气液两相流中气泡运动速度场的PIV分析与研究

粒子图像测速技术（PIV）作为一种无扰、瞬态、全场速度测量方法，已被广泛应用于液体或气体的单相流流速场测定。对于两相流PIV技术，目前还处于起步阶段，本文应用PIV技术的基本原理，对静止液体中的气泡运动速度进行了分析，并对有关气液两相流测量问题进行了探讨。     

风沙两相流PIV测量算法研究

在风沙两相流图像特征的基础上，提出了一种基于模式识别动态聚类方法中$K$-均值 算法的数字面具(Digital Mask)自动生成算法来求解风沙两相流动. 简化了传统生成Digital Mask过程中手动设置参数的操作，减少了人为误差，为批量处理风沙两相流PIV图像提供 了一种安全快捷的方法. 并将该算法应用于风沙两相流的PIV实际测量，分别得到了不同流 动状态下的气流、沙粒以及风沙气固两相流的速度场.     

两相流显微PIV/PTV系统的开发

开发了一个能同时测量两相流中两相速度和细颗粒尺寸分布的显微PIV/PTV系统,其硬件系统包括大功率连续激光器、显微镜、高速摄像机;软件系统由改进的球形颗粒图像识别算法、各种图像处理算法和各种先进的PIV/PTV算法组成。其中改进的圆弧识别算法能够进行更准确地进行曲线分割而能对充满噪音并相互重叠的颗粒图像给出较好的识别结果。应用该PIV系统,可以在微秒和微米数量级上捕获细颗粒/气泡图像,并能较准确地同时得到两相速度、颗粒尺寸和浓度分布。对焚香可吸入颗粒物进行了速度和尺寸的同时测量,得到了较满意的结果。     

激光多普勒测速技术在气液两相流中的应用

本文采用激光颗粒动态分析仪(PDA)测量了钢包底部喷吹气液两相流中气泡的直径和气泡上升速度的分布;采用激光多普勒测速仪(LDA)测量了气液两相区和液体单相循环区液体速度场的分布。测量结果表明:气泡在脱离喷嘴上浮一定距离后,其大小基本保持不变;在气液两相区中,气泡速度和液体速度的分布均服从高斯分布;液体在单相区作循环流动,在侧壁与底部交接处,存在液体流动的“死区”。     

采用PTV技术研究循环流化床内气固两相流动

采用PTV技术对循环流化床顶部颗粒稀疏流动区域进行了测量，其中采用先进的高速摄像技术获取流动的连续图像，并采用目前有望在气固两相流动测量中发挥较大作用的四种PTV算法：BICC法、VGT法、SPRING法和4-FRAME法，对所获取的图像进行颗粒配对处理，从而得到流场中运动颗粒的速度信息。所得到的结论为：本文所采用的PTV算法在图像处理中都产生少量的伪矢量，通过采取简单的伪矢量识别算法就可以剔除大部分伪矢量；本文实验条件下，测得循环流化床顶部区域内颗粒运动速度差别较小。本文工作为进一步详细实验测量研究奠定了理论与技术基础。     

两相流PIV粒子图像处理方法的研究

本文在单相PIV技术的基础上研究了两相流动PIV图像处理方法，采用摸板匹配法和灰度加权标定法对两相粒子进行了识别、区分和标定，采用灰度互相关法对区分后的单相粒子图像进行了处理，应用基于以上方法编制的Windows应用软件，首先对由美国Minnesota大学复杂流动实验室提供的两相流动粒子图片进行了处理，通过对比分析可见，应用本文所采用的方法能对两相粒子进行有效的识别和区分，然后以搅拌槽内液固两相流场为例对此方法进行了应用。     

