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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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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) more...

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. more...

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. more...

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. more...

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 ... more...

Spontaneous Raman scattering is used to experimentally determine the vibrational distribution functions of diatomic species in N2/CO and N2/CO/O2 gas mixtures optically pumped by a CO laser in the pressure range 410–760 torr. In N2/CO mixtures, as many as 38 vibrational levels of CO are observed, in addition to six levels of N2. The CO vibrational distribution function is highly non-Boltzmann, exhibiting the well-known Treanor plateau. In N2/CO/O2 mixtures, up to 13 vibrational levels of O2 are observed, which also exhibit a highly non-Boltzmann distribution. Experimental data are compared to predictions of a master equation kinetic model, which incorporates absorption of the laser radiation, species, and quantum state-specific vibration–vibration and vibration–translation energy exchange, as well as diffusion of vibrationally excited species out of the laser-excited volume. It is shown for the first time that modest power continuous wave lasers can be used to establish highly excited steady-state vibrati... more...

CO/N 2 , CO/Ar/O 2 , and CO/N 2 /O 2 gas mixtures are optically pumped using a continuous wave CO laser. Carbon monoxide molecules absorb the laser radiation and transfer energy to nitrogen and oxygen by vibration–vibration energy exchange. Infrared emission and spontaneous Raman spectroscopy are used for diagnostics of optically pumped gases. The experiments demonstrate that strong vibrational disequilibrium can be sustained in diatomic gas mixtures at pressures up to 1 atm, with only a few Watts laser power available. At these conditions, measured first level vibrational temperatures of diatomic species are in the range T V =1900–2300 K for N 2 , T V =2600–3800 K for CO, and T V =2200–2800 K for O 2 . The translational–rotational temperature of the gases does not exceed T =700 K. Line-of-sight averaged CO vibrational level populations up to v =40 are inferred from infrared emission spectra. Vibrational level populations of CO ( v =0–8), N 2 ( v =0–4), and O 2 ( v =0–8) near the axis of the focused CO laser beam are inferred from the Raman spectra of these species. The results demonstrate a possibility of sustaining stable nonequilibrium plasmas in atmospheric pressure air seeded with a few percent of carbon monoxide. The obtained experimental data are compared with modeling calculations that incorporate both major processes of molecular energy transfer and diffusion of vibrationally excited species across the spatially nonuniform excitation region, showing reasonably good agreement. more...

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. more...

Laser-induced fluorescence measurements (LIF) of nitric oxide and two photon absorption laser-induced fluorescence (TALIF) of oxygen and nitrogen atoms are performed in a diffuse plasma filament with significant vibrational loading. The results are compared with kinetic modeling calculations. The experimental data shows that significant NO concentrations are achieved within a few microseconds after the pulse, after which the NO evolution is controlled by the reverse Zel’dovich reactions. The modeling calculation results show that the dominant formation channel is through collisions of oxygen atoms with multiple electronically-excited nitrogen states. Aside from acting as an energy reservoir, the large concentration of vibrationally-excited nitrogen in the ground electronic state seems to have no effect on NO formation. The present experimental results are accurately reproduced by incorporating all electronically excited N2 ∗ levels. more...

Thomson scattering and kinetic modeling are used to study time evolution of electron density and electron temperature in a nanosecond pulse, diffuse filament electric discharge sustained between two spherical electrodes and operated at a low pulse repetition rate. The experiments have been done for three representative cases: (1) Helium, P=200 torr, discharge pulse energy 17 mJ/pulse; (2) Helium, P=100 torr, discharge pulse energy 0.6 mJ/pulse; and (3) 1% hydrogen in helium, P=100 torr, discharge pulse energy 0.8 mJ/pulse. In Case 1, peak electron number density and peak electron temperature are ne ≈ 3.5·10 15 cm -3 and Te ≈ 4 eV, respectively. The kinetic model predictions agree well with the temporal trends detected in the experiment (rapid initial rise of electron temperature and electron density during the discharge pulse and gradual decay in the afterglow), although peak electron temperature and electron density values during the pulse are somewhat overpredicted. Similar temporal trends, although at lower peak ne and Te, are observed at lower discharge pulse energies. The electron number density decay in a 1% H2 – He mixture is found to be much faster, by approximately a factor of 4, compared to helium at the same pressure and nearly the same discharge pulse energy. This is likely due to more rapid dissociative recombination of electrons in collisions with H2 + ions compared to dissociation recombination of electrons with He2 + ions. At these lower pulse energies, model predictions reproduce temporal trends fairly well, but overpredict peak electron density as well as the rate of electron cooling. D ow nl oa de d by I go r A da m ov ic h on F eb ru ar y 2, 2 01 4 | h ttp :// ar c. ai aa .o rg | D O I: 1 0. 25 14 /6 .2 01 413 58 52nd Aerospace Sciences Meeting 13-17 January 2014, National Harbor, Maryland AIAA 2014-1358 Copyright © 2014 by Andrew Roettgen. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. AIAA SciTech more...

