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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. more...

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. more...

A stable ethylene/oxygen/argon flame is sustained and nearly complete combustion is achieved in the combustion chamber of an M = 3 supersonic nozzle, at a stagnation pressure of P 0 =1 atm. Ultraviolet and visible emission is detected both from the combustion chamber and from the M = 3 flow of combustion products. Temperature in the combustor, inferred from the visible emission spectra, is To = 2000 ± 200 K. Electron density in M = 3 flow of combustion products has been measured using Thomson discharge n, = 1.4 ± 0.2·10 8 cm -3 , at an ionization fraction of n e /N = (0.65 ± 0.15) · 10 -9 . This corresponds to an electron density of n e0 = 2.2 ·10 9 cm -3 in the combustor. The chemi-ionization current measured in the M = 3 flow is found to be proportional to the equivalence ratio in the combustor. The time-resolved chemi-ionization current is in very good correlation with the visible emission from ethylene-air and propane-oxygen-argon flames in the combustor at unstable combustion conditions. The results show that nearly all electrons can be removed from the supersonic flow of combustion products by applying a moderate transverse electric field. No effect of electron removal on visible emission has been detected. A similar result was obtained for nitric oxide β bands and cyanogen violet band emission, when nitric oxide was injected into the combustion product flow. more...

A chemical flow reactor has been used to study the vibrational population distribution of carbon monoxide produced by a reaction between vapor-phase carbon generated in an arc discharge and oxygen, to determine feasibility of extracting the chemical energy released from this reaction by laser radiation. Additionally, a supersonic flow, electric discharge excited CO laser has been developed and characterized over a range of operating conditions. The same supersonic laser apparatus can be adapted to produce population inversion via oxidation of vapor-phase carbon, generating vibrationally excited CO. Resultant laser power and spectra are compared with the predictions of a kinetic model of a supersonic flow CO laser. 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...

The gas-phase Boudouard disproportionation reaction between two highly vibrationally excited CO molecules in the ground electronic state has been studied in optically pumped CO. The gas temperature and the CO vibrational level populations in the reaction region, as well as the CO 2 concentration in the reaction products have been measured using FTIR emission and absorption spectroscopy. The results demonstrate that CO 2 formation in the optically pumped reactor is controlled by the high CO vibrational level populations, rather than by CO partial pressure or by flow temperature. The disproportionation reaction rate constant has been determined from the measured CO 2 and CO concentrations using the perfectly stirred reactor (PSR) approximation. The reaction activation energy, 11.6 ± 0.3 eV (close to the CO dissociation energy of 11.09 eV), was evaluated using the statistical transition state theory, by comparing the dependence of the measured CO 2 concentration and of the calculated reaction rate constant on helium partial pressure. The disproportionation reaction rate constant measured at the present conditions is k f = (9 ± 4) × 10 −18 cm 3 /s. The reaction rate constants obtained from the experimental measurements and from the transition state theory are in good agreement. 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...

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. more...

A method for direct measurements of electron number density, ionization rate, and the electron–ion recombination rate coefficient in optically pumped non-equilibrium plasmas has been developed. In this method, a pulsed, non-self-sustained discharge created by applying square-shaped, below breakdown voltage pulses to two electrodes placed outside the plasma (a Thomson probe) is used to remove electrons from the plasma. The electron number density is inferred for CO/Ar and CO/N2 optical mixtures with small amounts of O2 additive present. The results are compared with microwave attenuation measurements. The electron–ion recombination rate coefficients in CO/Ar/O2 and in CO/N2/O2 plasmas are β = (3–4) × 10−8 cm3 s−1 and β = (2–3) × 10−7 cm3 s−1, respectively. Time-dependent measurements of the electron concentration and vacuum ultraviolet radiation (CO fourth positive system) are used to study the mechanism of CO(A1Π) population in the optically pumped plasma. The experimental results are compared with kinetic modelling calculations. The results systematically show that the intensity of the CO fourth positive radiation closely follows the electron number density in the laser-excited plasma region after the Thomson probe voltage is turned on or off. This demonstrates that electrons play a major role in the excitation of the A1Π electronic state of CO and provides additional evidence that vibrational-to-electronic (V–E) energy transfer in plasmas sustained without external electric fields is mediated by collisions with electrons. more...

