Ultrahigh-frame-rate nitric oxide planar laser-induced fluorescence imaging

We demonstrate the ability to generate ultrahigh frequency burst sequences of deep UV at 226 nm by mixing the optical parametric oscillator signal output at 622 nm with third harmonic at 355 nm from a pulse burst laser system. We obtained 226 nm burst sequences with uniform burst envelopes, and the average pulse energy is approximately 0.5 mJ. Nitric oxide planar laser-induced fluorescence image sequences at ultrahigh (100 kHz) frame rates have been obtained.     

Simultaneous CH planar laser-induced fluorescence and particle imaging velocimetry in turbulent nonpremixed flames

jet =18600). Here, PLIF images reveal a CH layer of thickness typically <1 mm from flame base to tip. Furthermore, in these permanently blue flames, we observe instantaneous flamefront strain rates – derived from the PIV data – in excess of ±10⁴ s^-1 without flame extinction. Received: 16 October 1997/Revised version: 30 October 1997     

High-speed CH planar laser-induced fluorescence imaging using a multimode-pumped optical parametric oscillator

We report on high-speed CH planar laser-induced fluorescence (PLIF) imaging in turbulent diffusion flames using a multimode-pumped optical parametric oscillator (OPO). The OPO is pumped by the third-harmonic output of a multimode Nd:YAG cluster for direct signal excitation in the A-X (0,0) band of the CH radical. The lasing threshold, conversion efficiency, and linewidth are shown to depend on the number of pump passes in the ring cavity of the OPO. Single-shot CH PLIF images are acquired at 10 kHz with excitation energy up to 6 mJ/pulse at 431.1 nm. Signal-to-noise ratios of ~25-35 are the highest yet reported for high-speed CH PLIF.     

Volume-resolved flame chemiluminescence and laser-induced fluorescence imaging

There is significant need for optical diagnostic techniques to measure instantaneous volumetric vector and scalar distributions in fluid flows and combustion processes. This is especially true for investigations where only limited optical access is available, such as in internal combustion engines, furnaces, flow reactors, etc. While techniques such as tomographic PIV for velocity measurement have emerged and reached a good level of maturity, instantaneous 3D measurements of scalar quantities are not available at the same level. Recently, developments in light field technology have progressed to a degree where implementation into scientific 3D imaging becomes feasible. Others have already demonstrated the utility of light field technology toward imaging high-contrast particles for PIV and for imaging flames when treated as single-surface objects. Here, the applicability and shortcomings of current commercially available light field technology toward volumetric imaging of translucent scalar distributions and flames are investigated. Results are presented from imaging canonical chemiluminescent and laser-induced fluorescent systems. While the current light field technology is able to qualitatively determine the position of surfaces by locating high-contrast features, the correlation-based reconstruction algorithm is unable to fully reconstruct the imaged objects for quantitative diagnostics. Current analysis algorithms are based on high-contrast correlation schemes, and new tools, possibly based on tomographic concepts, will have to be implemented to reconstruct the full 3D structure of translucent objects for quantitative analysis.     

平面激光诱导荧光实验中激励激光的光束整形

分析了激励激光光强分布对平面激光诱导荧光（PLIF）实验中荧光强度的影响。基于柱面微透镜列阵设计了一套激光片状光束匀滑整形系统,并根据PLIF实验的具体要求,通过光线追迹方法优化了系统参数。建立了片状光束整形实验系统,对染料激光进行了匀滑整形,获得了不均匀性〈4%的均匀片状光束,满足了PLIF实验所需。在此基础上建立了PLIF实验系统,获得了酒精灯火焰和CH4/air预混火焰中OH的二维荧光分布。     

Precise CCD image analysis for planar laser-induced fluorescence experiments

The goal of this paper is to describe essential criteria for image analysis of planar laser-induced fluorescence (PLIF) signals. A new image file conversion method is introduced and some typical illustrative examples showing the potential applications of the method in PLIF experiments are presented. Analysis of row, column, and total pixel counts, and the dark noise related to an image are discussed. Image segmentation, averaging and background correction can be easily done by the reported method. In our analysis it is straightforward to inspect the pixel counts and check for saturation of the camera sensors. It is also feasible to obtain a particular row or column for interpretation and it offers an easy way to check the validity of the captured images. Furthermore, the method offers a sensitive technique to check pulse-to-pulse variation of the excitation laser by using frame-to-frame fluorescence image data comparison, which is more illustrative than power checking by other means. The overall results show that the developed conversion method reported here can be effectively used to obtain more in-depth and quantitative information out of the raw data for the PLIF experiments.     

