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Time-resolved CH * chemiluminescence imaging was performed at 50 kHz in order to study flame motion and stabilization in the axial flow direction in a dual-mode scramjet at the University of Virginia Supersonic Combustion Facility. The combustor was operated in a fully premixed mode at a global equivalence ratio of 0.40 using ethylene fuel and air with stagnation temperature and stagnation pressure of 1200 K and 300 kPa, respectively. From the high-speed CH * measurements, information regarding the flame anchoring position and flame spreading angle is collected, providing insight on the effect of cavity flame holding and flame penetration into the freestream flow. Statistics have been collected on the variation of flame brush width as a function of axial position (or along the flame length). The flame spreading angle of 9.5 ○ calculated from the CH * chemiluminescence imaging is comparable to flame angles derived from previous LES calculations. In addition, the combustor is noted to be highly dynamic, exhibiting significant variations in the integrated CH * chemiluminescence signal and the flame brush width as a function of time. A characteristic frequency of 340 Hz has been determined from the CH * imaging which governs the periodic oscillations of the CH * signal, flame brush width, and convective motion of the flame brush front. It is likely due to an instability in which acoustic waves are reflected and convected between the shock train and flame front.

In the present study, a plasma flow reactor (PFR) facility designed to perform both ex situ and in situ experiments of stable (H2 and O2) and intermediate (OH radicals) species detection was used to examine the plasma-assisted characteristics of hydrogen oxidation at 1 atm pressure for temperatures ranging from 420 K to 1100 K. Experiments were performed at nearly isothermal conditions, by heavily diluting reactive mixtures in argon, in an attempt to mitigate temperature changes from exothermic chemical reactions. This technique allows experimental results to be interpreted from a perspective that plasma and thermal (neutral) heat release effects are decoupled, essentially isolating the effects of the plasma-chemistry and its influence on the neutral-chemistry. Results showed no thermal reaction until 860 K, at which point hydrogen was rapidly consumed within the flow residence time associated with the reactor. With the plasma discharge, the onset of oxidation was extended to lower temperatures (T < 860 K), while exhibiting a steady increase in the rate of oxidation starting from 470 K, and eventually consuming all the initial hydrogen by 800 K. Absolute measurements of OH mole fraction reveal that at conditions well below the dominance of the thermal chain-branched chemistry (at T = 668 K), the plasma induced a chain-propagating effect on OH formation, which was entirely confined to the boundaries of the plasma discharge section of the reactor. Furthermore, temporal measurements were also performed, showing that the extent of OH formation and subsequently its global effect on the fuel consuming chemistry can be manipulated based on the plasma perturbation timescale (i.e., the time scale at which the high-voltage pulses are administered to the reactive flow to generate the plasma discharge). Experimental results are compared to modeling calculations and show relatively good agreement, with the model predicting similar kinetic trends as a function of temperature. These results demonstrate new insight into the kinetics governing plasma-assisted combustion, and provide new experimental data to facilitate the development and validation of PAC-specific kinetic mechanisms.

Recent developments in the study of very high energy non-equilibrium fluid flows are reviewed. These are flows of molecular gases which exhibit substantial degrees of mode disequilibrium, specifically high energy in molecular vibrational and electronic modes, and high electron energies when the gases are weakly ionized. In contrast, the modes of molecular translation and rotation remain at lower energies. Attention is focused on high density, collision-dominated gases. Studies in two systems are presented: A small wind tunnel where an M = 5 steady air flow over small models is produced, and a flowing carbon monoxide gas laser, exhibiting very high energy loading of the vibrational quantum states. The development of non-intrusive optical diagnostics to measure vibrational and electronic state populations and rotational/translational mode temperatures in the flows, with high spatial and temporal resolution, is presented. Kinetic modeling and experimental validation studies in these environments are also discussed.

Time- and space-resolved temperature and absolute concentrations of OH and H are measured by UV Rayleigh scattering, Laser-Induced Fluorescence (LIF), and Two-Photon Absorption LIF (TALIF), respectively, in an Ar–O 2 –H 2 (80:20:2) mixture at P = 40 torr and T 0 = 300 K. The mixture is excited by a 50-pulse burst from a repetitive nanosecond pulse discharge (NSPD) in a point-to-point geometry, operated at 100 kHz pulse repetition rate and 5 Hz burst repetition rate. One-dimensional radial distributions of temperature and species concentrations across the discharge filament, during and after the burst, are obtained from Rayleigh scattering and fluorescence images of the laser beam. Both temperature and species concentration profiles are found to expand with time, with peak centerline values of T peak ≈ 1200 K, [H] peak ≈ 6.0 ⋅ 10 15 cm −3 , and [OH] peak ≈ 1.0 · 10 15 cm −3 . Secondary maxima in OH distributions are detected near the periphery of the filament after a few tens of discharge pulses. Experimental results are compared with predictions of a plasma chemical kinetics model. The model reproduces trends in temporal evolution of radial distributions of temperature, as well as OH and H concentrations. Based on a rate of species production analysis, the secondary peaks in OH radial distributions are found to be caused by radial diffusion of H atoms from the central region of the discharge filament, with subsequent formation of HO 2 and OH via reactions H + O 2 + M → HO 2 + M and H + HO 2 → OH + OH in the low-temperature peripheral regions. The results demonstrate significant potential of the present approach for quantitative, time- and spatially-resolved studies of coupled radical reaction kinetics and diffusion over a wide range of temperatures, pressures, and mixture compositions.