An experimental study of shock modie cation in an M =2:5 supersonic e ow of nonequilibrium plasma over a cone is discussed. The experiments are conducted in a nonequilibrium plasma supersonic wind tunnel. Recent experiments at the Ohio State University using a supersonic plasma e ow over a quasi-two-dimensional wedge showed that an oblique shock can be considerably weakened by a transverse rf discharge plasma. The previously observed shock weakening, however, has been found consistent with a temperature rise in the boundary layers heated by the discharge. In thepresent study, theboundary-layereffects on theshock waveare reduced by placing an entire cone model into a supersonic inviscid core e ow. The electron density in the supersonic plasma e ow in the test section is measured using microwave attenuation. The ionization fraction in the discharge is in the same range as in the previous plasma shock experiments, up to ne/N =(1.2‐3.0)£10 i7 . The results do not show any detectable shock weakening by the plasma. This strongly suggests that the previously observed shock weakening and dispersion in nonequilibrium plasmas are entirely due to thermal effects. more...

The paper discusses experiments on vibration-to-electronic energy transfer in CO laser pumped CO–Ar and CO–N2 plasmas. Ionization in these strongly nonequilibrium plasmas occurs by an associative mechanism, in collisions of two highly vibrationally excited CO molecules. The experiments show that removal of the electrons from the optically pumped plasmas using a saturated Thomson discharge results in considerable reduction of the UV/visible radiation from the plasma (CO 4th positive bands, NO c bands, CN violet bands, and C2 Swan bands). At some conditions, the removal of electrons results in a nearly complete extinguishing of the UV/visible glow of the plasma. This effect occurs even though electron removal results in an increase of the high vibrational level populations of the ground electronic state CO(X 1 R, v � 15–35). On the other hand, deliberate electron density increase by adding small amounts of O2 or NO to the optically pumped CO–Ar plasmas produced a substantial increase of the UV/visible radiation intensity, which strongly correlates with the electron density. The results of the present experiments indicate that the vibration-toelectronic (V–E) energy transfer process CO(X 1 R ! A 1 P), and, possibly, analogous processes populating radiating excited electronic states of NO, CN, and C2, in optically pumped plasmas, may be mediated by the presence of electrons which are created in the absence of an electric field, with low initial energies. Most importantly, this effect occurs at ionization fractions as low as ne=N � 10 � 9 –10 � 7 . 2002 Elsevier Science B.V. All rights reserved. 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...

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 kinetics of ionization and electron removal in optically pumped non-equilibrium plasmas sustained by a CO laser are studied using non-self-sustained dc and RF electric discharges. Experiments in optically pumped CO/Ar/N2 mixtures doped with O2 and NO demonstrated that associative ionization of CO produces free electrons at a rate up to S = 1015 cm-3 s-1. The ionization rate coefficient, inferred from the CO vibrational population measurements, is kion = (1.1-1.8)×10-13 cm3 s-1. It is shown that excited NO and possibly O2 molecules also contribute to the vibrationally stimulated ionization process. In a CO/Ar plasma, applying a dc bias to the cell electrodes resulted in the rapid accumulation of a deposit on the negative electrode due to a large cluster ion current. The average mass of an ion in this plasma, estimated by measuring the mass of the deposit, is m250 amu, which is consistent with the mass spectrometer analysis of the deposit. The deposit did not accumulate when small amounts of O2 and NO were added to the CO/Ar plasma, which presumably indicates the destruction of the cluster ions. It is demonstrated that adding small amounts of O2 to the optically pumped CO/Ar plasmas significantly increases the electron density, from ne = (4-7)×109 cm-3 to ne = (1-2)×1011 cm-3. This effect occurs at a nearly constant (within 50%) electron production rate S, indicating a substantial reduction in the overall electron removal rate. This reduction can be qualitatively interpreted as the destruction of rapidly recombining cluster ions in the presence of the O2 additive, and their replacement by monomer ions with a slower recombination rate. Further studies of the ion composition in optically pumped plasmas are suggested. more...