Quantitative dual-tracer planar laser-induced fluorescence measurements of molecular mixing

Simultaneous, planar laser-induced fluorescence (LIF) images of nitric oxide (NO) and acetone have been used to calculate instantaneous quantitative maps of molecularly mixed jet-fluid fraction in an axisymmetric shear layer. In this experiment, NO is seeded into high-purity nitrogen jet fluid and acetone is seeded into air coflow. On mixing at the molecular level, the NO LIF is strongly quenched by oxygen from the coflow, while the acetone signal is unaffected by the mixing process. The extent to which the jet fluid is mixed at the molecular level is determined on a pixel-by-pixel basis from the simultaneous NO and acetone planar LIF images. Jets at Reynolds numbers ranging from 1000 to 50 000 are investigated.     

Oxygen-distribution imaging with a novel two-tracer laser-induced fluorescence technique

We introduce a new technique for imaging oxygen concentrations in fuel/air mixtures that takes advantage of the different responses of toluene and 3-pentanone to collisional quenching by molecular oxygen. Since laser-induced fluorescence signals from both tracers upon excitation at 248 nm are spectrally well separated, simultaneous detection is possible. The technique is first applied to instantaneous imaging in turbulent mixing processes of interacting seeded air and nitrogen flows. Received: 1 August 2001 / Revised version: 29 October 2001 / Published online: 29 November 2001     

Single-shot laser-induced fluorescence imaging of formaldehyde with XeF excimer excitation

Single-shot formaldehyde laser-induced fluorescence (LIF) imaging measurements in a technical scale turbulent flame have been obtained using XeF excimer laser excitation in the ?¹A₂-˜X¹A₁ transition at 353.2 nm. Measurements have been carried out in a 150 kW natural gas swirl burner where formaldehyde distribution fields have the potential, in combination with OH concentration fields, to visualize the heat release distribution and therefore give an optimal visualization of flame-front positions. The extended areas where formaldehyde was detected in the swirl flame indicates the presence of low temperature chemistry in preheated gas pockets before ignition. Received: 31 January 2000 / Revised version: 2 March 2000 / Published online: 5 April 2000     

Planar laser-induced fluorescence imaging of the spatial vapor distribution around a monodisperse acetone droplet stream exposed to asymmetric radiant heating

The development of liquid fuelled microcombustors faces many challenges, one of which being high asymmetric heat flux across the combustion chamber. Typically, thin walls provide little resistance to convective heat transfer and therefore, allowing high heat loss rates. Insulating the walls results in high wall temperatures, which increases the likelihood that radiation plays an important role. Both of these effects have the potential to induce asymmetries and strong temperature gradients in the gas flow, relative to the more uniform environment of a conventional combustor. This investigation uses planar laser-induced fluorescence (PLIF) to reveal the spatial vapor distribution around a monodisperse acetone droplet stream that is exposed to asymmetric radiant heating. Droplets with diameters from 117 to 222 μm flow past a single-sided array of radiant heating elements to provide the asymmetric heating. A frequency-quadrupled Nd:YAG laser provides a 266 nm light sheet to excite the acetone vapor around the droplets, which are exposed to different experimental conditions by varying parameters such as the droplet diameter and temperature of the radiating elements for a fixed exposure time of the droplets in the heated region. A CCD camera captures the fluorescence of the excited acetone vapor molecules over a broadband wavelength range between 350 and 550 nm, to give the radial and axial vapor concentration around each droplet. After processing the PLIF images, we obtain contour plots of the spatial acetone vapor concentration around the droplets which depict asymmetric vapor distribution. The potential impacts on vaporization, combustion and pollutant formation are discussed.     

