IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011 3269

Ignition Time Measurements in RepetitiveNanosecond Pulse Hydrogen–Air Plasmas at

Elevated Initial TemperaturesZhiyao Yin, Keisuke Takashima, and Igor V. Adamovich

Abstract—Ignition time is measured in premixed preheatedhydrogen–air flows excited by a repetitive nanosecond pulse dis-charge in a plane-to-plane geometry. ICCD images of the plasmaand the flame demonstrate that mild preheating of the fuel–airflow greatly improves plasma stability and precludes filamentformation. At the initial temperatures of T0 = 100 − 200 C,hydrogen–air plasmas remain stable and uniform up to at leastP = 150 torr, and ignition occurs in a large volume. In contrast,ignition in less uniform preheated ethylene–air plasmas occurslocally, near the electrode edges, with flame propagating towardthe center of the plasma. Ignition time in hydrogen–air mixturesis measured at initial temperatures of T0 = 100 − 200 C, pres-sures of P = 40–150 torr, equivalence ratios of φ = 0.5–1.2, andpulse repetition rates of ν = 10–40 kHz. The results of ignitiontime measurements are compared with the predictions of thehydrogen–air plasma chemistry model, showing good agreement.Nitrogen emission spectra are used to measure time-resolvedtemperature in air and hydrogen–air plasmas. The results showthat ignition begins at the plasma temperature of T ≈ 700 Kand results in a rapid temperature rise. By turning off dominantplasma chemical radical generation processes in kinetic modelingcalculations, while keeping discharge energy loading the same,it is demonstrated that ignition is driven by additional energyrelease in reactions of plasma-generated radicals with hydrogen.To determine if plasma-generated radicals may reduce ignitiontemperature, discharge pulse burst was terminated before theonset of ignition, and ignition delay time was measured versusplasma temperature at the end of the burst. Experimental ignitiondelay time is in reasonably good agreement with kinetic modelingcalculations. The kinetic model predicts significant plasma-as-sisted ignition threshold temperature reduction at the presentconditions compared to thermal ignition, up to ∆T = 180 K.

Index Terms—Emission spectroscopy, ignition time, nanosecondpulse plasma, plasma assisted combustion.

I. INTRODUCTION

OVER THE recent years, plasma-assisted combustion hasbecome a rapidly developing research field [1]. One

of the main research goals in this area is elucidating funda-mental kinetic mechanisms of ignition via reactions of radi-

Manuscript received June 10, 2011; revised August 2, 2011; acceptedAugust 18, 2011. Date of current version December 14, 2011. This work wassupported by the U.S. Air Force Office of Scientific Research MURI “Funda-mental Aspects of Plasma Assisted Combustion,” Julian Tishkoff—TechnicalMonitor.

The authors are with the Michael A. Chaszeyka Nonequilibrium Ther-modynamics Laboratories, Department of Mechanical and Aerospace En-gineering, The Ohio State University, Columbus, OH 43210 USA (e-mail:yin.67@osu.edu; takashima.6@osu.edu; adamovich.1@osu.edu).

Digital Object Identifier 10.1109/TPS.2011.2171508

cal species generated in a low-temperature plasma. Nonequi-librium plasmas generated by high-peak-voltage repetitivelypulsed nanosecond pulse duration discharges appear especiallypromising for these applications since they operate at highvalues of the reduced electric field, E/N ∼ 100 Td and above(1 Td = 10−17 V · cm2). At these conditions, a significantfraction of the input discharge power is spent on electronicexcitation of molecules and molecular dissociation by electronimpact. These discharges also remain stable at high pressuresdue to their very low duty cycle (∼1/1000). Recent plasma-assisted combustion experiments using nanosecond pulse dis-charges include ignition and flameholding in room-temperaturequiescent and flowing air–fuel mixtures [2]–[9], ignition insupersonic flows [10]–[12], ignition delay time reduction inquiescent shock-preheated mixtures [13]–[15], and flame sta-bilization [16]–[20], as well as measurements of metastableand radical species concentrations [19], [21], [22] and CARSthermometry [23], [24].

A major difficulty in obtaining quantitative insight into thekinetics of low-temperature plasma-assisted ignition is isolatingconclusively the nonequilibrium ignition mechanism from amore trivial effect of thermal ignition, which may well occur inconstricted discharges and arc filaments formed in the plasma.Our recent experiments [8], [24] demonstrated that nanosecondpulse discharges, although considerably more stable comparedto dc, ac, RF, and microwave discharges, may still develop well-defined filamentary structures in room-temperature fuel–airmixtures, at pressures used for plasma-assisted ignition exper-iments (typically a few tens of torr and above). The approachused in this paper is mild preheating of the fuel–air flow totemperatures below autoignition temperature (up to T = 100 −200 C), combined with excitation by a repetitive nanosec-ond pulse discharge. As demonstrated by the present results,preheating substantially reduces discharge filamentation andallows sustaining stable, diffuse, and uniform nonequilibriumplasmas over a wide range of pressures.

The objectives of this paper are as follows: 1) to developan experimental apparatus for measurements of ignition time,temperature, and radical species concentrations in spatiallyuniform plasmas produced by a repetitive nanosecond pulsedischarge and 2) to measure ignition time and temperature inpreheated hydrogen–air mixtures over a wide range of pres-sures, discharge pulse repetition rates, and equivalence ratios.The principal difference of this approach compared to plasma-assisted ignition delay time measurements in shock tubes

0093-3813/$26.00 © 2011 IEEE

3270 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011

Fig. 1. Schematic of the high-temperature plasma-assisted ignition cell.

[13]–[15] is that, in this paper, the initial temperature of fuel–airmixtures is kept significantly below autoignition temperature,T = 100 − 200 C, to obtain data on plasma-assisted ignitionat low temperatures.

II. EXPERIMENTAL

The schematic of the experimental setup used for plasma-assisted ignition of premixed hydrogen–air and ethylene–airflows is shown in Fig. 1. Unlike in our previous plasma-assistedignition experiments [7], in this paper, the fuel–air flow andthe entire discharge cell are preheated in a tube furnace toimprove plasma stability. The plasma flow reactor consists of a280-mm-long 22 mm × 10 mm rectangular cross-sectionalquartz channel with a wall thickness of 1.75 mm. Two planequartz windows are fused to the ends of the channel. A quartztube coil that is 1 m long, fused to the surface of the quartz chan-nel for stability, serves as a flow-preheating inlet (see Fig. 1). Atthe present experimental conditions, the coil length is sufficientto preheat the fuel–air flow to the furnace temperature, whichwas verified by thermocouple measurements.

