1. Elsevier Editorial System(tm) for Proceedings of the Combustion Institute
Manuscript Draft
Manuscript Number: PROCI-D-13-01177R1
Title: Structure and extinction of water-laden methane/air non-premixed flames
Article Type: Research Paper
Keywords: Water-laden flames; Nonpremixed methane/air flames; Counterflow flames; Extinction
limits; Methane hydrates
Corresponding Author: Ms. Rosa Padilla, Mechanical and Aerospace Eng.
Corresponding Author's Institution: University of California Irvine
First Author: Rosa Padilla, Mechanical and Aerospace Eng.
Order of Authors: Rosa Padilla, Mechanical and Aerospace Eng.; Rosa Elida Padilla, Mechanical and
Aerospace Engineering; Valentina Ricchiutti, Masters; Sunny Karnani, Phd; Derek Dunn Rankin; Trinh
Pham, PhD
Abstract: An experimental and computational study investigates the influence of water vapor addition
into the methane stream of a nonpremixed flame using a counterflow configuration. Adding water into
the fuel stream simulates non-premixed combustion systems that naturally contain water, such as in
burning methane hydrates. Experimental results in the literature indicate that methane hydrate flames
contain approximately 1 mole of water per mole of methane entering the reaction zone. Similarly, the
current experiments show that in the counterflow configuration the fuel stream can contain just over
one mole of water per mole of methane before flame extinguishment occurs. The results indicate that
water has relatively little effect on peak temperature, but the location of the peak moves toward the air
nozzle with increasing water addition. Computationally, chemical kinetic calculations introducing
water vapor into the fuel stream using GRI MECH 3.0 and GRI 1.2 (reduced model) kinetics
mechanisms examine the critical conditions of water carrying capacity in flames and flame
temperatures at extinction. The results indicate that water reduces temperatures and the
concentrations of the radicals critical chain initiating, propagating, and branching reactions, ultimately
leading to flame extinguishment.
2. Response to Decision Letter
Please find the revised manuscript, where all of the suggestions and comments of the reviewers
were considered carefully and except for a very few cases changes were made to address them.
Changes have been highlighted in yellow, as indicated in your former email. The responses
submitted in the rebuttal are also used in addition to indicating the changes that were made to the
edited manuscript, but also as suggested in the previous email, rebuttals were included to those
suggestions not addressed in the manuscript. The major changes were:
1) Introduction now includes additional literature review on studies were water is introduced
from the air side and relevant applications with water from the fuel side. This was suggested by
reviewer 1.
2) Figure 3 now shows a comparison between the measured temperature profiles and those from
the computations with similar inlet boundary conditions for water free and water laden
methane/air flames. Temperature profile widths are compared between the two but discrepancies
such as maximum peak temperature is indicated in the edited version. The axis of the
experimental figure is adjusted to begin x=0 as fuel and x=L as the fuel. This was a concern by
reviewer 2 and 4.
3) Edits in the computational results and discussion section are enhanced by including Fig. 8 and
Fig 9. These figures mainly address reviewer 2’s comments, as he suggested performing
additional work in order to separate chemical from thermal effects due to the contribution of
water. Figure 8 highlights the chemical influence by using H2O and N2 as the diluent and then
comparing the relative concentration of major radicals, OH, O, and H at the same peak
temperature condition. Fig. 9 was not included in the original manuscript, but is also important
to distinguish the chemical versus thermal effect as it shows the net heat release of a water laden
methane/air flame in comparison to water free combustion and to a flame diluted with inert N2. It
shows a depressed heat release with water but also a transition from endothermic to neutral
behavior. Major reactions that produce OH, H, O, and HO2 are highlighted in this section to
show how water affects the split amongst the radicals.
4) Some other minor changes have been introduced in the edited version. These include,
new/modifications to references, grammar, and syntax (subscripts, word spacing, typos and style
changes in the figures). Also, more clarity in major and minor species is shown in Fig. 7, for
example by removing less important species like CO from the flame structure.
We also attach a detailed author's reply in response (in red) to the questions of the reviewers.
Sincerely,
Rosa E Padilla
Supplemental Material
3. Manuscript PROCI-D-13-01177
Structure and extinction of water-laden methane/air nonpremixed flames
The authors appreciate the overall positive evaluation from the reviewers and comments towards
improvements for the manuscript. The proposed changes in the manuscript are highlighted in
yellow, and a detailed reply (in red) is located below every reviewer's comment; some replies are
taken from the rebuttal and have been elaborated.
Reviewer #1
1. Although water was found to have a chemical influence here, this could be an artifact of
its pre-vaporization. Methane hydrates require flame heat to vaporize the water. In this
case there would be a larger thermal effect and a smaller chemical effect. This issue could
affect the conclusions but was not addressed.
Prevaporization will clearly affect the absolute water carrying capacity of the fuel stream
since the flame will not need to provide that heat but the chemical versus thermal
influence is not affected significantly by this difference, particularly in the sense we are
finding in this paper where it is really a thermochemical effect rather than one or the
other alone. In addition, the prevaporization, while artificial for natural hydrate burning
is not artificial for many of the cases planned where methane is proposed to be released
from hydrate beds by external combustion and in other important cases of highly water
laden fuel flames.
2. The authors should perform at least a few tests and computations with the actual fuel of
methane hydrates.
The reviewer suggests making a few test and computations with the actual fuel of
methane hydrates. It is not reproducible to use actual methane concentrations from
hydrates as fuel in this work because hydrates are non-uniform and unstable compounds
that make it very difficult to make experimental measurements at this point. The goal
with this paper was to isolate the role of the water chemistry in the process rather than to
predict exactly the water carrying limits of hydrate combustion. The results do provide
some insight of the water carrying capacity question but without a specific combustion
geometry the results will always depend on the flame configuration. The role of the water
in the extinguishment chemistry, however, will be less dependent on these effects.
3. Strain rate was held constant in the computations, but not in the experiments. This should
be held constant for both.
Reviewer was not clear on whether the strain rate was held constant while the water flow
rate was varied. However, the paper states distinctly that the methane flow rate was held
constant while water vapor addition was gradually increased, causing an increase in the
global strain rate. This discussion is on page 4. While there are absolute water-carrying
capacity changes associated with strain-rate it does not affect the role of water in the
4. flame chemistry at extinction. The paper is not evaluating strain-induced extinction, but
rather water-induced extinction with strain.
