Vortex-induced flame extinction in two-phase counterflow diffusion flames with CH planar laser-induced fluorescence and particle-image velocimetry

Here the interaction between a laminar two-phase, non-pre-mixed counterflow flame and a vortex is examined. Special emphasis is given to the influence of different flame and vortex parameters on the extinction behavior of the flame. Simultaneous planar laser-induced fluorescence of the CH radical layer produced by the flame and particle-image velocimetry measurements of the flowfield are used to characterize the flame-vortex interaction. These simultaneous diagnostics are used for the first time in this configuration. The extinction processes occurring during the flame-vortex interaction can be analyzed by this method, especially the influence of strain at the flame surface. The influence of the droplets on the extinction behavior appears clearly compared with a fully gaseous flame. The spray flame is weaker and extinguishes earlier than does a gaseous flame. In the measurements an additional broadband signal in the vicinity of the CH layer is probably due to the induced fluorescence of polycyclic aromatic hydrocarbons, excited at the same wavelength.     

Quantitative hydroxyl concentration time-series measurements in turbulent nonpremixed flames

Quantitative hydroxyl concentration time-series measurements have been obtained by picosecond time-resolved laser-induced fluorescence in a series of methane-air and hydrogen-argon-air nonpremixed flames. The recovery of a quantitative time series is complicated by the need to account for fluctuations in the fluorescence lifetime. We have recently developed instrumentation that enables the simultaneous measurement of fluorescence signal and lifetime. The present research represents the first application of this technique to turbulent flames. The correction for hydroxyl lifetime fluctuations is shown to be significant for mean concentrations and thus probability density functions but negligible for power spectral densities (PSD's). The hydroxyl PSD's were found to vary slightly with radial and axial location in the flames and to vary significantly with Reynolds number. However, the PSD's in the H(2)-Ar-air flames are nearly identical to those in the CH(4)-air flames.     

Measurements of hydroxyl concentrations and lifetimes in laminar flames using picosecond time-resolved laser-induced fluorescence

Picosecond time-resolved laser-induced fluorescence (PITLIF) can potentially be used to obtain measurements of minor species concentrations in rapidly fluctuating flames. Previous studies demonstrated this potential for atomic sodium by monitoring the temporal fluorescence signal with both an equivalent-time and a real-time sampling method. In this developmental study, PITLIF is used to determine hydroxyl concentrations in laminar CH(4)-O(2)-N(2) flames by the measurement of both the integrated fluorescence signal and the fluorescence lifetime. The quenching environment can be monitored with real-time sampling, and thus the necessary quenching rate coefficient is obtained in 348 us, which is fast enough for use in many turbulent flows. Fluorescence lifetimes of OH are also measured at different equivalence ratios in laminar flames by the use of the equivalent-time sampling technique. These results compare favorably with predicted lifetimes based on relevant quenching cross sections and calculated species concentrations.     

Quantitative technique for imaging mixture fraction, temperature, and the hydroxyl radical in turbulent diffusion flames

A technique for obtaining simultaneous quantitative images of the hydroxyl radical, OH, temperature, mixture fraction, and scalar dissipation rates in turbulent diffusion flames is described. Mixture fraction is obtained from images of Rayleigh and fuel Raman scattering. We quantified the OH laser-induced fluorescence (LIF) images using detailed calibration and a correction for quenching and population distribution effects based on the simultaneous mixture fraction and temperature images. This correction was derived from calculations of laminar counterflow diffusion flames for identical fuel mixtures. These laminar flame computations are further used to estimate the errors in the measured OH concentrations. The technique is applied to piloted, nonpremixed flames over a range of jet velocities. The measured mixture fraction, temperature, and OH concentrations are in good agreement with those obtained earlier in similar flames using the single-point Raman/Rayleigh/LIF technique.     

Real-time acquisition of laser-induced fluorescence decays

Picosecond time-resolved laser-induced fluorescence (PITLIF) has the potential to provide rapid measurements of minor-species concentrations by correction for local quenching conditions on the time scale of turbulence. Previous studies demonstrated that this technique could provide laser-induced fluorescence data and local quenching rates in flames but used equivalent-time sampling to obtain the required fluorescence decays. This precludes the use of PITLIF in turbulent systems. Fluorescence decays of sodium seeded into a laminar H(2)-O(2)-Ar diffusion flame are obtained from real-time data with an acquisition rate on the time scale of turbulence. The results obtained with this method are shown to be similar to those obtained from equivalent-time sampling.     

