Pure rotational CARS studies of thermal energy release and ignition in nanosecond repetitively pulsed hydrogen-air plasmas

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1 Available online at Proceedings of the Combustion Institute 33 (2011) Proceedings of the Combustion Institute Pure rotational CARS studies of thermal energy release and ignition in nanosecond repetitively pulsed hydrogen-air plasmas Y. Zuzeek, S. Bowman, I. Choi, I.V. Adamovich, W.R. Lempert * Michael A. Chaszeyka Nonequilibrium Thermodynamics Laboratories, Department of Mechanical Engineering, The Ohio State University, Columbus, OH 43210, USA Available online 1 September 2010 Abstract Pure rotational CARS thermometry, complemented by UV emission measurements and ICCD imaging, is used to study kinetics of low temperature plasma assisted fuel oxidation and ignition in a repetitive nanosecond pulse discharge in hydrogen-air mixtures, with number of pulses in a 40 khz burst varying from a few to a few hundred. Time-resolved OH emission, coupled with gated ICCD images of the plasma and the flame, demonstrate that volumetric ignition of H 2 air mixtures occurs in a spatially uniform plasma. The results are shown to agree well with predictions of a new hydrogen-air plasma chemistry model, which incorporates non-equilibrium plasma processes, H 2 air chemistry, non-empirical scaling of nanosecond pulse energy coupled to the plasma, and quasi-one-dimensional conduction heat transfer. In particular, the results demonstrate that the heating rate in low temperature hydrogen-air plasmas is much faster than in air plasmas, primarily due to energy release from exothermic reactions of fuel with O and H atoms generated in the plasma. Kinetic sensitivity analysis is used to identify dominant plasma and chemical processes of hydrogen oxidation, demonstrating that additional heat release in these reactions is a key factor in ignition kinetics. Kinetic modeling calculations demonstrate that removal of the radical generation processes by the nanosecond pulsed plasma from the model completely blocks subsequent exothermic chemical reactions, thus making ignition impossible. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Plasma assisted combustion; CARS spectroscopy; Nanosecond pulse; Ignition 1. Introduction The last decade has seen greatly increased activity in the discipline known as plasma assisted * Corresponding author. Address: Department of Mechanical Engineering, The Ohio State University, E445 Scott Laboratory, 201 W. 19th Ave., Columbus, OH 43210, USA. Fax: address: lempert.1@osu.edu (W.R. Lempert). combustion (PAC) [1]. One particular approach utilizes high peak voltage nanosecond pulse duration discharges, operated at high pulse repetition rate, on the order of khz. Such discharges have inherently high reduced electric fields, E/N, up to several hundred Townsend (1 Td = Vcm 2 ), such that a significant fraction of the total discharge energy goes into molecular dissociation and population of excited electronic states. Repetitively pulsed nanosecond discharges also exhibit stability at significantly higher pressures /$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi: /j.proci

2 3226 Y. Zuzeek et al. / Proceedings of the Combustion Institute 33 (2011) compared to other types of electric discharges, due to the very low duty cycle operation [2,3]. Several groups have recently reported the use of nanosecond pulse discharges for fundamental PAC studies [4]. Bozhenkov et al. [5] have shown more than one order of magnitude ignition delay time reduction in H 2 /Ar/air mixtures preheated in a shock tube. Other recent examples include large increases in the critical co-flow velocity [6] and NO production rate [7] in methane air PAC flames, and fuel lean flame stability limit extension [8] in premixed propane air flames, to / = 0.3. Uddi et al. [9] have used two photon absorption laser induced fluorescence (TALIF) to study low temperature generation and loss kinetics of atomic oxygen in air, methane air, and ethylene air mixtures at 60 Torr. The results were found to agree well with predictions of a hydrocarbon air plasma chemistry model, supplemented by GRI Mech 3.0 [10] and the hydrocarbon oxidation mechanism of Wang et al. [11]. Finally, Zuzeek et al. [12] have