4. The thermocouples should be corrected for radiation. This is a simple, well established,
and reliable correction.
Radiation correction statement was mentioned in the experimental section as the
following: Measured temperatures were corrected for radiative heat losses from the
thermocouple surface by assuming a spherical shape of the junction, a constant Nusselt
number of 2.0 and constant emissivity of 0.2. The accuracy of the thermocouple was 80
and higher when coated with Ceramabond as it was treated for catalytic effects.
5. I assume the authors confirmed there was no water condensation in the burner, but this
needs to be confirmed and stated.
Reviewer asks about water condensation in the burner. This was described in the
experimental section of the manuscript as the following statement: To confirm that water
is vaporized and condensation is avoided throughout the system, Type K thermocouples
monitor the temperatures at the boundaries of the fuel and oxidizer burners and
surrounding gas lines.
6. There are a large number of spelling mistakes and typos. Several papers are referred to
by author name, but not reference number. There are typos and style changes in the
figures.
Spelling mistakes and typos have been addressed and missing reference numbers were
included wherever needed in the manuscript.
7. Quantity RH2O/CH4 should be converted to XH2O throughout this paper.
This change is not made as it involves many changes and does not seem to be a standard
nomenclature warranting the change. In fact, our value is not the mole fraction of water
which would be implied by the request but the molar ratio between water and fuel, which
is the more common ratio for hydrates.
8. There is no soot here so the long discussion of soot on page 5 should be removed.
Reviewer suggested removing the soot discussion, but we believe that the removal of soot
precursors with the addition of water is an important element of flames being cooled by
water addition. This discussion is reduced but still kept in the Flame appearance section.
9. A more complete literature review is needed.
Literature review is found in the introduction. Other examples of water-laden fuel
nonpremixed flames were given in the first paragraph, such as emulsions, LNG, and
steam assisted flares. The following paragraph gives additional description on our choice
5. for using a counterflow burner to for studying detailed chemistry and its role in the
extinction water laden methane/air flames.
Reviewer #2
1. According to authors the goal of this work includes a study of effect of water additive to non-
premixed methane/air flames on combustion chemistry of methane. A study of the mechanism of
these processes is of current interest. Authors made a certain progress in solving this problem. At
the same time the difficulty consists in influence of water on the flame temperature that inevitably
changes concentration of active flame species. It is very difficult to separate thermal and chemical
effect from one another. To gain this aim I would recommend to simulate the structure of 2
flames (with and without water additive) using the same temperature profile as input data. By
this one could observe only chemical influence of water on methane combustion. In present work
the problem was not solved completely.
The reviewer suggested performing additional work in order to separate chemical from
thermal effects due to the contribution of water. This suggestion is followed and we have
now included (as in our prior work) a chemical specie in the reaction mechanism with
thermal and transport properties of water but without any reactions associated with it. It
was through this comparison that we were able to isolate the chemical effects from the
thermal effects of water. Because we were concerned with potential space limitations, we
did not include these additional findings but instead referred to a conference paper and
thesis where they were discussed in detail. In the revised paper we have included N2 as an
inert to accomplish the same goal but in a much more compact fashion so that it can now
fit into the paper length. We have found that N2 similarly to H2O, reduces the
temperature and the major radicals that prevent combustion from occurring. The big
difference, however, is that the ratio of O and H to OH is much different when water is
the diluent. Figure 8 shows a species profile comparison between O, OH, and H with H2O
addition in the fuel stream as well as for N2 in the fuel stream. Figure 9 shows the heat
release for the system with N2 and H2O as it is introduced from the fuel stream.
2. As authors mentioned chain-branching process, it would be useful to calculate the net rate
of chain-branching reactions for water-laden and water-free flames versus the distance. It
would demonstrate the effect of water additive.
The reviewer suggested additional investigations (these are underway), such as
computations of the net rates in chain branching reactions for water laden and water free
flames versus distance. We are performing a reaction path analysis and identifying the
dominant reactions influenced by water addition. In the paper we have only included the
most important reactions responsible for producing OH, O, H, and H2O. More
understanding for the role of water will be shown in the oral presentation, such as
identifying the reactions for which water is the most sensitive to temperature or that
show a transition from endothermic to exothermic will be covered but these are too
detailed for the limited length CI paper.
3. Figure 3 shows that the air temperature is about 500 K. At the same time authors said that
Toxidizer=298 K. That is why the experimental data cannot be compared with computational results
6. in Fig. 5. Besides, it is not clear why authors took air nozzle as x=0 unlike other figures.
Comment it please.
Figure 3 data has been modified to represent the fuel side as x=0 and x=L as the air side.
The computational temperature profiles, heat release and flame structure figures have all
been computed to the inlet boundary conditions that match the experiments. Figure 3 has
been modified to include the temperature profiles that have been computed with and with
water addition in the fuel stream. The inlet oxidizer temperature of 298 K for a higher
inlet fuel temperature has only been shown in Fig. 5b of the computations to show the
difference in the water carrying capacity with air preheating.
4. Figure 7 does not allow comparing concentrations of H and O in the flames without H2O
additive and water-laden flame. The figure should be revised. CO is not a radical,
therefore, I would recommend to remove its profile of concentration.
Figure 7 is improved and shows clearly only the reduction of OH, O, and H with water
addition. Figure 8 was added to the edited manuscript to show a comparison among OH,
O, and H with water addition versus nitrogen addition in the fuel stream. In addition, the
CO was removed from the minor species concentration profile of Fig. 7
Additional grammar and clarity in the figures was edited in the manuscript.
Reviewer #3
This study is very practical. I would expect that this research will help in the use of methane
hydrate.
Response
No response
Reviewer # 4
1. The procedure of the extinction experiments is described unclearly. It is difficult to
understand from the text how these experiments were performed: either at a fixed value
of global strain rate or this value was varying during the increasing H2O/CH4 ratio? This
is important question, because changing in the flame stretch during the experiment can
affect significantly the results of the observed value of water carrying capacity limit.
