atomic oxygen measurements in air and air/fuel ......single 25 nsec discharge pulse, and in burst...

American Institute of Aeronautics and Astronautics

1

Atomic Oxygen Measurements in Air and Air/Fuel Nanosecond Pulse Discharges by Two Photon Laser Induced

Fluorescence

M. Uddi1, N. Jiang2, E. Mintusov2, I. V. Adamovich3, and W. R. Lempert4

Department of Mechanical Engineering, The Ohio State University, Columbus, OH 43202

ABSTRACT

Xenon calibrated Two Photon Absorption Laser Induced Fluorescence (TALIF) is used to measure absolute atomic oxygen concentrations in air, methane-air, and ethylene-air non-equilibrium plasmas, as a function of time after initiation of a single 25 nsec discharge pulse, and in burst mode, in which a sequence of 2 – 100 discharge pulses is initiated at 100 kHz. Peak O atom mole fraction in air, after a single pulse, is ~0.4 x 10-4 at 60 torr, with decay occurring on a time scale of ~2 msec. Peak O atom mole fraction in a stoichiometric methane-air mixture is found to be approximately equal to that in pure air, but the rate of decay is found to be faster by a factor of approximately two to three. In Ф = 0.5 ethylene-air, peak atomic oxygen concentration is reduced by a factor of approximately four, relative to air, and the rate of decay increased by approximately one order of magnitude due to much faster rate of reaction of atomic oxygen with ethylene, compared to methane, at room temperature. Burst mode measurements show very significant (up to ~0.2%) build-up of atomic oxygen density in air, and some build-up (by a factor of approximately three) in methane-air at Ф=0.5. Burst measurements in ethylene-air at Ф=0.5 show essentially no build-up, due to rapid O atom reactions with ethylene in the time interval between the pulses. Discharge modeling calculations, incorporating full air species kinetics complemented with GRI Mech 3.0 hydrocarbon oxidation mechanism, are shown to provide good overall agreement with all the experimental data presented here. Reduced kinetic mechanisms are also identified, which provide additional insight into the key processes of O atom generation and decay.

1 Graduate Research Assistant, Student Member AIAA 2 Post-Doctoral Researcher, Member AIAA 3 Associate Professor, Associate Fellow AIAA 4 Professor, Associate Fellow AIAA


2

1. INTRODUCTION Over the last few years, significant progress has been made in generation and sustaining

stable and diffuse large volume (~100 cm3), high pressure (~0.1-1.0 atm) nonequilibrium plasmas using nanosecond duration, high voltage pulses [1-11]. The basic idea is to create volume ionization in a gas flow by application of ~20-50 kV, ~10 nsec duration pulses at a 10-100 kHz pulse repetition rate. Between the pulses, the plasma can be sustained, if necessary, by application of a relatively low voltage, sub-breakdown DC or RF field. This approach has two distinct advantages. First, since the ionizing pulse duration, ~10-8 sec, is much shorter than the characteristic time scale for development of Joule heating / ionization instability, ~10-3-10-4 sec [12], which leads to glow-to-arc transition, stable repetitively pulsed plasmas can be sustained at much higher pressures and power loadings compared to other types of nonequilibrium plasmas. Second, unlike conventional self-sustained discharges, with this technique the sustainer voltage, which accounts for as much as 90-95% of the total input power, can be independently controlled. Specifically, the sustainer voltage can be chosen to tailor the reduced electric field, E/N, where N is the number density, to a value optimum for excitation of the appropriate molecular energy states, such as vibrational levels of the ground electronic state nitrogen, N2(X1Σ,v), or low-energy electronic states of oxygen, O2(a1Δ) and O2(b1Σ). On the other hand, high voltage pulses result mainly in excitation of high-energy electronic levels of nitrogen and oxygen, as well as in their dissociation and ionization.

The use of repetitively pulsed discharges has formed the basis for several recent studies of oxygen-containing plasmas for Plasma Assisted Combustion (PAC) [1-7], Electric Discharge Chemical Oxygen Iodine Laser (E-COIL or DOIL) development [8, 9], and MHD supersonic flow control [10, 11]. For example, using such pulsers, Kim et al. [1] have reported large increases in the critical co-flow velocity, Vco, leading to lift-off of a methane/air jet diffusion flame (from ~1-2 times the laminar flame speed, SL, to as much as twenty times SL). The same group has also observed increased NO production in premixed methane/air PAC flames, presumably due to excited metastable species reactions such as O + N2* → NO + N [2]. Similar stability limit extension has been reported by Pilla et al [3], who demonstrated significant reduction of the lean flammability limit in a premixed propane-air flame, down to Φ=0.3. Several PAC studies have also been recently reported by the Starikovskii/Starikovskaya group [4,5]. In particular, they demonstrated ignition delay time reduction, using a single high voltage, nanosecond duration discharge pulse, by more than an order of magnitude in argon diluted H2/air mixtures preheated to ~900 K in a shock tube [4]. The same group has also observed an increase in blow-off velocity, by a factor of ~3 (from 3 to 9 m/sec), by application of a kHz pulse repetition rate nsec discharge to a pre-mixed, Φ=0.7 argon diluted methane/air flame [5]. Finally, and of particular relevance to this study, Bao et al [6], and Lou et al [7] have reported ignition and nearly complete combustion in low temperature oxidation of ethylene and methane in air at P=70-90 torr, at the conditions when the repetitively pulsed air plasma temperature (before fuel is added) was T=100-2000 C. Nanosecond pulser / DC sustainer discharges have also been used to generate non-equilibrium O2-helium plasmas containing substantial amounts of metastable singlet delta oxygen molecules, O2(a1Δ), and O atoms. In current E-COIL systems [9, 13], atomic oxygen contributes to the necessary dissociation of molecular iodine, but also plays an undesirable role of rapidly quenching the upper level of the lasing transition, I*(2P1/2) → I(2P3/2) [14]. This leads to laser gain and output power reduction. Therefore pulsed discharge optimization, such that the


3

resultant O atom concentration is not too high to result in significant I*(2P1/2) quenching, may be necessary.

Although successful engineering applications of nanosecond pulse discharges are rapidly emerging, there have been few detailed measurements of key radical species generated in these discharges, compared with kinetic modeling calculations. In a recent paper, we reported time-dependent absolute O atom number density measurements in O2-helium discharges [15] using the Two Photon Absorption Laser Induced Fluorescence (TALIF) technique [16-18]. Absolute O atom number density was determined using a xenon calibration technique recently described by Niemi, et al [19], which is similar to that reported by Niemi, et al [20] and Grinstead, et al [21] for absolute measurements of N atom number density in large scale arc jet facilities. In this paper we present two sets of measurements of absolute O atom number density in air, methane-air, and ethylene-air pulsed plasmas. In the first set, time resolved data is obtained for plasmas initiated by a single high voltage, approximately 20 nanosecond duration discharge pulse. In the second set, O atom concentrations are obtained immediately (within a few μs) after the final pulse of a rapid burst of 2–100 discharge pulses, initiated at 100 kHz pulse repetition rate (i.e. 10 microseconds apart). In both cases good agreement is found between the experimental data and kinetic model predictions.

