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TFC modeling of hydrogenated methane premixed combustion

Authors:
  • Université Oum El Bouaghi Larbi Ben Mhidi

Abstract and Figures

The use of hydrogenated fuels shows considerable promise for applications in gas turbines and internal combustion engines. Hydrogen addition to methane will have an important role to reach a fully developed hydrogen economy. The effects of this addition on the flame structure is evaluated in three fuels with the following compositions at constant global equivalent ratio: 100 % CH 4 , 10 % H 2 + 90 % CH 4 , 20 % H 2 + 80 % CH 4 . The turbulence is modeled by the standard k -ε model which is improved by the Pope correction in order to better predict round jet spreading. Combustion is modeled by the turbulent flame closure (TFC) model which is used with flamelet to give detailed chemistry. Computations were achieved by the ANSYS CFX code. A good agreement with experiments was found, it was noted that one can replace a significant fraction of basic fuel by hydrogen without making recourse to major modifications on the installations. Résumé -L'utilisation de combustibles hydrogénés est très prometteuse pour des applications dans les turbines à gaz et les moteurs à combustion interne. L'hydrogène en addition avec le méthane aura un rôle important pour arriver à une économie en hydrogène pleinement développé. Les effets de cet ajout sur la structure de la flamme est évalué à trois carburants à la suite de compositions à taux constant global équivalent: 100 % CH 4 , 10 % H 2 + 90 % CH 4 , 20 % H 2 + 80 % CH 4 . La turbulence est modélisé par le modèle standard k -ε, qui est inspiré par la correction de Pope, afin de mieux prédire la propagation du jet. La combustion est modélisée par le modèle de la fermeture de la flamme turbulente (TFC), qui est utilisé avec des flamelets de façon détaillée en chimie. Les calculs ont été réalisés par le code ANSYS CFX. Un bon accord a été trouvé avec les expériences. Il est à noter que l'on peut remplacer une fraction importante du carburant de base par de l'hydrogène sans recourir à des modifications importantes sur les installations.
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Revue des Energies Renouvelables CISM’08 Oum El Bouaghi (2008) 227 - 237
227
TFC modeling of hydrogenated methane premixed combustion
A. Mameri1*, A. Kaabi2 and I. Gökalp3
1 Institut des Sciences Technologiques, Centre Universitaire Larbi Ben M’Hidi, Oum El Bouaghi, Algérie
2 Département du Génie Climatique, Université des Frères Mentouri, Constantine, Algérie
3 Institut de Combustion, Aérothermique, Réactivité et Environnement, ‘ICARE’, CNRS, Orléans, France
Abstract - The use of hydrogenated fuels shows considerable promise for applications in
gas turbines and internal combustion engines. Hydrogen addition to methane will have an
important role to reach a fully developed hydrogen economy. The effects of this addition
on the flame structure is evaluated in three fuels with the following compositions at
constant global equivalent ratio: 100 % CH4, 10 % H2 + 90 % CH4, 20 % H2 + 80 %
CH4. The turbulence is modeled by the standard k -
ε
model which is improved by the
Pope correction in order to better predict round jet spreading. Combustion is modeled by
the turbulent flame closure (TFC) model which is used with flamelet to give detailed
chemistry. Computations were achieved by the ANSYS CFX code. A good agreement with
experiments was found, it was noted that one can replace a significant fraction of basic
fuel by hydrogen without making recourse to major modifications on the installations.
Résumé - L’utilisation de combustibles hydrogénés est très prometteuse pour des
applications dans les turbines à gaz et les moteurs à combustion interne. L’hydrogène en
addition avec le méthane aura un rôle important pour arriver à une économie en
hydrogène pleinement développé. Les effets de cet ajout sur la structure de la flamme est
évalué à trois carburants à la suite de compositions à taux constant global équivalent:
100 % CH4, 10 % H2 + 90 % CH4, 20 % H2 + 80 % CH4. La turbulence est modélisé par
le modèle standard k -
ε
, qui est inspiré par la correction de Pope, afin de mieux prédire
la propagation du jet. La combustion est modélisée par le modèle de la fermeture de la
flamme turbulente (TFC), qui est utilisé avec des flamelets de façon détaillée en chimie.
