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Nitric oxide PLIF measurement in a point-to-plane pulsed discharge in vitiated air of a
propane/air flame

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2014 Plasma Sources Sci. Technol. 23 065005(http://iopscience.iop.org/0963-0252/23/6/065005)
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Plasma Sources Science and Technology
Plasma Sources Sci. Technol. 23 (2014) 065005 (11pp)

doi:10.1088/0963-0252/23/6/065005

Nitric oxide PLIF measurement in apoint-to-plane pulsed discharge in vitiated
air of a propane/air flame
J B Schmidt1 , N Jiang1 and B N Ganguly2
1
2

Spectral Energies, LLC, Dayton OH 45433, USA
Air Force Research Laboratory, WPAFB OH 45433, USA

E-mail: JSchmidt@SpectralEnergies.com
Received 20 February 2014, revised 23 June 2014
Accepted for publication 15 July 2014
Published 18 August 2014
Abstract

The effect of a point-to-plane pulsed discharge on the vitiated downstream of a propane/air
flame has been investigated by phase-locked NO planar laser-induced-fluorescence (PLIF)
measurements. Phase-locked NO PLIF measurements with the variation of pulsed plasmaenergy, equivalence ratio and applied voltage rise time have been performed. Fast rise time
(25 ns) and slower rise time (150 ns) high-voltage pulsers are used to produce NO radical
densities greater than the ambient flame-produced NO radicals in lean, balanced and richpremixed flames. The pulsed plasma produced excess NO radical densities were found to
decay to 50% level with time constants greater than 250 µs in the burnt gas regions with gas
temperatures greater than 1000 K. The super-equilibrium NO populations were dependent onenergy deposited and overall equivalence ratio, but independent of voltage pulse rise time forsimilar energy deposition per pulse. Due to long NO radical density decay lifetimes,
super-equilibrium NO populations are convected away from production regions with theambient flow and observed in downstream exhaust gas regions.
Keywords: Optical plasma diagnostics, Glow discharges, Plasma reactions(Some figures may appear in colour only in the online journal)

the base of the premixed flame [6, 16, 17]. At this injection
point, the temperature of the fuel and oxidizer streams are
relatively low and have not reached the crossover temperature
where combustion reactions typically occur [22]. It has beenshown that when these reactive species are injected in theselower-temperature, upstream locations [8, 17], mole fractions
over 0.002 can have a strong impact on flame kinetics. In thisnear-burner surface plasma injection configuration, a portionof the discharge volume can lie in lower temperature, lower
gas conductivity regions. This reduction in conductivity can
result in a smaller amount of current passed through this region
for the same potential drop and discharge gap distance. For
these reasons, others have modified their geometry to havethe anode in the downstream gas and strike the discharge
downward passing near the high temperature flame reaction
zone [1, 2]. The Mini-PAC (plasma-assisted combustor) isa good example of this configuration [3, 4], however thisburner configuration uses the anode as a flame stabilizer.

1. Introduction

The use of plasma discharges to augment flame kinetics andenhance flame stability is an area of current research interest[1–21]. The potential impact of this study includes expanding
stability limits of complex burning configurations or ultra-lean
combustors, and expanding the hydrocarbon fuel base. A
number of studies performed in the last decade have shownthat different plasma geometries such as the corona discharge,
dielectric barrier, and short repetitively pulsed discharges
can affect NO (nitric oxide) production as well as otherspecies and improve overall combustion stability. In premixed
combustion, plasmas can produce reactive species from boththe fuel and oxidizer stream in flame areas that otherwise havelower levels of reactive species in the low-temperature pre-heatregion of a premixed, hydrocarbon flame.
Much work has also been done to determine the impactof the plasma when these reactive species are injected near0963-0252/14/065005+11$33.00

1© 2014 IOP Publishing Ltd Printed in the UK

Plasma Sources Sci. Technol. 23 (2014) 065005

J B Schmidt et al

Our experimental configuration specifically locates the anodeoutside of the flame reaction zone so that it does not act as aflame holder.
Nitric oxide is formed in non-combustion related
mechanisms through non-equilibrium plasma interaction.
Chemical reactions in air plasmas are initiated by electron
impact on the main air constituents; N2 and O2 . Whilethe electron impact neutral dissociation of N2 requires ahigher minimum energy than the dissociation of O2 , both
dissociative processes producing reactive atomic radicals areimportant for the initiation of chemical processes. Shown
below in equations (1)–(5) are some of the important reactions
and associated rate constants [20, 21, 24] involving N and O
atoms and excited state N2 , where N2 * is any electronically
excited state, including, but not limited to N2 (A). It is
understood that the pulsed plasma is also capable of producing
many of the reactants responsible for NO generation listed
in equations (1)–(5) such as N2 (A3 u+ ) [24], N [24], O andOH [25].
N + O + M ↔ NO + M kf = 2.7 × 10−13
N2 A3  +u + NO2 ↔ N2 + NO + O kf = 1.3 × 10−11

temperature, the formation of prompt NO increases withincreasing flame temperature. Formation of NO throughthe N2 O mechanism is shown in equations (15)–(17) and is
especially important in lean equivalence ratios ( = 0.80).
Since this sequence requires the O atom and the three-bodyrecombination reaction in equation (11), the formation of NO
is favored with increasing air pressure.
−315