Line Raman, Rayleigh, and laser-induced predissociation fluorescence technique for combustion with a tunable KrF excimer laser

We have applied a line UV Raman, Rayleigh, and laser-induced predissociation fluorescence technique for measurement of turbulent hydrocarbon flames. The species concentration of CO(2), O(2), CO, N(2), CH(4), H(2)O, OH, and H(2) and the temperature are measured instantaneously and simultaneously along a line of 11.4 mm, from which the gradients with respect to mixture fraction and spatial direction are obtained. The technique has been successfully tested in a laminar premixed stoichiometric methane flame and a laminar hydrogen diffusion flame. In addition the technique has been tested in a highly turbulent rich premixed methane flame. The data show that the technique can be used to provide instantaneous measurements of local profiles that describe the local flame structure in highly turbulent flames.     

Strategies for formaldehyde detection in flames and engines using a single-mode Nd:YAG/OPO laser system

This paper presents technical developments for the detection of formaldehyde (CH2O) using laser-induced fluorescence. The easily accessible third harmonic of the Nd:YAG laser at 355 nm was used for excitation of formaldehyde. In order to investigate potential background fluorescence, e.g., from large molecules such as polyaromatic hydrocarbons, special attention was paid to investigating the possibility of scanning the wavelength of a single-mode Nd:YAG laser under the gain profile, approximately 3 cm(-1), on and off resonance. Furthermore, a technique for simultaneous detection of formaldehyde and OH using one laser system is presented. The single-mode Nd: YAG laser at 355 nm in combination with an optical parametric oscillator (OPO) laser tuned to 283 nm was used for simultaneous two-dimensional imaging of both species using one charge-coupled device (CCD) detector equipped with a dual filter image separator. The techniques are demonstrated with measurements in laboratory flames and the combined measurements are also demonstrated in an engine.     

Nonlinear diffusion filtering of images obtained by planar laser-induced fluorescence spectroscopy

The application of nonlinear anisotropic diffusion filtering to reduce noise and enhance contours in images obtained by two-dimensional planar laser-induced fluorescence (PLIF) spectroscopy is presented. In this process the diffusion coefficient is locally adapted, becoming negligible as object boundaries are approached. Noise is efficiently removed, and object contours are strongly enhanced. The technique is demonstrated with PLIF images obtained from the OH radical recorded in turbulent flames. We show that nonlinear diffusion is suitable as a preprocessing step, before image segmentation becomes possible, and we demonstrate how the technique is applied for the quantitative extraction of flame reaction boundaries from PLIF data.     

激光燃烧诊断技术及应用研究进展

介绍了用于燃烧场温度、组分、火焰构造和流场速度等参数测量的相干反斯托克斯喇曼散射、自发振动喇曼散射、激光诱导荧光和OH示踪测速实验系统,给出了在预混火焰、高能固体推进剂瞬态燃烧场和超声速高温流场测量的部分实验结果,并分析了激光作用区域燃烧场温度、主要组分及流场速度的分布和火焰构造。     

Phase-conjugate holographic system for high-resolution particle-image velocimetry

A novel holographic particle-image velocimeter system has been developed for the study of threedimensional (3-D) fluid velocity fields. The recording system produces 3-D particle images with a resolution, a signal-to-noise ratio, an accuracy, and derived velocity fields that are comparable to high-quality two-dimensional photographic particle-image velocimetry (PIV). The high image resolution is accomplished through the use of low f-number optics, a fringe-stabilized processing chemistry, and a phase conjugate play-back geometry that compensates for aberrations in the imaging system. In addition, the system employs a reference multiplexed, off-axis geometry for the determination of velocity directions with the cross-correlation technique, and a stereo camera geometry for the determination of the three velocity components. The combination of the imaging and reconstruction subsystems makes the analysis of volumetric PIV domains feasible.     

Characterization of low losses in optical thin films and materials

Residual absorption in optical coatings and materials is directly measured by means of the laser-induced deflection (LID) technique. For transmissive coatings a measurement strategy is introduced that allows for the separation of different absorptions of the investigated sample (bulk, coating, surface) by use of only one sample. Laser irradiation yields absorption values between 2 x 10(-3) and 2.9 x 10(-2) for antireflecting and highly reflecting (HR) coatings at 193 nm and 30.6 x 10(-6) for a HR mirror at 527 nm. Use of laser-induced fluorescence at 193 nm excitation reveals trivalent cerium and prasodymium and hydrocarbons in different single layers and coatings. In addition to correlation with absorption data, the influence of a high fluorescence quantum yield on the absorption measurement is discussed.     