The initial development of a novel chemical carbon monoxide laser driven by the exothermicity of the chemical production of carbon monoxide via the reaction between gasphase atomic carbon and molecular oxygen is presented. A flowing chemical reactor has been constructed for the investigation of this chemical reaction, where sublimation of amorphous carbon is achieved within an electrically-driven arc, and injection of rf-discharge-activated oxygen results in the formation of carbon monoxide. Detection and quantification of the chemical product is performed via ex situ absorption spectroscopy. The production rate of carbon monoxide is determined to be ~4.3e 18 molecules/sec. more...

In recent decades significant interest has been paid to investigate the applications of non-equilibrium plasma discharges to a variety of practical combustion related applications. However in order to develop a more fundamental understanding of such phenomena, additional insight into important kinetic processes is of great importance. A two-photon absorption laser-induced fluorescence (TALIF) technique is developed utilizing wide-bandwidth, short-time duration, femtosecond (fs) laser pulses. Due to its increased bandwidth and short pulse duration, fs-TALIF has reduced impact from photo-dissociation and increased signal compared to traditional nanosecond TALIF schemes. This fs-TALIF technique is used to image key atomic species within nanosecond pulsed, non-equilibrium discharges at moderate pressures. These two dimensional results provides both spatial and temporal information that can be used in more predictive plasma kinetics models. 1 Research Engineer, Spectral Energies, LLC., 5100 Springfield St, Suite 301, Dayton, OH 45431, AIAA Member 2 Senior Research Scientist, Spectral Energies, LLC., 5100 Springfield St, Suite 301, Dayton, OH 45431, AIAA Associate Fellow 3 Senior Research Scientist & CEO, Spectral Energies, LLC., 5100 Springfield St, Suite 301, Dayton, OH 45431, AIAA Associate Fellow 4 Research Associate, The Ohio State University, Columbus, OH 43210, AIAA Member 5 Professor, The Ohio State University, Columbus, OH 43210, AIAA Associate Fellow 6 Principal Research Chemist, Air Force Research Laboratory, WPAFB, OH 45433, AIAA Associate Fellow more...

A femtosecond laser-based TALIF scheme is demonstrated allowing for two-dimensional imaging of atomic hydrogen, oxygen and nitrogen inside a non-equilibrium, pin-to-pin, low pressure discharge cell additionally assisting development of more predictive two-dimensional plasma kinetics models. more...

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 ... more...

This paper describes recent advances made in our laboratory in the development of a new high-energy pulse burst laser system (HEPBLS) for turbulent fluid dynamics/combustion and high-speed flow measurements. We discuss new results from HEPBLS demonstrating ultra-high pulse energies greater than 2.0 Joules/pulse at 1064 nm with inter-pulse separations of 100 s (10 kHz) for burst durations exceeding 100 pulses with small levels of pulse-to-pulse fluctuations and negligible “droop” over the long pulse burst. Second harmonic generation of 532 nm with conversion efficiency greater than 50% is demonstrated for 100-pulse burst durations at repetition rates of 10 kHz and 20 kHz. The utility of the new system is shown through example 10-kHz image sequences of planar Rayleigh scattering-based mixture fraction and temperature imaging in turbulent non-reacting jets and flames, respectively. more...

