49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition
4 - 7 January 2011, Orlando, Florida
AIAA 2011-94
Kinetic Mechanism For Low Pressure Oxygen / Methane
Ignition and Combustion
N.A. Slavinskaya 1
Institute of Combustion Technology, German Aerospace Centre (DLR), Pfaffenwaldring 38-40, 70569 Stuttgart,–
Germany
O.J.Haidn 2
Institute of Space Propulsion, German Aerospace Centre (DLR), Langer Grund, 74239Lampoldshausen, Germany
The detailed chemical kinetic model for high pressure CH4/O2 combustion has been enlarged for to
allow for modelling of low pressure and low temperature operating conditions 0.02 MPa < p < 0.1 MPa, 900
K < T < 1800 K and 0.5 < Ф < 3.0. The main model improvements have been performed for H2/CO submodels concerning the new data for reaction rates and verification of data for pressure depending reactions.
Sensitivity and rate of production analyses were applied to compare the main reaction paths at low – and
high pressure methane combustion. Developed model have been reduced with RedMaster code based on the
global sensitivity analysis. Obtained skeletal model keeps predictive capabilities of input detailed mechanism.
I
I. Introduction
n recent years, the propellant combination LOX/CH4 has received considerable attraction in USA, Europe and
Japan as a propellant combination for attitude control, upper stage or booster engines. In addition to these more
traditional rocket engine applications this propellant pair is also of interest for exploration missions [1-4].
Exploration missions towards Mars and beyond which base on classical storable propellants, i.e. mono-methylhydrazine (MMH) and nitrogen-tetroxide (NTO) have a considerable thermal management requirement which
ensures that throughout the mission the propellants in the tanks as well as import devices within the fluid lines such
as valves have sufficient temperature margin to avoid freezing of the propellants when injected into a combustion
chamber under flashing conditions. The absolute values of boiling and freezing temperatures of NTO and MMH of
394 K and 262 K, and, 361 K and 221 K, respectively, are rather high and require a thorough temperature
management. In comparison, the respective temperature values for boiling and freezing of CH4 and O2 are 111 K
and 91 K, and, 90 K and 55 K, respectively.
Another important aspect for the propellant choice of an in-space propulsion system is their specific impulse
since any reduction of propellant mass and a resulting smaller tank weight can be traded immediately into additional
payload. The propellant pair LOX/CH4 has a specific impulse advantage of about 20 sec which depending on the
operating conditions may exceed 10%. Hence, missions with a reduced requirement of thermal management and
propellant losses through evaporation will surely profit from a LOX/CH4 based propulsion system. In addition to the
necessary system analysis studies which would identify favourable missions for this propellant combination it is
absolutely mandatory to increase the knowledge about the detailed chemical kinetics which determine the ignition
and combustion behaviour of this propellant combination for low pressure conditions. The final step of the
development will be a reduced mechanism with sufficient predictive capabilities but still small enough to be
implemented in current CFD tools.
The requirement to predict with sufficient accuracy combustion performance and heat load to combustion
chamber walls of hydrocarbon - fuelled engines necessitates numerical tools with reliable chemical kinetic schemes.
The hierarchical structure of hydrocarbon oxidation reactions makes the methane chemistry the principal reaction
scheme for any hydrocarbon. Besides the interest in methane for space applications mentioned previously, this
propellant being a renewable bio-fuel has seen rizing interest for both economic and ecologic reasons. This strong
interest in methane has led to kinetic studies that highlight the important features of CH4 combustion. In Table 1 the
several successful studies have been collected together with the validation experimental data.
1
Correspondig author, Senior Research Fellow, Department of Chemical Kinetics, [email protected],
Member AIAA.
2
Head, Department of Technology, [email protected], Associate Fellow AIAA.
1
American Institute of Aeronautics and Astronautics
Copyright © 2011 by N.A. Slavinskaja and O.J. Haidn. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
The GRI 3.0 mechanism [5], see Table 1, is based on elementary reactions, where the fitted and theoretically
determined values are assigned to the rate parameters. The reaction rate fittings were carried out on the basis of
consecutive comparison with very broad spectrum of measured values. Extensive sensitivity tests reveal which
parameters need to be tuned to minimize the differences between the experimental data and the simulation results.