However, anyway, if to assume that the global strain rate was kept constant, changing its
value from 76 1/s (as used by authors) to another one will result in changes of the
observed value of water carrying capacity limit, and this fact makes the results presented
by authors useless. I would not say so if they represent the dependence of water carrying
capacity limit vs strain rate.
The procedure of extinction is described in the experimental section more clearly and it
reads as the following: Experimentally, flame extinction is achieved by setting VO and
7. VCH4 to a fixed value while VCH4 and the water to methane, molar ratio, is increased to
the point that the flame extinguishes by simply adding water to the fuel side stream.
2. Temperature measurements are questionable. Authors use a thermocouple, 200 micron
thick, which should distort the flame significantly. Moreover, they do not mention in the
text about using anticatalytic coating of the thermocouple, which is of much need,
because catalytic effects on its surface can considerably change the thermocouple
readings. As a reader, I would like very much to see the measured and calculated
temperature profiles compared in one plot, however authors do not make this opportunity.
A statement about catalytic effects was mentioned earlier in this document and a
temperature profile with measured and corrected temperature data is included in the
revised paper as suggested by reviewer 1 and 4. Reviewer 4 raises a concern about flame
distortion from thermocouples, but this was not apparent physically with the 200 micron
thermocouple wire, so this was not discussed in the manuscript.
3. Providing computational results on the flame structure without testing them against
experimental data represents a weak evidence of what is occurring in the flame.
Computations have been modified to include the experimental inlet boundary conditions,
T CH4=550 and TO=440, in order to have a closer agreement between results.
4. Finally, the text contains a plenty of misprints making the reading difficult.
Concerns from the reviewers about the document having typos was addressed throughout
the document.
8. 35th
International Symposium on Combustion
Structure and extinction of water-laden methane/air nonpremixed flames
R.E. Padillaa*
, V. Ricchiuttib
, S. Karnania
, D. Dunn-Rankina
, T. Phamc
a
Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA
92697, USA
b
Aerospace Sciences and Technology Department, Polytechnic University of Milan, Milan, Italy,
20156
c
Department of Mechanical Engineering, California State University, Los Angeles, CA 90032,
USA
*Corresponding author. Tel.: +1-949-824-8745; fax: +1-949-824-8585.
Mailing address: 4200 Engineering Gateway, University of California, Irvine, CA, 92697, USA
E-mail address: padilla.re@gmail.com
Colloquium: “New Technology Concepts, Reacting Flows, & Fuel Tech”
(alternate colloquia “Laminar Flames”)
Total length of paper: 4,475 +279 +45+1,395 = 6,194 words
Main text: 4,475 words
4 Equation: 45 words
Tables: 0 words
References: 279 words
Figures with captions: 1,395 words
Figure 1 = 138
Figure 2 = 154
Figure 3 = 118
Figure 4 = 117
Figure 5 = 212
Figure 6 = 215
Figure 7 = 205
Figure 8 = 118
Figure 9 = 118
Keywords: clathrate, water-laden flames, nonpremixed flames, counterflow
*Manuscript
Click here to view linked References
9. Structure and extinction of water-laden methane/air nonpremixed flames
R. E. Padillaa
, V. Ricchiuttib
, S. Karnania
, D. Dunn Rankina
, T. Phamc
aMechanical and Aerospace Engineering, University of California,Irvine,
4200 Engineering Gateway, Irvine, CA 92697-3975
bFacolt´a di Ingegneria Industriale e dell’ Informazione, Politecnico Di Milano,
34 Viva La Masa, 20156 Milan, Italy
cMechanical Engineering, California State University of Los Angeles,
5151 State University Dr., Los Angeles, 90032-8530
Abstract
An experimental and computational study investigates the influence of water vapor addition into the methane stream
of a nonpremixed flame using a counterflow configuration. Adding water into the fuel stream simulates non-premixed
combustion systems that naturally contain water, such as in burning methane hydrates. Experimental results in the
literature indicate that methane hydrate flames contain approximately 1 mole of water per mole of methane entering
the reaction zone. Similarly, the current experiments show that in the counterflow configuration the fuel stream can
contain just over one mole of water per mole of methane before flame extinguishment occurs. The results indicate
that water has relatively little effect on peak temperature, but the location of the peak moves toward the air nozzle
with increasing water addition. Computationally, chemical kinetic calculations introducing water vapor into the fuel
stream using GRI MECH 3.0 and GRI 1.2 (reduced model) kinetics mechanisms examine the critical conditions of
water carrying capacity in flames and flame temperatures at extinction. The results indicate that water reduces
temperatures and the concentrations of the radicals critical chain initiating, propagating, and branching reactions,
ultimately leading to flame extinguishment.
Keywords: Water-laden flames; Nonpremixed methane/air flames; Counterflow flames; Extinction limits; Methane
hydrates
1. Introduction
Currently, about 80% of the energy used around the world comes from fossil fuels, such as petroleum, coal, oil
shales, bitumen, and natural gas, all which are used for commercial, domestic and industrial applications [1]. A source
of relatively clean energy that has not been deeply explored is methane hydrates (gas hydrates), which are abundant
and promising based on their capability for storing methane at high density. According to the U.S. Geological Survey
methane hydrates represent more than 100 years equivalent at current natural gas use levels [1]. Gas hydrates are
composed of small guest molecules entrapped (enclathrated) in a lattice cavity of a water polyhedral crystal structure
that can vary in shape depending on the size of the guest molecule. Because hydrates are non-stoichiometric, with
complex interfaced cage structures (not all of which can be filled), the molar ratio of methane-to-water is only
approximate, with an ideal hydrate containing about 85% water (H2O) and 15% methane (CH4) by moles. Other
examples of water-laden fuel nonpremixed flames also occur in the combustion of water/fuel emulsions [2] where high
amounts of water are added to liquid spray fuels to mitigate nitrogen oxide (NOx) production. The rapidly rising
fuel plume above LNG pool fires has been shown to incorporate large amounts of water on the fuel side of the flame,
Preprint submitted to Proceedings of the Combustion Institute May 16, 2014
10. and some gel fuels have relatively high water content as part of their structure. Another application involves steam
assisted flares [3]. The current study examines the general problem of water-laden fuel nonpremixed combustion but
it uses methane hydrates as its main example.