Les calculs ont été réalisés par le code ANSYS CFX. Un bon accord a été trouvé avec les
expériences. Il est à noter que l’on peut remplacer une fraction importante du carburant
de base par de l’hydrogène sans recourir à des modifications importantes sur les
installations.
Keywords: Premixed flames - Hydrogenated fuels - Burning velocity - Turbulent flame
speed.
1. INTRODUCTION
Internal combustion engine and gas turbine manufacturers are faced with stricter
anti-pollution regulations. Lean premixed combustion is a well established technique to
achieve low emissions while maintaining high efficiency. According to the thermal
x
NO generation mechanism; low flame temperatures given by lean conditions, results
in low x
NO emissions. However, close to lean flammability limits, the stability of the
flame decreases and flame extinction phenomena may occur.
Since further reduction of x
NO will require even leaner mixtures, schemes for lean
stability extension must be considered. A solution to increase the flame stability at lean
condition is to add small amounts of hydrogen into the mixtures [1-4]. Several studies
* mameriabdelbaki@yahoo.fr
A. Mameri et al.
228
have been performed to estimate the impact of 2
H addition on the stability, on the
reactivity and on the pollutant emissions of the methane-air flames.
It has been shown that hydrogen addition, at constant global equivalence ratio,
extends the lean operating limit of natural gas engines, leading to a potential decrease in
pollutant formation. The origin of this effect is that the stretch resistance of these flames
is considerably increased by hydrogen blending, while other properties are
comparatively little modified.
In spark ignited engine [5], the authors show that mixture of natural gas blended
with hydrogen improves thermal efficiency, specific fuel consumption, reduces CO
and x
NO emissions, extend lean operating limit to lower equivalence ratios and finally
allows stable engine operation with lower pollutant emissions.
Hydrogen addition was experimentally tested in several atmospheric flame burners’
configurations. It was observed a decreasing in the height of the blue cone with
hydrogen addition. The increase in laminar burning velocity was identified as the main
effect of the behavior of this parameter. Also a significant reduction in CO emission
was obtained [6].
Experimental investigation on the flame stability of hydrogenated mixtures was also
performed for swirl stabilized flame configuration [7].
We perform a numerical study to understand and to complete F. Halter et al. [8]
measured data. The authors conducted experiments on a lean methane combustor at
several pressures with hydrogen enrichment.
2. EXPERIMENTAL SETUP
The ICARE (Institut de Combustion, Aérothermique, Réactivité et Environnement)
high pressure turbulent flame facility is composed from stainless steel cylindrical
combustion chamber (Fig. 1). The inner chamber diameter is 300 mm. The chamber is
composed of two superposed vertical portion each of 600 mm height, and each equipped
with four windows of 100 mm diameter for optical diagnostics.
Fig. 1: ICARE combustion installation Fig. 2: Bunsen burner
An axisymmetric bunsen type turbulent burner (Fig. 2) is located inside the
chamber. The burner internal diameter is 25 mm. It is fed by methane/hydrogen/air
mixtures.
CISM’2008: TFC modeling of hydrogenated methane premixed combustion
229
The global equivalence ratio is 0.6, the bulk mean flow velocity of the reactants, at
the exit is 2 m/s. The reactants flow exhibits a turbulence level about 10 %. The integral
length scale is about 3 mm.