Temperature and OH concentrations derived from OH laser-induced fluorescence (LIF) are known to be susceptible to effects such as collisional quenching, laser absorption, and fluorescence trapping. In this paper, a set of analytical and easy-to-implement methods is presented for treating these effects. The significance of these signal corrections on inferred temperature and absolute OH concentration is demonstrated in an atmospheric-pressure, near-stoichiometric CH4-air flame stabilized on a Hencken burner, for laser excitation of both the A2Σ+←X2Π (0,0) and (1,0) bands. It is found that the combined effect of laser attenuation and fluorescence trapping can cause considerable error in the OH number density and temperature if not accounted for, even with A–X(1,0) excitation. The validity of the assumptions used in signal correction (that the excited-state distribution is either thermalized or frozen) is examined using time-dependent modeling of the ro-vibronic states during and after laser excitation. These assumptions are shown to provide good bounding approximations for treating transition-dependent issues in OH LIF, especially for an unknown collisional environment, and it is noted that the proposed methods are generally applicable to LIF-based measurements.

OH Laser Induced Fluorescence (LIF) and picosecond (ps), broadband Coherent Anti-Stokes Raman Spectroscopy (CARS) are used for time-resolved temperature and time-resolved, absolute OH number density measurements in lean H 2 -air, CH 4 -air, C 2 H 4 -air, and C 3 H 8 -air mixtures in a nanosecond (ns) pulse discharge cell/plasma flow reactor. The premixed fuel–air flow in the reactor, initially at T 0 = 500 K and P = 100 torr, is excited by a repetitive ns pulse discharge in a plane-to-plane geometry (peak voltage 28 kV, discharge gap 10 mm, estimated pulse energy 1.25 mJ/pulse), operated in burst mode at 10 kHz pulse repetition rate. In most measurements, burst duration is limited to 50 pulses, to preclude plasma-assisted ignition. The discharge uniformity in air and fuel–air flows is verified using sub-ns-gated images (employing an intensified charge-coupled device camera). Temperatures measured at the end of the discharge burst are in the range of T = 550–600 K, using both OH LIF and CARS, and remain essentially unchanged for up to 10 ms after the burst. Time-resolved temperature measured by CARS during plasma-assisted ignition of H 2 -air is in good agreement with kinetic model predictions. Based on CARS measurement, vibrational nonequilibrium is not a significant factor at the present conditions. Time-resolved, absolute OH number density, measured after the discharge burst, demonstrates that OH concentration in C 2 H 4 -air, C 3 H 8 -air, and CH 4 is highest in lean mixtures. In H 2 -air, OH concentration is nearly independent of the equivalence ratio. In C 2 H 4 -air and C 3 H 8 -air, unlike in CH 4 -air and in H 2 -air, transient OH-concentration overshoot after the discharge is detected. In C 2 H 4 -air and C 3 H 8 -air, OH decays after the discharge on the time scale of ∼0.02–0.1 ms, suggesting little accumulation during the burst of pulses repeated at 10 kHz. In CH 4 -air and H 2 -air, OH concentration decays within ∼0.1–1.0 ms and 0.5–1.0 ms, respectively, showing that it may accumulate during the burst. The experimental results are compared with kinetic modeling calculations using plasma/fuel chemistry model employing several H 2 -air and hydrocarbon-air chemistry mechanisms. Kinetic mechanisms for H 2 -air, CH 4 -air, and C 2 H 4 -air developed by A. Konnov provide the best overall agreement with OH measurements. In C 3 H 8 -air, none of the hydrocarbon chemistry mechanisms agrees well with the data. The results show the need for development of an accurate, predictive low-temperature plasma chemistry/fuel chemistry kinetic model applicable to fuels C 3 and higher.

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.