The complexity of creating and combusting hydrates is in large part due to the high water mole fraction in this
fuel and to the non-uniformity that accompanies the condensed phase formation process. Consequently, in order
to extract the key burning characteristics of highly water-laden fuels, we use the geometrically simplified and more
stable non-premixed counterflow flame configuration. In this study, experiments and computations were conducted.
Specifically, we introduced water into the methane fuel stream, whereas the opposite burner contained air as the
oxidizer stream; these opposed reactants create a stagnation point flow. The flame structure is described as steady
and one dimensional in a small region near the stagnation streamline. Experiments and computations are made
normal to the flame sheet and along the centerline of the flame. Stagnation point flow analysis facilitates one
dimensional computational simulations of counterflow nonpremixed flames and allows comparisons of experimental
flame structure measurements. The flow of counterflow flames is generally characterized by either the global or local
strain rate, which is defined as the representative velocity gradient. These flames are then suitable for studying
combustion suppression or extinction because the strain rate can be a controlled parameter that depends on the
flow rate of the reactants. The inverse of the velocity gradient is used to define the characteristic flow time in
determining the Damk¨ohler (Da) number. The Da can be used to set a criteria of flame extinction since decreasing
the characteristic flow time below a critical value can cause flame extinguishment. Thus, the opposed flow laminar
diffusion flame is an appropriate flame system for studying detailed chemistry and its role in the extinction water
laden methane/air flames.
This study differs from previous studies involving water addition because water is introduced from the fuel stream
whereas in past research water was introduced from the air stream to simulate extinguishment in fires. Specifically,
the current study examines extinction limits and the water carrying capacity of a water laden methane air flames.
The water is pre-vaporized to simulate the fuel/water gas phase mixture burning off of a methane hydrate. The
water/methane mixture was heated to permit molar ratios of water appropriate for the highly water laden hydrate
system. We realize that by prevaporizing the water we do not require the flame to provide phase change heat to the
hydrate as would be required in a natural hydrate flame. Our goal, however, is to extract the relevant chemistry of
the process which is not affected by the slight preheating of the fuel stream needed to maintain realistic water vapor
concentrations in the gas phase.
Early research on fire supression focused on water mist systems using counterflow nonpremixed flames, and the
objective was to use water as a replacement for the banned halon 1301. Studies determined the water concentration
at which flame extinquished and they identified optimal droplet size and mist loading density for best suppression
performance [4]. Lentati and Chelliah [5] used a 1-D counterflow configuration numerical model to investigate
monodisperse liquid droplets on the air side as a fire suppresant. They found that the minimum water volume
needed for extinguishment was for droplet size of 15 µm. For this size, maximum evaporation rate occured in the
zone of maximum oxygen consumption and radical formation. Lentati and Chelliah [5] raised the issue of whether
or not water had a chemical influence on the combustion process. They reported that most studies show imprecise
understanding of the chemical role that water plays in combustion.
2
11. Adding water mist from the air side was found to have a physical effect on extinction by reducing oxygen
concentration and a diluent effect from the fuel and air side; it was also found that water lowered the temperature
of the flame by absorbing sensible heat and latent heat of evaporation. Lentati and Chelliah [5] reported that
water from the air side can cause a chemical effect by enhanced overall three-body recombination reactions and a
change in the water-gas shift reaction, as well as changes to concentrations of branching radicals. Dryer [6] described
physically that water lowered the temperature of a flame and caused dilution. He then showed that water reduced
NOx emissions as a consequence of lowering flame temperature and by changing concentrations of hydroxyl radicals
and oxygen. Suh and Atreya [7] conducted experiments and computations to study the physical and chemical effect
of H2O on the flame structure of a counterflow methane nonpremixed flame with water added from the air side.
Their study used experimental temperature profiles (rather than solving the energy equation) in their numerical
simulations to better understand the effect of heat losses from radiation on flame extinction. They found that as
water is added from the air side the OH radical concentration increases. An increase in flame temperature and CO2
is seen with air side water addition, while CO decreases as water is added. Mazas et al.[8] found a reduction in H and
O concentrations in laminar methane-air/water flames, and an increase in OH concentrations. The effect of water
vapor addition to the air stream has also been studied in exhaust gas recirculaton (EGR) for industrial burners and
internal combustion engines. Adding water provided lower NOx and CO emissions due to the low flame temperatures
and the fact that water causes dilution.
The current paper begins balancing our understanding of water’s role in non-premixed combustion by evaluating
the situation where water is added on the fuel side of a flame. The work addresses experimentally and computationally
the water carrying capacity of a flame with water addition up to the point of extinction. The study shows how adding
water vapor into the fuel stream can cause changes to the flame structure and extinction limits, and how the addition
of water vapor can lead to chemical effects, such as changes in concentration of chain branching radicals, and physical
effects, such as lowering the flame temperature.
2. Experimental and numerical configurations
The experimental configuration is illustrated in Fig.1. Two stainless steel tubes are aligned and opposed with
a gap distance of L = 0.0127 m and with an inner diameter of 0.015 m. The two tubes are aligned so that their
centerlines are along the same axis. Fine wire mesh and stainless steel honeycombs are applied on the exits of each
duct in order to create uniform velocity profiles.
In this configuration, methane (CH4) with the addition of water vapor is introduced to the bottom burner, and air
as oxidizer (21% oxygen (O2)/79% nitrogen (N2)) issues from the top burner. In addition, a coflow of nitrogen (N2)
is introduced to both burners in order to minimize the influence of ambient gas on the reaction zone and to prevent
heating the upper duct. The gases are supplied from standard compressed cylinders with purity >99.9%. Water
is introduced into the bottom burner using a syringe pump (New Era 0-30 cc) and is mixed with the methane fuel
stream. The fuel and water vapor mixture run through a Wattco recirculation heater at a temperature of 600◦
C. To
confirm that water is vaporized and condensation is avoided throughout the system, Type K thermocouples monitor
the temperatures at the boundaries of the fuel and oxidizer burners and surrounding gas lines. The bottom burner
is heated using a ceramic band and gas lines are heated by wrapping them with heating bands, both of which can
3
12. reach a maximum temperature of 1100◦
C (though the normal operating temperature in the experiments is much
lower), and gas lines are insulated with fiberglass. The mass flow rates of methane, air, and nitrogen are measured
using Cole-Parmer mass flow meters with an uncertainty of ±0.2% of full scale.
