HAL Id: hal-01591019
https://hal.archives-ouvertes.fr/hal-01591019
Submitted on 20 Sep 2017
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entic research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diusion de documents
scientiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
Eects of pressure and carbon dioxide, hydrogen and
nitrogen concentration on laminar burning velocities and
NO formation of methane-air mixtures
Peyman Zahedi, Kianoosh Youse
To cite this version:
Peyman Zahedi, Kianoosh Youse. Eects of pressure and carbon dioxide, hydrogen and nitrogen
concentration on laminar burning velocities and NO formation of methane-air mixtures. Journal of
Mechanical Science and Technology, 2014, 28 (1), pp.377-386. �10.1007/s12206-013-0970-5�. �hal-
01591019�
J
ournal of Mechanical Science and Technology 00 (2013) 0000~0000
www.springerlink.com/content/1738-494x
E
ffects of Pressure and Carbon Dioxide, Hydrogen and Nitrogen Concentration on
Laminar Burning Velocities and NO formation of Methane–air Mixtures
†
Peyman Zahedi
1,*
and Kianoosh Yousefi
1
1
Department of Mechanical Engineering, Mashhad Branch, Islamic Azad University, Mashhad, Iran
(Manuscript Received 000 0, 2013; Revised 000 0, 2013; Accepted 000 0, 2013)
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Abstract
In this research we have studied the effects of increasing pressure and adding carbon dioxide, hydrogen and nitrogen to Methane-air
mixture on premixed laminar burning velocity and NO formation in experimentally and numerically methods. Equivalence ratio was
considered within 0.7 to 1.3 for initial pressure between 0.1 to 0.5 MPa and initial temperature was separately considered 298 K. Mole
fractions of carbon dioxide, hydrogen and nitrogen were regarded in mixture from 0 to 0.2. Heat flux method was used for measurement
of burning velocities of Methane-air mixtures diluted with CO
2
and N
2
. Experimental results were compared to the calculations using a
detailed chemical kinetic scheme (GRI-MECH 3.0). At first, the results in atmosphere pressure for Methane-air mixture were calculated
and it was compared with the results of literature. Results were in good agreement with published data in the literature. Afterwards, by
adding carbon dioxide and nitrogen to Methane-air mixture, we witnessed that laminar burning velocity was decreased, whereas by
increasing hydrogen, the laminar burning velocity was increased. Finally, the results showed that by increasing the pressure, the premixed
laminar burning velocity decreased for all mixtures and NO formation indicates considerable increase, whereas the laminar flame
thickness decreases.
Keywords: Burning velocity; Dilution; Heat flux; Laminar; Methane-air mixture; NO formation; Pressure
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1. Introduction
With the depletion of crude oil reserves and the
strengthening of automotive emission legislations, the use of
natural gas (NG) as an alternative fuel has been promoted both
in combustion engines and power generation. Natural gas
contains mainly methane (typically 65-90 percent or more by
volume) along with higher hydrocarbons, inert gaseous
components like N
2
, CO
2
, water vapor and trace compounds.
The composition of natural gas varies widely from one source
to another in terms of the fractions of higher hydrocarbons
summarized as ‘‘C2+” gases (at present vary from 7 to 16
percent) and inert gaseous components like N
2
, CO
2
(at
present 20-25 percent maximum). This variation of both C2+
and inerts is expected to widen in the future [1]. Hence interest
in fuel-flexible gas turbine engines led to research on
premixed combustion parameters like laminar burning
velocity and ignition delay time. Fuel flexibility can impact
several important premixed burner design issues such as
flashback, blow off, auto ignition and stability.
Laminar burning velocities are important because they are
related not only to flashback and blow off issues, but they also
play a role in the stability of the flame in the combustor [2] of
course it will face with some problems such as: low thermal
efficiency or lean combustion capability [3]. The combustion
of lean hydrocarbon-air mixtures offers the potential of
reduced flame temperatures and
NO
X
emissions according to
the thermal mechanism. But there are two separated
deficiencies in getting use of that; the first one is significant
decrease of laminar burning velocity and the second one
increase of incomplete combustion. Many attempts have been
made to solve these two problems and one of the most
effective solutions has been fuel enrichment [4]. Laminar
burning velocity is important, because not only it is related to
feedback of the flame and combustion, but also it plays an
important role in stability of the flame in the combustion, and
it is considered to be a criterion for providing the details of the
reaction mechanism. Fast burning causes decrease in
combustion duration, increase in thermal efficiency and
decrease in fuel consumption [5].
