Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 17.6.1 - Numerical Investigation of Methane/Oxygen and Methane/LOX Counterflowing Spray Flames at Elevated Pressure
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Permanent Link: http://ufdc.ufl.edu/UF00102023/00429
 Material Information
Title: 17.6.1 - Numerical Investigation of Methane/Oxygen and Methane/LOX Counterflowing Spray Flames at Elevated Pressure Reactive Multiphase Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Urzica, D.
Gutheil, E.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: counterflow configuration
oxygenated flames
liquid oxygen
extinction conditions
flamelet library
 Notes
Abstract: The understanding of physical and chemical processes occurring in many applications in sciences and engineering is important to ensure stability and efficiency of their performance. Examples are the combustion process in direct injection engines, gas turbine combustors, and liquid rocket propulsion systems. In liquid rocket propulsion systems, the fuels methane and kerosene are being discussed as alternative fuels to hydrogen because of their high energy content. Methane has some advantages compared to kerosene because of its cleaner burning characteristics. The present study contributes to an improved understanding of methane/oxygen and methane/LOx (liquid oxygen) combustion compared to the hydrogen/oxygen system. A numerical investigation of laminar methane/air and methane/oxygen flames where differentmixtures of nitrogen and oxygen in the oxidizer stream are studied. Moreover, liquid oxygen spray flames with carrier gas methane against an oxygen stream are investigated in the counterflow configuration. These structures may be used in (spray) flamelet library or flamelet generated manifolds computations of turbulent combustion. The mathematical model is based on two-dimensional equations, which are transformed into one-dimensional equations using a similarity transformation. The numerical simulation concerns the axi-symmetric configuration with an adaptive numerical grid for the gas phase. Detailed models of all relevant processes are employed; in particular, a detailed chemical reaction mechanism is used which comprises 35 species involving 294 elementary reactions. The thermodynamic data for CH4 and O2 between 100 and 300K are implemented for normal and elevated pressures for use in cryogenic methane/LOx combustion. The CH4/O2 flame is studied for elevated pressures up to 2MPa. Both extinction strain rates and the scalar dissipation rate at stoichiometric conditions are evaluated for use in future turbulent flamelet computations. It is shown that oxygen dilution, pressure, and strain rate have a pronounced effect on flame structures. The use of liquid oxygen compared to gaseous oxygen has a pronounced effect on flame structure. The combustion of methane/LOx with preceding evaporation of liquid oxygen under cryogenic conditions has a pronounced effect of the liquid phase on gas temperature. Moreover, the spray flame is broadened with increase of initial droplet size.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
 Record Information
Bibliographic ID: UF00102023
Volume ID: VID00429
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: 1761-Urzica-ICMF2010.pdf

Full Text



7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30- June 4, 2010


Numerical Investigation of Methane/Oxygen and Methane/LOx Counterflowing
Spray Flames at Elevated Pressure


D. Urzica and E. Gutheil

Interdisciplinary Center for Scientific Computing, Heidelberg University, Heidelberg, Germany
urzica@iwr.uni-heidelberg.de and gutheil@iwr.uni-heidelberg.de
Keywords: counterflow configuration, oxygenated flames, liquid oxygen, extinction conditions, flamelet library