The mechanism was well validated on the experimental data for methane oxidation obtained in shock tubes, laminar
flames, and flow reactors. Leeds Methane Oxidation Mechanism [6] with its recently updated H2/CO sub
mechanism [3], Table 1, fully based on gas kinetics measurements, and, where possible, on evaluated rate
parameters, which are fully referenced and annotated [8, 9]. The validation parameter range is much less than by
GRI 3.0 mechanism and related to ignition delays data and premixed laminar flame speed data. Only laminar flame
velocity data have been used for validation of methane oxidation chemistry in mechanism proposed by Konnov [10]
for C1-C2 combustion, Table 1. The family of RAMEC mechanisms [11 - 13], Table 1, has been elaborated as
extensions and modification of GRI 3.0 mechanism [5] to model ignition delay times at very high pressure, low
temperature and lean mixtures conditions. A reaction mechanism presented through studies [15-16] has been
developed mostly for predicting methane autoignition under different pressures, temperatures, and equivalence
ratios, Table 1, has been also validated on the atmospheric laminar flame. This mechanism was further extended to
“San Diego” model for C1-C3 [17] chemistry generally with the same validation base for methane. A recently
proposed kinetic reaction mechanism [18, 19] for modelling the oxidation of hydrogen, CO, methane, methanol,
formaldehyde, and natural gas is today the most comprehensive methane reaction model, which has been validated
over a wide range of conditions including laminar burning velocities, flame structures, JSR and plug-flow reactors
concentration profiles, and shock tube data for ignition delays and concentration profiles.
Table 1. Methane kinetic mechanisms and their validation data base.
Mechanism
GRI 3.0 [5]
Ignition delay
Flame speed
p = 1 - 84 atm
p = 1 - 20 atm
T5 = 1356 -1700 K
To = 298, 400 K
 = 0.5 - 1.0
 = 0.6–1-.6
p = 1 - 4 ; 21 - 29 atm
Leeds Mechanism [6, 7] T5 = 1400 - 2050 K
 = 0.1 - 2.0
JSR
PFR
p = 1.07 atm
= 1.0
Shock tube
p = 1 – 2 atm
T5 =1400–2100 K
= 0.4–4.0
p = 1.0atm,
To = 298K
 = 0.6–1.4
p = 1 - 10 atm
Konnov
[10]
mechanism
RAMEC
[11-13]
mechanism p = 40 – 260atm
To = 298 K
 = 0.5–1-.6
T5 = 1040 -2870 K
 = 0.5-6.0
Li-Williams mechanism p = 1 – 150 atm
p = 1.0 atm
[14-16],
To = 298 K
T5 = 1000 -2000 K
San Diego Mechanism  = 0.4 – 6.0
Le Cong-Dagaut
mechanism [18,19]
p = 1- 60 atm
T5 = 1100–2800 K,
 = 0.5-1
 = 0.6–6.0
p = 1.0 – 20.0 atm p = 1 - 10atm p = 1, 2 atm
To = 298, 615K
To=900–1400K T = 1100 K
=1
 = 0.6–1.6
 = 0.1–0.6
p = 1–79 atm
T5=1400–2200 K
 = 0.5 - 1
The mechanisms described in Table 1 are validated mostly for higher pressure, i.e. gas turbine operating
conditions. A detailed mechanism development follows generally a problem oriented approach that is always
connected with an optimisation of thermo kinetic data on the selected set of experimental data. Even if the structure
of the different kinetic models is similar, either the reactions or the kinetic parameters involved may be different.
The direct application of any reaction mechanism for simulations of processes with parameters for which the model
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was not validated can lead to curios results. Therefore, the comprehensive mechanisms should be constructed on the
basis of detailed comparisons between predicted and experimental data obtained in a wide range of operating
conditions.