Nitric oxide planar laser-induced fluorescence (NO PLIF) imaging is demonstrated at a 10-kHz repetition rate in the Calspan University at Buffalo Research Center’s (CUBRC) 48-inch Mach 9 hypervelocity shock tunnel using a pulse burst laser–based high frame rate imaging system. Sequences of up to ten images are obtained internal to a supersonic combustor model, located within the shock tunnel, during a single ~10-millisecond duration run of the ground test facility. Comparison with a CFD simulation shows good overall qualitative agreement in the jet penetration and spreading observed with an average of forty individual PLIF images obtained during several facility runs.

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.

In this manuscript, we describe the development of two-dimensional, high-repetition-rate (10-kHz) Rayleigh scattering imaging as applied to turbulent combustion environments. In particular, we report what we believe to be the first sets of high-speed planar Rayleigh scattering images in turbulent non-premixed flames, yielding temporally correlated image sequences of the instantaneous temperature field. Sample results are presented for the well-characterized DLR flames A and B (CH4/H2/N2) at Reynolds numbers of 15,200 and 22,800 at various axial positions downstream of the jet exit. The measurements are facilitated by the use of a user-calibrated, intensified, high-resolution CMOS camera in conjunction with a unique high-energy, high-repetition-rate pulse-burst laser system (PBLS) at Ohio State University, which yields output energies up to 200 mJ/pulse at 532 nm with 100-μs laser pulse spacing. The spatial and temporal resolution of the imaging system and acquired images are compared to the finest spatial and temporal scales expected within the turbulent flames. One of the most important features of the PBLS is the ability to readily change the pulse-to-pulse spacing as the required temporal resolution necessitates it. The quality and accuracy of the high-speed temperature imaging results are assessed by comparing derived statistics (mean and standard deviation) to that of previously reported point-based reference data acquired at Sandia National Laboratories and available within the TNF workshop. Good agreement between the two data sets is obtained providing an initial indication of quantitative nature of the planar, kHz-rate temperature imaging results.

Pure rotational CARS thermometry, complemented by UV emission measurements and ICCD imaging, is used to study kinetics of low temperature plasma assisted fuel oxidation and ignition in a repetitive nanosecond pulse discharge in hydrogen-air mixtures, with number of pulses in a 40 kHz burst varying from a few to a few hundred. Time-resolved OH emission, coupled with gated ICCD images of the plasma and the flame, demonstrate that volumetric ignition of H2–air mixtures occurs in a spatially uniform plasma. The results are shown to agree well with predictions of a new hydrogen-air plasma chemistry model, which incorporates non-equilibrium plasma processes, H2–air chemistry, non-empirical scaling of nanosecond pulse energy coupled to the plasma, and quasi-one-dimensional conduction heat transfer. In particular, the results demonstrate that the heating rate in low temperature hydrogen-air plasmas is much faster than in air plasmas, primarily due to energy release from exothermic reactions of fuel with O and H atoms generated in the plasma. Kinetic sensitivity analysis is used to identify dominant plasma and chemical processes of hydrogen oxidation, demonstrating that additional heat release in these reactions is a key factor in ignition kinetics. Kinetic modeling calculations demonstrate that removal of the radical generation processes by the nanosecond pulsed plasma from the model completely blocks subsequent exothermic chemical reactions, thus making ignition impossible.

We report on the development of high-repetition rate planar laser-induced fluorescence (PLIF) imaging of the CH radical in turbulent flames. This paper presents what the authors believe to be the first multi-frame, high-speed CH PLIF image sequences captured in a turbulent nonpremixed jet-flame. The high-repetition rate CH PLIF measurements were made using a combination of a custom pulse burst laser system operating at a 10-kHz repetition rate, an in-house optical parametric oscillator (OPO) with frequency mixing for 390-nm laser pulse generation, and a high-framing rate ICCD camera for high-speed image capture. The 1064-nm output of the pulse burst laser is frequency-tripled (355 nm) and used to pump the OPO, which can be operated in a narrow bandwidth (300 MHz) or broadband (4 cm−1) mode. The OPO system produces a burst of laser pulses near 615 nm, which can be subsequently mixed with residual 1064-nm output from the pulse burst laser to generate a series of 390-nm pulses separated by less than 100 μs. The tunable ultraviolet output around 390 nm is then used to excite the B–X (0, 0) band allowing high-speed image sequences of the CH radical to be acquired from the B–X(0, 1), A–X(1, 1), and A–X(0, 0) bands near 430 nm. In this paper, we present results using the narrow bandwidth mode of the laser. By injection seeding the OPO with a single-frequency diode laser, we can obtain spectrally-narrow output at 390-nm, which allows imaging of CH with only 0.4 mJ/pulse of laser energy. Ten-kilohertz-CH PLIF image sequences from a turbulent nonpremixed flame are reported as an example of the potential of this diagnostic system. Although, the signal-to-noise ratio (SNR) is marginal, we discuss future improvements to the system to increase the SNR to levels comparable to that achieved with traditional low-repetition rate systems.

