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Combustion and Flame 161 (2014) 2038–2053
Contents lists available at ScienceDirect
Combustion and Flame
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e
Experiments and modeling of propane combustion with vitiation
Ponnuthurai Gokulakrishnan a,⇑, Casey C. Fuller a, Michael S. Klassen a, Richard G. Joklik a, Yash N. Kochar b,
Sarah N. Vaden b, Timothy C. Lieuwen b, Jerry M. Seitzman b
a
b
Combustion Science & Engineering, Inc., 8940 Old Annapolis Road, Suite L, Columbia, MD 21045, USA
School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA
a r t i c l e
i n f o
Article history:
Received 21 September 2013
Received in revised form 27 November 2013
Accepted 17 January 2014
Available online 14 February 2014
Keywords:
Propane oxidation
Reaction optimization
CO2 dilution
Vitiation
Exhaust gas recirculation
NO-sensitized oxidation
a b s t r a c t
The chemical species composition of a vitiated oxidizer stream can significantly affect the combustion
processes that occur in many propulsion and power generation systems. Experiments were performed
to investigate the chemical kinetic effects of vitiation on ignition and flame propagation of hydrocarbon
fuels using propane. Atmospheric-pressure flow reactor experiments were performed to investigate the
effect of NOx on propane ignition delay time at varying O2 levels (14–21 mol%) and varying equivalence
ratios (0.5–1.5) with reactor temperatures of 875 K and 917 K. Laminar flame speed measurements were
obtained using a Bunsen burner facility to investigate the effect of CO2 dilution on flame propagation at
an inlet temperature of 650 K. Experimental and modeling results show that small amounts of NO can
significantly reduce the ignition delay time of propane in the low- and intermediate-temperature
regimes. For example, 755 ppmv NOx in the vitiated stream reduced the ignition delay time of a stoichiometric propane/air mixture by 75% at 875 K. Chemical kinetic modeling shows that H-atom abstraction
reaction of the fuel molecule by NO2 plays a critical role in promoting ignition in conjunction with reactions between NO and less reactive radicals such as HO2 and CH3O2 at low and intermediate temperatures. Experimental results show that the presence of 10 mol% CO2 in the vitiated air reduces the peak
laminar flame speed by up to a factor of two. Chemical kinetic effects of CO2 contribute to the reduction
in flame speed by suppressing the formation of OH radicals in addition to the lower flame temperature
caused by dilution. Overall, the detailed chemical kinetic mechanism developed in the current work predicts the chemical kinetic effects of vitiated species, namely NOx and CO2, on propane combustion reasonably well. Moreover, the reaction kinetic scheme also predicts the negative temperature coefficient (NTC)
behavior of propane during low-temperature oxidation.
Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction
Vitiated air is used in many industrial applications including gas
turbines [1,2], industrial furnaces [3], compression ignition engines
[4] and combustors with flameless oxidation [5] to reduce emissions [6,7] as well as to improve flame stability [8]. Vitiated air
refers to an inlet oxidizer stream at high preheat conditions with
oxygen levels less than that of normal air. Vitiation is generally
achieved through exhaust gas recirculation (EGR) in which fresh
air is mixed with the exhaust stream and then introduced into
the inlet of a combustor. Therefore, vitiated air generally consists
of significant concentrations of combustion exhaust species such
as CO2, H2O, CO, NOx and unburned hydrocarbons in addition to
the O2 and N2 found in normal air.
⇑ Corresponding author. Fax: +1 410 884 3267.
E-mail address: gokul@csefire.com (P. Gokulakrishnan).
The presence of combustion products in the inlet oxidizer
stream has been found to influence the induction chemistry during hydrocarbon fuel oxidation as well as flame propagation in
combustors [9–11]. Previous three-level fractional factorial design
of experiments [12] in tubular flow reactors with methane [9]
and JP-8 [13] using vitiated air found that the effect of NO is
statistically more significant than that of CO2 and H2O on the
hydrocarbon induction chemistry during oxidation in the low
and intermediate temperature regimes. The presence of a small
amount of NO converts relatively less reactive radicals such as
HO2 and CH3O2 to more reactive radicals, thus enhancing the
oxidation of hydrocarbon fuels in the low- and intermediatetemperature regimes [14]. In general, the dominance of different
chain-branching reactions on hydrocarbon oxidation determines
the low-, intermediate- and high-temperature oxidation regimes.
A detailed discussion on this subject can be found elsewhere
[15–18].
http://dx.doi.org/10.1016/j.combustflame.2014.01.024
0010-2180/Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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In contrast to the induction chemistry of the ignition process,
flame propagation is largely controlled by high-temperature chemistry, which is dominated by the chain branching reaction between
H and O2. Therefore, the presence of NOx in the oxidizer stream will
have little impact on the combustion radical pool in the high-temperature oxidation regime. However, the presence of significant
amounts of CO2, H2O and N2 (and therefore lower O2 levels) in
the oxidizer stream will generally reduce the flame speed. The
dilution effect reduces the flame temperature and changes the
thermal and mass diffusive properties compared to standard air.
In addition, the presence of CO2 and H2O may influence the chemical kinetics, thereby further reducing the flame speed. Therefore,
the objectives of the current work are to investigate the chemical
kinetics effect of NOx on the autoignition and the chemical kinetics
effects of CO2 dilution on laminar flame speed at conditions relevant to vitiated combustion using propane as a prototypical hydrocarbon fuel.
Several experimental studies on the effect of NOx addition have
been reported in the literature with natural gas fuel components
such as methane [9,19–24], ethane [19,20,25,26], propane
[19,20,27], butane [19,28,29] and CH4/C2H6 mixtures [30–32].
Table 1 lists the experimental conditions used in the previous
works reported in the literature to investigate the effect of NOx
on the oxidation of natural gas fuel components. The experimental
conditions listed in Table 1 show that most of the previous works
were performed using heavily diluted fuel/oxidizer mixtures (i.e.,
either very high dilution ratios or very low equivalence ratios). In
the current work, flow reactor experiments were performed to
study the effect of NOx on propane ignition delay time at typical
vitiated combustion conditions found in practical devices (i.e.,
equivalence ratios: 0.5–1.5 and O2 levels: 14–21 mol%).
Laminar flame speeds have been previously measured for methane and other natural gas components at a range of equivalence
ratios and pressures, and with various diluents [33–40]. However,
most of these studies were limited to room temperature reactants
and standard oxygen levels (21 mol%). An early work on the
flame speed measurements for atmospheric pressure propane-air
at preheat temperatures from 302 K to 616 K was reported by Dugger [41] using the Bunsen flame surface area method coupled with
Schlieren photography. More recently, measurements by the stagnation flame method [39] at preheat temperatures up to 650 K
with N2 up to 41% dilution ratio have been reported. In the current
work, the effect of CO2 dilution on propane flame speed is investigated using the Bunsen burner flame surface area method at 650 K
preheat temperature with 10 mol% CO2, a level present in typical
vitiated air.
A detailed chemical kinetic mechanism was developed for propane combustion and validated at various conditions against the
experimental data obtained in the current work as well as data
from the literature. The kinetic mechanism includes detailed
low-temperature chemistry as well as vitiated chemistry to model
the influence of NOx on propane oxidation. The current reaction
mechanism is used to perform a kinetic analysis in order to understand the chemical kinetic implications of vitiated conditions on
hydrocarbon ignition and flame propagation by NOx and CO2
respectively. It is noteworthy that the current experimental and
modeling work elucidates the role of H-atom abstraction from
the fuel molecule by NO2 in promoting the oxidation of hydrocarbon fuels under vitiated conditions.
2. Effect of NOX on natural gas oxidation
There have been several studies reported in the literature to
investigate the sensitized oxidation of natural gas fuel components
in the presence of NOx using flow reactor experiments [9,19–31].
As shown in Table 1, most of these experiments were performed
to measure the reactor exit species concentrations or time-history
species profiles using heavily diluted fuel/oxidizer mixtures at lowand intermediate-temperature regimes. Dilution of fuel/oxidizer
mixtures reduces the overall heat release and hence decreases
the experimental uncertainty caused by heat release on species
measurements. In addition, trace amounts of fuel were used in
Table 1
Experimental conditions used in the previous works on the effect of NOx on gaseous hydrocarbon fuel oxidation at low-and intermediate temperatures compared with current
work.
a
b
c
Fuel type
Exp. system
data typea
Pressure
(atm)
Temp (K)
O2
(mole%)
NOx
(ppmv)
Equivalance
ratiob
Minimum dilution
ratioc
CH4
ST – IDT
TFR – species
TFR – species
TFR – species
TFR – species
JSR – Species
TFR – species
1.8
1
1
1
1
1,10
20, 50, 100
773–973
773–973
600–1100
750–1250
775–1100
800–1150
600–900
19.2
1–14
21
2.67–3.69
3.15–10.8
1–5
0.28–4.5
2000–38,000
25–200
20
186–211
0–350
200
179–214
0.5
Trace
Trace
Trace
0.24–0.84
0.1–1.0
0.04–1.15
C2H6
TFR – species
TFR – species
JSR – species
1
1
1
650–1300
600–1100
800–1200
4
21
1.54
485
20
750
Trace
Trace
1.0
4.25
0.00
12.57
C3H8
TFR – species
TFR – species
1
1
600–1100
773–1073
21
16
20
67
Trace
Trace
0.00
0.31
TFR – IDT
1
850–950
14–21
50–750
0.5–1.5
C4H10
TFR – species
TFR – species
1
1
600–1100
600–720
21
21
20
0.01–200
Trace
Trace
97%CH4 3%C2H6
TFR – species
10
800–1060
1.2
4–421
1.0
91% CH4/9%C2H6
JSR – species
10
800–1160
0.36–1.78
200
0.3–1.5
90% CH4/10%C2H6
ST – species
46, 49
1070–1495
0.16–0.19
44
0.5
0.00
0.50
0.00
4.68
0.86
3.19
3.45
Refs.