The aim of this paper is to discuss the extension of general kinetic scheme for C0 – C1 oxidation to low pressure
methane combustion. This scheme is the base sub model in the overall reaction mechanism of larger hydrocarbons
(up to C16H34) used for combustion simulations of various high-molecular weight fuels [20, 21]. This kinetic
mechanism has a strong hierarchical structure and is developed as continual data base of chemical kinetic data for
hydrocarbons. A previously developed C1 sub mechanism has been updated and applied to describe atmospheric and
high pressure (up to 60 bar) CH4/O2 combustion with initial temperature interval 900 K < T < 1800 K and
fuel/oxygen ratio = 0.5 – 3 [22]. In the present work, and based on a review of new thermo chemical data and on
detailed comparisons between predicted and experimental data, the core C0 – C1 sub model has been updated to
model the low pressure and low temperature CH4/O2 combustion under operating conditions 0.2 bar < p < 1.0 bar,
900 K < T < 1800 K and 0.5 <  < 3.0. The effects of pressure and temperature on the fundamental combustion
properties (auto-ignition delays and laminar flame speeds) and the active radical production are discussed.
II. Chemical Kinetic Model
The presented in [20-21] C0-C2 reaction submechanism is based on the H/O, C1, and C2 chemistry of the Leeds
methane oxidation reaction scheme, Table 1. The Leeds model [6, 7] has been selected as the base chemistry for the
mechanism since it was developed mostly on the basis of “first principals”, i.e. with minimal set of fitted kinetic
data. That is extremely useful for the iterative and time-consuming continuous development of the complex reaction
schemes with strongly hierarchical structure, because, first, that reduces the revision, tuning and adjustment of the
small chemistry kinetic parameters by each mechanism extension, and, second, it decreases the dependence of the
model from its validation data set, and, finally, it allows to widen the mechanism validity limits.
The mechanism update studied in this paper relates mostly to H2/CO chemistry. Recently, strong interest in the
use of syngas in gas turbines has led to active kinetic studies that highlight the important features of H2/CO
combustion [10, 16,20-33]. Since reactions involved in H2/CO combustion play an essential role in the hydrocarbon
oxidation and since a large amount of considerable studies have been successfully conducted, one might expect that
the kinetic mechanisms of H2/CO are largely established, the kinetic parameters governing each reaction are
accurately determined and reaction mechanisms are enough unique in predictions. This, unfortunately, is not the
case; considerable inadequacies exist in the available mechanisms. To demonstrate that, the reaction rates of 12
reactions, which have been identified as the most sensitive to the ignition delay time and flame speed by modelling
with presented mechanism, have been calculated with values used in [10, 16, 17, 24-28] models. The deviations
obtained for reaction rates calculated at T = 1000 K are visualised in Table 2. They have the values between 1.99 %
for OH + OH (+M) = H2O2 (+M) and 82.3 % for CO + O (+M) = CO2 (+M). Such unexpectedly large deviations can
be explained with the evolution of each individual mechanism development, problem oriented set of parameters for
validation and optimisation, the specific set of reactions and correlation of their rate coefficients, etc. To evaluate the
influence of the uncertainty of these reaction rates on ignition delays simulations, calculations with presented kinetic
model have been performed in the following way: for every bound in the reaction specific confidence interval
k~
f ,i
~
  i , k f ,i   i

~
the ignition delay time has been calculated, where k f ,i is the rate constant of the ith reaction
of Table 2 in our kinetic model and
i
the corresponding deviation. This leads to 24 ignition delay time values per
experiment. The obtained uncertainty of ignition time ~ign have been calculated with the standard deviation formula
 
ign
1 24 ~
 ign  ~ign, j 2

24 j 1
and given values in the range of 1.8 – 57.4 % of uncertainty. It has been observed that the sensitivity is higher for
low temperature experiments and for small 0.5). Obviously, for experiments, which are often and long time
used for mechanisms validation, the uncertainty is lower, and that is higher for new measured data. An analysis of
new literature data for the rate coefficients of the most important reactions of CO/H2 combustion, Table 2, has been
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performed. The importance of these reactions has been determined from sensitivity analyses of ignition delay times
and laminar flame speeds measured at process parameters covered the range of interest for different time points of
processes. Finally, to adopt the new advances in H2/CO chemistry the following upgrades have been made.
Three new reactions have been added to the model:
HO2 + HO2  H2O2 + O2
O + OH+M  HO2 + M
H2 + O2  OH + OH
with rate coefficients taken from [29], and [27], respectively.