In this manuscript, we demonstrate high-speed (10-kHz acquisition rate) 1D Raman scattering line imaging under turbulent flow conditions. Because of the inherently weak scattering process, Raman scattering measurements typically require high laser pulse energies that are not available from commercial high-repetition-rate laser systems. Using a custom pulse-burst laser system at OSU, we demonstrate the ability to generate a series of high-energy laser pulses with 100 μs pulse separations, which can be used for high-speed Raman scattering imaging. The ultimate goal is a capability of quantitatively measuring all major combustion species and deducing time-varying mixture fraction profiles in turbulent combustion environments. Toward this goal, we report initial results depicting the Raman scattering imaging of O2, N2, CH4, and H2 in a turbulent non-reacting CH4/H2 jet issuing into air. To the authors’ knowledge, this represents the first reported temporally-sequential 1D image sequences of major species measured via high-speed Raman scattering.

The paper presents results of plasma assisted combustion experiments in premixed hydrocarbon–air flows excited by a low-temperature transverse repetitively pulsed discharge plasma. The experiments have been conducted in methane–air and ethylene–air flows in a wide range of equivalence ratios, flow velocities, and pressures. The plasma was generated by a sequence of high-voltage (∼10 kV), short pulse duration (∼50 ns), high repetition rate (up to 50 kHz) pulses. The high reduced electric field during the pulse allows efficient electronic excitation and molecular dissociation. On the other hand, the extremely low duty cycle of the repetitively pulsed discharge, ∼1/500, greatly improves the discharge stability and helps sustaining diffuse and uniform nonequilibrium plasma. Generating this repetitively pulsed plasma in premixed hydrocarbon–air flows results in ignition and flameholding, occurring at low plasma temperatures, 140–300 °C, inferred from the nitrogen second positive band system spectra. At these conditions, the reacted fuel fraction, measured by the FTIR absorption spectroscopy, is up to 80%. The experiments demonstrate significant methane and ethylene conversion into CO, CO2, and H2O even at the conditions when there is no flame detected in the test section. At these conditions, fuel oxidation occurs due to plasma chemical reactions, without ignition. This provides additional evidence for the nonthermal fuel oxidation triggered by plasma-generated radicals.

We present new sets of V–V energy transfer rates for vibrational levels 0–5 in O2 at 300 K, using a stimulated Raman – spontaneous Raman pump/probe apparatus. Inferred rate coefficients, k 0 , 1 v , v - 1 , equal to 2.6 ± .5, 4.3 ± 1.1 × 10−13 cm3 s−1 for levels v = 2 and 3 are found to agree, within the combined quoted uncertainty, with the recent data of Kalogerakis. For v = 4 and 5, k 0 , 1 v , v - 1 is found to equal 7.1 × 10−13 cm3 s−1 and 2.0 × 10−13 cm3 s−1±, respectively. In agreement with earlier observations by Diskin, it is found that previous semi-classical trajectory calculations underestimate O2 V–V kinetics by approximately one order of magnitude. New calculations, invoking the same semi-classical formalism, require an increase in the attractive part of the intermolecular potential by a factor of approximately 1.75, in order to replicate the experimental data, a result which is considered non-physical. The purely repulsive Forced Harmonic Oscillator – Free Rotator model of Adamovich is found to under predict the absolute rates by a factor of approximately eight, but to predict the relative rates with reasonable accuracy.

This paper presents results of singlet oxygen generation experiments in a high-pressure, non-self-sustained crossed discharge. The discharge consists of a high-voltage, short pulse duration, high repetition rate pulsed discharge, which produces ionization in the flow, and a low-voltage dc discharge which sustains current in a decaying plasma between the pulses. The sustainer voltage can be independently varied to maximize the energy input into electron impact excitation of singlet delta oxygen (SDO). The results demonstrate operation of a stable and diffuse crossed discharge in O2–He mixtures at static pressures of at least up to P0 = 380 Torr and sustainer discharge powers of at least up to 1200 W, achieved at P0 = 120 Torr. The reduced electric field in the positive column of the sustainer discharge varies from E/N = 0.3 × 10 −16 to 0.65 × 10 −16 Vc m 2 , which is significantly lower than E/N in self-sustained discharges and close to the theoretically predicted optimum value for O2(a 1 �) excitation. Measurements of visible emission spectra O2(b 1 � → X 3 �) in the discharge afterglow show the O2(b 1 �) concentration to increase with the sustainer discharge power and to decrease as the O2 fraction in the flow is increased. Rotational temperatures inferred from these spectra in 10% O2–90% He flows at P0 = 120 Torr and mass flow rates of ˙ m = 0.73–2.2 g s −1 are 365–465 K. SDO yield at these conditions, 1.7% to 4.4%, was inferred from the integrated intensity of the (0,0) band of the O2(a 1 � → X 3 �) infrared emission spectra calibrated using a blackbody source. The yield remains nearly constant in the discharge afterglow, up to at least 15 cm distance from the discharge. Kinetic modelling calculations using a quasi-one-dimensional nonequilibrium pulser–sustainer discharge model coupled with the Boltzmann equation for plasma electrons predict gas temperature rise in the discharge in satisfactory agreement with the experimental measurements. However, the model overpredicts the O2(a 1 �) yield by a factor of 2–2.5, which suggests that the model’s description of nonequilibrium O2–He plasma kinetics at high pressures is not quite adequate. (Some figures in this article are in colour only in the electronic version)

