R134a闪蒸喷雾液滴动力学特征实验研究

闪蒸喷雾是典型的气液两相流动,在能源、化工、航天、医疗生物和食品生产等领域中应用非常广泛,对闪蒸喷雾形成的气液两相流中液滴粒径与速度的分布规律进行研究具有重要意义。本文搭建了闪蒸喷雾实验台,以R134a制冷剂为闪蒸喷雾工质,通过特定喷嘴形成闪蒸喷雾气液两相流。应用PDPA对闪蒸喷雾液滴直径和速度沿喷雾轴向方向和径向方向进行系统测量,拟合出了液滴轴向和径向无量纲速度沿径向无量纲距离变化的经验关联式。     

基于PIV图像处理法的管内低浓度液固两相流颗粒运动特性研究

为了研究管内低浓度液固两相流颗粒运动特性,提出一种基于PIV图像处理法的液固两相流颗粒速度场、涡量场及速度大小的分析方法。通过高速摄影仪获得不同工况下流场的运动图像,运用Canny算子边缘检测法分割图像提取粒子,由互相关函数获得粒子速度和方向,重建二维场。速度矢量场与颗粒运动轨迹相符,说明该方法可用于管内低浓度液固两相流动测量。研究发现:流量越大,流场越复杂,颗粒速度分布越混乱,涡量变化范围越大。流量、浓度和粒径对颗粒速度有显著影响。颗粒速度随浓度、粒径的增大而减小,而随流量增大正好相反。水平管中,颗粒粒径越小,颗粒速度增幅越大,最大约为2倍;流量越大,不同粒径组合下颗粒速度差越大,约达1.5倍。     

用于激光驱动飞片诊断的线成像速度干涉仪

设计并建立了一套完整的线成像激光干涉测速系统,用于激光驱动技术中小尺寸飞片或样品一条线上所有点的速度测量。它将激光压缩为线状照射到靶面,用成像物镜收集靶面的漫反射光并传递到广角迈克尔逊干涉腔中形成干涉,产生的梳妆干涉条纹作为信号载体,用变像管扫描相机记录条纹随时间的变化,用不同位置的条纹移动量反推出不同位置的速度分布,实现空间分辨。系统具有50 ps响应时间和20μm空间分辨能力。用该系统测量了激光驱动飞片的速度场,清晰的扫描干涉图像直观显示了飞片的运动过程和各点的速度差异。用傅里叶变换方法对干涉图像进行处理,得到了靶面一条线的速度和位移分布。     

小通道气液两相流电容测量方法研究

本文设计了一套用于小通道气液两相流的电容测量系统,并对内径为1.6 mm、2.5 mm和3.6 mm的玻璃管气液两相流进行了实验研究.文中首先对电容方法在小通道下的应用做了探索性尝试,设计了用于小通道气液两相流的电容传感器;然后利用所设计的电容传感器对小通道气液两相流弹状流电容动态数据进行采集、处理和分析;最后利用相关原理对气弹速度的测量进行了研究.研究结果表明所提出的电容方法可用于小通道气液两相流的分析研究中,是一种有效的小通道气液两相流参数检测手段,并为小通道两相流检测的研究提供了一定的借鉴.     

基于PSD原理的小管道两相流参数光学测量

针对小管道两相流参数测量问题,本文提出了一种基于PSD工作原理的光学测量新方法。该方法利用两相弯曲界面使光路发生偏转的特性,根据光电池输出信号判断相界面两侧的相分布情况和两相流的流动方向等信息。实验结果表明,该新方法可用于小管道气液两相流的研究,是一种有效的小管道两相流参数测量手段。     

Y型微通道气液两相流的数值模拟

针对Y型微通道气液两相流的数值模拟,建立了适用于微通道气液两相流的计算模型,采用CFD方法对微通道内流体的流动进行了数值研究,分析了微通道内流动状态、气泡形状以及生成周期,模拟了Y型微通道气液两相流弹状流的形成过程,并对弹状流的压力、速度、壁面剪切应力的分布、变化趋势及原因进行了深刻剖析,揭示了弹状流流动规律,为进一步加强弹状流应用打下基础,为微通道中的气液两相流动提供了可靠的理论依据。     

湍动雾化射流液雾粒径分布的数值模拟

对水在空气中湍动雾化射流的气液两相流场进行了数值模拟.其中,气体流场采用k-ε湍流模型进行模拟,给出了载气轴向速度的分布情况.对喷雾粒子的运动采用颗粒轨道法,建立粒子破碎和碰撞的数值模型,研究了液雾粒径在不同工况下沿轴向的变化趋势.数值模拟结果与实验结果在多种气液比下进行了比较,两者吻合较好.同时,分析了初始粒径和粒子总数及喷嘴尺寸对液雾粒径变化趋势的影响,讨论了有助于粒径均匀分布的条件.     