2. EXPERIMENTAL

2.1 Experimental Setup Atomic oxygen TALIF measurements are performed in a flowing discharge “flow reactor” fabricated from a single piece rectangular cross section quartz channel, 150 mm long x 22 mm span x 10 mm height, with flanges at the ends for connection of the gas inlet and exit lines and a test cell pressure sensor. The walls of the quartz channel are 1.75 mm thick. Two rectangular copper plate electrodes are attached directly to the outside surface of the channel and are covered by recessed acrylic plastic plates. The electrode plates are 14 mm wide and 63 mm long, and are rounded at the corners to reduce the electric field nonuniformity. The flow velocity is ~1 m/sec, corresponding to a residence time of the flow in the discharge region of approximately 0.08 seconds. High voltage, nanosecond duration pulses are generated in air, methane/air, and ethylene/air mixtures using high voltage pulsed plasma generators, several of which are available in our laboratory. Most of the work presented here utilizes a pulser manufactured by Chemical Physics Technologies (CPT) which is capable of producing 18 kV pulses with individual pulse duration of ~25 nsec and maximum pulse repetition rate of 40 kHz. For all time-dependent measurements reported in this paper, however, the pulser is operated at 10 Hz, which matches the repetition rate of the TALIF diagnostic laser and, more importantly, assures that each gas sample experiences only a single discharge pulse during its resonance time within the flow reactor. Figure 1 shows a typical high voltage waveform for an individual pulse from the CPT pulser. As will be described in more detail in section 3, we have also performed O atom TALIF measurements in which a “burst” of 2–100 pulses, 10 μsec apart, is repeated at 10 Hz pulse burst frequency. These measurements are performed with a pulser manufactured by FID GmbH, which is similar to the CPT pulser but has maximum output voltage of 32 kV (up to 64 kV at the end of a pulse transmission line to the load), individual pulse duration of 4-5 nsec, and maximum continuous repetition rate of 100 kHz

The TALIF apparatus, illustrated schematically in Fig. 2, utilizes a single 10 Hz repetition rate commercial injection-seeded Nd:YAG laser with 532 and 355 nm outputs. The 532 nm


4

beam pumps a tunable dye laser, which generates output at ~619 nm which is mixed with the 355 nm beam (in BBO) to generate the required UV wavelength of ~225 nm. A commercial autotracker controls the BBO phase matching angle as the dye laser is scanned. The 225 nm UV beam, with energy per pulse of ~40 μJ, is focused with a 300 mm focal length plano-convex lens into the plasma. In order to mitigate potential saturation effects, the TALIF collection volume, defined by the imaging optics, is centered approximately 2.5 cm upstream of the focused beam waist. TALIF signal, spectrally filtered with a 850±40 nm bandpass interference filter, is 1:1 imaged (f/2) onto a standard photomultiplier tube. The PMT signal is pre-amplified and averaged with a commercial boxcar averager. A photodiode detects diffuse UV scattering, providing a UV energy normalization channel that is continuously monitored during the laser scan. The timing of the experiments is controlled by a delay generator which triggers the pulser, laser and boxcar, in order to acquire signal at variable delay times with respect to discharge initiation. Initial experiments with the pulser were complicated due to electromagnetic interference which affected the laser (Q switch and flash lamp triggers), the autotracker, and the PC controlling the dye laser tuning. This was overcome by connecting these instruments to power circuits separate from that of the pulser. The pulser/test cell was also placed on a separate optical table, electrically (???) insulated from the table surface. 2.2 Details of O Atom TALIF Diagnostic Wavelengths of transitions from the ground state to the first excited states of common atomic species, such as H, O, C, and N, correspond to the vacuum ultraviolet (VUV) region of the spectrum. The strong single photon absorption by air in this spectral region has led to the development of what is known as the Two-photon Absorption Laser Induced Fluorescence (TALIF) technique [16,17,22], which has been utilized in a variety of plasma [18] and combustion [22,23] environments. The TALIF measurement of atomic oxygen has been described in detail previously [16,17] and will be therefore only be briefly summarized here. Referring to the energy level diagram shown on the left side of Fig. 3 (taken from Ref. [16]), the transition between the ground 2p 3P state and the excited 3p 3P state is two-photon allowed with a single photon wavelength of 225.7 nm. From this upper level the atom can make a single photon allowed downward transition to the 3s 3S state, with emission at 844.6 nm. At high laser intensities, the excited atom can also be ionized by a third 225.7 nm photon. At pressures above a few torr the 3p 3P state experiences non-negligible collisional quenching, with a resultant loss in fluorescence quantum yield. Assuming that the laser intensity is sufficiently low for the saturation effects to be ignored, and that the pressure is sufficiently high that the steady-state approximation can be invoked for the excited state number density, it is easy to show that the detected signal in photons per laser pulse, as a function of laser frequency, is given as (1) where γ is a constant which depends upon the geometry of collection and the transmission of the optical elements, including the spectrometer, η is the quantum yield of the detector photocathode, V is the collection volume, gND represents an additional loss due to insertion of a

neutral density filter, as will be discussed below, QA

Aa

+= 2121 is the fluorescence quantum yield,

σ(2) is the two-photon absorption cross section, g(ω) is the normalized two photon absorption line

( ) ( ) dt)t(IN)T(FGg)h(

agVS )()(

ND ∫∞

=0

200

22

2

21 ωνσηγω)


5

shape function, G(2) is a photon statistical factor [24], which takes into account the intensity beating of the stochastic multi-mode excitation field, F(T) is the Boltzmann factor for the lower level of the two photon absorption, N0 is the ground electronic state number density, and I0(t) is the time-dependent laser intensity at the measurement location. Note that while it is, in principle, possible to determine all of the quantities in Eq. (1), in practice this is quite difficult. As an alternative approach, the absolute number density of O atoms can be found by comparing the observed TALIF signal of O atoms with that from a stable reference species whose density and temperature are known. Goehlich et al [25], reported the use of O atom TALIF calibration procedure based upon the two photon xenon absorption resonance,

5p6 1So → 5p5 7p[3/2]2 (2)

at 225.5 nm, with subsequent 7p → 6s single photon allowed xenon fluorescence at 461 nm. A similar approach has also been presented for nitrogen atomic TALIF [20,21]. More recently, Niemi, et al [19] have reported an improved scheme which is based upon the two photon xenon absorption resonance, 5p6 1S0→ 6p’[3/2]2 (3) at 224.31 nm. The advantage of this approach is that the fluorescence emission is at 834.91 nm, corresponding to the 6p’[3/2]2 →6s’[1/2]1 transition of xenon. For both calibration schemes, the O and Xe absorption wavelengths are quite close to one another, so that mode characteristics of the UV beam, which are mostly determined by the dye laser output, are quite similar. In the latter scheme, however, the emission wavelengths are also very close to each other (844 nm vs 835 nm), so there is no need to modify either the collection bandpass filter, or the photomultiplier detector. Referring to Eq. (1), whereγ , V, η, and G(2) are now the same for O and Xe atoms, the ratio of spectrally integrated signal levels, each normalized to the square of the measured UV pulse energy, leads to the following expression for the O atom number density in terms of the known number density of xenon, (4) Note that in the present experiment, the voltage (and therefore the gain) applied to the photomultiplier detector was purposefully kept constant, despite xenon signal levels which were inherently higher by between three and four orders of magnitude. In order to maintain linearity a neutral density filter, with calibrated transmission gND = 10-4, was employed to attenuate the xenon signal. No neutral density filter was used for collection of the O atom signal. It should also be noted that the ground electronic state of the O atom is a triplet, whereas that of xenon is a singlet (see Fig. 3). All measurements reported here used the ground 3P2 component, for which the Boltzmann factor, F0, at 300 K is equal to 0.7429. Figure 4 shows typical O atom (left) and Xe calibration (right) TALIF spectra. The air spectrum was obtained in a flowing plasma at 60 torr whereas the xenon spectum was obtained with no flow, with the discharge cell filled to 55 torr of pure xenon. (The xenon density is found

( )( )

( )( ) ( )