Slack and Grillo [42]
Bromly et al. [21]
Hori et al. [20]
Bendtsen et al. [22]
Gokulakrishnan [9]
Dagaut et al. [24]
Rasmussen et al. [23]
Label
#
1
2
3
4
5
6
7
Hjuler et al. [25]
Hori et al. [20]
Dagaut et al. [26]
8
9
10
11
12
0.00
Hori et al. [20]
Nelson and Haynes
[27]
Current work
0.00
0.00
Hori et al. [19,29]
Bromly et al. [28]
14
15
16.39
Amano and Dryer
[30]
16
10.77
Dagaut et al. [31]
17
109.47
Sivaramakrishnan
et al. [32]
18
TFR – tubular flow reactor; JSR – jet stirred reactor; ST – shock tube; IDT – ignition delay time.
Fuel-oxidizer equivalence ratio less than 0.1 is denoted as trace.
Dilution ratio = (0.21/XO2) 1; XO2 – mole fraction of O2 in the oxidizer stream; e.g., for standard air dilution ratio = 0.
13
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some experiments to simulate the conditions relevant to typical
combustion exhaust systems in order to investigate the oxidation
of unburned hydrocarbons promoted by NOx in the exhaust gas
[19–22,25,28,29].
Figure 1 depicts the carbon to NOx molar ratio used in various
experiments as a function of the maximum total carbon in the inlet
fuel/oxidizer mixtures. It can be noted that most of the previous
experiments were performed with less than 1% total carbon in
the inlet mixture except for works reported by Slack and Grillo
[42] and Gokulakrishnan [9]. For relative comparison of total carbon values shown in Fig. 1, a stoichiometric mixture of CH4/air
would consist of an equivalent 8 mol% carbon and 16 mol% H2,
16 mol% O2 and 60 mol% N2. Also, the total carbon to NOx molar ratio is less than 25 in most experiments, except for that of Amano
and Dryer [30], Bromly et al. [28] and Gokulakrishnan [9].
The experimental and modeling results for conditions (listed in
Table 1) with less than 1% total carbon show that the chemical
kinetics effect in promoting fuel oxidation stems from the interaction between NOx and less reactive radical species such as HO2, CH3
and CH3O2, irrespective of the fuel type. The presence of NO or NO2
promotes the oxidation of the hydrocarbons, while the hydrocarbon radicals such as CH3O2 and CH3 convert NO into NO2, and
NO2 into NO, respectively, at low and intermediate temperatures.
This phenomenon is generally referred to as mutually-sensitized
oxidation.
During the NO-sensitized oxidation of hydrocarbon fuels, NO
promotes the oxidation by converting less reactive hydroperoxy
radicals (HO2) created from a chain–terminating reaction (R1) into
a chain–propagating hydroxy radical (OH) by the following catalytic cycle [14]:
H þ O2 þ M HO2 þ M
ðR1Þ
HO2 þ NO NO2 þ OH
ðR2Þ
NO2 þ H NO þ OH
ðR3Þ
H2 þ OH H2 O þ H
ðR4Þ
In addition, CH3O2 (formed via reaction (R5)) and CH3 are converted
to more reactive CH3O radicals by reactions (R6) and (R7).
10000
Total Carbon|max/NOX|ave
13 (current)
15
1000
16
5
100
18
10
11
3
1
0.002
14
2
12 17
4
6
7
10
1
9
8
0.02
0.2
2
20
Total Carbon|max [mole %]
Fig. 1. Comparison of the experimental conditions of various literature studies
listed in Table 1 with current work in terms of carbon-NOx molar ratio. Key: data
labels indicate row number listed in the far-right column in Table 1.
CH3 þ O2 þ M CH3 O2 þ M
ðR5Þ
CH3 O2 þ NO CH3 O þ NO2
ðR6Þ
CH3 þ NO2 CH3 O þ NO
ðR7Þ
The presence of NO makes the chain–terminating reaction (R5) into
a chain–propagating reaction (R6) by converting the CH3O2 radical
into an active methoxy radical (CH3O). The methyl radical (CH3) is
less reactive at low temperatures, whereas it is an important precursor for the formation of CH3O radicals at high temperatures.
The formation of NO2 through reactions (R2) and (R6) helps the oxidation of hydrocarbon fuels by converting the CH3 radical into CH3O
even at lower temperatures via reaction (R7). Therefore, NO promotes oxidation at low and intermediate temperatures by converting HO2 into OH, while NO2 readily reacts with CH3 radicals to form
CH3O radicals via fuel independent reaction pathways.
Experiments are performed in the current work to investigate
the effect of fuel dependent reaction pathways, in particular the
interaction between the fuel molecule and NO2, in promoting the
oxidation of propane. As noted in Fig. 1, most of the previous
experimental works were performed with less than 1% total carbon
to investigate the effect of NOx on sensitized oxidation of hydrocarbon fuels. The purpose of the current work is to investigate the
interaction between NOx and hydrocarbon fuels at concentrations
(4–13 mol% total carbon) relevant to practical devices under vitiated conditions. A detailed kinetic modeling analysis is also performed to understand the impact of H-atom abstraction by NO2
in promoting hydrocarbon fuel oxidation. The H-atom abstraction
by NO2 becomes an important reaction pathway for promoting
ignition at conditions with higher fuel concentrations than most
of the experimental conditions reported in the literature.
3. Experimental set-up
3.1. Ignition delay time test facility
The experimental facility used for ignition delay time measurements is a tubular reactor designed to operate with gaseous or
pre-vaporized liquid fuels at atmospheric and sub-atmospheric
pressures between 700 K and 1200 K under vitiated conditions.
Vitiated air is generated by metering and mixing standard air with
various laboratory grade gases of interest such as CO, CO2, N2, O2
and NO. The schematic of the flow reactor set-up is shown in
Fig. 2. The flow reactor consists of an annular premixing section
and a test section. Figure 3 shows the cross-sectional view of the
premixing section which contains a swirler to achieve near-perfect
mixing of the fuel and oxidizer streams by creating high-velocity
turbulent eddies.
The downstream portion of the premixing section consists of a
gradually expanding duct, which acts as a diffuser to connect to the
test section. The test section is a long ceramic alumina tube with an
internal diameter of 5 cm. The test section, totaling 5.3 m in length,
is heated and insulated to maintain uniform reactor temperature.
The first meter of the test section is enclosed in a ceramic tube furnace with three independently controlled electric zone heaters
that can maintain uniform gas temperatures up to 1200 K. Downstream of the tube furnace, the test section is heat-traced with ten
independently controlled zones that can maintain the internal gas
temperature up to 950 K. The mixing section and diffuser are also
heat traced and controlled such that the system can operate with
uniform temperature profiles between 700 K and 950 K.
The total mass flow rate of the oxidizer stream was maintained
at 2.0 g/s for each test condition, and the fuel flow was varied
depending on the desired equivalence ratio. In the current set of
experiments, the oxidizer stream consists of O2 and N2 with a
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2041
Fig. 2. Schematics of the flow reactor facility.
Fig. 3. Diagram of the fuel injection, mixing and diffuser sections of the flow reactor apparatus shown in Fig. 2. Dimensions are in millimeters.
varying amount of NOx up to 780 ppmv. Gas sample analyses of the
inlet vitiated oxidizer stream showed that the fraction of NO2 in
NOx increases with increasing NO addition up to 17% at 755 ppmv
NOx. The measured gas composition of the inlet vitiated stream is
provided in the supplementary data. The oxidizer stream is preheated prior to mixing with the fuel stream in order to maintain
the same inlet temperature of the fuel/oxidizer mixture as in the
test section. Gaseous propane is mixed with nitrogen in order to
minimize thermal decomposition during preheating to 650 K prior
to mixing with the oxidizer stream. The fuel stream is radially injected into the oxidizer stream prior to passing through a swirled,
annular mixing section and then enters the diffuser section as
shown in Fig. 3.