Temperature profiles across the centerline of the flame were taken using a Type B thermocouple (Platinum
Rhodium; 30% Rhodium in one lead and 6% in the other) with a junction diameter of 200 ± 20 µm and a wire
diameter of 70 µm, verified using a microscope. Measured temperatures were corrected for radiative heat losses from
the thermocouple surface by assuming a spherical junction, a constant Nusselt number of 2.0 and constant emissivity
of 0.2. The accuracy of the thermocouple was 80-150 K, and the thermocouple was coated with Ceramabond to
prevent catalytic effects. The thermocouple was placed on a translation stage that is automated using a Velmex
stepping motor controller, communicating with a computer through a National Instruments DAQ unit.
In experiments and numerical calculations the strain rate is defined as shown in Eq.1. The gap distance between
the fuel boundary and the oxidizer boundary is L = 0.0127 m and is illustrated in Fig.1. VO and VF are the velocity
of oxidizer and fuel, respectively. Experimentally, flame extinction is achieved by setting VO and VCH4
to a fixed
value while the water to methane molar ratio, RH2O/CH4
(and the total VF = VCH4+H2O) is increased to the point
that the flame extinguishes by adding water to the fuel side flow. The velocities of the reactant streams at the
boundaries are calculated as the ratio of their volumetric flow rates to the cross-sectional area of the ducts.
To characterize the global strain rate we define the following [9]:
a =
−2VO
L
1 −
VF
VO
(
ρF
ρO
)
1/2
(1)
where ρ is density, V is velocity, and F and O refer to the fuel and oxidizer, respectively. The water-free fuel
condition considered in this experiment is a = 76 s−1
, which corresponds to an initial fuel flow rate of 3 L/min and
an oxidizer flow rate of 6 L/min. The strain rate increases with water addition since the water addition increases the
overall fuel flow velocity at fixed methane flow rate. The extinction limits are determined as the maximum water
carrying capacity in terms of RH2O/CH4
. In the computational studies performed in this work two different sets of
initial fuel and water compositions were investigated, with boundary conditions that are similar to the experimental
conditions and a case when the initial gas temperature is higher, at TF = 720 K. The initial compositions:
• TF = 550 and TO = 440 K: CH4=1 mol, H2O=0-1.5 mol
• TF = 720 and TO = 298 K: CH4=1 mol, H2O=0-1.2 mol
Both:
O2=0.21 mol, N2=0.78 mol, Ar=0.01 mol
The initial fuel nozzle boundary temperature is found experimentally to be close to TF = 550 ± 3K, as measured
with type K thermocouples. The temperatures at each burner vary since there is a ceramic band on the fuel side
and heating bands across the gas lines, which cause a variation in temperatures at the exit of the burners, hence
the interest to see the effect of higher exit fuel temperatures of TF = 720 K. The studies were performed under
atmospheric pressure conditions for initial boundary conditions on the oxidizer side of TO = 298 K with a water-free
strain rate condition of a = 76 s−1
and varying water to methane ratios of RH2O/CH4
= 0−1.2
4
13. Kinetics simulations were performed for water-laden methane/air nonpremixed counterflow flames in order to
predict the water carrying capacity limit at which the flame cannot be sustained under the idealized conditions of
the 1-D counterflow system. The counterflow configuration is a standard one for kinetic modeling so the specifics
of the approach are not repeated here. Further details of the modeling approach can be found in Ricchiuti et al.
[10]. Using the reduced kinetics model decreases the computational time by replacing the differential equations of
intermediate species that are assumed to be in steady state by algebraic relations, but in addition it allows for a
better understanding of the chemical kinetics by having the ability to manipulate the fewer important parameters
that may influence global properties. GRI Mech 1.2 considers 104 equations, 5 elements (H, O, C, N, Ar) and 24
species. One aspect of the analysis is to determine the influence that net production rates of species and enthalpy
have as they influence the endothermic or exothermic direction of key reactions. The reactions most influenced by
water addition are reported in Richiuti et al. [10].
3. Experimental results and discussion
3.1. Flame appearance
Figure 2 shows a methane/air nonpremixed counterflow flame with no water added (RH2O/CH4
= 0) into the fuel
stream and a flame with a slight addition of water (RH2O/CH4
= 0.3, 0.9), at water-free strain rate a = 76 s−1
(flow
rate of methane is 3 L/min and of air is 6 L/min). The water free flame image shows a typical methane diffusion
flame; a blue zone (located on the oxidizer side) and a luminous region (located on the fuel burner side) are separated
by a dark region. The luminous zone disappears with higher increments of water, while the blue zone region extends
and its thickness decreases. The appearance of a blue flame or a luminous zone is controlled by the fuel and air
composition as well as the temperature of the flame. Similar behaviors are seen for premixed flames with no water
added; leaner flames reduce the luminous zone.
In the blue zone the maximum peak flame temperature is observed and thus a primary combustion reaction occurs.
The luminous zone has been spectroscopically investigated and was found to contain sodium at small concentrations
that are due to impurities in the water and/or burners. The luminous zone is also an indication of soot precursors.
Seungro et al.[11] showed that with water addition, the yellow zone disappears and their kinetic calculations suggest
OH radicals attack and oxidize soot precursors, e.g acetylene (C2H2), in the yellow zone, as the increased amount of
water vapor can provide more OH to the heated pre-flame zone. Axelbaum et al.[12] show how fuel dilution and flame
temperature are major factors in controlling soot reduction or creation. They investigated methane diffusion flames
in coflow configurations, and found that with mixture fraction Z>0.4, the soot (or luminous region) is decreased and
narrowed because there is a higher dilution of the fuel. The current study focuses on flames near the water laden
extinction limit so there is no soot influence in this case.