Two 1/4-in-diameter quartz-to-stainless-steel adaptors arefused to the inlet and the exit of the cell to connect it to thegas delivery system. Two rectangular plate copper electrodes,14 mm × 60 mm each, are placed on the top and bottom ofthe channel halfway between the ends and are held in place byceramic clamps, as shown in Fig. 1. The electrodes are roundedat the edges. A 1/16-in-thick silicone rubber sheet is placedbetween each electrode and the quartz channel to reduce the airgaps and prevent corona discharge formation outside the cell.A 60-mm right-angle fused silica prism is placed along the sideof the channel to provide optical access to the cell. The entireassembly is placed inside a tube furnace (Thermcraft, Ltd.) witha 6-in-diameter 12-in-long heating section. The furnace can be

heated up to 1200 C. Fuel and air flow rates through the cellare controlled by MKS mass flow controllers. Before enteringthe cell, fuel and air flows are premixed in an in-line flow mixerthat is 12 in long. Two pressure gauges are placed in the deliverylines upstream and downstream of the cell. At the presentconditions, the pressure drop across the cell is approximately8 torr. The test section pressure is taken to be the average of thetwo pressure gauge readings. Ignition measurements are madeat the initial fuel–air flow temperatures of T0 = 100 C and200 C, pressures of P = 50–150 torr, and equivalence ratiosof φ = 0.5–1.2, at the flow velocity of u = 12 cm/s. The slowflow velocity is chosen for more efficient flow preheating inthe furnace, as well as to reduce the pressure drop across thedischarge cell. At these conditions, the estimated flow residencetime in the discharge is 0.25–0.5 s. The electrodes are connectedto a Chemical Physics Technologies high voltage pulsed powersupply, which produces 25-kV peak voltage and 25-ns durationpulses, with a pulse repetition rate up to ν = 50 kHz. Inthis paper, the pulse generator was operated in burst mode,producing sequences of up to 1000 pulses at a pulse repetitionrate of ν = 10–40 kHz. To produce weak preionization of theflow in the cell, it was irradiated by a UV radiation source(Hg–Ar lamp) through one of the windows at the end of thecell. This approach helped producing breakdown by the firstdischarge pulse.

A separate low-temperature discharge cell of the same geom-etry, but without silicone rubber sheets between the electrodesand the quartz channel, is used for discharge pulse energymeasurements. In this cell, air gaps between each electrode andthe channel are removed using a thin layer of self-hardeningsilicone rubber compound with a lower temperature rating,which was also used to cover the entire electrode surface. En-ergy coupled to the plasma by a nanosecond pulse discharge isdetermined by simultaneous measurements of pulse voltage and

YIN et al.: IGNITION TIME MEASUREMENTS IN HYDROGEN–AIR PLASMAS 3271

TABLE IDOMINANT RADICAL SPECIES GENERATION PROCESSES IN THE PLASMA

TABLE IIREDUCED H2 − O2 REACTION MECHANISM

current using custom-designed short response time capacitivevoltage probes and shunt current probes [25]. The time integralof the product of voltage and current yields the pulse energycoupled to the plasma.

A PI-MAX gated ICCD camera with a UV lens (NikonNikkor 105 mm f/4.5) was used to take broad-band single-pulse images of the repetitively pulsed nanosecond plasma infuel–air mixtures, as well as broad-band flame images betweenthe pulses. The camera was triggered by a Stanford ResearchSystem DG535 four-channel delay/pulse generator that uses alow-voltage TTL pulse output generated by the high-voltagepulse generator 1.25 µs before each pulse, as synchronizationinput. Images were taken via a 60-mm-long right-angle prism,which captures the entire view of the discharge, as shown inFig. 1. Ignition was detected by monitoring time-resolved OHemission from the cell using a narrow-bandpass filter centeredat 310 ± 2 nm (bandpass of 10 ± 2 nm FWHM), a photomul-tiplier tube (PMT), and a digital oscilloscope. The responsetime of this emission diagnostics, approximately 10 µs, wascontrolled by a variable terminator resistor placed between thePMT and the oscilloscope and set at 50 kΩ.

The rotational temperature in the plasma as a function oftime during the pulse burst was inferred from visible emis-sion spectra (partially rotationally resolved 0 −→ 0 band ofthe N2(C3Πu −→ B3Πu) band system). The emission spec-troscopy setup consisted of an optical multichannel analyzerwith a 0.3-m Acton Research SpectraPro-300i spectrometer, a2400-g/mm grating, and a gated PI-MAX ICCD camera. To

discriminate the plasma emission between the discharge pulses,the camera was gated repeatedly to collect emission duringeight sequential pulses with a gate width of 2 µs, thus spanninga 200-µs period at a pulse repetition rate of ν = 40 kHz.To spatially resolve emission from the center and from theedges of the plasma, a translation stage was used to adjust thecollection optics and to image different regions of the plasmaonto the monochromator slit. The spatial resolution of thesemeasurements is approximately 6 mm. The relative uncertaintyof the temperature measurements is approximately ±5% for airand approximately ±15% for fuel–air plasmas.

III. PLASMA CHEMISTRY MODEL AND NANOSECOND

PULSE DISCHARGE MODEL

To obtain insight into the kinetic mechanism of plasma/chemical fuel oxidation and ignition, we used a hydrogen–airplasma chemistry model developed in our previous work[21]–[24], [26]. Briefly, the model incorporates a nonequi-librium air plasma chemistry model [27] expanded to in-clude hydrogen dissociation processes in the plasma andhydrogen–oxygen chemistry model (22 reactions among H,O, OH, H2, O2, H2O, HO2, and H2O2) developed by Popov[28]. The dominant radical species (O, H, and OH) generationprocesses in the plasma are listed in Table I.

Note that, although, in this paper, we are using the fullhydrogen–oxygen reaction mechanism of [28], a reduced ki-netic mechanism listed in Table II has been identified in

3272 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011

our recent work [32] using sensitivity analysis. The reducedmechanism remains accurate in the entire range of the presentexperimental conditions.

The species concentration equations are coupled with thetwo-term expansion Boltzmann equation for the energy dis-tribution function of plasma electrons, with electron impactcross sections taken from [30]–[32]. The full list of air plasmaprocesses incorporated into the model and their rates is givenin our recent paper [22]. The list of hydrogen–oxygen chemicalreactions and their rates is given in [28].