One of the difficulties of flow reactor ignition delay time measurements is the temporal monitoring of the premixed fuel/oxidizer mixture entering the flow reactor to determine the time of
the injection. A laser absorption technique [43] to monitor the fuel
injection system was installed at the inlet of the diffuser section to
monitor the time of injection of the fuel into the flow reactor. An
infrared beam at 3.39 lm supplied by a HeNe laser is directed
across a diameter of the flow channel through quartz optical ports
in the diffuser (Fig. 3). When the laser beam passes through a medium that contains a hydrocarbon fuel, the signal is attenuated due
to the absorption of the laser by the C–H bonds in the fuel. This
attenuation denotes the time of fuel entering the diffuser. The start
time for the autoignition is determined when laser signal attenuates to 50% of the difference between the maximum and minimum
values.
The time of ignition is measured by the detection of the chemiluminescence signal of OH emission that occurs during ignition. A
photomultiplier tube (PMT) is located at the end of the flow reactor
with direct line of sight down the axis of the test section through a
quartz window. The PMT is equipped with a 310 nm narrow band
pass filter (±5 nm) to observe OH chemiluminescence emission.
During an ignition event, the light emitted from the OH is registered by the PMT whose signal is recorded by the data acquisition
system in conjunction with the laser signals as well as the fuel
injection solenoid valve opening time. The initial time of light
emission by the OH radical is designated as the time of ignition.
The signal traces from a typical test case are shown in Fig. 4. The
ignition delay time is designated as the difference between the
time recorded to mark 50% attenuation of the laser signal obtained
at the diffuser inlet and the time of PMT excitation due to the
chemiluminescent radical emission of OH as shown in Fig. 4.
3.2. Laminar flame speed test facility
To assess the effect of the vitiated oxidizer on high-temperature
combustion phenomena, flame speed measurements were performed using a modified Bunsen flame technique [44]. A schematic
of the experimental facility used to produce axisymmetric jet
flames is shown in Fig. 5. The facility consists of a plenum fitted
with a contoured nozzle to produce laminar jets at high flow rates
Fig. 4. An example of solenoid, PMT and laser beam traces obtained in an ignition
test in the flow reactor. Measured ignition delay time is denoted by sig.
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(jet exit velocity to flame speed ratio 4.5). The reactants are preheated to the desired temperature in the plenum using electric
resistance heaters. The burner assembly is placed in a nitrogen
ventilated pressure chamber capable of withstanding 30 atm and
500 K.
The reactant gas flow rates are measured individually using a
bank of rotameters and the gases are allowed sufficient residence
time to mix thoroughly before passing through the nozzle. The
rotameters are calibrated at the supply pressure for the desired
flow rate range using a wet test meter for high flow rates and a
bubble flow meter for low flow rates. Overall, the flow rate calibration has an accuracy of better than 1%. The final temperature of the
reactants is monitored using a K-type thermocouple placed 25 mm
upstream of the exit. The flow entering the plenum passes through
a layer of ball bearings which breaks up the incoming jet. A ceramic
flow straightener (2 mm cell size) downstream removes any large
scale structures before the flow enters the converging nozzle. The
exit diameter of the contoured burner is 9 mm with an area-based
contraction ratio of 72. This ensures that the exit velocity profile is
uniform and the flow is laminar even at high Reynolds number
(Re 2000). A near-stoichiometric methane–air pilot flame is used
to anchor the flame at high flow rates. A sintered plate surrounding
the nozzle exit is used to produce a flat pilot flame.
Optical access for flame imaging is provided by three 5.1 cm
diameter quartz windows. Broadband chemiluminescence images
of the Bunsen premixed flame are acquired with a 16-bit ICCD
camera equipped with an f/4.5, 105 mm UV Nikkor lens. The camera is sensitive in the visible and ultraviolet range and capable of
capturing CH, OH and CO2 chemiluminescence from the flame.
The resolution of the imaging system ranged from 30–50 lm/pixel. Sample flame images for various propane mixtures are shown in
Fig. 6 for three experimental conditions with different dilution
ratios: (a) air with no dilution, (b) air with 40% N2 dilution; (c)
air with 10% CO2 dilution. The image exposure times are a few milliseconds, and reveal the flames to be essentially axisymmetric and
stable.
The chemiluminescence images are analyzed to determine the
reaction zone location with a gradient-based edge detection algorithm. The algorithm finds the inner edge of the reaction zone for
both the left and right half images, from which the reaction zone
area can be calculated. The reaction zone areas from 50 realizations
are then averaged to determine the flame area (Ab) at each operating condition. The unstretched, unburned flame speed (SL) can then
be calculated from SL ¼ Q_ =Ab , where Q_ is the measured volumetric
flow rate of the preheated reactants. This procedure, which involves determining the reaction zone area as opposed to the inner
edge of the preheat zone, has been shown to provide a better estimate of the unstretched (1-d) flame speed, as it is only weakly affected by the flame curvature and because the Bunsen flame strain
is primarily restricted to the low area flame tip [44].
4. Chemical kinetic model development
A detailed chemical kinetic model for propane is developed
with the focus on predicting the effect of vitiation on ignition,
flame propagation and emissions. The chemical kinetic mechanism
also contains detailed sub-models for methane and ethane, thus
making it a suitable kinetic mechanism for modeling natural gas
combustion. The kinetic model validation and predictions of the
other natural gas fuel components for ignition and NOx emissions
can be found elsewhere [18,45]. The detailed chemical kinetic
scheme for the oxidation of any hydrocarbon fuel follows a hierarchical structure with H2, CO and CH2O oxidation kinetics as the
backbone for the mechanism. The reaction rate parameters for
the sub-mechanism of H2, CO and CH2O are adopted from Li
et al. [46]. The majority of the reaction rate parameters for the oxidation kinetics of CH4, C2H6 and C3H8 were adopted from various
literature sources [47–51]. Most of the reaction rate parameters
and thermodynamic data for the low-temperature kinetic scheme
were estimated using the group additivity concept [52–54] based
on the values recommended by Curran et al. [47] and Miyoshi
[49] with a few exceptions discussed below. Most of the reaction
rate parameters for the nitrogen chemistry are adopted from Mendiara and Glarborg [55], with a few exceptions, which are discussed in detail below.
The fuel molecule first undergoes H-atom abstraction via reaction (R8) to produce two distinct propyl radicals,
C3 H8 þ X n=i-C3 H7 þ XH
ðR8Þ
where X represents O, H, OH, HO2, H2O2, O2, or CH3. The reaction rate
parameters for the H-atom abstraction reaction (R8) were estimated
using group additivity theory [52] with per-site rate parameters recommended by Curran et al. [47]. Subsequent reaction paths for the
propyl radicals formed in reaction (R8) will determine the low- versus high-temperature oxidation kinetics. At high temperatures, the
propyl radicals will undergo beta-scission reactions to form smaller
olefins (e.g., C2H4). Meanwhile, the O2 molecules react with H atoms
via reaction (R9) to form a branching route to generate O and OH radicals, which propel the oxidation of hydrocarbon fuel fragments generated from C3H8 to form CO via a series of chain propagation
reactions. CO will then be converted to CO2 via a relatively slow reaction process with reaction (R10) as the predominant route.
H þ O2 O þ OH
CO þ OH CO2 þ H
ðR9Þ
ðR10Þ
At low temperatures, a termination route for H-atom is favored via
reaction (R1) instead of chain branching reaction (R9). Meanwhile,
the propyl radicals formed in reaction (R8) react with molecular
O2 to form propylperoxy radicals, (i.e., C3H7O2) via reaction (R11a)
at low temperatures:
Fig. 5. Schematic of experimental setup for modified Bunsen flame technique.
n=i-C3 H7 þ O2 n=i-C3 H7 O2
ðR11aÞ
n=i-C3 H7 þ O2 C3 H6 þ HO2
ðR11bÞ
n=i-C3 H7 O2 C3 H6 OOH j; where j ¼ 1; 2; 3
ðR12aÞ
n=i-C3 H7 O2 C3 H6 þ HO2
ðR12bÞ
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Fig. 6. Instantaneous images of atmospheric pressure propane flames at 650 K preheat temperature with varying dilution ratios: (a) no dilution, U = 0.87 (b) N2 dilution
ratio = 0.40, U = 1.15 and (c) CO2 dilution ratio = 10%, U = 1.30.
The alkylperoxy radicals formed in reaction (R11a) then proceed
through internal isomerization to form three distinct hydroperoxyalkyl radicals (i.e., CH3CHCH2OOH, CH2CH2CH2OOH and CH3CH(OOH)CH2) via reaction (R12a). The C3H7O2 and C3H6OOH
radicals formed in reactions (R11a) and (R12a) then go through a
series of reaction steps, notably with HO2 and O2, respectively, to
generate hydroperoxyalkyl and hydroperoxyalkylperoxy (i.e., O2C3H6OOH) radicals, which lead to a branching sequence that will produce OH radicals in the process. However, as the temperature is
increased, equilibrium favors the reverse reaction for (R11a), while
reaction (R11b) becomes dominant. The equilibrium shift in reaction (R11a) results in the negative temperature coefficient (NTC) region, which is a peculiar chemical kinetic phenomenon normally
observed during cool flame oxidation of alkane fuels [16]. In addition, the competition between the internal-isomerization reaction
(R12a) and the termination reaction (R12b) also impact the NTC
behavior.