The paper discusses measurements of vibration-to-vibration (V–V) energy transfer rates for CO–CO using time-resolved step-scan Fourier transform infrared spectroscopy of optically pumped carbon monoxide. In the experiments, time evolution of all vibrational states of carbon monoxide excited by a CO laser and populated by V–V processes (up to v∼40) is monitored simultaneously. The V–V rates are inferred from these data using a kinetic model that incorporates spatial power distribution of the focused laser beam, transport processes, and multi-quantum V–V processes. Although the model predictions agree well with the time-dependent step-scan relaxation data, there is variance between the model predictions and the up-pumping data, however. Comparison of calculations using two different sets of V–V rates with experimental spectra showed that the use of the semi-empirical V–V rates of DeLeon and Rich provides better agreement with experiment. It is also shown that the multi-quantum V–V rates among high vibrational quantum numbers, calculated by Cacciatore and Billing, are substantially overpredicted. The results provide some new insight into nonequilibrium vibrational kinetics, and also demonstrate the capabilities of the step-scan Fourier transform spectroscopy for time-resolved studies of molecular energy transfer processes and validation of theoretical rate models. more...

between a rotating diatomic molecule and an atom has been developed. The model is based on analysis of classical trajectories of a free-rotating ~FR! molecule acted upon by a superposition of repulsive exponential atom-to-atom potentials. The energy transfer probabilities have been evaluated using the nonperturbative forced harmonic oscillator ~FHO! model. The model predicts the probabilities for vibrational energy transfer as functions of the total collision energy, orientation of a molecule during a collision, its rotational energy, and impact parameter. The model predictions have been compared with the results of three-dimensional close-coupled semiclassical trajectory calculations using the same potential-energy surface. The comparison demonstrates not only remarkably good agreement between the analytic and numerical probabilities across a wide range of collision energies, but also shows that the analytic FHO-FR model correctly reproduces the probability dependence on other collision parameters such as rotation angle, angular momentum angle, rotational energy, impact parameter, and collision reduced mass. The model equally well predicts the cross sections of single-quantum and multiquantum transitions and is applicable up to very high-collision energies and quantum numbers. Most importantly, the resultant analytic expressions for the probabilities do not contain any arbitrary adjustable parameters commonly referred to as ‘‘steric factors.’’ The model provides new insight into kinetics of vibrational energy transfer and yields accurate expressions for energy-transfer rates that can be used in kinetic modeling calculations. © 1998 American Institute of Physics.@S0021-9606~98!01642-0# more...

Shock propagation into weakly ionized gases shows several features differing markedly from conventional, nonionized-gas shock structure. Phenomenological analysis of general macroscopic features of the previously observed plasma shock effects allows only two possible interpretations: existence of an energy (momentum) flux toward the wave precursor or volumetric energy release (exothermic phase transition) in the upstream portion of the wave (precursor) followed by reverse transition in the downstream portion of the wave. It is shown that known microscopic mechanisms are not capable of producing such a flux or energy release: Typical processes involving electrons, ions, and excited species do not couple strongly to neutral atoms and molecules, and there is not enough energy stored in these species because of the low ionization fraction. The theoretical basis for phase transitions in low-density, weakly ionized plasmas also is unknown. Analysis of the steady two-wave system created by either of the two effects raises a question as to whether the observed plasma shocks are stable objects. Another question is whether there exists phase transition within the plasma shock. It also remains unclear to what extent twodimensional thermal inhomogeneity effects contribute to the observed phenomena. Answering these fundamental questions requires additional experimental studies of the problem. p D d E E/N F h j k more...

Nonequilibrium, reacting, ionized gas flow modeling is used to study the feasibility of magnetohydrodynamics (MHD) acceleration of airflows for the energy addition wind tunnel. The kinetic model incorporates equations of one-dimensional magnetogasdynamics, the master equation for vibrational level populations of diatomic species, equations of chemical and ionization kinetics, and the Boltzmann equation for electrons. The model is validated by comparison with the experiments in MHD accelerators. Calculations are made for two accelerator schemes, the first using an electron beam to sustain nonequilibrium ionization in unseeded air and the second using alkali-seeded air. Although at low pressures external ionization allows substantial increase of the flow total enthalpy, the obtained test section pressure is much lower than required, and the flow quality is poor. Calculations for alkali-seeded flows predict test section flow parameters closer to the target values, with O atom and NO concentrations lower than in the e-beam-controlled flows. Flow stability is analyzed using the linear stability theory. A thermodynamic energy addition criterion is used to demonstrate the advantage of direct kinetic energy increase in MHD acceleration over thermal energy addition methods more...