The model incorporates the equation for the temperature onthe discharge centerline, Tc, with heat transfer to the walls beingthe dominant energy loss mechanism

dTc

dt=

qθ

ρcp−α(T )

Tc−Tw

(L/π)2

=qpulseθν

ρccp− 1

ρccp

∑i

hi

(dni

dt

)− λ(T )

ρcp

Tc−Tw

(L/π)2(1)

where T is the spatially averaged temperature, T (t) =(1/L)

∫ L/2

−L/2 T (z, t)dz, θ = (Tc − Tw)/(T − Tw) is the tem-perature profile shape factor, qpulse is the coupled pulse energyper unit volume (in J/m3), ν is the pulse repetition rate ina burst, hi denotes the enthalpies of chemical and excitedspecies, dni/dt denotes the rates of species concentration (inkmole/m3) change in chemical reactions, α is the thermaldiffusivity, λ is the thermal conductivity, Tw is the wall temper-ature, L is the characteristic spatial scale (test section height),and L/π is the spatial scale for transient conduction heattransfer. In a transient conduction model for plane geometrywith constant thermal diffusivity at large Fourier numbers,greater than Fo = αt/(L/π)2 ∼ 0.1 (t ∼ 1 ms at the presentconditions), the transient spatial temperature distribution acrossthe plasma can be approximated by a cosine function, T (z) −Tw ≈ [Tc − Tw] · cos(πz/L), −L/2 ≤ z ≤ L/2, which givesthe spatially averaged temperature as T ≈ Tw + (2/π) · (Tc −Tw), i.e., θ = π/2. If the temperature dependence of the ther-mal diffusivity is taken into account, the temperature distrib-ution is T (z) − Tw ≈ [Tc − Tw] · (1 − 2z/L)3 such that T ≈Tw + (3/4) · (Tc − Tw) and θ = 4/3. The accuracy of thisapproximation has been verified by comparing it with a solutionof a 1-D unsteady heat transfer equation, for the same netenergy input rate, as discussed in greater detail in [23]. Forsingle-burst operation and short burst durations used in thispaper, the wall temperature was assumed to be the same as theinitial temperature of the mixture, Tw = T0.

The high-voltage pulse shape used by the plasma chemistrymodel is a Gaussian fit to the experimentally measured voltagepulse shape shown in Fig. 2

V (t) = Vn · exp

[−

(t − tn

τ

)2]

+ Vp · exp

[−

(t − tp

τ

)2]

(2)

with Vn = −22.5 kV, Vp = 17.5 kV, and τ = 10 ns. How-ever, the field in the plasma is much lower than the appliedfield, due to strong shielding of the applied voltage caused bycharge accumulation on the dielectric walls after breakdown.

Fig. 2. Experimental high-voltage pulse shape in room-temperature air atP = 65 torr and a Gaussian of (2), with τ = 10 ns.

The analytical model of energy coupling in nanosecond pulsedischarge plasma has been developed in [26]. Briefly, thismodel incorporates key effects of pulsed breakdown, chargeaccumulation on dielectric surfaces, and sheath development onnanosecond time scale. The model predicts that energy coupledto the plasma during the pulse is controlled primarily by thecapacitance of the dielectric layers and by the applied voltagepulse shape

Qpulse ≈ 12C

(V 2

b + V 2peak

√2π

νRCτ

)

=12

εε0A

2l

(V 2

b + V 2peak

√2π

νRCτ

). (3)

In (3), C is the capacitance of the dielectric layers covering theelectrodes, Vpeak and Vb are the peak pulse voltage and break-down voltage, respectively, τ is the pulse duration parameterof a Gaussian pulse [see (2)], νRC = 1/RC is the RC timeconstant of the load (i.e., the plasma and the dielectric layers)after breakdown, R is the resistance of the shielded plasma afterbreakdown, l and ε are the dielectric layer thickness and thedielectric constant, and A is the electrode surface area. Thecoupled pulse energy predicted by (3) is approximately propor-tional to the number density [26], i.e., the energy coupled permolecule, qpulse/N = Qpulse/ALN , remains nearly constant.

The coupled pulse energy predicted by (3) was found to bein good agreement with discharge pulse energy measurementsin room-temperature air plasma sustained in a low-temperaturedischarge cell without silicone rubber layers (see discussion inSection IV). The main difficulty with predicting the coupledenergy in the high-temperature discharge cell is evaluatingthe capacitance of the dielectric layers between the electrodesand the plasma, which consist of quartz channel walls andsilicone rubber sheets. The dielectric constant of silicone rubbervaries considerably depending on its chemical composition,ε = 3.2–9.8 (dielectric constant of fused quartz is ε = 3.8).

YIN et al.: IGNITION TIME MEASUREMENTS IN HYDROGEN–AIR PLASMAS 3273

Fig. 3. Comparison of experimental coupled pulse energy with nanosecondpulse discharge model prediction [see (3)] for the pulse voltage waveformshown in Fig. 2. Room-temperature air plasma in the low-temperature dischargecell, ν = 40 kHz, pulse #50 in the burst.

This results in a significant uncertainty in the predicted pulseenergy, Qpulse = 0.60–0.90 mJ at T = 300 K and P = 60 torr.The value of Qpulse = 0.72 mJ (coupled pulse energy permolecule of qpulse/N = Qpulse/ALN = 0.28 meV/molecule)is chosen because it provides better overall agreement with thepresent experimental data.

The pulse peak reduced electric field in the plasma chem-istry model is based on the breakdown voltage predictedby the nanosecond pulse discharge model for the Gaussianvoltage waveform [26], (E/N)peak = Vb/LN ≈ 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 mix-tures is spent on the generation of O and H atoms. Note thatgradual plasma temperature rise during the pulse burst resultsin breakdown voltage reduction, thereby decreasing the pulseenergy coupled to the plasma inversely proportional to tem-perature, Qpulse ∼ N ∼ 1/T . Peak pulse conduction currentand electron density, also approximated by a Gaussian pulsewith τ = 10 ns, are evaluated from the coupled pulse energy.This simple coupling of the nanosecond pulsed discharge modeland the plasma chemistry model incorporates the most essentialeffect of coupled pulse energy dependence on the voltage wave-form, the dielectric plate parameters, and the number density.

The present hydrogen–air plasma chemistry model has beenvalidated by comparing with O atom concentration measure-ments in single-pulse and repetitively pulsed nanosecond dis-charges in air [21], [26], as well as with purely rotational CARStime-resolved temperature measurements in repetitively pulsednanosecond discharges in air and in hydrogen–air [24], showinggood agreement.

IV. RESULTS AND DISCUSSION

Fig. 3 compares the results of coupled pulse energy mea-surements with the prediction of the nanosecond pulse dis-

Fig. 4. Schematic of ICCD camera gate timing used to separate plasmaemission from flame emission.

charge model of [26]. The pulse energy plotted in Fig. 3 hasbeen measured in the low-temperature discharge cell, in room-temperature air at P = 65 torr, pulse repetition rate of 40 kHz,and pulse #50 in a burst. It can be seen that the coupled pulseenergy predicted by the model for the voltage pulse waveformshown in Fig. 2 is in good agreement with the experimentaldata in the pressure range of P = 13–90 torr. At P ∼ 40–100torr, the predicted pulse energy scales approximately linearlywith pressure (number density), as shown in Fig. 3. At lowpressures, deviation from linear scaling is due to energy coupledto the plasma after breakdown, given by the second term in(2), which is primarily controlled by the pulse duration and isweakly dependent on pressure. These results provide validationof the nanosecond pulse discharge energy coupling model andjustify the constant energy loading per molecule approximationused in this paper.