Cinematographic imaging of hydroxyl radicals in turbulent flames by planar laser-induced fluorescence up to 5 kHz repetition rate

Temporally resolved measurements of transient phenomena in turbulent flames, such as extinction, ignition or flashback, require cinematographic sampling of two-dimensional scalar fields. Hereby, repetition rates must exceed typical flame-inherent frequencies. The high sensitivity planar laser-induced fluorescence (PLIF) has already proved to be a practical method for scalar imaging. The present study demonstrates the feasibility of generating tuneable narrowband radiation in the ultraviolet (UV) spectral range at repetition rates up to 5 kHz. Pulse energies were sufficiently high to electronically excite hydroxyl radicals (OH) produced in a partially-premixed turbulent opposed jet (TOJ) flame. Red-shifted fluorescence was detected two-dimensionally by means of an image-intensified CMOS camera. Sequences comprising up to 4000 frames per run were recorded. Besides statistically stationary conditions, extinction of a turbulent flame due to small Damköhler numbers is presented showing the potential of the technique.     

Delayed observation of laser-induced fluorescence for imaging of tumors

We report on a technique to improve fluorescence images of superficially growing tumors marked with photosensitizers. Exploiting the longer fluorescence decay times of porphyrin-based photosensitizers compared to average decay times of tissue autofluorescence, delayed detection of laser-induced fluorescence allows to suppress the autofluorescence background. The feasibility of delayed fluorescence imaging of tumors has been demonstrated in-vitro. It follows from an analysis of delayed fluorescence spectra that autofluorescence background falling into the photosensitizer fluorescence band can be reduced by up to one order of magnitude.     

Laser-induced fluorescence imaging of acetone inside evaporating and burning fuel droplets

Laser-induced fluorescence was used to visualize acetone fields inside individual droplets of pure acetone as well as droplets composed of methanol or 1-propanol initially mixed with acetone. Droplets were supported on a horizontal wire and two vaporization conditions were investigated: (1) slow evaporation in room air and (2) droplet combustion, which leads to substantially faster droplet surface regression rates. Acetone was preferentially gasified, causing its concentration in droplets to drop in time with resultant decreases in acetone fluorescence intensities. Slowly vaporizing droplets did not exhibit large spatial variations of fluorescence within droplets, indicating that these droplets were relatively well mixed. Ignition of droplets led to significant variations in fluorescence intensities within droplets, indicating that these droplets were not well mixed. Ignited droplets composed of mixtures of 1-propanol and acetone showed large time-varying changes in shapes for higher acetone concentrations, suggesting that bubble formation was occurring in these droplets.     

Comparison of laser-induced and planar laser-induced fluorescence measurements of nitric oxide in a high-pressure, swirl-stabilized, spray flame

We report spatially resolved linear laser-induced fluorescence (LIF) and planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) in a pre-heated, high-pressure (4.27 atm), lean direct-injection (LDI) spray flame. The feasibility of using PLIF in lieu of LIF is assessed with respect to measuring NO concentrations in high-pressure LDI spray flames. NO is excited via the resonant Q₂(26.5) transition of the γ(0,0) band while a non-resonant wavelength is employed to subtract background interferences. LIF detection is performed in a 2-nm region centered on the γ(0,1) band. PLIF detection is performed in a 68-nm window that captures fluorescence from several vibrational bands. An in situ NO doping scheme for fluorescence calibration is successfully employed to quantify the LIF signals. However, a similar calibration scheme for the reduction of PLIF images to quantitative field measurements is plagued by the laser-excited background. Excitation scans and calibration comparisons have been performed to assess the background contribution for PLIF detection. Quantitative radial NO profiles measured by LIF are presented and analyzed so as to correct the PLIF measurements to within the accuracy bars of the LIF measurements via a single-point scaling of the PLIF image. Received: 23 November 1999 / Revised version: 17 January 2000 / Published online: 27 April 2000     

