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.     

搅拌槽内的Rushton桨叶周围流场的PIV测量

In this paper, particle image velocimetry (PIV) was used to measure the mean and root mean square(RMS) velocity in the stirred tank with six-flat blade Rushton turbine and with no baffles. Two types of motion patterns were studied. One was that the impeller runs at constant speed, the other was that the impeller runs at time-dependent speed and in a periodic way. The emphasis of the paper was on the comparison of mean and RMS velocity vector maps and profiles between these two types of motion patterns, and especial attention was paid to the comparison of the mean velocity, time-averaged RMS velocity, phase averaged RMS velocity between the constant 3 RPS (revolution per second) and time-dependent operation. The Reynolds number was between 763 and i527. The study explained the mechanism that time-dependent RPS is more efficient for mixing than that of constant RPS.     

Flow Field Around Rushton Turbine in Stirred Tank by Particle Image Velocimetry Measurement

In this paper, particle image velocimetry (PIV) was used to measure the mean and root mean square(RMS) velocity in the stirred tank with six-flat blade Rushton turbine and with no baffles. Two types of motion patterns were studied. One was that the impeller runs at constant speed, the other was that the impeller runs at time-dependent speed and in a periodic way. The emphasis of the paper was on the comparison of mean and RMS velocity vector maps and profiles between these two types of motion patterns, and especial attention was paid to the comparison of the mean velocity, time-averaged RMS velocity, phase averaged RMS velocity between the constant 3 RPS (revolution per second) and time-dependent operation. The Reynolds number was between 763 and 1527. The study explained the mechanism that time-dependent RPS is more efficient for mixing than that of constant RPS.     

T型撞击流混合器内流动特性的PIV研究

采用粒子图像测速技术对入射管直径为3 mm、混合腔直径为16 mm的T型撞击流混合器内的流动特性进行了研究,考察了不同流速比和撞击轴线上方空间条件下混合腔内的速度和湍流动能分布. 结果表明,在相同入射管直径和流速下,撞击驻点位于混合腔中心处,无因次化的速度和湍流动能分布趋势基本一致. 高湍流动能区主要集中在撞击点附近区域,其无因次化数值是传统Rushton涡轮搅拌槽叶端处的3倍. 流速比对撞击驻点位置影响显著;减小撞击轴线上方空间可增加高湍流动能分布区域,利于物料混合.     

Particle Image Velocimetry of Human Cough

Cough generated infectious aerosols are of interest while developing strategies for the mitigation of disease risks ranging from the common cold to SARS. In this work, the velocity field of human cough was measured using particle image velocimetry (PIV). The project subjects (total 29) coughed into an enclosure seeded with stage fog. Cough flow velocity profiles, average widths of the cough jet, and maximum cough velocities were measured. Maximum cough velocities ranged from 1.5 m/s to 28.8 m/s. The average width of all coughs ranged between 35 to 45 mm. Wide variability in the data suggests that future cough simulations consider a range of conditions.     

Flow Field Measurement inside the Mouthpiece of the Spiros Inhaler Using Particle Image Velocimetry

Flow field distributions inside the mouthpiece of a Spiros inhaler, a dry powder inhaler delivering therapeutic powders to airways, were investigated by means of the particle image velocimetry technique. The aim of the study was to obtain flow patterns inside the mouthpiece at various flow rates for characterizing dynamics related to drug dispersion. Particle image velocimetry results revealed a strong turbulent flow structure in the mouthpiece at all the flow rates tested, ranging from 15 lpm to 35 lpm. Strong counterclockwise rotating flow, as viewed from downstream, prevailed due to upstream tangential air injection introduced into the mouthpiece by 4 holes equally spaced along the perimeter. The air jets were designed to introduce high kinetic energy along the wall of the mouthpiece in an attempt to prevent particle deposition onto the wall, thus reducing drug losses, by creating a circulating sheath of air along the surface. Experimental results indicated that excess core flow mixing prior to the full formation of the sheath of air caused particle collisions with the mouthpiece wall as a result of the strong tangential air injection.     

