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Formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) imaging was performed in a premixed, ethylene-fueled dual-mode scramjet combustor to study the flame structure and dynamic flame behavio...

A small-scale Mach 5 blow down wind tunnel, with ample access for optical diagnostics and ability to generate steady-state nonequilibrium flows, has been designed and operated. The wind tunnel uses transverse repetitively pulsed nanosecond discharge, fully overlapped with a transverse DC discharge and operated at high plenum pressures (P0=0.5-1.0 atm) to load internal energy modes of nitrogen and oxygen molecules. The discharge remains stable at energy loadings of up to ~0.1 eV/molecule in nitrogen (discharge power up to 2.5 kW). The wind tunnel generates nonequilibrium nitrogen and air flows with steady-state run time of 5-10 seconds, translational / rotational temperature of T0~300-400 K, and estimated upper bound nitrogen vibrational temperature of Tv0~2,000 K. Internal energy mode disequilibrium in the flow is controlled by injecting nitric oxide, hydrogen, or water vapor into the subsonic flow between the discharge section and the nozzle throat. The effect of energy mode disequilibrium is studied in a flow over a cylinder model placed in the Mach 5 test section. The flow field in the supersonic test section is well predicted by a 3-D compressible Navier-Stokes flow code, indicating good flow quality. The supersonic flow field over the model is visualized by schlieren imaging and NO PLIF imaging, using a burst mode laser operated in the vicinity of 226 nm, at a pulse repetition rate of 10-20 kHz. The laser was operated in the injection-seeded mode, generating narrow linewidth (~0.1 cm -1 ) output for single-line NO excitation in the flow. Nitric oxide was either injected into the flow in the plenum or generated in a repetitively pulsed nanosecond discharge in dry air. Both single-pulse PLIF images and images integrated over 10-50 laser pulses have been obtained. Two single-line NO PLIF images on a NO(X,v˝=0→A,v´=0) transition are used for measurements of 2-D temperature distributions in nitrogen flows in the supersonic test section. Another single-line NO PLIF image on a NO(X,v˝=1→A,v´=1) transition is used to estimate NO vibrational temperature behind the bow shock, TV(NO)=550 ± 100 K. The NO vibrational temperature increases when the energy loading in the discharge is increased. Kinetic modeling calculations indicate that low NO vibrational temperature is due to fairly low vibrational energy loading per nitrogen molecule in the discharge. Schlieren images of a supersonic flow over the cylinder model demonstrate that the shock stand-off distance is reduced by approximately 5% when the discharge in the wind tunnel is in operation and water vapor or hydrogen are injected into the flow between the discharge section and the nozzle throat. This effect is attributed to additional heating of the flow in the plenum during relaxation of vibrationally excited nitrogen in the presence of water vapor or hydrogen.

Picosecond coherent anti-Stokes Raman scattering is used for measurement of nitrogen vibrational distribution function in the plenum of a highly nonequilibrium Mach 5 wind-tunnel incorporating a high-pressure pulser– sustainer discharge. First-level vibrational temperatures of the order of 2000K are achieved in the 300 torr non-selfsustained plasma discharge generated by a high E=n ( 300 Td) nanosecond-pulsed discharge, which provides ionization in combination with an orthogonal low E=n ( 10 Td) dc sustainer discharge, which efficiently loads the nitrogen vibrational mode. It is also shown that operation with the nanosecond-pulsed plasma alone results in significant vibrational energy loading, withTv N2 of the order of 1100K.Downstream injection ofCO2, NO, andH2 results in vibrational relaxation, demonstrating the ability to further tailor the vibrational energy content of the flow. N2-NO vibration–vibration andN2-H2 vibration–translation rates inferred from these data agreewell with previous literature results to within the uncertainty in rotational-translational temperature.

This paper demonstrates significant potential of the use of high-voltage, nanosecond pulse duration, high pulse repetition rate discharges for aerospace applications. The present results demonstrate key advantages of these discharges: 1) stability at high pressures, high flow Mach numbers, and high-energy loadings by the sustainer discharge, 2) high-energy fractions going to ionization and molecular dissociation, and 3) targeted energy addition capability provided by independent control of the reduced electric field of the direct current sustainer discharge. These unique capabilities make possible the generation of stable, volume-filling, low-temperature plasmas and their use for high-speed flow control, nonthermal flow ignition, and gasdynamic lasers. In particular, the crossed pulsersustainerdischargewasusedformagnetohydrodynamic flowcontrolincoldM � 3 flows,providing firstevidenceof cold supersonic flow deceleration by Lorentz force. The pulsed discharge (without sustainer) was used to produce plasma chemical fuel oxidation, ignition, and flameholding in premixed hydrocarbon–air flows, in a wide range of equivalence ratios and flow velocities and at low plasma temperatures, 150–300 � C. Finally, the pulser-sustainer discharge was used to generate singlet oxygen in an electric discharge excited oxygen–iodine laser. Laser gain and output power are measured in the M � 3 supersonic cavity.

