RECENT studies have reported that low-temperature

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1 3288 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011 Hydroxyl Radical Kinetics in Repetitively Pulsed Hydrogen Air Nanosecond Plasmas Inchul Choi, Zhiyao Yin, Igor V. Adamovich, and Walter R. Lempert Abstract Absolute hydroxyl radical (OH) concentration is determined in stoichiometric hydrogen air mixtures at P = torr and initial temperature of T = 100 C 200 C, which are both functions of time, after the application of a single approximately 25-ns-duration approximately 20-kV discharge pulse and 60 μs after the final pulse of a variable-length burst of pulses, using single-photon laser-induced fluorescence (LIF). Relative LIF signal levels are put on an absolute number density scale by means of calibration with a standard atmospheric-pressure near-adiabatic Hencken flat-flame burner. By obtaining OH LIF data in both the plasma and the flame and correcting for differences in the collisional quenching and vibrational energy transfer rates, absolute OH number density has been determined. For a single discharge pulse, the absolute OH temporal profile is found to rise rapidly duringthe initial 0.1 ms after discharge initiation and decay relatively slowly, with a characteristic time scale of 1 ms. In repetitive burst mode, the absolute OH number density is observed to rise rapidly during the first approximately ten pulses (0.25 ms) and then level off to a near steady-state plateau. In all cases, a large secondary rise in OH number density is also observed, which is clearly indicative of ignition, with ignition time ranging from 5 to 10 ms, for initial temperatures of 100 Cand 200 C and pressures in the range of torr. Plasma kinetic modeling predictions capture this trend quantitatively, using both a full 22-hydrogen air-chemical-reaction set and a reduced 9-reaction set. Index Terms Hydroxyl radical, laser induced fluorescence, plasma-assisted combustion. I. INTRODUCTION RECENT studies have reported that low-temperature plasma-assisted ignition and plasma-assisted combustion (PAC) may produce volumetric ignition, reduce ignition delay time, improve flame stability, and expand the flammability limit of combustible mixtures [1]. One particular approach, which Manuscript received June 10, 2011; accepted July 28, Date of publication September 15, 2011; date of current version December 14, This work was supported in part by the U.S. Air Force Office of Scientific Research (J. Tishkoff, Technical Monitor) and in part by the National Science Foundation (A. Atreya, Technical Monitor). I. Choi was with the Michael A. Chaszeyka Nonequilibrium Thermodynamics Laboratories, Department of Mechanical Engineering, The Ohio State University, Columbus, OH USA. He is now with the Fuels, Engines, and Emission Research Center, Oak Ridge National Laboratory, Knoxville, TN USA. Z. Yin, I. V. Adamovich, and W. R. Lempert are with the Michael A. Chaszeyka Nonequilibrium Thermodynamics Laboratories, Department of Mechanical Engineering, The Ohio State University, Columbus, OH USA. Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TPS offers several advantages over conventional dc, RF, and microwave discharges, is to utilize high-peak-voltage nanosecondpulse-duration discharges, operated at high pulse repetition rate, on the order of khz. Such discharges are characterized by inherently high reduced electric fields E/N up to several hundreds of townsends. At these high reduced field strengths, a significant fraction of the total discharge energy goes into molecular dissociation and population of excited electronic states. Repetitively pulsed nanosecond discharges also exhibit increased stability at significantly higher pressures compared to other types of electric discharges, due to the inherent very low duty cycle operation. The basic idea is to create volume ionization by the discharge with individual pulse duration that is much shorter than characteristic time scales for onset of ionization instabilities and with pulse repetition rate that is greater than the rate of plasma decay. This approach enables sustaining large-volume diffuse discharges at relatively high pressures, on the order of atm in nitrogen and air [2], [3]. In addition to serving as ignition and flameholding devices, repetitively pulsed nanosecond discharge plasmas serve as an ideal method for the study of low-rotational translationaltemperature nonequilibrium air chemistry and fuel oxidation kinetics. Similar to traditional flash photolysis sources, nanosecond pulsed discharges in air/fuel mixtures generate pools of important radical and metastable state excited species, such as N 2 (A 3 Σ), O 2 (a 1 Δ), O,H,andR (where R represents any hydrocarbon), on time scales of μs. Since the energy deposited directly into translational rotational modes in such discharges is relatively low, single-pulse or repetitively pulsed nanosecond discharges provide a test bed for the study of low-temperature plasma chemical kinetic processes in air and hydrocarbon air mixtures. For example, Kim et al. [4] studied NO production in a methane/air premixed flame stabilized using an ultrashort repetitive discharge. The resulting flame was found to be characterized by an unusual structure impacting the production of NO. They found a locally increased production of NO up to about ppm. However, the spatially averaged NO fraction was less than 5 ppm, compared to about 10 ppm produced during conventional combustion, at an equivalence ratio of ϕ =0.72. The NO production was biased toward the cathode side of the discharge, and NO consumption in the preflame was detected. Recently, Uddi et al. [5] have used two-photon absorption laser-induced fluorescence (LIF) to obtain time- and space-resolved measurements of absolute atomic oxygen concentration after initiation of a single-pulse 20-kV-peak-voltage 25-ns-pulse-duration discharge in air, methane air, and ethylene air mixtures at 60 torr. The results /$ IEEE