Stimulated Raman scattering is used to vibrationally excite pure N2 in the pressure range 300–760 Torr at room temperature. The subsequent temporal evolution of the populations of vibrational levels as high as v=6 is probed by spontaneous Raman scattering. The experimental results are compared to predictions from a Master Equation kinetics modeling code, incorporating V–V energy transfer rate coefficients derived from several published theoretical models. Predictions using the rate coefficients given by the three-dimensional Forced Harmonic Oscillator Free Rotator (FHO-FR) model are found to give the best overall agreement with the experimental data, although agreement with the three-dimensional models of Billing and Fisher, and Bogdanov et al. is also found to be quite good.

We present results of a combined experimental computational study of free jet flow produced by a 1 mm (height)×5 mm (span) nominally Mach 2 supersonic jet. Two-dimensional maps of ux, the component of velocity parallel to the principal flow axis, are obtained experimentally, by acetone molecular tagging velocimetry (MTV), and computationally, by the direct simulation Monte Carlo (DSMC) method, at a stagnation pressure and temperature of 10 torr and 300 K, respectively. In all cases, direct comparison of the experimental data and the predictions from DSMC showed excellent agreement, with only minor deviations which, in most cases, can be ascribed to either the inherent uncertainty in the MTV or small uncertainties in the measured wall pressures.

A newly developed MHz rate imaging system that provides real-time flow visualization is described. The technique utilizes a custom-built Nd:YAG pulse burst laser and an ultra high-speed digital camera and is capable of capturing 17 images over 150 microseconds. The system was used to visualize a Mach 1.3 (M c =0.6) axisymmetric jet. Sample results indicate the potential of the technique to provide detailed information on the dynamic characteristics of large-scale structures. A two-dimensional cross-correlation technique was used to calculate the convective velocity of large-scale structures. Present results generally agree with the findings of earlier investigations that indicate a significant deviation of the convective velocity from theoretical predictions.

Single-walled carbon nanotubes (SWNTs) are synthesized in a gas-phase non-equilibrium plasma process.The carbon producing CO disproportionation reaction is driven very efficiently in a flow reactor, in which extreme disequilibrium between the vibrational and translational mode of the carbon monoxide gas is maintained even at low translational temperatures by using a powerful and efficient carbon monoxide gas laser.In the presence of metal catalysts, the vibrationally excited CO reacts to form CO2 and structured carbon molecules, notably SWNTs.The individual tubes form ropes or flat ribbons and these are aligned parallel to each other into larger structures of SWNT material without any post-synthesis treatment. 2002 Published by Elsevier Science B.V.

This article presents an experimental demonstration of a high-pressure unconditionally stable nonequilibrium molecular plasma sustained by a combination of a continuous wave CO laser and a sub-breakdown radio frequency (rf) electric field. The plasma is sustained in a CO/N2 mixture containing trace amounts of NO or O2 at pressures of P=0.4–1.2 atm. The initial ionization of the gases is produced by an associative ionization mechanism in collisions of two CO molecules excited to high vibrational levels by resonance absorption of the CO laser radiation with subsequent vibration-vibration (V-V) pumping. Further vibrational excitation of both CO and N2 is produced by free electrons heated by the applied rf field, which in turn produces additional ionization of these species by the associative ionization mechanism. In the present experiments, the reduced electric field, E/N, is sufficiently low to preclude field-induced electron impact ionization. Unconditional stability of the resultant cold molecular plasma ...

The use of caged dye photo-activated fluorophore velocimetry is described and representative examples are presented. After a brief survey of recently reported measurements, a more detailed example of the flow produced in a cylinder with a single rotating end wall is presented. Simultaneous stereoscopic image sets in the (r,z) and (r,θ) planes have been obtained over a Reynolds number range of roughly 102-105. At low Reynolds numbers (0-2000), the steady, axisymmetrical flow is found to quantitatively agree with predictions from a numerical flow solver. At higher Reynolds numbers (from 5×103 to 105), the flow develops considerable turbulent three-dimensional structure.