Abstract

The understanding of physical and chemical processes occurring in many applications in sciences and engineering
is important to ensure stability and efficiency of their performance. Examples are the combustion process in direct
injection engines, gas turbine combustors, and liquid rocket propulsion systems. In liquid rocket propulsion systems,
the fuels methane and kerosene are being discussed as alternative fuels to hydrogen because of their high energy
content. Methane has some advantages compared to kerosene because of its cleaner burning characteristics. The
present study contributes to an improved understanding of methane/oxygen and methane/LOx (liquid oxygen)
combustion compared to the hydrogen/oxygen system. A numerical investigation of laminar methane/air and
methane/oxygen flames where different mixtures of nitrogen and oxygen in the oxidizer stream are studied. Moreover,
liquid oxygen spray flames with carrier gas methane against an oxygen stream are investigated in the counterflow
configuration. These structures may be used in (spray) flamelet library or flamelet generated manifolds computations
of turbulent combustion. The mathematical model is based on two-dimensional equations, which are transformed into
one-dimensional equations using a similarity transformation. The numerical simulation concerns the axi-symmetric
configuration with an adaptive numerical grid for the gas phase. Detailed models of all relevant processes are
employed; in particular, a detailed chemical reaction mechanism is used which comprises 35 species involving 294
elementary reactions. The thermodynamic data for CH4 and 02 between 100 and 300K are implemented for normal
and elevated pressures for use in cryogenic methane/LOx combustion. The CH4/02 flame is studied for elevated
pressures up to 2MPa. Both extinction strain rates and the scalar dissipation rate at stoichiometric conditions are
evaluated for use in future turbulent flamelet computations. It is shown that oxygen dilution, pressure, and strain
rate have a pronounced effect on flame structures. The use of liquid oxygen compared to gaseous oxygen has a
pronounced effect on flame structure. The combustion of methane/LOx with preceding evaporation of liquid oxygen
under cryogenic conditions has a pronounced effect of the liquid phase on gas temperature. Moreover, the spray flame
is broadened with increase of initial droplet size.


Introduction

In the last years, methane (CH4) is considered as an al-
ternative fuel to hydrogen in liquid rocket propulsion
systems (Zurbach et al. (2002); Zurbach et al. (2003);
Candel et al. (2006); Singla et al. (2005, 2007); Cuoco
et al. (21 111ib); Yang et al. (2007); Pauly (2006);
Lux & Haidn (2009a); Lux & Haidn (2009b)). The
so called 'green propellant' has become attractive be-
cause of its limited effect on environment compared to
other hydrocarbons. Moreover, it has several advan-
tages compared to hydrogen including safety in trans-
port and storage as well as high energy content. There
are two major research groups in Europe which experi-
mentally investigate the CH4/02 and LOx/CH4 flames:


ONERA in France and DLR Lampoldshausen in Ger-
many. Both ONERA's MASCOTTE test facility (Zur-
bach et al. (2002)) and the DLR's M3 combustion
chamber (Cuoco et al. t(21l" '4, which initially were
developed for experimental investigations of liquid oxy-
gen/hydrogen combustion (LO\ H_ have been modi-
fied to allow for the study of LOx/CH4. In these in-
vestigations, pressure ranges from 0.1 to 5.5MPa, the
injection temperature for LOx is 85K, and for liquid
CH4 it is 125K, hence both temperatures are cryogenic.
A recent study (Zurbach et al. (2003)) concerns fuel
rich conditions. Candel et al. (2006) studied the flame
structure of both LOx/CH4 and LO\ H in the transcrit-
ical range for a pressure range between 0.1 to 7MPa











