Analysis of flow distribution from high-speed flow actuator using particle image velocimetry and digital speckle tomography

A variety of active flow control (AFC) methods are typically used in low-speed applications; however, the AFC techniques that are available for high-speed, supersonic applications are very limited. Under AFOSR (Air Force Research Laboratory) sponsorship, The Johns Hopkins University Applied Physics Laboratory (JHU/APL) is investigating a device that is intended for high-speed flow control; it is called the SparkJet actuator, which manipulates high-speed flows without active mechanical components. To date, actuator characterization has included computational and experimental techniques including parametric studies and flow visualization techniques to investigate the operation of the SparkJet device under various conditions. This paper focuses on the experimental flow measurement techniques that have been implemented. The results will be used for validating prospective computational studies that investigate the detailed characteristics of the SparkJet’s discharge and cooling stages after an energy deposition pulse. Current efforts include the use of high- resolution particle image velocimetry (PIV) to quantify the quiescent air operation of a single SparkJet pulse. However, the proper seeding of the SparkJet cavity continues to be challenging and has led to the use of digital speckle tomography (DST) to measure the temperature distribution in the core of the SparkJet plume. In this study, improved PIV techniques were used to acquire a higher-resolution image of the SparkJet-entrained flow. These PIV results show that the peak velocity in the entrained flow is around 53 m/s and the plume is sustained for 75–100 μs. Additionally, the DST data show a peak temperature of 1616.3 K at 75 μs and provide supporting information for interpreting the PIV data. These results are intended to calibrate and build confidence in a computational model.     

Particle image velocimetry investigations on multiphase flow in fluidized beds: A review

Fluidized Beds (FBs) are widely employed in the petroleum and coal energy sector because they offer excellent contact, both in terms of high surface area and long times. The last two decades has seen measurement on multiphase flows shift from conventional pressure sensors to direct flow image acquisition and processing. Particle Image Velocimetry or PIV, and PIV coupled with Digital Image Analysis or DIA, are used to directly and instantaneously acquire flow field data to make hidden flow patterns and flow structures discoverable. Research abounds on Gas-Solid FB hydrodynamics using PIV, but Liquid-Solid and Gas-Liquid-Solid systems are only slowly catching up. Similarly, the use of Geldart B and D particles for such studies is very common, whereas A and C type particle hydrodynamics is as yet largely unexplored by using imaging. Turbulence, high temperature, particle clusters, particle agglomeration and dense particle flows pose particular challenges to using PIV in FB. The two-zone FB and micro-FB warrant further attention. Small sized A & C type particles of rod-like, plate-like and angular shape provide huge scope for PIV investigations on FBs in the future. This review provides a concise account of several PIV studies on all types of FBs with focus on the past two decades, and also details the limitations of PIV measurements with future scope of work.     

颗粒团影响循环流化床内气固流场的PIV测试

利用自行构建的可用于循环流化床内流动特性测试的PIV系统对一台高4m截面200mm×200mm的冷态循环流化床进行了PIV测试研究。采用适合于较高粒子浓度的互相关处理MQD方法来处理PIV图像,获得了循环流化床内颗粒团运动对流场内颗粒速度的动态影响。结果表明:(1)颗粒团可能出现在流场截面内的各个位置,形状和速度都随时间的变化而变化;(2)颗粒团对床内的气固流动行为有较大的影响。只要有颗粒团出现,无论它的流动方向与主流颗粒的流动方向异同,气流和分散固粒都受到它的驱动向不同方向运动;(3)在颗粒团增长过程中,随着时间的增加,流场截面内颗粒平均速度虽然有波动,但总的变化趋势是增加的。     

Rushton桨搅拌槽内平均流场的二维PIV试验研究

用粒子图像测速技术(Particle image velocimetry,简称PIV)对带有Rushton桨叶的,无挡板搅拌槽内的流场进行研究。在叶轮转速240 r/min下,流动的雷诺数Re=1 527时,获取了桨叶处以及桨叶转过5个不同角度处的粒子图像。在对粒子图像进行处理和计算后,得到了相平均速度场和系综平均速度场,并给出了速度分布剖面图。结果表明,所研究的Rushton桨搅拌槽内径向喷射流动沿桨叶垂直方向是非对称的,而是向下方倾斜。在桨叶附近,径向流动速度高。随着流动远离桨叶,径向速度在降低。因此径向喷射流动作用就相当于自由射流:叶轮桨叶喷射出的流体进入周围大量低速运动的流体中,卷吸周围流体,并沿轴向和径向扩散,从而使更多的流体参与混合和反应。所作研究对于深入了解搅拌槽内流场结构具有实际意义。     