N2 + O ↔ NO + N

kf = 2 × 1014 e( RT )(6)

O2 + N ↔ NO + O−26
kf = 6.4 × 109 e( RT )(7)

N + OH ↔ NO + H(1)

kf = 3.8 × 1013

(8)

CH + N2 ↔ HCN + N(9)

C + N2 ↔ CN + N(10)

HCN + O ↔ NCO + H(11)

NCO + H ↔ NH + CO

(12)

NH + H ↔ H2 + N(13)

N + OH ↔ NO + H

(14)

(2)

N2 + O + M ↔ N2 O + M(15)+ O2 ↔ NO + NO

(3)

N2 O + H ↔ NO + NH

(16)

N2 (v > 2) + O2 ↔ NO + NO

(4)

N2 O + O ↔ NO + NO.

(17)

N2 ∗ +O ↔ NO + N.

(5)



N2 A

3

u+





When a pulsed plasma is applied to a combustion event, twodistinct regimes are formed to help determine the temporal
evolution of the produced NO populations; the initial
production dominated by plasma kinetics and the resulting
decay of the produced NO dominated by combustion
kinetics. This is supported by Uddi et al [21] where
initial NO concentrations from a nanosecond duration
pulse in low pressure fuel/air flows rise much faster thanZel’dovich mechanisms predict, but slower than reactions
with electronically excited nitrogen atoms and molecules anddecays are dominated by reverse Zel’dovich mechanisms andNO2 reactions with ozone. These results further concluded thatlong lifetime N2 and O2 metastables dominated NO production
at low pressure. The current study is focused on examining
these distinct regimes, the initial level of production and thetemporal decay of NO in a premixed, hydrocarbon flame todetermine the impact that short-duration, repetitively pulsed,
point-to-plane nanosecond plasma discharges can have on
premixed hydrocarbon flame kinetics at atmospheric pressure.

Recent work has shown that even higher, electronically excitedstates of nitrogen (N2 C and B states) may have a significant
role in generation of NO in plasma augmented combustion[27]. While many other reaction channels may be modified by
non-thermal plasmas some of these pathways are omitted here
due to smaller reaction rate constants or reactant populations
being sufficiently small in the applied temperature range.
These involve reaction pathways involving NO2 , NO3 , N2 O,
etc. All of these reactions and rates will be highly dependent
on parameters such as temperature, reduced electric field andtotal energy deposited.
Nitric oxide can also be formed from atmosphericnitrogen gas through three hydrocarbon-combustion-related
mechanisms with no fuel bound nitrogen components; thermalNO, prompt NO and the N2 O mechanisms. The thermal
NO mechanism consists of three reactions shown below inequations (6)–(8) with corresponding rate coefficients [20].
This formation mechanism tends to favor high temperature
gas and NO formation is considered to be unimportant at
temperatures below 1800 K [22]. Prompt NO formation or the
Fenimore mechanism arises from substantial NO productionin the cooler, upstream portion of the premixed hydrocarbon
flame that the thermal mechanism cannot predict and is shownin equations (9) and (10), assuming CH has already been
formed. For equivalence ratios less than ∼1.2 conversionof hydrogen cyanide (HCN) to NO follows the chain shownin equations (11)–(14). For equivalence ratios richer than
1.2 the combustion chemistry becomes more complex andother pathways to form NO become available [23, 24].
Since concentrations of O and CH increase with increasing

2. Experimental

The experimental setup used is shown in figure 1(a), whichclosely resembles the setup used in previous work [25, 26].
The setup consists of a premixed C3 H8 /air laminar-flow burner
producing an evenly distributed conical flame. The burner used
is a commercial atomic absorption type (Perkin-Elmer) with amodified cylindrical (44 millimeter (mm) diameter × 50 mm)
stainless-steel head. Pressed into the stainless-steel headis a bronze insert with machined orifices consisting of fiveconcentric rings of close spaced 1 mm diameter holes. Thisgeometry is shown in figure 1(b). The propane and air are
2

Plasma Sources Sci. Technol. 23 (2014) 065005

J B Schmidt et al

Figure 1. (a) Electrical and flow schematic of experimental setup. (b) Top view of burner geometry. Dimensions are in mm.