This paper addresses the analysis, validation, and application of analytic, nonperturbative, semiclassical vibration-translation (V-T) and vibration-vibration-translation (V-V-T) rate models for atom-diatom and diatom-diatom vibrational molecular energy transfer collisions. These forced harmonic oscillator (FHO) rate models are corrected and validated by comparison with recent experiments, and with three-dimensional semiclassical trajectory calculations for N 2 -N 2 , O 2 -O 2 , and N 2 -O 2 , which are considered to be the most reliable theoretical data available. A remarkably good overall agreement is shown for both the temperature and quantum number dependence of single-quantum and double-quantum V-V-T transitions in the temperature range 200 < T < 8000 K and for vibrational quantum numbers 0 < ν < 40. It is demonstrated that the multiquantum vibrational energy transfer processes occur via a sequential FHO mechanism, as a series of virtual single-quantum steps during one collision. An important exception, asymmetric multiquantum V-V exchange at low temperatures, that occurs via a direct first-order mechanism, is discussed. Analytic thermally averaged FHO V-T and V-V rates are suggested. The FHO model gives new insight into vibrational kinetics and may be easily incorporated into kinetic modeline calculations under conditions when first-order theories are not applicable. 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...

Energy transfer between highly vibrationally excited CO molecules and low-energy electrons is studied using kinetic modeling. The results are compared with those of experimental measurements in optically pumped CO. The effect of vibrational energy transfer by electrons from the high towards the low vibrational levels of CO, previously observed in the experiments, is reproduced in calculations. The best agreement with the experiment is obtained for an electron concentration in the plasma of , which is consistent with the previous measurements of the vibrationally stimulated ionization rate. The results of kinetic modelling calculations provide better insight into the kinetics of energy exchange between vibrationally excited molecules and electrons. more...

High-temperature vibrational relaxation of diatomic molecules is analyzed using master equation and Fokker‐ Planck diffusion equation modeling. Both approaches are based on the nonperturbative forced harmonic oscillator vibrational energy transfer rate model. The results show that the effect of the retarded diffusion at the intermediate vibrational energies during the relaxation process (bottleneck), predicted in the previous studies, does not exist at the high temperatures. It is also shown that the interpretation of the high-temperature vibrational relaxation as a diffusion-type process is inadequate because of the strong effect of multiquantum energy transfer. The obtained results, applied to interpretation of the rates of thermal dissociation, suggest an important role of rotations in molecular activation at the high vibrational energies. more...

The infrared radiation of nitric oxide (NO) behind a shock wave in O2-N2 mixtures has been calculated by two different techniques, and compared with recent shock-tube experiments. The first technique (model I) utilizes the Park model. This model incorporates the vibrational relaxation of O2 and N2 and assumes a Boltzmann distribution of vibrational energy during the relaxation process. Model II uses a master equation solution, employing recently published state-to-state vibration-translation and vibration-vibration transition probabilities. Vibration-chemistry coupling is provided through the MacheretFridman-Rich model (MFR). The calculations are compared with experimental results for shock waves in the range of 3-4 km/s. Results of the two model calculations are compared at speeds up to 9 km/s, for both normal shocks and bow shocks. The two models predict nearly the same NO production rates behind all of the normal shocks, and show the prominent effect of N2 vibrational coupling in the reaction N2 + O —> NO + N. For high-altitude bow shocks, where extreme vibrational nonequilibrium is present, there are large differences in the results calculated by the Park and MFR coupling techniques. more...