3.2. Experimental temperature profiles and extinction limits
Figure 3 illustrates the experimental temperature profiles with and without water addition (RH2O/CH4
= 0, 1.4) in
the fuel stream for an initial water-free strain rate of a=76 s−1
. The peak flame temperature with no water added
(RH2O/CH4
= 0) is 1775 K. With water addition the maximum flame temperature decreases and the flame extinguishes
at a peak temperature of 1615 K. With preheated water addition in the fuel stream (VF incorporates water and
5
14. methane), the velocity from the bottom burner increases while VO remains fixed so the temperature profile moves
toward the air side since the location of the stagnation plane moves with water addition in the experiments. Figure
4 represents the computed axial velocity profiles for RH2O/CH4
= 0 and at the point of extinction RH2O/CH4
= 1.5.
The velocity profile for both cases are similar, with a dip at the flame location 0.0066 m from the fuel exit. It is
important to note that unlike the experiments the computations are carried out for a fixed global strain rate so the
fuel velocity does not increase with water addition.
Figure 3 shows experimentally that the width of the reaction zone decreases with water addition. This decrease
is due to the increase in total fuel-side flow velocity which produces an increase in global strain rate. We have
found that the temperature profile width near water-laden extinction is relatively insensitive to the initial water-
free strain rate since it is the global strain rate with water addition that governs the transport at the condition
of interest. Similarly, Katzlinger et al.[13] investigated a nonpremixed flame for extinction characteristics with CO
addition to either the fuel or oxidizer stream and they found that the global strain at extinction was not influenced
by CO addition. A paper by Su et al.[7] reported that width changes in temperature profiles may be due to velocity
variations and reactant compositions. The modest variation at the condition of interest in the reaction thickness
means that the Damk¨ohler number will not be influenced significantly by the variation of velocity with water addition
into the fuel stream, which can allow us to establish a critical condition of extinction for water laden methane air
nonpremixed flames. Figure 3, as compared with Figure 5, shows a qualitative agreement between the experimental
and computational temperature profiles. The temperature in both cases decrease with water addition. Also, the
position in the maximum peak temperature for both water-free cases align closer to each other than with water
addition. As water is added a distinct shift is observed in the measured and computational data. The overal thermal
width is shown in the experimental data to increase in lower strained flames and to allow a higher water carrying
capacity; this means that water can be distributed in a larger region making it more difficult to extinguish the flame.
4. Computational results and discussion
4.1. Temperature and extinction limits
Figure 5 shows a computed temperature profile for a water laden methane/air nonpremixed counterflow flame
under fuel inlet temperatures TF = 720 K, and TF = 550 K, and at a water-free strain rate of a = 76 s−1
. The
temperature of the flame with varying water to methane molar compositions RH2O/CH4
is illustrated across a total
gap distance of x = 0.0127 m, between the fuel at x = 0 m and oxidizer burner at x = 0.0127 m. The figure
shows that for a water-free case RH2O/CH4
= 0 the temperature will reach a maximum temperature of 2051 K at
a flame position of x = 0.00525 m. The peak temperature of the profiles with increasing water to methane molar
ratios, RH2O/CH4
= 0 − 1.5, decreases as more water is added to the fuel. The maximum temperature just prior to
extinction for RH2O/CH4
= 1.1 is 1452 K and 1890 K for RH2O/CH4
= 1.4, with TF = 720 and TF = 550 K, respectively.
The inlet boundary condition for TF = 720 K has a temperature at extinction much lower than experiments, but
the water carrying capacity is relatively close to the measured value. Cases with a lower inlet fuel temperatures
of TF = 550 K yield relatively high temperatures at extinction and high water carrying capacity RH2O/CH4
= 1.5,
as compared to the experiments. The temperature drops with gradual water additions was 20-30 K for both cases.
One of the advantages of using the simulations to provide insight is that small changes near extinction can be seen
6
15. to create a drastic drop in temperature which indicates that when water is introduced to the fuel stream at higher
inlet temperatures, the decomposition of water (endothermic process) creates an enhanced pool of radicals but at
the expense of thermal drain from the exothermic zone that drops the local temperature and shifts the important
chemistry. These subtleties are masked in the experiment because of the natural uncertainties and fluctuations.
The reactions that have been affected most heavily with water addition can be found in [10]. These findings
are important for our understanding of methane hydrate flames because they show that water evaporated into the
fuel stream will have a modest effect unless the evaporated water reaches a critical value. Since in a hydrate flame
the evaporated water depends on flame standoff distance there will be an important balance stabilizing the flame
location. It appears, therefore, that a methane hydrate will be easier to ignite (as there is little evaporation initially)
than to maintain burning but that any hydrate that is ignited should burn continuously.
4.2. Flame structure and influence of water vapor on major radicals
Figure 6 shows the distribution of temperature and species mole fractions across the flame for RH2O/CH4
= 0, 1.4
with an inlet fuel temperature of TF = 550 K. With no water added, a typical flame structure in a methane diffusion
flame is shown; the flame front is considered to reside at the maximum temperature, which corresponds to a maximum
concentration of major products H2O and CO2. In contrast to a flame with water vapor addition at RH2O/CH4
= 1.4
an increase in water concentration from the fuel stream and then a gradual decrease in the reaction zone can be seen.
Note that with water addition in the fuel stream and the maximum water concentration is not in the reaction zone.
The flame structure also shows important influences of water in some of the major chain-branching species O, OH,
and H, as shown in Fig.7. The chemical effects leads to changes in the concentration of these major chain branching
radicals, and as mentioned earlier, water physically leads to a lower temperature. Figure 7 shows that the decrease
in temperature with water vapor addition causes the concentrations of OH, H and O to decrease. Similarly, these
observations are also in agreement with Mazas et al.[8] and Suh et al, as we observe that OH concentration increases
with water vapor addition. Das et al.[14] who applied steam injection in laminar syngas flames, also observed this
effect. Fig. 6 shows that the flame position moves very little with water addition if the total fuel-side flow rate is
kept constant, as occurs in the computations. This is in contrast to the experimental observation of flame location
changes with water addition but only because the total fuel-side flow varied in the experiments as water was added
to a fixed methane flow rate. As has been well-established, OH, O, and H, are mainly created through the chain
branching reactions shown below, and which belong to the H2-O2 submechanism of GRI 1.2.