(1)
(2)
(3)
For the set of reactions,
2H + Ar/N2/H2/H2O/H  H2 + Ar/N2/H2/H2O/H
OH + H2  H2O + H
H + O2 (+ M/Ar/O2/H2O)  HO2 (+ M/Ar/O2/H2O)
H + HO2  H2 + O2
H + HO2  2OH
(4)
(5)
(6)
(7)
(8)
Table 2. Mean values and deviations for reaction rates calculated from data [10, 16,17, 24-28] at T=1000K
Reaction
H + O2 = OH + O
OH + H2 = H2O + H
H2 + O =OH + H
H+HO2 = H2 + O2
H2O2 + H = HO2 + H2
OH + OH (+M) =H2O2(+M)
H + O2 (+M) = HO2 (+M)
O2 + CO = CO2 + O
CO + O (+M) =CO2 (+M)
CO + OH =CO2 + H
CO + HO2 =CO2 + OH
HCO (+M) = H + CO (+M)
 %
Mean value
8,22E-14
2,12E-12
3,54E-13
2,92E-11
1,11E-12
3,05E-32
1,01E-32
1,29E-22
6,58E-34
2,55E-13
9,54E-16
5,83E-14
8,19
10,53
20,82
35,10
51,36
1,99
11,67
33,90
82,31
43,36
56,95
30,20
rate coefficients have been updated. For reaction (4), a value based on the recommendation in [30] has been applied.
For reaction (5), rate coefficients obtained from ab initio data presented in [31] have been adopted. A review of data
in the literature for reaction (6) resulted in the replacement of rate coefficients used in Leeds mechanism [6,7] with
recommendations provided in [23]. Values of reaction rates for (7) and (8) from [8] have been modified within the
range of their stated uncertainty: increased and decreased, respectively, by a factor of 0.3. For reaction
CO+HO2  CO2+OH
(9)
the recently obtained with ab initio study and master equation modelling [32] value of rate coefficient has been
adopted with factor 0.5. Reaction rate for the one of most important reaction for a laminar flame
CO+OH  CO2+H
(10)
has been changed to values from experimental work [33]. Reaction rate of
H+HCO  CO+H2
from [8] has been modified with factor 1.2.
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(11)
III. Results and Discussion
All computed data on
ignition delay times, concentration
profiles and laminar flame speed
have been obtained by means of
the SENKIN and PREMIX
program from the CHEMKIN II
package
[34].
The
sharp
temperature rise was taken as
ignition
criterion.
Computer
simulations of the laminar
premixed flames were performed
with thermal diffusion, for the
assumption of a free flame. Care
was taken in the computations to
reach final solution (no evolution
of laminar flame speed when the
number of mesh points is
increased); typically, around 500
mesh points were computed.
F u e l, p , T ,  = v a r
I g n it io n d e la y
S ij 
 ln  i
 ln p j
L a m in a r F la m e
fo r s e t o f t i m e p o i n t s
U n im p o rta n t r e a c ti o n s /s p e c ie s fo r a l l tim e p o in ts
S a ve o f u n im p o r ta n t r e a c ti o n /s p e c ie s fo r p ro c e ss
U n im p o rt a n t r e a c t io n s/ s p e c i e s fo r a ll p ro c e s s e s
Fig.1: Principal scheme for RedMaster code
Sensitivity and rate of production analysis were applied to compare the main reaction paths at low – and high
pressure methane combustion. The developed model have been reduced with RedMaster code [22] based on the
global sensitivity analysis, Fig.1. With this tool the input detailed mechanism ( 47 species and 311 reactions) was
reduced to skeletal model with 24 species, H2, CH4, C2H4, C2H6, O2, H2O, H2O2, CO, CO2, CH2O, H, CH2, CH2(S),
CH3, O, OH, HO2, HCO, CH3O, CH2OH, CH3O2, N2, and AR, and 103 reactions.
The presented mechanism has been validated applying the experimental data base collected in Table 3.
Unfortunately, there is only a very limited set of measured data for ignition delays and laminar flame speeds
obtained for low pressure conditions. Therefore, ignition delay data measured at pressure 1.76 - 2.40 bar behind
shock tube in [36] have been included in the validation data base.