mixed in-line for 1 m before entering the burner section andthen pass through three increasingly smaller gauge wire gauze
to homogenize the mixture. Flame equivalence ratios [22]and flow speeds are accurately measured with a pair of digital
thermal mass-flow controllers for both fuel and air; accurate
to within 0.8% of the reading. The incoming flow speed
for the conditions tested is just over 2 m per second [26].
The burner is directly grounded and an inductive Pearson
probe, labeled as A in figure 1(a), was used for current.
A 1 mm diameter molybdenum wire is suspended above theburner surface to serve as an anode for the application of thepositive voltage. The thin anode tip temperature is nearly inequilibrium with the gas temperature and the soot build-up
is minimal even for fuel rich flames. The pulsed voltage isvaried in magnitude (up to +8 kV) and applied using a currentlimiting ballast resistor at 1 kHz repetition rates. As withprevious studies [25], two different high-voltage switches areused that achieve the same applied voltage levels, but withdifferent voltage pulse rise times. The fast rise-time switch
has voltage rise times near 25 nanoseconds (ns) and uses
a stacked metal–oxide–semiconductor field-effect transistor(MOSFET) configuration. This is an order of magnitude
faster than the slow rise-time switch which has voltage risetimes near 150 ns and uses a transformer-coupled insulatedgate bipolar transistor (IGBT). The current and voltage traces
were recorded for each test conditions. Figure 2 shows typicalvoltage and current levels for the fast rise-time switch.
Laser-induced fluorescence (LIF) theory and its applications to combustion are described in greater detail elsewhere[28–30]. NO is one of the more widely used species for planar
laser-induced-fluorescence (PLIF) measurements in combustion flows. The NO molecule is chosen for this fluorescencestudy due to its importance in hydrocarbon/ air combustion
chemistry as well as in air plasma chemistry and its advantages of being relatively stable, naturally present in flames,
and its well-understood spectroscopic properties [30, 31].

Figure 2. Typical voltage and current for tested flow conditions
using the fast rise-time switch applied to a  = 1.0 flame. +6 kV
applied for 1 microsecond results in 6 A of current over 10 mm gap.
Total power deposited is 15 W.

In this work we use a high-voltage discharge in a premixed,hydrocarbon flame to augment the density of flame-produced
NO through normally non-existent non-equilibrium air plasma
chemistry channels. Figure 3 shows a schematic of the laser
diagnostic system used for the NO PLIF measurements in
an atmospheric pressure burner. NO fluorescence images
were acquired with a gated ICCD by excitingtransitions
within the A 2  + ← X 2  v  = 0, v  = 0 γ -band, near
226 nanometers (nm). In this band, transitions from a large
range of rotational levels are easily accessible with pulsed dyelasers. The (0,0) band also has relatively strong absorption,
thereby providing high fluorescence signals, while allowing
for efficient rejection of laser scattering through non-resonant
detection of broadband fluorescence (primarily v  = 0 ).
A dye laser (Continuum ND6000) using a combination of
Sulforhodamine 640, Rhodamine 640 Perchlorate and Cresyl
Violet 670 dye in a methanol solvent is pumped with the second
harmonic of a 10 Hz Nd : YAG laser (Spectra-Physics PRO250)
3

Plasma Sources Sci. Technol. 23 (2014) 065005

J B Schmidt et al

Figure 3. Laser diagnostic setup up used for NO PLIF. A 2  + ← X 2  (v  = 0, v  = 0) γ -band, near 226 nm, is excited through frequency
mixing in a BBO crystal and imaged by an ICCD with filters used to discriminate against scatter and unwanted fluorescence signal.

of the fluorescence decay. A Stanford Research Systems
Digital Delay Generator (DG645) is used to control timing
between the laser, camera systems and the applied voltage
pulses, enabling synchronization between the laser pulses,
camera gates, and applied voltage.
In order to determine absolute NO concentrations, acalibration is performed by flowing a 1% NO in helium mixture
through the above-mentioned burner system. Concentrations
were varied through dilution with helium and total flow
rates were controlled with the same mass-flow controllers
mentioned previously.
This configuration allows forconcentrations up to 10 000 parts per million (ppm) to
be calibrated and the same collection system to be used,
simplifying the calibration process. Since NO populationsin flame exhaust gas conditions are calibrated with known
populations of NO at room temperature, corrections must
be made to the above calibration to account for the thermal
redistribution of rotational spectra.
Collisional quenching must be accounted for as
it is a major source of reduction to quantum yield.
Assuming that collisional quenching of the upper state isindependent of rotational quantum number, equation (18)
provides the total fluorescence signal as a function of
excitation laser frequency, ν, for a broadband detector,
where Copt describes the total optical collection system,
Ep is the total laser energy per pulse, A and Bi are
Einstein coefficients for spontaneous emission and stimulated
absorption, fBi (T ) is the equilibrium Boltzmann fraction
based on rotational/translational temperature, Q(χp , P , T )is the A-state quenching rate, gi (n, χp , P , T ) is the line shapefunction, and nNO = χNO P /kB T is the NO number density