3D Particle image velocimetry test of inner flow in a double blade pump impeller

The double blade pump is widely used in sewage treatment industry,however,the research on the internal flow characteristics of the double blade pump with particle image velocimetry(PIV) technology is very little at present.To reveal inner flow characteristics in double blade pump impeller under off-design and design conditions,inner flows in a double blade pump impeller,whose specific speed is 111,are measured under the five off-design conditions and design condition by using 3D PIV test technology.In order to ensure the accuracy of the 3D PIV test,the external trigger synchronization system which makes use of fiber optic and equivalent calibration method are applied.The 3D PIV relative velocity synthesis procedure is compiled by using Visual C++ 2005.Then absolute velocity distribution and relative velocity distribution in the double blade pump impeller are obtained.Test results show that vortex exists in each condition,but the location,size and velocity of vortex core are different.Average absolute velocity value of impeller outlet increases at first,then decreases,and then increases again with increase of flow rate.Again average relative velocity values under 0.4,0.8,and 1.2 design condition are higher than that under 1.0 design condition,while under 0.6 and 1.4 design condition it is lower.Under low flow rate conditions,radial vectors of absolute velocities at impeller outlet and blade inlet near the pump shaft decrease with increase of flow rate,while that of relative velocities at the suction side near the pump shaft decreases.Radial vectors of absolute velocities and relative velocities change slightly under the two large flow rate conditions.The research results can be applied to instruct the hydraulic optimization design of double blade pumps.     

Particle image velocimetry in a centrifugal pump: Details of the fluid flow at different operation conditions

Centrifugal pumps are present in the daily life of human beings. They are essential to several industrial processes that transport single- and multi-phase flows with the presence of water, gases, and emulsions, for example. When pumping low-viscous liquids, the flow behavior in impellers and diffusers may affect the centrifugal pump performance. For these flows, complex structures promote instabilities and inefficiencies that may represent a waste of energetic and financial resources. In this context, this paper aims at characterizing single-phase water flows in one complete stage of a centrifugal pump to improve our understanding of the relationship between flow behavior and pump performance. For that, a transparent pump prototype was designed, manufactured and installed in a test facility, and experiments using particle image velocimetry (PIV) were conducted at different conditions. The acquired images were then processed to obtain instantaneous flow fields, from which the flow characteristics were determined. Our results indicate that the flow morphology depends on the rotational speed of the impeller and water flow rate: (i) the flow is uniform when the pump works at the best efficiency point (BEP), with streamlines aligned with the blades, and low vorticity and turbulence in the impeller; (ii) the velocity field becomes complex as the pump begins to operate at off-design conditions, away from BEP. In this case, velocity fluctuations and energy losses due to turbulence increase to higher numbers. Those results bring new insights into the problem, helping validate numerical simulations, propose mathematical models, and improve the design of new impellers.     

A study on the slip velocity on a pair of asymmetric electrodes for AC-electroosmosis in a microchannel