Table 3. Experimental data base for low-pressure CH4 combustion.
No Pressure
1
0.5 atm
Composition Experimental data
CH4/ air
Laminar flame speed

T0,K
0.5 – 1.5
300
Ref.
Hassan et al., 1997 [35]
2 1.76 - 2.40 bar CH4/ O2/ Ar Ignition delay time
3
0.7- 0.9 atm CH4/ O2/ Ar Ignition delay time
1-2
1
1700 - 2200 Petersen et al., 2004 [37]
4
0.54 - 1.0 atm CH4/ H2/Air
Ignition delay time
0.5
1130 - 2000 Petersen et al., 2007 [38]
5
25 – 30 Torr CH4/ O2/ Ar
Concentration profiles 0.81– 1.28
400 - 2000
Berg et al., 2000 [39]
450 - 1800
Turbieza et al., 2004[40]
300
Ombrello et al., 2011
[41]
2
6
40 Torr
CH4/ O / Ar
Concentration profiles
1
7
0.16 atm
CH4/ air
Laminar flame speed
0.8 – 1.3
1500- 1800 Seery et al.,1970, [36]
Effects of positive flame stretch on the laminar burning velocities of methane/air flames were studied experimentally
in [35] for = 0.60 –1.35 and pressures of 0.5– 4.0 atm, at normal temperatures. In [41], experiments for methane-air
flames at 125 Torr from a Hencken burner have been performed to show their unique structure for detailed flame
studies. Flames were stabilized at a significant height above the burner surface with the properties of being steady,
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full model (47/311)
skeletal model (24/103)
T0= 300 K
50
40
30
20
10
0.50
Exp., Hassa et al., 1997, p=0.5 atm
Exp., Ombrello et al., 2011, p=0.16 atm
0.75
1.00
1.25
1.50

Fig. 2: Laminar flame speed versus equivalence ratio
of CH4/air flames for T0 = 298 K and p = 0.5 and
0.16 bar. Symbols – experimental data [35, 41], lines
– simulations with presented full and skeletal
mechanisms.
the data. Ignition delay times for pressures between
0.54 – 1.0 atm were measured in [37-38] behind
reflected shock waves. Ignition was deduced from end
wall
pressure
measurements
and
CH*
chemiluminescence [38 and from measurements a
combination of OH* and CH*chemiluminescence [37].
Comparison with these ignition delay data,
Figs.4a-b, are reasonable well. Models slightly overpredict the data [37, 38] for for temperature higher than
1800 K. To complete as much as possible the low
pressure methane oxidation data set, the CH and OH
concentration profiles have been simulated and
compared with those measured in [39] for extreme low
pressure conditions: 25 Torr.
laminar, nearly one-dimensional, minimally curved,
weakly stretched, and near adiabatic. The comparisons of
low pressure flame speed simulations with experimental
data [35 and 41] are presented on Fig. 2. Both, detailed and
reduced models describe generally satisfactory the
experimental data. Trends of the experimental results [35]
and simulations are similar, but for lean mixtures slight
over-predictions occur. Calculations of data [41] show the
trend, which slithly deviates from the experimental results
at the flame speed peaking . The calculated flame speeds
are in exellent agreement with data [41] for lean mixtures
and lower than the data for  > 1.0. Lastly, if a 10%
uncertainty is estimated and applied to the results of [35
and 41] the flame speed simulations appear to be in a
reasonable agreement with experimental data.
Figure 3 compares the predicted ignition times to those
measured [34] for stoichiometric and fuel-rich mixtures
with equivalence ratios between 1-2 and pressures ranging
from 1.76 - 2.40 bar. The simulation results agree well with
lgnition delay, s
Laminar flame velosity, cm/s
60
10
3
10
2
Exp. Seery et al.,1970  = 1, 2
p = 1.85 - 2.40 bar,
p = 1.76 - 1.83 bar
2 bar / 1.8 bar
/
pw, full mechanism
/
pw, skeletal mechanism
CH4/O2/Ar
5.5
6.0
6.5
-1
10000/T ,K )
10
3
10
2
7.0
Fig. 3: Comparison of modelled ignition delays of
CH4/O2/Ar mixtures with experimental data [36]. Lines
– simulations with presented full, skeletal mechanisms.