2221o o

o ND Xe2Xe 21 Xe o

a Xe XeS 1N g NS a O F TO

⎛ ⎞σ ⎛ ⎞υ⎜ ⎟= ⎜ ⎟⎜ ⎟⎜ ⎟ υσ ⎝ ⎠⎝ ⎠


6

using the ideal gas equation p=nkbT). This relatively high pressure was used in order to maintain comparable saturation intensities (see quenching data in Table I) for the O atom and xenon measurements, enabling both measurements to be performed without changing the UV laser energy. As described in Bamford, et al [16], the O atom spectrum consists of three closely spaced transitions corresponding to the fine structure of the 3p 3P upper electronic state. The solid curve is a least squares fit to a sum of three spectral components, assuming a line shape which is a convolution of the true Doppler profile of the O atom absorption with an assumed 3 GHz linewidth Gaussian profile of the dye laser. The line positions and relative intensities of the individual components are taken from Baskin [26] and Bamford [16], respectively. The xenon spectrum is similarly least squares fit, but to a single transition. The resulting least squares fits are numerically integrated to yield the experimental signal levels, which are used in Eq. (4). Experimental precision, based on repeated measurements under identical conditions, was found to be ~±10%. Implementation of the calibration procedure requires that the values of the two photon cross section and the fluorescence quantum yield be known for both xenon and atomic oxygen. Table I shows the fundamental spectroscopic and quenching values that were used for analysis of the raw intensity data, along with estimated uncertainties, taken from references [19,20,27]. Table II shows the calculated values of the fluorescence quantum yield, a21, for the xenon calibration, and for each of the oxygen mixtures for which data will be given below. Note that for air and methane-air the fluorescence quantum yields are identical to within better than two percent. Since, to our knowledge, O atom quenching by ethylene has not been experimentally determined, and since the ethylene mole fraction is only 0.033 for the Ф = 0.5 ethylene-air mixture studied here, we have also used a value of a21 = 0.022 for that case. Based on a simple propagation of errors, we estimate a systematic uncertainty in the absolute O atom calibration, based on the cited uncertainties in the cross section and quenching rates given in Table I, of ±40%. Note that no uncertainty value is given in reference [20] for the N2 quenching, so we have, somewhat arbitrarily, assigned in a fractional uncertainty of ±5% equal to that for the O2 quenching uncertainty given in reference [19] by the same authors.

2( )σ̂ τ (ns) )10( 1310 −− scmki Xe O2 N2 CH4

Xe 344 94 0 98 10. . −± × cm4 [19] 40.8±2.0 [19] 3.6±0.4 [19] O 342 66 0 80 10. . −± × cm4 [16] 34.7±1.7 [19] 9.5 ±0.5 [19] 5.9 [20]

5.5±0.15 [27]

Table I. Natural lifetimes, τ , TALIF absorption cross sections, 2( )σ̂ , and quenching coefficients, ik , of Xe and O atom excited states at T=300 K.

Table II.

Fluorescence quantum yields for O and Xe TALIF measurements.

O Xe Mixture Air Air-CH4

(Φ=1.0) Air-C2H4 (Φ=0.5)

Pure Xe (55 torr)

a21 0.022 0.022 0.022 0.027


7

The calibration procedure also requires that both the O atom and xenon TALIF measurements be performed in the non-saturated regime, in which the observed signal scales as the square of the UV laser pulse energy. This was confirmed by a series of measurements in which the laser energy was varied between ~10 and 250 μJ / pulse. For both O and Xe, the desired quadratic dependence was observed over the entire range. For all data presented in this paper, the pulse energy was equal to ~40 μJ.

3. RESULTS AND KINETIC MODELING

3.1 Description of Kinetic Model The kinetic model of the air plasma used in the present work incorporates a set of ordinary

differential equations for number densities of neutral, charged, and excited species produced in the plasma, N, N2, O, O2, O3, NO, e-, N2+, O2+, O+, NO+, O-, O2-, N2(A3Σ), N2(B3Π), N2(C3Π), N2(a'1Σ), O2(a1Δ), O2(b1Σ), and O(1D), as well as the energy equation for prediction of the temperature of the mixture. This set of equations is coupled with the steady, two-term expansion Boltzmann equation for the electron energy distribution function (EEDF) of the plasma electrons [28] using experimental cross sections of electron impact electronic excitation, dissociation, ionization, and dissociative attachment processes [29,30]. The Boltzmann equation yields the rate coefficients of these electron impact processes by averaging the cross sections over the EEDF. In addition, the kinetic model incorporates chemical reactions of excited electronic species, electron-ion recombination and ion-ion neutralization processes, ion-molecule reactions, and electron attachment and detachment processes. Rate coefficients of these processes are taken from Ref. [31]. The high voltage pulse shape used by the code is a Gaussian fit to the experimentally measured pulse shape with the pulse FWHM of τ=25 nsec (see Fig. 1).

The effect of short pulse duration on the EEDF in the pulsed discharge was estimated in our previous paper [32] by solving a two-term expansion Boltzmann equation with the unsteady term incorporated [28], uncoupled from the set of equations for species number densities. The results of calculations for air at P=0.1 atm and E/N=50·10-16 V·cm2 (500 Td) have shown that the EEDF reaches near steady state over ~0.1 nsec, starting from the Maxwell distribution at Te=2.5 eV as an initial condition. Since this is much shorter than the pulse duration produced by the pulsed plasma generator, ~25 nsec, the assumption of the quasi-steady-state EEDF controlled by the instantaneous value of E/N at the present conditions is justified.

The main difficulty of modeling nanosecond pulse discharge plasmas using the present model is that the model does not incorporate the Poisson equation for the electric field and therefore does not take into account charge separation and sheath formation near the electrodes. Neglecting this effect may considerably overestimate the reduced electric field in the plasma, because the voltage drop across the sheaths in the nanosecond pulse discharge may be quite significant, up to several kV [33]. To circumvent the difficulty of evaluating the electric field without solving the Poisson equation, in the present work the reduced electric field in the plasma was considered to be an adjustable parameter, varied until the peak O atom number density in the air plasma predicted by the model was close to the experimentally measured value.

3.2 Air Results The upper data/solid curves in Figs. 5 and 6 compare experimental and predicted time-dependent O atom mole fractions generated by a single-pulse discharge in air at P=60 torr, on linear and semi-log scales, respectively, using the kinetic model described in Section 3.1. (Note


8

that in the modeling calculations, the high voltage pulse is initiated at t=10-7 sec). It can be seen that to within the experimental precision the model agrees well with the experiment, correctly reproducing O atom rise and decay after the pulse for delay times ranging from 0.5 µsec to 5 msec. In these calculations, the reduced electric field in the plasma was assumed to be E/N=24.7·10-16 V·cm2 (247 Td). This corresponds to the peak voltage of 6.58 kV for the distance between the electrodes of 1.4 cm (neglecting voltage drops in the sheaths). As expected, the peak voltage used is significantly lower than the actual pulse voltage. The predicted total energy deposited into the plasma is 0.61 mJ, approximately half of which goes to O2 dissociation. Sensitivity analysis showed that O atoms are primarily formed both by electron impact during the discharge pulse, O2+e→O+O+e, and by collisions of electronically excited nitrogen molecules with O2 for times up to ~10 µsec after the pulse, N2(A3Σ)+O2→N2(X1Σ)+O+O. The dominant O atom decay process is recombination with oxygen resulting in ozone formation, O+O2+M→O3+M. Because of that, the O atom decay time is nearly independent of the absolute O atom number density produced by the pulse. In order to provide more physical insight a reduced air plasma O atom generation and decay mechanism has been developed. This simplified mechanism, which also provides excellent agreement with the air TALIF data of Figs. 5 and 6, is summarized in Table III.