T型微通道反应器内气液两相流动机制及影响因素

基于液滴或气泡的多相微流控是近年来微流控技术中快速发展的重要分支之一.本文利用高速显微摄影技术和数字图像处理技术对T型微通道反应器内气液两相流动机制及影响因素进行实验研究.实验采用添加表面活性剂的海藻酸钠水溶液作为液相,空气作为气相.研究T型微通道反应器内气液两相流型的转变过程,并根据微通道内气泡的生成频率和生成气泡的长径比对气泡流进行分类.研究发现当前的进料方式下,可以观测到气泡流和分层流2种流型,且依据气泡生成频率和微通道内气泡的长径比可将气泡流划分为分散气泡流、短弹状气泡流和长弹状气泡流3种类型,并基于受力分析确定3种气泡流的形成机制分别为剪切机制、剪切–挤压机制和挤压机制.考察不同液相黏度和表面张力系数对不同类型气泡流范围的影响规律.结果表明:液相黏度相较于表面张力系数而言,对气泡流生成范围影响更大.给出不同类型气泡流流型转变条件的无量纲关系式,实现微通道生成微气泡过程的可控操作.     

T型微通道反应器内气液两相流动机制及影响因素

基于液滴或气泡的多相微流控是近年来微流控技术中快速发展的重要分支之一.本文利用高速显微摄影技术和数字图像处理技术对T型微通道反应器内气液两相流动机制及影响因素进行实验研究.实验采用添加表面活性剂的海藻酸钠水溶液作为液相,空气作为气相.研究T型微通道反应器内气液两相流型的转变过程,并根据微通道内气泡的生成频率和生成气泡的长径比对气泡流进行分类.研究发现当前的进料方式下,可以观测到气泡流和分层流2种流型,且依据气泡生成频率和微通道内气泡的长径比可将气泡流划分为分散气泡流、短弹状气泡流和长弹状气泡流3种类型,并基于受力分析确定3种气泡流的形成机制分别为剪切机制、剪切-挤压机制和挤压机制.考察不同液相黏度和表面张力系数对不同类型气泡流范围的影响规律.结果表明:液相黏度相较于表面张力系数而言,对气泡流生成范围影响更大.给出不同类型气泡流流型转变条件的无量纲关系式,实现微通道生成微气泡过程的可控操作.      

基于SPH法的返回舱入水载荷模拟分析

返回舱水上着落前期面临较大砰击,为研究过程中受到的砰击载荷,基于光滑粒子流体动力学(SPH)气-液两相流算法,首先通过模拟平板和楔形体两种算例模型的入水过程,并与相关文献的试验结果进行对比,验证算法的有效性。在此基础上,对返回舱的入水过程进行模拟。结果显示,两种算例模型的计算结果与相关文献试验结果吻合良好。返回舱入水速度和倾角对砰击有较大影响并且过程中存在二次砰击现象。砰击载荷随入水速度增大而增加。第一次砰击载荷峰值随倾角增大而减小,第二次砰击载荷峰值随倾角增大先增后减。结果表明,SPH气-液两相流算法能够较好地模拟返回舱入水过程。     

风沙两相流测量技术研究进展

围绕风沙两相流的测量, 归纳了过去几十年来在风沙动力学研究中所使用的风速测量技术和输沙率测量装置.着重讨论了高频测量在目前风沙动力学研究中的必要性, 分析了传统风速和输沙率测量装置的优缺点.对新一代光学测量技术------PIV在风沙两相流测量中的应用进行了较为详细的探讨.指出PIV测速技术在风沙两相流研究中具有广泛的应用前景, 使用PIV测速技术可以得到风沙流结构、两相速度场等宏观信息, 同时也可以进行单个颗粒运动状态的研究.      