激光熔覆中金属粉末粒子与激光相互作用模型

为了对同轴激光熔覆过程中运动的金属粉末粒子的速度和温度进行理论分析,并研究各工艺参量的影响,建立了运动中金属粉末粒子的运动模型和热模型.模拟结果表明,粉嘴几何尺寸、粒子直径以及气/粉两相流初始速度是影响粒子运动行为的重要因素;粉嘴几何尺寸、激光焦点位置、激光发散角、激光功率、粒子直径以及气/粉两相流初始速度是影响粒子热行为的重要因素.在相同的工艺参量下(粉嘴出口内径r=2 mm,粉嘴倾角α=60°,初始气流速度v0=0.8 m/s),基于数字粒子图像测速(DPIV)技术,对316L不锈钢粉末粒子运动模型进行了实验验证.结果表明,运动理论模型是可靠的.该模型是掌握同轴激光熔覆过程中金属粉末粒子运动行为的有效工具;同时,热模型也是分析粉末粒子温度随不同参量变化的重要工具.     

方管内气液两相流界面波统计特性分析

以空气、水为工质，利用电导探针测量技术结合统计分析理论研究了水平矩形管内气液两相流界面波特性．首先对管内气液两相流的界面波进行了分类和定义；其次研究了气液两相流界面波在幅域、时域和频域的统计特性，同时考察了界面波特性参数（波速、频率和波高）等随气液两相流量的变化规律．     

中心体喷嘴自由水射流的波动与能量分布特性

在15 MPa射流压力条件下,对一中心体喷嘴进行自由水射流实验.用高速数码摄像技术捕捉喷嘴出口附近的瞬态流束界面波动特征;用相位多普勒粒子分析技术(PDPA)测量充分发展的射流场中的平均速度、均方根速度及液滴粒径分布.研究表明,中心体喷嘴射流流束的界面波动明显,远离喷嘴出口,流束界面波动频率降低,但波动幅值增大;射流轴...     

Particle Image Velocimetry Applied to a Spray Jet

Experiments are described in which particle image velocimetry (PIV) is applied to the measurement of liquid droplets in a spray jet. The two velocity components in planes formed by the light sheet originating from a double-pulsed ruby laser are determined. The PIV records are evaluated with the method of Young's fringes. It is shown that this procedure allows the simultaneous measurement of the droplet size within a certain size range.     

An image-shifting technique based on grey-scale classification for particle image velocimetry

The image-shifting techniques are used to overcome the directional ambiguity of particle image displacement in the measurement of particle image velocimetry (PIV). This paper proposes an image-shifting technique based on grey-scale classification for PIV. By calculating the unified grey-scale statistical frequency of each interrogated unit, the directional ambiguity is resolved without any special requirement of the camera, and the particle image displacement is calculated synchronously. This image-shifting technique can be realized by controlling the difference in the light intensity of two lasers. Using this new technique, a PIV system was developed and used to measure the diesel spray flow. The displacement vector map of fuel particle in the spray flow was obtained, and the structure of the spray flow was investigated. The application confirmed that the image-shifting technique is viable and effective.     

Simultaneous Measurement of Droplet Size and Three-Components of Velocity in Spray

The combination of the optical diagnostics and image processing technique has produced various methods of practical measurement in engineering. The measurement of size and velocity of droplets in fuel spray is one typical example. In general, the spray is a complicated and three-dimensional two-phase flow; therefore multi-dimensional field measurement of the spray field is indispensable. In the present paper, a technique of simultaneous measurement of droplet size and three-components of velocity is proposed and applied to spray issuing from the swirl nozzle as a practical application. The focused image of glare points is captured by a stereoscopic arrangement, and droplet size and the three-components of velocity are evaluated from the doublet image. Fundamental optical arrangements to capture the image of the glare points with a stereo view are established, and the feasibility and the accuracy of the technique are confirmed. © 2004 The Optical Society of Japan     

3D imaging of evaporating fuel droplets by stereoscopic PIV

A gun-type burner is a widely used oil burner for industrial and domestic applications. The oil is pressure-atomized and mixed with air generating a recirculating, swirling flow. Because of the surrounding flame, fuel droplets evaporate, being difficult to obtain information on droplets’ dynamics. Several laser techniques have been applied to this burner for spray diagnosis. PDA provides information about droplet size and velocity but can say little about the instantaneous spatial structures in the flow. Planar laser techniques as PIV can describe the 2D instantaneous spatial structures, but cannot provide information about the 3D structures in the flow. Then Stereoscopic PIV was applied. This technique allows us to measure the full 3D velocity vector map in a whole fluid plane. This paper has a double purpose. Firstly, to visualize the 3D structures which are present in the burner; secondly, to show that Stereoscopic PIV is an applicable technique for the diagnosis of an evaporating spray.     

