运用粒子图像测速仪研究双层桨搅拌槽内流体流动

The flow fields in a dual Rushton impeller stirred tank with diameter of 0.48 m （T） were measured by using Particle Image Velocimetry （PIV）. Three different size impellers were used in the experiments with diameters of D = 0.33T, 0.40T and 0.50T, respectively. The multi-block and 360° ensemble-averaged approaches were used to measure the radial and axial angle-resolved velocity distributions. Three typical flow patterns, named, merging flow, parallel flow and diverging flow, were obtained by changing the clearance of the bottom impeller above the tank base （C1） and the spacing between the two impellers （C2）. The results show that while C1 is equal to D, the parallel flow occurs as C2≥0.40T, C2≥0.38T and C2≥0.32T and the merging flow occurs as C2≤0.38T, C2≤0.36T and C2≤0.27T for the impellers with diameter of D=0.33T, 0.40T and 0.50T, respectively. When C2 is equal to D, the diverging flow occurs in the value of C1≤0.15T for all three impellers. The flow numbers of these impellers were calculated for the parallel flow. Trailing vortices generated by the lower impeller for the diverging flow were shown by the 10° angle-resolved velocity measurements. The peak value of turbulence kinetic energy （ k/V^2tip = 0.12-0.15 or above） appears along the center of the impeller discharging stream.     

叶片形状对涡轮桨搅拌槽内尾涡特性的影响

Particle image velocimetry technique was used to analyze the trailing vortices and elucidate their rela-tionship with turbulence properties in a stirred tank of 0.48 m diameter,agitated by four different disc turbines,in-cluding Rushton turbine,concaved blade disk turbine,half elliptical blade disk turbine,and parabolic blade disk turbine.Phase-averaged and phase-resolved flow fields near the impeller blades were measured and the structure of trailing vortices was studied in detail.The location,size and strength of vortices were determined by the simplified λ2-criterion and the results showed that the blade shape had great effect on the trailing vortex characteristics.The larger curvature resulted in longer residence time of the vortex at the impeller tip,bigger distance between the upper and lower vortices and longer vortex life,also leads to smaller and stronger vortices.In addition,the turbulent ki-netic energy and turbulent energy dissipation in the discharge flow were determined and discussed.High turbulent kinetic energy and turbulent energy dissipation regions were located between the upper and lower vortices and moved along with them.Although restricted to single phase flow,the presented results are essential for reliable de-sign and scale-up of stirred tank with disc turbines.     

半椭圆管盘式涡轮桨搅拌槽内湍流结构的时间解析PIV研究(英文)

The turbulence structure in the stirred tank with a deep hollow blade (semi-ellispe) disc turbine (HEDT) was investigated by using time-resolved particle image velocimetry (TRPIV) and traditional PIV. In the stirred tank, the turbulence generated by blade passage includes the periodic components and the random turbulent ones. Traditional PIV with angle-resolved measurement and TRPIV with wavelet analysis were both used to obtain the random turbulent kinetic energy as a comparison. The wavelet analysis method was successfully used in this work to separate the random turbulent kinetic energy. The distributions of the periodic kinetic energy and the random turbulent kinetic energy were obtained. In the impeller region, the averaged random turbulent kinetic energy was about 2.6 times of the averaged periodic one. The kinetic energies at different wavelet scales from a6 to d1 were also calculated and compared. TRPIV was used to record the sequence of instantaneous velocity in the impeller stream. The evolution of the impeller stream was observed clearly and the sequence of the vorticity field was also obtained for the identification of vortices. The slope of the energy spectrum was approximately &;#61485;5/3 in high frequency representing the existence of inertial subrange and some isotropic properties in stirred tank. From the power spectral density (PSD), one peak existed evidently, which was located at f0 (blade passage frequency) generated by the blade passage.     