基于激光诱导叶绿素荧光寿命成像技术的植物荧光特性研究

针对植物荧光遥感探测中信号易受干扰的问题, 提出了一种用于评估植物生长状况及环境监测的荧光寿命成像技术. 采用凹透镜对355 nm波长的激光扩束, 再照射植物激发叶绿素荧光, 由增强型电荷耦合器件接收荧光信号. 采用时间分辨测量法, 连续用相同激光脉冲照射植物以激发相同的荧光信号, 同时不断改变激光脉冲触发探测器启动的延时时间, 从而能够得到完整的离散荧光信号分布图像. 对植物特定位置点产生的离散荧光信号进行拟合, 再运用一种改进型的迭代解卷积法可反演高精度的荧光寿命; 进而反演图像各点的荧光寿命以生成植物的荧光寿命分布图. 该方法所绘制的荧光寿命图比荧光强度图能更准确地反映植物内部的叶绿素含量, 并对活体植物叶绿素荧光寿命的物理特性进行了初步研究, 证明叶绿素荧光寿命与植物生理状态存在一定关联; 并且叶绿素荧光寿命与活体植物所处环境存在着复杂的关系. 未来将与生物物理学家们合作, 继续探寻叶绿素荧光寿命与植物生存环境的关系.     

Measurement of water film thickness by laser-induced fluorescence and Raman imaging

We present two non-intrusive, laser-based imaging techniques for the quantitative measurement of water fluid film thickness. The diagnostics methods are based on laser-induced fluorescence (LIF) of the organic tracer ethyl acetoacetate added to the liquid in sub-percent (by mass) concentration levels, and on spontaneous Raman scattering of liquid water, respectively, both with excitation at 266 nm. Signal intensities were calibrated with measurements on liquid layers of known thickness in a range between 0 and 500 μm. Detection via an image doubler and appropriate filtering in both light paths enabled the simultaneous detection of two-dimensional liquid film thickness information from both methods. The thickness of water films on transparent quartz glass plates was determined with an accuracy of 9% for the tracer LIF and 15% for the Raman scattering technique, respectively. The combined LIF/Raman measurements also revealed a preferential evaporation of the current tracer during the time-resolved recording of film evaporation.     

Investigation of droplet combustion in strained counterflow diffusion flames using planar laser-induced fluorescence

Experimental investigation of an isolated droplet burning in a convective flow is reported. Acetone droplets were injected in a steady laminar diffusion counterflow flame operating with methane. Planar laser-induced fluorescence measurements applied to OH radical and acetone was used to measure the spatial distribution of fuel vapour and the structure of the flame front around the droplet. High-magnification optics was used in order to image flow areas with a ratio of 1:1.2. The different combustion regimes of an isolated droplet could be observed from the configuration of the envelope flame to that of the boundary-layer flame, and occurrence of these regimes was found to depend on the droplet Reynolds number. Experimental results were compared with 1D numerical simulations using detailed chemistry for the configuration of the envelope flame. Good agreement was obtained for the radial profile of both OH radical and fuel vapour. Influence of droplet dynamics on the counterflow flame front was also investigated. Results show that the flame front could be strongly distorted by the droplet crossing. In particular, droplets with high velocity led to local extinction of the flame front whereas droplets with low velocity could ignite within the flame front and burn on the oxidiser side. PACS 33.50.-j; 42.62.-b; 47.55.D-; 47.70.Pq; 47.80.Jk     

Experimentally based correction procedures for planar laser-induced fluorescence measurements of mean species concentration

Experimentally-based correction procedures are demonstrated which enhance the quantitative nature of planar laser-induced fluorescence (PLIF) images for mean species concentration by correcting for the influence of the electronic quenching rate coefficient. Implementation of these methods requires only the ability to make PLIF and laser-saturated fluorescence (LSF) measurements. Though applied herein to NO, these procedures are broadly applicable both in terms of species and users. Moreover, they are generally effective regardless of the error gradients associated with spatial variations in the electronic quenching rate coefficient. In such general environments, these methods produce quenching-corrected, spatially resolved PLIF images of mean species concentration with a total uncertainty equivalent to that of a single LSF measurement. Received: 22 November 1999 / Revised version: 3 March 2000 / Published online: 16 June 2000