Optical coherence tomography based particle image velocimetry (OCT-PIV) of polymer flows

The measurement of spatially resolved velocity distributions is crucial for modelling flow and for understanding properties of materials produced in extrusion processes. Traditional methods for flow visualization such as particle image velocimetry (PIV) rely on optically transparent media and cannot be applied to turbid polymer melts. Here we present optical coherence tomography as an imaging technique for PIV data processing that allows for measuring a sequence of time resolved images even in turbid media. Time-resolved OCT images of a glass-fibre polymer compound were acquired during an extrusion process in a slit die. The images are post-processed by ensemble cross-correlation to calculate spatially resolved velocity vector fields. The results compared well with velocity data obtained by Doppler-OCT. Overall, this new technique (OCT-PIV) represents an important extension of PIV to turbid materials by the use of OCT.     

Whole field spatial frequency analysis of double-or multiple-exposed PIV images

Speckle Velocimetry or high-image density PIV gives a velocity vector map of a twodimensional flow field by point-by-point spatial frequency analysis of local pattern at an interrogation spot in a double- or a multiple-exposed image of laser speckle or pseudo-speckle pattern generated by pulsed laser-light-sheet illuminations of a plane in the flow densely seeded with fine particles. The whole field spatial frequency analysis of the double- or multiple-exposed PIV image gives more useful information of the flow field. Optical Fourier transform is a conventional technique not only for the local spatial frequency analysis but for the whole field analysis. Filtering of spatial frequency is one of the typical techniques for the latter which can reconstruct a velocity contour or a component velocity contour map of the flow. Fundamentally this technique is a simple and efficient analogue method to get more information in the velocity field analysis of the flow than a digital image processing, but in practical applications the measurement is restricted to simple flows because of optical noise and low spatial resolution. In order to improve the technique, the fundamental characteristics of the filtering and the noise yielded in the filtering process must be investigated. Meanwhile, wavelet analysis can also be applied to the whole field spatial frequency analysis of PIV image. In this paper the filtering technique is examined by numerical convolution integral, and the results obtained are compared with ones obtained by the wavelet analysis.     

Visualization of blast waves created by exploding bridge wires

Blast waves created by small exploding bridge wires are used as a test bed for the development of a particle image velocimetry (PIV) technique that uses polymers, seeded with scattering particles, as dynamic witness plates. Combined with pulsed, incoherent schlieren photography, the PIV method permits visualization of the instantaneous velocity vector field in a plane cutting through the blast wave.     

Flow measurement in a transonic centrifugal impeller Using a PIV

Laser velocimetries, such as LDV or laser-2-focus (L2F) velocimetry, have been widely used for a flow measurement in a high-speed rotating impeller. A particle image velocimetry (PIV) is one of the popular velocity measurement techniques for the ability to measure a velocity field. And a PIV offers an extensive velocity field in an extremely shorter measurement time than the laser velocimetries. In the present experiment, a PIV was applied to a flow measurement in a transonic centrifugal impeller. A phase locked measurement technique every 20% blade pitch was performed to obtain a velocity field over one blade pitch of the inducer. The measured velocity field at the inducer of impeller clearly showed a shock wave generated on the suction surface of a blade. The validity of the present technique was also discussed.     

A digital image analysis technique for quantitative characterisation of high-speed sprays

A digital image analysis technique developed as a particle or droplet sizing tool and capable of measuring non-spherical objects has been examined in terms of its suitability for quantitative measurements in moderately dense sprays and in particular the potential capability for the characterisation of small diameter, high-speed two-phase flows by employing high-intensity pulsed lasers for illumination. In order to evaluate robustness of the image analysis technique (PDIA), measurement certainty and also to assess whether measurement performance is sensitive to the optical set-up, the technique was applied to data obtained from a hollow cone spray via two independent optical configurations which employed firstly a diode laser and secondly an Nd:YAG laser. The calibration response of the two optical set-ups revealed significant differences in terms of the depth-of-field characteristics and thus effective measurement volume dimensions. Despite these differences, a comparison of PDIA spray data revealed excellent agreement between the two datasets for measured diameters in the range 10–90 μm in the number distributions which not only confirmed robustness of the technique but also the potential of PDIA for the measurement of fast, small diameter objects. Subsequent comparisons of the PDIA data were made with PDA data obtained within the same spray in space and time and showed excellent agreement between the two techniques for droplets larger than approximately 25 μm in diameter. Discrepancies between PDIA and PDA were observed in the volume size distributions for the larger droplets measured whose diameters were greater than approximately 40 μm. This discrepancy is due to the ability of PDIA to measure the diameter of non-spherical droplets which were shown to exist in significant numbers at this measurement location within the spray. In contrast, the well-established technique PDA, which relies on the assumption of droplet sphericity clearly does not detect the presence of these larger deformed droplets.