and a subcritical injection temperature of liquid oxy-
gen. Combustion of cryogenic oxygen and methane in-
jected at pressures between 4.5 and 6MPa were inves-
tigated experimentally by Singla et al. (2005). The
coaxial injector delivers oxygen at a temperature of 85K
and methane at a temperature of 120K or 288K. Sta-
bilization of flames formed by cryogenic liquid oxy-
gen/hydrogen or methane has been investigated through
planar laser induced fluorescence (PLIF) of OH (Singla
et al. (2007)). In the LO\ H. experiments, injection
conditions are transcritical, since the chamber pressure
(6.3MPa) is above the critical value, whereas the temper-
ature (80K) is below the critical value. In the LOx/CH4
experiments, the chamber pressure (2MPa) and LOx in-
jection temperature (80K) are below critical values.
In the DLR experiments (Cuoco et al. (2 41,1,1, a
comparison between spray combustion for coaxially in-
jected CH4/LOx and H2/LOx, at similar injection con-
ditions was performed where the Weber number and the
momentum flux ratio were varied. Experimental investi-
gations (Cuoco et al. (2" 1114b on the development of
LOx/CH4 flames, especially ignition and flame stabi-
lization, strongly coupled with the distribution of liquid
oxygen phase before and after the occurrence of igni-
tion, were carried out. Yang et al. (2007) experimen-
tally investigated cryogenic reactive coaxial sprays with
oxygen and hydrogen or methane in order to determine if
concepts from H2/LOx injector design can be transferred
to LOx/CH4. The experimental study of the ignition
and flame stabilization of a gaseous methane/oxygenjet
for different configurations and injector conditions is de-
scribed by Pauly (2006).
Lux & Haidn (2009a) investigated flame stabiliza-
tion in CH4/LOx combustion using a single shear coax-
ial injector during both ignition and steady-state oper-
ation. Three main operating points of sub-, near-, and
supercritical conditions with respect to thermodynamic
critical point of oxygen were considered, whereas LOx
was injected at about 120K and CH4 was injected at near
ambient temperature of about 270K. Taking into account
the slightly different experimental setup and operating
conditions in comparison with previous H2/LOx inves-
tigations (Lux & Haidn (2'21 "11 these results are in
good conformance with those for H2/LOx flames. Lux
& Haidn (2009b) experimentally studied the effect of a
moderately recessed LOx tube on the flame stabilization
in CH4/LOx combustion at the same operation condi-
tions as used by Lux & Haidn (2009a), using an op-
tically accessible combustion chamber equipped with a
single-shear coaxial injector. It was observed that a re-
cessed LOx post significantly increases the flame expan-
sion shortly after injection.
Even though applications typically involve turbulent
flow fields, laminar flames are studied extensively to in-


7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30- June 4, 2010


vestigate principal flame characteristics in a simple ge-
ometry. The evidence gained from these investigations
then is included in both experimental and numerical con-
figurations.
Combustion of the laminar CH4/air system in the
counterflow configuration is extensively studied in the
literature. Several authors (Chelliah et al. (1993); Du &
Axelbaum (1996); Sohn et al. (2002)) studied the be-
havior of laminar CH4/air in counterflow configurations
at different conditions using different chemical reaction
mechanisms. For instance, Chelliah et al. (1993) pre-
sented numerical results for laminar CH4/air diffusion
flames using a starting kinetic mechanism, a reduced 5-
step mechanism and a 4-step mechanism, and the extinc-
tion results are compared with experimental results. Du
& Axelbaum (1996) experimentally investigated the ef-
fect of flame structure on extinction of CH4/02/N2, and
their numerical simulation includes a 58-step C1 mech-
anism. Sohn et al. (2002) investigated numerically the
flame structure, extinction characteristics and nitric ox-
ide (NO) formation in diffusion flames at different pres-
sures in the axisymetric counterflow configuration. The
chemical reaction mechanism used was GRI 2.11 in-
cluding NOx reactions.
Other studies (Chen et al. (2006, 2004); Naik et
al. (2003); Beltrame et al. (2001)) specifically investi-
gated the effect of enriched oxygen flames. For example,
Chen et al. (2006) and Chen et al. (2" '4) investigated
oxygen-enhanced methane counterflow flames through
both optical diagnostics and numerical simulations. The
effect of strain rate and the influence of oxygen concen-
tration in the oxidizer on the flame structure were stud-
ied for nitrogen-diluted methane 21' r. CH4 and X, '.
N2). The strain rate varies from 60 to 168s 1 while in
the oxidizer stream, the nitrogen in air is replaced by
oxygen ranging from 2 '. 02 to 100% 02. For the
simulations, the GRI 3.0 mechanism was used. Bel-
trame et al. (2001) studied soot formation in oxygen-
rich counterflowing methane-oxygen diffusion flames.
They found that soot formation in methane flames is en-
hanced by oxygen enrichment. With increase of the oxy-
gen in the oxidizer stream, the soot zone narrows and is
shifted towards the stagnation plane. An extension of
the GRI 2.11 mechanism including chemical reactions
with species up to C6, thus consisting of 365 reactions
among 62 chemical species, was used. A soot map that
separates non-soot from soot regions for laminar coun-
terflow methane-oxygen-nitrogen diffusion flames at at-
mospheric pressure is given by Naik et al. (2003). Soot
formation is studied at a constant strain rate of 20s 1
as a function of the methane content ranging from 25%
to 100% in nitrogen in one stream and as a function of
oxygen diluted in nitrogen from 35% to 100% in the ox-
idizer stream.




