to produce 624 nm light. This beam is then mixed with the third
harmonic from the original Spectra-Physics Nd : YAG laser
inside a BBO crystal through sum frequency generation. In
this configuration, over 1 mJ per pulse laser energy is producedwith a measured spectral linewidth of 0.25 cm−1 centered near
226 nm. With this bandwidth, multiple primary rotationaltransitions are excited, including Q1 14.5, 15.5, 16.5, P1 23.5,
R1 8.5, 9.5, R2 15.5, as well as several secondary transitions.
A Pellin–Broca prism, located near the BBO crystal, is used
to spectrally separate the residual dye laser and frequencytripled YAG output from the desired UV light. The UV beam isconverted into a sheet by using a f = −300 mm cylindrical andf = 1000 mm spherical lens as a single-direction telescope.
This optical setup produces a UV sheet that is 0.7 mm thickand 10 mm tall within the test section. Because it is only asingle direction telescope the UV sheet has a slowly diverging
height. However, the rate of divergence of the beam in thisdirection is assumed to have negligible effects on the energy
density of the sheet within the laser waist due to the long focal
length of the spherical lens used.
The fluorescence signal is imaged with an ICCD camera(512 × 512 array) equipped with an UV f/4.5 lens. A 250 nm
long-pass filter from Asahi and color short-pass filter from
Oriel were used to block the laser scattering and unwanted
fluorescence signal. The total region imaged is 17 mm by
17 mm providing a spatial resolution of 30.11 pixel per mm.
The intensifier gate widths are set to provide 200 ns of uniform
gating for the NO measurements, fully collecting the entire
NO fluorescence decay while eliminating flame emission. The
laser pulses are carefully positioned temporally to occur justafter the intensifier gates are fully on, to ensure uniform gating
4

Plasma Sources Sci. Technol. 23 (2014) 065005

J B Schmidt et alwith χNO being the NO mole fraction. Additionally, if it isassumed that the temperature does not increase due to thenanosecond discharge, which has been shown to be the case[21], then a simplified equation relating the number density ofNO during the discharge (nNO,dis ) to the collected calibrationlevels (nNO,cal ) is given in equation (19) and depends only on
fluorescence signals (Sf ) and quantum yields.
SLIF (v) = ×i

Simulated NO absorption spectra generated fromLIFBASE [40] is integrated over the excitation laser bandwidth
for both temperature ranges. While significant vibrationaltemperature changes can occur within the discharge volume,
this is not corrected for as the actual vibrational temperatureis unknown during the discharge and will be highly spatiallydependent. In this effort, the accuracy of the calibrated
measurement is limited by the assumption of thermalequilibrium population distribution. For this measurement,
the error is approximately 9.4% and is largely dominated bypulse-to-pulse fluctuations of the laser system and instability
in the mass-flow controllers at lower flow speeds.

COpt EP Aυ  =0→υ  =2 AA 
+
Q
χ
,
P
,
T
p

υ
υ =0→υ =2

P
fBi (T )Bi gi (ν, χp , P , T ) × χNO
kB T

P Sf,disnNO,dis = χNO,calkB T cal Sf,cal





υ  Aυ =0→υ =2 + Q χp , P , T dis


× 


υ  Aυ =0→υ =2 + Q χp , P , T cali fBi (T )Bi gi (ν, χp , P , T ) cal× i fBi (T ) Bi gi (ν, χp , P , T ) dis(18)

3. Results and discussion

The digital delay generator permits temporal scanning of
the laser diagnostic system with respect to the applied
voltage pulses. This feature is utilized to determine NO
population versus time delay from voltage onset for a given
data condition. With this PLIF collection system, flameproduced NO populations are comparable to the detection
noise threshold. For this reason a flame background image
is collected in addition to the camera noise background image.
However, plasma-produced NO populations are significantly
higher and excited state NO emission is within the passband of the collection system. For each data point collected,
both a plasma/flame only image and a plasma/flame/PLIF
image are collected. Each raw image set is backgroundsubtracted to eliminate flame background and camera noise
signals. Plasma/flame emission levels are then subtracted fromthe plasma/flame/PLIF images to leave only NO PLIF signal.
Datasets presented below show both the corrected PLIF signal
as well as the plasma emission for comparison.
Within a processed image, specific regions are integrated
to determine total PLIF signal. Figure 4 shows the spatiallayout of the imaged area with respect to the flame/anodeand the three 0.5 mm × 0.5 mm regions of interest integrated.
The first point (A in figure 4) is located directly under theanode and is where the NO PLIF emission as well as theNO plasma emission is the strongest. The second region ofinterest (B in figure 4) is below region of interest A whilestill within the discharge volume. This middle-of-the-plasmavolume location shows both strong PLIF and plasma emission,
but is far enough away from the anode tip to allow the emission
to be more spatially uniform and absent from strong gradients.
Area B is displayed below, and most of the studies discussed
below originate from area B unless otherwise noted. This area
is selected because it is in the plasma discharge volume and it
best shows the effect of the plasma on the flame, but is awayfrom the tip of the anode where the strongest, non-uniform
emissions are observed from plasma/PLIF. The third region ofinterest (C in figure 4) is located in the exhaust gas region and issufficiently far away from the plasma volume so that no plasma
emission is observed from the initial discharge. This location is
selected to observe convection/diffusion effects downstream.
Figure 5 below shows typical corrected single-shot NO
PLIF measurements with a false color scale collected at various
time delays after the pulsed discharge. The anode is located at(19)