H + HO2 ⇐⇒ 2OH (2a)
H + O2 ⇐⇒ O + OH (2b)
O + H2 ⇐⇒ H + OH (2c)
The results for similar flow rate conditions, CH4 = 3 L/min and VO = 6 L/min and inlet boundary conditions
close to the experiments, TCH4
= 550 K and TO = 440 K were used to compute the net reaction rates [forward
minus backward] of OH as a function of distance for the major production and destruction reactions for a water
7
16. free flame and for a water-laden flame just prior to extinction. The results showed that H + O2 ⇐⇒ O + OH is
responsible for hydroxyl radicals production, and showed how water addition affects this reaction. With increase
in water, reactions 2OH ⇐⇒ O + H2O and OH + H2 ⇐⇒ H + H2O appear to be the reactions that drive up the
relative concentration of OH at the expense of O and H atoms. Fig. 9 compares the computed net heat release rate
profiles for three cases, RH2O/CH4
= 0, 1.4 and RN2/CH4
= 1.8. The dominant heat release is on the oxidizer side of
peak temperature for all three cases. Preheating of the oxidizer gives a secondary peak. We also find that without
water RH2O/CH4
= 0 we have a distinct endothermic region on the fuel side. With water addition in the fuel stream
RH2O/CH4
= 1.4 the net endothermic region disappears as temperature decreases just prior to flame extinguishment.
Likewise, the OH, H and O radical concentrations decrease as temperature decreases. It is also noted that the peak
values of OH and H occur on the oxidizer side of the flame. To verify that extiguishment is not purely thermal, an
inert with a lower heat capacity, N2 was added in the fuel stream instead of water but at a level that produced the
same peak temperature as occurred in the water case. Again the endothermic region disappeared and as in the water
case the radical concentrations decreased as the temperature dropped with dilution. The big difference, however,
is that with nitrogen dilution the O and H atom concentrations are much higher relative to the OH concentration
than in the case of water laden flames. The water addition makes the system preferentially selects OH radicals over
O and H, and it is this relative loss of these more reactive radicals that ultimately leads to extinguishment for the
water diluted system.
5. Conclusions
The extinction and structure of counterflow nonpremixed water-laden methane/air flames were experimentally
and computationally studied to understand the influence of water in the combustion process. Thermal profiles show
a decrease in temperature with water vapor addition. Simulations showed temperature drops at extinction down to
about 1400 K, with maximum water carrying capacity of 1.2 water/methane molar ratio for TF = 720 K and 1.5 closer
to experimental inlet fuel boundary conditions, TF = 550 K. In experiments flame temperatures reached 1650 K with
maximum water to methane molar ratios of up to 1.4. The experimental and computational results are comparable
to the maximum amount of water vapor observed in the combustion of methane hydrates, which indicates that the
water laden nonpremixed flame in a counterflow burner configuration may be suitable for understanding the chemical
and thermal effects of water in methane hydrate combustion. Methane hydrates flame temperatures are found to be
between 1850 to 2050 K, for water to methane molar ratio in the range of 1-1.3, similar to the water/methane molar
fractions found in the water laden methane-air counterflow flame configuration study presented in this paper. The
temperature profiles show a narrowing with increasing water addition but this effect is simply due to the increasing
strain rate caused by the higher velocity fuel inlet with water added. Similarly, the peak temperature moves toward
the air burner with water addition because the fuel (methane plus water vapor) velocity is increasing while the air
velocity remains fixed. Calculations show a similar temperature trend and profile with water addition.
Simulations also show that adding water into the fuel side for low inlet gas temperatures produces a relatively
small decrease in peak temperature (20-30 K). With higher inlet gas temperatures, as was demonstrated in this study,
a larger drop in temperature was observed with water addition, and this drop drove the system toward reduced the
overall radical pool. In addition, the excess water created a larger concentration of OH relative to O and H than
8
17. occurs for inert diluents, and this shift in the radical species diminishes the reactivity and leads to extinguishment
even with relatively high flame temperature. We expect a similar phenomenon to occur for hydrate flames and other
nonpremixed water-laden fuel combustion systems.
6. Acknowledgments
This work is supported by grants from the National Science Foundation (grant number CBET-0932415 as part
of the Center for Energy and Sustainability at California State University Los Angeles) and from the W.M. Keck
Foundation as part of the UCI Deep Ocean Power Science Laboratory.
9
18. References
[1] A. Demirbas, Methane Hydrates-Green Energy Technology, Springer-Verlag London Limited, 2010.
[2] D. Wu, W. Wang, L. Wang, S. Cao, J. Yan, An experimental investigation of spray characteristics of diesel-
methanol water emulsion, Master’s thesis, School of Engineering and Power Engineering, Xi’and Jia Tong
University.
[3] P. J. Smith, J. N. Thornock, S. T. Smith, M. Hradisky, Controlling steam assisted flare operations in light of
federal regulations, CrSim Inc., Draper, Utah (2013) 1–5.
[4] G. Thomas, Trans Inst Chem. Eng B: Part B-Process Safety and Ennvironmental Protection 78 (2000) 339–354.
[5] A. M. Lentati, H. K. Chelliah, Dynamics of water droplets in a counterflow fiel and their effect on flame
extinction, Combust. Flame 115 (1998) 158–179.
[6] F. Dryer, Water addition to practical combustion systems-concepts and applications, Proc. Combust. Inst.
(1977) 279–295.
[7] A. Atreya, J. Suh, The effect of water vapor on counterflow diffusion flames, International Conference on Fire
Research and Engineering (1995) 103–108.
[8] A. N. Mazas, B. Fiorina, D. Lacoste, T. Schuller, Effects of water vapor addition on the laminar burning velocity
of oxygen-enriched methane flames, Combust. Flame 158 (2011) 2428–2440.
[9] H. K. Chelliah, C. K.Law, T. Ueda, M. D. Smooke, F. Williams, An experimental and theoretical investigation
of the dilution, pressure and flow field effects on the extinction of methane-air nitrogen diffusion flames, Proc.
Combust. Inst 23 (1991) 503–511.
[10] V. Ricchiuti, CANTERA simulations of water-laden methane/air nonpremixed counterflow flames, Master’s
thesis, Polytechnic University of Milan (2013).
[11] O. C. Kwon, S. Lee, R. Padilla, D. Dunn-Rankin, T. K. Pham, Extinction limits and structure of counterflow
nonpremixed water-laden methane/air flames, in: International Symposium on Combustion, 2012.
