Peter A. DeBarber

Single-pulse, simultaneous multipoint multispecies Raman measurements in turbulent nonpremixed jet flames

An imaging technique based on Raman scattering induced by a KrF excimer laser has been developed for quantitative simultaneous multiple-point measurements of absolute multispecies concentrations (O(2), N(2), H(2), and H(2)O) and temperature in turbulent flames. These multiparameter measurements along the laser line have a temporal resolution of 17 ns, a spatial resolution of 0.7 mm for each point, and single-shot standard deviations of ~8%. The UV Raman imaging system has been applied in turbulent nonpremixed hydrogen-air flames, providing what are to our knowledge the first reported single-pulse, simultaneous multiple-point quantitative images of multiple species and temperature.

Ozone Tagging Velocimetry Using Narrowband Excimer Lasers

Ozone tagging velocimetry (OTV), a nonintrusive, unseeded, time-of-flight velocity measurement technique, consists of a write step, where a 193-nm pulsed excimer laser creates an O 3 line via O 2 uv absorption, and a subsequent read step, where a 248-nm excimer laser photodissociates the O 3 and fluoresces the vibrationally exdted O 2 product, revealing the tag line displacement. For the flrst time, instantaneous OTV images and velocity measurements are reported in airflows at room temperature. The narrowband lasers are tuned to the O 2 Schumann-Runge transitions improving the OTV signal strength by a factor of six over that obtained using two broadband lasers. This improvement is less than expected from absorption ratio estimates, due in part to incomplete laser locking efficiency and possibly to laser bleaching. Diffusion of the O 3 tag line is shown to be important only for write-read delay times of the order of milliseconds or greater Modeling of O 3 concentration vs time shows O 3 is long lived at room temperature and relatively insensitive to water vapor, but O 3 peak concentration and lifetime greatly decrease at high temperature, though high pressure increases peak O 3 concentration

Influence of fluorescence reabsorption and trapping on solid-state optical cooling

We analyze the random process of fluorescence reabsorption and trapping in solid-state optical materials in general and its influence on the efficiency of optical cooling of solids by anti-Stokes fluorescence in particular. Using the absorption and fluorescence spectra of Yb3+:ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) as input data, we employ a random-walk model to test analytical approximations of the fluorescence escape efficiency and cooling efficiency, including reflections at the boundary.

Resonant holographic interferometry with ZnTe:V:Mn.

The photorefractive semiconductor ZnTe:V:Mn is investigated for use in real-time resonant holographic interferometry applications. Experimental results of two-wave and four-wave mixing with pulsed dye and cw diode lasers are presented; in addition holographic image transfer, as well as two-wavelength resonant holographic interferometry, are demonstrated. Species-specific interferograms of potassium seeded into various combustion environments are captured at video-frame rates. Calculations of the species measurement sensitivity and dynamic range are presented, and design considerations for resonant holographic interferometry systems employing photorefractive materials are outlined.

Temperature measurement by degenerate four-wave mixing with strong absorption of the excitation beams

We have made simultaneous temperature measurements by degenerate four-wave mixing (DFWM) and absorption spectroscopy of OH in a CH(4)-air, lifted-diffusion flame. After we corrected the DFWM data for laser beam absorption of as much as 60%, the DFWM-based temperatures were in good agreement with temperatures derived strictly from the absorption data, as well as a one-dimensional reacting flow simulation.

Trace species concentration and temperature measurements at high pressure using laser-induced grating spectroscopy

We have recorded laser-induced grating signals from mixtures of NO2 and air over a pressure range extending from less that 100 kPa (1 atm) to 10 MPa (100 atm). Signals generated from concentrations of NO2 at the part-per-million level have been successfully detected with high signal-to-noise rations. The measurements were made using the technique of laser-induced thermal acoustics (LITA). Analysis of the acquired data was made using a comprehensive theory which includes the hydrodynamic response of the fluid and finite beam-size effects. The observed pressure dependence of the peak amplitude signals is consistent with the theory. Additionally, least squares fits between the theory and the temporally resolved signal yield accurate values of the local sound speed and thermal diffusivity. Determination of the local sound speed provides a measurement of the local temperature.

Ozone tagging for unseeded velocimetry

This paper builds upon our discovery of a new method called ozone tagging velocimetry (OTV) for performing unseeded molecular velocimetry. A major advantage of OTV is that the measurement may be performed without the requirement of seeding for subsonic and supersonic air flows. The OTV approach consists of tagging the flow with ozone produced by an ArF laser pulse. The stable ozone tag is convected with the flow and subsequently observed using laser-induced fluorescence by a delayed KrF laser pulse. We outline several considerations for the implementation of OTV and present results demonstrating its practice in controlled laboratory experiments. (Author)

Practical applications in UV Raman line imaging of water vapor

High-temperature water vapor imaging based on Raman scattering for hypersonic exhaust diagnostics has been developed. A temperature-independent UV Raman line imaging instrument that is calibrated with room air and readily scaled to large combustion facilities has been tested in a laboratory H2/air combustor. The H2O concentration measurements exhibit a single-shot standard deviation of approximately 8%. In this paper we investigate practical issues concerning the application of UV Raman line imaging for quantitative water vapor measurements.

Temperature sensitivity of the two-photon photodissociation fluorescence spectrum of water vapor in a hydrogen-air burner

High temperature water vapor detection in hypersonic exhaust has been the subject of ongoing research in a collaboration between MetroLaser and Vanderbilt Universitys Department of Mechanical Engineering. In this paper we examine some of the temperature sensitivity issues surrounding water vapor diagnostics based on two-photon fluorescence measurements. We present work which shows a large temperature sensitivity for the two-photon excited fluorescence features for H2O in a hydrogen and air flat flame burner. The spectra are modeled for various temperatures, results are experimentally verified, and recommendations are made.

Two-photon photodissociation of water vapor to measure hydrogen/air mixing in hypersonic propulsion

Two-photon photodissociation fluorescence spectroscopy is a technique to measure nonintrusively and in real time the concentration of water vapor in combustion devices. The presence of water vapor provides key information of the mixing process of hydrogen and air, and of the density distribution which is needed in propulsion computations. Two different tunable lasers are investigated as excitation sources in the predissoaation of water vapor. The subsequent fluorescence follows two distinct mechanisms. The direct fluorescence from undissociated H20 is one mechanism. The second mechanism is the fluorescent emission from the electronidly excited OH photofragment. Both mechanisms exhibit reasonable fluorescence yields, however, it is only the direct fluorescence from H20 that is capable of producing quantitative concentration measurements. W