We address the application of the analytical nonperturbative semiclassical vibration-translation and vibration-vibration-translation forced harmonic oscillator rate models for kinetic modeling calculations. Master equation modeling of nonequilibrium dissociating gas flows, based on the forced harmonic oscillator multiple-jump rate model, is applied for simulation of vibrational relaxation and dissociation of N 2 and O 2 -Ar mixtures behind strong shock waves. The comparison with the first-order rate model (Schwartz, Slawsky, and Herzfeld [SSH] theory) shows that the SSH and forced harmonic oscillator theories predict strongly different vibrational distribution functions only for times less than or equal to the vibrational relaxation time T vT . Consequently, replacing SSH rates by the forced harmonic oscillator rate model has very little effect on the calculated dissociation rate, since the dissociation incubation time is T lne ∼T VT . The incubation time calculated using impulsive and forced harmonic oscillator dissociation models also agrees well with experimental data. Thus, it is shown that vibrational relaxation, as well as nonequilibrium dissociation at hypersonic temperatures, may be satisfactorily described using the first-order, SSH vibration-vibration-translation rate model, despite the fact that this temperature region is normally clearly beyond the applicability of the SSH theory. more...

Picosecond Unstable-resonator Spatially Enhanced Detection Coherent Anti-Stokes Raman Scattering (USED-CARS) is used for measurement of nitrogen Q-branch (ΔJ = 0) spectra in the subsonic plenum and supersonic flow of a highly nonequilibrium Mach 5 wind tunnel. Spectra are processed to infer simultaneous rotational / translational (Trot) and 1 st level vibrational (Tvib) temperatures in the 200 – 370 Torr plenum. Operation of the nominally high reduced electric field (E/npeak ~ 500 Td), nsec pulsed discharge alone results in fairly significant vibrational loading, Tvib ~ 720 K / Trot ~ 380 K; addition of an orthogonal low E/n (~10 Td) DC sustainer discharge produces substantial vibrational loading, Tvib ~ 2000 K / Trot ~ 450 K. Effects of injection of CO2, NO, and H2 downstream of the pulser-sustainer discharge are examined, which result in vibrational relaxation accompanied by simultaneous gas heating, Tvib ~ 800-1000 K / Trot ~ 600 K. CARS measurements within very low density flows in the Mach 5 expansion nozzle are also performed, with Tvib measured in both the supersonic free-stream and downstream of a bow shock created by a 5 mm diameter cylindrical test object in the Mach 5 flow. Measurements within 300 μm of the cylinder leading edge show that for pure N2, or N2 with 0.25 Torr CO2 injection, no vibrational relaxation is observed behind the bow shock. more...

The paper addresses several problems of kinetics of spatially nonhomogeneous, vibrationally excited gas flows. It is shown that vibrational energy transfer in most diatomic molecules does not affect the transport coefficients of gas, despite very fast vibration-to-vibration (V—V) rates among high vibrational levels. The influence of diffusion and the vibration-to-electronic (V—E) energy transfer on the vibrational distribution function (VDF) and on the vibrational energy balance is discussed. The diffusion and V—E corrections of the VDF and to the gas heating rate in vibrational relaxation is obtained for the “strong” excitation regime. The effect of the non-local diagnostics on the VDF is connsdered. It is shown that the spatial integration may significantly influence the inferred distribution function, as has been previously reported in experiments. A new approach for kinetic modeling of nonhomogeneous, vibrationally nonequilibrium flows is suggested. A two-dimensional kinetic model is developed which combines vibrational and translational energy balance and gas motion equations with the analytical theory of anharmonic oscillators. The comparison of the model calculations with both 1D and 2D experiments shows reasonable agreement within the applicability of the gas flow model. more...

Nitric oxide is vibrationally excited in a flowing gas mixture of NOAr using resonance absorption of single-line CO laser radiation. The fluorescence in the infrared and in the ultraviolet is analyzed under steady-state pumping conditions. Quantitative analysis of the NO IR overtone spectrum, Δ v =2, allows inference of the vibrational distribution function of the X 2 Π ground state up to vibrational level v =15. It is shown that the mechanism of vibrational excitation is anharmonic vibration-vibration pumping. In particular, the higher vibrational levels, v ⩾8, are populated by near-resonant vibration-to-vibration exchange processes. It is suggested that the electronically excited NO molecules in A 2 Σ and B 2 Π states, which are observed, can be produced both by resonant vibration-to-electronic energy transfer processes and in energy pooling reactions. Previous rate measurement experiments in NO are analyzed and discussed in light of the present data, and further state-resolved measurements are proposed. more...