a
2400
2200
2000
1800
1600
1400
b
2400
CH4/O2/AR
2000
1800
1600
1400
Exp., Petersen et al., 2007
calc. full mechanism (47/311)
calc. skeletal mechanism (24/103)
1000
Exp.,Petersen et al., 2004: ign
Exp.,Petersen et al., 2004: max OH
Calc. full mechanism (47/311)
Calc.skeletal mechanism (24/103)
Ignition delay, mks
p = 0.8- 1.0 atm
 = 1.0
Ignition delay, mks
2200
1000
CH4/O2/N2
100
p = 0.54 - 0.92 atm
 = 0.36
100
0.50
0.55
0.60
0.5
1000/T, 1/K
0.6
0.7
1000/T, 1/K
Fig. 4: Comparison of modelled ignition delays of a) CH4/O2/Ar mixtures and b) CH4/O2/air with experimental data
[37, 38]. Lines – simulations with presented full and skeletal and short mechanisms.
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25
2000
p=25 Torr
1600
15
1400
1200
10
1000
5
800
0.01
OH mole fraction
CH Exp. [37]
CH, T, pw
CH, ppm
p=25 Torr
1800
=1.07 =1.28
Temperature, K
20
600
0
0.0
0.2
0.4 0.6 0.8 1.0 1.2
Height above burner, cm
1E-3
1E-4
OH Exp. Berg et al.,, 2000
OH, pw, full mechanism
OH, pw, skeletal mechanism
1E-5
400
1.6
1.4
=1.07 =1.28
0.0
0.2
0.4 0.6 0.8 1.0 1.2
Height above burner, cm
1.4
1.6
a
b
Fig. 5: Comparison of modelled a)CH and b)OH concentration profiles with measured in CH4/O2/N2 laminar premixed
flames data [39], p = 25 Torr, = 1.07 and = 1.28. Lines – simulations with presented full and skeletal mechanisms.
The CH and OH concentration profiles have been measured in rich and lean premixed laminar-flow methane
flames, using laser-induced fluorescence calibrated with a Rayleigh scattering technique for LIF calibration.
8.0x10
-2
6.0x10
-2
4.0x10
-2
Exp., Tubiez et al., 2004
calc. full mechanism (47/311)
calc. skeletal mechanism (23/103)
Mole fraction
Mole fraction
Simulations of CH and OH radical concentrations measured in laminar premixed flames [39] are shown in the
Fig.5. Although the data [39] have some uncertainties in experimental conditions, it was important for this study to
model these measurements, because CH and OH radicals are mostly used as indicators for ignition process
development. As Fig.5 shows, the proposed model reproduces satisfactory the CH and OH concentration changes.
The shifts along the flame length can follow from experimental errors.
CH4/O2/Ar laminar flame
2.0x10
p = 0.05 atm
 = 1.05
-2
CH4
0.0
0
a
6.0x10
5
10
15
Distance above the burner, mm
20
1.6x10
-1
1.2x10
-1
8.0x10
-2
4.0x10
-2
0.0
0
b
-2
3.0x10
-2
2.5x10
-2
2.0x10
-2
1.5x10
-2
1.0x10
-2
5.0x10
-3
2.0x10
-2
0.0
0
c
5
10
15
Distance above the burner, mm
20
H2
-2
Mole fraction
Mole fraction
CO
4.0x10
O2
5
10
15
Distance above the burner, mm
20
d
0
4
8
12
16
Distance above the burner, mm
20
Fig. 6: Comparison of modelled CH4, O2, CO and H2 concentration profiles with measured in CH4/O2/Ar
laminar premixed flame data [40], p = 0.05 atm, = 1.05. Lines – simulations with presented full and
skeletal mechanisms.