Spontaneous Raman spectroscopy has been used, in conjunction with a master equation kinetic model, to study vibrational energy loading and relaxation in nitrogen (P=100 torr), in a pin-topin, nanosecond pulsed electric discharge. A highly non-equilibrium vibrational distribution (up to v=12 significantly populated) was observed for time delays after the pulse ranging from 135 nsec to 10 msec. Approximately 35% of the total discharge energy was found to couple directly to the N2 vibrational mode via direct electron impact, a result found to be in good agreement with discharge model predictions. Following the discharge pulse, the total vibrational quanta in all observed levels (v=0-12) was seen to rise by a factor of approximately 70 percent, indicating further energy coupling into N2 vibrations post-discharge, consistent with previous Coherent Anti-Stokes Raman Spectroscopy (CARS) studies performed with the same discharge geometry. As a first attempt to account for this, a simple phenomenological E-V coupling model was incorporated into the master equation model. Significantly better overall agreement with the data, including the temporal evolution of the vibrational distribution function and total vibrational quanta was observed when a 30% energy defect into the N2(X,v) vibrational mode, during E-V transfer from quenching and pooling processes (involving the N2(A) and N2(C) excited electronic states), was assumed. more...

OH Laser Induced Fluorescence (LIF) is used for temperature and absolute OH number density measurements in an atmospheric pressure, near stoichiometric CH4-air flame generated by a Hencken burner. OH rotational temperature is inferred with excitation scans of both the OH A-X (0,0) and (1,0) bands. OH LIF signal is corrected by considering transition-dependent total radiative decay rate, laser attenuation, and fluorescence trapping. The relative OH concentrations are put on an absolute scale by calibrating the optical collection constant using Rayleigh scattering. The measured absolute OH number density in the flame is compared with laser absorption measurements done at the same locations, showing good agreement and thus demonstrating the efficacy of our calibration approach employing Rayleigh scattering—for a low temperature and pressure, lean, fuel-air mixtures excited by a repetitively pulsed nanosecond (nsec) discharge. Here, a premixed fuel-air flow, initially at T0=500 K and P=100 torr, is excited by the discharge in a plane-to-plane geometry, operated in burst mode at 10 kHz pulse repetition rate. 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-nsec-gated images, employing an intensified charge-coupled device camera. Time-resolved, absolute OH number density, measured after the discharge burst, demonstrates that OH concentration in C2H4-air, C3H8-air, and CH4 is highest in the leanest mixtures, while in H2-air, OH concentration is nearly independent of the equivalence ratio. In C2H4-air and C3H8-air, unlike in CH4-air and in H2-air, transient OH-concentration overshoot after the discharge is detected. In C2H4-air and C3H8-air, OH decays after the discharge on the time scale of ~0.02-0.1 msec, suggesting little accumulation during the burst of pulses repeated at 10 kHz. In CH4-air and H2-air, OH concentration decays within ~0.1-1.0 msec and 0.5-1.0 msec, 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 H2-air and hydrocarbon-air chemistry mechanisms. Kinetic mechanisms for H2-air, CH4-air, and C2H4-air developed by A. Konnov provide the best overall agreement with OH measurements. In C3H8-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 C3 and higher. more...

Picosecond CARS is employed to study vibrational loading in diffuse air and nitrogen plasma filaments while using a nanosecond discharge that couples a large fraction of energy into the vibrational mode. It was found that, as has been previously reported, the total energy coupled to the vibrational mode increases long after the pulse, reaching a peak more than 100μsec later at 100 torr. Spatially resolved Two Photon Absorption Laser Induced Fluorescence (TALIF) is used to measure absolute number densities of atomic nitrogen and oxygen within the plasma filament. The concentrations of these species remain approximately constant at long time scales, indicating that neither N atom recombination or N2(A 3 Σ) quenching into the lower vibrational levels of molecular nitrogen can account for the measured rise in vibrational quanta. Master equation modeling was applied, confirming that an electronic-vibrational coupling is needed to explain the data. The evidence presented here is consistent with the hypothesis that N2(A 3 Σ) is quenched into highly vibrationally excited levels of the ground state, after which vibrational population is re-distributed downward due V-V transfer. more...

Femtosecond, two-photon-absorption laser-induced fluorescence (fs-TALIF) is employed to measure space- and time-resolved distributions of atomic hydrogen and oxygen in moderate-pressure, non-equilibrium, nanosecond-duration pulsed-discharge plasmas. Temporally and spatially resolved hydrogen and oxygen TALIF images are obtained over a range of low-temperature plasmas in mixtures of helium and argon at 100 Torr total pressure. The high-peak-intensity, low-average-energy fs pulses combined with the increased spectral bandwidth compared to traditional ns-duration laser pulses provide a large number of photon pairs that are responsible for the two-photon excitation, which results in an enhanced TALIF signal. Krypton and xenon TALIF are used for quantitative calibration of the hydrogen and oxygen concentrations, respectively, with similar excitation schemes being employed. This enables 2D collection of atomic-hydrogen and -oxygen TALIF signals with absolute number densities ranging from 2 × 1012 cm−3 to 6 × 1015 cm−3 and 1 × 1013 cm−3 to 3 × 1016 cm−3, respectively. These 2D images are the first application of TALIF imaging in moderate-pressure plasma discharges. 1D self-consistent modeling predictions show agreement with experimental results within the estimated experimental error of 25%. The present results can be used to further the development of higher fidelity kinetic models while quantifying plasma-source characteristics. more...

