A numerical method for the analysis of the fast axial flow glow discharge CO₂ laser has been developed. The method is based on the self-consistent solution to the 1-D steady-state glow discharge equations, the gas dynamic equations, and the vibrational relaxation equations. The discharge equations include the continuity ones for the electrons, the positive and negative ions, and Poisson's equation for the electric field. The three-mode relaxation model for the vibrational kinetics and the plane-parallel optical resonator model have been used. This approach does not require previous assignment of the discharge power distribution and enables obtaining the discharge structure including the near-electrode regions in addition to the laser characteristics.

The amount of water vapor formed in scramjet engines in high-enthalpy environments is a key parameter used to infer combustion efficiency. A H₂O(V) concentration measurement system for combustion diagnostics in these high-temperature, high-velocity conditions has been developed at the NASA Langley Research Center and tested at General Applied Science Laboratories (GASL) in Ronkonkoma, NY. The system was first tested in an instrumentation shock tunnel using a H₂/O₂/N₂ combustible mixture in the driven section for calibration and development purposes. The system uses a distributed feedback InGaAsP diode laser operating near 1350 nm. A wavelength modulation spectroscopy technique is used to achieve high detection sensitivity. The diode laser is modulated at radio frequency (rf), while the wavelength is scanned over two water vapor absorption lines by injection current tuning at a 5 kHz repetition rate. The detected rf signal is then demodulated at the modulation frequency (one f demodulation). Preliminary experimental results and data analysis are presented.

A multichannel infrared absorption technique has been developed and applied to the study of solid propellant flames. Infrared absorption spectra of HCN, H₂CO, CH₄, CO, CO₂, N₂O, and H₂O have been obtained in the dark zone of solid propellant flames during steady state combustion conditions through the use of a 1024 element platinum silicide (Pt-Si) array detector. The experiment consists of a quartz tungsten halogen lamp as the source, a 0.320 m spectrometer with a 75 grooves/mm grating, the Pt-Si array detector and a windowed strand burner where cylindrical propellant samples are burned cigarette fashion in the presence of an inert buffer gas. Results for a nitramine propellant M43 are presented along with estimates for the temperature and species concentrations through the use of the HITRAN database and associated PC programs.

A rugged, easy to implement, line-of-sight absorption instrument, which utilizes a low- pressure water vapor microwave discharge cell as the light source, has been developed to make simultaneous measurements of the OH concentration and temperature at 10 spatial positions. The design, theory, and capability of the instrument are discussed. Results of the measurements obtained on a methane/air flat flame burner are compared with those obtained using a single-frequency, tunable dye laser system.

The atmospheric effects of stratospheric aircraft component of the NASA High Speed Research Program will require measurements of trace gas concentrations in the exhausts of high speed civil transport engines. In parallel with the development of these engines by NASA and its industrial partners, a portable infrared tunable diode laser apparatus has been assembled and tested which is capable of both in situ and extractive sampling of combustion gas flows. Infrared diode laser absorption is sensitive, quantitative, and applicable to a wide range of molecular species. In the present apparatus, sensitive detection is achieved by rapid frequency scanning and real-time nonlinear least squares fitting and background subtraction. Sensitivity is further increased for extractive sampling by an advanced design multiple pass cell which gives longer path lengths in smaller volumes. Observations of a laboratory flat flame burner are reported. These observations and spectroscopic models are used to predict detection sensitivities in exhausts and other combustion systems.

Exciplex-based liquid/vapor visualization systems used for fuel distribution studies allow 2D fluorescent images of the two phases to be taken separately. Although the exciplex technique is not new, an improved quantitative calibration method has been developed, relating fluorescence intensity to mass concentration of liquid and vapor fuel. Multiple droplet summation of optically thin droplets should be used to generate liquid phase calibrations for use in studies with high-pressure injectors. In addition to eliminating the need to correct for optical density, the spatial-averaging effect of this method resulted in a repeatable calibration.

Temperature profiles determined from LIF spatial profiles of six lines [A - X (1-0) band: R₁(3), R₁(10), R₂(5), R₂(8), R₂(10), and P₁(2)] agree well with values obtained from LIF spectral scans (282.6 - 281.0 nm) at fixed burner height in the luminous and post-flame zones. Typical uncertainties by this method are +/- 50 - 75 K which are satisfactory for purposes of flame modeling. Fewer lines did not consistently give good agreement. In the preheat zone, the profile method is subject to several systematic errors due in part to the high thermal and OH concentration gradients which affect any temperature measurement in this region. Temperature profiles can be obtained much faster and with similar accuracy using this method than those derived from an excitation spectrum for each desired position in the flame. The method is particularly attractive when temperature profiles for several different flames are desired.

We apply light scattering to the production of species-, space-, and time-resolved images from a Sandia transparent engine, which operates with propane and air. By appropriate tuning of our KrF laser, and by treating the scattered light with an appropriate filter, we can image (a) the total density (and thus the temperature), via Rayleigh scattering, and (b) densities of OH, and of hot O₂ (i.e., vibrational states 6 and 7) via laser-induced fluorescence.