Simultaneous velocity field measurements in two-phase flows for turbulent mixing of sprays by means of two-phase PIV

This paper describes a novel derivative of the PIV method for measuring the velocity fields of droplets and gas phases simultaneously using fluorescence images rather than Mie scattering images. Two-phase PIV allows the simultaneous and independent velocity field measurement of the liquid phase droplets and ambient gas in the case of two-phase flow mixing. For phase discrimination, each phase is labelled by a different fluorescent dye: the gas phase is seeded with small liquid droplets, tagged by an efficient fluorescent dye while the droplets of the liquid phases are tagged by a different fluorescent dye. For each phase, the wavelength shift of fluorescence is used to separate fluorescence from Mie scattering and to distinguish between the fluorescence of each phase. With the use of two cross-correlation PIV cameras and adequate optical filters, we obtain two double frame images, one for each phase. Thus standard PIV or PTV algorithms are used to obtain the simultaneous and independent velocity fields of the two phases. Because the two-phase PIV technique relies on the ability to produce two simultaneous and independent images of the two phases, the choice of the labelling dyes and of the associated optical filter sets is relevant for the image acquisition. Thus a spectroscopic study has been carried out to choose the optimal fluorescent dyes and the associated optical filters. The method has been evaluated in a simple two-phase flow: droplets of 30–40 μm diameter, produced by an ultrasonic nozzle are injected into a gas coflow seeded with small particles. Some initial results have been obtained which demonstrate the potential of the method.     

Flow structure of microbubble-laden turbulent channel flow measured by PIV combined with the shadow image technique

The turbulence structure of a horizontal channel flow with microbubbles is experimentally investigated using combined particle image velocimetry (PIV) in order to clarify the mechanism of drag reduction caused by microbubbles. A new system which simultaneously measures the liquid phase and the dispersed bubbles is proposed, based on a combination of particle tracking velocimetry (PTV), laser-induced fluorescence (LIF) and the shadow image technique (SIT). To accurately obtain the velocity of the liquid phase, tracer particles which overlap with the bubble shadow images are almost entirely eliminated in the post-processing. Finally, the turbulence characteristics of the flow field are presented, including measurements for both phases, and the bubble effect on the turbulence is quantified.     

Visualisation of air–water bubbly column flow using array Ultrasonic Velocity Profiler

In the present work, an experimental study of bubbly two-phase flow in a rectangular bubble column was performed using two ultrasonic array sensors, which can measure the instantaneous velocity of gas bubbles on multiple measurement lines. After the sound pressure distribution of sensors had been evaluated with a needle hydrophone technique, the array sensors were applied to two-phase bubble column. To assess the accuracy of the measurement system with array sensors for one and two-dimensional velocity, a simultaneous measurement was performed with an optical measurement technique called particle image velocimetry(PIV). Experimental results showed that accuracy of the measurement system with array sensors is under 10% for one-dimensional velocity profile measurement compared with PIV technique. The accuracy of the system was estimated to be under 20% along the mean flow direction in the case of two-dimensional vector mapping.     

Full-field bubbly flow velocity measurements using a multiframe particle tracking technique

The study is an examination of two-phase dispersed air bubble flow about a cylindrical conductor emitting a constant heat flux. The technique of Particle Image Velocimetry is utilized in order to obtain a full-field non-invasive measurement of the resulting bubbly flow velocity field. The employed approach utilizes a flow visualization technique in which the instantaneous velocity profile of a given flow field is determined by digitally recording particle or bubble images within the flow over multiple successive video frames and then conducting a completely computational analysis of the data. The use of particle tracking algorithms which perform a point-by-point matching of seed images from one frame to the next allows construction of particle or bubble pathlines and instantaneous velocity field. Results were initially obtained for a synthetically created flow field and a single phase liquid convective field seeded with flow-following tracer particles. The method was additionally extended to measurements within a gas/liquid system in which bubble rise velocities over a substantial two-dimensional flow area were determined in order to demonstrate the effectiveness of the developed digital data acquisition and analysis methodology.A version of this paper was presented at the 12th Symposium on Turbulence, University of Missouri-Rolla, 24–26 September, 1990     

Local gas and liquid phase velocity measurement in a miniature stirred vessel using PIV combined with a new image processing algorithm

