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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...

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.

Ignition delay time, time-resolved temperature, and time-resolved absolute OH concentration are measured in mildly preheated H2–air mixtures (T = 473–500 K), excited by a repetitive nanosecond pulse discharge in a plasma flow reactor. The measurements are done in decaying plasma after the discharge pulse burst (pulse repetition rate 10–40 kHz). Ignition delay increases steeply as the number of pulses in the burst is reduced, exhibiting threshold-like behavior. Temperature and OH concentration during the ignition process are measured by two-line OH LIF, as functions of time delay after the discharge burst. Absolute calibration is done using a near-adiabatic flame. The results show that the temperature at the end of the discharge burst is 700–750 K, rising to 1500–1600 K within a few ms after the burst, indicative of ignition. During ignition, OH concentration increases by a factor of 20–50, from (0.5–1.1) � 10 14 cm � 3 at the end of the burst to peak value of (2.3–2.6) � 10 15 cm � 3 . Kinetic modeling calculations show good agreement with the experimental results, demonstrating quantitative insight into kinetics of plasma-assisted ignition. Modeling calculations demonstrate that ignition is induced primarily by accumulation of H atoms and gradual temperature rise in the discharge, at temperatures significantly lower than autoignition temperature, by up to 200 K.

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.

In this manuscript, we demonstrate high-speed (10-kHz-acquisition rate) planar laser-induced fluorescence (PLIF) imaging of formaldehyde (CH2O) in turbulent non-premixed flames. Using the unique pulse-burst laser system (PBLS) at Ohio State University, high-energy laser pulses (∼100 mJ/pulse) at 355 nm with 100 μs pulse separation are generated and used to measure the time-varying CH2O distributions in attached and lifted methane-based turbulent flames. By taking advantage of the tunable, narrow spectral linewidth of the PBLS at 355 nm, the laser output can be frequency-tuned and adjusted to overlap with absorption “peaks” within the tail of the A–X transition of CH2O near 355 nm, thus increasing the acquired signal by as much as a factor of three. The reported signal-to-noise ratio (SNR) exceeds 55, which represents one of the highest SNR reported to date for kilohertz-rate imaging of scalars for comparable spatial resolution. Potential applications and pairings with other diagnostic approaches for high-speed reaction rate and multi-scalar imaging also are discussed.

Absolute hydroxyl radical (OH) concentration is determined in stoichiometric hydrogen-air mixtures at P = 54-94 torr and initial temperature of T = 100°C-200°C, which are both functions of time, after the application of a single approximately 25-ns-duration approximately 20-kV discharge pulse and 60 μs after the final pulse of a variable-length burst of pulses, using single-photon laser-induced fluorescence (LIF). Relative LIF signal levels are put on an absolute number density scale by means of calibration with a standard atmospheric-pressure near-adiabatic Hencken flat-flame burner. By obtaining OH LIF data in both the plasma and the flame and correcting for differences in the collisional quenching and vibrational energy transfer rates, absolute OH number density has been determined. For a single discharge pulse, the absolute OH temporal profile is found to rise rapidly during the initial ~0.1 ms after discharge initiation and decay relatively slowly, with a characteristic time scale of ~1 ms. In repetitive burst mode, the absolute OH number density is observed to rise rapidly during the first approximately ten pulses (0.25 ms) and then level off to a near steady-state plateau. In all cases, a large secondary rise in OH number density is also observed, which is clearly indicative of ignition, with ignition time ranging from 5 to 10 ms, for initial temperatures of 100 °C and 200°C and pressures in the range of 54-94 torr. Plasma kinetic modeling predictions capture this trend quantitatively, using both a full 22-hydrogen-air-chemical-reaction set and a reduced 9-reaction set.

In this study, we describe the development of two-dimensional, high repetition-rate (10-kHz) Rayleigh scattering imaging as applied to turbulent flows. In particular, we report what we believe to be the first sets of high-speed 2D Rayleigh scattering images in turbulent non-reacting jets, yielding temporally correlated image sequences of the instantaneous mixture fraction field. Results are presented for turbulent jets of propane issuing into a low-speed co-flow of air at jet-exit Reynolds numbers of 10,000, 15,000, and 30,000 at various axial positions downstream of the jet exit. The quantitative high-speed mixture fraction measurements are facilitated by the use of a calibrated, un-intensified, high-resolution CMOS camera in conjunction with a unique high-energy, high-repetition rate pulse-burst laser system (PBLS) at Ohio State, which yields output energies of ∼200 mJ/pulse at 532 nm with 100-μs laser pulse spacing. The quality, accuracy, and resolution of the imaging system and the resulting image sets are assessed by (1) comparing the mean mixture fraction results to known scaling laws for turbulent jets, (2) comparing instantaneous images/mixture fraction profiles acquired simultaneously with the high-speed CMOS camera and a well-characterized, high-quantum efficiency CCD camera, and (3) comparing statistical quantities such as the probability density function of the mixture fraction results using the high-speed CMOS camera and the CCD camera. Results indicate accurate mixture fraction measurements and a high potential for accurately measuring mixture fraction gradients in both time and space.