Both for plasma imaging and for temperature measurements,we used the same ICCD camera gate timing, illustrated inFig. 4. To obtain an image or an emission spectrum of theplasma generated by an individual nanosecond pulse, the cam-era gate, which is 2 µs long, was set to open shortly before thepulse and close after the pulse to collect N2 second positiveemission from the plasma. For temperature measurements,emission spectra were accumulated over eight consecutivedischarge pulses, gating the camera on each pulse. Note thatthis approach is critical for accurate translational–rotationaltemperature inference from N2 emission spectra, which re-quires the emitting N2(C3Πu) state to be populated by directelectron impact from the ground electronic state of nitrogen[33]. In particular, recent experiments in nanosecond pulsedischarges [34] provided direct evidence that rotational tem-perature inferred from N2 second positive emission spectra, atthe conditions when N2(C3Πu) state is populated by processesother than electron impact, is significantly overestimated (byup to 200 K–300 K). Flame images, on the other hand, weretaken after the discharge pulse burst was turned off, as shownin Fig. 4, slightly before ignition, to distinguish the flame signal(produced mainly by OH, CH, and C2 emission) from muchstronger N2 emission. The camera gate for flame imaging wasset to 30 µs. In addition, the camera gain was increased tocompensate for lower flame emission intensity compared toplasma emission.

3274 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011

Fig. 5. Time-resolved UV emission (310 nm) from a stoichiometrichydrogen–air mixture during ignition by a repetitive nanosecond pulsed dis-charge, plotted together with OH number density predicted by the plasmachemistry model. T = 200 C, P = 104 torr, and ν = 40 kHz.

Fig. 5 shows a typical UV emission trace from a stoichio-metric hydrogen–air mixture at a pressure of P = 104 torr andan initial temperature of T0 = 200 C, excited by a repetitivenanosecond pulse discharge at ν = 40 kHz. The emission wasdetected using a PMT and a narrow-bandpass filter, as discussedin Section II. Before ignition occurs, the emission (primarilyN2 second positive system bands, v′ − v′′ = 1) decays to nearzero between the discharge pulses (see the inset in Fig. 5).Ignition time, 5.1 ms, was determined from the beginning ofthe OH(A2Σ −→ X2Π) emission “footprint,” which no longerdecays to zero between the discharge pulses (see Fig. 5),indicating a self-sustained combustion process. This was alsoverified by ICCD images of the flame, which confirm that theflame originates at the same time as the OH emission “foot-print” (see discussion of Fig. 7 hereinafter). The same approachhas been used in our previous work [24], where its validity wasdemonstrated by comparing the emission traces and flame im-ages with time-resolved temperature measurements by purelyrotational CARS. OH number density, predicted by the kineticmodel at these operating conditions, is also plotted in Fig. 5.In the calculations, ignition time was defined as the time whenOH number density begins a steep rise, from the intercept of thesteepest slope tangent line to the OH number density curve.

In Fig. 5, ignition time was measured while the pulseddischarge was operating throughout the entire ignition process.This was done to improve run-to-run reproducibility. In manycases, ignition would still occur, after some delay, even if thepulse burst is terminated before the onset of ignition footprint.Based on the results of the present experiments, maintainingthe pulsed discharge during the ignition process allows mea-suring the minimum ignition time at a given set of conditions,i.e., pressure, initial temperature, equivalence ratio, and pulserepetition rate. During the following discussion, the minimumignition time will be also expressed as the number of dischargepulses necessary to ignite the flow during continuous dischargeoperation.

Fig. 6 shows a series of broad-band ICCD camera imagestaken in stoichiometric hydrogen–air and hydrocarbon–air plas-

Fig. 6. ICCD camera images of filamentary and uniform nanosecond pulsedischarge plasmas in (top row) stoichiometric hydrogen–air, (middle row)ethylene–air, and (bottom row) methane–air mixtures at different initial tem-peratures and pressures. Emission intensity distributions are plotted above eachimage on the same scale.

mas at different pressures and initial temperatures. In all casesshown in Fig. 6, the discharge was operated at a pulse repetitionrate of 40 kHz, and the camera was gated during the dischargepulse #200 (for hydrogen–air and ethylene–air) and pulse #400(for methane–air). The pressure in the flow reactor was chosento maintain approximately the same initial number density atdifferent initial flow temperatures, T = 20 C, 100 C, and200 C. Emission intensity distributions along the centerlineof the discharge, extracted from the ICCD images, are plottedabove each image to illustrate nonuniform structures formed inthe plasma. All intensity distributions are plotted on the samescale. From the images in Fig. 6, it is quite clear that, in allthree fuel–air mixtures, initially at room temperature, multiplewell-defined constricted filaments are formed in the plasma. Itis also obvious that preheating the flow considerably improvesplasma uniformity in all three mixtures. In particular, preheat-ing the hydrogen–air mixture up to 100 C–200 C makes theplasma almost completely uniform in the entire field of view.For comparison, hydrocarbon–air plasmas at 100 C remainnonuniform and filamentary, although the filaments becomeless pronounced. Even at 200 C, higher emission intensity nearthe edges of the electrodes, at the left and the right boundary ofthe images, is apparent. Plasma images such as those shown inFig. 6 have been used to map plasma uniformity qualitatively,in a wide range of pressures and initial temperatures. Based onthese plasma uniformity tests, ignition time measurements werecarried out in hydrogen–air mixtures at T0 = 100 C − 200 Cand P = 50 − 150 torr. Methane–air and ethylene–air plasmaswere found to be too filamentary for quantitative ignition timemeasurements.

Fig. 7(a) shows a 310-nm UV emission trace froma hydrogen–air mixture at φ = 1.0, P = 104 torr, andT0 = 200 C, excited by a nanosecond pulse discharge atν = 40 kHz. The discharge was turned off after 197 pulses (att = 4.93 ms) to prevent strong plasma afterglow emission fromsaturating the flame images. It was verified that turning theplasma off immediately before ignition did not affect ignition

YIN et al.: IGNITION TIME MEASUREMENTS IN HYDROGEN–AIR PLASMAS 3275

Fig. 7. (a) UV emission trace (310 nm) and (b) ICCD camera images ofa nanosecond pulsed plasma and a flame in a stoichiometric hydrogen–airmixture (T = 200 C, P = 104 torr, and ν = 40 kHz). Plasma images:Camera gate of 2 µs. Flame images: Camera gate of 30 µs (see Fig. 4). Ignitiontime is 5.0 ms.

time. Fig. 7(b) shows collages of ICCD images of the plasma(camera gate 2 µs) and the flame (camera gate of 30 µs), withplasma images shown during individual discharge pulses andflame images shown after the pulse burst is over. It can beseen that the plasma remains diffuse and uniform during theentire burst, and the flame originates first near the edge ofthe discharge, then rapidly occupying the entire field of view(60 mm long) in less than 0.2 ms (see Fig. 7). The fact that theflame does not originate at a single point (or multiple isolatedpoints) in the discharge, as occurs in filamentary discharges[8], provides additional evidence of spatial uniformity of theplasma. Although the plasma and flame images shown in Fig. 7were taken during different runs, comparison of OH emissiontraces shows that the results are reproduced extremely wellrun-to-run, with ignition time varying by only about 0.1 ms(i.e., 2%).