3. COMPUTATIONAL MODEL
To gain a more complete understanding of the impact of 2
H addition to lean 4
CH
flames, the Ansys CFX 11 code is used to compute the multi component turbulent
reacting flow. The Reynolds averaged equations are given by:
Continuity
0)U(
t=ρ+
ρ (1)
Momentum
{}
M
'' S)uu()UU(
t
U+ρτ=ρ+
ρ (2)
Energy
h
''
tot
tot S)huT()hU(
t
p
t
h+ρλ=ρ+
ρ (3)
Where kU
2
1
hh 2
tot ++=
The other equations are similar and they can be cast in the following general form:
φφ +φρφΓ=φρ+
φρS)u()U(
t
'' (4)
Where φ is a general scalar.
The perfect gas state equation is given by:
ρ=
jj
j
W
Y
TRp (5)
The Reynolds stresses '' uu ρ and fluxes ''
uφρ represent the convective effect
of turbulent velocity fluctuations. These terms need to be modelled.
3.1 εk turbulence model
Today, even with the successful development of DNS and LES for turbulent flows,
the most popular models for industrial modeling are the two-equation Reynolds
averaged Navier-Stokes (RANS) models. Of these, the
ε
k two equations model
accounts for 95 % or more of the industrial use at the present [9]. This form of model is
easy to solve, converges relatively quickly, is numerically robust and stable, is able to
solve large domains and high Reynolds numbers and requires minimal computational
expense, which is important for industrial models. The standard
ε
k model with the
standard constants predicts the velocity field of a two dimensional plane jet quite
accurately, but results in large errors for axisymmetric round jets. Although the standard
εk model matches the spreading rate of the round jet more accurately than other two
equations models, it still overestimates it by 15 % [10]. In this model, the Reynolds
stresses are given by a Newtonian type closure which looks like:
A. Mameri et al.
230
)U.k(
3
2
))U.(U.(uu t
T
t
'' µ+ρδ+µ=ρ (6)
Where t
µis the turbulence ‘viscosity’ (also called the eddy viscosity). By analogy with
the turbulence viscosity, the turbulence diffusivity is defined, and the Reynolds fluxes
are given by:
φΓ=φρ..u t
'' (7)
Where t
Γ is the turbulence ‘diffusivity’, it is related to the turbulence viscosity by:
t
t
tPr
µ
=Γ (8)
Where t
Pr is the turbulent Prandtl number.
In the
ε
k model, the turbulent viscosity is computed by the relation:
ε
ρ=µ µ
2
t
k
..C (9)
The
k
and
ε
equations are given by:
ερ+
σ
µ
+µ=ρ+
ρ
k
k
tPk.)kU(
t
k (10)
()
ερ
ε
+
ε
σ
µ
+µ=ερ+
ερεε
ε2k1
tCPC.
k
.)U(
t (11)
To tailor the εk model for solving round jet flows, McGuirk and Rodi [11],
Morse [12], Launder et al. [13], and Pope [10] suggested modified turbulence model
constants. The best correction in our case is the Pope’s one; it is a new source term
added to the ε equation. It is given by the following relation:
p
2
3pope .
k
..CS χ
ε
ρ= ε (12)
With
ε
=χ y
v
x
u
.
x
v
y
u
.
k
4
12
3
p (13)
for two dimensional axisymmetric geometry.
In this work, a limited version of the Pope correction introduced by Davidenko [14]
is adopted and incorporated in the CFX code. Its expression is:
()
)(sign,min.
k
..CS plimp
2
3pope χχχ
ε
ρ= ε (14)
With lim
χ, a limiting value of p
χ
.
3.2 Combustion model
The model for premixed or partially premixed combustion can be split into two
independent parts:
• Model for the progress of the global reaction: Burning Velocity Model (BVM),
also called Turbulent Flame Closure (TFC);
CISM’2008: TFC modeling of hydrogenated methane premixed combustion
231
• Model for the composition of the reacted and non-reacted fractions of the fluid:
Laminar Flamelet with PDF.
Reaction Progress
A single progress variable c
is used to describe the progress of the global reaction:
iburndifreshi Y
.c
Y
.)c
1(Y
+= (15)
The reaction progress variable is computed by solving a transport equation:
c
jc
t
jj
i
x
c
~
.D.
xx
)c
~
u
~
(
t
c
~ω+
σ
µ
+ρ
=
ρ
+
ρ (16)
The burning velocity model (BVM), also known as turbulent flame closure (TFC), is
used to close the combustion source term for reaction progress.