The control of the vibrational distribution of nitrogen by energy transfer to CO2 is studied in two closely related experiments. In the first experiment, the time-resolved N2(v = 0–3) vibrational level populations and temperature in the afterglow of a diffuse filament nanosecond pulse discharge are measured using broadband coherent anti-Stokes Raman spectroscopy. The rotational–translational temperature in the afterglow is inferred from the partially rotationally resolved structure of the N2(v = 0) band. The measurements are performed in nitrogen, dry air, and their mixtures with CO2. N2 vibrational excitation in the discharge occurs by electron impact, with subsequent vibration–vibration (V–V) energy transfer within the N2 vibrational manifold, vibration–translation (V–T) relaxation, and near-resonance V–V' energy transfer from the N2 to CO2 asymmetric stretch vibrational mode. The results show that rapid V–V' energy transfer to CO2, followed by collisional intramolecular energy redistribution to the symmetric stretch and bending modes of CO2 and their V–T relaxation, accelerate the net rate of energy thermalization and temperature increase in the afterglow. In the second experiment, injection of CO2 into a supersonic flow of vibrationally excited nitrogen demonstrates the effect of accelerated vibrational relaxation on a supersonic shear layer. The nitrogen flow is vibrationally excited in a repetitive nanosecond pulse/DC sustainer electric discharge in the plenum of a nonequilibrium flow supersonic wind tunnel. A transient pressure increase as well as an upward displacement of the shear layer between the supersonic N2 flow and the subsonic CO2 injection flow are detected when the source of N2 vibrational excitation is turned on. CO2 injection leads to the reduction of the N2 vibrational temperature in the shear layer, demonstrating that its displacement is caused by accelerated N2 vibrational relaxation by CO2, which produces a static temperature and a pressure increase in the test section. This demonstrates the significant potential of accelerated vibrational relaxation for nonequilibrium flow control, by injection of rapid 'relaxer' species at a desired location, resulting in the rapid thermalization of vibrational energy in nitrogen and air flows, and producing a significant effect on the flow field. more...

Thomson scattering is used to study temporal evolution of electron density and electron temperature in nanosecond pulse discharges in helium sustained in two different configurations, (i) diffuse filament discharge between two spherical electrodes, and (ii) surface discharge over plane quartz surface. In the diffuse filament discharge, the experimental results are compared with the predictions of a 2D plasma fluid model. Electron densities are put on an absolute scale using pure rotational Raman spectra in nitrogen, taken without the plasma, for calibration. In the diffuse filament discharge, electron density and electron temperature increase rapidly after breakdown, peaking at n e ≈ 3.5 1015 cm−3 and T e ≈ 4.0 eV. After the primary discharge pulse, both electron density and electron temperature decrease (to n e ~ 1014 cm−3 over ~1 µs and to T e ~ 0.5 eV over ~200 ns), with a brief transient rise produced by the secondary discharge pulse. At the present conditions, the dominant recombination mechanism is dissociative recombination of electrons with molecular ions, . In the afterglow, the electron temperature does not relax to gas temperature, due to superelastic collisions. Electron energy distribution functions (EEDFs) inferred from the Thomson scattering spectra are nearly Maxwellian, which is expected at high ionization fractions, when the shape of EEDF is controlled primarily by electron–electron collisions. The kinetic model predictions agree well with the temporal trends detected in the experiment, although peak electron temperature and electron density are overpredicted. Heavy species temperature predicted during the discharge and the early afterglow remains low and does not exceed T = 400 K, due to relatively slow quenching of metastable He* atoms in two-body and three-body processes. In the surface discharge, peak electron density and electron temperature are n e ≈ 3 1014 cm3 and T e ≈ 4.25 eV, attained after the surface ionization wave reaches the grounded electrode. The sensitivity of the present diagnostics is too low to measure electron density in the plasma during surface ionization wave propagation (estimated to be below n e ≈ 1013 cm−3). After peaking during the primary current pulse, the electron density decays due to dissociative recombination. Electron temperature decreases rapidly over ~150 ns after the primary current pulse rise, to T e ≈ 0.5 eV, followed by a much more gradual electron cooling between the primary and the secondary discharge pulses, due to superelastic collisions providing moderate electron heating in the afterglow. more...