[12] R. L. Axelbaum, D. L. Urban, B. C. P.B. Sunderland, Flame design, ACME Flight Experiments (2010) 1–39.
[13] G. Katzlinger, V. Amin, K. Seshadri, E. Pucher (Eds.), The influence of carbon monoxide on the structure and
extinction of nonpremixed methane flames, 2013.
[14] A. Das, A. K. Kumar, C. J. Sung, Combust. Flame 158 (2011) 345–353.
10
19. Figure 1: Experiment schematic of the counterflow configuration
Figure 2: Photographs of counterflow nonpremixed H2O/CH4/air flames of RH2O/CH4
(a), 0.3 (b) and 0.9 (c)
0 0.002 0.004 0.006 0.008 0.01 0.012
400
600
800
1000
1200
1400
1600
1800
Distance (m)
Temperature(K)
R
H
2
O/CH
4
=0
R
H
2
O/CH
4
=0.5
R
H
2
O/CH
4
=0.6
R
H
2
O/CH
4
=0.8
R
H
2
O/CH
4
=1.4
Figure 3: Temperature profiles as a function of distance for flow rates, CH4 = 3 L/min, Oxidizer=6 L/min
11
20. 0 0.002 0.004 0.006 0.008 0.01 0.012
−0.6
−0.5
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
Distance (m)
Velocity(m/s)
R
H
2
O/CH
4
=0
R
H
2
O/CH
4
=1.4
R
H
2
O/CH
4
=1.5
Figure 4: Computed velocity profile as a function of distance for TF = 550 K and TO = 440 K
0 0.002 0.004 0.006 0.008 0.01 0.012
400
600
800
1000
1200
1400
1600
1800
2000
2200
Distance (m)
Temperature(K)
R
H
2
O/CH
4
=0
R
H
2
O/CH
4
=0.7
R
H
2
O/CH
4
=1.0
R
H
2
O/CH
4
=1.4
Flame structure with experimental boundary conditions TF= 550 K and TO
= 440 K
0 0.002 0.004 0.006 0.008 0.01 0.012
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
Distance (m)
Temperature(K)
R
H
2
O/CH
4
=0
RH
2
O/CH
4
=0.4
R
H
2
O/CH
4
=0.7
RH
2
O/CH
4
=1.1
R
H
2
O/CH
4
=1.2
Flame structure with higher inlet fuel boundary conditions TF= 720 K and
TO = 298 K
Figure 5: Computed temperature profiles as function of distance for water free strain rate a = 76 s−1
12
21. 0 0.002 0.004 0.006 0.008 0.01 0.012
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Distance (m)
Speciesmolarfraction
0 0.002 0.004 0.006 0.008 0.01 0.012
400
600
800
1000
1200
1400
1600
1800
2000
2200
Temperature(K)
O
2
H
2
O
CH
4
CO
2
T
RH
2
O/CH
4
=0
Flame structure with no water,RH2O/CH4
= 0
0 0.002 0.004 0.006 0.008 0.01 0.012
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Distance (m)
Speciesmolarfraction
0 0.002 0.004 0.006 0.008 0.01 0.012
400
600
800
1000
1200
1400
1600
1800
2000
2200
Temperature(K)
O
2
H
2
O
CH
4
CO
2
T
RH
2
O/CH
4
=1.4
Flame structure with water addition, RH2O/CH4
= 1.4
Figure 6: Effect of H2O addition on structure of major radicals in a water laden CH4/air flame
13
22. 0 0.002 0.004 0.006 0.008 0.01 0.012
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
Distance (m)
Speciesmolarfraction
0 0.002 0.004 0.006 0.008 0.01 0.012
400
600
800
1000
1200
1400
1600
1800
2000
2200
Temperature(K)
OH
O
H
T
RH
2
O/CH
4
=0
RH2O/CH4
= 0
0 0.002 0.004 0.006 0.008 0.01 0.012
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
Distance (m)
Speciesmolarfraction
0 0.002 0.004 0.006 0.008 0.01 0.012
400
600
800
1000
1200
1400
1600
1800
2000
2200
Temperature(K)
OH
O
H
T
RH
2
O/CH
4
=1.4
RH2O/CH4
= 1.4
Figure 7: Effect of H2O addition on structure of radicals in the water-laden CH4/air and water free flame
0 0.002 0.004 0.006 0.008 0.01 0.012
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
Distance (m)
Speciesmolarfraction
0 0.002 0.004 0.006 0.008 0.01 0.012
400
600
800
1000
1200
1400
1600
1800
2000
2200
Temperature(K)
OH
O
H
T
R
N
2
/CH
4
=1.8
Figure 8: Effect of N2 addition on structure of OH, O, and H in a CH4/air flame
14
23. 2 3 4 5 6 7 8
x 10
−3
0
1
2
3
4
5
6
7
x 10
8
Distance (m)
NetHeatProductionRate(J/m3
−s)
2 3 4 5 6 7 8
x 10
−3
400
600
800
1000
1200
1400
1600
1800
2000
2200
Temperature(K)
T
Q
Q
R
H
2
O/CH
4
=0
R
H
2
O/CH
4
=1.4
R
N
2
/CH
4
=1.8
Figure 9: Computed net heat production as a function of distance for TF = 550 K and TO = 440 K
15
24. List of Figures
1 Experiment schematic of the counterflow configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2 Photographs of counterflow nonpremixed H2O/CH4/air flames of RH2O/CH4
(a), 0.3 (b) and 0.9 (c) . . 11
3 Temperature profiles as a function of distance for flow rates, CH4 = 3 L/min, Oxidizer=6 L/min . . . 11
4 Computed velocity profile as a function of distance for TF = 550 K and TO = 440 K . . . . . . . . . 12
5 Computed temperature profiles as function of distance for water free strain rate a = 76 s−1
. . . . . . 12
6 Effect of H2O addition on structure of major radicals in a water laden CH4/air flame . . . . . . . . . . 13
7 Effect of H2O addition on structure of radicals in the water-laden CH4/air and water free flame . . . 14
8 Effect of N2 addition on structure of OH, O, and H in a CH4/air flame . . . . . . . . . . . . . . . . . . 14
9 Computed net heat production as a function of distance for TF = 550 K and TO = 440 K . . . . . . 15
16
