CW laser-induced fluorescence of toluene for time-resolved imaging of gaseous flows

A laser-induced fluorescence diagnostic is presented for high-speed measurements in gaseous flows. The technique employs a toluene tracer excited at 266 nm by a cavity-doubled 532 nm diode-pumped 5.5 W CW laser. The high power (600 mW) of UV light produced by cavity doubling, together with the high fluorescence yield of toluene, yields strong signal levels needed for high-speed recording. Fluctuation detection limits for tracer mole fraction were investigated by applying the diagnostic to an atmospheric temperature and pressure nitrogen jet. For single-point measurements with a photomultiplier tube, the detection limit for fluctuations in the toluene mole fraction was 0.028%, achieved with 430 mW of laser power and 8.5 kHz bandwidth for a 1×0.4×0.4 mm collection volume. Line (1-D) imaging with a kinetic-readout camera (512 pixels/row) achieved a detection limit of 0.23% with 440 mW of laser power, 9.7 kHz frame rate, and 0.3×0.2×0.4 mm collection volume per pixel, while planar (2-D) imaging with a 512×512 pixel intensified camera achieved a detection limit of 0.88% with 205 mW of laser power, 100 μs exposure time, and 0.4×0.4×0.4 mm volume per pixel. Line and planar imaging were applied to a turbulent jet with Re of about 10000.     

Entrained droplets in underexpanded gas jet in water

Abstract  

A visualization study was performed to investigate the flow of an underexpanded nitrogen gas jet injected into water. The stagnation pressure was varied in the range 0.5–8.0 MPa. The gas jet length and expansion angle were obtained from time-averaged images captured using a high-speed camera. The gas jet length and expansion angle increased approximately linearly with increasing stagnation pressure. The entrainment velocity and the velocity of entrained water droplets in the gas jet were obtained by particle image velocimetry.     

Multi-kHz mixture fraction imaging in turbulent jets using planar Rayleigh scattering

In this study, we describe the development of two-dimensional, high repetition-rate (10-kHz) Rayleigh scattering imaging as applied to turbulent flows. In particular, we report what we believe to be the first sets of high-speed 2D Rayleigh scattering images in turbulent non-reacting jets, yielding temporally correlated image sequences of the instantaneous mixture fraction field. Results are presented for turbulent jets of propane issuing into a low-speed co-flow of air at jet-exit Reynolds numbers of 10,000, 15,000, and 30,000 at various axial positions downstream of the jet exit. The quantitative high-speed mixture fraction measurements are facilitated by the use of a calibrated, un-intensified, high-resolution CMOS camera in conjunction with a unique high-energy, high-repetition rate pulse-burst laser system (PBLS) at Ohio State, which yields output energies of ∼200 mJ/pulse at 532 nm with 100-μs laser pulse spacing. The quality, accuracy, and resolution of the imaging system and the resulting image sets are assessed by (1) comparing the mean mixture fraction results to known scaling laws for turbulent jets, (2) comparing instantaneous images/mixture fraction profiles acquired simultaneously with the high-speed CMOS camera and a well-characterized, high-quantum efficiency CCD camera, and (3) comparing statistical quantities such as the probability density function of the mixture fraction results using the high-speed CMOS camera and the CCD camera. Results indicate accurate mixture fraction measurements and a high potential for accurately measuring mixture fraction gradients in both time and space.     