where the total quenching rate, shown in equation (20), is given
by a sum over all quenching species in the volume at thoseconditions.
nk Q k(20)
Q=
kThis situation is complicated by the temperature gradientsthat are present in the ‘discharge’ condition. This was
originally simplified by defining three zones; the pre-heatregion upstream of the reaction zone, the thin reaction zone,
and the exhaust region downstream of the reaction zone.
However, this system is greatly simplified as the majority ofthe observed effects occur in the exhaust region downstream
with no observed effects occurring in the pre-heat region.
From this, only two regions were considered; the reaction
zone and downstream exhaust products. In the reaction
zone, both mole fraction and temperature depend only onequivalence ratio. In the exhaust products region, mole
fraction and temperature are dependent on both equivalence
ratio and distance from the reaction zone. Mole fractions
and a general temperature is determined and assigned toeach zone. These are determined by a global simulation
conducted in CHEMKIN [32] utilizing the hydrocarbon
oxidation mechanism from Lawrence Livermore National
Laboratory based on a known equivalence ratio for aburner-stabilized, premixed, propane/air flame. These major
quenching species include Ar, CH4 , C2 H4 , C2 H6 , CO, CO2 ,He, H, H2 , H2 O, N2 , NO, N2 O, NO2 , O2 and OH. Spontaneousradiative decay rates and collisional quenching rates werecollected from the literature at a range of temperatures for
similar atmospheric pressure flame conditions [21, 33–39]
and equilibrium Boltzmann fraction was determined fromLIFBASE [40]. The collected fluorescence images were thenassigned regions based on flame structure and corrected basedon the appropriate parameters for that region. Interpolation
was used in the exhaust gas region to obtain a linearly varying
parameter set used for correction.
5

Plasma Sources Sci. Technol. 23 (2014) 065005

J B Schmidt et al

Figure 4. Spatial layout of anode/flame configuration with sample set of data. UV beam is passed under the anode and above the burner faceto eliminate scatter. Thermal NO populations are augmented by plasma-produced NO (visible in the sample data image). Three specificlocations (under anode (A), middle discharge (B), and exhaust gas (C)) within the image sets are used and integrated to find the total signal.
Imaged region measures 20 mm by 20 mm.

the top of the image with the discharge striking downward
to the grounded burner surface. Initially observed in thisimage sequence is the distribution of the NO fluorescencesignal. Much larger signals are observed up to 2 mm fromthe anode compared to what is observed in the middle of thedischarge. This variation is likely related to local temperaturedriven density gradients in the plasma volume. As the gas
density decreases with increased axial distance from the burnerhead pre-heat zone, the reduced electric field is increased. This
thermally driven increase in reduced electric field strength near
the anode allows for stronger electron impact excitation rates.
To verify this, three important plasma species were selected(N2 , OH and O) and their emission was collected at various
times after the discharge. These single-shot images are shownin the lower portion of figure 5. These emissions were collectedin the same geometry as the NO PLIF images of the flame andappear not to be affected by the thermal fields. Initially, allthree species show emission within the discharge volume that is
relatively constant aside from the very near (less than 0.7 mm)
electrode locations. These emission levels also show that the
lifetime of the plasma discharge and the generated excitedspecies are quite short with most of the collected emission
decaying within 500 ns. It is suggested that while the excitedstates distribution is not affected, these thermal effects havea very large effect on the produced NO populations. The
significantly increased temperature near the anode causes NO
production to be greatly augmented and populations observed
here are much larger due to a significantly higher reduced
electric field in this lower gas density area compared to the
pre-heat zone near the burner. The second item to be notedin the NO PLIF images is the convective transport of the NO
populations that occurs. This is more evident in the larger
time delays from the discharge. Between 25 and 100 µs thenear anode populations are shown to spread downstream withthe overall gas flow. While peak signal has decreased, theintegrated signal from the larger area has only decayed byapproximately 15%. The much larger 500 µs delay time shows

Figure 5. Typical single-shot, two-dimensional NO PLIF images atvarious delays from the discharge are shown at the top. Below are
single-shot images of emission from various important combustion
species produced by the discharge. The short lifetime provides anupper-bound of the lifetime of the discharge.

