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At least 343 records · Page 19

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NASA NTRS1990

Saturated fluorescence measurements of the hydroxyl radical in laminar high-pressure flames

The efficacy of laser saturated fluorescence (LSF) for OH concentration measurements in high pressure flames was studied theoretically and experimentally. Using a numerical model describing the interaction of hydroxyl with nonuniform laser excitation, the effect of pressure on the validity of the balanced cross-rate model was studied along with the sensitivity of the depopulation of the laser-coupled levels to the ratio of rate coefficients describing: (1) electronic quenching to (sup 2) Sigma (+) (v double prime greater than 0), and (2) vibrational relaxation from v double prime greater than 0 to v double prime = 0. At sufficiently high pressures and near-saturated conditions, the total population of the laser-coupled levels reaches an asymptotic value, which is insensitive to the degree of saturation. When the ratio of electronic quenching to vibrational relaxation is small and the rate of coefficients for rotational transfer in the ground and excited electronic states are nearly the same, the balanced cross-rate model remains a good approximation for all pressures. When the above ratio is large, depopulation of the laser-coupled levels becomes significant at high pressures, and thus the balanced cross-rate model no longer holds. Under these conditions, however, knowledge of the depletion of the laser-coupled levels can be used to correct the model. A combustion facility for operation up to 20 atm was developed to allow LSF measurements of OH in high pressure flames. Using this facility, partial saturation in laminar high pressure (less than or equal to 12.3 atm) C2H6/O2/N2 flames was achieved. To evaluate the limits of the balanced cross-rate model, absorption and calibrated LSF measurements at 3.1 and 6.1 atm were compared. The fluorescence voltages were calibrated with absorption measurements in an atmospheric flame and corrected for their finite sensitivity to quenching with: (1) estimated quenching rate coefficients, and (2) an in situ measurement from a technique employing two fluorescence detection geometries.

Carter, Campbell D.↗

NASA NTRS1990

Discovery of molecular hydrogen fluorescence in the diffuse interstellar medium

The first detection of molecular hydrogen fluorescence in the diffuse interstellar medium is reported. Using the Berkeley UVX Shuttle Spectrometer, H2 Lyman band fluorescence has been observed in four directions, each with high significance. Molecular hydrogen fluorescence is detected in all directions that have previously been found to contain significant CO emission. A simple equilibrium model has been developed that includes attenuation of the incident UV radiation field by H2 line and dust continuum absorption. Evidence is found that the gas in the CO emission portions of the clouds may be clumpy, with a filling factor less than 0.2 and an average density greater than 30/cu cm in most cases.

Martin, Christopher↗

NASA NTRS1989

Laser-fluorescence measurements of nitric oxide in low-pressure H2/O2/NO flames

The concentration profiles of NO in low-pressure (76 Torr) H2/O2/Ar flames to which nitric oxide is added are measured by pulsed laser-induced fluorescence. Temporally resolved fluorescence measurements are used to determine the collisional deexcitation rates needed to convert time-integrated fluorescence signal into oxide concentration. Five flames are studied with H2/O2 equivalence ratios of 0.88, 0.98, 1.22, 1.37, and 1.50. In these flames the collisional deexcitation rate decreases rapidly above the burner surface as the density decreases with increasing temperature. A 20 percent decrease is observed for the lean flames, and a 30 percent decrease for the rich flames. Within the precision of the measurement technique (+ or - 10 percent), no significant removal of nitric oxide is observed in these flames.

Cattolica, R. J.↗

NASA NTRS1991

Imaging of molecular hydrogen and oxygen by single and two-photon fluorescence using laser and flashlamp sources

Two flow visualization techniques, i.e., simultaneous two-dimensional fluorescence imaging of H2 and O2 in a diffusion flame, and quasi-linear fluorescence imaging of O2, are presented. The first uses an injection-locked argon-fluoride excimer laser and a partial overlap of a two-photon ground state absorption in H2 with a single photon absorption from a vibrational level in O2. The second uses a simple, high-intensity ultraviolet flashlamp which provides a flux of photons in the 180-195 nm range, sufficient to produce a quasi-one-dimensional fluorescence image of hot/room temperature oxygen. Both techniques do not require that a seed material be introduced into the flow, they can image major flow constituents, and provide an instantaneous snapshot of the flow.

Diskin, Glenn S.↗

NASA NTRS1991

The ultraviolet continuum and the fluorescent H2 lines in low-excitation Herbig-Haro objects

Long-exposure IUE observations are used here to determine the spatial distribution of the fluorescent H2 line emission in the low-excitation objects HH 43 and HH 47. In HH 43, the spatial distribution of the fluorescent H2 line emission has a width equal to or smaller than the width of the point-spread function of IUE, while both the short-wavelength UV continuum and the optical forbidden S II lines show distributions which are wider by more than a factor of two. In HH 47, the results are qualitatively analogous, but the differences between the fluorescent HH 23 emission distribution on the one hand, and the UV continuum and optical forbidden S II distribution on the other, are smaller. These distributions indicate that the appropriate conditions for the formation of fluorscent H2 lines are fulfilled only in rather narrow regions for these low-excitation objects. A continuous energy distribution in HH 43 and HH 47 is found which differs only slightly from that in the high-excitation objects HH 1 and HH 2.