Interaction of an Optical Discharge with a Shock Wave

The results of an experimental study of the impact of the focused pulsed-periodic radiation from a CO₂ laser on a gas-dynamic structure in a supersonic jet are presented. The radiation of the CO₂ laser is propagated across the stream and focused by a lens on the axis of the supersonic jet. To register the flow structure, a shadow scheme with a slit and a flat knife located along the flow is used. The image is fixed by a speed camera with an exposure time of 1.5 μs and a frame rate of 1000 1/s. In the flow, the plasma initiated by the pulsedperiodic laser is visualized in order to identify and determine the period of plasma development, as well as the motion of the initial front of the shock wave. It is shown that at the transverse input of laser radiation into the stream the periodic structure of the thermal trace is created with the formation of an unsteady shock wave from the energy release zone. At small repetition rates of laser radiation pulses, the thermal spot interacts with the flow in the pulsed mode. It is shown that elliptic nonstationary shock waves are formed only at low subsonic flow velocities and in a stationary atmosphere. The process of nonstationary ignition by an optical discharge of a methane–air mixture during a subsonic outflow into a motionless atmosphere is shown experimentally. The results of optical visualization indicate burning in the trace behind the optical discharge region.     

Observation of Electrospinning Behavior of Nanoscale Fibers by a High-Speed Camera

Electrospinning is regarded as an effective method to generate nanoscale fibers. However, we believe that the mechanism of the formation of nanoscale fibers by Electrospinning has not yet been fully investigated. In this study, the jet behavior under various conditions was observed with a high-speed camera with short exposure periods. The behaviors of stable and unstable jets were compared. The effect of bending instability on the deposition pattern of jets was examined at different voltages and concentrations. In addition, the diameters of nanoscale fibers were analyzed as a function of sampling position.     

Nanosecond resolution photography system for laser-induced cavitation based on PIV dual-head laser and industrial camera

The detailed study of the initial and collapse processes of the laser-induced cavitation requires nanosecond resolution (both nanoseconds exposure and nanoseconds interframe time) of the photography measurement system. The high-speed video cameras are difficult to achieve nanoseconds interval time. The framing and streak cameras are able to reach the nanosecond resolution, but their complex technology and expensive prices make them far from being commercially available. The present study builds a nanosecond resolution photography system based on PIV dual-head laser and conventional industrial camera. The exposure time of the photography system is controlled by the laser pulse width, which is 5 ns. The two heads of the PIV laser are operated independently thus the smallest time interval between two laser pulses can be set to less than 10 ns. A double-pulse per-exposure imaging technique is used to record the information from two laser pulses on single frame on a low-speed industrial camera. The nanosecond resolution photography system was applied to the laser-induced cavitation experiments to verify the reliability of the measurement results. The measurement of the shock wave velocity demonstrates the ability of the system to capture ultrafast phenomena, which reduces from 3611 m/s to approximately 1483 m/s within 400 ns. The experimental results also reveal the asymmetric evolution of laser-induced cavitation bubbles. The major axis of the ellipsoidal bubble has twice reversals along the laser propagation and perpendicular direction from the laser-induced breakdown to the first collapse.     

Sustained simultaneous high-speed imaging of scalar and velocity fields using a single laser

A high-speed technique that combines planar laser induced fluorescence (PLIF) detection of biacetyl and particle image velocimetry (PIV) for simultaneous imaging of scalar and velocity fields is demonstrated at a frame rate of 12 kHz for up to 32500 consecutive frames. A single diode-pumped, frequency-tripled Nd-YAG laser was used for excitation. Wavelength-separated recording was achieved for Mie scattering from silicone oil droplets with a CMOS camera and for the red-shifted fluorescence from biacetyl with an image-intensified CMOS camera. Interference between PIV and PLIF tracers was found to be negligible. Cross-talk between PIV and PLIF signals was low and a strategy to completely eliminate it was devised and is discussed. The signal-to-noise ratio is about 9 for single-shot scalar images. Example image sequences were recorded in an atmospheric pressure air jet at Re=2000. PACS 42.62.Fi; 33.50.Dq; 06.30.Gv; 06.60.Jn     

Comparative visualized investigation of impact-driven high-speed liquid jets injected in submerged water and in ambient air