2 CHOI et al.: HYDROXYL RADICAL KINETICS IN HYDROGEN AIR NANOSECOND PLASMAS 3289 Fig. 1. Schematic diagram of the OH LIF apparatus and the high-temperature PAC kinetics cell. were found to agree well with predictions of a hydrocarbon air plasma chemistry model, supplemented by GRI Mech 3.0 [6], and the hydrocarbon oxidation mechanism developed by Wang et al. [7]. In particular, Uddi et al. concluded that O atoms were primarily created by dissociating collisions of ground electronic state O 2 with metastable excited electronic states of N 2, a result which is consistent with that recently reported by Stancu et al. [8], [9] who studied an atmosphericpressure nanosecond pulsed discharge in a pin-to-pin electrode configuration. More recently, Zuzeek et al. [10] have used pure rotational coherent anti-stokes Raman spectroscopy (CARS) thermometry to study oxidation and heat release in nanosecond repetitively pulsed H 2 air mixtures at 40-torr pressure and an initial temperature of T = 300 K. Good agreement was found between experimentally determined rotational translational temperature, as a function of the variable number of pulses in a 40-kHz burst, between 1 and 1000, and predictions of a plasma chemistry model which incorporates nonequilibrium plasma discharge processes, low-temperature H 2 air chemistry developed by Popov [11], a model for energy coupling in repetitively pulsed nanosecond discharge plasmas developed by Adamovich et al. [12], and quasi-1-d conduction heat transfer. As a follow on to that work, this paper presents new experimental and modeling studies of fundamental low-temperature H 2 air plasma chemistry. In particular, new kinetic data are reported on the generation and loss of hydroxyl radical (OH), by single-photon LIF. OH is an important transient species influencing the rate of oxidation and heat release in low-temperature plasmas due to the relatively fast rate, at low temperature, of exothermic H abstraction by OH, such as the process OH + H 2 H + H 2 O. It also has a significant presence in the chainbranching reactions for a high-temperature combustion system. Absolute OH radical concentration measurements, calibrated using an atmospheric-pressure Hencken flat-flame burner, are performed in stoichiometric H 2 air mixtures in the pressure range P =54 94 torr and initial temperature in the range T = 400 K 500 K. Measurements are performed both as a function of time delay after initiation of a single nanosecond discharge pulse and as a function of the number of pulses in a 40-kHz burst, ranging from 1 to II. EXPERIMENTAL The experimental apparatus is shown in Fig. 1. For these measurements, the fuel air flow as well as the entire discharge cell was preheated in a tube furnace to improve plasma stability and to provide temperature-dependent data. The main body of the test cell is fabricated from a single piece of rectangular cross-sectional quartz tubing with two plane quartz windows fused to the ends of the channel. The channel is 280 mm long 22 mm wide 10 mm high, with 1.75-mm-thick walls. A 1-m-long quartz coil is fused to the surface of the cell as a preheating inlet. Two 1/4-in-diameter quartz-to-stainless-steel adaptors are fused on the cell to connect the inlet and exit to the gas delivery system. Two rectangular copper plate electrodes

3 3290 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011 Fig. 2. Experimental high-voltage pulse shape in air at P =60 torr and an analytic fit (double Gaussian pulse with the pulsewidth parameter of τ = 10 ns). are attached directly to the outside of the quartz channel using ceramic clamps, as shown in Fig. 1. The electrodes are 60 mm long 14 mm wide and are rounded at the corners to reduce the electric field nonuniformity. A 1/16-thick silicone rubber sheet is placed between each electrode and the quartz channel to reduce the air gaps and prevent corona discharge formation outside of the cell. The entire assembly is placed inside a tube furnace (Thermcraft, Ltd.) with a 6-in-diameter 12-in-long heating section. The measurements were conducted in mixtures of ultradry grade air (H 2 O < 2 ppm) with hydrogen. Flow rates of air and fuel are metered and controlled by commercial mass flow controllers (MKS Instruments). OH LIF measurements are made at the initial H 2 air flow temperature of T 0 = 100 C 200 C, pressure of torr, and equivalence ratio of ϕ =1.0, at a flow velocity of u =0.5 m/s. Roomtemperature flow residence time in the discharge region is approximately 120 ms, based on the flow velocity. The slow flow velocity is chosen for more efficient flow preheating in the furnace, as well as to reduce the pressure drop across the discharge cell. Repetitively pulsed plasmas are generated by a high-voltage nanosecond pulsed discharge pulser (Chemical Physics Technologies, Belarus). It produces 25-ns pulses with 20-kV peak voltage, at pulse repetition rates up to ν =50kHz in air and premixed hydrogen air flows. This power supply generates negative-polarity pulses, while the terminal connected to the positive electrode can be grounded or left floating. Fig. 2 shows a typical single-pulse voltage waveform during a burst of nanosecond pulses in air at P =60torr and a pulse repetition rate of 40 khz. For many of the measurements presented in this paper, the pulse generator was operated in a repetitive burst mode, generating sequences of between 1 and 1000 pulses at a pulse repetition rate of 40 khz. The burst repetition rate is 5 Hz, which matches the pulse repetition rate of the diagnostic lasers and ensures that each gas sample in the flowing discharge cell experiences only a single burst. To produce breakdown in the Fig. 3. Saturation test of hydroxyl radical LIF. Detection was centered at P 1 (1) transition. discharge section on the first pulse, the test cell was irradiated by a deuterium ultraviolet (UV) lamp (Resonance Ltd.) through one of the windows at the end