O atom generation Rate Reference

N2 + e- = N2(A3Σ) + e- σ* [29]

N2 + e- = N2(B3Π) + e- σ [29] N2 + e- = N2(C3Π) + e- σ [29] N2 + e- = N2(a'1Σ) + e- σ [29] O2 + e- = O(3P) + O(3P,1D) + e- σ [30] N2(C3Π) + O2 = N2 (a'1Σ) + O2 3.0·10

-10 cm3/s [31] N2(a'1Σ) + O2 = N2 (B3Π) + O2 2.8·10

-11 cm3/s [31] N2(B3Π) + O2 = N2 (A3Σ) + O2 3.0·10

-10 cm3/s [31] N2(A3Σ) + O2 = N2 + O + O 2.5·10

-12 cm3/s [31] O atom decay Rate

O + O2 + M = O3 + M 5.9·10-34 cm6/s [31]

O + O3 = O2 + O2 8.3·10-15 cm3/s [31]

* Calculated by the Boltzmann solver from the experimental cross section

Table III. Reduced O atom kinetics mechanism in the air plasma As can be seen in the experimental data of Figs. 5 and 6, O atom mole fraction in the air discharge peaks at approximately at 4·10-5 (O atom number density of 0.8·1014 cm-3) and decays (1/e) over ~2 msec. This suggests that at the conditions of the low-temperature repetitively pulsed nanosecond discharge ignition experiments [6,7] in which the flow experiences ~100 discharge pulses at pulse repetition rates up to 50 kHz, i.e. within ~2 msec during its residence time in the discharge, significant build-up of atomic oxygen is predicted to occur in air plasmas, up to ~0.5%. Similar results were obtained at three different pressures, P=40, 60, and 80 torr.


9

3.3 Air/Fuel Results To estimate the effect of charged species kinetics processes on the neutral species chemistry,

a series of modeling calculations was conducted using a simplified approach, in which charged species kinetics processes (ionization, recombination, attachment, detachment, and ion-molecule reactions) were not taken into account. Instead, electron density was modeled as a Gaussian shape pulse with the same width as the experimental electric field pulse (see Fig. 1) and peak value adjusted to result in the same pulse energy as predicted by the full model (with charge species kinetics incorporated). The results showed that neutral species concentrations in the air plasma (electronically excited nitrogen, O atoms, and ozone) predicted by both approaches are very close. This demonstrates that (i) charged species kinetics do not significantly affect neutral species chemistry, and (ii) the simplified approach can be used for modeling pulsed plasma chemical reactions in hydrocarbon-air mixtures with complex ion chemistry, if the pulse energy has been measured. The simplified approach described above (without charged species kinetics) has been used in all subsequent calculations.

To model O atom generation and decay in methane-air and ethylene-air plasmas, the air plasma kinetic model was combined with the GRI Mech 3.0 [35] hydrocarbon oxidation mechanism and supplemented with methane and ethylene dissociation by electron impact and in reactions with electronically excited nitrogen molecules. Cross sections and rate coefficients of these reactions were taken from [36-44]. In particular, experimental electron impact dissociation cross section of methane into CH3 + H [36], and theoretical electron impact dissociation cross sections of ethylene into C2H3 + H, C2H2 + H2, C2H2 + H + H, and C2H + H2 + H [37], have been used. In methane and ethylene dissociation reactions by electronically excited nitrogen molecules, the quantum yield for H atom production was assumed to be equal to one, i.e. dissociation into CH3 + H and C2H3 + H was assumed.

In the modeling calculations in fuel-air mixtures, the high-voltage pulse energy was assumed to be the same as in air, 0.61 mJ. This assumption is based on the results of pulse energy measurements in a repetitively pulsed nanosecond discharge [6], which showed that pulse energies in air and ethylene-air mixtures are nearly the same. The lower data/dashed curves in Figs. 5,6 show experimental and modeling results for single pulse discharges in a stoichiometric methane-air mixture and a lean ethylene-air mixture at the equivalence ratio of Φ=0.5, both at P=60 torr. Again, it can be seen that for both air/fuel mixtures the model is in good overall agreement with the experimental data.

In the methane-air mixture, it can be seen that the O atom decay rate is increased by a factor of ~2-3 compared to pure air (see Fig. 5). To identify kinetic processes responsible for the rapid O atom decay in the methane-air mixture, modeling calculations were conducted for three different cases: (1) Aair plasma model combined with the GRI-Mech 3.0 mechanism hydrocarbon oxidation (baseline model), (2) baseline model plus electron impact dissociation of methane, CH4 + e- → CH3 + H + e-, and (3) baseline model plus electron impact dissociation of methane and methane dissociation in collisions with electronically excited nitrogen molecules, N2* + CH4 → N2 + CH3 + H (full model). Figure 7 compares the results of these calculations with the experiment. It can be seen that it is H and CH3 radical production from methane dissociation that greatly accelerates the rate of O atom decay, particularly that resulting from collisions with N2*. The model prediction for Case 3 (full model) is in good agreement with the experiment.

Sensitivity analysis shows that the increased atomic oxygen loss rate in methane-air occurs primarily due to reactions of H atoms and CH3 radicals, generated by methane dissociation, in


10

particular H+O2+M→HO2+M, O+HO2→OH+O, and O+CH3→H+CH2O. In fact, as clearly seen in Fig. 7, when methane dissociation processes are removed from the model it is found that the predicted O atom decay rate in methane-air is almost exactly the same as in air. In this regard it is noted that the rate coefficient of the reaction between CH4 and O at room temperature, O+CH4→OH+CH3, is very low, k=4.8·10-18 cm3/sec, and does not contribute to the observed O atom decay. Note that the rates of reactions of O atoms with HO2 and CH3, which control the O atom decay time, are both proportional to the product of two radical concentrations. Therefore in this case the O atom decay time strongly depends on the absolute radical density produced by the pulse and would decrease with the pulse energy.

From Fig. 8, it can also be seen that although the case (3) model correctly reproduces the O atom decay rate, peak O atom density in methane-air, relative to that in pure air, is underpredicted by about 30%. The model predicts that this occurs due to rapid quenching of excited nitrogen molecules in collisions with methane resulting in its dissociation, e.g. N2(C3Π)+CH4→N2+H+CH3, so that the contribution of excited nitrogen reactions to oxygen dissociation is reduced. This difference suggests presence of an additional oxygen dissociation mechanism at these conditions, perhaps in collisions with methane predissociated by electronically excited nitrogen. As in the air plasma case, a reduced kinetics mechanism for O atom production and decay has been developed for methane-air. In addition to the air processes given in Table III, this reduced mechanism adds the processes listed in Table IV, which are divided into three groups, (i) fuel dissociation reactions, both by electron impact (during the discharge pulse) and collision (after the discharge pulse), (ii) rapid reactions of ground electronic state species controlling the rate of O atoms decay, and (iii) relatively slow exothermic fuel oxidation reactions controlling energy release after the pulse. Figures 8 and 9 show a comparison of O atom (Fig. 8) and heat release (Fig. 9) for the complete and reduced methane-air mechanism. It can be seen that the predictions of O atom production and decay are nearly indistinguishable for the two models, whereas the reduced model predicts approximately 10% additional heat release for times on the order of 20 msec after the discharge pulse. Note that O atom concentration measurements in the present experiments do not allow validation of the slow fuel oxidation mechanism, which would require measuring the temperature rise of the mixture after the pulse (or after a “burst” of multiple pulses). Also, the reduced mechanism of Table IV does not accurately reproduce concentrations of fuel oxidation product species such as CO, CO2, CH2O, and H2O, predicted by the full mechanism. Finally, validating the model predictions of these species concentrations would also require measuring their number densities in the experiment, e.g. by diode laser absorption or by FTIR absorption spectroscopy.