A new blowdown nonequilibrium plasma magnetohydrodynamic (MHD) supersonic wind tunnel operated at complete steady state has been developed and tested at Ohio State. The wind tunnel can be operated at Mach numbers up to M = 3-4 and mass flow rates of up to 45 g/s at a stagnation pressure of 1 atm

Results are presented of nonequilibrium rf plasma-assisted combustion experiments in premixed air-fuel flows. The experiments have been conducted in methane-air, ethylene-air, and CO-air mixtures. The results show that large volume ignition by the uniform and diffuse rf plasma can be achieved at significantly higher flow velocities (up to u = 25 m/s) and lower pressures (P = 60-130 torr) compared to both a spark discharge and a dc arc discharge. The experiments also demonstrated flame stabilization by the rf plasma, without the use of any physical obstacle flameholders. Fourier transform infrared (FTIR) absorption spectra of combustion products show that a significant fraction of the fuel (up to 80%) burns in the test section. Temperature measurements in the diffuse rf discharge using FTIR emission spectra show that the flow temperature in the plasma before ignition (T = 250-550°C at P = 60-120 torr) is considerably lower than the autoignition temperatures for ethylene-air mixtures at these pressures (T = 600-700°C). Visible emission spectroscopy measurements in C 2 H 4 -air flows in the rf discharge detected presence of radical species such as CH, C 2 , and OH, as well as O atoms. In CO-air flows, O and H atoms have been detected in the rf plasma region and CO 2 emission (carbon monoxide flame bands) in the flame downstream of the rf plasma.

A pulse burst laser and either one or two high-speed charge-coupled-device cameras were used to perform onecomponent time-resolved planar-Doppler-velocimetry (PDV) measurements in a rectangular Mach 2.0 jet. The measurements were carried out on a streamwise plane passing through the jet centerline and covering approximately 6‐12 jet heights downstream of the jet exit. The pulse burst laser operated at 0.532-µ mw avelength and produced 28 pulses at 250 kHz with approximately 9 mJ/pulse energy. Velocity image sequences consisting of 28 frames showed dynamics of the velocity field over a time span of 108 µs (approximately 4.5 convective timescales). A typical sequence of images is presented, which demonstrates the process of entrainment of low-speed fluid into the high-speed region of the jet. Mean and standard deviation statistics of the velocity calculations produced expected trends and showed good agreement between the single- and two-camera experiments. An error analysis revealed speckle as the predominant source of noise, as in a conventional PDV technique. At a transmission ratio of 0.5, the estimated total error is 13 m/s for the single-camera experiment and 15 m/s for the two-camera experiment. I. Introduction P LANAR Doppler velocimetry is a powerful optical diagnostic technique that can be used to measure all three components of instantaneous velocity over a two-dimensional plane within a flowfield with high spatial resolution. This is accomplished by using an atomic or molecular vapor filter to measure the frequency shift of light as it is scattered by particles contained in the flowfield. The �

A new spectrally e ltered light scattering apparatus is presented based on a diodelaser injected seeded titanium: sapphire laser and rubidium vapor e lter at 780.24 nm. It is shown that the realizable line center attenuation of quasi elastically scattered light, limited by a residual broad spectral linewidth, unseeded, component to the laser output, isashigh as 1 :67£10 5 using the laser system alone. Preliminary measurementsindicatethat incorporation of a set of dispersing prisms and a stimulated Brillouin scattering phase conjugate mirror external to the laser provides an additional approximate factor of 10 extinction. The utility of the system for measurement of electron density and temperature by Thomson scattering is demonstrated in a 3950-Pa (30-torr) argon dc discharge. At 100-mA current, an electron density of 3 :7£1013 cmi3 is measured on the discharge centerline with a 2 ae value of statistical uncertainty equal to 8 :0£10 11 cm i3 . The corresponding electron temperature is 0 :63§ 0:025 eV.

The application of acetone-based molecular tagging velocimetry (MTV) is demonstrated in sonic and supersonic jets produced by a 1-mm-exit-diameter nozzle. Measurements are performed in the static pressure range 1.3-53 mbar, with spatial resolution of approximately 10 μm. The statistical uncertainty (2σ) in velocity is found to be of order 6-10 m/s, approximately independent of flowfield pressure. Acetone laser-induced fluorescence temporal decay curves were also obtained, with 1/e lifetime found to range from 200 ns at 1.3 mbar to less than 50 ns at 24 mbar. These relatively short lifetimes were nonetheless sufficient to obtain MTV data over the entire pressure range