Time evolution of electron density and electron temperature in a nanosecond pulse, diffuse filament electric discharge in H2–He and O2–He mixtures at a pressure of 100 Torr is studied by Thomson/pure rotational Raman scattering and kinetic modeling. The discharge is sustained between two spherical electrodes separated by a 1 cm gap and powered by high voltage pulses ~150 ns duration. Discharge energy coupled to the plasma filament 2–3 mm in diameter is 4–5 mJ/pulse, with specific energy loading of up to ~0.3 eV/molecule. At all experimental conditions, a rapid initial rise of electron temperature and electron density during the discharge pulse is observed, followed by the decay in the afterglow, over ~100 ns–1 µs. Electron density in the afterglow decays more rapidly as H2 or O2 fraction in the mixture is increased. In He/H2 mixtures, this is likely due to more rapid recombination of electrons in collisions with and ions, compared to recombination with ions. In O2/He mixtures, electron density decay in the afterglow is affected by recombination with and ions, while the effect of three-body attachment is relatively minor. Peak electron number densities and electron temperatures are n e = (1.7–3.1) 1014 cm−3 and T e = 2.9–5.5 eV, depending on gas mixture composition. Electron temperature in the afterglow decays to approximately T e ≈ 0.3 eV, considerably higher compared to the gas temperature of T = 300–380 K, inferred from O2 pure rotational Raman scattering spectra, due to superelastic collisions. The experimental results in helium and O2–He mixtures are compared with kinetic modeling predictions, showing good agreement. more...

The effect of ns discharge pulses on the AC barrier discharge in hydrogen in plane-to-plane geometry is studied using time-resolved measurements of the electric field in the plasma. The AC discharge was operated at a pressure of 300 Torr at frequencies of 500 and 1750 Hz, with ns pulses generated when the AC voltage was near zero. The electric field vector is measured by ps four-wave mixing technique, which generates coherent IR signal proportional to the square of electric field. Absolute calibration was done using an electrostatic (sub-breakdown) field applied to the discharge electrodes, when no plasma was generated. The results are compared with one-dimensional kinetic modeling of the AC discharge and the nanosecond pulse discharge, predicting behavior of both individual micro-discharges and their cumulative effect on the electric field distribution in the electrode gap, using stochastic averaging based on the experimental micro-discharge temporal probability distribution during the AC period. Time evolution of the electric field in the AC discharge without ns pulses, controlled by a superposition of random micro-discharges, exhibits a nearly 'flat top' distribution with the maximum near breakdown threshold, reproduced quite well by kinetic modeling. Adding ns pulse discharges on top of the AC voltage waveform changes the AC discharge behavior in a dramatic way, inducing transition from random micro-discharges to a more regular, near-1D discharge. In this case, reproducible volumetric AC breakdown is produced at a well-defined moment after each ns pulse discharge. During the reproducible AC breakdown, the electric field in the plasma exhibits a sudden drop, which coincides in time with a well-defined current pulse. This trend is also predicted by the kinetic model. Analysis of kinetic modeling predictions shows that this effect is caused by large-volume ionization and neutralization of surface charges on the dielectrics by ns discharge pulses. The present work demonstrates that this effect may be used to control the phase of AC barrier discharges, triggering reproducible breakdowns at well-defined moments. more...

The design and implementation of a new picosecond Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy instrument for measurement of nitrogen Vibrational Distribution Function (VDF) in a highly nonequilibrium Mach 5 flow is described. First level vibrational temperatures of the order of 2000 K are achieved in the 300 Torr nonself-sustained plasma wind tunnel plenum, generated by a high E/n (300 Td) nanosecond pulsed discharge, which provides ionization, in combination with an orthogonal low E/n (~10-30 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, with Tv of the order of 1100 K. Downstream injection of CO2, NO, and H2 results in vibrational relaxation, demonstrating the ability to further tailor the vibrational energy content of the flow. more...