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.

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.

The paper presents effect of vibrational relaxation between a vibrationally excited supersonic flow and jet stream of a rapid vibration-translation (V T ) relaxer. For the discharge excited supersonic flow of nitrogen up to TV =2000 K, mixing of CO2 downstream of the expansion corner results in the decrease of expansion angle by up to ∆θ = 8◦, as a consequence of heat release during the vibrational relaxation and the resultant pressure rise by up to 55%. Single / dual pump Coherent Anti-Stokes Raman Scattering measurements demonstrated the decrease of the vibrational temperature across the mixing layer, which is accompanied with the increase of local CO2 mixture ratio. A two-dimensional vibrational temperature distribution coincides with the spatial profile of the modified shear layer measured in the schlieren visualization. The region of the vibrational relaxation is also consistent with the layer of the V T relaxer penetration into the supersonic flow downstream of the trailing shocks, with the subsequent flow residence on the corresponding time scale of the V T relaxation.

Formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) and high-speed CH* chemiluminescence have been performed in a premixed, ethylene-fueled dual-mode scramjet combustor in order to study the dynamic flame behavior and structure. CH2O PLIF is used as a “flame” marker to provide information on the structure and position of the lower-temperature portion of the primary reaction zone. Low-repetition-rate (10 Hz) measurements were taken at several cross-flow planes along the axial flow direction. Statistics, including the mean of the CH2O topology, indicate sparse concentrations of formaldehyde that are intermittently detected by the PLIF measurement. Comparisons between previous Large Eddy Simulations (LES) and the current experimental measurements indicate descrepancies between the expected and measured formaldehyde concentrations. Time-resolved CH* chemiluminescence was collected at 50 kHz in order to study flame motion and stabilization in the axial flow direction. From these 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. Measurements of lean blowout (LBO) events were also acquired revealing the flame dynamics at the moment of blowout.

We present a new set of V–V rate coefficients for vibrational levels 0–5 in H2 at 300 K, measured using a stimulated Raman–spontaneous Raman pump/probe apparatus. The measured rate of the non-resonant process, H2(v = 1) + H2(v = 1) → H2(v = 0) + H2(v = 2), is consistent with the previously reported experimental value of Kreutz et al. However, semi-classical predictions of such non-resonant processes, using the identical inter-molecular potential and methodology to that given by Cacciatore and Billing, results in rates which are too slow, by a factor of approximately 3. For the “resonant” V–V process, H2(v = 1) + H2(v = 0) → H2(v = 0) + H2(v = 1), the semi-classical rate is found to be too slow by an even larger factor, of approximately 30, compared to the experimental rate, but consistent with the previously reported experimental result of Farrow and Chandler. Further, unlike the semi-classical model prediction in which the (1, 1 → 2, 0) process rate is predicted to exceed that of the (1, 0 → 0, 1) process, the experimental data shows it to be a factor of approximately 2.5 less, suggesting that semi-classical methods that treat the rotational motion classically are unsuitable for the highly anharmonic H2 molecule. The ratio of pure rotation and rotation–vibration Raman cross sections for scattering from levels 0 and 1 is also determined, with results which agree with calculations of Schwartz and LeRoy, but are somewhat larger than previous experimental results of Cureton.

The paper presents results of a high-pressure, non-self-sustained crossed discharge–M = 3 supersonic laser cavity operation. A stable and diffuse pulser–sustainer discharge in O2–He flows is generated at pressures of up to P0 = 120 Torr and discharge powers of up to 2.1 kW. The reduced electric field in the dc sustainer discharge ranges from 0.6 × 10−16 to 1.2 × 10−16 V cm2. Singlet delta oxygen (SDO) yield in the discharge, up to 5.0–5.7% at the flow temperatures of 400-420 K, was inferred from the integrated intensity of the (0, 0) band of the O2(a 1Δ → X 3Σ) infrared emission spectra calibrated using a blackbody source. The yield increases with the discharge power and remains nearly independent of the O2 fraction in the mixture (in the 10–20% range). Static pressure and temperature measurements in the supersonic cavity show that a steady-state M = 3 flow in the cavity can be sustained for up to 20 s, at the flow temperature of T = 120 ± 15 K. The results suggest that the measured SDO yield exceeds the threshold yield at the cavity temperature by up to a factor of 2.5. PLIF iodine vapour visualization in the supersonic cavity, which showed the presence of large-scale structures, suggests the need to improve iodine vapour mixing with the main oxygen–helium flow.