Experiments conducted in a new, small-scale, nonequilibrium plasma wind tunnel recently developed at Ohio State University are discussed. The facility provides a steady-state supersonic e ow of cold nonequilibrium plasma with well-characterized, near uniform, properties. The plasma is produced in aerodynamically stabilized highpressureglowdischargethatformstheplenumofthesupersonicnozzle.Thepossiblemodie cation ofthesupersonic e ow due to ionization is studied by measuring the angle of oblique shocks attached to the wedge located in the nozzle test section. The results do not show any detectable shock weakening or attenuation in weakly ionized nitrogen plasma, compared to the measurements in a nonionized gas e ow. Experiments in supersonic e owing nitrogen and helium afterglow also demonstrate a novel technique for high-density supersonic e ow visualization. It allows identifying all key features of the supersonic e ow, including shocks, boundary layers, e ow separation regions, andwakesby recording intensevisibleradiation of theweakly ionizedplasmas.Interpretation ofradiation intensitydistributionsinnonequilibriumsupersonice owingafterglowmayprovideinformationonkeymechanisms of energy storage and ultraviolet radiation in high-altitude rocket plumes. In addition, these e ow visualization experiments can be used for validation of multidimensional computer e ow codes used for internal e ow simulation.

We demonstrate the use of the photoactivated nonintrusive tracking of molecular motion flow tagging technique to obtain quantitative velocity measurements in free falling water droplets and in electrohydrodynamic Taylor cones. A simple ray tracing procedure is outlined to remove the optical distortion caused by the droplet surface curvature. This correction is applied to free falling droplet images, and the vertical component of velocity is measured across the droplet. Maximum vertical velocities in the droplet are 15.9 ± 3.3 mm/s. Without ray tracing, the optical distortion is shown to cause errors in the sign of velocity as well as errors of over 100% in velocity magnitude. Preliminary velocity measurements in a Taylor cone are also presented. Centerline velocities in the Taylor cone are approximately 4 n/s.

Filtered Rayleigh scattering is an optical diagnostic technique that allows for simultaneous planar measurement of velocity, temperature, and pressure in unseeded flows. An overview of the major components of a filtered Rayleigh scattering system is presented. In particular, a detailed theoretical model is developed and discussed with associated model parameters and related uncertainties. Based on this model, results for two experimental conditions are presented: ambient room air and a Mach 2 freejet. These results include two-dimensional, spatially resolved measurements of velocity, temperature, and pressure derived from time-averaged spectra. ILTERED Rayleigh scattering (FRS), a recently developed flow diagnostic technique,1'2 achieves large suppression of background scattering allowing planar flowfield visualization and obtains quantitative measurements of velocity, temperature, and density in unseeded gaseous flows. This technique makes use of Rayleigh scattering from molecules in the flow and is driven by a high-power, narrow linewidth, tunable, injection seeded laser. When imaging the scattered light onto a charge-coupled device (CCD) camera, unwanted background scattering from stationary objects may be filtered out by tuning the frequency of the narrow linewidth laser to coincide with an atomic or molecular absorption line and by placing a cell containing the atomic or molecular species between the camera and the flow. This cell acts as a notch filter, absorbing all background scatter at the laser frequency. Scattered light that is Doppler shifted, however, passes through the filter and is imaged on the camera. Quantitative measure of flow properties is achieved by measuring the total intensity, Doppler shift, and spectral profile of the Rayleigh scattered light. The total intensity is directly proportional to density; the Doppler shift is directly proportional to velocity; and the spectral profile is a function of temperature and pressure. The scattering intensity, Doppler shift, and spectral profile are determined by passing the scattered light through the notch absorption filter and then by imaging it onto an intensified CCD camera. Because the filter absorbs light in a narrow frequency band, it converts the spectral information contained in the Doppler shift and Rayleigh profile into intensity information at the camera. By collecting data (camera pixel intensity) for varying conditions, v, T, and P may be determined. Previous work has concentrated on the use of this technique for background suppression when visualizing flows and for the measurement of velocity. The background suppression feature of FRS has been used to image flowfields that otherwise would be completely obscured by the strong scattering from wind-tunnel surfaces. The authors have used this technique to image the flowfield inside a Mach 3 inlet and to generate volumetric images of the crossing shocks and boundary layer.3 Elliott et al.4 have also used this technique to observe structures in compressible mixing layers. The use of FRS to measure velocity was initially demonstrated using scattering

The observation of small-scale velocity structures in high Reynolds number turbulent flowfields is important for the understanding of energy scaling and dissipation phenomena. The Raman excitation plus laser-induced electronic fluorescence (RELIEF) technique is shown to be a promising tool to observe these scales. Lines on the order of 100 μ thick and 1 cm long are written into a contained turbulent jet and observed with a resolution on the order of tens of microns. The images that are recorded give instantaneous velocity profiles it very short time delays between tagging and interrogation are used. This time-of-flight approach is a direct measure of velocity, so approximations such as constant density, constant temperature, or the Taylor hypothesis are not required