ρ
ρ=ω
jj
Tuc x
c
~
.D.
x
c
~
..S. (17)
Where the turbulent flame velocity is given by:
4/l
t
4/1
u
2/1
L
4/3'
Tl..S.u.G.AS
λ= (18)
And the stretch factor:
σ
+
ε
ε
σ
= 2
ln.
2
1
erfc
2
1
Gcr
(19)
The integral and Kolmogorov length scales are given by:
ε
=2/3
t
k
l and 4/1
4/3
v
ε
=η
Flamelet libraries
Under flamelet regime hypothesis [15], the species transport equations are
simplified to:
k
2
k
2
k
lk
Z
Y
Le2t
Yω=
χρ
ρ (20)
A detailed chemical mechanism of 64 species and 752 equations was adopted.
The simplified energy equation is:
k
N
1k
k
p
2
2
l.h.
C
1
Z
T
2t
Tω=
χρ
ρ
=
(21)
With the laminar scalar dissipation:
2
l)Z(.D2 =χ (22)
An external program CFXRIF solves these equations to obtain a laminar flamelet
table, which is integrated using a beta PDF to have the turbulent flamelet library.
This library provides the mean species mass fractions as functions of mean mixture
fraction Z
, variance of mixture fraction 2''
Z
~ and turbulent scalar dissipation rate χ
:
(
)
st
2''
ii ~
,Z
,Z
Y
Y
χ= (23)
A. Mameri et al.
232
On the other hand two transport equations are solved in the CFD code, the first gives
mixture fraction:
σ
µ
+µ
=
ρ
+
ρ
jz
t
jj
j
x
Z
xx
)Z
u
(
t
Z
(24)
And the second gives the mixture fraction variance:
χρ
σ
µ
+
σ
µ
+µ
=
ρ
+
ρ~
x
Z
~
2
x
Z
~
xx
)Z
~
u
~
(
t
Z
~2
jz
t
j
2''
z
t
jj
j
2''
(25)
The turbulent dissipation scalar is modelled by:
2''
Z
~
.
k
~
.C
~ε
=χ χ (26)
To interpolate species mass fractions from the turbulent flamelet table, the CFD
program use the mixture fraction, mixture fraction variance and the turbulent scalar
dissipation computed above.
4. FLAME GEOMETRY AND NUMERICAL PROCEDURE
The computational domain is the half of the chamber with 3 degrees thick (Fig. 3).
200 × 300 × 1 grid nodes are taken inside the domain. The grid is refined near walls and
in the high gradients regions (Fig. 4). Boundary conditions for the main flame, pilot
flame, at wall, exit and symmetry axis are taken from experiments; the most important
are shown for the flame in figure 5.
Fig. 3: Combustion chamber geometry
Fig. 4: Near burner meshing
CISM’2008: TFC modeling of hydrogenated methane premixed combustion
233
Fig. 5: Inlet velocity and turbulence kinetic energy
A double precision computation is done with the CFX code. The high resolution
scheme with an automatic time scale control was used. To reach the target maximum
residual of 10-7 for all equations, the computation takes about 3000 to 5000 iterations to
converge.
5. COMPUTATION RESULTS
5.1 Cold jet test
Before computing the reacting flow, a cold jet test case is performed with the same
boundary conditions. The Pope correction constants 3
Cε and lim
χ
were adjusted to
have the best agreement with the experimental axial turbulent kinetic energy variation
and mean velocity decay.
The potential core length is well reproduced, the figures 6 and 7 show this
agreement. We note that the core length is 75 mm or three times the jet diameter
(3D/X =).