recently presented time-resolved Coherent Anti- Stokes Raman Spectroscopy (CARS) measurements of heat release in air and ethylene air mixtures. Results were found to agree well with predictions of the plasma chemistry kinetic model of Uddi et al. [9], modified to incorporate a new model for energy coupling in repetitively pulsed nanosecond plasmas [13], as well as quasi-onedimensional heat transfer. This paper presents new experimental CARS thermometry results for heat release in nanosecond repetitively pulsed H 2 air discharges [14]. Sensitivity analysis predicts that the initial heating rate, at temperatures of approximately 300 K, is controlled by the low temperature processes O+HO 2? OH + O 2 and OH + H 2? H 2 O+ H, where HO 2 is formed by three body recombination of O and H 2. At intermediate temperatures, K, OH formation from chain branching processes increases, with rapid concurrent increase in heat release due to plasma chemical reactions, leading to rapid temperature rise and, in some cases, volumetric ignition. 2. Experimental Experiments are conducted in a 220 mm long 22 mm width 10 mm height quartz channel/plasma flow reactor [9,12], at 40 Torr pressure and flow velocity 0.8 m/s, controlled by mass flow controllers. Rectangular copper plate electrodes, 14 mm wide by 65 mm long and rounded at the corners to reduce the electric field nonuniformity, are flush mounted to the top and bottom of the 1.75 mm thick quartz channel, which acts as a dielectric barrier. A Chemical Physics Technologies power supply produces 20 kv peak voltage, 25 ns duration pulses at 40 khz repetition rate. For all measurements presented in this paper, the pulser was operated in a repetitive burst mode, generating sequences of up to 1000 pulses at pulse repetition rate of 40 khz and burst repetition rate of 10 Hz. This rate matches the repetition rate of the CARS system, and ensures that each gas sample in the flowing discharge cell experiences only a single burst. To produce breakdown in the discharge on the first pulse, the test cell was irradiated by a deuterium UV lamp, providing pre-ionization. The equivalence ratio in air fuel mixtures was varied from / = 0.05 to 1.0. The pure rotational CARS apparatus, patterned after that of Alden et al. [15], has been described in detail previously and will be only summarized here. A broad-band Ti:sapphire laser [16] creates two, orthogonally polarized, 15 ns, 30 mj/pulse, CARS pump beams at 780 nm, which are overlapped in a focused planar phase matching geometry with a 532 nm probe beam, using a 400 mm focal distance lens. Pure rotational CARS is generated with vertical polarization, which allows for discrimination from stray horizontally polarized probe light using a polarizer. As detailed in [12] this results in detected CARS resonant and non-resonant intensities which are a factor of 9/16 and 1/9, respectively, of the values which would be obtained if all polarizations were aligned parallel [17]. The CARS beam is spectrally resolved (resolution 1 2 Å) with a ½ m spectrometer (1800 lines/ mm grating), and a gated ICCD camera. Typically, for burst durations of 10 ms or less, the CARS signal was averaged up to 600 bursts (60 s). For longer bursts, five such spectra were added together. Inference of rotational temperature and O 2 mole fraction is performed using the Sandia National Laboratories CARS code [18]. Prior to fitting, experimental spectra are corrected to account for the finite spectral width (400 cm 1 FWHM) of the broad-band Ti:sapphire pump beams, as described in [12]. CARS spectra are modeled as pure air since the H 2 (S(0)) transition falls outside the spectral region of nitrogen and oxygen. Uncertainty in rotational translational temperatures, as discussed in Section 4, is generally found to be in the range K. In order to detect ignition and gauge discharge uniformity, time-resolved emission measurements and plasma imaging have been performed. Emission measurements detect time-resolved OH emission with a photomultiplier tube and bandpass filter centered at 310 ± 2 nm, captured with a digital oscilloscope. Time response of emission measurements (10 ls) was controlled using a 50 kx termination resistor. Broad-band visualization images are obtained with a gated ICCD camera and UV lens.