Figure 1: Schematic of the counterflow configuration.


In the present study, the numerical investigation of
laminar oxygenated CH4/air as well as CH4/02 flames
as well as intermediate degrees of oxygen dilution in
the counterflow configuration is performed. For the
CH4/02 system a flamelet library for turbulent combus-
tion is generated for use in turbulent spray computations
at normal and elevated pressure. Evaluation of extinc-
tion conditions are presented, which are very important
for use in turbulent flamelet computations. Moreover,
the nitrogen/oxygen ratio is varied. Physical properties
of methane at cryogenic inlet conditions are considered
which are typical for liquid rocket propulsion systems.
These properties are parameterized for use in LOx/CH4
computations. Moreover, structures of LOx/CH4 spray
flames for different initial droplet sizes are discussed.

Mathematical Model

An axisymmetric counterflow configuration is consid-
ered, where either gaseous or liquid fuel versus ox-
idizer, hot products or inert gas, may be fed from
each side of the stagnation plane in any combina-
tion (Continillo & Sirignano (1990); Gutheil & Sirig-
nano (1998); Schlotz & Gutheil (2000); Gutheil (2001,
2005)), c.f. Figure 1. The mathematical model is based
on Eulerian/Lagrangianformulation of non-dimensional
equations (Continillo & Sirignano (1990); Gutheil &
Sirignano (1998)), where a similarity transformation
transferring the two-dimensional equations into a one-
dimensional system is applied. The governing equations
for viscous flow with variable transport properties are
presented by Gutheil & Sirignano (1998).
The gas-phase model includes a detailed chemical re-
action mechanism (Warnatz et al. (1999)) for CH4/air,
which is also used for CH4/02 as well as LOx/CH4. The
mechanism consists of 294 elementary reactions among


7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30- June 4, 2010


35 species. The gas phase transport coefficients are com-
puted from NASA polynomials which cover the tem-
perature range between 300 and 5000K. For the present
cryogenic high-pressure conditions, the set of physical
properties must be extended by data from the JSME ta-
bles (J.S.M.E. (1983)) for the temperature range be-
tween 80 and 300K and for pressures up to 20MPa.
Figures 2 and 3 show properties of methane for pres-
sures of 0.1 and 20MPa and cryogenic temperatures for
use in liquid oxygen/methane spray flames. The figures
are generated from the JSME tables (J.S.M.E. (1983)).
In these figures, a strong dependence of pressure and
temperature of thermal conductivity and dynamic vis-
cosity can be observed. Previous studies have shown
that the liquid phase is dominant in characterizing both
laminar (Continillo & Sirignano (1990); Gutheil &
Sirignano (1998)) and turbulent (Hollmann & Gutheil
(1998)) flames, and they may not be neglected.
The chemical reaction terms cause the system of con-
servation equations to be strongly non-linear and stiff.
The system of equations is solved using a numerical
scheme described by Continillo & Sirignano (1990);
Gutheil & Sirignano (1998); Schlotz & Gutheil (2000);
Gutheil (2001, 2005). For the solution of the gas
phase equations, an adaptive grid is used. The veri-
fication of the mathematical model and mechanism is
achieved through comparison of the results for CH4/air
and CH, N 02 flames with results from the litera-
ture (Chelliah et al. (1993); Bollig et al. (1998)) as
shown by Urzica & Gutheil (2009).

Results and Discussion

In this section, numerical results for CH4/02 and
LOx/CH4 flames as well as conditions with different
mixture ratios of nitrogen and oxygen in the oxidizer
stream are presented and discussed.