There exist three effects of diluents on the laminar burning
velocity (including dilution, thermal-diffusion, and chemical
effects). The dilution effect is that, when diluent is added, the
percentage of fuel and oxidant in the mixture is decreased,
which decreases the laminar burning velocity. The thermal-
diffusion effect is that, when diluent is added, the specific heat
capacity and thermal diffusivity of the mixture vary and affect
the laminar burning velocity. The chemical effect is that the
*
Corresponding author. Tel.: +989151024207, Fax.: +985118812422
E-mail address: peimanzahedi@gmil.com
†
Recommended by Editor 000 000-
© KSME & Springer 2013
Author's Proof
0000 P. Zahedi and K. Yousefi / Journal of Mechanical Science and Technology 00 (2013) 0000~0000
diluent will participate in chemical reactions and affect the
laminar burning velocity [6].
There are two types of methods used for determining
laminar velocity: the stationary flame methods and moving
flame methods. In the former category of methods, the flame
front remains stationary in space, Bunsen burner method, heat
flux method etc. In the latter, the flame moves with respect to
some fixed point (i.e., the point of ignition): soap bubble
method, constant volume method, constant pressure method
etc [7, 8]. The burning velocity directly affects the flame
propagation speed and hence, the operation of the SI engine.
Faster burning in SI engines leads to a more robust and
repeatable combustion and permits engine operation with
substantially larger amount of exhaust gas recirculation (EGR),
bringing the reduction in NO
X
emission [9]. Enrichment of
Methane fuel by hydrogen, nitrogen and carbon dioxide has
been investigated by many researchers through experimental
methods [10, 11], and the studies have indicated that laminar
burning velocity of Methane-Hydrogen mixture will be
increased, by increase of hydrogen fractional ratio [11, 12].
This paper is an experimental research indicating the effect of
dilution of Methane-air mixture with N
2
, CO
2
and also
numerically with H
2
on flame burning velocity. Also the effect
of pressure has been identified in these diluted mixtures.
Moreover, NO formation under these different conditions has
been investigated numerically.
2. Experimental Setup
The Heat Flux method for the stabilization of adiabatic
premixed laminar flames on a flat flame burner has been
proposed by Bosschaart et al. [13]. This method was
extensively used for measuring laminar burning velocities
of gaseous fuels [14-16] and has recently been extended
towards liquid fuels [17]. The designed burner is consisting
of three separated parts which are overlapped on each other
and have formed one unique burner. These three parts are as
the following: Burner Head, Burner Plate and Plenum
chamber
as shown in Fig. 1. The brass burner plate (3 mm
of thick) on which the flame is stabilized is a perforated
disc with a hexagonal pattern of 0.5 mm diameter holes
with 0.7 mm pitch as shown in Fig. 2. It has been shown
that this kind of pattern stabilizes a flat flame on the burner
[18].
An essential ingredient in the heat flux method is the
attachment of thermocouples to the burner plate to
determine the temperature distribution. Five K-type
thermocouples of 0.4 mm wire were attached into selected
holes using special glue on perforated plate on the upstream
side. The thermocouples were positioned at different radii
and different circumferential locations to measure the
temperature profile on the burner plate. The upper half of
the burner head has a heating jacket and a Teflon insulation
ring that separates it from the lower half with cooling jacket.
The burner head has a heating jacked supplied with
thermostatic water to keep the temperature of the burner
plate constant. The heating jacket, water temperature was
maintained at 358 K and the plenum chamber has a separate
cooling system supplied with water at room temperature
(around 307 K). This fixes the initial temperature of the
fresh gas mixture. The heating jacket keeps the burner plate
edges at a certain temperature higher than the initial gas
temperature, thus warming up the (unburned) gases flowing
through. Conductive heat transfer of the flame to the burner
plate cools the gas flow on its turn. If the flame is stabilized
under sub-adiabatic conditions, the gas velocity is lower
than the adiabatic flame burning velocity and the sum of the
heat loss and heat gain is higher than zero. Then,
the center
of the burner plate is hotter than the heating jacket. If the
unburned gas velocity is higher than the adiabatic burning
velocity (super-adiabatic conditions), the net heat flux is
lower than zero and the center of the burner plate is cooler
than the heating jacket.