3 Y. Zuzeek et al. / Proceedings of the Combustion Institute 33 (2011) Hydrogen-air nanosecond pulsed discharge and plasma chemistry model The hydrogen-air nanosecond pulsed discharge model used in this work is patterned after that described by Uddi et al. [9,19]. Briefly, the present model incorporates a non-equilibrium air plasma chemistry model [20], expanded to include hydrogen dissociation processes in the plasma, and the hydrogen oxygen chemistry model (22 reactions among H, O, OH, H 2,O 2, H 2 O, HO 2, and H 2 O 2 ) developed by Popov [14]. Species concentration equations are coupled with the two-term expansion Boltzmann equation for the energy distribution function of plasma electrons, with electron impact cross sections taken from [21 23]. The full list of air plasma processes and their rates is given in [19]. The list of hydrogen oxygen chemical reactions and their rates is given in [14]. As in our previous work [12], the pulse shape used by the plasma chemistry model is a Gaussian fit to a negative polarity voltage pulse. However, as described in a recent paper by Adamovich et al. [13] due to strong shielding of the applied voltage caused by charge accumulation on the quartz channel walls, the field in the plasma after breakdown is much lower than the applied field. Summarizing the results of [13], the model predicts that energy coupled to the plasma during an individual nanosecond discharge pulse is controlled by the capacitance of the dielectric layers, C, and by the breakdown voltage, V b, so that q pulse CV 2 b /2, with the energy coupled per molecule being nearly independent of the number density. The coupled pulse energy predicted by the nanosecond pulse discharge model in room temperature (300 K) air was found to agree well with the value inferred from previous TALIF measurements, 0.76 mj/pulse (0.28 mev/molecule/pulse) [9,19]. Gradual plasma temperature rise during a pulse burst results in breakdown voltage reduction, thus decreasing the energy coupled to the plasma approximately linearly with density. In the present model, the pulse peak reduced electric field, (E/N) peak 250 Td, was adjusted to keep the coupled pulse energy per molecule constant (0.28 mev) during the burst. At these conditions, almost 50% of the coupled pulse energy in a hydrogen-air mixture at / = 0.5 is spent on generation of O and H atoms. As described in our previous work [12], energy loss due to conduction to the quartz channel walls, which is very significant in hydrogen-air mixtures, is described by incorporating a conduction heat transfer term into the energy equation [13]. Finally, it is noted that UV preionization produces very low initial electron density, which has negligible effect on the model predictions. 4. Results and discussion 4.1. Discharge emission and imaging measurements Figure 1 shows an example OH emission data trace obtained in stoichiometric H 2 air at 70 Torr pressure and 40 khz burst rate. As can be seen in the upper left inset, which is an expanded time scale spanning the first 150 ls, in the absence of ignition a series of sharp intensity spikes spaced 25 ls apart (40 khz rate) is observed, which fall to near zero in the interval between successive pulses. In the event of ignition, an OH intensity footprint appears, such as that seen at 10 ms, which does not decay between the pulses, and which serves as an ignition marker [2]. As presented in [24], in this manner ignition has been observed, and ignition delay determined, in H 2 air mixtures over a range of equivalence ratios from u = 0.3 to 1, and pressures from 50 to 100 Torr. In order to confirm diffuse volumetric ignition, and to justify the use of a quasi-one-dimensional approximation for kinetic and heat transfer analysis, UV ICCD camera images have been obtained (primarily N 2 second positive band and, in some cases, OH A? X emission) as a function of pressure, burst duration, and pulse number within, and/or after, a burst. As a representative example, Fig. 2 shows a collage of six images obtained from a u =1 H 2 air mixture at 40 Torr, where the emission fills the entire volume defined by the electrode area and channel height. The discharge pulse repetition rate is 40 khz and the images shown are obtained during a burst of pulses, with pulse numbers in the burst ranging from 1 to 700 (17.5 ms). Note that the individual images shown were obtained from different bursts Emission (arb. units) Fig. 1. Time-resolved OH emission for stoichiometric H 2 air mixture at 70 Torr, excited by a nanosecond pulse burst discharge (m = 40 khz). Inset is expanded time scale spanning first 150 ls.