In this paper we will describe recent advances made in our laboratory in the development of high-speed (10-kHz acquisition rate) 1D 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. In this paper we will review our initial results depicting high-speed Raman scattering imaging of O2, N2, CH4, and H2 in a turbulent non-reacting CH4/H2 jet issuing into air (Appl Phys B, 101(1), 2010, p. 1-5). This work represented the first temporally-sequential 1D image sequences of major species measuring via high-speed Raman scattering. This paper will also describe our most recent work towards the application in combustion environments by examining system performance through single-shot measurements (at 10 Hz) in wellcharacterized laminar flames. more...

In this paper we will describe recent advances made in our laboratory in the development of high-repetition-rate PLIF, Rayleigh and Raman scattering imaging capabilities that typically are not possible with commercial high-speed laser systems. Using a custom pulse burst laser system, pulse energies approaching 200 mJ at 532-nm with inter-pulse spacing of 1 to 100 s have been achieved. Such high pulse energies enable planar (2D) Rayleigh scattering for imaging the time-varying mixture fraction and temperature fields in turbulent non-reacting jets and flames and 1D Raman scattering measurements for major species concentrations in turbulent flames. In addition, the pulse burst laser output can pump an optical parametric oscillator (OPO) for tunable visible laser light that is frequency-mixed with residual output to generate tunable UV laser output for a variety of PLIF measurements. Finally, this paper also will describe the development and pending application of a new higher-energy, long-duration, next-generation pulse-burst laser system. more...

The paper reports recent progress in experiments on supersonic flow characterization in a small scale, Mach 5 nonequilibrium flow wind tunnel. The wind tunnel generates nonequilibrium Mach 5 flows at steady state. Transverse repetitively pulsed nanosecond discharge, fully overlapped with transverse DC discharge, is used to load internal energy modes of nitrogen in the nozzle plenum, at P0~0.5 atm and at low translational/rotational temperatures. NO PLIF images on two single-line NO(X,v´=0→A,v˝=0) transitions are used to infer twodimensional rotational temperature distributions in NO-seeded nitrogen flows in the supersonic test section. These measurements are conducted in a cold flow (i.e. without the discharge in plenum) and in a flow vibrationally excited by the pulsed / DC discharge. The results do not indicate detectable temperature difference between these two cases, outside the experimental uncertainty. Single-line NO PLIF images on a NO(X,v´=1→A,v˝=1) Q1+P21 (J=3.5) transition are used to infer the NO vibrational temperature in a nitrogen Mach 5 flow excited by the discharge in plenum and seeded with NO, TV(NO)=1050 ± 170 K. The N2 vibrational temperature, inferred from this results, is Tv(N2)=850 K. Emission from NO γ bands and NO2 chemiluminescence are detected in a Mach 5 flow of nitrogen excited by the pulsed discharge in plenum over a cylinder model, with counterflow injection of NO-N2 mixture through the model. NO electronic emission most likely occurs due to energy transfer from N2(A 3 Σ) state generated in the discharge to the NO(A 2 Σ) state. Schlieren visualization is used to examine the effect of N2 vibrational relaxation on a Mach 5 bow shock stand-off distance in the supersonic section, by seeding the nonequilibrium nitrogen flow downstream of the discharge with water vapor and hydrogen. The results suggest that shock stand-off distance change detected in these experiments is affected by water vapor condensation, and is unlikely to be related to accelerated N2 vibrational relaxation. Feasibility of Mach 5 flow control over a cylinder model by a surface dielectric barrier discharge (DBD) powered by repetitive nanosecond duration voltage pulses has been tested. Preliminary results demonstrate that the nanosecond pulse discharge initiated on the surface of the model generates shock waves upstream of the baseline Mach 5 bow shock, on a microsecond time scale. The shocks generated by the discharge pulses perturb the baseline shock and gradually merge with it. The results show that strong supersonic flow perturbations by using this approach can be generated at a repetition rate of up to 100 kHz. more...