Fig. 8 shows a 310-nm UV emission trace from anethylene–air mixture at φ = 1.0, P = 84 torr, and T0 = 200 C,excited by a nanosecond pulse discharge at ν = 40 kHz.Compared to the stoichiometric H2–air mixture (see Fig. 7),the plasma is not as uniform, with brighter emission regionsapparent near the electrode edges. Indeed, in this case, theignition begins in these “hot spots,” and flame propagates sym-metrically toward the center of the plasma over approximately2 ms, before filling the entire discharge region. This behav-ior is completely different from hydrogen–air ignition, whichoccurs nearly simultaneously in the entire plasma volume.However, preheating of the flow does significantly improve

Fig. 8. (a) UV emission trace (310 nm) and (b) ICCD camera images of ananosecond pulsed plasma and a flame in a stoichiometric ethylene–air mixture(T = 200 C, P = 84 torr, and ν = 40 kHz). Plasma images: Camera gate of2 µs. Flame images: Camera gate of 30 µs (see Fig. 4). Ignition time is 13 ms.

the ethylene–air plasma uniformity, compared to our previouswork in initially room-temperature mixtures [8], where ignitionoccurred in multiple “hot spots” in filamentary plasmas. Webelieve that more ethylene–air flow preheating, up to T ∼300 C, would further improve plasma uniformity and makepossible quantitative ignition measurements.

Fig. 9 plots the ignition time and the corresponding numberof discharge pulses to ignite the hydrogen–air mixture versuspressure (P = 50–150 torr) at two different pulse repetitionrates, ν = 40 and 20 kHz, and two different initial tempera-tures, T0 = 100 C and 200 C. In these measurements, thepulsed discharge was operating during the ignition process.As discussed earlier, in this regime, minimum ignition timeat a given pressure, initial temperature, equivalence ratio, andpulse repetition rate is determined. In the entire parameter rangein Fig. 9, the estimated flow residence time in the discharge,0.25–0.5 s, is significantly longer than the ignition delay time,3–30 ms. The lower bound of a pressure range tested in theseexperiments corresponds to the ignition threshold, and theupper limit corresponds to the conditions when the plasma isno longer uniform. From the results plotted in Fig. 9, it isapparent that the general trend is ignition time reduction as thepressure and the initial temperature increase. The same trendwas observed at ν = 30 and 10 kHz. No ignition was detectedat pulse repetition rates below ν = 10 kHz at T0 = 200 C andbelow ν = 15 kHz at T0 = 100 C.

3276 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011

Fig. 9. Number of discharge pulses to ignite a stoichiometric hydrogen–air mixture versus test section pressure, for different pulse repetition rates, (left) ν =40 kHz and (right) ν = 20 kHz.

Fig. 10. Number of discharge pulses to ignite a stoichiometric hydrogen–air mixture versus pulse repetition rate at different pressures and initial temperatures,(left) T0 = 100 C and (right) T0 = 200 C.

Fig. 9 also compares ignition time measurements with thepredictions of the kinetic model described in Section III. It canbe seen that the model correctly captures the trend of ignitiondelay time reduction as the temperature and the pressure areincreased. However, ignition delay predicted by the model atlow pressures and pulse repetition rates is significantly shortercompared to the experimental results, by up to 30% at ν =20 kHz (see Fig. 9). Also, the threshold pressure at whichignition no longer occurs is underestimated by the model by20%–30%. Finally, as the pressure is increased, ignition delaypredicted by the model nearly levels off, while experimentalignition delay continues to decrease gradually, as evident fromFig. 9. The agreement between the model and the data be-comes less close at low pulse repetition rates. All this suggeststhat, during burst mode operation, the coupled pulse energyper molecule, qpulse/N (see Section III), is not constant, aspredicted by the nanosecond pulse discharge model [26], butmay rather vary with temperature and pressure in a morecomplex way.

Fig. 10 plots the number of discharge pulses to ignite thehydrogen–air flow versus pulse repetition rate at different pres-

sures and two different initial temperatures (T0 = 100 C and200 C). During these measurements, the discharge was againmaintained during the entire ignition process. From Fig. 10, itcan be seen that, at a given pressure and initial temperature,there appears to be an optimum pulse repetition rate at whichthe number of pulses to ignite reaches minimum. The optimumpulse repetition rate decreases with increasing pressure. Thistrend becomes less noticeable at a higher initial temperature,T0 = 200 C, and higher pressures. Greater number of pulsesneeded to produce ignition at low pulse repetition rates canbe explained by heat transfer losses to the channel walls andradical species recombination on the walls, which may becomesignificant at these conditions. However, a similar trend ob-served at high pulse repetition rates is not fully understood andsuggests that, at these conditions, the pulse energy coupled tothe plasma, or the amount of radical species produced, maybe reduced. This may occur due to the high residual electrondensity remaining from the previous pulse. The nanosecondpulse discharge model of [26] predicts that, in this case, thecoupled pulse energy would drop considerably due to rapidshielding of the applied electric field in the initially ionized

YIN et al.: IGNITION TIME MEASUREMENTS IN HYDROGEN–AIR PLASMAS 3277

Fig. 11. Ignition time in hydrogen–air mixtures versus equivalence ratio at different pulse repetition rates and initial temperatures. (Left) T0 = 100 C andP = 84 torr. (Right) T0 = 200 C and P = 104 torr.

plasma. The other factor that may reduce the coupled pulseenergy at high pulse repetition rates is charge accumulation onthe dielectric surfaces, which would produce a similar plasmashielding effect. Since these effects have not been incorporatedin the present plasma chemistry model, the predicted ignitiondelay time is nearly independent of the pulse repetition rate.Further measurements of the coupled pulse energy versus pulsenumber in the burst are necessary to quantify these effects andto evaluate the rates of radical generation during individualdischarge pulses.