Fuel dissociation Rate Reference

CH4 + e- = CH3 + H + e- σ [36] N2(A3Σ) + CH4 = N2 + CH3 + H 3.3·10-15 cm3/s [38] N2(B3Π) + CH4 = N2 + CH3 + H 3.0·10-10 cm3/s [39] N2(C3Π) + CH4 = N2 + CH3 + H 5.0·10-10 cm3/s [40] N2(a'1Σ) + CH4 = N2 + CH3 + H 3.0·10-10 cm3/s [41]

O atom decay reactions Rate O + CH3 = H + CH2O 8.4·10-11 cm3/s [35] H + O2 + M = HO2 + M 5.7·10-32 cm6/s [35] O + HO2 = OH + O2 3.3·10-11 cm3/s [35]


11

Energy release reactions Rate OH + CH4 = CH3 + H2O 8.1·10-15 cm3/s [35] OH + HO2 = O2 + H2O 5.6·10-11 cm3/s [35] HO2 + CH3 = OH + CH3O 6.3·10-11 cm3/s [35] CH3O + O2 = HO2 + CH2O 1.8·10-15 cm3/s [35] H + O2 = O + OH 3.6·10-22 cm3/s [35]

Table IV. Reduced kinetic mechanism for the methane-air plasma

(complements processes of Table III).

The lower curve/solid points in Fig. 6 shows the experimental TALIF data for ethylene-air at Ф = 0.5 (Note that this relatively low value of equivalence ratio was chosen purposefully to avoid ignition). Comparison with the methane-air data (Fig. 5) indicates a dramatic increase in the rate of O atom decay, by approximately two orders of magnitude, ~10 µsec vs. 2 msec. This rapid decay can be attributed, primarily, to the rapid rate of reaction between C2H4 and O atoms at room temperature, C2H4+O→CH3+HCO and C2H4+O→CH2CHO+H, with k=4.9·10-13 cm3/sec and k=2.6·10-13 cm3/sec, respectively. Much faster rate of O atom reaction with ethylene, relative to methane, also causes the predicted O atom decay to be less sensitive to CxHy radical and H atom generation in ethylene plasma dissociation processes. Finally, as also clearly evident from Fig. 7, the rapid consumption of O atoms results in peak mole fraction in ethylene-air which is significantly lower than in air, 1.4·10-5 vs. 4·10-5.

Figure 10 shows the results of modeling calculations in ethylene-air for the same three cases as in Figure 8: (1) Air plasma model combined with the GRI-Mech 3.0 mechanism hydrocarbon oxidation (baseline model), (2) baseline model plus electron impact dissociation of ethylene, CH4 + e- → products + e-, and (3) baseline model plus electron impact dissociation of ethylene and ethylene dissociation in collisions with electronically excited nitrogen molecules, N2* + C2H4 → N2 + C2H3 + H (full model). Similar to the methane-air case, it can be seen that H and C2H3 radical production in collisions with excited nitrogen reduces the peak O atom number density since, again, the contribution of excited nitrogen reactions to oxygen dissociation is diminished. As in methane-air, the model prediction for Case 3 (full model) is in good agreement with the experiment, although the difference in decay rate between Case (1) and Case (3) is substantially less than in methane-air.

Analogous to methane-air, a reduced O atom generation and decay mechanism for ethylene-air is summarized in Table V. Similar to Table IV, chemical reactions of species in their ground electronic states in Table V are divided into two groups, (i) rapid reactions controlling the rate of O atoms decay, and (ii) relatively slow exothermic fuel oxidation reactions controlling energy release. While not plotted, the reduced and full ethylene-air models give very close results for both O atom generation and decay, and heat release. In particular, as discussed below and in contrast to the methane-air case, both ethylene-air models predict significant additional net energy release (by about a factor of two), compared to total discharge input energy, due to low temperature net exothermic fuel oxidation processes.

Fuel dissociation Rate Reference

C2H4 + e- = products* + e- σ [37] N2(A3Σ) + C2H4 = N2 + C2H3 + H 9.7·10-11 cm3/s [42] N2(B3Π) + C2H4 = N2 + C2H3 + H 3.0·10-10 cm3/s estimate N2(C3Π) + C2H4 = N2 + C2H3 + H 3.0·10-10 cm3/s estimate


12

N2(a'1Σ) + C2H4 = N2 + C2H3 + H 4.0·10-10 cm3/s [43] O atom decay reactions Rate

O + C2H4 = CH3 + HCO 4.9·10-13 cm3/s [35] O + C2H4 = H + CH2CHO 2.6·10-13 cm3/s [35] C2H3 + O2 = HCO + CH2O 5.0·10-12 cm3/s [35] C2H3 + O2 = O + CH2CHO 2.6·10-12 cm3/s [35]

Energy release reactions Rate O + CH2CHO = H + CH2 + CO2 3.0·10-13 cm3/s [35] H + O2 + M = HO2 + M 5.7·10-32 cm6/s [35] O + HO2 = OH + O2 3.3·10-11 cm3/s [35] OH + HO2 = O2 + H2O 5.6·10-11 cm3/s [35] OH + C2H4 = C2H3 + H2O 8.1·10-15 cm3/s [35] HO2 + CH3 = OH + CH3O 6.3·10-11 cm3/s [35] CH3O + O2 = HO2 + CH2O 1.8·10-15 cm3/s [35] O2 + CH2CHO = OH + HCO + HCO 3.9·10-14 cm3/s [35] HCO + O2 = HO2 + CO 1.1·10-11 cm3/s [35] HO2 + HO2 = O2 + H2O2 3.3·10-12 cm3/s [35] CH2 + O2 = H + H + CO2 6.7·10-13 cm3/s [35]

* Three comparable dissociation channels into C2H3 + H, C2H2 + H2, and C2H2 + H + H [37].

Table V. Reduced kinetic mechanism for the ethylene-air plasma (complements processes of Table III).

Figure 11 shows modeling predictions, using the full kinetic models, of the total deposited

discharge pulse energy and energy dissipated as heat in air, methane-air, and ethylene-air at the conditions of Figs. 5,6. It can be seen that in air up to 40-60% of the input pulse energy is stored in products of plasma chemical reactions and does not thermalize after the pulse. Comparison of Fig. 11 with Fig. 12, which plots predicted species mole fractions in the air plasma at the conditions of Figs. 5,6, indicates that a significant fraction of the input pulse energy is stored in O atoms (from ~1 µsec to ~1 msec after the pulse) and in ozone (at ~10 msec after the pulse). In methane-air, ~10 msec after the pulse all input energy is thermalized because much less ozone is formed. Note that in this case methane oxidation results in a modest additional energy release, about 15% in excess of the input pulse energy (see Fig. 11). Finally, in ethylene-air the predicted additional energy release due to ethylene oxidation is quite significant, approximately 80% of the input pulse energy. This suggests that the energy release, accumulated over multiple pulses, may result in significant additional heating of the ethylene-air mixture, almost by a factor of 3 higher than in air. This effect would accelerate the plasma chemical fuel oxidation process and possibly result in ignition. Note that at the present experimental conditions, 1 mJ energy thermalized in the gas mixture results in heating by approximately 1.20 K. Therefore, as discussed above, at the conditions of non-thermal ignition experiments of Ref. [6] (measured pulse energy of ≈2 mJ), this effect would result in additional heating of ethylene-air flows by ~200 K, compared to heating of air flows by the plasma. Such low-temperature exothermic chemistry could play a critical role in explaining recent non-equilibrium PAC results. A reliable validation of this effect can be done by rotational temperature measurements with the pulser operating in a “burst” mode, in which sequences of ~100-200 discharge pulses is be repeated at ~10 Hz burst rate. Such


13

measurements are planned for the near future using Coherent Anti-Stokes Raman Spectroscopy (CARS).