5.2 Reacting flow computations
Three cases are taken into consideration; the first is the methane-air combustion
using TFC with a detailed chemical mechanism. The second and third cases are the
blended hydrogen-methane combustion using the same model with a chemical
A. Mameri et al.
234
mechanism of 64 species and 752 reactions. The fuel compositions, for the second and
third cases, are: 10 % H2 + 90 % CH4 and 20 % H2 + 80 % CH4.
The following figures showing general field are taken for the case of 10 % H2. The
other cases are similar, and the difference resides in the flame length and maximum
variables values. Figure 8 shows the flow structure near the burner. Streamlines are
parallel in the potential core, and they are deviated in the preheat zone and the velocity
increases under the gas expansion effect.
Fig. 6: Axial velocity
Fig. 7: Axial turbulent kinetic energy
Fig. 8: Near burner flow field
CISM’2008: TFC modeling of hydrogenated methane premixed combustion
235
Fig. 9: Near burner temperature field
In figure 9, we have the temperature distribution for the fuel with 10 % H2. In this
case, the maximum temperature is about 1668 K (the adiabatic one is 1674 K). For the
case of the pure methane, we obtain 1664 K which is the same as the adiabatic one. The
last fuel (20 % H2) has an adiabatic temperature of 1680 K.
Fig. 10: Top- Turbulent flame velocity; Bottom- Laminar flame velocity
The figure 10 shows the difference between laminar and turbulent flame velocity.
The laminar flame velocity is a fuel property; it depends only on the chemical
composition. The turbulent flame velocity depends also on the flow conditions. It
represents the interaction of the flame with the turbulence. In this case, it is nearly thirty
times the laminar one.
Fig. 11: Axial reaction progress
A. Mameri et al.
236
We can see the good agreement between measured and computed axial reaction
progress variable c for all fuels (Fig. 11). We note that maximum reaction depends on
fuel composition, it becomes close to the burner when we add more hydrogen to the
fuel. This means that flame velocity becomes more important.
6. CONCLUSION
In this study turbulent premixed combustion of pure methane and hydrogenated
methane is computed. We have used a detailed chemistry with a turbulent flame closure
model. The most important result found is that hydrogen enrichment with small
amounts doesn’t increase significantly combustion temperature. On the other hand it
increases flame velocity and stabilises combustion process. Also hydrogen doesn’t
contain carbon, it is a clean fuel. The replacement of a fraction of the methane by
hydrogen results in lower pollutant emissions. Turbine gas and IC engines can be fed
with hydrogenated fuels without recourse to major modification in installations.
NOMENCLATURE
A: Zimont model constant 2
Cε:
ε
k model coefficient (= 1.92)
1
Cε: εk model coefficient (= 1.44) µ
C:
ε
k model coefficient (= 0.09)
3
Cε: POPE correction coefficient (= 0.79) I: Chemical species I
h, tot
h: Static and total enthalpy k
N: Number of species in the mixture
k
: Turbulence kinetic energy k
R: Production rate of species
k
t
Pr :Turbulent Prandtl number I
S: Source term of the I species equation
M
S, E
S: Source terms of the momentum and energy equations
R
: Universal gas constant t: Time
U: Velocity vector U(u, v, w) T: Static temperature
u: Axial velocity component v: Radial velocity component
I
W: Species I molar weight i
Y: Species I mass fraction
Greek symbols
α: Hydrogen mass fraction t
Γ
: Turbulent diffusivity coefficient
δ: Kronecker delta c
σ
: Transport coefficient for c (=0.9)
σ: Standard deviation – Distribution of
ε
µ
: Dynamic viscosity
k
σ, ε
σ: Transport coefficient for
k
and
ε
(= 1.0 and 1.3)
z
σ, ''z
σ: Transport coefficient for Z
and 2''
Z
(= 0.9 and 0.9)
t
µ: Turbulent viscosity c
ω
: Rate of production of c
k
ω: Rate of production of species
k
p
χ, lim
χ: Vortex stretching rate invariant and limited invariant (Pope correction)
CISM’2008: TFC modeling of hydrogenated methane premixed combustion
237
REFERENCES
[1] I. Yamaoka and H. Tsuji, ‘An Anomalous Behavior of Methane–Air and Methane–Hydrogen–
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[2] I. Wierzba and B.B. Ale, ‘Rich Flammability Limits of Fuel Mixtures Involving Hydrogen at
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[3] G.S. Jackson, R. Sai, J.M. Plaia, C.M. Boggs and K.T. Kiger, ‘Influence of H2 on the