In numerical studies on microscale electroosmotic flows, the electric double layer (EDL) effect is usually predicted by solving the traditional Navier-Stokes equation subjected to the slip velocity induced by the electric-charged wall as a boundary condition. Recently, Suh and Kang (Physical Review E 77, 2008) presented the asymptotic solutions of the ion transport equations near a polarized electrode under the action of an AC field, and then proposed a new theoretical model of the slip velocity on the electrode considering the ion adsorption effect. In the present paper, we apply the model to a two-dimensional AC-electroosmotic flow in a microchannel to calculate the slip velocity on a pair of coplanar asymmetric electrodes embedded on the bottom wall, and then experimentally measure the slip velocity using the micro-PIV technique to validate the theoretical model. Comparison shows an excellent overall match between the theoretical and experimental results, except for on the narrow electrode at low frequencies. Next, we numerically perform parametric studies regarding the AC frequency, effective Stern-layer thickness and ion adsorption effect to further understand the characteristics of the AC electroosmotic flow. Results show that, as the frequency increases, the slip velocity also increases. In addition, the velocity decreases with increasing either Stern-layer thickness or ion adsorption effect. This paper was recommended for publication in revised form by Associate Editor Dongshin Shin Sangmo Kang received a B.S. and M.S. degrees from Seoul National University in 1985 and 1987, respectively, and then had worked for five years in Daewoo Heavy Industries as a field engineer. He also achieved a Ph.D. degree in the field of Mechanical Engineering from the University of Michigan in 1996. Dr. Kang is currently a Professor at the Division of Mechanical Engineering at Dong-A University in Busan, Korea. Dr. Kang’s research interests are in the area of micro- and nanofluidics and turbulent flow combined with the computational fluid dynamics.     

EXPERIMENTAL INVESTIGATION OF ROTATING STALL FOR A LOW-SPEED CENTRIFUGAL COMPRESSOR

Unsteady flows and rotating stall of a low-speed centrifugal compressor are investigated by measuring vaneless diffuser wall static pressure fluctuation and internal flow fields at different small flow fluxes. During the experiment, firstly the real time static pressure fluctuations on the vaneless diffuser shroud at different circumferential and radial position were acquired by high-frequency dynamic pressure transducers. Discrete Fourier transformation analysis and cross-correlation analysis were applied to the experimental results to ascertain the rotating stall beginning operation conditions and stall cells numbers and rotating speed. Secondly, the vaneless diffuser inlet flow angle distribution along diffuser width direction was acquired by single hotwire, which was compared with SENOO＇s analysis results. At last, the internal flow fields of the centrifugal compressor were investigated with a particle image velocimetry （PIV） system at different small flow fluxes. The flow field development of vaneless diffuser and blade flow passage are given at rotating stall conditions. The experiments enrich the understanding of rotating stall flow phenomenon of the low-speed centrifugal compressor and provide full experiment data for designing high performance centrifugal compressor.     

Finite Element Numerical Simulation and PIV Measurement of Flow Field inside Metering-in Spool Valve

The finite element method (FEM) and particle image velocimetry (PIV) technique are utilized to get the flow field along the inlet passage, the chamber, the metering port and the outlet passage of spool valve at three different valve openings. For FEM numerical simulation, the stream function ψ -vorticity ω forms of continuity and Navier-Stokes equations are employed and FEM is applied to discrete the equations. Homemade simulation codes are executed to compute the values of stream function and vorticity at each node in the flow domain, then according to the correlation between stream function and velocity components, the velocity vectors of the whole field are calculated. For PIV experiment, pulse Nd: YAG laser is exploited to generate laser beam, cylindrical and spherical lenses are combined each other to produce 1.0 mm thickness laser sheet to illuminate the object plane, Polystyrene spherical particle with diameter of 30-50 μm is seeded in the fluid as a tracing particles, Kodak ES1.0 CCD camera is employed to capture the images of interested, the images are processed with fast Fourier transform (FFT) cross-correlation algorithm and the processing results is displayed. Both results of numerical simulation and PIV experimental show that there are three main areas in the spool valve where vortex is formed.Numerical results also indicate that the valve opening have some effects on the flow structure of the valve. The investigation is helpful for qualitatively analyzing the energy loss, noise generating, steady state flow forces and even designing the geometry structure and flow passage.     

基于PIV测量的涡轮流量计响应分析

应用粒子成像测速技术获得了涡轮流量计叶片入口流场的速度分布信息,并基于该测量结果,运用T-G模型理论得出流量计的响应.通过与以往所采用的几种典型的入口速度分布计算得到的结果比较分析表明,基于PIV测量的结果更接近于涡轮流量计的真实响应.还比较分析了涡轮入口速度分布对涡轮流量计响应的影响机理,相关结果可望为改进涡轮流量计响应的计算分析方法以及优化设计提供有价值的参考.     