Gas Flames

Pure fuel enters from one side of the counterflow con-
figuration, and it is directed against the oxidizer stream.
In order to validate the mathematical model, the results
of the CH4/air flame were compared with results from
the literature (Chelliah et al. (1993); Du & Axelbaum
(1996); Sohn et al. (2002); Bollig et al. (1998)), and
they were found to be in very good agreement (Urzica &
Gutheil (2009)). Small discrepancies between the sim-
ulation results and the results presented by Chelliah et
al. (1993) were attributed to the different chemical re-
action mechanisms used in these papers. In particular,
Urzica & Gutheil (2009) added C2 chemistry, which
typically causes lower extinction temperatures due to
the fact that a higher number of chemical species is in-








7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30- June 4, 2010


0 02
02;
E o:



01,
LA 01E



01'
0 00

o01
0 Oz

- r


Temperature [K]


Figure 2: Pressure and temperature dependence of ther-
mal conductivity for methane in the temperature range
100 < T < 300K.


volved leading to a reduced maximum gas temperature
because of the species' energy content. The inclusion
of C2 chemistry is inevitable if soot precursors are to be
studied.

Successive replacement of nitrogen by oxygen in the
air stream opposing the methane flow leads to laminar
CH4/02 flames. This procedure enables the study of
flame structures with different degrees of oxygen dilu-
tion in the oxidizer stream. Figure 4 shows the maxi-
mum temperature and the mass fractions of species CH3,
CO, O, OH, C2H2, C2H6, H as well as CHO2 as a func-
tion of the oxygen mass fraction in the oxidizer stream at
a fixed strain rate of 100s- The oxygen mass fraction
ranges from 0.233 diluted in nitrogen (CH4/air) to pure
02. With an increase of oxygen content in the oxidizing
gas stream, maximum flame temperature increases sub-
stantially from 1874K to 2965K. The maximum mass
fractions of the major species plotted in Figure 4 in-
crease non-linearly as nitrogen is removed from the sys-
tem. At the same time, formaldehyde decreases with
nitrogen removal which is due to the reduced stability
with increased temperature of this species.

The pollutants and soot formation in laminar flames
are of particular interest. The detailed chemical reaction
mechanism used here is favorably enabled to predict for-
mation of species such as CO, C2H2 as well as CH2O.
Maximum mass fraction of carbon monoxide and acety-
lene increase by a factor of about 10 as nitrogen is re-
moved (CO increases from 0.043 to 0.46 and C2H2 from
0.0078 to 0.075, respectively). This increase of acety-
lene formation with oxygen content confirms the results


Temperature [K]


Figure 3: Pressure and temperature dependence of dy-
namic viscosity for methane in the temperature range
100 < T < 300K.




3000 T -10



CCO
2800 03 04 05 06 07 08 09 110
c2H21 1o1













Mass Fraction of 02 in the Oxidizer Stream [-]


Figure 4: Maximum flame temperature and mass frac-
tion of various species with increase of oxygen content
in the oxidizing stream at a strain rate of lOOs 1 (Urzica
2600& Gutheil (2009)).


of Beltrame et a (2001) who found soot fo action to
CH3




















increase with oxygen content, since acetylene is consid-
ered an important soot precursor.
E 2200 -
H 0o
2000 C2H,
CH20
2 03 04 05 06 07 08 09 1







The maximum mass fractions of othxidizer species such
Figure 4: Maximum flame temperature and mass frac-





tion of various specie as itrogen is removed, however, the
in the oxidizing stream at a strain rate of 100sr. (Urzica
Urzica & Gutheil (2009) compared the outer flame

Belstructure of Cal. (2001) who foundlames at strain rates
increase with oxygen content, since acetylene is consid-
ered an important soot precursor.
The maximum mass fractions of other species such
as CH2O decrease as nitrogen is removed, however, the
reduction is considerably smaller.
Urzica & Gutheil (2009) compared the outer flame
structure of CH4/air and CH4/0 flames at strain rates


22E-
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E 1 8E-
L)
Z 1 6E-
1 4E-
' 1 2E-
0
U
A 1 OE-
80E-
.2
E 60E-
40 E-
20E-