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E
xperimental mole fraction profiles of stable and reactive species have been obtained in [40] by coupling molecularbeam/mass spectrometry (MB/MS) with gas chromatography/mass spectrometry (GC/MS) analyses. The
0.2
0.08
H2O
Mole fraction
Mole fraction
0.06
CH4/O2/Ar laminar flame
p = 0.05 atm
 = 1.05
0.1
Exp., Tubiez et al., 2004
calc. full mechanism (47/311)
calc. skeletal mechanism (23/103)
0.04
CO2
0.02
0.00
0.0
0
a
5
10
15
Distance above the burner, mm
20
0
b
5
10
15
Distance above the burner, mm
20
0.0004
0.004
C2H4
Mole fraction
Mole fraction
0.003
0.002
C2H6
CH2O
0.0002
0.0001
0.001
c
0.0003
0.0000
0.000
0
5
10
15
20
d
0
2
4
6
8 10 12 14 16
Distance above the Burner, mm
18
20
Distance above the Burner, mm
Fig. 7: Comparison of modelled H2O, CO2, C2H6, CH2, C2H4 concentration profiles and measured CH4 /O2 /Ar
laminar premixed flame data [40], p=0.05 atm, Ф=1.05. Lines – results of full and skeletal mechanism.
flames have been stabilized on a flat flame burner at a very low pressure (40 Torr). Temperature profiles were
measured with a coated thermocouple in the sampling conditions. The work provides a detailed experimental data
set on the nature and concentrations of the stable and reactive species produced by oxidation of various
representative natural gas mixtures.
In the Figures 6 - 8 the simulated and experimental concentration profiles of the reactants, stable intermediates,
reactive radicals and final products in laminar stoichiometric premixed CH4/O2/Ar flame [40] obtained at a pressure
0.05 atm are shown. As can be seen from these figures, the model represents fairly well the data for the methane
oxidation. The present modeling is in excellent agreement with the measurements for stable molecules and CO,
Figs.6 and 7, and in reasonable agreement with data for radicals, Fig.8. So, measured values for OH and H radicals
are slightly undepredicted, Fig.8 a, b, whereas the discreepamce (an underprediction) between simulations and
experimental data for CH3 and HCO radicals is a factor 2.
Finally, one can conclude, the proposed simulation results obtained with the developed chemical kinetic scheme
demonstrate an overall good agreement with experimental data measured at low pressure conditions in different
kinetic reactors using different techniques. A reasonable progress has been made in the extension of the core C1
submechanism to describe the low pressure methane oxidation. Although it is not possible to present the analyses of
the mechanism for all of the validation cases, we performed analyses forselected conditions to get sample of those
details that are characteristic of the mechanism and to compare this details for low and high pressure combustion.
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American Institute of Aeronautics and Astronautics
-4
1.0x10
-4
8.0x10
-5
6.0x10
-5
4.0x10
-5
2.0x10
-5
0.015
HCO
Mole fraction
Mole fraction
0.020
1.2x10
H
0.010
0.005
0.000
0.0
a
0
2
4
6
8
10
12
14
16
18
0
20
2
4
6
8
10
12
14
16
18
20
Distance above the Burner ,mm
b
Distance above the Burner, mm
0.010
0.004
CH4/O2/Ar laminar flame
p = 0.05 atm
 = 1.05
0.006
Mole fraction
Mole fraction
0.008
OH
0.004
Exp., Tubiez et al., 2004
calc. full mechanism (47/311)
calc. skeletal mechanism (23/103)
0.002
5
10
15
Distance above the Burner, mm
0.003
0.002
0.001
0.000
0
CH3
0.000
20
0
5
10
15
20
d
c
Distance above the Burner, mm
Fig. 8: Comparison of modelled OH, H, HCO, CH3 concentration profiles with measured CH4/O2/Ar
laminar premixed flame data [40], p=0.05 atm, Ф=1.05. Lines – results with full and skeletal mechanism.
HO2 OH O
O2
CH3
O2
H
M
CH4
CH3
O2
CH3O
C2H6 C2H5
H2/O2 subsystem
CH2O
C2H4
C2H3
C2H2
C3H4 , C4H4
CH2HCO
HCO
CH2OH
CH3OH
CH2CO
C3H3 , C3H5
CO
PAH
CH3O2
CH2
CO2
A simplified scheme summarizing the
main reactions paths of the methane
oxidation
realised
in
reaction
mechanism is presented in Fig. 9. The
sensitivity analyses performed for the
ignition delays indicate that the model
results regarding OH mole fractions are
not sensitive to the pressure, Fig. 10.