Fig. 11 plots ignition time versus equivalence ratio for twosets of conditions that have approximately the same initialnumber density, T0 = 100 C and P = 84 torr (left) and T0 =200 C and P = 104 torr (right). In Fig. 11, ignition time isplotted instead of the number of pulses necessary for ignition,to separate data sets obtained at different pulse repetition rates.The results of Fig. 11 show that the effect of the equivalenceratio on ignition time is strongest at low (near threshold) pulserepetition rates, ν = 10 kHz at T0 = 100 C (nearly 60% igni-tion delay variation with the equivalence ratio at φ = 0.5–1.2)and ν = 15 kHz at T0 = 200 C (nearly 40% variation). Athigher pulse repetition rates, the effect of equivalence ratio onignition delay becomes significantly weaker, only about 10% atν = 40 kHz. The modeling calculation results, also plotted inFig. 11, also predict very weak dependence of ignition delaytime on the equivalence ratio. However, the model does notreproduce the effect of ignition delay time increase at low pulserepetition rates, near ignition threshold.

Uncertainty in temperature dependence of the rates of plasmachemical processes is known to be one of limitations of low-temperature plasma-assisted combustion kinetic models. Anoverwhelming majority of these rates have been measured atroom temperature, and their scaling with temperature remainsuncertain. This includes quenching rate coefficients of excitedelectronic states of nitrogen (primarily by O2 and H2), whichmay well affect the rate of O and H atom generation in theplasma. To estimate the effect of temperature dependence ofthe rates of N2(C3Π, B3Π, a′1Σ, A3Σ) quenching by O2 andH2 (reactions P4–P7 and P9–P11 in Table I), we incorporated

the scaling of these rates with temperature proportional to thecollision frequency, i.e., ∼ (T/300)1/2 [35], into the model. In[35], incorporating such scaling for the N2(A3Σ) + O reactioninto the kinetic model improved the agreement with the ex-perimental Vegard–Kaplan band emission intensity distributionin upper atmosphere. Ignition time predicted by the modelincorporating this temperature scaling was then compared withthe baseline model predictions, using N ∗

2 quenching rates givenin Table I. The model incorporating quenching rates scaledwith temperature predicted ignition time only a few percentshorter compared to the baseline model. The weak effect ofN ∗

2 quenching rate temperature dependence on ignition timeis due to the fact that, at the present conditions, the yield ofradicals in the plasma (O and H atoms) is primarily limitedby the net rate of N ∗

2 production by electron impact during theburst, with discharge pulses generated every 25–100 µs (at ν =10 − 40 kHz), i.e., by the number of pulses. The characteristictime for N ∗

2 quenching between the pulses, with O and H atomformation, is much shorter, ∼ 0.1 − 1.0 µs. Thus, ignition timeat the present conditions is controlled by a relatively slow rate ofO and H accumulation during the burst, over ∼100–500 pulses,and is not very sensitive to the temperature dependence of N ∗

2

quenching rates.As discussed in Section II, in this paper, time-resolved tem-

perature in the plasma was determined from N2 second positivesystem emission spectra, N2(C3Π−→B3Π, v′=0−→v′′=0)band. As noted earlier, in all present temperature measure-ments, the emission signal was accumulated from eight con-secutive discharge pulses, by repeatedly gating the camera witheach gate width of 2 µs. At ν = 40 kHz, the pulses are 25 µsapart such that the spectra are accumulated over a 200-µsperiod. These spectra are taken at a relatively low spectralresolution to improve signal-to-noise for the short camera ex-posure time and to provide better time resolution. The relativeuncertainty of the temperature measurements is ±5% in airand ±15% in hydrogen–air mixtures. The initial temperaturein the plasma, inferred from the emission during the first eightdischarge pulses, T = 370 ± 50 K and T = 470 ± 50 K, wasfound to be consistent with the flow temperature without the

3278 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011

Fig. 12. Time-resolved temperature in air and hydrogen–air plasmas versus the number of discharge pulses in the burst. (Left) T0 = 100 C and P = 84 torr.(Right) T0 = 200 C and P = 84 torr.

plasma, measured by a thermocouple, within the uncertainty oftemperature inference from the spectra.

Fig. 12 plots time-resolved temperature in hydrogen–airplasma excited by a nanosecond pulse discharge burst, inferredfrom the N2 emission spectra. For these measurements, theemission was collected from a region in the center of thedischarge, extending from the top electrode to the bottomelectrode. Temperature is plotted for two sets of conditions withapproximately the same initial number density, T0 = 100 Cand P = 84 torr (left) and T0 = 200 C and P = 104 torr(right). Note that initial temperatures in the plasma are con-sistent with the thermocouple measurements taken without theplasma. As can be seen from Fig. 12, the air plasma temperatureincreases by approximately ∆T ≈ 200 K during the burst of400 pulses (10 ms long). In hydrogen–air, the temperature risebecomes more rapid, due to additional energy release in reac-tions of hydrogen with plasma-generated radicals, O, H, andOH. In this case, the temperature first increases gradually up toT ≈ 700 K, at which point there is a sudden (within ∼1 ms)well reproducible jump to nearly T ≈ 1600 K. Both thesetrends, i.e., gradual temperature rise at a rate faster than that inair and a subsequent temperature jump, are consistent with theresults of purely rotational CARS temperature measurementsin hydrogen–air mixtures, initially at room temperature [24].The timing of the temperature jump is consistent with ignitiontime measurements from time-resolved OH emission, shown inFigs. 9 and 11, confirming the ignition and combustion of asignificant fraction of the fuel.

To estimate the degree of temperature nonuniformity alongthe plasma, N2 emission spectra were also taken from an∼6-mm-wide region near the periphery of the discharge, i.e.,near the electrode edges. As can be seen from Fig. 12, thetemperature inferred from these spectra is very close to theone in the center of the discharge, except after ignition occurs.This provides additional evidence of plasma uniformity at thepresent conditions. Fig. 12 also plots the spatially averagedtemperatures in the discharge in air and in a stoichiometrichydrogen–air mixture, predicted by the kinetic model, whichare in good agreement with the experimental data. Note,however, that the model predicts somewhat lower peak

temperature during ignition. Finally, to illustrate the critical roleof nonthermal plasma chemistry in ignition kinetics, Fig. 12plots the average temperature in the hydrogen–air plasma pre-dicted by the kinetic model, with the dominant plasma chem-ical radical generation processes, listed in Table I, turned off,but keeping the energy loading by the discharge the same.It can be seen that, in this case, the temperature remainsvery close to the air plasma temperature, and ignition doesnot occur. This demonstrates that a more rapid temperaturerise in hydrogen–air plasma detected in the present experi-ments, which eventually results in ignition, is entirely due toenergy release in reactions of plasma-generated radicals withhydrogen.