The low-temperature oxidation of long-chained hydrocarbon
fuels has been extensively studied in the context of internal combustion engines [56]. However, due to the lack of experimental
or theoretical studies of the elementary reaction kinetics of lowtemperature oxidation of long-chained hydrocarbon fuels, the
group additivity method [52,57,58] has been used to develop
detailed kinetic models for primary reference fuels for gasoline oxidation [47,59]. The group additivity method relies on functional
group similarities among hydrocarbon radical species in order to
estimate the reaction rate parameters of larger molecules based
on the oxidation of smaller hydrocarbon molecules [49,60,61].
The propyl + O2 system [62,63] is one of the ideal candidates to
construct the low-temperature reaction scheme for larger hydrocarbon molecules. However, there is considerable uncertainty in
terms of the reaction rate parameters of the low-temperature
oxidation of hydrocarbon systems due to lack of experimental validation [64–66]. Theoretical and experimental studies on the lowtemperature kinetics of propyl + O2 system to predict OH and HO2
radical concentrations were reported by Taatjes and co-workers
[48,67–69]. In the current work, the kinetics rate parameters for
propyl + O2 (i.e., reactions (R11a) and (R11b)) and unimolecular
C3H7O2 reaction scheme (i.e., reactions (R12a) and (R12b))
were adopted from Huang et al. [48], while the rate parameters
for C3H6OOH, O2C3H6OOH and C3H7OOH reaction schemes were
adopted from Curran et al. [47] and Miyoshi [49].
The current chemical kinetic model also includes a detailed
reaction scheme for the vitiated kinetics of NOx. At low and intermediate temperatures, NOx interacts with relatively less reactive
radicals such as HO2, CH3 and CH3O2 to form highly reactive
combustion radicals, such as OH and CH3O, which promote the oxidation of hydrocarbon fuels. In addition, the H-atom abstraction
from the fuel molecules by NO2 can also play a significant role in
the ignition of hydrocarbon fuels when NOx is present in the inlet
oxidizer stream. In the current vitiated kinetics mechanism, two
reaction channels were considered for the H-atom abstraction by
NO2 from the fuel molecule:
C3 H8 þ NO2 n=i-C3 H7 þ HNO2
ðR13aÞ
C3 H8 þ NO2 n=i-C3 H7 þ HONO
ðR13bÞ
The reaction rate parameters were estimated for reactions (R13a)
and (R13b) by group additivity theory [52] based on the primary
and secondary H-atom abstraction rates recommended by Chan
et al. [70]. The theoretical rate estimation by Chan et al. [70], using
ab initio molecular dynamics simulations, considered cis-HONO and
trans-HONO isomers for reaction (R13b). In the current mechanism,
the rate parameters for reaction (R13b) were estimated considering
the sum of cis-HONO and trans-HONO isomers. Rasmussen et al.
[23] has discussed in detail the H-atom abstraction of CH4 by NO2
and the merits of assuming a single channel for HONO formation.
NOx reactions with C3H7 and C3H7O2 were also included in the
current model via reactions (R14) and (R15):
n=i-C3 H7 þ NO2 n=i-C3 H7 O þ NO
ðR14Þ
n=i-C3 H7 O2 þ NO n=i-C3 H7 O þ NO2
ðR15Þ
The reaction rate parameters used in the current model for the
interaction of NOx with C3H8, C3H7 and C3H7O2 are listed in Table 2.
The complete kinetic mechanism consists of 136 species and
966 elementary reactions. The chemical kinetic simulations presented in this paper were performed using Cantera [71].
Author's personal copy
2044
P. Gokulakrishnan et al. / Combustion and Flame 161 (2014) 2038–2053
5. NTC behavior of propane oxidation
2000
100
CO
HO2 þ HO2 H2 O2 þ O2
ðR16Þ
H2 O2 þ M 2OH þ M
ðR17Þ
Table 2
Arrhenius rate parameters used in the current work for reactions relevant to vitiated
kinetics of NOx.
Reaction
A
n
E
Refs.
NO + HO2 NO2 + OH
CH3 + NO2 CH3O + NO
CH3O2 + NO NO2 + CH3O
CH4 + NO2 HONO + CH3
C2H6 + NO2 HONO + C2H5
C3H8 + NO2 HONO + nC3H7
C3H8 + NO2 HONO + iC3H7
CH4 + NO2 HNO2 + CH3
C2H6 + NO2 HNO2 + C2H5
C3H8 + NO2 HNO2 + nC3H7
C3H8 + NO2 HNO2 + iC3H7
nC3H7 + NO2 nC3H7O + NO
iC3H7 + NO2 iC3H7O + NO
nC3H7O2 + NO nC3H7O + NO2
iC3H7O2 + NO iC3H7O + NO2
2.10E+12
1.00E+13
1.40E+12
6.50E+14
6.50E+14
3.00E+14
2.00E+13
6.00E+14
6.00E+14
9.60E+14
6.00E+13
1.00E+13
1.00E+13
1.75E+12
1.63E+12
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
497
0
715
45,800
41,400
32,000
28,500
37,600
33,200
33,800
30,300
0
0
695
715
[55]
[55]
[55]
[55]
[55]
Currenta
Currenta
[55]
[55]
Currenta
Currenta
Currentb
Currentb
[51]
[51]
Rate constant, k = ATnexp(E/RT), units: cm3/mol s.
E – activation energy in cal/mol; T – temperature in Kelvin; R – universal gas
constant.
a
Estimated from H-atom abstraction rates recommended by Chan et al. [70].
b
Same as CH3 + NO2 and CH3O2 + NO rates from Mendiara and Glarborg [55].
80
C3H8
1200
60
800
40
400
20
0
Propane Consumed [%]
1600
CO [ppmv]
Paraffinic fuels generally show NTC behavior during the low
temperature (<900 K) slow oxidation process known as the cool
flame region in which alkylperoxy radicals (RO2) and hydroperoxyalkyl radicals (QOOH) play a critical role in the fuel oxidation. Reactivity species profiles from flow reactor experiments
[72,73] and ignition delay time data from Rapid Compression Machine experiments [74] demonstrated that propane exhibits an
NTC behavior during low-temperature oxidation. Figure 7 shows
the flow reactor reactivity experimental data of Koert et al.
[72,75] and Ramotowski [73] for propane conversion and CO formation, respectively, measured during low-temperature propane
oxidation. The experiments were performed using a propane/air
mixture with 3.9% N2 dilution at an equivalence ratio of 0.4 and
10 atm. The experimental data show that preignition of propane
occurs between 650 K and 750 K with an NTC region between
700 K and 750 K at these conditions.
At low temperatures, the preignition chemistry of propane is
influenced by the formation of propylperoxides via reaction
(R11a) and the internal isomerization of C3H7O2 to form C3H6OOH
(i.e., (R12a)), which subsequently leads to chain-branching reactions to form OH radicals. On the other hand, competing product
channels to form HO2 and C3H6 via reactions (R11b) and (R12b)
act as chain-terminating pathways at these temperatures [64,76].
As the temperature is increased, the reaction equilibrium favors
the reverse reaction of (R11a) and forward reactions of (R11b)
and (R12b) so that the low temperature reaction route starts to
diminish. This marks the beginning of the NTC region (around
700 K) shown in Fig. 7. As the temperature is further increased,
the oxidation proceeds via an intermediate temperature kinetic regime where HO2 radicals from reactions (R1) and (R11b) react with
other combustion radicals, primarily with another HO2 to form
H2O2 via reaction (R16). Through the branching reaction (R17),
H2O2 then decomposes to form the OH radicals necessary for the
oxidation of fuel molecules in the intermediate combustion
regime.
0
650
675
700
725
750
775
800
Inlet Temperature [K]
Fig. 7. Reactivity experimental data for propane [72,75] and CO [73] obtained at
10 atm compared with the current model predictions. Key: symbols – experimental
data and lines – current model predictions: optimized model predictions (solid
lines); un-optimized model predictions using Huang et al. [48] rates (dashed-lines)
and Miyoshi [49] rates (dotted-lines) for the reactions listed in Table 3.
Therefore, it is important to accurately model the product channel
distribution between reactions (R11a) and (R11b) as well as between reactions (R12a) and (R12b) to simulate the NTC behavior
of low-temperature propane oxidation. The sensitivity of the preignition model predictions to these reactions is demonstrated in Fig. 7
in which the un-optimized model predictions are based on the rate
parameters of Huang et al. [48] and Miyoshi [49] for reactions
(R11a), (R11b), (R12a), and (R12b). The model using the Miyoshi
[49] reaction rate parameters yields very little propane oxidation
between 650 K and 800 K, while the predictions with reaction rate
parameters of Huang et al. [48] produced a much larger propane
conversion than the experimental data in Fig. 7. A similar modeling
trend is observed for the time-history experimental data [73] for
propane oxidation shown in Fig. 8.