Engineering tools for understanding the hydrodynamics of dissolution tests

In this article, three well-established engineering tools are used to examine hydrodynamics in dissolution testing apparatuses. The application of these tools would provide detailed information about the flow, shear, and homogeneity in dissolution tests. Particle image velocimetry successfully measures two-dimensional cross-sections of the velocity field in an experimental device under both laminar and turbulent conditions. The velocity field is also calculated with computational fluid dynamics (CFD), which can rapidly provide data that is difficult or impossible to obtain experimentally. The occurrence of segregated regions within a USP Apparatus II under mild agitation conditions is revealed by CFD simulations and confirmed by laser-induced fluorescence experiments. The results clearly demonstrate that under current operation settings, the USP Apparatus II operates in a regime where the flow is in incipient turbulence, which is a highly time-dependent condition that might explain possible inconsistencies in dissolution results. It is further demonstrated that proposed changes advocating lower speeds or smaller vessels displace the system toward laminar flow conditions characterized by segregation, compromising the robustness of the test and making it vulnerable to variability with respect to sample location.     

Characteristics of the turbulent/non-turbulent interface of a non-isothermal jet

The turbulent/non-turbulent interface of a jet is characterized by sharp jumps ('discontinuities') in the conditional flow statistics relative to the interface. Experiments were carried out to measure the conditional flow statistics for a non-isothermal jet, i.e. a cooled jet. These experiments are complementary to previous experiments on an isothermal Re=2000 jet, where, in the present experiments on a non-isothermal jet, the thermal diffusivity is intermediate to the diffusivity of momentum and the diffusivity of mass. The experimental method is a combined laser-induced fluorescence/particle image velocimetry method, where a temperature-sensitive fluorescent dye (rhodamine 6G) is used to measure the instantaneous temperature fluctuations. The results show that the cooled jet can be considered to behave like a self-similar jet without any significant buoyancy effects. The detection of the interface is based on the instantaneous temperature, and provides a reliable means to detect the interface. Conditional flow statistics reveal the superlayer jump in the conditional vorticity and in the temperature.     

Laser-based diagnostics for the measurement of liquid water film thickness

Three different diagnostic techniques are investigated for measurement of the thickness of liquid water films deposited on a transparent quartz plate. The methods are based on laser-induced fluorescence (LIF) from low concentrations of a dissolved tracer substance and spontaneous Raman scattering of liquid water, respectively, both excited with 266?nm of radiation, and diode laser absorption spectroscopy (DLAS) in the near-infrared spectral region. Signal intensities are calibrated using liquid layers of known thickness between 0 and 1000?μm. When applied to evaporating liquid water films, the thickness values derived from the direct DLAS and Raman scattering measurements correlate well with each other as a function of time after the start of data recording, while the LIF signal derived thickness values decrease faster with time due to selective tracer evaporation from the liquid. The simultaneous application of the LIF with a tracer-free detection technique can serve as an in situ reference for quantitative film thickness measurements.     

Effect of ion motion on zeta-potential distribution at microchannel wall obtained from nanoscale laser-induced fluorescence

The present study has experimentally investigated the two-dimensional distribution of zeta-potential at the wall, which dominates electroosmotic microchannel flow. Nanoscale laser-induced fluorescence imaging using fluorescent dye and the evanescent wave with total internal reflection was developed for the zeta-potential measurement. The fluorescent dye in the vicinity of the wall is excited by the evanescent wave, which decays exponentially from the wall. The zeta-potential is obtained from the fluorescent intensity because the distribution of fluorescent dye near the wall is related to the zeta-potential by the Boltzman distribution. Two kinds of solution at different Na+ concentrations were mixed in a T-shaped microchannel composed of PDMS and silica glass. The zeta-potential distribution at the silica glass wall was measured with the uncertainty of 4.7 mV. The motion of Na+ in the microchannel was estimated by the numerical analysis using the velocity information obtained by micrometer-resolution particle image velocimetry. It is concluded that electroosmotic flow was generated by the zeta-potential distribution at the silica glass and PDMS wall, which was dependent on the Na+ transport in the flow field.     