of the cell. A schematic diagram of the OH LIF experimental apparatus is shown in Fig. 1. A Nd:YAG laser (Continuum, Model Powerlite 8010) generates 1064-nm output, from which a 532-nm beam is generated using second-harmonic generation crystals. While the flashlamp operates at 10 Hz, the frequency of the laser output is maintained at 5 Hz to match the burst repetition rate of the power supply and to ensure that the gas sample in the cell is refreshed with every pulse of the probing laser. The 532-nm beam pumps a tunable pulsed dye laser (Laser Analytical Systems, Model LDL 20505), which generates output at 566 nm with a linedwidth of 0.25 cm 1 as measured using a Fizeau wavemeter (High Finesse, Model Angstrom WS/6). The output of the dye laser was frequency doubled to the UV using a β barium borate crystal with a beam diameter of 2 mm. The 283-nm beam is split into two beams with a UV beam splitter (Lattice Electro Optics, R = 97%, and AOI =45 ). Approximately 3% of the total energy from the 283-nm beam is used to excite the well-known A 2 Σ + X 2 Π (1, 0) electronic transition in OH at 283 nm, followed by fluorescence detection in the A 2 Σ + X 2 Π (1, 1) band centered at 315 nm. Care was taken to restrict the laser pulse energy to within the linear fluorescence regime, below 5 μj per pulse. Saturation behavior of the OH fluorescence signal is shown in Fig. 3. Spectral irradiance of the 283-nm laser beam is 10 4 W/(cm 2 cm 1 ), which is below the saturation laser spectral irradiance of 10 6 W/(cm 2 cm 1 ) [13]. A 60-mm right angle fused silica prism is placed against the side of the test cell channel to provide optical detection from the cell, as shown in Fig. 1. The collection optics consists of two fused silica lenses with 50.8-mm diameter (100- and 300-mm focal lengths, respectively). A standard photomultiplier tube (PMT) (Hamamatsu, Model R106) in combination with a small monochromator (Horiba, Model H-10) is used to capture excitation spectra as the 283-nm laser beam is spectrally scanned across the Q 1 (1) OH absorption transition. The PMT signal was detected using a boxcar integrator (Stanford Research Systems SR 250) with a prompt gate width of 10 ns and an analog time constant

4 CHOI et al.: HYDROXYL RADICAL KINETICS IN HYDROGEN AIR NANOSECOND PLASMAS 3291 III. HYDROCARBON AIR NANOSECOND PULSED DISCHARGE AND PLASMA CHEMISTRY MODEL Fig. 4. Typical OH LIF excitation spectrum used for absolute OH calibration in atmospheric-pressure ethylene air Hencken flat-flame burner at the equivalence ratio of ϕ =0.95. Fig. 5. Centerline OH fluorescence signal as a function of height above Hencken flat-flame calibration burner surface. Ethylene air; ϕ =0.95. equivalent to 30 laser shots (6 s). The boxcar gate width was narrow enough to exclude the A 2 Σ + X 2 Π (0, 0) band emission by vibrational energy transfer (VET), both in the discharge and in the atmospheric-pressure flame, to be discussed hereinafter. Variation in the UV beam pulse energy was monitored with a photodiode (Thorlabs, DET210) and used to normalize the observed OH signal to laser pulse energy. Relative OH concentrations for each series of measurements were put on an absolute scale by calibration with an atmospheric-pressure near-adiabatic ethylene air flat-flame Hencken burner (Technologies for Research, RD5X5). For these measurements, OH excitation spectra were obtained over multiple rotational transitions, from Q 1 (1) to Q 1 (5), asshown in Fig. 4 which shows a typical OH calibration spectrum obtained in an atmospheric-pressure ϕ =0.95 ethylene air mixture. All OH calibration measurements used in this work were collected from the centerline of the burner, at an axial location 10 mm above the surface, as shown in Fig. 5, in order to assure that the measured OH calibration signal corresponds to the chemical equilibrium value of OH concentration at the measured temperature. The hydrogen air nanosecond pulsed discharge model used in this work is similar to that described by Zuzeek et al. [10]. Briefly, the air plasma chemistry model incorporates a set of equations for number densities of ground electronic state species (N, N 2, O, O 2, O 3, NO, NO 2, N 2 O, and NO 3 ) and excited species [N 2 (A 3 Σ), N 2 (B 3 Π), N 2 (C 3 Π), N 2 (a 1 Σ), O 2 (a 1 Δ), O 2 (b 1 Σ), O 2 (c 1 Σ), N( 2 D ), N ( 2 P), and O( 1 D)] produced in the plasma [14] and is expanded to include hydrogen dissociation processes in the plasma and 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 [11]. The dominant neutral species in air plasma are the excited electronic states of nitrogen, which are formed by electron impact excitation, and O atoms, which are formed by electron impact dissociation of oxygen and by collisional quenching of excited nitrogen by oxygen, as well as ozone. The species concentration equations are coupled with the two-term expansion Boltzmann equation [15] for the energy distribution function of plasma electrons. The Boltzmann equation solver uses electron impact cross sections [16] [18] and calculates the electron energy distribution function, as well as the rate coefficients of electron impact ionization, dissociation, and electronic excitation, used by the rest of the model. The full list of air plasma processes incorporated into the model and their rates is given in our recent paper [19]. The list of hydrogen oxygen chemical reactions and their rates is given in [11]. The dominant radical species (O, H, and OH) generation processes in the plasma are listed in Table I. The high-voltage pulse shape used by the plasma chemistry model is a fit to the experimentally measured voltage pulse shape, shown in Fig. 2. However, the field in the plasma is much lower than the applied field, due to strong shielding of the applied voltage caused by charge accumulation on the dielectric walls after breakdown. The analytical model of energy coupling in nanosecond pulsed discharge plasmas has been developed by Adamovich et al. [12]. Briefly, this model incorporates key effects of pulsed breakdown, charge accumulation on dielectric surfaces, and sheath development on nanosecond time scale. The model predicts that energy coupled to the plasma during the pulse is controlled primarily by the capacitance of the dielectric layers and by the applied voltage pulse shape ( Q pulse 1 ) 2π 2 C Vb 2 + Vpeak 2 ν RC τ ( = 1 ) εε 0 A 2π Vb 2 + Vpeak 2 (1) 2 2l ν RC τ where C is the capacitance of the dielectric layers, V peak and V b are the peak pulse voltage and breakdown voltage, respectively, τ is the pulse duration parameter of a Gaussian pulse, V peak exp[ (t/τ) 2 ], ν RC =1/RC is the RC time constant of the load (i.e., the plasma and the dielectric layers) after the breakdown, R is the resistance of the shielded plasma after breakdown, l and ε are the dielectric layer thickness and the