5 MPa
S10MPa 20MPa









7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30- June 4, 2010


3400
3200
3000
2800
2600
2400
2200-
2000~
1800
1600 E
1400 1
1200
1000
800
600


1 ------- 0.1MPa
9 2MPa

8 It
7 T

7 \
6 H20


4I

2 \ \


i ., ..... ..
'- s




2 -15 -1 -05 0 05 1 15 2 25 3 35 4 45
Axial Position [mm]


Figure 5: Profiles of temperature and mass fractions of
H20, CO2 and CO in the CH4/02 flame, at strain rate
100s 1 and pressures of 0.1MPa and 2MPa.


006

C 005
.0

U.
0 04

S003

002

001


Axial Position [mm]


Figure 6: Profiles of temperature and mass fractions of
CH3, CH20, OH and O in the CH4/02 flame, at strain
rate 100s 1 and pressures of 0.1MPa and 2MPa.


\
175,0001s
-- 320,0001s \
667,5001s /





I--
I



\






-1
N


015 -0 01 -0 005 0
Axial Position [mm]


0 005 0 o


E 006
0 005 O "
00050
0 0 0 05

0 0 004
0 003
u4 uL 003

0002 0 2
S002002


0001


_------ 175,0001s /
. ------ 300,000/s /
_---- 338,5001s /


It
/ \
-/\


/ / \\
- i/ \ \
'"


- 02 -001


Figure 7: Profiles of C2H2 (solid line) and CHO2
(dashed line) mass fractions in the CH4/02 flame at
2MPa and different strain rates.


Figure 8: Profiles of C2H2 (solid line) and CHO2
(dashed line) mass fractions in the CH4/02 flame at
0.9MPa and different strain rates.


008

007

0 06
I
0 05
16
o 004


U)
0 003

S002

001


0007


0006


0 005 O
I
0004 6
02
0003 U


0002 c


0001


0 001 0 02
Axial Position [mm]


n "-6-














30 E-04


S3000
20E-04
1 I

1 5E-04 o E 2500

sL E
E
| 2000


7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30- June 4, 2010




2 OE+06



1 5E+06-

S0.1MPa,air
0.1MPa,02 -1 E+06
1 OE+06 0
2MPa,0, y g

5 OE+05



0- OOE+00


Axial Position [mm]


Figure 9: Profiles of C2H2 (solid line) and CHO2
(dashed line) mass fractions at strain rate 100s 1 and dif-
ferent pressures.


near extinction and standard conditions. With replace-
ment of nitrogen by oxygen, the flame thickness de-
creases dramatically by about a factor of seven. This is
due to the fact that the damping of chemical reactions
by nitrogen is removed with replacement of nitrogen
through oxygen leading to enhanced chemical reactions.
This enhancement is accompanied by a pronounced in-
crease of flame temperature.
Figures 5 and 6 show profiles of the gas tempera-
ture as well as major and minor species at a strain rate
of 100s 1 and pressures of 0.1MPa (dashed lines) and
2MPa (solid lines). The temperature increases and flame
thickness becomes narrower with increased pressure due
to flame compression. The increase of gas temperature
leads to higher values of mass fraction of reaction prod-
ucts. For both pressures, the flame resides on the oxi-
dizer side of the flame which is typical for gas flames of
this type. For the methane/oxygen flame, however, the
flame is shifted towards the stagnation plane.
Figures 7 and 8 show profiles of mass fraction of
C2H2 for the CH4/02 flame at 2 (Fig. 7) and 0.9MPa
(Fig. 8) for different strain rates. With increased strain
rate, the mass fraction of C2H2 decreases while that of
CHO2 increases, and the narrower profiles are conse-
quence of the decreased flame thickness resulting from
pressure increase. The profile of CHO2 shows a non-
monotonic behavior near the maximum flame tempera-
ture being a consequence of the stability of this species.
In Figure 9, the pressure dependence of the profiles of
C2H2 and CH2O mass fractions for the CH4/02 flame is
studied at a fixed value of strain rate, 100s 1. It can be