Fig. 13 plots temperatures in air and hydrogen–air plasmas,initially at T0 = 100 C and P = 84 torr, versus the pulsenumber in the burst (rather than versus time), for different pulserepetition rates. It can be seen that, in air, the temperaturerise versus the number of pulses is nearly independent of thepulse repetition rate, suggesting that, at these conditions, theeffect of coupled pulse energy variation with pulse repetitionrate, as well as that of heat transfer losses on energy balance,is relatively minor. In hydrogen–air, the number of dischargepulses until the beginning of steep temperature rise, indicativeof ignition, is minimum at the pulse repetition rate of ν =20 kHz, rather than at the highest value tested, ν = 40 kHz[see Fig. 13(b)]. This result is consistent with Fig. 10, whichdemonstrates the existence of a minimum in the number ofpulses before ignition at intermediate pulse repetition rates. Onthe other hand, the rate of temperature rise before ignition isalmost the same for ν = 20 and 40 kHz, showing that, in fact,the net energy loading (i.e., coupled discharge energy minusheat transfer losses) in both cases is approximately the same. Asmaller number of pulses necessary for ignition at ν = 20 kHzsuggest that the concentration of radicals at this pulse repetitionrate is higher than that at nu = 40kHz, contrary to the kineticmodel prediction. Resolving this issue requires OH numberdensity measurements versus the number of pulses for thesetwo sets of conditions. At lower pulse repetition rates, ν = 10and 15 kHz, the rate of temperature increase is somewhat lower,which is most likely due to more significant heat transfer losses

YIN et al.: IGNITION TIME MEASUREMENTS IN HYDROGEN–AIR PLASMAS 3279

Fig. 13. Time-resolved temperature in air and stoichiometric hydrogen–air plasmas versus the number of discharge pulses in a burst, for different pulse repetitionrates. T0 = 100C.

to the walls on a longer time scale. At ν = 10 kHz, ignitiondoes not occur.

To determine if plasma-generated radicals may reduce ig-nition temperature, an additional series of experiments wasconducted, where the pulse burst was terminated before theonset of ignition. This was done to measure ignition delaytime, defined as a delay between the end of the pulse burstand the onset of ignition, as shown in Fig. 14(a), versustemperature. During these experiments, the temperature at theend of the burst, T f , was measured by taking N2 emissionspectra from the last eight pulses in the burst, as discussedearlier. Experimental ignition delay time is plotted versus T f

in Fig. 14(b) and (c), along with plasma-assisted and thermal(i.e., without plasma processes) ignition delays predicted bythe kinetic model. To provide adequate comparison betweenplasma-assisted and thermal ignition, thermal ignition delaytime was calculated assuming that the hydrogen–air flow isheated by a volumetric heat source matching the rate of tem-perature rise in the plasma before ignition (e.g., see Fig. 12),turned off after the desired temperature is reached. Note thatthermal ignition delay time calculated for a slower heating rateis shorter since, in this case, the fuel–air mixture is kept atelevated temperatures (prior to ignition) for a longer time. Heattransfer losses are incorporated both for plasma-assisted andthermal ignition processes. Ignition delay time predicted bythe model is plotted versus centerline and spatially averagedtemperatures achieved at the end of the discharge pulse burst,or at the end of the heating period, respectively.

The results show that, when the temperature measured at theend of the burst exceeds T f = 700 K, ignition occurs, with de-lay time of τ = 1.0–2.2 ms at ν = 40 kHz and τ = 0.5–1.0 msat ν = 20 kHz [see Fig. 14(b) and (c)]. For T f < 700 K, i.e.,with fewer pulses in the burst, no ignition was detected. Thedata obtained in this series of experiments are rather limitedbecause, for long burst durations (a few hundreds of pulses),the plasma generator software varies the pulse burst durationonly in 1-ms increments. At ν = 40 kHz, the model predicts thesame plasma-assisted ignition delay for the spatially averagedtemperature at the end of the burst of T f = 670 K–700 K, andat ν = 20 kHz, for T f = 710 K–740 K, it is in fairly good

agreement with the experimental data [see Fig. 14(b) and (c)].However, as discussed earlier, the model does not reproducethe effect of more rapid ignition at ν = 20 kHz compared toν = 40 kHz (see also Figs. 10 and 13).

In contrast with plasma-assisted ignition, with predictedignition threshold of T f ≈ 660 K (centerline temperature ofTf = 720 K) at ν = 40 kHz and of T f ≈ 680 K (Tf = 740 K)at ν = 20 kHz, thermal ignition does not occur below T f ≈800 K (Tf ≈ 900 K). Thermal ignition delay time of τ =1.0–2.0 ms is predicted to occur at ∆T f = 130 K–150 K higher(∆Tf = 170 K–180 K higher) compared to plasma-assistedignition at ν = 40 kHz [see Fig. 14(b)]. At ν = 20 kHz (τ =0.5–1.0 ms), this difference is reduced to ∆T f = 80 K–100 K[∆Tf = 100 K–120 K, see Fig. 14(c)]. Smaller differencebetween plasma-assisted and thermal ignition temperatures (forthe same ignition delay) predicted by the model at ν = 20 kHzis expected since, at lower pulse repetition rates, the modelpredicts lower number densities of radicals generated in theplasma (O, H, and OH).

Note that the predicted ignition threshold temperature re-duction (up to ∆Tf = 180 K for plasma-assisted ignition atν = 40 kHz) cannot be explained simply by enthalpy storagein the radical species, O, H, and OH (which may result in ad-ditional heating during their recombination). Kinetic modelingshows that, at ν = 40 kHz and T f = 700 K, recombination ofthe radicals generated by the discharge during the pulse burst(i.e., without chain reactions of fuel oxidation) would resultin a temperature rise of only about 10 K, without producingignition. This demonstrates that the main effect of the plasma onignition temperature reduction (or on ignition delay reductionat the same temperature) is primarily due to chain reactioninitiated by the radicals generated by the plasma, rather thandue to heating of the flow.

The results of Fig. 14 show reasonably good agree-ment between the experimental ignition delay time and theplasma-assisted ignition delay time predicted by the model.However, the uncertainty of time-resolved temperature infer-ence from fairly low resolution N2 emission spectra in thepresent experiments is significant and comparable to the pre-dicted effect of plasma on ignition threshold temperature [see

3280 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 12, DECEMBER 2011

Fig. 14. (a) Definition of ignition delay time. (b and c) Experimental ignition delay time versus plasma temperature at the end of the pulse burst, compared withthermal ignition delay time versus temperature at the end of the heating period. Stoichiometric hydrogen–air mixtures.

Fig. 14(b) and (c)]. Additional spatially resolved temperatureand radical species number density measurements at the endof the discharge pulse burst are needed to quantify the effectof plasma-generated radical kinetics on ignition temperaturereduction.

V. SUMMARY

This paper discusses ignition delay time and temperaturemeasurements in spatially uniform nonequilibrium plasmas inpreheated hydrogen–air mixtures. For this, a premixed fuel–airflow is preheated in a tube furnace to temperatures belowautoignition temperature and excited by a repetitive nanosecondpulse discharge in a plane-to-plane geometry. The experimentalapparatus has ample optical access and lends itself for plasmaand flame imaging, emission spectroscopy, and excited radicalspecies concentration measurements using optical diagnostics,such as laser-induced fluorescence. In this paper, ICCD imagingof the plasma and the flame demonstrated that mild preheatingof the fuel–air flow, up to T0 = 100 C − 200 C, greatlyimproved plasma stability and precluded filament formation.