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Abstract – To minimize the harmful emissions and the unburnt residues exits of combustion, a surplus of air is used. In this mode of combustion known as poor, one consumes all fuel theoretically, which gives us a ,complete combustion ,and less unburnt residues [1-3]. This mode ,of combustion supports the appearance of instabilities[4-6], for example, the flash back, the top spin and even the extinction of the flame. In order to solve this problem, one uses the hydrogen which is a clean fuel and which has a great ,calorific value [7-9]. The addition of hydrogen ,to fuels with well defined proportions increases the reactivity of the ,mixture and tends to stabilize the flame. This work of digital simulation supplements experimental work of T. Lachaux [10] and F. Halter [11], where one adds hydrogen,to methane in a mixed pre turbulent combustion poor. It was found that the addition ofa proportion of 10% in volume ,of hydrogen ,would not change the characteristics of the flame.
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The book Turbulent Combustion by Norbert Peters is a concise monograph on single-phase gaseous low Mach number turbulent combustion. It is compiled from the author's review papers on this topic plus some additional material. Norbert Peters characterizes turbulent combustion both by the way fuel and air are mixed and by the ratio of turbulent and chemical time scales. This approach leads naturally to detailed models, which are based on results of turbulence modelling and asymptotic flame theory. In both areas Norbert Peters has contributed significantly over the last two decades. The book has four sections. In chapter 1 he discusses briefly the state of the art of combustion models as they are used by different authors. Important turbulent and chemical scales are introduced, which are then used to introduce and explain the different combustion models. He distinguishes between premixed and non-premixed combustion and also between infinitely fast and finite rate chemistry. The current turbulent combustion models are described in order of their complexity and physical accuracy. He explains Eddy BreakUp and Eddy Dissipation Models, the fundamentals of the PDF transport equation, and the laminar flamelet concept applied to non-premixed and premixed turbulent combustion. Then, the Conditional Moment Closure, the Linear Eddy Model, and combustion models used in Large Eddy Simulations are described very briefly. Chapter 2 is devoted to premixed turbulent combustion. After introducing some characteristic dimensionless numbers Peters uses the level set approach and the flamelet concept to formulate a combustion model valid in the thin zone and corrugated reaction zone regimes. He shows parallels between this more fundamental model and standard models like the Bray-Moss-Libby model. He also presents models for the turbulent burning velocity, the Flame Surface Area Ratio, and discusses the effects of gas expansion. Very helpful for the reader's understanding is the presentation of three worked examples of a slot burner, a propagating spherical flame and an oscillating counter flow. Peters' model for premixed turbulent combustion is based on the equations for the mean and the variance of the $G$-equation, some closure relations as well as the flamelet equation for premixed combustion. A numerical example is used to discuss its accuracy. Non-premixed turbulent combustion is the subject matter of chapter 3. Peters uses the mixture fraction variable and asymptotic flame theory to explain the regimes of non-premixed turbulent combustion. Two worked examples of a counterflow diffusion flame and the one-dimensional unsteady laminar mixing layer help the reader to understand the theory. After discussing turbulent jet diffusion flames and introducing the flamelet equation he develops steady and unsteady flamelet models for non-premixed turbulent combustion. In particular, the Eulerian Particle Flamelet Model and the RIF (Representative Interactive Flamelet) Model are discussed. These models have been used to predict pollutant formation in a gas turbine and a direct injection Diesel engine, respectively. Finally, partially premixed combustion is discussed in chapter 4. Lifted turbulent diffusion flames are reviewed and the prediction of the lift-off height is identified as a key problem. This leads directly to the introduction of the concept of a triple flame. Different models for partially premixed combustion are then presented and the numerical simulation and the scaling of lift-off heights in turbulent jet flames are studied. There is no doubt that this book is well written and is an important