4 3228 Y. Zuzeek et al. / Proceedings of the Combustion Institute 33 (2011) Fig. 2. ICCD images of the plasma and the flame in H 2 air excited by a m = 40 khz discharge at / = 1 and P = 40 Torr. Emission spans entire discharge volume. and do not, therefore, illustrate discharge development during a single burst. The top four images in Fig. 2 were obtained with a 2 ls intensifier gate, which was timed to include the 25 ns discharge event. While some filamentary structure is observed for the first pulse, all images for pulse numbers exceeding 2 4 are observed to be uniform, including images obtained for pulse numbers up to 1000 (not shown in Fig. 2). This result is in contrast to our previous nanosecond pulse plasma images obtained in ethylene air mixtures, in which large scale filaments were detected after pulses, depending upon equivalence ratio [12,24]. While some evidence of filamentation was observed after pulses at pressures of 70 Torr or higher, these structures gradually diffuse, resulting in uniform plasmas after 300 pulses (7.5 ms), i.e. before ignition occurred (see Fig. 1) [24,25]. High thermal diffusivity of hydrogen, as well as its relatively high ionization potential (15.6 ev), compared to ethylene (10.5 ev), is the most likely the explanation of the observed improvement in discharge stability. The bottom two images in Fig. 2 (labeled Flame ) were taken with an 18 ls gate and timed to collect only emission that occurs after the discharge emission has decayed (3 ls delay after the discharge pulse). The observed uniform emission (principally OH) is further evidence that volumetric ignition has occurred in this case after 10 ms. Figure 3 is similar to Fig. 2, except that the images shown were obtained at ls (with a 100 ls gate) after the final pulse in a 15 ms (600 pulse) burst for u = 1 at 70 Torr. Uniform OH emission, again spanning the complete discharge volume, is clearly evident, indicating continuous, combustion-induced, emission for a period of several milliseconds after application of the final discharge pulse Pure rotational CARS CARS is a well known diagnostic technique for high spatial and temporal resolution measurement of temperature and major species distributions in flames and plasmas, and is ideally suited as a method to provide more quantitative data which Fig. 3. ICCD images in / =1 H 2 air at 70 Torr, obtained at differing times after final pulse of 15 ms 40 khz burst. Emission spans entire discharge volume.

5 Y. Zuzeek et al. / Proceedings of the Combustion Institute 33 (2011) can be benchmarked against plasma chemical oxidation kinetic modeling codes. Figure 4 shows a comparison of experimental CARS temperatures obtained in this work as a function of burst duration at 40 khz discharge repetition rate in air and in H 2 air at / = 0.05, 0.5, and 1.0, along with predictions from the plasma kinetic modeling code. In all cases, P = 40 Torr and the initial temperature is T = 300 K. Note that the raw CARS spectra are similar in intensity and precision to those of our previous work in ethylene air mixtures [12] and, therefore, will not be reproduced here. Note also that the Sandia CARS code does not provide any estimate of uncertainty in the fitting parameters. To provide an estimate of experimental precision, ten spectra were obtained and fit for each measurement condition presented in Fig. 4. The statistical uncertainty (95% confidence interval) in the mean, defined as 2.23 ( t value) times the experimental standard deviation, divided by the square root of 10, is shown for representative data in Fig. 4. Focusing first on the experimental data in Fig. 4, it can be seen that the initial heating rate, for burst durations less than 3 4 ms ( pulses), is approximately equal for each of the mixtures to within the experimental uncertainty. For burst durations exceeding approximately 5 7 ms ( pulses) clear trends become discernable. In particular, the rate of heating for mixtures at u = 0.5 and 1.0 can be clearly seen to exceed that for pure air and the u = 0.05 mixture. A somewhat closer examination of the data in Fig. 4 shows that accelerated heating for the u = 0.5 and 1.0 mixtures begins to occur at temperatures of approximately K. This will be discussed in more detail in Section 5. T, K Number of pulses Fig. 4 also shows that at burst durations in the approximate range ms ( pulses), the experimental CARS temperatures (as well as the modeling predictions) for both u = 0.5 and 1 exhibit a maximum, and then decay. Furthermore, as shown in Fig. 5, the O 2 mole fractions for the stoichiometric mixture, obtained from the Sandia CARS code (which floats this parameter) decrease very rapidly in the region of ms duration bursts. Figure 5 also shows that the O 2 mole fraction decay agrees well with the kinetic model predictions, which also predicts large concurrent decrease in H 2 mole fraction. The observed temperature maximum, coupled with the observed rapid loss in O 2, is a very strong indication that ignition, accompanied by almost complete fuel oxidation, has occurred in these mixtures, a result which is consistent with the emission data and images shown in Figs While the experimental temperature rise in the region of ms is more gradual then that predicted, Figs. 4 and 5 show good overall agreement between the model and the experiments. It is believed, although not definitely determined, that the burst to burst reproducibility of the ignition event is not highly reproducible in time, leading to averaging of the temperature profile in the vicinity of the maximum. Nonetheless the overall quantitative agreement demonstrates that the plasma kinetic model adequately describes the energy balance in the repetitively pulsed nanosecond discharge, including the energy coupling to the plasma, energy release during exothermic plasma chemical reactions, and energy loss by heat transfer to the test section walls. It is stressed that this level of agreement is attained without recourse to any adjustable parameters in the model. Mole fractions P=40 torr φ=0 (air) φ=0.05 φ=0.5 φ= P=40 torr, φ=1 O 2, CARS spectra O 2, model H 2, model Fig. 4. Comparison of CARS temperatures in a repetitively pulsed nanosecond discharge in air and in hydrogen-air at P = 40 Torr with kinetic model predictions. m = 40 khz, burst repetition rate 10 Hz Fig. 5. Experimental and calculated O 2 mole fractions at the conditions of Fig. 4. Also shown are calculated H 2 mole fractions for the same conditions.