Fig. 1. Schematic of heat flux set up.
Fig. 2. Burner plate with a hexagonal pattern with d=0.5 mm
and P=0.7 mm.
By varying the flow rate of the gas mixture, an
appropriate value of gas velocity could be found where net
heat flux is zero. This will manifest in the form of uniform
radial distribution of temperature. The flow velocity at
which the net heat flux is zero is shown to be adiabatic
burning velocity. The temperature distribution of the burner
plate is measured with the thermocouples attached to it and
radial temperature profile of the plate obtained by solving
the energy Eq. (1)
2
4
)( r
x
q
TrT
p
centerp
λ
−=
(1)
Author's Proof
0000 P. Zahedi and K. Yousefi / Journal of Mechanical Science and Technology 00 (2013) 0000~0000
Here T
p
is the temperature profile across the burner plate,
T
center
is the plate center temperature, q is the net heat flux
into the plate,
λ
is the thermal conductivity of burner plate,
x
p
is the plate thickness and r is the radial coordinate. Eq.
(1) is expressed in the following general form:
2
)( arTrT
centerp
+=
(2)
T
he coefficient a depends on the unburned gas velocity v.
By plotting a against v, the adiabatic burning velocity S
L
can be found by interpolation to a=0, as described by Boss-
chaart [12].
3. Numerical
Method
T
he PREMIX code
[19] was used to calculate laminar
flame velocities and to compare to our experimental results.
PREMIX uses a hybrid time integrating/ Newton iteration
technique to solve the steady state mass, species and energy
conservation equations and can simulate the propagating
flame. Equations were solved by using the TWOPNT, a
boundary value problem solver in the CHEMKIN package
[20].
One of the critical elements for simulation is the proper
reaction mechanism that can describe the essential
fundamental reaction paths followed by the overall reaction.
The chemical kinetic mechanism used is the GRI-MECH
mechanism. GRI-MECH is an optimized detailed chemical
reaction mechanism for the calculation of natural gas
chemical reaction process and the latest version is GRI-
MECH 3.0 [21]. GRI-MECH 3.0 consists of 325
elementary chemical reactions with associated rate
coefficient expressions and thermo chemical parameters for
53 species. It includes the detailed combustion reaction
mechanism for hydrogen. The ranges of GRI-MECH 3.0 are
1000-2500 K in temperature, 10 Torr to 10 atm in pressure
and 0.1-5.0 in equivalence ratio. The initial flow rate of the
unburned mixture was chosen equal to 0.04 g/cm
2
s,
according to the measurement of stoichiometric methane/air
flame speed by Egolfopoulos et al. [22].
To start the
iteration the temperature profile estimation obtained by Van
Maaren et al. [14] for a stoichiometric methane/air flame
was adopted, as suggested by Uykur et al. [23]. The
temperature profile resulting from the first simulation step
was used for the next step. At the inlet boundary
temperature (298 K), pressure (0.1, 0.3 and 0.5 MPa) and
composition of the unburned mixture were assigned. At the
exit boundary it was specified that all gradients vanish. It is
observed that by using adaptive grid parameters
GRAD=0.02 and CURV=0.1, the burning velocity obtained
is grid independent. Hydrogen mole fractions, nitrogen and
carbon dioxide (from 0 to 20 percent) with other
components existing in unburned mixtures of Hydrogen-
Methane/air, Nitrogen-Methane/air and Carbon Dioxide-
Methane/air will be estimated. The length of calculations
have been regarded 2 cm before the spot of reaction or
generally equal to 12 cm. The conservation of mass is
expressed by the general continuity equation [10]:
0).( =∇+
∂
∂
u
t
ρ
ρ
(3)
W
here,
ρ
is the mixture mass density and u is the gas
mixture velocity. The conservation of momentum, with no
body forces other than gravity, is covered by
guu
t
ρρ
ρ
+Π∇=∇+
∂
∂
.).(
(4)
W
here
Π
is the stress-tensor and g is the acceleration due
to gravity. The stress-tensor consists of a hydrodynamic and
viscous part:
τ
+−=Π pI
, in which p is the pressure,
I the
unit tensor and
τ
the viscous stress-tensor. The equation
describing the conservation of energy is written in terms of
specific enthalpy h,
qupu
t
p
uh
t
h
.)(:.).( ∇−∇+∇+
∂
∂
=∇+
∂
∂
τρ
ρ
(5)
W
here q is the total heat flux. The term
)(: u∇
τ
represents
the enthalpy production due to viscous effects. When
chemical reactions are to be considered, conservation
equations for the species mass fractions Y
i
are used. They
are defined as
ρρ
/
ii
Y =
with
i
ρ
the density of species i.