5 3292 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011 TABLE I DOMINANT RADICAL SPECIES GENERATION PROCESSES IN THE PLASMA dielectric constant, respectively, and A is the electrode surface area. To calculate the coupled pulse energy, the pulse waveform shown in Fig. 2 is approximated by a superposition of two Gaussian pulses (one negative and one positive) with τ =10ns. The coupled pulse energy predicted by (1) is approximately proportional to the number density [12], i.e., the energy coupled per molecule remains nearly constant. The main difficulty with predicting the coupled pulse energy at the present conditions is evaluating the capacitance of the dielectric layers between the electrodes and the plasma, which consist of quartz channel walls and silicone rubber sheets. The dielectric constant of silicone rubber varies considerably depending on its chemical composition ε = (the dielectric constant of quartz is ε =4.3). This results in a significant uncertainty in the predicted pulse energy Q pulse = mj/pulse at T = 300 K and P =60torr. The value of Q pulse =0.72 mj (coupled pulse energy per molecule of 0.28 mev/molecule) is chosen because it provides better overall agreement with the present experimental data. Note that the gradual plasma temperature rise during the pulse burst results in breakdown voltage reduction, thereby decreasing the pulse energy coupled to the plasma as a function of the pulse number in the burst. Therefore, the total coupled pulse energy during the burst decreases inversely proportional to temperature. The pulse peak reduced electric field in the plasma chemistry model is based on the breakdown voltage predicted by the nanosecond pulsed discharge model for the Gaussian voltage waveform [12], (E/N) peak 400 Td [(T [K]/300) (60/P[torr])] 1/2 N 1/2. At these conditions, about 50% of the coupled pulse energy in hydrogen air mixtures is spent on the generation of O and H atoms. Peak pulse conduction current and electron density, also approximated by a Gaussian pulse with τ =10ns, are evaluated from the coupled pulse energy. This simple coupling of the nanosecond pulsed discharge model and the plasma chemistry model incorporates the most essential effect of coupled pulse energy dependence on the voltage waveform, the dielectric plate parameters, and the number density. As described in our previous work, energy loss due to conduction to the quartz channel walls, which is very significant in hydrogen air mixtures (due to the high thermal diffusivity of hydrogen), is described by incorporating a conduction heat transfer term into the energy equation [12]. IV. RESULTS AND DISCUSSION A. Hencken Calibration Burner For a pulsed laser, assuming that the steady-state approximation is valid, the fluorescence signal S F (ω) measured in a LIF experiment in a single laser pulse for the linear regime is given by ( ) Ω S F (ω) =Bhυg(ω)n OH f B ΦF fl εηv I(t) LIF dt 4π 0 (2) where B is the Einstein absorption coefficient divided by the speed of light (in square meters per joule), h is Planck s constant, υ is the transition absorption frequency, I(t) LIF is the laser spectral power density (J/m 2 s cm 1 ), g(ω) is the spectral line shape function of the absorbing transition, which includes a contribution from the laser itself, n OH is the OH number density (in per cubic centimeter) in the ground electronic state, which is the desired result of the experiment, f B is the temperature-dependent Boltzmann fraction of the lower quantum state of the transition, Φ is the fluorescence quantum yield, and F fl is the fraction of fluorescence collected within the detector spectral bandwidth (in this case, a small monochromator). The remaining terms are Ω, the solid angle of fluorescence collected by the detector; ε and η, the transmission and photoelectron efficiencies of the detector system; and V, the interaction volume observed. However, as discussed in detail by Cathey et al. [20], analysis of OH LIF in the presence of VET requires some additional considerations. Specifically, it is necessary to account for the modification to the effective fluorescence quantum yield due to the fact that, for this case, direct excitation of the v =1 level of the excited OH state is followed by partial VET to the v =0level. Figs. 6 and 7