6 3230 Y. Zuzeek et al. / Proceedings of the Combustion Institute 33 (2011) Kinetic sensitivity analysis To provide insight into the chemical processes which are primarily responsible for the observed low temperature oxidation, heat release, and ignition in hydrogen-air plasmas, a sensitivity analysis has been performed, the principal results of which will be summarized below. A more complete analysis can be found in [24,25]. At the lowest temperatures relevant to this work, K, a detailed analysis is given in [25]. Summarizing it is found that at 300 K plasma chemical oxidation proceeds, principally, by the following mechanism. First, O and H are formed during, and after, the discharge pulse (on a time scale of ns) primarily via electron impact and dissociating collisions with electron metastable states of N 2. Subsequent fuel oxidation during the post-discharge stage (on a time scale from 1 ls to1 ms) occurs primarily via a three process sequence, H þ O 2 þ M! HO 2 þ M ð1þ O þ HO 2! OH þ O 2 ð2þ OH þ H 2! H þ H 2 O ð3þ In other words, at this low temperature, OH formation via the well known high temperature chain processes, Oð 3 PÞþH 2! H þ OH ð4þ H þ O 2! O þ OH ð5þ is extremely slow, while processes (2) and (3) are relatively fast, with activation barriers of kj/mol (1660 K) and essentially zero, respectively [14]. As the initial temperature is increased, to K, the rates of processes (4) and (5), as well as of the following three processes, H þ HO 2! H 2 O þ O ð6þ H þ HO 2! OH þ OH ð7þ H þ HO 2! H 2 þ O 2 ð8þ increase rapidly, greatly accelerating the rate of overall plasma chemical oxidation. More quantitatively, modeling calculations predict that at 300 K the total H 2 O produced from a single discharge pulse is essentially identical to the atomic oxygen initially produced by the discharge. This is clear from inspection of processes (1) (3), which do not produce any chain propagation or branching. Indeed, the reaction path (1) (3) is equivalent to a net reaction O+H 2? H 2 O. However, modeling calculations predict that at 600 K the rate of reactions (4) and (7) increase rapidly, increasing the amount of H 2 O produced by a single discharge pulse by approximately one order of magnitude. The effect of this on plasma chemical heat release can be seen in Fig. 6, which shows model predictions for total thermalized energy from a single discharge pulse as a function of initial temperature. At 300 K the additional thermalized energy (beyond the 0.28 mev/molecule coupled directly from the discharge) is minimal, approximately 10%. However, as the temperature is increased, the additional thermal energy due to net exothermic plasma chemical reaction increases rapidly, reaching a factor of approximately three greater than the pulse input energy at 600 K. It is this rapid increase in plasma chemical oxidation which causes the observed rapid temperature rise of the u = 0.5 and 1.0 mixtures, relative to u = 0.05 and pure air, observed in Fig. 4 after 5 ms (200 pulses), and leads, ultimately, to the observed ignition. Figures 7 and 8 summarize the most significant sensitivity analysis results for burst mode kinetics and oxidation. Figure 7 plots time-resolved temperatures predicted by the model at u = 1.0, P = 40 Torr using both the full H 2 air chemistry model, and a reduced kinetic mechanism consisting of reactions (1) (8) plus the additional process, OH þ HO 2! H 2 O þ O 2 ð9þ The full and reduced kinetic mechanisms result in very similar predictions, both of which agree well with the experimental data. Figure 8 shows predictions of the full kinetic model for the temporal evolution of key species for the same conditions as Fig. 7. It can be seen that H, O, and OH mole fractions are predicted to peak at 18 ms, consistent with the observed maximum in the experimental CARS temperatures observed in Fig. 4. In addition, while not plotted here, it is shown in [24] that the reduced kinetic model Energy, mev/molecule H 2 -air, P=40 torr input output, T=300 K output, T=400 K output, T=500 K output, T=600 K 0.0 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 1E-2 1E-1 1E+0 Time, sec Fig. 6. Thermalized energy per molecule vs. time after a single-pulse nanosecond discharge. Hydrogen-air, / =1, P = 40 Torr, initial temperature T = K. Energy input by the discharge is 0.28 mev/molecule.