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.

This work describes the kinetic mechanism of coupled molecular energy transfer and chemical reactions in low-temperature air, H 2 –air and hydrocarbon–air plasmas sustained by nanosecond pulse discharges (single-pulse or repetitive pulse burst). The model incorporates electron impact processes, state-specific N 2 vibrational energy transfer, reactions of excited electronic species of N 2 , O 2 , N and O, and ‘conventional’ chemical reactions (Konnov mechanism). Effects of diffusion and conduction heat transfer, energy coupled to the cathode layer and gasdynamic compression/expansion are incorporated as quasi-zero-dimensional corrections. The model is exercised using a combination of freeware (Bolsig+) and commercial software (ChemKin-Pro). The model predictions are validated using time-resolved measurements of temperature and N 2 vibrational level populations in nanosecond pulse discharges in air in plane-to-plane and sphere-to-sphere geometry; temperature and OH number density after nanosecond pulse burst discharges in lean H 2 –air, CH 4 –air and C 2 H 4 –air mixtures; and temperature after the nanosecond pulse discharge burst during plasma-assisted ignition of lean H 2 -mixtures, showing good agreement with the data. The model predictions for OH number density in lean C 3 H 8 –air mixtures differ from the experimental results, over-predicting its absolute value and failing to predict transient OH rise and decay after the discharge burst. The agreement with the data for C 3 H 8 –air is improved considerably if a different conventional hydrocarbon chemistry reaction set (LLNL methane– n -butane flame mechanism) is used. The results of mechanism validation demonstrate its applicability for analysis of plasma chemical oxidation and ignition of low-temperature H 2 –air, CH 4 –air and C 2 H 4 –air mixtures using nanosecond pulse discharges. Kinetic modelling of low-temperature plasma excited propane–air mixtures demonstrates the need for development of a more accurate ‘conventional’ chemistry mechanism.

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)

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 �

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.

Time-resolved, absolute NO and N atom number densities are measured by NO Laser Induced Fluorescence (LIF) and N Two-photon Absorption LIF in a diffuse plasma filament, nanosecond pulse discharge in dry air, hydrogen-air, and ethylene-air mixtures at 40 Torr, over a wide range of equivalence ratios. The results are compared with kinetic modeling calculations incorporating pulsed discharge dynamics, kinetics of vibrationally and electronically excited states of nitrogen, plasma chemical reactions, and radial transport. The results show that in air afterglow, NO decay occurs primarily by the reaction with N atoms, NO + N → N2 + O. In the presence of hydrogen, this reaction is mitigated by reaction of N atoms with OH, N + OH → NO + H, resulting in significant reduction of N atom number density in the afterglow, additional NO production, and considerably higher NO number densities. In fuel-lean ethylene-air mixtures, a similar trend (i.e. N atom concentration reduction and NO number density increase) is observed, although [NO] increase on ms time scale is not as pronounced as in H2-air mixtures. In nearstoichiometric and fuel-lean ethylene-air mixtures, when N atom number density was below detection limit, NO concentration was measured to be lower than in air plasma. The results show the need for further kinetic modeling to provide quantitative insight into NO kinetics in hydrocarbon-air plasmas.

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.

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.

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

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.

The paper presents the results of temporally and spatially resolved electric field measurements in a nanosecond pulse discharge in atmospheric air, sustained between a razor edge high-voltage electrode and a plane grounded electrode covered by a thin dielectric plate. The electric field is measured by picosecond four-wave mixing in a collinear phase-matching geometry, with time resolution of approximately 2 ns, using an absolute calibration provided by measurements of a known electrostatic electric field. The results demonstrate electric field offset on the discharge center plane before the discharge pulse due to surface charge accumulation on the dielectric from the weaker, opposite polarity pre-pulse. During the discharge pulse, the electric field follows the applied voltage until 'forward' breakdown occurs, after which the field in the plasma is significantly reduced due to charge separation. When the applied voltage is reduced, the field in the plasma reverses direction and increases again, until the weak 'reverse' breakdown occurs, producing a secondary transient reduction in the electric field. After the pulse, the field is gradually reduced on a microsecond time scale, likely due to residual surface charge neutralization by transport of opposite polarity charges from the plasma. Spatially resolved electric field measurements show that the discharge develops as a surface ionization wave. Significant surface charge accumulation on the dielectric surface is detected near the end of the discharge pulse. Spatially resolved measurements of electric field vector components demonstrate that the vertical electric field in the surface ionization wave peaks ahead of the horizontal electric field. Behind the wave, the vertical field remains low, near the detection limit, while the horizontal field is gradually reduced to near the detection limit at the discharge center plane. These results are consistent with time-resolved measurements of electric field components, which also indicate that vertical electric field reverses direction after the ionization wave.