The density of the mixture
ρ
is related to the density of the
various species by
∑
=
=
s
N
i
i
1
ρρ
(6)
w
ith N
s
the number of species. This leads to a conservation
equation for every species mass fraction in the mixture [10]:
],1[,).().(
siiii
i
NiYUuY
t
Y
∈=∇+∇+
∂
∂
ωρρ
ρ
(7)
With U
i
is the diffusion velocity of species i. The chemical
source term
i
ω
in this equation, is characteristic for the reac-
tive nature of the flow. Note that Eq. (7) together with the
continuity Eq. (3) gives an over-complete system, so instead
of N
s
only N
s
-1 equations in Eq. (7) have to be solved. The
mass fraction of one of the species can be computed using
the following constraint:
1
1
=
∑
=
s
N
i
i
Y
(8)
An abundant species, e.g. nitrogen, is commonly chosen
for this species. By definition chemical reactions are mass
conserving, so therefore the following relations hold:
0
1
=
∑
=
s
N
i
ii
UY
ρ
(9)
0
1
=
∑
=
s
N
i
i
ω
(10)
Finally, state equations are needed to complete the set of
differential equations (4) - (7). The first state equation in-
troduces the specific enthalpy h as a function of temperature
T. This relation is given by
Author's Proof
0000 P. Zahedi and K. Yousefi / Journal of Mechanical Science and Technology 00 (2013) 0000~0000
dTTchhwithhYh
T
T
p
ref
iii
N
i
i
ref
i
s
)(
1
∫
∑
+==
=
(11)
And holds for perfect gases. In this equation h
i
represents
the enthalpy of species i and
ref
i
h
the formation enthalpy of
species i at a reference value for the temperature
ref
T
and
i
p
c
the specific heat capacity at constant pressure of species
i. The mixture heat capacity is defined by
i
s
p
N
i
ip
cYc
∑
=
=
1
(12)
The species heat capacity c
pi
is commonly tabulated in
polynomial form. In most combustion problems the mixture
and its components are considered to behave as perfect
gases. The ideal-gas law relates the density, temperature
and pressure to each other by
RT
Mp
=
ρ
(13)
With R the universal gas constant and
M
the mean molar
mass. This
M
can be determined from
1
1
−
=
=
∑
s
N
i
i
i
M
Y
M
(14)
Where
i
M
is the molar mass of species i.
4. Va
lidation of Experimental Setup
In this section, initial experiments done on methane–air
and to validate the experimental facility built are presented.
There is literature abundant available on unstretched lami-
nar burning velocity of methane for validation of the setup
[24-27].
Fig.
3. Laminar burning velocities plotted versus equivalence ratio for
CH4/air mixtures at P
ini
=0.1 MPa and T
ini
=298 K. Line represent calcu-
lations with CHEMKIN and Symbols represent experimental data.
The present experimental and numerical results of adiabatic
burning velocities of methane-air are plotted in Fig. 3, some
experimental data of previous studies are mentioned in this
figure which have been conducted in different methods.
Fig. 4. Flame photo and temperature distribution of methane-air
mixture
Fig. 5. Flame photo temperature distribution of diluted methane-air
mixture with nitrogen
Flame photos of methane-air mixture and diluted mixture
with nitrogen at
1=
φ
besides of their temperature
distribution data in burner plate are shown in Figs. 4 and 5.
As mentioned before, the velocities that are assumed lower
than adiabatic velocities are defined as sub-adiabatic
conditions, for instance 32 and 33 cm/s burning velocities in
Fig. 4 and the velocities which are higher than adiabatic
velocity are under super-adiabatic conditions, for example
37 cm/s burning velocity in Fig. 4.
Author's Proof