contribution to combustion literature. Peters uses asymptotic theory and scale separation to develop combustion models from first principles. Also, the book contains a comprehensive review of the current literature on turbulent combustion. It is clearly a `must have' for experienced combustion modellers and experimentalists. The book has not been written for non-experts and beginners; these readers would probably have liked more worked examples and exercises, a list of symbols, some material on the mathematical techniques of asymptotic analysis, and a more detailed discussion of the standard combustion models that are often used in the literature. Also, the numerical aspects of turbulent combustion modelling are not discussed in the book. Nevertheless, the book will be a useful source for advanced courses on combustion. Markus Kraft
Article
Experimental studies on the behavior, structure and extinction of laminar, premixed methane-hydrogen-air flames diluted with nitrogen in a stagnation flow are made using counterflow, twin flames established in the forward stagnation region of a porous cylinder. The experiments are made under the condition of a constant stagnation velocity gradient (30 s−1). The extinction limits of the twin flames and the flame separation distance at extinction are measured over the whole composition range of fuel-air-nitrogen flames. The concentration distributions of stable species across the twin flames are also measured for flames near extinction. An anomalous relationship between the flame separation distance δc at extinction and the equivalence ratioof the mixture is found for slightly fuel-rich flames. Asis increased, δc increases and attains its maximum value at about =1.15. Then δc decreases, and in the range > about 1.3, it becomes almost independent of(methane flames) or it increases again gradually asis increased (methane-hydrogen flames). This anomalous relationship between δc andcan be explained on the basis of preferential diffusion of atomic hydrogen across the divergent stream tube boundary toward the unburned gas mixture (i.e. back diffusion).
Article
The performance, emissions and combustion characteristics of lean mixtures of natural gas and hydrogen were studied in a conventional spark ignited engine. Specifically, mixtures of natural gas blended with 5, 10 and 15 percent by volume hydrogen were considered. Engine performance parameters included power (BHP), thermal efficiency (BTE), specific fuel consumption (BSFC), coefficient of variation in mean effective pressure, cumulative energy release schedule, and emissions of CO, NOx and hydrocarbons. Major conclusions of the work include: (t) at equivalence ratios leaner than 0.80, improvements in BHP, BSFC, and BTE were significant wilh hydrogen addition; (ii) significant extension of the lean operating limit to lower equivalence ratios was demonstrated with increasing hydrogen concentrations in natural gas; (iii) emissions of CO, NO^ and hydrocarbons decreased as equivalence ratio was reduced until partial burning became predominant; (iv) hydrogen addition appeared to allow stable engine operation with lower pollutant emissions over a relatively broad range of lean equivalence ratios; and (v) the impact of hydrogen blending on performance and emissions was dependent on the volume fraction of hydrogen, although the functional relationship appeared to be non-linear.
Article
The stability characteristics of a premixed, swirl-stabilized flame were studied to determine the effects of hydrogen addition on flame stability under fuel-lean conditions. The burner configuration consisted of a centerbody with an annular, premixed methane/air jet. Swirl was introduced to the flow using 45-degree swirl vanes. The combustion occurred within an air- cooled quartz chamber at atmospheric pressure. The results, using methane/hydrogen fuel mixtures, showed that the addition of up to 41% hydrogen significantly extended the lean burning limit. Planar Laser-Induced Fluorescence (PLIF) measurements of the OH radical were applied to study the behavior of the OH mole fraction near the lean stability limit. The results showed that as the lean stability limit was approached the overall OH mole fraction decreased, the flame width decreased and length increased, and the high OH region took on a more intermittent, shredded appearance. For operating conditions near the lean stability limit, the addition of a moderate amount of hydrogen to the methane/air mixture resulted in a significant increase in the OH concentration and a more robust appearing flame.