6 CHOI et al.: HYDROXYL RADICAL KINETICS IN HYDROGEN AIR NANOSECOND PLASMAS 3293 Fig. 6. Time-integrated OH LIF spectrally resolved emission spectrum of the (0, 0) and (1, 1) bands from Hencken burner. C 2 H 4 air N 2, ϕ =0.95, P = 760 torr, and 2-ns gate. show typical time-resolved OH emission spectra resulting from VET in the atmospheric-pressure Hencken burner and in the plasma discharge cell in burst mode discharge (20 pulses) at P =40 torr and T = 300 K. Note that these spectra were obtained by replacing the small monochromator/pmt combination shown in Fig. 1 with a 1/4-m OMA system which includes a gatable ICCD camera for detection. It is integrated by using a 2-ns gate in WinSpec software (Princeton Instruments) with changing delay time after laser excitation using a delay generator. The ratio of the VET rate to the quenching rate is greater in the low-pressure plasma discharge cell than in the atmospheric-pressure Hencken burner. In general, LIF photons would be detected from both the (0, 0) and (1, 1) emission bands. To account for this, following the notation in [20], which assumes that the steadystate approximation is valid, we can rewrite the product of the fluorescence quantum yield and the fluorescence detection efficiency ΦF fl as ΦF fl = ε 1A 1 + ε 0 A 0 Q V,10 /(Q e,0 + A 0 ) Q V,10 + Q e,1 + A 1 (3) where ε i, A i, and Q e,i are the respective fluorescence collection efficiency, fluorescence emission rate, and electronic quenching rate for the A-state v = i (zero or one) level and Q V,10 is the rate for VET from v =1 to 0. Note that the steadystate assumption is valid only during the 10-ns laser pulse, hence the use of a prompt gate for detection. Since there is population change through VET from v =1to v =0,we will observe, in general, the (0, 0) band as well as the (1, 1) band in the OH emission spectra. To quantify this, the small monochromator, used as a spectral filter, was placed in front of the OMA system used to obtain the spectra in Figs. 6 and 7. The spectrum on the left side of Fig. 8 was obtained without the filtering monochromator. However, by use of a narrow width slit on the small monochromator (0.5 mm) and adjustment of the center wavelength set to 315 nm, it is possible to greatly reduce the contribution of the OH LIF signal from the (0, 0) band, by Fig. 7. Time-integrated OH LIF spectrally resolved emission spectrum of the (0, 0) and (1, 1) bands from plasma discharge cell in burst mode discharge (20 pulses). H 2 air, ϕ =1, P =40torr, and 2-ns gate. Fig. 8. Time-integrated OH LIF spectrally resolved emission spectrum of the (0, 0) and (1, 1) bands from plasma discharge cell. H 2 air, ϕ =1.0, P =40 torr, room temperature, and 20-ns gate. (Left) Before (0, 0) band filtering. (Right) After (0, 0) band filtering. 85%, as shown on the right side of Fig. 8. The significance of this will be discussed hereinafter. While it is, in principle, possible to determine values for each of the constants in (2), in practice, it is more accurate to employ some sort of optical calibration. In this work, relative LIF experimental data are put on an absolute number density scale using an atmospheric-pressure near-adiabatic flat-flame Hencken burner as a calibration device, where the measured OH rotational distribution is used to determine the burner temperature. Fig. 9 shows the rotational Boltzmann plot corresponding to the experimental spectrum shown in Fig. 4. The Einstein B coefficients for individual vibronic transitions and the rotational termsofthex 2 Π and A 2 Σ + states of OH are taken from the LIFBASE program written by Luque and Crosley at the

7 3294 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011 TABLE II PARAMETERS FOR OH COLLISIONAL QUENCHING predicted by the CEA code. The total quenching rate is then the sum of the contributions from each collision partner Fig. 9. Rotational Boltzmann plot corresponding to OH LIF spectrum shown in Fig. 4. Inferred temperature is 2260 K ± 150 K. Stanford Research Institute [21]. The slope of the straight line that fits the experimental points is equal to 1/kT, which gives the temperature of 2260 K. It is slightly lower than the adiabatic flame temperature of 2337 K, as would be expected. Heat transfer to the burner surface or chemical nonequilibrium would lower the peak flame temperature. Adiabatic flame temperature for ethylene air at the equivalence ratio of ϕ = 0.95 was calculated using the NASA Glenn chemical equilibrium code (CEA) [22]. Under these conditions, the OH mole fraction predicted by the CEA code is The least squares residual analysis yields an uncertainty of approximately ±100 K in the inferred rotational temperature, which corresponds to an estimated systematic uncertainty of 40% in the OH mole fraction inferred from the LIF spectra, using the CEA code. B. OH LIF Diagnostic Details Utilizing the flat-flame burner calibration technique requires that OH plasma/burner signal intensity ratio be corrected for differences in temperature, or more specifically by the rotational Boltzmann fraction for the absorbing transition, and upper electronic state quenching and VET rates, which depend on pressure, temperature, and mixture composition. Specifically, the OH number density in the plasma is related to the OH number density in the flat flame according to S F,PAC Φ Flame f B,Flame n OH,PAC = n OH,Flame. (4) S F,Flame Φ PAC f B,PAC Furthermore, assuming that ε 0 in (3) is equal to zero (which is admittedly not precisely true for our (0, 0) band filter), the fluorescence quantum yield is given by the simple expression Φ= A 1 Q V,10 + Q e,1 + A 1 = A 1 Q V,10 + Q e,1. (5) All parameters in (4) are necessary to obtain absolute OH number densities from LIF measurements. Evaluation of the quenching rate in the atmospheric-pressure flat-flame burner is done using literature values for individual species quenching rate coefficients, in combination with species mole fractions Q e,1 = i n i k i. (6) The collisional quenching cross section of OH in the A state by a number of different collision partners has been measured over a range of temperatures by several investigators [23], [24]. Tamura et al. [23] developed an empirical correlation to tabulate the experimental results k i = a i σ q T 0.5 (7) where a i is the fit coefficient for collider species i, σ q is the collisional cross section, and T is the local flame temperature. Temperature dependence of the collisional cross section can be expressed as the empirical two-parameter equation σ q = σ q e ε kt. (8) The values of a i, σ q, and ε/k for major collider species of OH are summarized in Table II. In the Hencken burner, the major species of interests are H 2,H 2 O, O 2, and N 2. The rates for VET are from Paul et al. [24], [25] for the Hencken burner calibration