涡轮桨搅拌槽内湍流特性的V3V实验及大涡模拟

分别用体三维速度测量技术（volumetric three-component velocimetry measurements,V3V）和大涡模拟（large eddy simulation,LES）方法对涡轮桨搅拌槽内流场进行研究,发现在完全湍流状态下,涡轮桨搅拌槽内流场的量纲1相平均速度及湍动能分布同Reynolds数无关。用V3V方法实现了Rushton桨叶附近三维流场的重构;探讨尾涡的三维结构及运动规律;分析了叶片后方30°截面轴向、径向和环向速度沿径向分布规律。用V3V实验结果对比了2D-PIV（particle image velocimetry）数据中的尾涡涡对位置和涡量,涡对位置吻合度较好,但2D-PIV中涡量较V3V小37.5%;通过大涡模拟得到完整的尾涡结构,发现在叶片上边缘后侧存在一个和尾涡形成方式相同但不成对出现的涡结构;将大涡模拟结果和2D-PIV及V3V实验结果对比发现,大涡模拟在速度分布及尾涡运动轨迹方面均同实验结果吻合较好,表明大涡模拟能较好地预测涡轮桨搅拌槽内流场。     

涡轮桨搅拌槽内流场特性的V3V实验

用体三维速度测量技术（volumetric three-component velocimetry measurements,V3V）实验研究了涡轮桨搅拌槽内桨叶附近流场。通过速度数据得到三维流场特性,确定尾涡三维结构;分析了叶片后方30°截面轴向、径向和环向速度沿径向分布规律;对比了V3V和2D-PIV（particle image velocimetry）径向和轴向速度,发现速度分布吻合较好,特别是尾涡所在的射流区。用2D-PIV方法对尾涡发展规律进行研究,发现受流体自由液面影响,尾涡轨迹向上倾斜,并与水平方向成10°,上、下尾涡运动轨迹不对称,下尾涡运动比上尾涡稍快,衰减亦较快,这与V3V实验结果一致;叶片后方60°尾涡依然清晰可见。用V3V和2D-PIV方法对桨叶附近湍流各向同性假设进行了分析,发现桨叶区和尾涡所在位置湍动能被各向同性假设近似法高估了25%~33%,桨叶区和尾涡所在位置趋向于各向异性。     

Liquid flow in impeller region of an unbaffled agitated vessel with an angularly oscillating impeller

The characteristics of a liquid flow were studied in the impeller region of an unbaffled agitated vessel with an angularly oscillating impeller whose unsteady rotation proceeds while periodically reversing its direction at a set angle. The measurement of the velocity of the liquid flow was performed by particle tracking velocimetry (PTV), abreast of that of the torque of the shaft to which the impeller was attached. When a disk turbine impeller with six flat blades was used with variations in operating conditions, such as the frequency and amplitude of impeller angular oscillation, a series of images obtained during one oscillation cycle were analyzed to characterize the internal and discharge streams inside and outside the impeller rotational region. Energy data were inferred on the basis of the circumferential and radial velocities of an internal flow. Results showed that although the total head provided to the liquid by the impeller blades is almost similar, independent of the amplitude of impeller angular oscillation, namely, the acceleration of its movement, the transformation of energy from the pressure head to the velocity head is more efficient at a larger amplitude. In addition, the discharge flow was characterized in terms of volumetric flow rates calculated from the radial and axial velocities. The operation at a smaller amplitude was shown to transform the flow more successfully from the radial direction to the upward and downward axial directions near the vessel wall.     

Hydrodynamic analysis of an axial impeller in a non-Newtonian fluid through particle image velocimetry

The rheology of non-Newtonian fluids in agitated vessels is complex making equipment sizing more an art than a science. To increase our understanding and to resolve the data gap for computational fluid dynamics validation, we present a detailed particle image velocimetry study of the hydrodynamics of Carbopol encountered in a 70 L mechanically mixed (A310) tank at three different rotational speeds (100, 250, 500 rpm). Bulk flow visualizations show that the flow field below the impeller is highly influenced by the rheological behavior of the fluid. Moreover, an analysis of the shear rate and viscosity revealed important spatial heterogeneities. An estimation of the Reynolds number classified the rotational speeds as the onset of the transitional regime (100), in the transitional regime (250) and turbulent conditions (500 rpm). This data set consists of local mean and fluctuating velocities at different locations below the impeller, which are available for validation and further study.     