Very often, detailed kinetic mechanisms are validated against
experimental data in order to improve the fidelity of the model
predictions by tuning sensitive reactions within their uncertainty
limits. For example, Smith et al. [77] and Qin et al. [78] demonstrated the optimization of detailed kinetic mechanisms using
response surface methodology [79]. Mittal et al. [80] and
Klippenstein et al. [81] used uncertainty analysis to identify and
improve sensitive reactions in chemical kinetic mechanisms for
syngas and methanol ignition, respectively. The reaction rate
parameter uncertainty for high-temperature oxidation reactions
can range over a factor of 4 (e.g., [82]), while the uncertainty factor
for low-temperature oxidation reactions can vary up to an order of
magnitude (e.g., [83]).
In the current work, reaction rate parameters for the low-temperature chemistry were optimized using newly developed software, known as rkmGen [84], by minimizing the error between
the model calculations and the experimental data for propane
decomposition shown in Figs. 7 and 8. rkmGen couples Cantera
[71] with simulated annealing [85,86] to perform the nonlinear
integration–optimization process for reaction rate parameter estimation. A brief summary of the optimization process is provided
here. The experimental conditions of Koert et al. [72] and Ramotowski [73] are used as the initial conditions for the optimization
using a plug-flow reactor simulation between 650 K and 750 K.
The propane mole fractions from the experiments shown in
Fig. 7 (i.e., X(Ti)|exp) and Fig. 8 (i.e., X(tj)|exp) are provided as the target data to compute the objective function, Eq. (1), that is to be
minimized:
Author's personal copy
2
735
1.6
725
1.2
715
C3H8
0.8
705
CO
Temperature [K]
C3H8 or CO [mol %]
P. Gokulakrishnan et al. / Combustion and Flame 161 (2014) 2038–2053
Temp.
0.4
695
0
685
0
50
100
150
Time [ms]
200
250
Fig. 8. Time–history experimental data for propane, CO and temperature obtained
at 10 atm compared with the current model predictions. Key: symbols – experimental data [73] and lines – current model predictions: optimized model
predictions (solid lines); un-optimized model predictions using Huang et al. [48]
rates (dashed-lines) and Miyoshi [49] rates (dotted-lines) for the reactions listed in
Table 3.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
"
#2
u
XM XðT i ÞjC H ;exp XðT i ÞjC H ;model
1u
1
t
3 8
3 8
þ
I¼
i¼1
M
N
XðT i ÞjC3 H8 ;exp
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
"
#2ffi
u
uXN Xðt j Þ
Xðtj ÞC H ;model
C3 H8 ;exp
t
3 8
j¼1
Xðt j ÞC H ;exp
3
ð1Þ
8
where M and N are number of target data points. The propane mole
fraction, given by Eq. (2), is computed using Cantera:
XjC3 H8 ;model ¼
Z
t
f ðC o ; T; P; WÞ
ð2Þ
t¼0
where Co – initial species concentrations, T – temperature, P – pressure and W 2 fA; n; Eg.
Simulated annealing is a stochastic nonlinear global optimization technique that is analogous to the solid state physics of
annealing in which a controlled cooling process achieves the thermodynamic equilibrium of a metal at its lowest energy level in
order to produce a high quality crystalline structure [85]. In the
current optimization process, the system ‘‘energy’’, defined by
the objective function (i.e., Eq. (1)), is minimized using the ‘‘cooling
schedule’’ given by Eq. (3) [86] in order to find the optimal reaction
rate parameters, W, in Eq. (2).
h
Xl ¼ X0 eðCl Þ
2045
where uk is the probability for kth function evaluation in the lth
annealing stage and the value of l varies between 1 and +1. The
minimization of the objective function is carried out by selecting
a new parameter set, Wl, that yields Il < Il 1 based on the k number
of function evaluations for l at the lth annealing temperature. In order to avoid local minima, a new parameter set is accepted, even if
Il > Il 1, based on the Boltzmann probability distribution function.
In the current optimization, around 40,000 function evaluations
were performed to reach the optimum parameters. Figure 9 shows
a visual representation of the optimization process for the parameter estimation of reaction (R11a).
The optimization results from simulated annealing were obtained by perturbing the reaction rate parameters such that
the rate constants lie within an order of magnitude change from
the nominal values used in the kinetic mechanism. Table 3 lists
the optimized rate parameters for the reaction scheme of propyl + O2 given by (R11a), (R11b), (R12a), and (R12b). A comparison
of the optimized reaction rate constants at 700 K with those of the
rates recommended by Huang et al. [48], and the theoretical uncertainty estimates of Goldsmith et al. [83] for n-propyl + O2 are also
provided in Table 3. It should be noted that the optimization process made very little change in the original activation energies of
Huang et al. [48] as shown in Table 3, while most of the change
in the rate constants stem from the pre-exponential factors. A comparison of the optimized reaction rate parameters shown in Table 3
with the various literature data is provided in the supplementary
data.
Figure 7 shows the optimized model predictions compared with
the reactivity experimental data for propane conversion and CO,
while Fig. 8 compares the optimized model predictions with the
time-history experimental data for propane oxidation. The current
model was optimized for propane decomposition and predicts the
preignition propane oxidation reasonably well including CO formation. However, further improvements are needed to predict CO
concentration more accurately in the NTC region. In order to further verify the model accuracy for the product channel distribution
between reactions (R11a) and (R11b), the optimized kinetic model
was used to simulate the experimental conditions of Slagle et al.
[87] to predict C3H6 formation. Slagle et al. [87] performed flow
reactor experiments to measure nC3H7 decomposition and C3H6
formation profiles at low-pressures between 297 K and 635 K by
generating nC3H7 radicals via CO2 laser photolysis of C6F5C4H9.
ð3Þ
where X0 – initial annealing temperature, Xl – lth annealing
temperature, h – quenching factor and C is given by Eq. (4).
C ¼ aeðbhÞ
ð4Þ
where a and b are constants.
At the lth annealing stage, the new parameter estimation is
given by
Wl ¼ Wl1 þ lk K
ð5Þ
where K is the search domain for simultaneous parameter perturbation of the reaction rate coefficients. The values for K are fixed
and those used in the current optimization are: 1.0, 0.1 and 10.0
for log(A), n and E respectively. The value for lk is obtained from
Eq. (6).
"
lk ¼ Xl ðuk 0:5Þ
1þ
1
Xl
#
j2uk 1j
1
ð6Þ
Fig. 9. Simulated annealing optimization process for the parameter estimation of
reaction n-C3H7 + O2 ? n-C3H7O2 (R11a). Key: black circle – optimum parameters at
each annealing stage; gray dots – parameter perturbations for the function
evaluations; green circle – initial parameters; blue circle – final parameters. (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
Author's personal copy
2046
P. Gokulakrishnan et al. / Combustion and Flame 161 (2014) 2038–2053
Table 3
Optimized Arrhenius rate parameters of propyl + O2 system relevant to low-temperature propane oxidation.
Log(kopt/knominal)a
Log(f)b
552
2319
0.79
0.07
0.36
0.57
2.32
8796
9478
0.56
0.15
0.62
9.27E + 07
1.82E10
7.69E + 22
0.72
6.67
4.05
24,363
13,412
33,335
0.33
0.38
1.05
0.71
6.45E + 24
1.58E + 30
3.83
5.83
34,783
35,968
0.16
1.05
0.81
#
Reactions
A
n
(R11a)
nC3H7 + O2 nC3H7O2
iC3H7 + O2 iC3H7O2
2.72E + 15
1.84E + 23
1.30
3.35
(R11b)
nC3H7 + O2 C3H6 + HO2
iC3H7 + O2 C3H6 + HO2
3.66E + 11
2.11E + 20
(R12a)
nC3H7O2 C3H6OOH_1
nC3H7O2 C3H6OOH_2
iC3H7O2 C3H6OOH_3
(R12b)
nC3H7O2 C3H6 + HO2
iC3H7O2 C3H6 + HO2
E
Rate constant, k = ATnexp(E/RT); units: cm3/mol s for bimolecular reactions and 1/s for unimolecular reactions. A – pre-exponential factor; E – activation energy in cal/mol; T
– temperature in Kelvin; R – universal gas constant.
a
Change in optimized rate constants relative to Huang et al. [48] values at 700 K and 10 atm.
b
Uncertainty factor, f, based on 3-standard deviations of theoretical rate estimation for n-propyl + O2 system reported by Goldsmith et al. [83] at 700 K and 1 atm.
Figure 10 compares the current model predictions with the experimental data for nC3H7 and C3H6 time-history profiles at 635 K. The
model agrees with the experimental data for C3H6 production reasonably well, however, the model slightly under-predicts the
nC3H7 decomposition profile between 2 and 8 ms.
6. Ignition delay time predictions without NOx addition
Several experimental studies have been reported in the literature over the years for propane ignition using shock tube [88–
96], rapid compression machine (RCM) [74] and flow reactors
[97,98]. The current model predictions are compared with selected
shock tube experimental data [88,92,93] and RCM data [74] at various pressures in Fig. 11. For the purpose of modeling, shock tube
and RCM experimental systems are assumed to be adiabatic
zero-dimensional constant volume reactors with negligible postshock or post-compression pressure gradients. Chaos and Dryer
[99] and Petersen and co-authors [96,100] discussed in detail the
potential drawbacks of these assumptions when the shock tube
ignition delay time is longer than few milliseconds. At these conditions, the induction chemistry is sensitive to any experimental perturbations caused by system non-uniformities that can adversely
1
affect the ignition delay time measurements, especially at high
pressures in low- and intermediate-temperature regimes. Nevertheless, the current adiabatic zero-dimensional reactor simulation
results shown in Fig. 11 can still be compared quantitatively with
the measured data for ignition delay times shorter than 1 ms, while
qualitative comparisons can be made with longer ignition delay
time data at 30 atm pressure.