flame at 2300 K ± 100 K. There are only very limited data available in the literature. The collider species of interests are CO, CO 2,N 2, and O 2. The rates for VET are estimated from the data of Paul et al. to be approximately 30% lower than the total quenching rates at flame temperature. For the low-pressure low-temperature plasma measurements, it was determined that the procedure outlined previously resulted in predicted quenching rates that were considerably higher than those directly measured experimentally by capturing the temporal rise and decay of the OH LIF signal. As an example of this direct measurement, Fig. 10 (left) shows a typical time-resolved OH fluorescence PMT signal trace on a semilog scale, recorded using an ordinary digital oscilloscope. It can be seen that the decay is approximately single exponential, for times greater than 10 ns after the peak signal. The experimental fluorescence decay rate was determined from data recorded for times greater than 10 ns after the peak of laser excitation. As Roy et al. suggested in [26], the PMT could still respond to the laser excitation pulse for times closer to the start of laser excitation. Fig. 10 (right) shows the same data on an expanded time scale covering ns, along with a least squares fit to an assumed single exponential function, which results in an inferred experimental decay time τ obs of 25 ns, with an estimated uncertainty of ±20%. The sum of

8 CHOI et al.: HYDROXYL RADICAL KINETICS IN HYDROGEN AIR NANOSECOND PLASMAS 3295 Fig. 10. (Left) Typical time-resolved LIF signal in burst mode discharge (360th pulse) with UV laser pulse. H 2 /air, ϕ =1.0, P =54torr, and T = 100 C. (Right) Expanded time scale between 30 and 50 ns, with exponential fit. Fig. 11. ICCD camera images of filamentary and uniform nanosecond pulsed discharge plasmas in stoichiometric hydrogen air mixtures at different initial temperatures. Centerline emission intensity is plotted above each image. the quenching rate and the VET rate is related to the observed fluorescence decay rate according to Q V,10 + Q e,1 = 1 1 (9) τ obs τ rad where τ rad is the spontaneous radiative lifetime of the gas mixture [27], which, for the OH A 2 Σ + (v =1)state, is equal to 1150 ns [28]. For this measurement, this sum is determined experimentally from the time-resolved LIF fluorescence decay curves. C. OH Measurement Results and Discussion The use of a quasi-1-d approximation for kinetic and heat transfer analysis is predicated on the assumption that the discharge remains diffuse and spatially uniform during the entire pulse burst sequence. This has been confirmed in our previous hydrogen air measurements [29], in which an extensive set of UV ICCD camera images was obtained (primarily N 2 second positive band emission) as a function of pressure, burst duration, and pulse number within a burst. As a representative example, Fig. 11 shows a group of images of stoichiometric hydrogen air plasmas [29]. All images were taken at the pulse repetition rate of v =40kHz, and ICCD gating was set at the 200th pulse in the burst. Pressure was set to maintain approximately the same number density at different initial temperatures of the hydrogen air mixture. Emission intensity profiles extracted from ICCD images are plotted above the images on the same scale to assist distinguishing local intensity maxima. From the images, it is quite clear that, at room temperature, multiple well-defined constricted filaments are formed in the plasma. When the flow and the discharge cell are preheated up to 100 C 200 C, the hydrogen air plasma becomes com- Fig. 12. Comparison of experimental time-resolved absolute hydroxyl radical concentration after a single-pulse discharge with kinetic model predictions. H 2 air, ϕ =1.0, (left) P =54and (right) 74 torr, and T 0 = 100 C. pletely uniform and occupies the entire field of view. After a thorough mapping of plasma uniformity by such method, OH LIF measurements were carried out at T 0 = 100 C 200 C and P =54 94 torr. Fig. 12 shows experimental absolute OH number density as a function of time after initiation of a single discharge pulse in a hydrogen air mixture at ϕ =1.0 and P =54and 74 torr, at 100 C (symbols), along with predictions of the full model (solid line) and a reduced model (dashed line) that will be discussed in more detail hereinafter. Experimentally, the absolute OH temporal profile is found to rise rapidly during the initial 0.1 ms after discharge initiation, to peak values of cm 3 at P =54torr and of cm 3 at P = 74 torr, and then decay relatively slowly, with a characteristic time scale of 1 ms thereafter. The corresponding modeling prediction for the relative shape of the OH temporal evolution is similar, although the OH number density predicted by the plasma kinetic model peaks somewhat later than that in the experiment, 0.2 and 0.15 ms after the pulse, at 54 and 74 torr, respectively. The peak predicted OH number density is about a factor of 40% higher than the experimental value at 54 torr and 15% higher at 74 torr. Note that the measured absolute OH fluorescence signal from the discharge cell may be lower than that predicted due to geometrical issues in the detecting optics, as the Hencken calibration burner and the plasma cell are installed on different tables. In addition, the absolute OH number density is strongly dependent on coupled pulse energy and its scaling with temperature and pressure, which are