Transitional flow in a Rushton turbine stirred tank

The way in which the single phase flow of Newtonian liquids in the vicinity of the impeller in a Rushton turbine stirred tank goes through a laminar‐turbulent transition has been studied in detail experimentally (with Particle Image Velocimetry) as well as computationally. For Reynolds numbers equal to or higher than 6000, the average velocities and velocity fluctuation levels scale well with the impeller tip speed, that is, show Reynolds independent behavior. Surprising flow structures were measured—and confirmed through independent experimental repetitions—at Reynolds numbers around 1300. Upon reducing the Reynolds number from values in the fully turbulent regime, the trailing vortex system behind the impeller blades weakens with the upper vortex weakening much stronger than the lower vortex. Simulations with a variety of methods (direct numerical simulations, transitional turbulence modeling) and software implementations (ANSYS‐Fluent commercial software, lattice‐Boltzmann in‐house software) have only partial success in representing the experimentally observed laminar‐turbulent transition. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3610–3623, 2017     

Effect of impeller clearance on liquid flow within an unbaffled vessel agitated with a forward–reverse rotating impeller

For an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller whose rotation proceeds with repeated acceleration, deceleration, and stop–reverse processes, liquid flow was studied through visualisation and measurement using particle tracking velocimetry (PTV). A disk turbine impeller with six flat blades was used with varied height settings. The impeller clearance and its forward–reverse rotation cycle characterised the impeller region flow: the radially outward flow in the deceleration process for the larger clearance relative to the vessel diameter of 1/3, and the axially downward flow in the acceleration process for the smaller clearance relative to the vessel diameter of 1/8. The flow patterns within the vessel resulting from the impeller's larger and smaller clearances were outlined, respectively, by double loops and a single loop of circulation, resembling the pattern produced by unidirectionally rotating turbine‐type impellers. The discharge flow was revealed to contain a comparable level of periodic circumferential velocity component, irrespective of the impeller clearance.     

Study of trailing vortices and impeller jet instabilities of a flat blade impeller in small-scale reactors

The design of economically feasible and profitable production processes has driven companies toward integrated continuous manufacturing by reducing working volumes and increasing operating frequencies. Thus, the development of robust small-scale devices capable of multivariate process optimization is essential. The aim of this work is to characterize the flow developed in a ml-scale stirred tank operating at intermediate 𝑅𝑒 ~3732, and assess trailing vortex stability with respect to baffle presence and size. Velocity characteristics are computed via computational fluid dynamics (CFD) and validated experimentally for an unbaffled (UB) and two baffled configurations. Proper orthogonal decomposition (POD) is used to extract dominant spatial–temporal flow features affecting the underlying flow patterns. Results show very good agreement between simulations and experiments, while POD analysis revealed the existence of highly energetic and periodic modes, linked to interactions between impeller jet and reactor walls. These modes are responsible for an impeller jet instability, which is amplified by the presence and size of baffles.     

翼型桨叶片尾涡结构的PIV实测与LES模拟研究

在塔径383 mm、高径比为1的翼型桨搅拌反应器内,采用粒子图像测速(PIV)技术和大涡模拟(LES)研究了一种翼型桨的叶片尾涡结构. 对比二维PIV技术测定的翼型桨搅拌反应器平均速度场数据,验证了LES方法的可靠性. 并通过叶片端的速度矢量场、叶片区内的涡量大小及湍动能分布清楚地识别了翼型桨叶片尾涡结构. 发现该叶片尾涡为单涡结构,其尾涡轴心轨迹几乎就在桨叶尖划过的圆柱面上. 此外,还探讨了叶片尾涡对湍动能分布、湍流特性等的影响,表明最大无因次湍动能k/Vtip2约为0.04~0.06,其位置约在0.5相似文献   

几种框式桨搅拌槽内流动特性的比较研究

用粒子成像测速（PIV）技术对传统框式桨、传统框式组合桨和新型框式组合桨的流动特性进行研究,对比了三种框式桨在相同工况下搅拌槽内的速度、流型和湍动能。结果表明：传统框式桨搅拌槽内流体流动以水平环流为主,在框式桨上方和框式桨中间区域流体流动不充分;传统框式组合桨搅拌槽内框式桨上方由于二折叶桨的作用使得框式桨上部流体流速变大,槽内流体上下部的流动得到加强,但在框式桨中心区域依旧存在流动死区;新型框式组合桨搅拌槽内两层桨叶间的连接流得到了加强,框式桨底部和中间区域物质和能量的交换更加充分。在考察的三种框式桨中,新型框式组合桨的混合效果更好。研究结果可为新型框式组合桨应用于化工合成工业中提供参考。