Finally, Fig. 13 shows predicted stable fuel oxidation product mole fractions at the conditions of Fig. 6. The total mole fraction of stable combustion products predicted by the model is 0.8·10-4, which is a factor of two higher than the O atom mole fraction generated in air, 0.4·10-4 (see Figs 5,6). This suggests existence of a low-temperature chain mechanism of ethylene oxidation, although with a short chain length.

Note that although the present model is in satisfactory agreement with the O atom concentration measurements in air, methane-air, and ethylene-air plasmas, this is not meant to imply that the thermalized pulse energy and fuel oxidation product concentrations predicted by the model, such as those plotted in Fig. 13, are necessarily accurate. Since the GRI Mech 3.0 hydrocarbon oxidation mechanism has been developed to fit high-temperature hydrocarbon ignition and combustion data, such as ignition delay time and autoignition temperature, its predictions may clearly not be accurate at room temperature. Further experiments, in particular time-resolved H atom, OH and CH3 radical measurements after the pulse, as well as temperature rise and stable species concentration (CO, CO2, CH2O, and H2O) measurements are key for validating the model predictions and for providing insight into the mechanism of ignition by a low-temperature plasma demonstrated in our previous experiments [6,7]. Such measurements are planned for the near future.

3.4 Burst Mode Results Oxygen atom densities were also measured, using the FID pulser (64 kV peak voltage, 5 nsec pulse duration) in a pulse “burst” mode. (At the present time, the CPT pulser is not capable of operation in this manner). Each burst consists of a sequence of between two and 100 discharge pulses, initiated 10 microseconds apart at the highest pulse repetition rate the FID pulser is designed for, 100 kHz. In a manner identical to that of the single pulse data, the TALIF excitation laser is slowly scanned to obtain a full O atom spectrum, as the burst sequence is repeated at 10 Hz repetition rate. O atom measurements are made within few μs delay (40μs forair, 10μs for air/ CH4 and 3μs for air/ C2H4) after initiation of the final pulse of the 2-100 pulse burst. Both in methane-air and in ethylene-air mixtures, the equivalence ratio was kept low, Φ=0.5, to avoid ignition. Figure 14 compares the experimental results of atomic oxygen number density as a function of the number of pulses in the burst with the predictions of the full kinetic model. (Note that in Fig. 14 time is plotted as the independent variable, which should be interpreted as the number of pulses within the burst by multiplication by 100 pulses / msec). It can be seen that significant build up of atomic oxygen density is both measured and predicted when the pulser is operated in burst mode in air flow, with maximum of ~ 3.65 x 1015cm-3 (~0.2% mole fraction) for 100 pulses in the burst. For air/CH4 flow, the build up is ~ 3.1 x 1014cm-3 (~0.016% mole fraction) showing that a significant fraction of the oxygen atoms produced during each individual pulse is lost to oxidation reactions on the time scale of the measurement. In air/C2H4 flow, the oxygen atom density remains almost constant, at ~3 x 1013 cm-3, essentially independent of the number of pulses in the burst, indicating very rapid consumption by C2H4, and other plasma species, within the 10 μs time delay between the pulses. Such rapid plasma oxidation chemistry could, potentially, lead to initiation of important chain and branching reactions, leading to low temperature ignition. From Fig. 14 it can be seen that the model satisfactory predicts the measured oxygen atom densities, both relative to one another in the different mixtures and as function of number of


14

pulses, albeit there is some under prediction, especially for higher number of pulses in the burst. In addition to systematic error in the calibration, which as stated previously is estimated to be of the order of +/- 40%, it is noted that the O atom density inferred from the experiment using equation 4 assumes constant temperature of 300 K when calculating the Boltzmann factor, F0. Since F0 decreases for increasing T and since the temperature is expected to rise as the number of pulses in the burst is increased, it can be seen from Eq. 4 that ignoring temperature rise results in an inferred O atom number density which is systematically low. Figure 15 shows the temperature of the mixture predicted by the code for the same conditions as Fig. 14. It can be seen that considerable temperature rise is predicted, particularly for ethylene – air where an increase of ~300 K is predicted for rapid input of 100 pulses. As stated above, CARS rotational temperature measurements are planned for the near future to verify confirm this prediction.

4. SUMMARY AND CONCLUSIONS Xenon calibrated Two Photon Absorption Laser Induced Fluorescence has been used to measure time-resolved absolute atomic oxygen concentrations in air, methane-air, and ethylene-air pulsed plasmas, initiated using both a single ~20 kV voltage, ~25 nsec duration pulse, in order to study the nascent kinetics of non-equilibrium pulsed discharge plasmas at a low (~300 K) temperature, and in burst mode, in which a sequence of 2 – 100 discharge pulses is initiated at 100 KHz. Calibrated TALIF spectra show peak atomic oxygen concentrations generated by a single pulse in air at 60 torr of ~ 0.8 x1014 cm-3, corresponding to a peak mole fraction of ~0.4 x 10-4, with decay occurring on a time scale of ~2 msec. Peak atomic oxygen concentration in a stoichiometric methane-air mixture is found to be approximately equal to that in pure air, but the rate of decay is found to be faster by a factor of approximately two to three. This result is primarily due to reactions with H atoms and methyl radicals formed both by electron impact and by reactions with electronically excited N2. In Ф=0.5 ethylene-air, peak atomic oxygen concentration is reduced by a factor of approximately four, relative to air, and the rate of decay increased by more than an order of magnitude, mainly due to the rapid rates of reactions of atomic oxygen with ethylene at room temperature. Burst mode measurements show very significant (up to ~0.2%) build-up of atomic oxygen density in air, and some build-up (by a factor of approximately three compared to a single pulse) in methane-air at Ф=0.5. Burst measurements in ethylene-air at Ф=0.5 show essentially no build-up, due to rapid O atom reactions with ethylene in the time interval between the pulses. Kinetic modeling calculations provide good overall agreement with all of the present experimental data, both for a single pulse and for pulse burst, and identify key processes of O atom generation and decay in room temperature air-fuel mixtures. Future work will focus on performing temperature measurements to validate modeling predictions of net exothermic low temperature oxidation.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the sponsorship of the U.S. Air Force Office of Scientific Research, and NASA – Glenn Research Center.


15

References 1. Kim, W., Do, H., Mungal, M.G, and Cappelli, M, IEEE Trans on Plasma Science 34, p. 2545 (2006). 2. Kim, W., Do, H., Mungal, M.G, and Cappelli, M, Proceedings of Combustion Institute 31, p. 3319

(2007). 3. Pilla G, D. Galley, D. Lacoste, F. Lacas, D. Veynante, and C. Laux, AIAA Paper 2006-3243, 37th

AIAA Plasmadynamics and Lasers Conference, San Francisco, CA, 2006. 4. S.A. Bozhenkov, S.M. Starikovskaya, and A.Yu. Starikovskii, Combustion and Flame, vol. 133,

2003, pp. 133-146 5. A. Yu. Starikovskii, Proceedings of the Combustion Institute 30 (2005) 2405-2417. 6. A. Bao, Yu.G. Utkin, S. Keshav, G. Lou, and I.V. Adamovich, “Ignition of Ethylene-Air and

Methane-Air Flows by Low-Temperature Repetitively Pulsed Nanosecond Discharge Plasma”, 7. A. Bao, Yu.G. Utkin, S. Keshav, G. Lou, and I.V. Adamovich, “Ignition of Ethylene-Air and

Methane-Air Flows by Low-Temperature Repetitively Pulsed Nanosecond Discharge Plasma”, IEEE Transactions on Plasma Science, vol. 35, pp. 1628-1638, 2007

8. G. Lou, A. Bao, M. Nishihara, S. Keshav, Y.G. Utkin, J.W. Rich, W.R. Lempert, and I.V. Adamovich, Proceedings of the Combustion Institute, vol. 31, Issue 2, January 2007, pp. 3327-3334.