Strain Rate a [1/s]


Figure 10: Profiles of maximum flame temperature
(filled symbols) and scalar dissipation rate at stoichiom-
etry (unfilled symbols) for strain rates up to extinction.


seen that for the CH4/02 flame at 0.1MPa, the mass frac-
tion profiles of C2H2 and CH20 are broader in compar-
ison with the CH4/02 flame at higher pressures and the
peak is shifted towards the fuel side. Pressure increase
causes a narrowing of flame thickness and thus a nar-
rower profile of acetylene. For pressures up to 2MPa, the
maximum mass fraction of C2H2 and CH20 decreases.
Thus, both nitrogen removal and pressure increase cause
lower profiles of formaldehyde.

Figure 10 shows a parameter study that was done
to generate a flamelet library for the methane/oxygen
system at different pressures. For comparison with
data from the literature (Urzica & Gutheil (2009), also
methane/air is shown. In flamelet computations, the
scalar dissipation rate plays an important role since it
is relevant to determine the regime of extinguished and
burning flamelets. The scalar dissipation rate varies with
space (and mixture fraction), and typically the flamelet
computations include the value of scalar dissipation rate
at stoichiometric conditions (Peters (2006); Hollmann
& Gutheil (1996)). Therefore, the figure shows both
maximum flame temperature, T, and scalar dissipation
rate at stoichiometry, Xst. Methane/oxygen flames at el-
evated pressures are investigated, which are relevant for
liquid rocket propulsion applications. In particular, the
flames at pressures 0.1, 0.9, and 2MPa are studied. For
all cases, extinction conditions are determined. It is well
known that increased pressure leads to increased flame
stability and thus to both increased extinction strain rates
and temperatures, which is reflected in the extinction
data shown in Fig. 10.


3300
3000
2700
2400
22100
|1800
01500
I-
1200
900
600
300


0 07

- 006
-I
S0 05

-.2 004

LL 003
- )
- m
a 0 02

S001







7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30- June 4, 2010


3000

- 2500

-Z
22000

E1500
-M
-U)
0o1000

500


Axiale Position [mm]


Figure 11: Profiles of gas temperature and H20, CO2
and CO mass fractions in the LOx/CH4 spray flame,
at strain rate 100s- and pressure of 0.1MPa, initial
droplet radius 15pm.


Spray Flames


In the present section, a stream of monodisperse LOx
droplets with carrier gas methane is directed against an
oxygen stream. The initial droplet velocity equals the
methane velocity, vo vo = 0.49m/s, and together
with the strain rate on this side of the configuration, the
velocity of the oxygen is determined from the compu-
tation. All present results are for atmospheric pressure
and initial LOx temperature of 85K and methane tem-
perature of 300K. The results show variation of initial
droplet radius for fixed strain rate.

Figures 11, 12 and 13 show the comparison of the
outer flame structure of CH4/LOx spray flames for three
different initial droplet radius of 15, 25, 35pm, respec-
tively, at a strain rate of 10s 1 at the fuel side of the
configuration. The droplets vaporize completely before
they reach the stagnation plane at axial position zero.
However, they move closer to the stagnation plane as
initial droplet radius is increased. The present computa-
tions do not show droplets crossing the stagnation plane
and no droplet reversal is seen due to the small strain
rate, which is typical for spray flames in the counter-
flow configuration (Gutheil & Sirignano (1998); Gutheil
(2001, 2005)). The flame resides on the oxidizer side of
the configuration, and the spray flame broadens with in-
creased initial droplet size. Larger droplet size also in-
creases drag which can be seen from the velocity profiles
of the gas and the droplets. As droplets vaporize, drag
is reduced and small droplets eventually follow the gas


08




0



02


3000

- 2500

-2000

E1500

g01000
500
500


Axiale Position [mm]