A model is suggested for the analytical calculation of dissociation rates behind shock waves where the vibrational temperature T v is less than the gas temperature T . The model is based on an analysis of the threshold translational energy for collision-induced dissociation as a function of initial vibrational and rotational energies. The threshold function method combined with a classical impulsive model for energy exchange yields explicit formulae for the rate coefficient k ( T v , T ) and the mean vibrational energy removed in dissociation. The mechanism of nonequilibrium dissociation is predicted to change during vibrational relaxation: dissociation from low vibrational levels dominates at low T v / T , while dissociation from all levels contributes almost equally as T v / T approaches unity. The formulae obtained exhibit an explicit dependence on the mass ratio of the dissociating molecule and its collision partner, the lighter mass of the partner making dissociation from high levels more favorable. Dissociation in a molecular gas at T > T v is demonstrated to occur predominantly via noncollinear collisions with simultaneous transfer of rotational and translational energy to the vibrational mode of the dissociating molecule. more...

Vibrational and electronic excitation of nitric oxide is studied experimentally using optical pumping by a CO laser. A mixture of nitric oxide and argon diluent is vibrationally excited in an optical absorption cell, by resonance absorption of CO laser radiation operating on a single line, in near resonance with one of NO(v=0 → v=1) fundamental band absorption transitions. Higher NO vibrational levels, not directly accessible to laser excitation, are populated by collisional vibration-vibration (V-V) energy exchange processes. Steady-state vibrational level populations and translational-rotational temperature in the cell are measured by Fourier transform infrared emission spectroscopy. At steady state, vibrational levels up to v~10 are populated. Steady-state vibrational temperatures up to Tv=3000-4500 K are maintained at low translational-rotational temperatures of T=330-440 K. At these conditions, UV emission (NO β and γ bands) is detected from the optically pumped cell, both at steady state and during pulsed laser excitation using a mechanical chopper to interrupt the laser beam. UV emission delay time is measured during pulsed excitation, relative to the laser pulse rise time. The results provide insight into kinetics of vibration-to-electronic (V-E) energy transfer in nitric oxide. Similar optical pumping technique can be used to study kinetics of dissociation of diatomic molecules at the conditions of extreme vibrational and electronic disequilibrium. more...

A stable ethylene/oxygen/argon flame is sustained and nearly complete combustion is achieved in the combustion chamber of an M = 3 supersonic nozzle, at a stagnation pressure of P 0 =1 atm. Ultraviolet and visible emission is detected both from the combustion chamber and from the M = 3 flow of combustion products. Temperature in the combustor, inferred from the visible emission spectra, is To = 2000 ± 200 K. Electron density in M = 3 flow of combustion products has been measured using Thomson discharge n, = 1.4 ± 0.2·10 8 cm -3 , at an ionization fraction of n e /N = (0.65 ± 0.15) · 10 -9 . This corresponds to an electron density of n e0 = 2.2 ·10 9 cm -3 in the combustor. The chemi-ionization current measured in the M = 3 flow is found to be proportional to the equivalence ratio in the combustor. The time-resolved chemi-ionization current is in very good correlation with the visible emission from ethylene-air and propane-oxygen-argon flames in the combustor at unstable combustion conditions. The results show that nearly all electrons can be removed from the supersonic flow of combustion products by applying a moderate transverse electric field. No effect of electron removal on visible emission has been detected. A similar result was obtained for nitric oxide β bands and cyanogen violet band emission, when nitric oxide was injected into the combustion product flow. more...

The paper presents results of singlet oxygen generation experiments in a high-pressure, non-selfsustained crossed discharge. The discharge consists of 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. 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 V·cm 2 to E/N=0.65·10 -16 V·cm 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 Σ→X 3 Σ) in the discharge afterglow show the O2(b 1 Σ) 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 g/sec to 2.2 g/sec are 365 K to 465 K. Singlet delta oxygen 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 Σ) 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 modeling 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. more...