9 3296 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011 Fig. 13. Comparison of experimental time-resolved absolute hydroxyl radical concentration after a single-pulse discharge with kinetic model predictions. H 2 air, ϕ =1.0, (left) P =68and (right) 94 torr, and T 0 = 200 C. difficult to measure directly. As stated previously, the kinetic model assumes constant coupled pulse energy per molecule [12], 0.28 mev/molecule for the pulse voltage waveform in Fig. 2, which so far has not been verified experimentally. Finally, as discussed in detail previously, the absolute calibration depends upon the literature values for the VET rates and quenching rates in the Hencken burner, which may be inaccurate, and the assumption that ε 0 is zero. Examination of the modeling predictions for OH production and loss channels in more detail reveals that, at low temperatures (T = 300 K 400 K), the principal OH formation and loss processes on a long time scale after the nanosecond discharge pulse, ms, are as follows: H + O 2 + M HO 2 + M (10) O + HO 2 OH + O 2 (11) OH + H 2 H + H 2 O. (12) It is interesting to note that, at these low temperatures, process (10), which is typically considered a radical chain terminating reaction, is actually a principal contributor to net exothermic low-temperature oxidation chemistry. Using the same reaction rate data as in [11], the characteristic time for HO 2 production by reaction (10) at 54 torr and ϕ =1.0 at 100 Cis 30 μs. Similarly, assuming that approximately 50% of the input pulse energy goes to oxygen dissociation [5] (i.e., that O atom mole fraction is 50 ppm), the characteristic time scale for OH production (i.e., loss of HO 2 ) by reaction (11) is 0.2 ms, which is consistent with the OH rise time measured in the experiment (see Fig. 12). The characteristic 1/e OH decay time due to H 2 O formation by reaction (12) is 0.4 ms, which also correlates reasonably well with the time scale for OH decay detected in the experimental data shown in Fig. 12. Finally, steady-state concentration of H 2 O predicted by the model after a single discharge pulse at 100 Cisveryclose to that of atomic oxygen created by discharge processes (electron impact dissociation and N 2 quenching, processes P1 and P3 P7, Table I). This demonstrates nearly complete absence of chain-branching processes in a low-temperature hydrogen air plasma. Indeed, the reaction paths (10) (12) are equivalent to a net recombination reaction O + H 2 H 2 O. Fig. 13 shows experimental absolute OH number density as a function of time after initiation of a single discharge pulse in a hydrogen air mixture at ϕ =1.0 and P =68and 94 torr in 200 C. The absolute OH temporal profile is found to rise rapidly during the initial 0.1 ms after discharge initiation and decay slowly, which is very similar to the T = 100 C condition. Again, the relative shapes of the data and modeling predictions are similar, although the model prediction at P =68torr exhibits peak OH at somewhat longer time after discharge initiation than what is observed in the experiment. At P = 94 torr, the agreement, both relative and absolute, is quite good. Steady-state concentration of H 2 O predicted by the model after a single discharge pulse at 200 C, P =68 94 torr, exceeds that of atomic oxygen created by discharge processes by a factor of This shows that chain-branching processes in hydrogen air plasmas become increasingly more important at elevated temperatures. Figs. 14 and 15 show absolute OH concentration data as well as predicted OH and temperature data when the discharge in stoichiometric hydrogen air mixtures is operated in repetitive burst mode at 40 khz, at P =54and 74 torr at T = 100 C and at P =68and 94 torr at T = 200 C, respectively. The experimental results are plotted as a function of number of pulses in a burst, which range from a single pulse/burst (0.025 ms) to 1000 pulses/burst (25 ms). In each case, the experimental data were obtained 60 μs after the initiation of the final pulse in the burst, corresponding to the approximate time for peak OH formation from a single pulse, as shown in Figs. 12 and 13. As would be expected, based on the approximately millisecond decay time exhibited in the single-pulse data in Figs. 12 and 13, the absolute OH number density rises rapidly during the first approximately ten pulses (0.25 ms) and then levels off to a near steady state after 0.5 ms (20 pulses). It can also be seen that, in both cases, a large (log scale) secondary rise in OH number density is observed, which is clearly indicative of ignition. Sudden OH number density jumps predicted by the model for short burst durations ( pulses) are due to rapid OH generation during each discharge pulse, process P12 in Table I. As expected, this stepwise rise of the timeresolved OH number density predicted by the model becomes nearly undetectable for long burst durations. Comparing the experimental and modeling calculation results, it can be seen that ignition time, defined as the position of the maximum OH LIF signal, is predicted by the model quantitatively. On the other hand, the model again predicts OH number density somewhat larger than the experimental OH temporal profile after 0.1 ms. In addition to issues discussed previously, Adamovich et al. discussed in [12] that the residual electron density from previous discharge pulses can be quite significant, therefore reducing the coupled pulse energy. The other factor that may well contribute to the coupled pulse energy reduction is charge accumulation on the dielectric surfaces, which would produce a similar plasma shielding effect. D. Reduced Kinetic Mechanism The solid curves in Figs correspond to predictions utilizing the full set of 22 hydrogen air chemical reactions suggested by Popov [11]. It is useful, however, to consider a reduced reaction set in order to both simplify the mechanism and provide additional physical insight into the most important processes. In our previous work [10], it was found