PIV测量用模型泵的水力设计及性能分析

为了研究离心泵内部流动情况,参考IS80-50-315型离心泵,设计了一台PIV测量用有机玻璃模型泵,并对该泵的可拍摄性和水力性能进行了分析。介绍了模型泵、片光和CCD相机的布置及其流场拍摄方法。结果表明,该泵适用于PIV拍摄,且水力性能满足实验要求。     

圆柱绕流的数值模拟与PIV测试研究

利用计算流体力学软件Fluent对均匀来流绕固定圆柱的流动情况进行了数值模拟,考察和研究了气流绕过圆柱时的边界层的形成以及旋涡的形成及脱落过程,利用粒子图像测试(PIV)技术对绕流情况进行了实验研究.通过数值模拟结果与PIV实验结果的定性和定量分析比较,证明Fluent软件的模拟结果具有相当的准确性,能够准确的预测流体的流动特性.     

非稳态载粒多相流曳力研究进展

介绍不稳定曳力的理论研究现状,并形成了初步的理论体系。实验研究方面,利用激波管和高速摄像法进行了曳力系数可视化研究,但仍需要更全面、更精确的数据补充;在实验研究的基础上又进行了模拟研究,有助于揭示非稳态多相流的瞬时微观特性,但由于模型本身的局限性,仍无法准确描述非稳态流场和颗粒的复杂耦合作用。最后指出DNS模型修正和可视化实验技术将是后续研究需要突破的方向。     

Studying Fluid Characteristics Atop Surface Patterned Membranes via Particle Image Velocimetry

Surface patterning is a recent promising approach to promote performance of pressure-driven membranes in water treatment and desalination. Nevertheless, knowledge about foulant deposition mechanisms, especially at early stage of filtration, is still lacking. The applicability of particle imaging velocimetry to study fluid characteristics atop surface patterned thin-film composite membranes was investigated at different operating conditions. This work is an important first step toward reliable understanding of the impacts of topographical membrane surface modification on hydrodynamic conditions and foulant deposition mechanisms.     

Particle Image Velocimetry of a Swirling Fluidized Bed at Different Blade Angles

The particle motion within a stable swirling operation regime of a fluidized bed is investigated. Particle image velocimetry was used to elaborate the hydrodynamics of multi‐sized particles fluidized at different air distributor blade angles. The experimental results revealed that the superficial velocity is the most prominent aspect affecting the hydrodynamics of a swirling fluidized bed (SFB) followed by the bed weight, particle shape, and blade inclination angle. Particles of different sizes and shapes fluidized well in the SFB which emphasizes its superiority over contemporary techniques. The slug‐wavy regime in the SFB is promising and has considerable potential, especially for diffusion‐controlled reactions and processes in the industry. The particle velocity increased with air flow rate at shallow bed height but decreased with bed weight.     

Multiphase Particle in Cell Simulations of Fluidized Beds: Studies on Bubble Rise Velocity and Minimum Fluidization Velocity

Multiphase particle in a cell simulation of fluidized beds for different Geldart group particles (A, B and D) were investigated. The minimum fluidization velocities predicted by the Barracuda Virtual Reactor® were compared and contrasted by theoretical predictions with the Wen-Yu model. The optimal drag force coefficients based on the Wen-Yu model for MP-PIC simulations was further evaluated for different particle sizes and densities for Group A catalysts. A neural network model was constructed for the optimal model constants as a function of the explanatory variables for group A catalysts. This work illustrates the importance of choosing the correct value for the drag force coefficients to obtain realistic simulation results for fluidized beds. This work clearly demonstrates that the Barracuda simulation results are very sensitive to the value of the Wen-Yu parameter. Using multiphase particle in cell simulations in conjunction with a neural network model can be directly used in further studies as a rapid tool to get the correct values for realistic drag force coefficients based on the Wen-Yu model.     