We present experimental results using Filtered Rayleigh Scattering to make planar measurements of velocity, temperature and pressure in ambient air and in a Mach 2 free jet. The ambient air measurements are used to identify and calibrate experimental uncertainties. The Mach 2 measurements demonstrate the usefulness of the technique for making accurate planar measurements in a high speed flow. The measured values for velocity, temperature and pressure in the Mach 2 jet ranged, through a shock system, from 205 to 235 m/s, 150 to 170 K and 700 to 1000 torr, with estimated uncertainties of ±5.4 m/s, ±3.2 K and ±38 torr (±2 to 3%, ±2% and ±4–5%, respectively).

Absorption spectra from a mixture of 320 ppm CO2 in synthetic air (79% N2, 21% O2) were collected in the region from 3500 cm−1 to 4000 cm−1 under conditions in the range of 100–1000 atm and 295–900 K. At 295 K, both bands of the (1001), (0201) Fermi dyad show the collapse of P and R branches into a single nearly Lorentzian spectral feature as a result of interbranch line-mixing. At elevated temperatures, the presence of interbranch mixing is also clearly evident as is the presence of several hot bands. The experimental data are modeled using two methods for simulating line-mixed spectra; first, the usual line-by-line approach which relies on the binary impact approximation, and second, a simple band-averaged model proposed by Hartmann and L’Haridon [J. Chem. Phys. 103, 6467 (1995)]. The energy corrected sudden (ECS) approximation is used to generate the relaxation matrix in the first approach. Comparison with the measurement shows that the ECS method does not fit the high density data satisfactorily when ...

In many cases, the widely used matrix inversion approach to describe the spectral interference in collisionally perturbed molecular spectra is not feasible if the particular molecular interactions do not allow the sudden impact approximation (infinitely short collision duration). To overcome this problem, we present a time domain model that describes collisional broadening and narrowing phenomena without requiring the sudden approximation. The key element of the model is a Monte Carlo type sampling process to quantify the temporal autocorrelation of the molecular dipole moment. The spectrum is then obtained numerically via fast Fourier transform. The model does not require a frequency-dependent relaxation operator; the finite collision duration is simply an adjustable parameter in the time domain process. Our approach, which is generally applicable to any set of transition lines, is derived from concepts of both conventional quantum-mechanical and semiclassical theory of line interference. Coherent transf...

▪ Abstract The various tools for flow visualization have been significantly expanded over the past several years through the use of molecular scattering and molecular laser-induced fluorescence. These approaches have added the capability of sampling individual small volume elements within a flow, and by using cameras for detection, they are easily extended to sample lines and cross-sectional planes. This localized measurement capability means that these approaches can be made quantitative even in complex and/or unsteady flow fields. If the molecular species is naturally occurring, such as oxygen or nitrogen in air, then no seeding is required. Furthermore, in these applications, images of the flow can be frozen in time by using a short pulse laser for illumination. The distribution of the molecules reflects the true physics of the flow, so even raw images taken in this manner give an immediate understanding of flow field properties. With proper calibration, the images can be further analyzed to yield quantitative information about the flow. In the case of flow tagging, the analysis gives velocity profiles when lines are written, and deformation, vorticity, and dilation with grid patterns. Molecular scattering can be used to give quantitative values of density, temperature, and two-dimensional velocity. This paper presents three such molecular-based approaches: laser-induced fluorescence from oxygen, flow tagging by oxygen excitation, and Rayleigh scattering. These three approaches are chosen because all three can be used in naturally occurring air with no seeding. The raw data from each of these approaches gives an immediate appreciation of the flow structure and further analysis yields accurate values of velocity, temperature, and density. These approaches use readily available laser sources; however, they will be greatly enhanced with new source technologies that are currently under development.

Instantaneous cross-sectional images of turbulent air flows with densities on the order of one atmosphere or less can be obtained in a straightforward manner using far ultraviolet Rayleigh scattering. These images give quantitative values for the air density and show the details of turbulent structure, shock structure, and shock wave/boundary layer interactions. Two-dimensional spatial correlations taken from multiple images give the shape and extent of average turbulent structure as well as the coupling between turbulent structure and other flow features. This technique may be extended to observe velocity fields by either double pulsing the illumination source or by using a narrow linewidth atomic or molecular filter window in front of the detector array. The latter approach also yields temperature. Used in conjunction with flow marking techniques such as RELIEF, coupling between turbulent structure and velocity fluctuations can also be determined. These diagnostic techniques can be extended to combusting flows to observe instantaneous structure, mixing, flame front location, and velocity fields.