the final stages of the convective transport as it merges withthe hot exhaust over 5 mm downstream.
Figure 6 shows NO radical density decay times versus
delay from the onset of the voltage pulse for different applied
voltages from the fast rise-time voltage pulse for a  = 0.80
premixed, propane/air flame. The region of integrated PLIF
signal displayed here from area B is shown in figure 4. The
dashed line denoted by (P) in figure 6 show plasma emission
whereas the solid lines show signal from NO PLIF denoted by(F). It is important to note that the plasma emission levels may
not be an accurate measure of local NO concentration as opticalemission relies greatly on spectral resolution, reduced electricfield dependent electron impact excitation rates of NO excited6Plasma Sources Sci. Technol. 23 (2014) 065005

J B Schmidt et al

speed of approximately 2 m s−1 , a volume of gas can moveapproximately 2 mm between 1 kHz discharges. If this volume
of gas were to move entirely axially, multiple discharges could
affect the same volume of gas. However, when these reactants
undergo the thermal expansion across the reaction zone forthis constant pressure combustion process, they accelerate andtravel normal to the reaction zone. This acceleration to over
5 m s−1 [26] after thermal expansion of combustion combinedwith the narrow discharge volume occurring near the reaction
zone, it is concluded that only a single discharge process occurs
for a given volume of gas. However, diffusion occurring within
this 1 mm timescale could cause the next volume of gas within
the discharge to have artificially higher concentrations of NO
due to pulse-to-pulse cumulative build-up.
The initial NO production during the plasma pulse cancome from a variety of plasma-initiated sources. However,based on the previously listed pathways, reactant populations
and related rates of reaction it is suggested that vitiated airplasma chemistry is largely responsible for the large initial
production of NO in the downstream region of the flame. While
it is possible that combustion-related chemical reactions could
be modified or augmented due to the additional energy of thepulsed plasma, these reactions are generally well-balanced in
the high temperature region of the flame where the discharge
occurs. In addition, the reaction rates with the newly formed,
super-equilibrium products from discharge are typically larger
than the combustion-based reaction rates; especially those
involving vibrationally excited nitrogen or water vapor [27].
From this diagnostic alone, the results of the plasma-modified
combustion reactions cannot be neglected, but are suggested
to have minimal impact on local gas temperatures. The slow
decay of the produced NO is another matter. The consumptionof the super-equilibrium NO should more heavily rely on
the reverse reactions of the combustion-related production
mechanisms previously listed, specifically those from thethermal NO and N2 O mechanism, since there is no additionalenergy actively being deposited into the flame zone from thepulsed discharge. These reactions include equations (5)–(7),(13), (15) and (16). However, a relatively slow decay times
longer than 700 µs suggest these reverse reactions are notsignificantly balancing the super-equilibrium NO produced by
the pulsed discharge at a rate that would be useful for many
combustion applications.
The zero dimensional CHEMKIN [32] code mentioned
previously can be used to predict the equilibrium NO
populations of a flame condition based on a known equivalence
ratio for a burner-stabilized, premixed, propane/air flame. This
code suggests that the NO mole fraction just past the reaction
zone in the hot portions of the flame should be between
10−6 and 10−5 . Again, this thermal-equilibrium level cannot
be measured with the current diagnostic system. Longer
decay times than expected in thermal-equilibrium conditions
combined with the minimum detectable signal levels and the
CHEMKIN code calculation results suggest NO populations
far above thermal equilibrium are augmented and maintainedby the kHz repetition rate point-to-plane discharge in flamevitiated air.
To better understand these chemical kinetics modifications, a similar test is performed by applying +8 kV for plasma

Figure 6. Decay rates of super-equilibrium NO produced in the
middle of the plasma volume as a function of delay from the onset
of applied voltage for three voltage levels for a  = 0.80,
propane/air flame. Solid lines represent corrected PLIF signals (F)and dashed lines show plasma emission only (P).