Cold gas plasmas are common excitation environment in high energy electric –discharge excited lasers. Such plasmas are typically low ionization fraction, diffuse, and extremely non-thermal, with low gas kinetic temperatures but with deliberately high levels of excitation of one or more modes of internal molecular motion. We briefly review on-going programs at OSU in using these plasmas in development of high power lasers (CO overtone laser), in optically pumped plasmas in air and other molecular gases. The paper presents results of diagnostic measurements of the vibrational and electronic state populations, V-V transfer rates, electron concentrations and electron recombination rates in these plasmas. Measurement methods reviewed include FTIR, step-scan FTIR spectroscopy for infrared active vibrational modes; visible and vacuum UV emission spectroscopy foe electronic modes; laser Raman spectroscopy (stimulated and spontaneous) for measurements in infrared inactive vibrationally excited species such au N2, O2, H2. Finally, the use of Thomsondischarge probes to infer the electron number density, ionization and electron recombination rates in optically pumped high density molecular plasmas is presented. The plasma studied show that markedly non-Boltzman distribution of vibrational, electronic and free electron more...

The kinetic modeling and design of a carbon monoxide (CO) gas laser is presented. In contrast to the more widely known fundamental band CO laser, this laser is designed to operate on the first overtone bands, the Δν = 2 vibrational quantum transitions. Lasing on these bands is known to produce multi-line output at wavelengths from 2.5 microns to beyond 4.0 microns in the infrared. The present study is to develop a compact, wall-cooled CO overtone laser, that can develop average powers O[100 W]. A kinetic modeling code has been developed to guide the design, and to calculate both continuous wave (c.w.) and Q-switched performance. There are distinct advantages in Q-switched operation of this laser, which is potentially one of the few truly efficient lasers scalable to very high average powers. more...

The paper presents results of nonequilibrium RF plasma assisted ignition and combustion experiments in premixed methane-air, ethylene-air, and CO-air flows. The results show that large volume ignition of these mixtures by the uniform and diffise RF plasma can be achieved at significantly higher flow velocities (up to u=25 rnls) and lower pressures (P=60-130 tom) 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. Temperature measurements in the stable diffise RF discharge using Fourier transform infrared spectroscopy show that the flow temperature in the plasma prior to ignition (~=250-550' C at P=60-120 tom) is considerably lower than the autoignition temperatures for both ethylene-air and CO-air mixtures at these pressures (~=600-700' C). Spatially resolved temperature measurements show the transverse temperature nonuniformity in the RF discharge to be insignificant. Visible emission spectroscopy measurements in C~H4-air flows in the RF discharge detected presence of radical species such as CN, CH, C2, and OH, as well as 0 atoms. In CO-air flows, 0 and H atoms have been detected in the RF plasma region and COz emission (carbon monoxide flame bands) in the flame downstream of the RF plasma. more...

The Air Force Research Laboratory, Air Vehicles Directorate, has sponsored computational and experimental research in the control of hypersonic boundary layer transition through a Small Business Innovation Research contract since the year 2000. This work has been presented in several previous papers. more...

Kinetic modeling of propagating and stationary normal shocks in nonequilibrium nitrogen afterglow plasma is used to simulate the results of shock emission measurements in nitrogen afterglow. Emission intensity overshoot behind the shock predicted by the model is in satisfactory agreement with the experimental results and is consistent with previous analytic estimates. The model demonstrates that the first and the second positive band emission overshoot behind the shock are produced by energy transfer processes among the triplet electronic states of nitrogen generated in the electric discharge. On the other hand, charge separation and ambipolar electric field produced across the shock layer do not result in electron heating and additional electron impact excitation of electronic states. The calculations show that emission overshoot makes possible accurate detection of a stationary shock layer in supersonic flowing afterglow experiments. more...

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

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

Spontaneous radiation on the CO fundamental vibration‐rotation bands, created by sidelight emission from a liquid‐nitrogen—cooled CO–He laser, has been studied spectroscopically. The CO rotational temperature and the relative populations of vibrational states v=1 to v=14 have been inferred. The vibrational population distribution is significantly non‐Boltzmann. more...