Figure 12: Profiles of gas temperature and H20, CO2
and CO mass fractions in the LOx/CH4 spray flame,
at strain rate 100s 1, and pressure of 0.1MPa, initial
droplet radius 25pm.


velocity. As initial droplet size is increased, the gas tem-
perature is pronouncedly affected by evaporation caus-
ing an increased dip in the left wing of the tempera-
ture profile. The maximum flame temperature increases
by 30K as initial droplet radius is increased from 15 to
35pm.
Considering the gas-phase species, it is observed that
profiles of both major and minor species are quite close
for different initial droplet sizes. Principal flame char-
acteristics such as CO formation prior to CO2 formation
are maintained. Carbon monoxide is produced and then
consumed at high temperatures in the rate-limiting reac-
tion CO + OH ; CO2 + H.
A comparison of the LOx/CH4 spray flame and the
CH4/02 flame, see Fig. 14, shows that in both config-
urations, the main reaction zone resides on the gas side
of the counterflow configuration and the principal flame
structure is not too different. At the same time, the pres-
ence of the spray strongly influences the gas tempera-
ture in regions where the spray is present. Moreover, the
presence of the spray causes considerable broadening of
the reaction zone.
It is interesting to see that in methane/LOx flames,
so far, only single flames are found whereas in both
H2/LOx (Schlotz & Gutheil (2000)) flames and dur-
ing combustion of fuel sprays in air (Continillo & Sirig-
nano (1990); Gutheil & Sirignano (1998); Gutheil
(2001, 2005)), double flames where identified for small
strain rates below about 500/s. In fact, Gutheil (2005)
identified two different flame structures with one and







7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30- June 4, 2010


Axiale Position [mm]


Figure 13: Profiles of gas temperature and H20, CO2
and CO mass fractions in the LOx/CH4 spray flame,
at strain rate 100s 1, and pressure of 0.1MPa, initial
droplet radius 35pm.


two reaction zones, respectively, for spray flames at
low strain. Future studies will investigate if the occur-
rance of a single flame is a principal characteristics of
methane/LOx flames or if different structures (Gutheil
(2005)) may be found besides the low-strain structures
with a single reaction zone shown in the present paper.
Future computations will include methane/LOx
flames at elevated strain where the spray has an in-
creased effect on both the flame structure and extinction
conditions due to droplets crossing the stagnation plane
and droplet oscillation. Also, structures at elevated pres-
sures being relevant for liquid rocket propulsion will be
investigated for use in turbulent spray computations ei-
ther through spray flamelet models or flamelet generated
manifolds.


Conclusions

Both normal and high-pressure laminar flame structures
of CH4/air as well as CH4/02 were investigated. The
present detailed chemical reaction mechanism used for
the methane/air and the methane/oxygen system con-
tains 35 species with 294 elementary reactions (Warnatz
et al. (1999)). In contrast to standard chemical kinetic
mechanisms in the literature, the present study includes
C2 reactions, which enable the prediction of soot precur-
sors such as acetylene.
Simulations of CH4/O2 flames at elevated pressure
were performed for both low and increased strain rates,
in particular, extinction conditions were determined in-


Axial Position [mm]


Figure 14: Profiles of gas temperature and H20, CO2
and CO mass fractions in the CH4/02 laminar flame, at
strain rate 10s 1, and pressure of 0.1MPa.



eluding extinction strain rates, which are required in
turbulent spray computations using spray flamelet mod-
els or flamelet generated manifolds. The present flame
structures for CH4/02 are ready to be used in turbulent
flame computations.
Simulations of LOx/CH4 combustion in the counter-
flow configuration at atmospheric pressure were per-
formed. Simulations of LOx/CH4 spray flames for dif-
ferent initial droplet sizes are presented, and their evap-
oration strongly influences the gas temperature on the
spray side of the flame. Moreover, increased initial
droplet size considerably broadens the spray flame.
Double flames which are found in both the H2/LOx
system and in fuel sprays burning in air could not be
identified in the present study, and future research is
warranted. Moreover, extinction conditions fo LOx/CH4
flames at elevated pressures will be studied.


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