Article
A combined experimental and numerical investigation on the effects of H2 addition to lean-premixed CH4 flames in highly strained counterflow fields (with strain rates up to 8000 s−1) using preheated flows indicate significant enhancement of lean flammability limits and extinction strain rates for relatively small amounts of H2 addition. Numerical modeling of the counterflow opposed jet configuration used in this study indicated extinction strain rates which were within 5% of experimentally measured values for equivalence ratios ranging from 0.75 to less than 0.4. Both experimental and numerical results indicate that increasing H2 in the fuel significantly increases flame speeds and thus extinction strain rates. Furthermore, increasing H2 decreases the dependency of extinction equivalence ratio on the strain rate of the flow. For all of the mixtures investigated, extinction temperatures depend primarily on equivalence ratio and not fuel composition for the range of H2 content studied, which suggests that extinction can be correlated to flame temperature and O2 concentration. Nonetheless, H2 addition greatly increases the maximum allowable strain rate before extinction temperatures are reached. Inspection of the model-predicted species profiles suggest that the enhancement of CH4 burning rates with H2 addition is driven by early H2 breakdown increasing radical production rates early in the flame zone to enhance CH4 ignition under conditions where otherwise CH4 combustion might be prone to undergo extinction.
Article
The rich flammability limits of binary fuel mixtures of hydrogen with methane, ethylene and propane in air were determined experimentally for upward vertical flame propagation at elevated initial mixture temperatures up to 350°C at atmospheric pressure. A conventional stainless steel flammability test tube apparatus was used. Generally, the flammability limits of hydrogen–methane and hydrogen–propane mixtures obeyed Le Chatelier’s Rule at elevated temperatures reasonably well (when the corresponding limits of pure fuels were used) if the hydrogen concentration in the mixture was less than 70%. However, the limits of hydrogen–ethylene mixtures deviated very significantly from those calculated using the Rule. It was also found that the rich flammability limit of hydrogen–methane mixtures at initial temperatures higher than 200°C is a function of the residence time. The longer this time and the higher hydrogen concentration in the fuel mixture the smaller was the value of the rich limit. It was suggested that the narrowing of the rich limit is due to surface reactions on the stainless steel wall during the waiting time that tends to change the mixture composition just prior to spark ignition. The flammability limits of these mixtures exposed to longer residence time do not obey Le Chatelier’s Rule.
Article
Hydrogen addition to methane will have an important role to reach a fully developed hydrogen economy. The effects of this addition on the flame structure and CO emissions were evaluated in two different atmospheric burners. Four fuels with the following composition were used: 100%CH4, 2%H2+98%CH4, 6%H2+94%CH4 and 15%H2+85%CH4. In a single-port atmospheric burner, a decreasing trend in the height of the blue cone with hydrogen addition was determined. The increase in the laminar burning velocity was identified as the main effect on the behavior of this parameter. In a drilled-port atmospheric burner, a significant reduction in CO emissions with hydrogen addition was achieved under two operating conditions: (1) keeping the primary air ratio constant and (2) keeping the primary air ratio and the thermal input constant. The results obtained were consistent with previous experimental studies. This reduction is attributed to a higher concentration of OH radicals as a result of hydrogen addition.
Article
The use of hydrogenated fuels shows considerable promise for applications in gas turbines and internal combustion engines. In the present work, the effects of hydrogen addition in methane/air flames are investigated using both a laminar flame propagation facility and a high-pressure turbulent flame facility. The aim of this research is to contribute to the characterization of lean methane/hydrogen/air premixed turbulent flames at high pressures, by studying the flame front geometry, the flame surface density and the instantaneous flame front thermal thickness distributions. The experiments and analyses show that a small amount of hydrogen addition in turbulent premixed methane–air flames introduces changes in both instantaneous and average flame characteristics.