Boehm, K. H.↗

NASA NTRS1992

Fluorescence Imaging Reveals Surface Contamination

In technique to detect surface contamination, object inspected illuminated by ultraviolet light to make contaminants fluoresce; low-light-level video camera views fluorescence. Image-processing techniques quantify distribution of contaminants. If fluorescence of material expected to contaminate surface is not intense, tagged with low concentration of dye.

Schirato, Richard↗

NASA NTRS1991

Fluorescence microscopy for the characterization of structural integrity

The absorption characteristics of light and the optical technique of fluorescence microscopy for enhancing metallographic interpretation are presented. Characterization of thermally sprayed coatings by optical microscopy suffers because of the tendency for misidentification of the microstructure produced by metallographic preparation. Gray scale, in bright field microscopy, is frequently the only means of differentiating the actual structural details of porosity, cracking, and debonding of coatings. Fluorescence microscopy is a technique that helps to distinguish the artifacts of metallographic preparation (pullout, cracking, debonding) from the microstructure of the specimen by color contrasting structural differences. Alternative instrumentation and the use of other dye systems are also discussed. The combination of epoxy vacuum infiltration with fluorescence microscopy to verify microstructural defects is an effective means to characterize advanced materials and to assess structural integrity.

Street, Kenneth W.↗

NASA NTRS1992

Lidar Tracking of Multiple Fluorescent Tracers: Method and Field Test

Past research and applications have demonstrated the advantages and usefulness of lidar detection of a single fluorescent tracer to track air motions. Earlier researchers performed an analytical study that showed good potential for lidar discrimination and tracking of two or three different fluorescent tracers at the same time. The present paper summarizes the multiple fluorescent tracer method, discusses its expected advantages and problems, and describes our field test of this new technique.

Eberhard, Wynn L.↗

NASA NTRS1992

Alexandrite as a high-temperature pressure calibrant, and implications for the ruby-fluorescence scale

The wavelength shifts of the R1 and R2 fluorescence lines of alexandrite (BeAl2O4:Cr(+3)) have been experimentally calibrated against the ruby-fluorescence scale as a function of both hydrostatic and nonhydrostatic pressures between 0 and 50 GPa, and simultaneously as a function of temperatures between 290 and 550 K. It is found that the pressure-temperature cross derivative of the fluorescence wavelength shifts are negligible for both ruby and alexandrite.

Jahren, A. H.↗

NASA NTRS1992

Quantitative fluorescence measurements of the OH radical in high pressure methane flames

A method for quantifying laser-induced fluorescence signals from the OH radical in high-pressure flames is presented. The fluorescence signal per unit OH mole fraction is modeled as a function of temperature, pressure, and overall flame stoichiometry. Known values of the collisional quenching cross sections as a function of temperature are used to model the electronic quench rate. The reverse A - X (1.0) Q15 transition is used with broadband collection to measure single-point fluorescence produced by a pulsed Nd:YAG-pumped, frequency-doubled dye laser. Laser absorption and thermocouples are used to measure absolute OH concentration and temperature, respectively, which are used to confirm the validity of the model. Measurements are made in CH4/O2/N2 flames up to 10 atm.

Battles, B. E.↗

NASA NTRS1991

A laser fluorescence anemometer system for the Langley 16- by 24-inch water tunnel

A laser fluorescence anemometer which comprises a three-component laser Doppler velocimeter system with a fourth channel to measure fluorescent dye concentration has been installed in the NASA Langley 16- by 24-in water tunnel. The system includes custom designed optics, data acquisition, and traverse control instruments and a custom software package. Feasibility studies demonstrated how water tunnels can be used in conjunction with advanced optical techniques to provide nonintrusive detailed flow field measurements of complex fluid flows with a minimum of expense. The measurements show that the laser fluorescence anemometer can provide new insight into the structure, entrainment, control and of mixing vortical and shear layer flows.

Owen, F. K.↗

NASA NTRS1993

Fluorescence measurements of the thermal control experiments coatings on LDEF S0069 and A0114

Fluorescence measurements were made on the thermal control coatings from the Long Duration Experiment Facility (LDEF) S0069, Thermal Control Surfaces Experiment (TCSE); and the A0114, Interaction of Atomic Oxygen with Material Surfaces in Low Earth orbit. Fluorescence was observed in two types of thermal control coatings and is attributed to pigments or binders. In addition, fluorescence measurement on the silver Teflon from the front cover of TCSE led to confirmation of damage (cracking) to the metal layers during application.

Zwiener, J. M.↗

NASA NTRS1991

Concentration measurements in an axisymmetric jet using laser-induced fluorescence

Errors associated with the use of an image processing technique to correct images of an axisymmetric jet produced by laser-induced fluorescence are analyzed. In particular, attention is given to errors related to the alignment of the optical system, spreading of the laser light sheet, laser light attenuation due to light absorption by the fluorescent dye, and solubility effects. Although the above effects are negligible in the case of a small low-concentration jet, they are not negligible in all fluorescent dye experiments.