Figure 11 compares the ignition delay time experimental data
for propane mixtures with the current kinetic model predictions
at 1 am [92], 10 atm [88] and 30 atm [74,93] pressures. The ignition delay time data of Burcat et al. [88] shown in Fig. 11 were obtained at varying pressures between 8 atm and 14 atm, while the
simulation results were performed using an average pressure of
10 atm. Overall, the model predictions agree fairly well with the
experimental data for ignition delay times shorter than 1 ms as
shown in Fig. 11. The model under-predicts the ignition delay time
compared to RCM experimental data at 30 atm, however the model
exhibits a qualitative agreement with the experimental data for the
NTC profile between 700 K and 800 K. The model needs to take into
account the system non-idealities at longer ignition delay times
(>1 ms) in order to accurately predict the experimental data at
lower temperatures.
1000
nC3H7
C3H6
Ignition Delay Time [ms]
Fraction of Initial nC3H7 [-]
0.8
0.6
0.4
10
30 atm
10 atm
1 atm
0.1
0.2
0.001
0.6
0
0
4
8
Time [ms]
12
0.8
1.0
1.2
1.4
1.6
16
Fig. 10. Time–history profiles of nC3H7 and C3H6 from current model predictions
compared with the experimental data of Slagle et al. [87] for nC3H7 + O2 reaction
system at 635 K. Initial conditions: [nC3H7] = 5 1011 molecules/cm3;
[O2] = 2.06 1015 molecules/cm3; [He] = 6.0 1016 molecules/cm3; Key: lines –
model predictions; symbols – experimental data [87].
1000/T - 1/K
Fig. 11. Ignition delay time predictions for various propane mixtures compared
with the experimental data at 1 atm (squares [92]), 10 atm (triangles [88]) and
30 atm (open circles [74] and closed circles [93]). Key: symbols – experimental data;
lines – current model predictions; solid line – 1 atm; dotted line – 10 atm, dashed
line – 30 atm.
Author's personal copy
2047
P. Gokulakrishnan et al. / Combustion and Flame 161 (2014) 2038–2053
7. Effect of NOX on propane oxidation
Ignition delay time measurements were obtained using the CSE
flow reactor facility to investigate the effect of NOx on the autoignition of propane. Atmospheric pressure experiments were performed with reactor temperatures of 875 K and 917 K at 0.5, 1.0
and 1.5 equivalence ratios. The concentration of NOx in the inlet
oxidizer stream ranged from 0 to 780 ppmv at varying O2 levels between 14 mol% and 21 mol%. Experimental data for a stoichiometric propane/air mixture at 875 K with varying NOx concentration
is shown in Fig. 12. Experimental data show that the presence of
NOx in the inlet stream reduces the ignition delay time significantly. For example, the addition of 755 ppmv of NOx in the oxidizer stream was able to reduce the ignition delay time of
stoichiometric propane/air mixture by 75%.
Figure 12 also shows current model predictions compared to
the experimental data. Adiabatic and constant-pressure plug-flow
reactor (PFR) simulations were used to model the flow reactor.
The current model predicts the effect of NOx on ignition delay time
fairly well. Figure 12 also compares the modeling results of two
detailed propane kinetic mechanisms [101,102] reported in the literature [20,103] with the current experimental data. The kinetic
modeling results using the Hori et al. [20] mechanism show an
opposite trend for the effect of NOx on ignition delay time compared to the experimental data. The modeling results using the Faravelli et al. [103] mechanism show that the ignition delay time
decreases with NOx but does not have good quantitative agreement
with the current experimental data.
As discussed above, the premixing section of the flow reactor
shown in Fig. 3 was designed to achieve near-perfect mixing of
the fuel and oxidizer within a short period of time relative to the
reaction time. However, in the ideal plug-flow reactor simulations,
it was assumed that perfect mixing occurs instantaneously at the
reactor inlet. Gokulakrishnan et al. [104] discussed in detail the
effect of premixing on induction chemistry in flow reactor experiments through PSR-PFR reactor modeling and demonstrated the
numerical validity of the ‘time shifting’ approach [105] to account
for the chemical kinetic perturbation that occurs during fuel and
oxidizer premixing. In the current work, simulations were performed to investigate the mixing effects on ignition delay time
using a reactor network model that consists of a series of
perfectly-stirred reactors (PSRs) to represent the premixing and
diffuser sections of the system using geometrical volume and flow
rates. Following the series of PSRs, an adiabatic PFR is used to model the test section of the flow reactor. The simulation results of the
reactor network model show that there was no significant difference in the ignition delay time predictions compared to single
PFR simulation. The agreement between the PFR simulation results
and the chemical reactor network of PSRs and PFR indicate that the
chemical kinetic perturbation in the premixing section has a negligible impact on the induction chemistry for the current experimental conditions.
Figure 13 shows the ignition delay time of a stoichiometric mixture of propane/air as a function of NOx addition at reactor temperatures of 875 and 917 K. Figure 14 shows the ignition delay time of
stoichiometric propane/air mixtures as a function of NOx at varying
O2 levels of 20, 17 and 14 mol% in the oxidizer stream. The results
show that as the level of O2 is lowered, the percent reduction in the
ignition delay time increases. However, both the experimental and
modeling results show that the ignition delay times become insensitive to the O2 levels when the concentration of NOx exceeds
400 ppmv. Figure 15 shows the ignition delay time data as a function of NOx concentration in the vitiated air at various equivalence
ratios with a reactor temperature of 875 K. The experimental
results show that the effect of NOx in promoting the oxidation is
greater under fuel-lean conditions than fuel-rich conditions. For
example, with a 90 ppm NOx addition the ignition delay time
was reduced by 50% at an equivalence ratio of 0.5, while it has
a 30% and 10% reduction at equivalence ratios of 1.0 and 1.5
respectively. It can also be noted that the relative effect of NOx in
reducing the ignition delay time diminishes as the amount of
NOx is increased.
Figures 16 and 17 show model comparisons to the time-history
experimental data for propane and NO conversion, respectively,
reported by Hori et al. [20]. The experiments were performed in
an atmospheric pressure tubular flow reactor at 800 K with
50 ppm C3H8/20 ppm NO in air (item#11 in Table 1) to investigate
the mutual sensitized oxidation of propane by NO. The current
model predicts the trends for propane and NO to NO2 conversions
fairly well as shown in Figs. 16 and 17, respectively. However, the
current model shows higher values for the steady-state NO to NO2
conversion compared with the experimental data. The modeling
results using the chemical kinetic mechanisms of Hori et al. [20]
and Faravelli et al. [103] are also shown for comparison.
1000
800
917 K, 20% O2, φ = 1.0
875 K, 20% O2, φ = 1.0
600
600
Ignition Delay Time [ms]
Ignition Delay Time [ms]
800
400
200
400
200
0
0
200
400
600
NOX [ppm]
800
1000
0
0
Fig. 12. Ignition delay time experimental data (symbols) for stoichiometric
propane/air at 875 K as function of NOx is compared with kinetic modeling results.
Key: solid line – current model; dotted line – current model without H-atom
abstraction reactions of propane by NO2 (i.e., (R13a) and (R13b)); dashed line: Hori
et al. model [20,101]; dashed-dotted line – Faravelli et al. model [102,103].
200
400
600
NOX [ppm]
800
1000
Fig. 13. Ignition delay time for stoichiometric propane/air mixture at 875 K and
917 K. Key: symbols – experimental data; lines – modeling results (solid line: 875 K;
dashed line: 917 K).
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1000
60
875 K, 20% O2, φ = 1.0
875 K, 17% O2, φ = 1.0
40
C3H8 [ppmv]
Ignition Delay Time [ms]
50
875 K, 14% O2, φ = 1.0
800
600
400
20
200
10
0
0
0
200
400
600
NOX [ppm]
800
1000
Fig. 14. Ignition delay time for stoichiometric mixture with varying O2 at 875 K.
Key: symbols – experimental data; lines – modeling results (solid line: 20 mol% O2;
dashed line: 17 mol% O2; dotted line: 14 mol% O2).
0
0.4
0.8
Time [s]
1.2
1.6
Fig. 16. Model predictions for propane time-history profile for the experimental
conditions of Hori et al. [20] with 50 ppm C3H8/20 ppm NO in air at 800 K. Key:
symbols – experimental data [20]; solid-line: current model; dashed line: Hori et al.
model [20,101]; dash-dotted line – Faravelli et al. model [103,102].