Two-point time-series measurementsof hydroxyl concentration in a turbulent nonpremixed flame

Quantitative two-point hydroxyl time-series measurements have been performed in a turbulent nonpremixed flame by using two-point picosecond time-resolved laser-induced fluorescence. The current diagnostic system has been improved from its preliminary version to address optical aberrations and fluorescence lifetime fluctuations. In particular, with a newly designed collection system, the aberration-limited blur spot is reduced from 6 mm to 180 microm. Additional photon-counting channels enable the recovery of absolute OH concentrations through a triple-bin integration method. The present research represents what we believe to be the first application of this two-point technique to turbulent flames. Two independent schemes have been applied to remove uncorrelated noise in the derived two-point statistics. We show that optical aberrations can have a significant effect on space-time correlations. However, the sampling rate and fluctuations in the fluorescence lifetime barely affect the spatial autocorrelation function and thus the integral length scale. Such length scales for hydroxyl are found to rise linearly with increasing axial distance at peak [OH] locations. Along the jet centerline, the integral length scale varies slightly below the flame tip but increases rapidly above the flame tip. In addition, the OH length scale demonstrates the same trend as the OH time scale along the jet centerline, but the opposite trend at peak [OH] locations.     

Engineering Tools for Understanding the Hydrodynamics of Dissolution Tests

Abstract

In this article, three well-established engineering tools are used to examine hydrodynamics in dissolution testing apparatuses. The application of these tools would provide detailed information about the flow, shear, and homogeneity in dissolution tests. Particle image velocimetry successfully measures two-dimensional cross-sections of the velocity field in an experimental device under both laminar and turbulent conditions. The velocity field is also calculated with computational fluid dynamics (CFD), which can rapidly provide data that is difficult or impossible to obtain experimentally. The occurrence of segregated regions within a USP Apparatus II under mild agitation conditions is revealed by CFD simulations and confirmed by laser-induced fluorescence experiments. The results clearly demonstrate that under current operation settings, the USP Apparatus II operates in a regime where the flow is in incipient turbulence, which is a highly time-dependent condition that might explain possible inconsistencies in dissolution results. It is further demonstrated that proposed changes advocating lower speeds or smaller vessels displace the system toward laminar flow conditions characterized by segregation, compromising the robustness of the test and making it vulnerable to variability with respect to sample location.     

Quantification of the evaporative cooling in an ethanol spray created by a gasoline direct-injection system measured by multiline NO-LIF gas-temperature imaging

Two-dimensional gas-phase temperature fields were quantitatively measured in an evaporating ethanol spray with multiline excitation thermometry based on laser-induced fluorescence of nitric oxide (NO-LIF). This technique yields absolute temperature fields without calibration and simultaneously detects the spray position. The accuracy of the presented temperature measurements is +/-1 K. Systematic errors of the scanned multiline thermometry approach due to time averaging in turbulent systems were investigated and found to be negligible. The pulsed spray was generated by a gasoline direct-injection nozzle with swirl injecting ethanol into air in a flow cell at room temperature and atmospheric pressure. The gas temperature inside the spray cloud was found to decrease by 10 K at approximately 5-10 ms after injection. Different injection pressures influence the evaporation behavior.     

Simultaneous Two-Dimensional Flow Velocity and Gas Temperature Measurements by use of a Combined Particle Image Velocimetry and Filtered Rayleigh Scattering Technique

For the first time, to the best of our knowledge, two-dimensional instantaneous measurements of the flow velocity and the gas temperature have been performed in a turbulent flame with simultaneous use of particle image velocimetry and planar filtered Rayleigh scattering. These single-shot measurements provide simultaneous information on the local flame structure (curvature and temperature gradients) and on the local flow conditions (vortices, flow divergences, and strain rates). The applicability of the technique is demonstrated in a turbulent lean CH(4)-air V flame.     

Detection of uranium in solids by using laser-induced breakdown spectroscopy combined with laser-induced fluorescence

Detection of uranium in solids by using laser-induced breakdown spectroscopy has been investigated in combination with laser-induced fluorescence. An optical parametric oscillator wavelength-tunable laser was used to resonantly excite the uranium atoms and ions within the plasma plumes generated by a Q-switched Nd:YAG laser. Both atomic and ionic lines can be selected to detect their fluorescence lines. A uranium concentration of 462 ppm in a glass sample can be detected by using this technique at an excitation wavelength of 385.96 nm for resonant excitation of U II and a fluorescence line wavelength of 409.0 nm from U II.