This paper is a comparative study on the characteristics of high-speed liquid jets injected in surrounding water and air using shadowgraph technique. One of the main objectives is to investigate the effects of liquid’s physical properties, used to generate the high-speed liquid jets, on jet generation’s characteristics. Moreover, comparative investigations on effects of those liquid jets after injected in water and air are reported. The high-speed liquid jets were generated by the impact of a projectile launched by a horizontal single-stage power gun. The impact-driven high-speed liquid jets were visualized by shadowgraph technique and images were recorded by a high-speed digital video camera. The process of impact-driven high-speed liquid jet injection in air and water, oblique shock waves, jet-induced shock waves, shock waves propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud regeneration were clearly observed. It was found that different properties of liquid (surface tension and kinematic viscosity) affect the jet maximum velocity and shape of the jet. Bubble behaviors were only found for the jet injected in water. From the shadowgraph images, it is found that the maximum average jet velocity, expansion and contraction velocities of bubble in axial direction increase when the value of the multiplied result of surface tension by kinematic viscosity increases. Therefore, surface tension and kinematic viscosity are the significant physical properties that affect characteristics of high-speed liquid jets.     

Evolution of gas-puff Z-pinch during implosion and pinch phase

The four-frame high-speed electrooptical camera backed by an automatic image capturing and processing system has been used to investigate the evolution of the gas-puff Z-pinch. The sequence of four images of the pinch continuum radiation (the exposure of each frame was 1 ns, the time separation of frames was 10 or 20 ns) enabled the study of the pinch-forming phase (zipper effect, compression), the stagnation of plasma on the axis, as well as the pinch decay (disintegration, collapsing of the plasma column) and the determination of the velocities of individual processes     

超声速气流中液体横向射流的非定常特性与振荡边界模型

针对液体圆柱射流垂直喷入超声速横向气流中的非定常分布特性开展实验研究, 并建立穿透深度方向上的射流振荡分布模型. 利用脉冲激光背景成像方法“冻结”拍摄马赫2.1(Ma=2.1)气流中煤油射流/喷雾瞬态图像, 结合最大类间方差法(Otsu)和Canny算法提取瞬态图像特征, 基于统计方法并引入间歇因子(γ)定量描述射流振荡分布特性; 通过研究多参数协同作用下的射流振荡分布规律, 提出振荡分布数学模型, 研究的参数变量包括超声速来流总压(642-1010 kPa)、 液体喷注压降(0.36-4.61 MPa)、液体喷嘴流道直径 (0.48 mm/1.0 mm/1.25 mm/1.52 mm)、距离喷嘴的流向距离(10-125 mm)以及液气动量通量比(0.11-7.49). 研究中利用射流振荡分布模型成功预测出水射流在Ma=2.1气流中的的振荡分布, 预测分布与实验结果符合良好.     

激光聚变极端环境下X光高速摄影技术研究进展

在激光惯性约束聚变（ICF）研究中，通过X光高速摄影获取的图像数据能够反映等离子体中由于做功和能量输运导致的流体状态的时空演化信息，与之相关的诊断技术与工程研究一直以来都是ICF诊断能力建设的重要组成部分。介绍了作为我国ICF工程的主要实施单位之一，中国工程物理研究院激光聚变研究中心近年来在X光高速摄影技术研究方面取得重要进展，包括:（1）面向神光系列激光装置，开发了系列工程化的100 ps曝光高速摄影相机，整体达到国际先进水平，并在高灵敏探测、透射式带通滤波和结构小型化等方面形成中国特色；（2）提出微扫描门控、同视扫描分幅等10 ps曝光X光高速摄影新技术，为突破时间分辨瓶颈做出有益尝试；（3）在国内率先开展抗辐射加固高速摄影相机理论设计、技术验证与工程设计；（4）针对激光聚变靶碎片对设备安全的威胁，在国内首次开展靶碎片的理论建模与仿真研究，并开展首次验证实验，取得重要进展。     