7 Y. Zuzeek et al. / Proceedings of the Combustion Institute 33 (2011) T, K Number of pulses described directly above results in essentially indistinguishable results for predicted species mole fractions. Finally, to illustrate the critical effect of radical generation by the nanosecond discharge, Fig. 7 also plots time-resolved temperatures in u = 1.0, P = 40 Torr H 2 air predicted by the model using only chemistry processes (reactions (1) (9)), i.e. with all nanosecond discharge plasma processes, principally O and H formation via both electron impact, and dissociating collisions with electron metastable states of N 2, removed. In both cases, the coupled pulse energy per molecule, i.e. the discharge input power is the same. It can be seen clearly that removal of the radical generation processes in the pulsed discharge plasma completely blocks subsequent exothermic chemical reactions and makes ignition impossible. 6. Summary and conclusions φ=1 full set reduced set no O and H generation by plasma Fig. 7. Temperatures predicted in a nanosecond pulsed discharge burst (H 2 air, / =1, P = 40 Torr, m = 40 khz) for full and reduced reaction sets, and with O and H generation processes in the plasma removed. CARS temperatures shown for comparison. Species mole fractions 1.0E+0 1.0E-1 1.0E-2 1.0E-3 1.0E-4 1.0E-5 1.0E-6 H 2 H O HO 2 H 2 O OH Fig. 8. Predicted species mole fractions at the conditions of Fig. 7 for full reaction set. This paper presents, for the first time to our knowledge, quantitative data on the kinetics of heat release in low temperature, non-equilibrium plasma assisted fuel oxidation. Specifically, pure rotational CARS is used to obtain time-resolved rotational temperature and O 2 mole fraction in a repetitively pulsed discharge operated in burst mode in air and in hydrogen-air mixtures at 40 Torr pressure and 40 khz discharge repetition rate, conditions in which UV ICCD camera images show that the discharge remains stable and diffuse. In particular, it is found that for u = 0.5 and 1.0 mixtures at 40 Torr the CARS temperatures exhibit a clear maximum after discharge pulses, indicating that ignition has occurred for this condition. Timeresolved OH emission, coupled with gated ICCD images of the plasma and the flame, demonstrate that volumetric ignition of H 2 air mixtures occurs in a spatially uniform plasma. The experimental results agree well with predictions from a new low temperature H 2 air plasma chemistry model which incorporates non-equilibrium plasma discharge processes, low temperature H 2 air chemistry, non-empirical scaling of nanosecond discharge pulse energy coupled to the plasma as a function of plasma number density, and onedimensional conduction heat transfer. Quantitative agreement is attained without recourse to any adjustable parameters. Sensitivity analysis indicates that at low initial temperatures, heating in excess of that resulting from direct energy input from the pulser, is controlled primarily by a sequence of reactions (1) (3), all of which are relatively rapid at 300 K. At higher initial temperatures, K, the rate of chemical oxidation in reactions (4) and (6) (8) increases rapidly, producing chain branching and additional heat release, approximately