10 CHOI et al.: HYDROXYL RADICAL KINETICS IN HYDROGEN AIR NANOSECOND PLASMAS 3297 Fig. 14. Comparison of experimental absolute hydroxyl radical concentration as a function of burst duration at pulse repletion rate of 40 khz with kinetic model predictions. H 2 air, ϕ =1.0, and (top) P =54and (bottom) 74 torr at T 0 = 100 C. that a nine-reaction set reduced mechanism reproduced experimental pure rotational CARS thermometry data obtained in a 40-kHz-repetition-rate burst mode H 2 air discharge at P = 40 torr and an initial temperature of T = 300 K. Similar to the time-resolved temperature behavior predicted by the model in Figs. 14 and 15, in [10], a modest temperature rise was observed for the first approximately 15 ms (600 pulses), followed by a more rapid increase and temperature maximum at 17 ms (740 pulses). Through a sensitivity analysis, it was found in [10] that, at elevated temperatures, T = 400 K 600 K, the rate of fuel oxidation increases significantly, primarily due to additional reactions controlling chain branching H + HO 2 H 2 O + O (13) H + HO 2 OH + OH (14) H + HO 2 H 2 + O 2 (15) O + H 2 H + OH (16) H + O 2 O + OH (17) as well as the exothermic chain termination process OH + HO 2 H 2 O + O 2. (18) Additional energy released from these net exothermic plasma chemical reactions is predicted to increase considerably, reaching a factor of approximately three greater than the energy input directly by the discharge at 600 K [10]. Similar to the results in [10], Figs show that the reduced reaction set of (10) (18) (dashed curves) results in predictions for the temporal evolution of OH, in both single-pulse mode and burst mode, that are nearly identical to those of the full 22-reaction set model, including the prediction of ignition time. V. S UMMARY AND CONCLUSION Time-resolved absolute hydroxyl radical concentration measurements have been obtained in repetitively pulsed nanosecond hydrogen air plasmas. Linear-regime LIF is used to obtain relative OH concentrations which are put on an absolute basis using an atmospheric-pressure flat-flame Hencken burner calibration method. Absolute OH number density data are obtained in stoichiometric hydrogen air mixtures at P = torr and initial temperature of T = 400 K 500 K. Data are obtained both as a function of time after initiation of a single nanosecond discharge pulse and as a function of number of pulses in a 40-kHz-repetition-rate pulse burst, with the number of pulses in the burst varying from 1 to For a single discharge pulse, absolute OH number density is found to rise rapidly during the initial 0.1 ms after discharge initiation and decay relatively slowly, with a characteristic time scale of 1 ms. In repetitive burst mode, the absolute OH number density is observed to

11 3298 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011 Fig. 15. Comparison of experimental absolute hydroxyl radical concentration as a function of burst duration at pulse repletion rate of 40 khz with kinetic model predictions. H 2 air, ϕ =1.0, and (top) P =68and (bottom) 94 torr at T 0 = 200 C. rise rapidly during the first approximately ten pulses (0.25 ms) and then level off to a nearly steady-state plateau. In all cases, a large (log scale) secondary rise in OH number density is also observed, which is clearly indicative of ignition, with ignition time of 5 10 ms, for initial temperatures of 100 C and 200 C and pressures ranging from 54 to 95 torr. Kinetic modeling calculations capture this trend quantitatively. The experimental results are found to agree well, albeit not perfectly, with the predictions of a hydrogen air plasma chemistry model, which incorporates nonequilibrium plasma discharge processes, low-temperature H 2 air chemistry, nonempirical nanosecond discharge pulse energy coupling, and quasi-1-d conduction heat transfer. Uncertainty in coupled pulse energy, as well as insufficient VET rate data, and some uncertainty in key chemical process rates are likely reasons for the discrepancy between experimental OH temporal profiles and kinetic model predictions. In addition, geometric issues related to absolute calibration could also contribute to the lack of agreement between the measured absolute OH number density and model predictions. Finally, the results of sensitivity analysis show that a reduced nine-reaction set involving the species O, H, OH, HO 2,H 2,O 2, and H 2 O results in predictions for the temporal evolution of OH number density and ignition time that are essentially identical to those of the full 22-hydrogen airreaction model by Popov [11]. REFERENCES [1] S. M. Starikovskaia, Plasma assisted ignition and combustion, J. Phys. D, Appl. Phys, vol. 39, no. 16, pp. R265 R299, Aug [2] I. Choi, M. Uddi, Y. Zuzeek, I. V. Adamovich, and W. R. Lempert, Stability and heating rate of air and ethylene air plasmas sustained by repetitive nanosecond pulses, presented at the 47th Aerospace Sciences Meeting Exhibit, Orlando, FL, Jan. 5 8, 2009, AIAA Paper [3] M. Nishihara, K. Takashima, N. Jiang, W. R. Lempert, I. V. Adamovich, and J. W. Rich, Development of a Mach 5 nonequilibrium wind tunnel, presented at the 48th Aerospace Sciences Meeting and Exhibit, Orlando, FL, Jan. 4 7, 2010, AIAA Paper [4] W. Kim, H. Do, M. G. Mungal, and M. Cappelli, Investigation of NO production and flame structure in plasma enhanced premixed combustion, Proc. Combustion Inst., vol. 31, no. 2, pp , Jan [5] M. Uddi, N. Jiang, E. Mintusov, I. V. Adamovich, and W. R. Lempert, Atomic oxygen measurements in air and air/fuel nanosecond pulse discharges by two photon laser induced fluorescence, Proc. Combustion Inst., vol. 32, no. 1, pp , [6] GRI-Mech 3.0. [Online]. Available: version30/text30.html [7] H. Wang, X. You, A. V. Joshi, S. G. Davis, A. Laskin, F. Egolfopouls, and C. K. Law, USC Mech version II. High-temperature combustion reaction model of H 2 /CO/C1 C4 compounds, [Online]. Available: [8] G. D. Stancu, M. Janda, F. Kaddouri, D. Pai, D. A. Lacoste, J. C. Rolon, and C. O. Laux, Two photon absorption laser induced fluorescence study of repetitively pulsed nanosecond discharges in atmospheric pressure air, in 39th AIAA Plasmadynamics Lasers Conf., Seattle, WA, Jun , 2008, AIAA [9] G. D. Stancu, F. Kaddouri, D. A. Lacoste, and C. O. Laux, Investigations of rapid plasma chemistry generated by nanosecond discharges in air at atmospheric pressure using advanced optical diagnostics, presented at

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