过冷沸腾气泡行为的实验研究

本文采用拍摄速度为10000帧/秒的高速摄影仪对不锈钢箔表面的过冷沸腾现象进行了可视化实验研究。实验结果与用微液层模型理论预测的结果一致。高过冷度区域的沸腾换热机理主要是由气泡生长、消失过程中温度边界层的强制排除(所谓强制对流)引起的。气泡周期主要由等待时间构成,这在过冷度高的情况下尤为显著。对等热流密度换热面,微液层模型预测的气泡周期与实验值比较吻合。     

高压热气流与整装式液体工质相互作用的实验研究

为了探索整装式液体发射药燃烧稳定性的控制方法,本文设计了多级渐扩型圆柱观察室,采用高速录像系统,开展了含能气体射流在液体模拟工质中扩展形态的实验研究.实验结果表明,渐扩型结构尺寸、喷气压力、喷孔直径对泰勒空腔的扩展形态以及气液间的湍流掺混强度有显著影响,通过这些参数的合理匹配,可以在一定程度上实现对射流发展过程的控制.     

双盘靶X光辐射光谱时空特性

在“星光－Ⅱ”激光装置上，利用三倍频激光辐照金双盘靶，研究Ｘ光辐射光谱时空特性。将４针孔透射光栅阵列和皮秒分幅相机结合，实现了Ｘ光辐射时间、空间、能谱三维联合测量，得到了双盘靶初级和次级辐射光谱结构，观测到Ｘ光辐射弛豫过程，并得到Ｎ带Ｏ带和零级光谱强度随时间和空间的变化特征、等离子体喷射二维空间分布、等离子体膨胀速率等重要结果。     

神龙一号参数测量用高速两分幅相机

采用长焦距镜头的后工作空间全口径分光原理,利用门控型像增强器、CCD相机、基于大规模可编程集成电路的高速快门控制触发系统等部件,研制了具有较高时间分辨能力和高灵敏度的两分幅高速相机,并在此基础上建立了束参数的高速测量系统。两分幅相机的最高快门速度约3 ns,幅间间隔时间则具有以0.5 ns的步进进行调节的能力;快门时间及幅间间隔时间可以分别独立调节,最大可到1 s;同时具有较好的线性度和空间响应的均匀性,等效背景噪声低到约5 electronspixel-1s-1,并且分幅相机灵敏度调节范围大。该系统一次可以拍摄两幅图像,图像阵列可达到1 0241 024,满足神龙一号的各种测量要求。     

High-speed non-intrusive measurements of fuel velocity fields at high-pressure injectors

Using a single high-speed camera and a frequency modulated laser, a novel approach is presented for fast velocity field measurements in unsteady spray flows. The velocity range is from zero up to several 100 m/s, which requires a high measurement rate and a large dynamic. Typically, flow measurements require to seed tracer particles to the fluid. A paradigm shift to seeding-free measurements is presented. The light scattered at the phase boundaries of the fluid droplets is evaluated. In order to validate the high-speed measurement system, a detailed uncertainty analysis is performed by means of measurements as well as simulations. Thereby, variations of the scattered light intensity, which are based on the high temporal velocity gradients, are found to be the main contribution to the uncertainty. The eventually measurement results, obtained at a measurement rate of 500 kHz, exhibit spray velocities ranging from 0 m/s up to 400 m/s in less than 1 ms, and the detection of unsteady and irregular flow phenomena with a characteristic time of several μs is achieved. This demonstrates the high measurement rate, the high temporal resolution and the large measurement range of the proposed high-speed measurement system.     