A method which combines standard two-dimensional particle image velocimetry (PIV) with a new image processing algorithm has been developed to measure the average local gas bubble velocities, as well as the local velocities of the liquid phase, within small stirred vessel reactors. The technique was applied to measurements in a gas–liquid high throughput experimentation (HTE) vessel of 45 mm diameter, but it is equally suited to measurements in larger scale reactors. For the measurement of liquid velocities, 3 μm latex seeding particles were used. For gas velocity measurements, a separate experiment was conducted which involved doping the liquid phase with fluorescent Rhodamine dye to allow the gas–liquid interfaces to be identified. The analysis of raw PIV images enabled the detection of bubbles within the laser plane, their differentiation from obscuring bubbles in front of the laser plane, and their use in lieu of tracer particles for gas velocity analysis using cross-correlation methods. The accuracy of the technique was verified by measuring the velocity of a bubble rising in a vertical glass column. The new method enabled detailed velocity fields of both phases to be obtained in an air–water system. The overall flow patterns obtained showed a good qualitative agreement with previous work in large scale vessels. The downward liquid velocities above the impeller were greatly reduced by the addition of the gas, and significant differences between the flow patterns of the two-phases were observed.     

Probing the velocity fields of gas and liquid phase simultaneously in a two-phase flow

The feasibility of simultaneous measurements of the instantaneous velocity fields of gaseous and liquid phase is demonstrated in a laminar, unsteady two-phase flow. Thus, the instantaneous relative velocity field can be measured in such media. This is achieved by combining Particle Image Velocimetry (PIV) and a gas-phase velocimetry technique, which is based on laser-induced fluorescence (LIF) from a gaseous tracer. The wavelength shift of LIF is exploited to separate it from Mie scattering from the liquid phase. The new technique and the PIV measurement system work independently in this approach. Thus, the measurement accuracy and precision of the new technique can be validated by comparing it to the PIV results in regions of the flow field where the relative velocity vanishes. Received: 18 October 1998/Accepted: 16 October 1999     

Particle tracking velocimetry of the flow field around a collapsing cavitation bubble

The velocity field in the vicinity of a laser-generated cavitation bubble in water is investigated by means of particle tracking velocimetry (PTV). Two situations are explored: a bubble collapsing spherically and a bubble collapsing aspherically near a rigid wall. In the first case, the accuracy of the PTV method is assessed by comparing the experimental data with the flow field around the bubble as obtained from numerical simulations of the radial bubble dynamics. The numerical results are matched to the experimental radius–time curve extracted from high-speed photographs by tuning the model parameters. Trajectories of tracer particles are calculated and used to model the experimental process of the PTV measurement. For the second case of a bubble collapsing near a rigid wall, both the bubble shape and the velocity distribution in the fluid around the bubble are measured for different standoff parameters γ at several instants in time. The results for γ > 1 are compared with the corresponding results of a boundary-integral simulation. For both cases, good agreement between simulation and experiment is found.     

Two-color particle image velocimetry technique for an internal combustion engine

A two-color particle image velocimetry (PIV) technique has been applied to a single-cylinder motored research engine. Two-color PIV is a quantitative planar velocity measurement technique that can unambiguously determine the velocity magnitude and direction.

The work includes the development of an interrogation system, a series of computer simulations to determine the performance of the technique under various conditions, the comparison of these results to similar ones obtained for an autocorrelation PIV system, and a test of the technique by reconstructing the velocity field of a uniform jet flow.

The technique was then applied to the in-cylinder flow field of a motored single-cylinder, cup-in-head, research engine. A total of 27 instantaneous velocity fields were obtained at a single measurement plane for a single operating condition of the engine. The data were analyzed to yield ensemble-averaged velocity and velocity fluctuation.     

Laminar separation on a forward facing step

The separation ahead of a forward facing step was investigated under laminar flow conditions using the hydrogen bubble technique to visualise and PTV to evaluate the 3-D velocity field in an Eulerian representation in the vicinity of the step. Short-time averaged velocity fields allowed the construction of streamlines showing that the separation is topologically of an open bubble type with a span-wise quasi periodicity. The entrained fluid is continuously released by the front vortex which breaks out of the separation bubble in longitudinal streaks. The topology of this dynamic behaviour is described and confirmed by streamlines and vorticity lines.