states and collisional quenching rates. Nonetheless, the PLIF
signal has been corrected to remove this plasma emission aspreviously mentioned. As applied voltage levels are increased
the initial production of NO in the middle of the discharge
volume is also increased. This trend is observed in both NO
signals; from plasma only and from NO PLIF. It is important
to note that while plasma produced NO is distinguishable
with this collection system, NO PLIF signal with the plasma
on is sufficiently stronger compared to a condition with theplasma turned off. The condition with the highest applied
voltage show peak NO concentrations of 2500 ppm. This is
an increase of nearly two orders of magnitude over normal
combustion produced NO and is similar to results observed ina corona-type discharge [20]. From both the plasma produced
NO emission and also NO PLIF signals, it is possible to lookat the decay of the super-equilibrium NO populations as thetiming of the PLIF data acquisition is changed with the delay
generator. The highly repeatable nature of this discharge isrelied upon for consistency of the sequential measurements tobuild a stitched-temporal dataset. From this temporal evolutionof NO signal it is possible to look at the decay rates of boththe plasma and NO PLIF signals. The plasma emission showsa rapid decay of signal to the detection threshold level. Thissignal level appears to depend on the applied voltage levelwith a higher applied voltage level resulting in a higher signal
level suggesting the long decay times of the plasma-produced
NO cause a gradual build-up depending on the initial plasmabased production. The long decay times coupled with the local
temperatures in excess of 1000 K suggest that diffusion of the
produced NO could cause some of the decay behavior and must
be considered. The +8 kV and +6 kV NO PLIF signals show
very similar decay rates, especially after 250 µs, even thougheach started at a different initial super-equilibrium population
providing a stronger case for NO radical density decay via
diffusion.
A simple estimate of flow residence time within thevolume of the discharge is performed to ensure that a volume
of gas is only excited once. Based on the cold reactants flow
7

Plasma Sources Sci. Technol. 23 (2014) 065005

J B Schmidt et al

Figure 7. Decay rates of super-equilibrium NO produced in the
middle of the plasma volume as a function of delay from the onset
of applied voltage for three equivalence ratios. Solid lines represent
corrected PLIF signals (F) and dashed lines show plasma emission
only (P).

Figure 8. Decay rates of super-equilibrium NO produced in the
middle of the plasma volume as a function of delay from the onset
of applied shown for two different pulsers (25 and 150 ns voltage
rise times) in a  = 0.80 flame. Solid lines represent corrected
PLIF signals (F) and dashed lines show plasma emission only (P).

generation in flames over a range of equivalence ratios. Overall
equivalence ratios are varied from 0.8 to 1.2 and both plasma
and NO PLIF emissions are collected for various delay times
from the onset of the applied voltage to show how small deviations from unity can affect chemistry modification. With thesesmall equivalence ratio changes, the total amount of electrical
power that can be coupled to the flame is changed. This is likely
due to a change in local temperature and a change in gas composition. The equivalence ratio of  = 1 flame shows the highest
power deposited at just over 15 W for a 1 kHz pulse repetition
rate. The lean flame shows a 15% reduction from this level andthe rich flame shows 23% reduction. Shown in figure 7 are initial build up and decay times of super-equilibrium populationsin the middle of the plasma discharge recorded by delaying
the collection system from the onset of the applied voltage. In
all cases the NO PLIF emission is stronger than plasma emission which are still higher than thermal-equilibrium flame NO
signal levels. The lean flame condition shows the greatest
augmentation of both plasma and NO PLIF emission whereas
minimal build-up or decays are shown for both the balanced
and rich equivalence ratios since NO production is strongly
dependent on the level of excess air for both equilibrium andnon-equilibrium production/recombination mechanisms [24].
Results of the balanced and rich equivalence ratios suggestthat even with sufficient levels of electrical power deposited in
the flame, the flame chemistry is balanced such that NO radical
density decay is unchanged.
A study of super-equilibrium NO production is performed
to test the effectiveness of each pulser. Previous studies
between these two types of pulsers on OH super-equilibrium
production [25] has suggested that the faster rise-time voltage
pulse is more efficient in super-equilibrium species production
as measured by level of production per deposited plasmaenergy level. Utilizing the two optimum production conditions
shown previously, the two pulsers are used to apply +8 kV to apremixed, propane/air flame with overall equivalence ratio of
0.8. Shown in figure 8 are the production and decays of NO
for the two pulsers as a function of delay from the onset ofthe applied voltage. Unlike that previously seen with superequilibrium OH, initial NO production levels, as obtained fromNO PLIF, are very similar with the same energy deposited per
pulse with each pulser. The change in voltage pulse rise time
and therefore more rapid heating of the gas in the discharge
volume seems to show minimal effects. In addition, decay
rates of NO PLIF signal for both pulser configurations arequite similar. The largest difference between the two pulsers
appears to be in the plasma emission. Initially, the faster risetime pulser shows a greater level of super-equilibrium NO
production. As dictated by the pass-band of the collection
filters, this increase in signal could come from an excited stateof NO (NO*) and could result due to the increased heating of
the gas in the plasma discharge [41] and/or by direct electron
impact production of NO*.
Lastly, other regions of interest are spatially averaged
to observe the effects listed previously in figure 4. Effects
at region of interest A in figure 4 are quite similar to
those observed in region B and discussed above. The maindifferences being that initial production levels are as much as
25% higher than those of region B, but decay rates showedvery similar trends as compared against time delay after onset
of applied voltage, applied voltage level, equivalence ratio,
and pulser configuration. Area C in figure 4 shows different
behavior than A or B. This is due to the fact that area C
is located out of the plasma discharge volume and does notdirectly see the effects from the energy deposited or the rapid
thermal heating. Figure 9 shows the super-equilibrium NO
emission levels for three different equivalence ratios. Minimal
production is observed for all three equivalence ratios byeither the plasma or the PLIF emission. Again, this can be
attributed to the distance between the plasma volume and thesampling region. However, all three equivalence ratios show
modification of the emission levels delayed in time by 500 µs
from the onset of the applied voltage. Based on the local
flow speeds of over 5 m s−1 [26] and the distance between
the plasma and the region of interest, this agrees with initialsuper-equilibrium NO being produced in the plasma discharge
8Plasma Sources Sci. Technol. 23 (2014) 065005