Shoe, B.↗

NASA NTRS1993

Absolute intensity measurements of impurity emissions in a shock tunnel and their consequences for laser-induced fluorescence experiments

Absolute intensity measurements of impurity emissions in a shock tunnel nozzle flow are presented. The impurity emission intensities were measured with a photomultiplier and optical multichannel analyzer and calibrated against an intensity standard. The various metallic contaminants were identified and their intensities measured in the spectral regions 290 to 330 nm and 375 to 385 nm. A comparison with calculated fluorescence intensities for predissociated laser-induced fluorescence signals is made. It is found that the emission background is negligible for most fluorescence experiments.

Palma, P. C.↗

NASA NTRS1993

Wavelength dispersive analysis with the synchrotron x ray fluorescence microprobe

A wavelength dispersive spectrometer (WDS) was tested on the synchrotron x ray fluorescence microprobe at Brookhaven National Laboratory. Compared to WDS spectra using an electron microprobe, the synchrotron WDS spectra have much better sensitivity and, due to the absence of bremsstrahlung radiation, lower backgrounds. The WDS spectrometer was successfully used to resolve REE L fluorescence spectra from standard glasses and transition metal K fluorescence spectra from kamacite.

Rivers, M. L.↗

NASA NTRS1993

Laser-induced fluorescence of space-exposed polyurethane

The object of this work was to utilize laser-induced fluorescence technique to characterize several samples of space-exposed polyurethane. These samples were flown on the Long Duration Exposure Facility (LDEF), which was in a shuttle-like orbit for nearly 6 years. Because of our present work to develop laser-induced-fluorescence inspection techniques for polymers, space-exposed samples and controls were lent to us for evaluation. These samples had been attached to the outer surface of LDEF; therefore, they were subjected to thermal cycling, solar ultraviolet radiation, vacuum, and atomic oxygen. It is well documented that atomic oxygen and ultraviolet exposure have detrimental effects on many polymers. This was a unique opportunity to make measurements on material that had been naturally degraded by an unusual environment. During our past work, data have come from artificially degraded samples and generally have demonstrated a correlation between laser-induced fluorescence and tensile strength or elasticity.

Hill, Ralph H., Jr.↗

NASA NTRS1993

Fluorescence of dissolved organic matter: A comparison of north Pacific and north Atlantic Oceans during April 1991

Profiles of airborne-laser-induced fluorescence emission from dissolved organic matter in the upper ocean have been produced and compared for the Southern California Bight (SCB) and the Mid-Atlantic Bight (MAB). Findings were as follows. (1) The fluorescent components of dissolved organic matter (FDOM) are present in easily measurable quantities from near shore to well over 300 km offshore in the SCB and are likewise easily measurable in the coastal, shelf, slope, and Gulf Stream waters of the MAB. (2) The reange of FDOM in the MAB is considerably greater than that in the SCB. (3) The lowest FDOM levels observed in the SCB were higher than those found in the Gulf Stream. (4) The onshore-to-offshore spatial gradient of the FDOM was found to be considerably lower in the SCB than in the MAB, with the highest levels of FDOM being found immediately adjacent to the coast in the MAB. This suggests that the water adjacent to the SCB shoreline is not as strongly influenced by terrestrial and estuarine sources of FDOM as the MAB is. (5) The spatial distribution of the FDOM within both the SCB and the MAB is frequently coherent with the spatial distribution of chlorophyll determined form the concurrent airborne- laser- induced phytoplankton pigment fluorescence measurements. However, distinct noncoherency is sometimes observed, especially at water mass boundaries.

Hoge, Frank E.↗

NASA NTRS1995

In situ phytoplankton absorption, fluorescence emission, and particulate backscattering spectra determined from reflectance

An inverse model was developed to extract the absortion and scattering (elastic and inelastic) properties of oceanic constituents from surface spectral reflectance measurements. In particular, phytoplankton spectral absorption coefficients, solar-stimulated chlorophyll a fluorescence spectra, and particle backscattering spectra were modeled. The model was tested on 35 reflectance spectra obtained from irradiance measurements in optically diverse ocean waters (0.07 to 25.35 mg/cu m range in surface chlorophyll a concentrations). The universality of the model was demonstrated by the accurate estimation of the spectral phytoplankton absorption coefficents over a range of 3 orders of magnitude (rho = 0.94 at 500 nm). Under most oceanic conditions (chlorophyll a less than 3 mg/cu m) the percent difference between measured and modeled phytoplankton absorption coefficents was less than 35%. Spectral variations in measured phytoplankton absorption spectra were well predicted by the inverse model. Modeled volume fluorescence was weakly correlated with measured chl a; fluorescence quantum yield varied from 0.008 to 0.09 as a function of environment and incident irradiance. Modeled particle backscattering coefficients were linearly related to total particle cross section over a twentyfold range in backscattering coefficents (rho = 0.996, n = 12).

Roesler, Collin S.↗