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.

This research work studied the effects of oxidant constitution on soot formation in diffusion flames by simultaneously measuring the soot properties and the species concentration. The soot properties are measured by the laser light scattering and extinction method and the hydroxyl concentration is measured by the laser-saturated fluorescence (LSF) method. The temperature distributions in the flames were measured by the two-line LSF technique and by fine wire thermocouple. . The hydroxyl fluorescence profiles for all four flames presented here show that the OH fluorescence intensities peak near the flame front. The OH fluorescence intensity drops sharply towards the dark region of the flame and continues declining to the sooting region. The OH fluorescence profiles also indicate that OH fluorescence decreases with increasing height in the flames for all flames investigated. Varying the oxidizer composition resulted in corresponding variation in the maximum OH concentration and flame temperature. Furthermore, it appears that the maximum OH concentration for each flame increases with increasing flame temperature. Soot particles are formed on the fuel side of the flame front where the number density is high for all four flames. The fuel/oxygen/argon flame (Flame C) shows the largest soot particle size, and shortest flame height. In the higher portion of the Flame C, the soot volume fraction was observed to decrease indicating that soot is being oxidized before leave the flame. The fuel/oxygen/carbon dioxide flame (Flame B) shows the least soot formation in the flame. The experiment demonstrated that soot formation can be reduced by changing the inert of the oxidant while keeping the fuel flow rate and oxygen flow rate constant. The carbon dioxide dramatically reduced the soot formation in the flames. The temperature effect may play the major role in this reduction. Some researchers have doubted that the hydroxyl radical is the dominant oxidizer of the soot particles in flames. In this investigation, both calculation and measurement data show that the highest OH concentration flame has the most soot formation inside the flame.

Resonance-enhanced multiphoton ionization and laser-induced fluorescence were used to obtain relative concentration measurements of six species in low-pressure stoichiometric hydrocarbon flame systems. Relative density profiles of O, H, OH, CH, HCO, and CH₃ were compared to model calculations for methane/oxygen, ethane/oxygen, and ethylene/oxygen flame systems. Good agreement between measured profiles and model predictions was found for the methane/oxygen system, allowing for the methane/oxygen system to be used as a standard of comparison by which relative concentration profiles of other hydrocarbon flame systems may be put on an absolute scale. With this approach, one finds that peak HCO densities in the ethane/oxygen and ethylene/oxygen systems appear to be over estimated by current model predictions of C₁ and C₂ chemistry.

A tunable laser system that operates in the ultraviolet (UV) has been utilized to ignite premixed reactive gaseous flows of H₂/O₂, D₂/O₂ and CH₄/N₂O in a jet burner at atmospheric pressure. Multiphoton UV photodissociation of the fuel or oxidizer molecules produced ground state radicals (H and O atoms). Resonance-enhanced multiphoton excitation and ionization of these radicals formed a laser-produced microplasma that served as an ignition source. Time-resolved absorption techniques were utilized to determine minimum ignition energies for the gaseous mixtures when the laser was tuned to resonance two-photon excitation transitions of H and O atoms near 243 nm and 225.6 nm respectively. The minimum ignition energy was found to be wavelength dependent and was the least when the laser was tuned to the resonant two-photon excitation transitions. These results suggest that ignition is not only dependent upon the absorption of a certain minimum amount of energy but is also photochemically enhanced by the production of ground and excited-state radicals and ions which participate in exothermic chemical chain-branching reactions in the early stages of ignition. Therefore, a new laser ignition mechanism is proposed.

Laser ignition experiments of ball powder have been performed in a specially designed and constructed `optical' loader, employing five different lasers; viz. Excimer (ArF 193 nm), Nd:Yag, CO₂ Pulsed, CO₂ cw Chopped and Ruby. This study provides pertinent technical information necessary for the further development of laser ordnance ignition systems towards practical hardware devices (military and commercial). Specifically, it has experimentally demonstrated that direct ignition of ball powder can be successfully obtained with well-controlled laser pulses, provided the fluence exceeds a critical threshold value. This threshold fluence depends on laser wavelength, beam quality and pulse length. Furthermore, means for relaxing the optical alignment, and improving the efficiency of coupling the laser output to the energetic material are evaluated.

An experimental study on mechanisms of soot inception and nucleation in diffusion flames is presented. Microscopic amounts of soot samples are collected from benzene, acetylene, and methane diffusion flames. The molecular composition of these samples are analyzed using a laser desorption time-of-flight mass spectrometer. Preliminary results clearly indicate that soot samples generated by different fuels have different chemical structures. Surprisingly, methane soot contained the largest carbon clusters and acetylene soot generated the smallest carbon clusters.

Planar Lorenz/Mie scattering is being used to study passive scalar mixing of multiple jets in crossflow. Mean and instantaneous concentration distributions of jet mixture fraction are acquired in planes perpendicular to the crossflow direction and used to determine mixing performance. The results are in agreement with measurements made by physical probes and other optical techniques. The utility of the technique as an engineering design tool is demonstrated.