We discuss two recent diagnostic-development efforts in our laboratory: femtosecond pure-rotational Coherent anti-Stokes Raman scattering (CARS) for thermometry and species detection in nitrogen and air, and nanosecond vibrational CARS measurements of electric fields in air. Transient pure-rotational fs-CARS data show the evolution of the rotational Raman polarization in nitrogen and air over the first 20 ps after impulsive pump/Stokes excitation. The Raman-resonant signal strength at long time delays is large, and we additionally observe large time separation between the fs-CARS signatures of nitrogen and oxygen, so that the pure-rotational approach to fs-CARS has promise for simultaneous species and temperature measurements with suppressed nonresonant background. Nanosecond vibrational CARS of nitrogen for electric-field measurements is also demonstrated. In the presence of an electric field, a dipole is induced in the otherwise nonpolar nitrogen molecule, which can be probed with the introduction of strong collinear pump and Stokes fields, resulting in CARS signal radiation in the infrared. The electric-field diagnostic is demonstrated in air, where the strength of the coherent infrared emission and sensitivity our field measurements is quantified, and the scaling of the infrared signal with field strength is verified. more...

This project focuses on an integrated experimental/modeling study of hydrocarbon oxidation mechanisms under conditions of extreme thermal non-equilibrium. Large, and highly nonequilibrium, initial pools of important hydrocarbon combustion intermediate and electronically excited air species are created in a short pulsed fuel-air plasmas, created using approx 10-20 nsec duration - high (approx 20 kV) voltage pulsers, capable of operation at repetition rates as high as 40-50 kHz. The time evolution of critical species and temperature, after application of a single nanosecond discharge pulse, or a rapid "burst" of pulses, depending upon the experiment, are experimentally determined using advanced laser-based optical diagnostic methods. Results are compared to theoretical predictions from discharge and plasma chemical kinetic codes, developed as part of this program, in order to provide model validation and to improve understanding of fundamental nonequilibrium plasma kinetics in air and air/fuel mixtures. more...

In this paper we will describe recent advances made in our laboratory in the development of high-repetition-rate Rayleigh and Raman scattering imaging capabilities. High-repetition-rate 1D Raman and 2D Rayleigh scattering imaging capabilities are being developed to image the time-varying mixture fraction and temperature fields in turbulent non-reacting and reacting flows. Initial results using a custom pulse-burst laser system at Ohio State University have demonstrated the ability to capture ten sequential 2D Rayleigh scattering images at a repetition rate of 10 kHz in both turbulent non-reacting jets and non-premixed jet-flames with pulse energies approaching 200 mJ at 532 nm. This paper will also describe the development and pending application of a new higher-energy, long-duration, next-generation burst-mode laser system and the use of higher resolution cameras for high-speed Raman/Rayleigh imaging. more...

We report the use of a tunable ArF laser at 193.26 nm to record simultaneous single-laser-shot, planar images of molecular hydrogen and hot oxygen in a turbulent H(2)-air diffusion flame. Excitation spectra of fuel and oxidantrich flame zones confirm a partial overlap of the two-photon H(2) (6,0) Q(1) E, F ? X and single-photon O(2) (10,2) R(17) B ? X (Schumann-Runge) absorption bands. UV Rayleigh scattering images of flame structure and estimated detection limits for the H(2) two-photon imaging are also presented. more...

A new analytic model of energy coupling to repetitive nanosecond pulsed discharge plasmas has been developed and shown to agree well with previous estimates from single pulse nanosecond discharges in air. The new, quasi-one dimensional model provides accurate expressions for electric field, electron density, and coupled pulse energy, including effects of dielectric surface charge accumulation, and corresponding sheath development. Primary conclusions which result directly from this new model are: (i) the pulse energy coupled to the plasma during an individual nanosecond discharge pulse is controlled primarily by the capacitance of the dielectric layers and by the breakdown voltage, and (ii) the pulse energy coupled to the plasma during a burst of nanosecond pulses decreases as a function of the pulse number in the burst, rather than remaining constant. This occurs primarily because of plasma temperature rise, and resultant reduction of breakdown voltage, with the result that the coupled pulse energy varies approximately proportionally to the number density. The model has been validated by a series of experimental temperature measurements in air and ethylene-air discharges, where the nanosecond discharge was operated in repetitive burst mode. Experimentally determined temperature, by pure rotational Coherent Anti-Stokes Raman Spectroscopy (CARS) and N2 emission spectroscopy, as a function of number of pulses in a burst, was found to agree well with predictions of the model. It is found that heating rate in fuel-air plasmas is much faster compared to air plasmas, primarily due to energy release in exothermic reactions of fuel with O atoms generated by the plasma. 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, heating rate in lean fuel air-mixtures is significantly reduced when all fuel is oxidized. more...