9. A. Hicks, S. Norberg, P. Shawcross, W.R. Lempert, J.W. Rich, and I.V. Adamovich, “Singlet Oxygen Generation in a High Pressure Non-Self-Sustained Electric Discharge”, Journal of Physics D: Applied Physics, vol. 38, 2005, pp. 3812-3824

10. A. Hicks, Yu.G. Utkin, W.R. Lempert, J.W. Rich, and I.V. Adamovich, “Continuous Wave Operation of a Non-Self-Sustained Electric Discharge Pumped Oxygen-Iodine Laser”, Applied Physics Letters, vol. 89, 2006, p. 241131

11. M. Nishihara, N. Jiang, J.W. Rich, W.R. Lempert, I.V. Adamovich, and S. Gogineni, “Low-Temperature Supersonic Boundary Layer Control Using Repetitively Pulsed MHD Forcing”, Physics of Fluids, 2005, vol. 17, No. 10, p. 106102

12. M. Nishihara, J.W. Rich, W.R. Lempert, I.V. Adamovich, and S. Gogineni, “Low-Temperature M=3 Flow Deceleration by Lorentz Force”, Physics of Fluids, vol. 18, No. 8, 2006, p. 086101

13. Raizer, Yu.P., "Gas Discharge Physics", Springer-Verlag, Berlin, 1991. 14. D.L. Carroll, J.T. Verdeyen, D.M. King, J.W. Zimmerman, J.K. Laystrom, B.S. Woodard, G.F.

Benavides, K. Kittell, D.S. Stafford, M.J. Kushner, and W.C. Solomon, “Continuous-Wave Laser Oscillation on the 1315 nm Transition of Atomic Iodine pumped by O2(a1Δg) Produced in an Electric Discharge”, Appl. Phys. Lett., vol. 86, 2005, pp. 111104-111104-3

15. D.L. Carroll, J.T. Verdeyen, D.M. King, J.W. Zimmerman, J.K. Laystrom, B.S.Woodard, G.F. Benavides, K.W. Kittell, and W.C. Solomon, “Path to the Measurement of Positive Gain on the 1315-nm Transition of Atomic Iodine Pumped by O2(a1Δ) Produced in an Electric Discharge”, IEEE J. Quantum Electron., vol. 41, 2005, pp. 213-223.

16. Uddi M., N. Jiang, K. Frederickson, J. Stricker, I. Adamovich, and W. Lempert, “Spatially and Temporally Resolved Atomic Oxygen Measurements in Short Pulse Discharges by Two Photon Laser Induced Fluorescence”, AIAA Paper 2007-1357, 45th Aerospace Sciences Meeting and Exhibit, 8-11 January 2007, Reno, NV

17. Bamford J D, Jusinski L E, Bischel W K, “Absolute two photon absorption and three photon ionization cross sections for atomic oxygen”, 1986, Vol 34, No 1, Physical Review A, pp 185- 198

18. Bischel W K, Perry B E, Crosley D R, “Detection of fluorescence from O and N atoms induced by two photon absorption”, 1982, Vol 21, No 8, Applied Optics, pp 1419- 1429

19. Tserepi A D, Miller T A, “Spatially and temporally resolved absolute O- atom concentrations in etching plasmas”, 1995, 77(2), Journal of Applied Physics, pp 505-511

20. Niemi K., Gathen V. S. and H F Dobele H. F., Plasma Sources Sci. Technol. 14 (2005) 375–386 21. Niemi, K, Gathen, S. V., and Doebele, H.F., 2001, J. Phys. D 34, p. 2330-2335. 22. Grinstead, J.H., Driver, D.M., and Raiche, G.A., 2002, AIAA-2002-0398, 40th AIAA Aerospace

Sciences Mtg, Reno, NV


16

23. Goldsmith J E M, Laurendeau N M, “Two photon excited fluorescence measurements of OH concentration in a hydrogen in a hydrogen-oxygen flame”, 1986, Vol 25, No 2, Applied Optics, pp. 276-283

24. Bittner, J., Kohse-Hoinghaus, K., Meier, U., Kelm, S., and Just, Th., 1998, Comb. and Flame 71, p. 41.

25. Louden, R., The Quantum Theory of Light, 2nd Ed., Clarendon Press, Oxford UK, 1983. 26. Goehlich A, Kawetzki, and Dobele H F, “On absolute calibration with xenon of laser diagnostic

methods based on two photon absorption”, Vol 108, No 22, 8 June 1998, Journal of Chemical Physics, pp 9362- 9370

27. Baskin S., and Stoner J. A., Jr., Atomic Energy Levels and Grotrian diagrams” (North- Holland, Amsterdam, 1975), Vol. I, p. 165

28. J. Bittner, K.Kohse-Hoinghaus, U. Meier, and Th. Just, Chem. Phys Lett 143, p. 571 (1988). 29. L. G. H. Huxley and R. W. Crompton, “The Diffusion and Drift of Electrons in Gases”, Wiley, New

York, 1974. 30. Y. Itikawa, M. Hayashi, A. Ichimura, K. Onda, K. Sakimoto, K. Takayanagi, M. Nakamura, M.

Nishimura, and T. Takayanagi, “Cross Sections for Collisions of Electrons and Photons with Nitrogen Molecules”, J. Phys. Chem. Ref. Data, vol.16, 1986, pp. 985-1010

31. Y. Itikawa, A. Ichimura, K. Onda, K. Sakimoto, K. Takayanagi, Y. Hatano, M. Hayashi, H. Nishimura, and S. Tsurubichi, “Cross Sections for Collisions of Electrons and Photons with Nitrogen Molecules”, J. Phys. Chem. Ref. Data, vol.18, 1989, pp. 23-42

32. I.A. Kossyi, A. Yu. Kostinsky, A. A. Matveyev, and V.P. Silakov, “Kinetic Scheme of the Nonequilibrium Discharge in Nitrogen-Oxygen Mixtures, Plasma Sources Sci. Technol., vol. 1, 1992, pp. 207-220

33. M. Uddi, N. Jiang, K. Frederickson, E. Mintoussov, I.V. Adamovich, and W.R. Lempert, “Oxygen Atom Measurements in a Nanosecond Pulse Discharge by Two Photon Absorption Laser Induced Fluorescence”, AIAA Paper 2007-4027, 38th AIAA Plasmadynamics and Lasers Conference, 25-28 June 2007, Miami, FL

34. S.O. Macheret, M.N. Shneider, and R.C. Murray, “Ionization in Strong Electric Fields and Dynamics of Nanosecond Pulse Plasmas”, Physics of Plasmas, vol. 13, 023502, 2006.

35. Atkinson, R., Baulch, D.L. , Cox, R.A., Hampson,Jr.,R.F. , Kerr,J.A.,and Rossi, M.J.,1997 ,Journal of Physical and Chemical Reference Data , 26 , p. 521.

36. http://www.me.berkeley.edu/gri_mech/version30/text30.html, GRI-Mech 3.0. 37. S. Motlagh and J. Moore, “Cross Sections for Radicals from Electron Impact on Methane and

Fluoroalkanes”, J. Chemi. Phys., vol. 109, 1998, pp. 432-438. 38. R.K. Janev and D. Reiter, “Collision Processes of C2,3Hy and C2,3Hy+ hydrocarbons with Electrons and

Protons”, Physics of Plasmas, vol.11, 2004, pp. 781-829. 39. M.F. Golde, G.H. Ho, W. Tao, and J.M. Thomas, “Collision Deactivation of N2(A3Σu+, v=0-6) by

CH4, CF4, H2, H2O, CF3Cl, and CF2HCl”, J. Phys. Chem., vol. 93, 1989, pp. 1112-1118 40. L. G. Piper, “Energy Transfer Studies on N2(X1Σg+,v) and N2(B3Πg)”, J. Chem. Phys., vol. 97, 1992,

pp. 270-275. 41. F. Albugues, A. Birot, D. Blanc, H. Brunet, J. Galy, P. Millet, and J.L. Teyssier, “Destruction of the

Levels C3Πu (v΄=0, v΄=1) of nitrogen by O2, CO2, CH4, and H2O”, J. Chem. Phys., vol. 61, 1974, pp. 2695-2699.

