尤其在混合的初始阶段,混合均匀程度及混合速度均有一定的优势。与刚性桨混合时间的对比表明,柔性桨的宏观混合时间较短,有助于提高流体混合效率。研究结果为该桨的工业应用奠定了基础。     

疏水Rushton搅拌桨的减阻性能

为了减小搅拌阻力与功耗,本研究提出了疏水叶片搅拌桨的设想。首先采用数值模拟的方法,对非疏水Rushton桨搅拌容器内的流场进行了模拟,通过与文献中实验结果的对比,验证了数值模型和模拟方法的可靠性。随后研究了湍流状态下疏水Rushton搅拌桨的流体动力学性能,分析了不同疏水状态下的流场结构、剪应力和压力分布以及减阻效果和搅拌功耗,并与非疏水桨进行了对比。结果表明,疏水处理后Rushton桨搅拌容器内的流场没有明显变化,但流体的轴向泵送能力有所增强,高速度区域略有扩大,超疏水时效果更明显。疏水处理可降低Rushton桨的剪应力和桨叶前后表面间的压差,具有减阻效果,超疏水时减阻幅度高达39.56%。另外,疏水Rushton桨的搅拌功耗有所降低,与非疏水桨相比,超疏水桨的功率准数降低了8.53%,具有显著的节能效应。     

柔性Rushton搅拌桨的功耗与流场特性研究

基于传统的Rushton桨,开发了一种柔性叶片Rushton搅拌桨。采用数值模拟方法研究了柔性桨的功耗及层流和湍流流场特性,并分别采用扭矩测量法和粒子图像测速法进行了实验验证。结果表明,对于实验规模的搅拌容器,当介质黏度与甘油接近时,可用橡胶作为柔性桨叶制作材料。Reynolds数≤100时,柔性桨的功耗大于刚性桨;Reynolds数大于该值后,柔性桨的功耗小于刚性桨。柔性桨叶对被搅拌流体具有自适应特性,流固耦合作用下产生的变形增加了流体的径向流动能力。搅拌低黏度流体时,柔性桨能提升近桨区流体的速度,增加桨叶远端流体的循环流动能力;搅拌高黏度流体时,近桨区和桨叶远端流体的速度均大于刚性桨。就尾涡而言,柔性桨产生的涡量较小,耗能少。     

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。     

搅拌容器系统的振动分析

采用有限元法对载荷工况下的搅拌容器系统进行了预应力模拟,通过模态分析得出该系统在预应力状态下的1～10阶固有频率,同时把转换和计算得到的系统在工作状态下电机的转动频率、轴的转动频率、流体通过挡板的频率、流体通过叶片的频率和流体的冲击频率作为激振频率,并与固有频率进行对比得出:激振频率分别落在1～10阶固有频率±20%区间之外,即在工作状态下搅拌容器系统不会发生共振。     

几种单层桨搅拌槽内宏观混合特性的比较

为了丰富对向心桨的混合特性的认识,比较了向心桨、Rushton桨、三斜叶桨和穿流桨的单层桨搅拌槽内的宏观混合特性,考察了搅拌转速、桨叶离底高度对搅拌槽混合时间和功率特性的影响。结果表明,四种桨的宏观混合时间均随着搅拌转速的增加而减少,搅拌功率均随转速的增加逐渐增大。当转速相同时,四种桨型中Rushton桨的功率消耗最大,三斜叶桨功率消耗最小,向心桨的功率消耗仅仅比三斜叶桨高。桨叶离底高度的变化对四种桨型的混合时间和功率的影响不尽相同。混合效率的影响因素大小顺序为：搅拌转速>桨型>桨叶离底高度。在考察的四种桨型中,向心桨的混合效率最高。研究成果可为向心桨等新型搅拌桨的工业应用积累实验数据,为其优化设计和放大提供理论依据。     

小型风力机叶片全覆冰仿真和模态试验对比研究

风力机叶片覆冰后会影响风力发电机组正常工作,甚至会使风力机过载、叶片折断,进而造成安全事故和资源浪费。应用ANSYS模态仿真和模态试验两种研究方法,探究风力机叶片在不同覆冰厚度下模态参数的变化,得到叶片前四阶振型和固有频率。结果表明,仿真和试验模态测试结果接近,振型相符,基频的误差在3%以内。覆冰后叶片一阶固有频率振型为挥舞,从第二阶开始,出现挥舞和摆振方向的耦合模态振型。前四阶振型随着覆冰厚度增加振动形式保持不变,主要振动形式为挥舞摆动,叶尖处的振幅逐渐变小。随着覆冰厚度的增加,叶片固有频率逐渐降低,在覆冰厚度为30 mm时,叶片基频比不覆冰降低28. 7%。     

Comparison of flow patterns of dual rushton and CD-6 impellers

The flow patterns (parallel, merging and diverging) of dual CD-6 impeller has been modeled using computational fluid dynamics simulation in FLUENT software compared with dual Rushton impeller. Simulated results have been verified with experimental observations on Rushton impeller. Comparison of simulated and experimental results of Rushton impeller has been observed similar flow patterns, whereas CD-6 impeller exhibits different flow pattern from the Rushton impeller in merging flow pattern. Analysis has shown that formation of flow patterns of Rushton impeller and CD-6 impeller is different in the case of merging flow pattern.     