进口节流式滑阀内流场的有限元计算与PIV研究

用有限元法(FEM)和粒子图像测速技术(PIV)对三种不同开口度下进口节流滑阀沿进口流道、节流口、阀腔以及出口流道的流场进行了数值计算和试验可视化研究。数值模拟的数学模型采用的是连续性方程和Navier-Stokes方程的流函数—涡量式,有限元法用于方程的离散。自行编制的有限元计算程序用来计算求解区域离散点上的流函数及涡量值,再根据流函数及涡量与速度分量之间的关系求出各结点上的速度矢量。对于粒子图像测速试验,光源采用双脉冲Nd:YAG激光器,再用柱透镜和球面镜调制得到1.0mm片光照射流场。30～50μm的聚苯乙烯小球用做示踪粒子,Kodak ES1.0 CCD照相机拍摄流场图像,所得图像用FFT相关算法进行处理,结果用Tecplot输出,数值计算和PIV试验表明,滑阀内部有三个部位产生涡旋。数值计算又表明滑阀开度对阀内部流场结构有影响。研究对于定性分析阀内能量损失、噪声以及对阀的结构和流道的设计有重要实际意义。     

Particle Image Velocimetry Measurement of the Flow Field in the Play of the Drilling Pump Valve

The failure of a drilling pump is always due to the break of the drilling pump valve, which is one of the most important but also the weakest parts of the drilling pump. Over the decades, the degradation of drilling pump valves has been investigated extensively and various failure mechanisms have been proposed. However, no experimental test on the fluid has been successfully performed to support some of these mechanisms. In this paper, tests of the flow within the valve play are carried out to investigate the factors resulting in the failure of the valve. In the tests, particle image velocimetry(PIV) technology is employed to measure the flow field distribution of the valve play in the model. From these tests, the distributions of velocity and vorticity of fluid in various valves with different valve angles and different valve lifts are obtained, from which the features of flow fields are derived and generalized. Subsequently, a general rule of the influence of valve angles and valve lifts on the flow velocity is concluded according to chart analyses of maximal velocities and mean velocities. Finally, an analysis is made on the possibility of valve failure caused by erosion and abrasion in a working valve, with the application of the failure mechanisms of drilling pump valves. PIV measurement improves the study on the failure of the drilling pump valve, and the results show good agreement with previous computational fluid dynamics(CFD) simulations.     

3D and 2D experimental views on the flow field of gas-evolving electrode cold model for electrolysis magnesium

In the magnesium electrolysis process, chlorine gas bubbles release at the surfaces of anode and affect electrolyte flow patterns. This paper presents an experimental apparatus to simulate the flow field induced by chlorine gas evolution at the gas-evolving electrodes of magnesium electrolysis cell. The three-dimensional flow structures were determined by using volumetric three-component velocimetry (V3V) technique, which has the ability to capture the out-of-plane velocity component. The three-dimensional flow structures in the region with a depth about 120 mm can be obtained. To achieve this, approximately 15,000 three-dimensional velocity vectors were detected in the flow measurements and constituted the three-dimensional flow field, which eliminated the perspective error caused by the out-of-plane motion in Particle Image Velocimetry (PIV) method. In experiments, comparisons are made between the V3V and PIV results. The in-of-plane velocities data obtained by V3V technique have the same trend with the PIV results, and V3V provides more details in the third direction for the flow field accurately.     

X-ray visualization measurement of slurry flow in solid propellant casting

Simulated solid propellant slurry containing lead sphere tracers is experimentally cast into a double-circular cylindrical container. During the casting, the temperature and the pressure environment has been mimicked to an actual composite solid propellant casting of solid rocket motors. X-rays are projected on to the slurry flow from two directions and penetration images are recorded by a flat-panel detector and an X-ray image intensifier. By suppressing the mutual interference of the X-rays, the two-directional X-ray photography has been successfully conducted. Using the time series data of the X-ray images from the two directions, the identification of each tracer particle in space and time has been done and their three-dimensional paths have been computed. From these procedures, the flow field or the velocity field inside the slurry flow, which is usually invisible, has been estimated.     