At the initial temperatures of T0 = 100 C − 200 C,hydrogen–air plasmas remain stable and uniform up to at least

P = 150 torr. Ignition of hydrogen–air mixtures by a repeti-tive nanosecond pulse discharge occurs in the entire plasmavolume, nearly simultaneously. Ignition time is measured ina spatially uniform repetitive nanosecond pulse discharge inpreheated hydrogen–air mixtures, at initial temperatures of T =100 C − 200 C, pressures of P = 50–150 torr, equivalenceratios of φ = 0.5–1.2, and discharge pulse repetition rates ofν = 10–40 kHz. As expected, ignition delay time has a ten-dency to decrease as the initial temperature and the pressureof the fuel–air mixture are increased. At high pulse repetitionrates, ν = 20–40 kHz, ignition delay time is nearly independentof the equivalence ratio. At low pulse repetition rates, nearignition threshold (ν = 10–15 kHz), ignition delay in fuel-reach mixtures increases considerably. The number of pulsesnecessary for ignition exhibits a minimum at a certain optimumpulse repetition rate, which shifts toward lower values at higherinitial temperatures and pressures.

Compared to hydrogen–air plasmas, ethylene–air andmethane plasmas are significantly less uniform, even when pre-heated up to T = 200 C. At T = 200 C, ethylene–air plasmabecomes diffuse and nearly uniform, except for two regionsnear the electrode edges. Ethylene–air flame imaging has shownthat, unlike in hydrogen–air, ignition in ethylene–air occurs first

YIN et al.: IGNITION TIME MEASUREMENTS IN HYDROGEN–AIR PLASMAS 3281

in these two regions, with the flame propagating symmetri-cally toward the center of the plasma. Since ethylene–air andmethane–air plasmas exhibit fairly well-defined filamentarystructure in the entire temperature range tested (room temper-ature to T = 200 C), ignition time in these mixtures has notbeen measured.

The results of ignition time measurements in hydrogen–airmixtures are compared with predictions of the hydrogen–airplasma chemistry model, with the coupled pulse energy pre-dicted by the analytic nanosecond pulse discharge model. Mea-surements of the coupled pulse energy in room-temperature airare in good agreement with the nanosecond pulse dischargemodel predictions. The plasma chemistry model correctly re-produces the main trends observed in the experiments, i.e.,ignition delay time reduction as the initial temperature andpressure are increased, weak dependence of the number ofpulses necessary for ignition on pulse repetition rate, and weakdependence of ignition delay on the equivalence ratio. However,quantitative agreement between the model predictions and theexperiment becomes less close at low pressures and pulserepetition rates, i.e., near ignition threshold. Also, the modeldoes not reproduce the existence of an optimum pulse repetitionrate.

Time-resolved N2(C3Π −→ B3Π, v′ = 0 −→ v′′ = 0) emis-sion spectroscopy is used to measure the temperature inair and in hydrogen–air plasmas. The results show that, inhydrogen–air, the temperature rise is more rapid than thatin air, due to additional energy release in reactions of hy-drogen with plasma-generated radicals. Ignition begins at thehydrogen–air flow temperature of T ≈ 700 K and results ina rapid temperature rise up to T ≈ 1600 K (within ∼1 ms).Temperature measured near the electrode edges is essentiallythe same as in the center of the discharge, within experimentaluncertainty. This provides additional evidence that the plasmais uniform throughout the discharge region. The critical roleof nonthermal plasma chemistry in ignition kinetics was il-lustrated in modeling calculations, by turning off dominantplasma chemical radical generation processes, while keepingthe energy loading by the discharge the same. In this case, thepredicted hydrogen–air plasma temperature remains very closeto the air plasma temperature, and ignition does not occur. Thisdemonstrates that, at the present conditions, ignition is drivenby additional energy release in reactions of plasma-generatedradicals with hydrogen.

To determine if plasma-generated radicals may reduce igni-tion temperature, discharge pulse burst was terminated beforethe onset of ignition, to measure ignition delay time afterthe burst versus plasma temperature at the end of the burst,T f . For T f = 700 ± 100 K, ignition delay time is τ = 1.0–2.2 ms at ν = 40 kHz and τ = 0.5–1.0 ms at ν = 20 kHz, inreasonably good agreement with kinetic model predictions. Themodel also predicts significant ignition threshold temperaturereduction for plasma-assisted ignition at the present conditions,compared to thermal ignition, up to ∆Tf = 180 K, primarilydue to chain reactions of radicals (O, H, and OH) generated inthe plasma. Due to uncertainty of temperature inference fromN2 emission spectra, spatially resolved temperature and radicalconcentration measurements at the end of the discharge pulse

burst are needed to quantify the effect of plasma-generatedradical kinetics on ignition temperature reduction.

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Zhiyao Yin received the B.S. degree in naval archi-tecture from Huazhong University of Science andTechnology, China, in 2008. In 2008, he enrolledin a B.S.-Ph.D. program in Mechanical Engineer-ing at The Ohio State University, Columbus. He iscurrently working toward the Ph.D. degree in theNonequilibrium Thermodynamics Laboratories, De-partment of Mechanical and Aerospace Engineering,The Ohio State University.

His research is focused on studies of the kineticsof nanosecond pulse plasmas, as well as plasma-

assisted fuel oxidation, ignition, and combustion, using laser diagnostics.

Keisuke Takashima (Udagawa, original familyname) received the B.S. degree in mechanical en-gineering, the M.S. degree in energy sciences, andthe Ph.D. degree in energy sciences from TokyoInstitute of Technology, in 2005, 2006, and 2009,respectively.

He is currently a Postdoctoral Researcher with theNonequilibrium Thermodynamics Laboratories, De-partment of Mechanical and Aerospace Engineering,The Ohio State University, Columbus. His researchinterests include the kinetics of nanosecond pulse

discharges, MHD, high-speed flow control, optical diagnostics, and develop-ment of high-voltage nanosecond pulse generators operating at high pulserepetition rates.

Igor V. Adamovich received the M.S. degreein aerothermodynamics from Moscow Institute ofPhysics and Technology, Russia, in 1987, and thePh.D. degree in chemical physics from The OhioState University, Columbus, in 1993.

He is currently a Professor with the Nonequilib-rium Thermodynamics Laboratories, Department ofMechanical and Aerospace Engineering, The OhioState University. His research interests are in plasma-assisted combustion, high-speed flow control byplasmas, nonequilibrium hypersonic flows, genera-

tion of stable high-pressure nonequilibrium plasmas, and kinetics of gases andplasmas at extreme thermodynamic disequilibrium.