This Letter presents results from a new master-oscillator, power-amplifier pulse-burst laser system demonstrating ultrahigh pulse energies greater than 2.0 J/pulse at 1064 nm with interpulse separations of 100 μs (10 kHz) for burst durations of 100 pulses. Each pulse generates peak powers exceeding 130 MW and an average power of approximately 20 kW is generated over a 100-pulse-burst. Pulse energies decrease by less than 10% over a 100 sequential pulses, demonstrating negligible “droop” over long-duration pulse trains. Second-harmonic generation of 532 nm with conversion efficiency greater than 50% is demonstrated for 100-pulse-burst durations.

We present the results of a diagnostic study of the use of coherent four wave mixing for in situ measurement of an electric field in air or in nitrogen-containing plasmas. Static electric fields in air at a nominal pressure of 625 Torr and temperature of 300 K are detected using vibrational CARS of nitrogen. It is shown that the ratio of the infrared signal to the vibrational N(2) CARS signal is equal to approximately 10(-8) at 8.33 kV/cm, a factor of approximately 50 less than that predicted assuming equal third-order nonlinear susceptibilities. It is also shown that the spatial resolution of a typical collinear geometry measurement is approximately 1 cm. Finally, it is shown that achieving sensitivities of the order of 1 kV/cm requires that the coherent Raman pumping be performed in the highly saturated and Stark broadened regime.

Pure rotational CARS thermometry is used to study low-temperature plasma assisted fuel oxidation kinetics in a repetitive nanosecond pulse discharge in ethene–air at stoichiometric and fuel lean conditions at 40 Torr pressure. Air and fuel–air mixtures are excited by a burst of high-voltage nanosecond pulses (peak voltage, 20 kV; pulse duration, ~ 25 ns) at a 40 kHz pulse repetition rate and a burst repetition rate of 10 Hz. The number of pulses in the burst is varied from a few pulses to a few hundred pulses. The results are compared with the previously developed hydrocarbon–air plasma chemistry model, modified to incorporate non-empirical scaling of the nanosecond discharge pulse energy coupled to the plasma with number density, as well as one-dimensional conduction heat transfer. Experimental time-resolved temperature, determined as a function of the number of pulses in the burst, is found to agree well with the model predictions. The results demonstrate that the heating rate in fuel–air plasmas is much faster compared with air plasmas, primarily due to energy release in exothermic reactions of fuel with O atoms generated by the plasma. It is found that the initial heating rate in fuel–air plasmas is controlled by the rate of radical (primarily O atoms) generation and is nearly independent of the equivalence ratio. At long burst durations, the heating rate in lean fuel air–mixtures is significantly reduced when all fuel is oxidized.

Experiments were conducted to measure convective velocity of large-scale structures in the shear layer of a Mach 2.0 free jet with a convective Mach number of 0.87 and Reynolds number based on a nozzle diameter of 2.6×106. Real-time flow visualization and planar Doppler velocimetry (PDV) measurements along with space-time correlation were used. The results reveal that regardless of the measurement technique employed, whether qualitative flow visualization or quantitative PDV, the method of seeding can have a significant effect on the ability to accurately identify, track, and measure convective velocity of large-scale structures. This is apparent by the relatively large spread in convective velocity results obtained in the same jet by using various methods of seeding and qualitative and quantitative visualization. Convective velocity measurements made under the best seeding conditions available with PDV indicate a convective velocity of structures that is close to the value predicted by theory up to the c...

This letter discusses the effect of nitric oxide on gain and output power of a pulser-sustainer discharge excited oxygen iodine laser. Adding small amounts of NO to the laser mixture (a few hundreds of ppm) considerably increases gain and output power due to (i) O atom titration and resultant slower I* atom quenching and (ii) improved stability of the dc sustainer discharge, which allows stable operation at significantly higher discharge powers. Gain on the 1315nm iodine atom transition and laser power in the M=3 transverse laser cavity are 0.049%∕cm and 1.24W, at a flow temperature of T=100K.

A series of time resolved microwave attenuation measurements are performed of the electron number density of an electron beam generated, CO laser excited nonequilibrium O2∕N2 plasma. Resonant absorption of infrared radiation from the CO laser produces the nonequilibrium state, in which the heavy species vibrational modes are disproportionately excited, compared to the rotational and translational modes (Tvib≈2000–3000K vs TR∕T≈300K). It is shown that this results in an increase in the plasma free electron lifetime by two orders of magnitude compared to the unexcited cold gas, an effect which is ascribed to complete mitigation of rapid three-body electron attachment to molecular oxygen. A series of heavy species filtered pure rotational Raman scattering measurements are also presented, which exhibit minimal temperature change (+50K), indicating that the observed lifetime increase cannot be due to heavy-species thermal effects. Finally, computational modeling results infer an increase in the rate of O2− det...