1200
1
875 K, 20% O2, φ = 0.5
875 K, 20% O2, φ = 1.0
1000
875 K, 20% O2, φ = 1.5
0.8
800
NO2/NOX
Ignition Delay Time [ms]
30
600
400
0.6
0.4
0.2
200
0
0
200
400
600
NOX [ppm]
800
1000
Fig. 15. Ignition delay time for propane/air as a function of NO at varying
equivalence ratios. Key: symbols – experimental data; lines – modeling results
(solid line: / = 0.5; dashed line: / = 1.0; dotted line: / = 1.5).
In order to further investigate the role of H-atom abstraction on
propane ignition, constant pressure simulations were performed to
examine propane oxidation with 0 and 100 ppmv of NO addition.
Argon is used as the bulk diluent (instead of N2) in order to investigate the fate of nitrogenous species during propane oxidation in
the presence of NO with and without the H-atom abstraction reactions (R13a) and (R13b). Figure 18 shows the modeling results for
the temperature profile of stoichiometric propane oxidation in a
21 mol% O2/79 mol% Ar mixture at 875 K. The modeling results in
Fig. 18 show that the presence of 100 ppm NO reduces the ignition
delay time from 0.76 s to 0.51 s. It also shows that 70% of the
reduction in ignition delay time is due to the H-atom abstraction
by NO2. Ignition delay time model predictions without the H-atom
abstraction by NO2 for stoichiometric propane/air mixture as a
function of NOx are shown in Fig. 12. It can be noted that the contribution of H-atom abstraction reactions by NO2 to the reduction
in ignition delay time increases as the amount of NOx in the oxidizer is increased.
Figure 19 compares the time-history profiles of the major intermediate nitrogenous species, namely NO2, HNO2, CH3NO2 and
HONO, formed with and without the H-atom abstraction of
0
0
0.4
0.8
Time [s]
1.2
1.6
Fig. 17. Model predictions for NO to NO2 conversion profile for the experimental
conditions shown in Fig. 16. Key: symbols – experimental data [20]; solid-line:
current model; dashed line: Hori et al. model [20,101]; dash-dotted line – Faravelli
et al. model [102,103].
propane by NO2 for conditions in Fig. 18. Most of the fixed nitrogen
introduced as NO in the vitiated air is converted into NO2, HONO
and CH3NO2 prior to ignition as shown in Fig. 19. However, a significant amount of HONO has formed through reaction (R13b)
when H-atom abstraction of propane by NO2 is included. It is also
interesting to note that all of the nitrogenous species convert back
to the initial NO by the time of ignition. In the post-ignition phase,
however, some of the NO is converted into N2 in order to attain
equilibrium in the absence of bulk N2 dilution.
As discussed in Section 2, the addition of NOx promotes hydrocarbon oxidation through the conversion of relatively less reactive
radicals into more reactive OH and CH3O radicals via reactions
((R2), (R6), and (R7)). During this sensitized-oxidation process,
NO is converted into NO2 when NO reacts with HO2 (via (R2))
and CH3O2 (via (R6)). Subsequently, NO2 reacts with C3H8 to produce HNO2 and HONO via reactions (R13a) and (R13b), respectively, in addition to propyl radicals. HNO2 then undergoes
unimolecular decomposition to produce HONO via reaction (R18):
HNO2 þ M HONO þ M
ðR18Þ
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3000
Temperature [K]
2500
2000
1500
1000
500
0
0
0.2
0.4
0.6
Time [s]
0.8
1
Fig. 18. Time–history profile for a stoichiometric propane mixture in 21 mole%O2/
79mole%AR at 875 K and 1 atm. Key: dotted line – with 0 ppm NO; dashed line –
100 ppm NO without H-atom abstraction reactions; solid line – 100 ppm NO with
full reaction mechanism.
HONO produced via reactions (R13b) and (R18) disassociates into
NO and OH via reaction (R19), which helps to accelerate propane
oxidation by regenerating NO while producing OH radicals.
HONO þ M NO þ OH þ M
ðR19Þ
Therefore, H-atom abstraction of NO2 from the fuel molecule plays a
major role in promoting the ignition at intermediate temperatures
when NOx is present in the oxidizer stream.
A sensitivity analysis coupled with a Principal Component Analysis (PCA) [14,106] was used to identify the important reactions
responsible for the effect of NOx on promoting the fuel oxidation
at the experimental conditions used in the current work (i.e.,
Fig. 12) as well as the experimental conditions of Hori et al. [20]
(i.e. Fig. 16). The sensitivity coefficients of species with respect to
each reaction were computed at discreet time intervals over the
simulation period. An eigenvalue–eigenvector decomposition was
then performed on a time-integrated sensitivity matrix using Singular Value Decomposition (SVD) in order to transform the data
variation into a new set of linear, uncorrelated variables, known
as the principal components (PCs). In essence, the purpose of performing a PCA is to reduce the dimensionality of the original
1
NO2
N-atom Molar Ratio
0.8
0.6
NO
N2
CH3 NO2
0.4
HONO
0.2
0
0
0.2
0.4
0.6
Time [s]
0.8
1
Fig. 19. Time–history profile for the distribution of N atom in nitrogenous species
during the ignition of propane with 100 ppm NO for the conditions shown in Fig. 18.
Key: solid line – full reaction mechanism; dashed line – without the reactions for Hatom abstraction by NO2.
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sensitivity data set, which consists of a large number of interrelated variables (i.e., sensitivity coefficient of each reaction with
respect to every species), into uncorrelated PCs so that the only a
very few PCs will retain most of the variation in the original data
set. The ranking of the PCs are determined by the magnitudes of
the eigenvalues generated by SVD of the sensitivity coefficient data
set. The magnitudes of the elements of the eigenvector corresponding to the each PC give a measure of the contribution of each reaction to the variation expressed by the PC. Very often the first PC
contains most of the variation in the original sensitivity data set,
and hence the ranking of the reactions based on the eigenvector
values identify the important rate-liming reactions. For example,
the ratio of the first eigenvalue, k1, to the sum of all the eigen
P
values, i:e:k1 = ni¼1 ki , for the conditions shown in Figs. 12 and
16 are 0.98 and 0.96, respectively. Here n is the number of PCs
which is equivalent to the number of species in the kinetic mechanism. Therefore, the eigenvectors of the first PC only are examined
to investigate the important reactions at these conditions.
Figure 20 shows the first 24 reactions with the highest eigenvector values with respect to the first PC computed for a stoichiometric C3H8/air mixture at 875 K with the addition of 100 ppm NOx
(i.e., carbon-NO molar ratio of 975). Figure 21 shows the first 24
reactions with the highest eigenvector values with respect to the
first PC obtained for the experimental condition of Hori et al.
[20] using 50 ppm C3H8/20 ppm NO in air at 800 K (i.e., carbonNO molar ratio of 7.5). Comparing the ranking of the reactions in
Fig. 20 reveals that reaction between C3H8 and NO2 (i.e., (R13a))
has the largest eigenvector value among the reactions that involve
NO or NO2. In Fig. 20, the order of the ranking for reactions with
nitrogenous species is: (R13a), (R7), (R2), (R19), and (R13b), while
the order of ranking in Fig. 21 is: (R2), (R19), and (R6). Comparison
of the important reactions in Figs. 20 and 21 reveals that H-atom
abstraction by NO2 (e.g., reactions (R13a) and (R13b)) plays a critical role in promoting propane oxidation at a carbon-NO ratio of
975, while it has negligible impact at condition with a carbonNO ratio of 7.5.
For the current experimental conditions shown in Fig. 12, species flux analysis shows that more than 90% of the NO in the inlet
stream is converted to NO2 via (R2) prior to ignition. More than
50% of the NO2 formed via reaction (R2) is then converted back
to NO via reaction (R7) in which CH3 is converted into CH3O. Reaction (R7) also competes with the recombination reaction (R20) for
NO2 to produce CH3NO2, especially at fuel-rich conditions:
CH3 þ NO2 þ M CH3 NO2 þ M
ðR20Þ
Some of the remaining NO2 reacts with C3H8 to produce HNO2 and
HONO via reactions (R13a) and (R13b), respectively. However, almost all of the HNO2 produced via reactions (R13a) is then converted into HONO via reaction (R18), of which more than 90% of
the HONO dissociates to produce NO and OH via reaction (R19).
Therefore at the current experimental conditions, the formation of
HONO due to H-atom abstraction by NO2 from propane molecule
plays a critical role in re-generating NO, while producing an OH radical in the process.
Constant pressure ignition delay time simulations were performed to investigate the impact of dilution on the effect of NO
on propane ignition by varying the N2 concentration while keeping
the propane/O2 molar ratio constant at stoichiometric conditions.
Figure 22 shows the ignition delay time as a function of O2 mole
fraction in the oxidizer stream at varying levels of NO addition.