Gas recirculation rate and its influences on mass transfer in multiple impeller systems with various impeller combinations

Both the numerical and experimental approaches were used to study the effects of the gas recirculation and non‐uniform gas loading on the mass transfer rate for each impeller in a multiple impeller system. By combining the calculated gas velocity and local gas holdup, the gas recirculation rate around each impeller was estimated. The local mass transfer coefficients for systems equipped with various combinations of the Rushton turbine impeller (R) and pitched blade impeller (P) were determined by using the dynamic gassing out method. It is found that the Rushton turbine impeller has to be served as the lowest impeller in order to have a better gas dispersion and to give a higher overall K_La for a multiple impeller gas‐liquid contactor. The upper pitched blade impeller always enforces the circulating flow around the Rushton turbine impeller just beneath it and gives a higher overall average mass transfer rate. However, the system equipped with only the pitched blade impellers results in a much lower mass transfer rate than the other systems owing to the poor gas dispersion performance of the pitched blade impeller.     

Gas-liquid hydrodynamics in a vessel stirred by dual dislocated-blade Rushton impellers☆

Towards the objective of improving the gas dispersion performance, the dislocated-blade Rushton impeller was applied to the gas–liquid mixing in a baffled stirred vessel. The flow field, gas hold-up, dissolved oxygen, power consumption before and after gassing were studied using the computational fluid dynamics (CFD) technique. Dispersion of gas in the liquid was modelled using the Eulerian–Eulerian approach along with the dispersed k–εturbulent model. Rotation of the impeller was simulated with the multiple reference frame method. A modified drag coefficient which includes the effect of turbulence was used to account for the momentum exchange. The predictions were compared with their counterparts of the standard Rushton impeller and were validated with the experimental results. It is concluded that the dislocated-blade Rushton impeller is superior to the standard Rushton impeller in the gas–liquid mixing operation, and the findings obtained here lay the basis of its application in process industries.     

Effect of Particle Image Velocimetry Setting Parameters on Local Velocity Measurements in an Agitated Vessel

Although they are obtained under the same conditions, results on the flow field in an agitated vessel achieved using particle image velocimetry (PIV) may vary due to differences in the PIV conditions. The influence on turbulence characteristics of the main PIV setting parameters, i.e., PIV spatial resolution, sampling frequency, and recording time, was investigated. Tests were performed with three different liquids in a developed turbulent field for a Rushton turbine impeller using two‐dimensional time‐resolved PIV. To obtain the relevant velocity gradients, a minimum recording time is needed. No effect of sampling frequency was observed if the sampling frequency was higher than approximately 17 times the impeller frequency, which is about three times the impeller blade frequency.     

Characterisation of three different impellers using the LDV-technique

A characterisation of three commonly used impellers was made in this study by measuring local mean velocities and the fluctuations of these velocities with the LDV technique. The data was used to estimate volumetric flow, velocity fluctuations and turbulent intensity in the impeller region of the tank. The impellers investigated were a high flow impeller, a pitched blade turbine and a Rushton turbine. The cylindrical vessel used was made of Perspex, had a dished bottom (DIN 28013), was equipped with four baffles and had an inner diameter of 0.45 m. It was found that the bulk velocities could be scaled with the tip-speed of the impeller (ND). The flow rate at constant impeller speed increased in the order high flow impeller — Rushton turbine — pitched blade turbine. The corresponding order for the turbulence fluctuation is: high flow impeller — pitched blade turbine — Rushton turbine. The velocity profile of the flow out from the high flow impeller was furthermore, not as smooth as could be expected.     

Gas-Liquid Mixing in a Grid-Disc Impeller Stirred Tank

To enhance the gas-liquid mixing performance in stirred tanks, the grid-disc impeller was designed by replacing the solid disc of the standard Rushton impeller with a grid disc. Gas-liquid hydrodynamics of the new impeller was studied by employing the Eulerian-Eulerian two-phase model coupled with the dispersed k-ε turbulence model. Rotation of the impeller was simulated with the multiple reference frame method. Flow field, gas holdup, and power consumption were investigated and compared with the standard Rushton impeller. The numerical method was validated by comparing the gas holdup with literature. The grid-disc impeller performed better than the Rushton impeller in terms of gas dispersion performance, axial pumping capacity, and energy requirement, indicating its potential for gas-liquid mixing applications.