A high-speed image sensing technique with adjustable frame rate based on an ordinary CCD

In the sinusoidal phase modulating interferometer technique, the high-speed CCD is necessary to detect the interference signals. The reason of ordinary CCD's low frame rate was analyzed, and a novel high-speed image sensing technique with adjustable frame rate based on an ordinary CCD was proposed. And the principle of the image sensor was analyzed. When the maximum frequency and channel bandwidth were constant, a custom high-speed sensor was designed by using the ordinary CCD under the control of the special driving circuit. The frame rate of the ordinary CCD has been enhanced by controlling the number of pixels of every frame; therefore, the ordinary of CCD can be used as the high frame rate image sensor with small amount of pixels. The multi-output high-speed image sensor has the deficiencies of low accuracy, and high cost, while the high-speed image senor with small number of pixels by using this technique can overcome theses faults. The light intensity varying with time was measured by using the image sensor. The frame rate was up to 1600 frame per second (f/s), and the size of every frame and the frame rate were adjustable. The correlation coefficient between the measurement result and the standard values were higher than 0.98026, and the relative error was lower than 0.53%. The experimental results show that this sensor is fit to the measurements of sinusoidal phase modulating interferometer technique.     

Picosecond X-ray plasma radiation measurements

Direct picosecond measurements of X-ray laser plasma radiation were performed with a high-speed X-ray image-converter camera (ICC). This camera operates in the single-frame mode with an exposure time ranging from 5 ns to 0.5 μs and in the streak mode with 5 × 10⁹ to 5 × 10⁷ cm/s streak velocities. Its temporal resolution in the streak mode was calculated to be about 7 ps. A plasma was created by focussing 10 ps 1 to 2 joule laser pulses onto a titanium target placed in a vacuum chamber. The halfwidth of the recorded X-ray pulses varied from 30 to 60 ps.     

Detailed measurements of transient two-stage ignition and combustion processes in high-pressure spray flames using simultaneous high-speed formaldehyde PLIF and schlieren imaging

This study investigates the low- and high-temperature ignition and combustion processes in a high-pressure spray flame of n-dodecane using simultaneous 50-kHz formaldehyde (HCHO) planar laser-induced fluorescence (PLIF) and 100-kHz schlieren imaging. The PLIF measurements were facilitated through the use of a pulse-burst-mode Nd:YAG laser, producing a 355-nm pulse-train with 300 pulses at 70 mJ/pulse, separated by 20-µs, in a 6-ms burst. The high-speed HCHO PLIF signal was imaged using a non-intensified CMOS camera with dynamic background emission correction. The acquisition rate of this HCHO PLIF diagnostic is unique to the research community, and when combined with high-speed schlieren imaging, provides unprecedented opportunity for analysis of the spatiotemporal evolution of fuel jet penetration and low- and high-temperature ignition processes relevant to internal combustion engine conditions. The present experiments are conducted in the Sandia constant-volume preburn vessel equipped with a new Spray A injector. The influences of ambient conditions are examined on the ignition delay times of the two-stage ignition events, HCHO structures, and lift-off length values. Consistent with past studies of traditional Spray A flames, the formation of HCHO is first observed in the jet peripheries where the equivalence ratio (Φ) is expected to be leaner and hotter and then grows in size and in intensity downstream into the jet core where Φ is expected to be richer and colder. The measurements demonstrate that the formation and propagation of HCHO from the leaner to richer region leads to high-temperature ignition events, supporting the identification of a phenomenon coined “cool-flame wave propagation” during the transient ignition process. Subsequent high-temperature ignition is found to consume the previously formed HCHO in the jet head, while the formation of HCHO persists in the fuel-rich zone near the flame base over the entire combustion period.     

层/湍流等离子体射流的稳定性与三维特性的实验观测研究

采用普通照相和短时间曝光成像的ICCD照相技术,观测了低于大气压条件下产生的纯氩和氩-氢直流电弧等离子体射流的高温区的瞬时形貌及其变化,结合电弧弧根在阳极表面贴附行为的观测结果,对射流的稳定性与三维特性和弧根行为之间的关联进行了分析.结果表明,层流等离子体射流的高温区长度明显长于湍流射流情形,并且具有很好的轴对称性和时间稳定性;湍流射流的高温区瞬时形貌则表现出明显的三维特征;等离子体射流的三维特性与弧根在阳极表面的贴附行为没有直接的联系.