The simulation results are also compared with the current experimental data. Overall, the addition of NO decreases the ignition delay time at all dilution levels at these conditions. However, the
influence of NO in promoting the ignition is decreased as the O2 level is reduced below 10 mol%, especially at high NO concentrations. It can also be noted that high-levels of NO addition show
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C3H8+NO2=iC3H7+HONO
HO2+C3H6=H2O2+C3H5
O2+C2H3=HCO+CH2O
HO2+C3H5= O2+C3H6
C3H8+NO2=nC3H7+HNO2
OH+C3H6=H2O+C3H5
O2+iC3H7=HO2+C3H6
HO2+CH3=O2+CH4
OH+NO(+M)=HONO(+M)
HO2+NO=OH+NO2
nC3H7O2=>O2+nC3H7
CH3+NO2<=>CH3O+NO
O2+nC3H7=> nC3H7O2
HO2+CH2O=H2O2+HCO
C3H8+NO2=iC3H7+HNO2
2 HO2=O2+H2O2
O2+nC3H7=HO2+C3H6
HO2+C3H8=H2O2+nC3H7
HO2+C3H8=H2O2+iC3H7
H2O2(+M)=2OH(+M)
0.0
0.2
0.4
0.6
0.8
1.0
Eigenvectors of the First Principal Component
Fig. 20. Magnitude of the eigenvector elements of the 1st Principal Component obtained from the PCA of the sensitivity coefficients for the oxidation of stoichiometric
propane/air mixture with 100 ppm NO at 875 K (i.e., current experimental conditions in Fig. 12). Key – black bars indicates reactions that involved NO or NO2 species.
O2+iC3H7=>iC3H7OO
CH3O=H+CH2O
OH+C3H6=H2O+aC3H5
O2+iC3H7=HO2+C3H6
CH3O2+NO=CH3O+NO2
O2+C3H8=HO2+nC3H7
H+O2(+M)=HO2(+M)
OH+HO2=H2O+O2
H+ O2=O+OH
O2+C3H8=HO2+iC3H7
OH+C3H8=H2O+iC3H7
nC3H7O2=HO2+C3H6
OH+NO(+M)=HONO(+M)
O2+QOOH_2=O2QOOH_2
nC3H7O2=>O2+nC3H7
HO2+NO=OH+NO2
OH+C3H8=H2O+nC3H7
O2QOOH_2=OH+CHOC2H4OOH
O2+nC3H7=>nC3H7O2
O2+nC3H7=HO2+C3H6
0.0
0.2
0.4
0.6
0.8
Eigenvectors of the First Principal Component
1.0
Fig. 21. Magnitude of the eigenvector elements of the 1st Principal Component obtained from the PCA of the sensitivity coefficients for the oxidation of 50 ppm propane in air
with 20 ppm NO at 800 K (i.e., Hori et al. [20] experimental conditions as in Fig. 16). Key – black bars indicates reactions that involved NO or NO2 species.
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200
10000
0 ppm NO
90 ppm NO
Laminar Flame Speed [cm/s]
Ignition Delay Time [ms]
430 ppm NO
755 ppm NO
1000
0.1
Oxidizer O2 [mol-frac]
1
50
1
0.8
0.6
0.4
0.2
0
10
100
1000
Carbon-NO Ratio
0.4
0.8
1.2
1.6
2.0
Equivalence Ratio
Fig. 22. Ignition delay time for stoichiometric propane/O2/N2 mixture as a function
of O2 concentration in the oxidizer stream at varying NO concentrations at 875 K
and 1 atm. Key: symbols – current experimental data; lines – modeling results: solid
line - with 0 ppm NO; dashed line – with 90 ppm NO; dot-dash line – with 430 ppm
NO; dotted line – 755 ppm NO.
IDT/IDT0ppmNO
100
0
100
0.01
1
150
10000
Fig. 23. Normalized ignition delay time as a function of carbon–NO molar ratio for
the simulation results shown in Fig. 22. Key: dashed line – with 90 ppm NO; dotdash line – with 430 ppm NO; dotted line – 755 ppm NO.
an inhibiting effect on propane ignition below 2.5 mol% O2 levels.
This is due to the fact that the reverse reaction (R19) is favored
as a termination route for NO scavenging at these conditions. The
contour plots in Fig. 23 show the simulation results presented in
Fig. 22 as a function of carbon-NO molar ratio at varying NO levels.
The effect of NO in promoting the ignition increases as the carbonNO molar ratio is increased until a turn-over point. However, the
effect of NO in reducing the ignition delay time diminishes as the
carbon-NO molar ratio is further increased beyond the turn-over
carbon-NO ratio as shown in Fig. 23. A similar trend is observed
for all levels of NO addition, however, the turn-over carbon-NO ratio shifts towards higher values as the NO is decreased.
8. Effect of CO2 on laminar flame speed
The presence of diluents in the oxidizer stream plays a different
role on laminar flame speed than what has been demonstrated for
Fig. 24. Effect of CO2 on propane laminar flame speed at 650 K preheat temperature
and 1 atm. Key: symbols – experimental data; lines – model predictions; solid line –
model predictions for propane/air mixtures; closed circles – propane/air at 650 K
(current); open circles – propane/air [39]; diamonds and dashed line – propane/air
with 10% CO2 (current); dot-dashed line – current model predictions with
chemically inert 10 mol% CO2 diluent; dotted line – current model predictions
with chemically active or inert 10 mol% N2 diluent; triangles – propane/air at 300 K
[107].
ignition delay time. Figure 24 shows the effect of CO2 dilution on
propane laminar flame speed obtained in the current work with
a 650 K preheat temperature. The laminar flame speed data for
propane/air reported in the literature at 300 K [107] and 650 K
[39] inlet temperatures are included along with the current model
predictions for comparison. The model predictions agree fairly well
with the experimental data for propane/air with and without 10%
CO2 dilution. Comparisons of the laminar flame speed data for
C3H8/air with that of CO2 dilution show that addition of 10 mol%
CO2 reduces the peak flame speed by more than a factor of two.
One of the goals of this study is to improve the understanding of
the role of vitiation on combustion chemistry. Vitiation alters the
flame speed not only through a change in the thermal and transport properties but also by affecting the elementary reaction rates
of the flame chemistry. To better isolate the kinetic effects from the
thermodynamics effects, laminar flame speed simulations were
performed for N2 and CO2 dilution with their chemical kinetics disabled by declaring them chemically inert species. Laminar flame
speed calculations performed with chemically inert CO2, shown
in Fig. 24, indicate a significant difference in laminar flame speed
from that of the chemically active case. The case with 10 mol% inert CO2 dilution produces as much as a 25% higher flame speed
than the chemically active case, with the effect most pronounced
at near-stoichiometric conditions. No such difference is observed
for the similar simulations using N2 dilution. This indicates that
CO2 is kinetically active in reducing the flame speed whereas the
effects of N2 dilution are primarily due to its thermal properties.
A sensitivity analysis shows that the presence of CO2 in the oxidizer stream inhibits the production of H atom via reaction (R10).
This will result in a reduction in the combustion radical pool generated via branching reaction (R9) (i.e., OH and O), which is also the
most important rate-limiting reaction step for laminar flame
speed. Therefore, the presence of CO2 in the oxidizer stream chemically inhibits the radical production, which contributes to the
reduction in flame speed in addition to the thermal diluent effects.
Since the rapid production of the radicals is enhanced at the high
temperatures corresponding to near stoichiometric conditions, this
is also the region where the CO2 kinetic inhibition effect is most
pronounced.
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9. Conclusions
The effect on ignition and flame propagation of hydrocarbon
fuels of chemical species present in typical vitiated air was investigated using propane as the prototype fuel. New experimental
data are reported on the effect of NOx on ignition delay time and
the effects of CO2 dilution on laminar flame speed at high preheat
conditions. The experimental data show that the presence of NOx
in the vitiated air reduces the ignition delay time significantly.
For example, 755 ppm NOx in the inlet oxidizer stream can reduce
the ignition delay time of stoichiometric propane/air by 75% at
875 K. Although the presence of NOx has little effect on the flame
propagation, the levels of CO2 in the vitiated air can have significant impact on the flame speed. Experimental measurements of
propane flames with a preheat temperature of 650 K showed that
the addition of 10% CO2 to the oxidizer stream can reduce the peak
laminar flame speed of propane by a factor of two.
A detailed chemical kinetic mechanism was developed to model
the vitiated kinetics of natural gas fuels including propane at relevant conditions to many practical devices. The reaction mechanism
has been validated over a range of experimental conditions for vitiated and unvitiated conditions. One of the important findings of
the current work is that the H-atom abstraction of propane by
NO2 plays a significant role in promoting hydrocarbon ignition
when the vitiated air contains NO. Chemical kinetic analyses also
show that the carbon/NO ratio influences the effect of NO in promoting the fuel oxidation. Laminar flame speed experiments and
modeling show that CO2 is more effective in reducing the laminar
flame speed than N2. Analysis also shows that chemical kinetics
play significant role in reducing the flame speed during CO2 dilution compared to N2 dilution in addition to the change in thermal
and transport properties.
Acknowledgments
The authors acknowledge the financial support from the U.S. Air
Force for the experimental work performed at the Georgia Institute
of Technology through an SBIR with Combustion Science & Engineering, Inc. (Contract # FA8650-08-M-2879; Program Monitors:
Dr. Barry Kiel and Dr. David Blunck). The authors would also like
to thank Michael J. Ramotowski of Solar Turbines for helpful discussions on propane pre-ignition experiments used for model
validation.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.combustflame.
2014.01.024.
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