J B Schmidt et al

Figure 9. Decay rates of super-equilibrium NO produced in theexhaust gas as a function of delay from the onset of applied voltage
for three equivalence ratios. Solid lines represent corrected PLIF
signals (F) and dashed lines show plasma emission only (P).

Figure 10. Point-based Raman scattering for temperature results.
Region of interrogation is located 2 mm below the anode in the
discharge volume.

are not the suspected cause for this significant increase in NO
production or slowed decay times.

volume (area B in figure 4) and being convected downstream
with the local flow to area C. The lean equivalence ratio shows
the strongest emission modification later in time while the richequivalence ratio showing the weakest modification. This also
agrees with the previous findings.
Point-based temperature measurements by spontaneous
Raman scattering were conducted previously with this burner
setup. This measurement, described in detail elsewhere [25],determines the ratio of total Stokes- and anti-Stokes-shiftedsignal in the first excited vibrational level to find temperatureassuming a known number density and constant pressure ofnitrogen. The collection volume that is approximately 0.1 mm3
is located 2 mm below the anode in the discharge volume
coinciding with the location of peak NO fluorescence signal.
It was suspected that a significant increase in temperaturein this location could cause a significant increase in NO
production as well as slower decay times of the produced
species. The results, displayed below in figure 10, show
the temperatures recorded are significantly reduced from anadiabatic temperature expected at these equivalence ratios.
This likely occurs for two reasons. The most significant reason
is that this is a burner-stabilized flame which relies on a large
thermal mass to help pre-heat the incoming premixed reactants
to increase flame stability limits. The flame rejects large
amounts of energy through standard transport processes to theburner head, especially at lower flow rates, which removes
heat from the flame reducing the expected peak temperatures.
Additionally, the point-based collection location is just outside
the flame reaction zone, roughly halfway from the burner
surface to the flame tip. Peak temperatures for a premixed
conical flame occur closest to the tip of the flame and at thislocation the products have begun to form and cool. In addition
to the reduced temperatures observed, the temporal evolutionof temperature in this location is very stable. The only temporal
behavior appearing in the plot occurs a few µs after the onsetof discharge. This is most likely caused by a shockwave that is
generated by the fast voltage rise time of the discharge causing
small flame perturbations [25]. Outside of these behaviors, ourresults suggest that thermal effects induced from the plasma

4. Conclusion

In conclusion, we have shown that with approximately 15 mJof electrical energy deposited in 2 cm3 with a fast voltage
rise-time (25 ns) pulser, we were able to produce nearly 100
times more than the thermally produced NO concentration
in a premixed, propane/air flame. The largest production
region is observed within 3 mm of the anode, but theplasma location is too far downstream from the pre-heatzone to have significant effect on the flame kinetics. The
temporally stitched NO density measurements show a 50%
density decay time lasting over 100 µs in downstream regionsin the flame where gas temperatures are known to haveexceeded the crossover temperature. Previously conducted
point-based Raman temperature measurements suggest that
increased thermal contributions from the pulsed discharge are
not responsible for the increased NO number densities andreduced decay times observed. We conclude NO populations
produced by the pulsed discharge are not consumed throughreverse reaction mechanisms at rates fast enough to preventa build-up of super-equilibrium NO. These significant NO
radical populations last in the downstream region for over
700 µs and maybe subject to diffusion which would artificially
increase decay rates. The larger portion of the discharge
volume is in the post-reaction-zone of the flame and the largest
NO production occurs downstream in a hot, low density region
where the reduced electric field is higher than the colder preheat zone. This suggests the present plasma configuration
producing large amounts of super-equilibrium NO is not the
optimum for plasma-assisted combustion enhancement where
hot burnt gases already exist. This configuration may be morefavored for plasma-initiated ignition of fuel at reduced gas
pressure since a large volume ignition kernel can be produced
sufficiently far away from a surface.
9Plasma Sources Sci. Technol. 23 (2014) 065005

J B Schmidt et al

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Acknowledgments

Funding for this research was provided by the Air Force Officeof Scientific Research (Dr. Jason Marshall, Program Manager)
and by the Air Force Research Laboratory under Contract No.
FA8650-10-C-2008.
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