a factor of three greater than the pulse input energy at 600 K. As the temperature increases, primarily due to the plasma chemical heat release, ignition occurs by the well known chain mechanism of reactions (4) and (5). Finally, kinetic modeling calculations demonstrate clearly that removal of the radical generation processes in the pulsed discharge plasma completely blocks subsequent exothermic chemical reactions and makes ignition impossible. It is anticipated that the results of this work will provide a foundation for future experimental and computational studies of more complex

8 3232 Y. Zuzeek et al. / Proceedings of the Combustion Institute 33 (2011) plasma assisted combustion environments and geometries, such as those which might be employed in future advanced concepts for high speed ignition and flame holding. Acknowledgements The authors wish to acknowledge the sponsorship of the US Air Force Office of Scientific Research (Julian Tishkoff Technical Monitor) and the National Science Foundation (Phillip Westmoreland Technical Monitor) for support of this research. References [1] S.M. Starikovskaia, J. Phys. D: Appl. Phys. 39 (2006) R265 R269. [2] I.V. Adamovich, I. Choi, N. Jiang, et al., Plasma Sources Sci. Technol. 18 (2009) [3] A. Hicks, S. Tirupathi, N. Jiang, et al., J. Phys. D: Appl. Phys. 40 (2007) [4] A.Y. Starikovski, Proc. Combust. Inst. 30 (2005) [5] S.A. Bozhenkov, S.M. Starikovskaya, A. Yu. Starikovskii, Combust. Flame 133 (2003) [6] W. Kim, H. Do, M.G. Mungal, M. Cappelli, IEEE Trans. Plasma Sci. 34 (2006) [7] W. Kim, H. Do, M.G. Mungal, M. Cappelli, Proc. Combust. Inst. 31 (2007) [8] G. Pilla, D. Galley, D.A. Lacoste, F. Lacas, D. Veynante, C.O. Laux, IEEE Trans. Plasma Sci. 34 (2006) [9] M. Uddi, N. Jiang, E. Mintusov, I.V. Adamovich, W.R. Lempert, Proc. Combust. Inst. 32 (2009) [10] text30.html, GRI-Mech 3.0. [11] H. Wang, X. You, A.V. Joshi, S.G. Davis, A. Laskin, F. Egolfopouls, C.K. Law, < [12] Y. Zuzeek, I. Choi, M. Uddi, I.V. Adamovich, W.R. Lempert, J. Phys. D: Appl. Phys. 43 (2010) [13] I.V. Adamovich, M. Nishihara, I. Choi I, et al., Phys. Plasmas 16 (2009) [14] N.A. Popov, Plasma Phys. Rep. 34 (2008) [15] M. Alden, P.E. Bengtsson, H. Edner, Appl. Opt. 25 (1986) [16] N. Finkelstein, J. Gambogi, W.R. Lempert, et al., AIAA , AIAA 32nd Aerospace Sciences Meeting and Exhibit, Reno, NV, [17] F. Vestin, M. Afselius, P.-E. Bengtsson, Proc. Combust. Inst. 31 (2007) [18] Code provided by R. Palmer, Sandia National Laboratories Combustion Research Facility. [19] M. Uddi, N. Jiang, I.V. Adamovich, W.R. Lempert, J. Phys. D: Appl. Phys. 42 (2009) [20] I.A. Kossyi, A.Y. Kostinsky, A.A. Matveyev, V.P. Silakov, Plasma Sources Sci. Technol. 1 (1992) [21] W. L. Morgan, J.P. Boeuf, L.C. Pitchford, BOLSIG Boltzmann Solver, Kinema Software, Monument, CO, [22] Y. Itikawa, M. Hayashi, A. Ichimura, et al., J. Phys. Chem. Ref. Data 16 (1986) [23] Y. Itikawa, A. Ichimura, K. Onda, et al., J. Phys. Chem. Ref. Data 18 (1989) [24] Y. Zuzeek, S. Bowman, I. Choi, I. Adamovich, W. Lempert, AIAA , 48th AIAA Aerospace Sciences Meeting, 4 7 January, Orlando, FL, [25] S. Bowman, I. Choi, I. Adamovich, W. Lempert, AIAA , 48th AIAA Aerospace Sciences Meeting, 4 7 January, 2010, Orlando, FL.