42. H. Umemoto, R. Ozeki, M. Ueda, and M. Oku, “Reactions of N2(a΄1Σu-) with H2, CH4, and Their Isotopic Variants: Rate Constants and the Production Yield of H(D) atoms”, J. Chem. Phys., vol. 117, 2002, pp 5654-5659.

43. J.M. Thomas, F. Kaufman, and M.F. Golde, “Rate Constants for Electronic Quenching of N2(A3Σu+, v=0-6) by O2, NO, CO, N2O, and C2H4”, J. Phys. Chem., vol. 86, 1987, pp. 6885-6892.

44. F. Fresnet, G. Baravian, L. Magne, S. Pasquiers, C. Postel, V. Puech, and A. Rousseau, “Kinetic of the NO Removal by Nonthermal Plasma in N2/NO/C2H4 Mixtures”, Appl. Phys. Lett., vol. 77, 2000, pp. 4118-4120.


17

45. H. Umemoto, “Production Yields of H(D) Atoms in the Reactions of N2(A3Σu+ ) with C2H2, C2H4, and Their Deuterated Variants”, J. Chem. Phys., vol. 127, 014304, 2007.


18

-5.0E-8 0.0E+0 5.0E-8 1.0E-7

-20

-10

0

10

Time, seconds

Voltage, kV

Figure 1. Typical single pulse voltage waveform for the CPT pulser in air at P=60 torr.

Figure 2. Schematic diagram of the O atom TALIF apparatus

Nd:Yag SHG THG

1064 Dump

Dye Laser

BBO

Boxcar

SRS272

PMT

UV Separator

DELAY LINE

Gas Out

Gas In

Pulser

Quartz Test Cell

Filters

619nm Mirrors 355nm Mirrors

225nm Mirrors 840nm Collection lens

355nm Mirror 619nm Transmit UV focusing lens

Photodiode


19

Figure 3. Oxygen (left) and xenon (right) energy level diagrams, taken from references [16] and [19] respectively, illustrating states relevant to the TALIF diagnostics.

Figure 4. Typical O atom TALIF signal (left) and Xe atom TALIF signal (right), along with least squares fits to standard spectral model.

00.5

11.5

22.5

33.5

224.309 224.3095 224.31 224.3105 224.311

TALIF absorption wavelength (nm)

TALI

F si

gnal

(arb

uni

ts)

ExperimentFit

00.5

11.5

22.5

33.5

224.309 224.3095 224.31 224.3105 224.311

TALIF absorption wavelength (nm)

TALI

F si

gnal

(arb

uni

ts)

ExperimentFit


20

Figure 5. Oxygen atom mole fraction vs. time after a single high-voltage pulse in air and in a methane-air mixture at P=60 torr and Φ=1.0.

0.0E+0 1.0E-3 2.0E-3 3.0E-3 4.0E-30.0E+0

1.0E-5

2.0E-5

3.0E-5

4.0E-5

5.0E-5

Time, seconds

O atom mole fraction

Air

Air-methane, Φ=1.0

1.0E-7 1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-20.0E+0

1.0E-5

2.0E-5

3.0E-5

4.0E-5

5.0E-5

Time, seconds

O atom mole fractionAir

Air-ethylene, Φ=0.5

Figure 6. Oxygen atom mole fraction vs. time after a single high-voltage pulse in air and in an ethylene-air mixture at P=60 torr and Φ=0.5.

0.0E+0 1.0E-3 2.0E-3 3.0E-3 4.0E-3 5.0E-30.0E+0

1.0E-5

2.0E-5

3.0E-5

4.0E-5

5.0E-5

Time, seconds


Air-methane, Φ=1.0

Experiment

Case 1

Case 2

Case 3

Figure 7. Oxygen atom mole fraction vs. time after a single high-voltage pulse in a methane-air mixture at the conditions of Fig. 5: comparison with different kinetic mechanisms.


21

1.0E-7 1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-2 1.0E-10.0E+0

5.0E-4

1.0E-3

1.5E-3

Time, seconds

Pulse energy balance, JInput energy

Heat (air)

Heat (methane-air, Φ=1)

Heat (ethylene-air, Φ=0.5)

Figure 11. Input pulse energy and thermalized energy after a single high-voltage pulse in air, methane-air, and ethylene-air mixtures at the conditions of Figs. 5,6.

1.0E-7 1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-20.0E+0

1.0E-5

2.0E-5

3.0E-5

4.0E-5

Time, seconds


Air-ethylene, Φ=0.5

Experiment

Case 1

Case 2

Case 3

Figure 10. Oxygen atom mole fraction vs. time after a single high-voltage pulse in an ethylene-air mixture at the conditions of Fig. 6: comparison with different kinetic mechanisms.

Figure 9. Thermalized energy after a single high-voltage pulse in a methane-air mixture at the conditions of Fig. 5: comparison of the full and the reduced mechanisms.

Figure 8. Oxygen atom mole fraction vs. time after a single high-voltage pulse in a methane-air mixture at the conditions of Fig. 5: comparison of the full and the reduced mechanisms.

0.0E+0 4.0E-3 8.0E-3 1.2E-2 1.6E-2 2.0E-20.0E+0

2.0E-4

4.0E-4

6.0E-4

8.0E-4

Time [seconds]

Energy [J]

Air/CH4 , P=60 torr

Full mechanism

Reduced mechanism

0.0E+0 1.0E-3 2.0E-3 3.0E-3 4.0E-3 5.0E-30.0E+0

1.0E-5

2.0E-5

3.0E-5

4.0E-5

Time [seconds]


Air/CH4 ,P=60 torr

Experiment

Full mechanism

Reduced mechanism


22

0.0 0.2 0.4 0.6 0.8 1.01E+13

1E+14

1E+15

1E+16

Time, msec

O atom concentration, cm-3

P=60 torr

air

air/CH4, Φ=0.5

air/C2H4, Φ=0.5

Figure 14. Experimental and modeled O atom density at 40μs (air), 10μs (air/ CH4) and 3μs (air/ C2H4) delay after the last pulse in the burst, as functions of the number of pulses in the burst. Pulse repetition rate in the burst is 100 kHz (100 pulses / msec).

Figure 12. Dominant species mole fractions after a single high-voltage pulse in air at the conditions of Figs. 5,6.

1.0E-7 1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-21.0E-7

1.0E-6

1.0E-5

1.0E-4

Time, seconds

Species mole fractions

N2(A3Σ)

O O3

Figure 13. Fuel oxidation product mole fractions after a single high-voltage pulse in ethylene-air at the conditions of Fig. 6.

1.0E-7 1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-2 1.0E-11.0E-7

1.0E-6

1.0E-5

1.0E-4

Time, seconds

Species mole fractions

CH2OCO

H2O

CO2


23

0.0 0.2 0.4 0.6 0.8 1.0300

400

500

600

Time, msec

Temperature, K

Figure 15. Modeled gas temperature at 40μs (air - lower), 10μs (air/ CH4 - middle) and 3μs (air/ C2H4 - upper) delay after the last pulse in the burst as functions of the number of pulses in the burst. The conditions are the same as mixture in Fig. 14.

atomic oxygen measurements in air and air/fuel ......single 25 nsec discharge pulse, and in burst...

Documents