Complex index of refraction values of RP-1 liquid rocket fuel are reported at laser wavelengths of 0.193 micrometers (ArF excimer), 0.5145 micrometers (argon-ion), 0.532 micrometers (Nd-YAG, frequency doubled), 1.064 micrometers (Nd-YAG), and 10.5915 micrometers (CO₂). The imaginary part of the index of refraction (k) was determined by the traditional transmission method. The real part (n-r)) is determined by reflectance measurements, critical angle, Mueller matrix, and Michelson interferometer techniques. Reflectance measurements are used to obtain n_r at a wavelength of 0.193 micrometers . Critical angle method is used to determine n_r at 0.5145 micrometers and 0.532 micrometers . The real part of the refractive index is obtained from Snell's law by measuring the critical angle. The real part of the refractive index at 1.064 micrometers is derived based on elements of the Mueller matrix. Specular measurements were performed using a TMA scatterometer to obtain the Mueller matrix. A Michelson interferometer is used to determine n_r at 10.5915 micrometers .

We previously developed a holographic interferometry (HI) technique capable of detecting a sudden and sensitive temperature change occurring nearly instantaneously in a distributed area. In that study and our more recent work, it has been shown that HI is a very useful temperature measurement technique and the estimated errors are so small that they cause no significant effects when the HI data are used for quantitative analysis. This paper summarizes recent progress made on the application of HI to highly transient phenomena, such as the studies of flame spread and liquid pool fires.

A new method for the determination of the temperature of single droplets is presented. The method utilizes the analysis of the near field scattered light distribution. This distribution exhibits sharp peaks, the so-called glare points, if observed through a lens with finite aperture. The ratio of the brightnesses of the glare points of the zero and the first orders shows a monotonic dependence on the index of refraction. The ratio is measured and the temperature is calculated by means of the known dependence of the index of refraction on the temperature.

We present a simplified 1D theory for detecting locations of normal shocks in a converging- diverging nozzle. The theory assumes that the flow is quasi 2D and the flow is accelerated in the throat area. Optical aspects of the model consider propagation of electromagnetic fields transverse to the shock front. The theory consists of an inverse problem in which it reconstructs from the measured intensity an index of refraction profile for the shock. From this profile and the Dale - Gladstone relation, the density in the flow field is determined, thus determining the shock location. Experiments show agreement with the theory. In particular, the location is determined within 10 percent of accuracy. Both the theoretical as well as the experimental results are presented to validate the procedures in this work.

The flow field and fuel/air mixing patterns produced in an optically accessible, premixing/prevaporization section of a high pressure/high temperature combustor were examined via focused Schlieren high-speed photography. A focal plane, approximately 8 mm thick and centered within this section downstream of the fuel injectors, was imaged at a framing rate of 8,000 frames/second. High-speed focused Schlieren images were obtained for three different Venturi-type fuel injector configurations under identical experimental parameters. The results demonstrate the efficacy of this technique to discern fuel spray patternization, fuel-air mixing efficiencies, and mixing times of various fuel injector arrangements.

Being the preliminary task of holographic image processing, various approaches for the holographic image analysis have been proposed. This presentation describes an on—line image processing system for the automatic distribution analysis of particles in particle holograms. In this system, the particle holograms are obtained using in—line retrieval method. To find the approximate locations of particles, or seed points, the holographic images are first processed by the Sobel operator, and automatically thresholded. Then, using the seeds and a bi—threshold region growing technique, the edges and the preliminary parameters of the candidate particles are obtained by edge tracking in the original images. To remove the speckle noises caused by in—line retrieval method, the variance of the radius of the candidates and the texture of the candidate areas in the Sobeled images are analyzed. After that, the radial intensity profile of the candidates and the clearness of candidate neighboring areas in the original images are analyzed to remove particles out—of—focus. At last, the candidates are verified, and the statistical data of the verified particles are calculated. Experiment results are given to show the efficiency of the approach described in this presentation. Key Words: Particle Hologram, In—Line Retrieval Method, Segmentation, Region Growing.

In practical applications, it is possible to acquire multidirectional interferometric data of different instants. In such cases, refractive-index fileds can be reconstructed based on the multidirectional interferometric data of two consecutive times. The advantages of using the interferometric data of different instants include: more a priori information and knowledge about refractive-index fields can be incorporated in reconstruction and the problem of sovling velociety, temperature and pressure fields will be simplified. In this paper. An iterative direct 3D reconstruction algorithm from the interferometric data of two consecutive times is presented. The problem of calculating physical fields of fluid is discussed. The experimental results are also provided.

Interferometric optical tomography is a valuable method for combustion and flow diagnostics. However, it meets serious experimental difficulties when investigated objects are large (about 50 sm or greater) and/or perturbations of refractive index are greater than 0.01. In the latter case the interference fringes become too thin, while in the former case too large and expensive optical elements (lenses and holograms) are required. These difficulties can be overcome by using optical ray-deflection tomography with fan-shaped beams. In the paper a scheme of the tomographic experiment, reconstruction algorithms, and experimental results are presented.