The computed sensitivity coefficients
for OH concentation have been
calculated for similar temperature and
mixture ratio conditions for two
different pressures, 0.74 and 10.9 atm.
In the conditions of Fig. 10, the same
termination or pseudo-termination
reactions inhibit methane oxidation and
the same propagation and branching
reactions favour methane oxidation.
Fig. 9: The main reactions paths of the methane oxidation realised in
reaction mechanism.
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American Institute of Aeronautics and Astronautics
T0 = 1526.0 K
O2+CH3<=>CH2O+OH
O2+CH2O<=>HCO+HO2
HCO+M<=>H+CO+M
H+O2<=>OH+O
CH4+OH<=>CH3+H2O
CH4+O2<=>CH3+HO2
CH4+H<=>CH3+H2
CH3+OH<=>CH2(S)+H2O
CH3+O2<=>CH3O+O
CH3+HO2<=>CH3O+OH
CH3+HCO<=>CH4+CO
CH2O+OH<=>HCO+H2O
CH2O+CH3<=>CH4+HCO
2CH3<=>C2H5+H
2CH3(+M)<=>C2H6(+M)
-4
-3
p = 0.74 atm
-2
-1
0
1
2
3
4
ln(Sensitivity of the concentration of OH)
a
T0 = 1535.0 K
p = 10.9 atm
O2+CH3<=>CH2O+OH
O2+CH2O<=>HCO+HO2
HCO+M<=>H+CO+M
H+O2<=>OH+O
CH4+OH<=>CH3+H2O
CH4+O2<=>CH3+HO2
CH4+H<=>CH3+H2
CH3+O2<=>CH3O+O
CH3+HO2<=>CH3O+OH
CH3+HCO<=>CH4+CO
CH2O+OH<=>HCO+H2O
CH2O+CH3<=>CH4+HCO
C2H6+OH<=>C2H5+H2O
2CH3<=>C2H5+H
2CH3(+M)<=>C2H6(+M)
b
-4
-3
-2
-1
0
1
2
3
ln(Sensitivity of the concentration of OH)
4
Fig. 10: Comparison of ignition delay (OH conc.) sensitivity coefficients to reaction rates at low and
high pressure conditions. a) p = 0.74 atm, T0 = 1526 K, Ф = 0.36; b) p = 10.9 atm, T0 = 1535 K, Ф =0.5.
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American Institute of Aeronautics and Astronautics
IV. Conclusions and Outlook
The chemical kinetic model of the low pressure methane combustion was developed on the base of well
validated thermo-chemical data as the extension in the common hydrocarbon combustion mechanisms. The scheme
is able to reflect the main properties of methane combustion and describes satisfactory good experimental data for
ignition delays and laminar flame speed as well as concentrations of the reactants, stable intermediates, reactive
radicals and final products.
Starting from an input model with 47 species and 311 reactions a skeletal mechanism with 24 species and 103
reactions was produced without any loss of predictive capabilities compared to the original scheme. Both detailed
and reduced models describe satisfactory experimental data under conditions: 0.02 MPa < p < 0.1 MPa, 900 K < T
< 1800 K and 0.5 < Ф < 3.0.
The performed model sensitivity analysis did not reveal the principal difference between low pressure and
high pressure methane high temperature oxidation reaction paths.
Furthermore, the developed mechanism could easily be extended to become the base mechanism for studies
of low temperature, low pressure reactions in the exhaust plume of a CH4/LOX rocket engine of a launch vehicle in
the strato- and mesosphere. In this case however, the mechanism would have to be expanded to include also
reactions of nitrogen and its derivates to enable for post combustion processes of the exhaust gases with the
surrounding air. For applications to low temperature long-time reactions in the outer atmosphere, the mechanism
would have to expanded to include radiation activated reactions and additional important molecules such as ozone.
Acknowledgments
Part of this work was performed within the “ ISP-1” project, coordinated by SNECMA, and supported by the
European Union within the 7th Framework Program for Research & Technology. (Grant agreement N° 218849.)
Lots of thanks to Dr. Eric L. Petersen and Dr. Timoty M. Ombrello for the sent experimental data.
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