EXPERIMENTAL INVESTIGATION OF TIP CLEARANCE FLOW FOR AN AXIAL FLOW FAN ROTOR

The flow field in the tip region of an axial ventilation fan is investigated with a particle image velocimeter (PIV) system at the design condition. Flow fields with three different tip clearances are surveyed on three different circumferential planes, respectively. The phase-locked average method is used to investigate the generation and the development of a tip leakage vortex. The result from PIV system is compared with that from a particle dynamics anemometer(PDA) system. Both data are in good agreement and the structure of the tip leakage vortex for the rotor is illustrated. The characteristic of a leakage vortex is described in both velocity vectors and vortical contours. The unsteadiness of the leakage vortex and the position of the vortex are surveyed in detail, which interprets the discrepancy between the numerical simulation and PDA experimental results to a certain extent. The center loci of tip leakage vortex at different times and the mean center loci of the leakage vortex are displayed particularly. Finally, the trajectories of the tip leakage vortex by the experimental measurement are compared with predictions from the existing models for high speed and high-pressure compressors and turbines when appropriately interpreted. A good agreement is obtained.     

双流道泵内非定常流动数值模拟及粒子图像测速测量

为探讨双流道泵内部的非定常流动机理,采用Fluent软件,基于滑移网格技术、 湍流模型计算了一双流道泵在不同工况下的内部流动,并将计算结果与粒子图像测速仪(Particle image velocimeter, PIV)实测结果进行比较。结果表明：计算所得双流道泵内部流场符合叶轮机械内部流动的一般规律,且与PIV实测结果总体变化趋势一致;由于双流道泵结构特殊,其进口处的流动状态与普通叶轮相差较大,出口处的流动状态与普通叶轮类似;叶轮进口处,流体基本沿流道吸力面流动,流道工作面上的相对速度很小,存在严重的脱流和旋涡;叶轮出口处,压力面和吸力面的速度趋于相等,射流—尾迹现象并不明显;由于叶轮—蜗壳动静干涉,两个叶轮流道内的静压分布有所不同;同一流道内,静压随着半径的增加而逐步增大,压力面侧静压大于吸力面侧;蜗壳流道内静压随半径增大,最大静压值在隔舌处。此项研究不仅加深了人们对双流道泵内非定常流动图画的理解,从而进一步完善双流道泵设计方法,同时也可为其他类型泵的内流研究提供借鉴。     

PIV-based estimation of viscosity and pressure fields for a steady pseudoplastic flow

Targeting a pseudoplastic fluid, we propose a method to simultaneously estimate viscosity and pressure fields from the velocity field data of particle image velocimetry (PIV). As a two-dimensional case study, we chose a steady two-dimensional wake structure behind a circular cylinder where local viscosity and pressure are dynamically coupled. The method involves PIV, momentum conservation equation of non-Newtonian fluids and rheological constitutive equations. The CMC (Carboxy Methyl Cellulose) aqueous solution of weight concentration of 0.1% is used as a test case of pseudoplastic fluid. Viscosity distribution is obtained through power law model and Carreau-Yasuda model as the constitutive equation. Pressure distribution is then calculated by substituting the viscosity into the momentum conservation equation. Applied results show pressure-lowering at the vortex cores and viscosity-lowering at their perimeters, stabilizing vortex attachment to the cylinder in the pseudoplastic fluid. We also analyze error propagation characteristics to conclude the feasibility of the present method and highlight the difference in error propagation characteristics during pressure estimation between Newtonian and pseudoplastic fluid flows.     

液浮转子式陀螺的间隙流场

研究了液浮转子式陀螺仪转子在高速转动时其间隙流场的流动状态.采用计算流体力学中的雷诺应力模型,对陀螺仪转子与定子间的封闭流场进行数值模拟,并运用粒子成像测试技术(Micro PIV)观测运动流场,测量液浮陀螺的间隙流场速度.模拟结果表明,转子上下表面与侧壁面均形成了不同类型的二次流动,该现象随转速及空间尺寸的增大而增强;当转速增大到7500 r/min时,二次流速可达到0.3 m/s,故减小空间尺寸有利于提高转子的运动稳定性.另外,文中给出了运用粒子成像测试技术(Micro PIV)得到的流场矢量分布与流线分布图.实验证实,Micro PIV测试技术可实现对转子转速在0～1000 r/min内的流场速度的精确观测,满足限域空间内对高速运动流场的非接触测量要求.