Influence of multiple gas inlet jets on fluidized bed hydrodynamics using Particle Image Velocimetry and Digital Image Analysis

A rectangular fluidized bed setup was developed to study the evolution of inlet gas jets located at the distributor. Experiments were conducted with varying distributor types and bed media to understand the motion of particles and jets in the grid-zone region of a fluidized bed. Particle Image Velocimetry and Digital Image Analysis were used to quantify the parameters that characterize these jets. A grid-zone phenomenological model was developed to compare these parameters with those available in the literature. It was determined from this study that jet penetration length behavior is consistently different for fluidization velocities below and above the minimum fluidization. For velocities above minimum fluidization, jet lengths were found to increase more rapidly with increase in orifice velocity than for operating conditions below minimum fluidization.     

Particle Holdup Profiles in Horizontal Gas‐liquid‐solid Multiphase Flow Pipeline

Sand holdup is one of the most important hydrodynamic parameters that is needed for performance estimation, design, operation and control of oil‐gas‐sand multiphase production and pipeline transportation systems. The performance of oil‐gas‐sand multiphase flow can be reliably evaluated by measuring the sand holdup in such oil‐gas‐sand multiphase production and pipeline transportation systems. In the present work, a local sand holdup has been measured under conditions analogous to the horizontal oil‐gas‐sand three‐phase slug flow in pipelines. Accurate local sand particle holdup measurements were performed by the digital imaging technique. The results revealed the influence of operating conditions such as gas and liquid velocities and sand particle loading on the distribution of the local sand particle holdup in the horizontal air‐water‐sand multiphase slug flow pipe. Explanations for the observed trends are provided, shedding light on the general structures and mechanisms of the distribution of the local sand holdup in a horizontal oil‐gas‐sand three‐phase slug flow. Such information on the horizontal air‐water‐sand three‐phase slug flow mechanisms are essential to advance the mechanistic approach for predicting local sand holdup distribution and the subsequent effect on sand deposition during multiphase petroleum production and transfer operations.     

Local Velocity and Concentration of the Single Components in Water/Oil Mixtures Monitored by Means of MRI Flow Experiments in Steady Tube Flow

In this paper the spatially resolved determination of velocities in two‐phase systems consisting of water and oil by means of nuclear magnetic resonance (NMR) imaging (MRI) techniques is described and applied to steady tube flows with regard to the total flow rate. As MRI offers the possibility to study the flow of multiphase materials spatially resolved with various forms of contrast, even optically opaque water/oil mixtures can be studied in the interior of the material. Besides snapshot images of the actual flow pattern also local velocity and concentration fields of both phases can be obtained separately. The insight into the inner micro flow processes and microstructure allows to characterize fluid mixtures or emulsions. Flow rate and preparation/mixing method were varied in order to realize changes of the flow pattern and the structure of the mixture during flow. Physical models of the flow behavior and physical stability of these complex systems can be based upon this information.     

湍动流化床过渡段中颗粒速度分布的光纤测量与模拟

流化床中颗粒速度的测量一直是个技术上的难点。今针对光纤测量信号提出一种基于互相关原理的时均速度计算方法。考虑到数据分段计算时其可靠性方面的差异,将互相关系数作为第二权重因子引入时均速度计算公式。利用PV6D光纤探针在200 mm直径流化床中测得的原始信号,对比了三种数据处理方法所得的颗粒时均速度,并分别计算其床层截面颗粒净流率,说明提出的方法可靠性更高。据此测量了流化床过渡段不同高度处颗粒时均速度的径向分布,并采用修正的三段曳力模型来描述具有团聚行为的颗粒曳力,进行计算流体力学模拟。测量与模拟结果均表明,从过渡段到稀相段,床中心区的颗粒速度先降低后升高,其径向分布也从陡峭变平缓,然后再次趋于陡峭。颗粒速度分布的上述规律主要由固含率与气速的径向分布共同作用所致。     

光信号互相关法测量循环流化床内颗粒速度的应用研究

光信号互相关方法被广泛地应用于流化床中固体颗粒速度测量.基于互相关原理,本文讨论了各种因素对计算结果的影响,详细地叙述了光纤信号的处理过程.计算窗口重叠法得到的颗粒速度能更详尽地反映固体颗粒的动态变化,而采用的中位数法确定的时均速度具有较强的数值稳定性.计算结果能够较好地反映循环流化床颗粒速度的径向分布特点,为循环流化床颗粒速度的测量提供可靠的数据.