This letter discusses operation of an electric discharge excited oxygen-iodine laser using a high-pressure, non-self-sustained pulser-sustainer discharge. Small signal gain on the 1315 nm iodine atom transition and the laser output power are measured in the M = 3 supersonic cavity downstream of the discharge section. In a 15% O 2 – 85 % He mixture, at a discharge pressure of 60 torr and discharge power of 1.5 kW , the highest gain measured in the M = 3 cavity is 0.022 % ∕ cm , at the flow temperature of T = 100 ± 10 K . At these conditions, the laser output power is 0.28 W .

This paper presents results of cold magnetohydrodynamic (MHD) flow deceleration experiments using repetitively pulsed, short pulse duration, high voltage discharge to produce ionization in M=3 nitrogen and air flows in the presence of transverse direct current electric field and transverse magnetic field. MHD effect on the flow is detected from the flow static pressure measurements. Retarding Lorentz force applied to the flow produces a static pressure increase of up to 17%–20%, while accelerating force of the same magnitude results in static pressure increase of up to 5%–7%. The measured static pressure changes are compared with modeling calculations using quasi-one-dimensional MHD flow equations. Comparison of the experimental results with the modeling calculations shows that the retarding Lorentz force increases the static pressure rise produced by Joule heating of the flow, while the accelerating Lorentz force reduces the pressure rise. The effect is produced for two possible combinations of the magne...

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short-pulse duration, high-voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization results show that as the Reynolds number increases, the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using laser differential interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0∼200–250Torr. A crossed discharge (pulser+dc sustainer) in M=3 flows of air and nitrogen produced a stable, diffuse, and uniform plasma, with the time-average dc current up to 1.0A in nitrogen and up to 0.8A in air. The electrical conductivity and the Hall parameter in these f...

A real-time flow visualization system that produces 17 images over a time span of 150 μs is used to visualize the mixing layers of Mach 1.3 (Mc=0.59) and Mach 2.0 (Mc=0.87) ideally expanded high Reynolds number axisymmetric jets. These image sequences reveal details about the influence of compressibility on the dynamics of turbulence structures. In general, the behavior observed in axisymmetric jets is similar to that observed in planar shear layers at similar convective Mach numbers. In addition, large streamwise vortices are apparent in cross-stream images of the flow. Large-scale structures become more three-dimensional and less organized with increasing compressibility and more difficult to identify and track. Planar space–time correlations are used to track structures as they convect downstream. The histogram of the convective velocity for the Mach 1.3 jet revealed a broad distribution of convective velocities with an ensemble average of 266 m/s, which is much higher than the theoretical prediction of 206 m/s. The Mach 2.0 jet, however, exhibited a bimodal convective velocity distribution with an ensemble average of 402 m/s for the “fast” and 190 m/s for the “slow” mode. These modes are equally spaced from the theoretical convective velocity of 303 m/s. Approximately 2/3 of the measured velocities were in the slow mode.

High resolution depolarized Rayleigh spectra of nitrobenzene in CCl4 have been measured at several concentrations and temperatures. All spectra display a shear wave dip at the zero frequency. They are found to fit well to the form of a two‐variable theory in the low frequency regions studied. The various parameters such as the collective molecular reorientation rate Γ, the shear wave relaxation rate q2νs, and the dynamic coupling coefficient R are obtained as a function of temperature and concentration. While the R value is found to be independent of temperature, it changes significantly with the nitrobenzene concentration. Moreover, the R value for pure nitrobenzene is found to be greater than 0.5, higher than any reported previously for a low viscosity liquid. The τRay values are found to follow the classical Stokes–Einstein relation, with zero intercept. The pair orientational correlation affects the Rayleigh relaxation time.

An extensive Rayleigh–Brillouin scattering study of polystyrene and cyclohexane mixtures at various temperatures and polymer concentrations has been carried out. The experimental results have been analyzed in terms of the equations of generalized hydrodynamics developed recently by Lin and Wang. Over the entire concentration range, the temperature dependence of the Brillouin frequency and linewidth data can be satisfactorily interpreted in terms of the equations in the recent theory. The pronounced effects of the Brillouin data have been related to a quantity, referred to as the effective structural relaxation time τs for the mixture. The use of the single relaxation time model is justified for Brillouin scattering. It has been found that τs is not significantly affected by the presense of polymer molecules at low polymer concentration, but it increases rapidly with increasing polymer concentration at high concentration. At low polymer concentration, the relaxation frequency τs−1 is fast compared with the...