NUMERICAL STUDY OF THE INHIBITION OF PREMIXED AND DIFFUSION

FLAMES BY IRON PENTACARBONYL*

M.D. Rumminger†, D. Reinelt‡, V. Babushok, and G.T. Linteris§

National Institute of Standards and Technology, Gaithersburg MD 20899, USA

(marc.rumminger@nist.gov, dirk.reinelt@ze.basf-ag.de, vbabushok@nist.gov, linteris@nist.gov)

Published in Combustion and Flame 116 (1-2), pp. 207-219

ABSTRACT

Iron pentacarbonyl (Fe(CO)5) is an extremely efficient flame inhibitor, yet its inhibition

mechanism has not been described. The flame-inhibition mechanism of Fe(CO)5 in

premixed and counterflow diffusion flames of methane, oxygen and nitrogen is

investigated. A gas-phase inhibition mechanism involving catalytic removal of H

atoms by iron-containing species is presented. For premixed flames, numerical

predictions of burning velocity are compared with experimental measurements at three

equivalence ratios (0.9, 1.0, and 1.1) and three oxidizer compositions (0.20, 0.21, and

0.24 oxygen mole fraction in nitrogen). For counterflow diffusion flames, numerical

predictions of extinction strain rate are compared with experimental results for

addition of inhibitor to the air and fuel stream. The numerical predictions agree

reasonably well with experimental measurements at low inhibitor mole fraction, but at

higher Fe(CO)5 mole fractions the simulations overpredict inhibition. The

overprediction is suggested to be due to condensation of iron containing compounds

since calculated supersaturation ratios for Fe and FeO are significantly higher than

unity in some regions of the flames. The results lead to the conclusion that inhibition

occurs primarily by homogeneous gas-phase chemistry.

 

*

 Official contribution of the National Institute of Standards and Technology; not subject to copyright in

the United States.

† National Research Council/NIST postdoctoral fellow

‡ Currently with BASF Aktiengesellschaft, 67056 Ludwigshafen, Germany

§ Corresponding author
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INTRODUCTION

A world-wide ban on the production of the principal halogenated fire suppressants,

including halon 1301 (CF3Br), has created a need for new, environmentally-acceptable

fire suppressants; however, an agent with all of the desirable properties of CF3Br is

proving difficult to find. Understanding the inhibition mechanisms of known, effective

flame inhibitors will help direct the search.

Certain metallic compounds have been found to be substantially more effective

flame inhibitors than halogen-containing compounds [1-3]. In particular, iron

pentacarbonyl (Fe(CO)5) was found to be one of the strongest inhibitors—up to two

orders of magnitude more effective than CF3Br at reducing the burning velocity of

premixed hydrocarbon/air flames [4,5]. Although Fe(CO)5 is highly toxic and

flammable, understanding its inhibitory effect could lead to development of effective

non-toxic agents.

The first detailed experimental studies of flame inhibition by iron pentacarbonyl

are the studies of Wagner and co-workers [1,4,5]. Iron pentacarbonyl's inhibition action

was studied by measuring the burning velocity of premixed flames with inhibitor

added to the reactants [5]. In that research, Bonne et al. found Fe(CO)5 to be

significantly more effective than Br2 in premixed hydrogen/air and n-hexane/air

flames and found that its inhibition effectiveness decreased as the pressure was

reduced below atmospheric. They also measured the emission intensity and absorption

of Fe and FeO at various heights in a low pressure (0.079 atm) flat premixed flame.

Experiments of Lask and Wagner [1] showed that Fe(CO)5 was more effective at

reducing flame speed in hydrocarbon/air flames than in hydrogen/air flames, and

more effective in hydrocarbon/air flames than in hydrocarbon/oxygen flames.

Reinelt and Linteris [6] studied the flame inhibition effect of iron pentacarbonyl

in premixed flames by measuring the burning velocity, and in counterflow diffusion

flames by measuring the extinction strain rate. In the premixed flames, behavior at low

and high Fe(CO)5 mole fractions was distinctly different: at low Fe(CO)5 mole fraction

the burning velocity was strongly dependent on inhibitor mole fraction, whereas at

high Fe(CO)5 mole fraction, the burning velocity was nearly independent of inhibitor

mole fraction. In the counterflow flames, the degree of inhibition depended upon the

location of the flame relative to the stagnation plane, as well as the location of the

inhibitor addition. In a methane/air flame, little change in extinction strain rate

resulted from addition of the Fe(CO)5 to the fuel stream. In contrast, when Fe(CO)5 was

added to the oxidizer stream the inhibition was strong for low Fe(CO)5 mole fractions,

and the rate of extinction strain rate decrease was lower as the inhibitor mole fraction

increased—but did not go to nearly zero as in the premixed flames. Reduction in

extinction strain rate occurred for Fe(CO)5 mole fraction up to 500 ppm (all uses of ppm

in this paper signify mole fraction · 106). A critical part of the research on Fe(CO)5 is to

understand iron pentacarbonyl’s diminishing effectiveness at high mole fraction in

order to avoid similar behavior in future fire suppressants.
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Despite the experimental work performed to date, the inhibition mechanism of

Fe(CO)5 has not been thoroughly described. For example, the relative effects of

homogeneous and heterogeneous chemistry have not been conclusively determined,

although each has been suggested [1, 5, 7]. Heterogeneous effects are of interest

because Fe(CO)5 can form condensed-phase particulates upon heating [8, 9]. In

premixed flames, some authors found that particles are formed upstream of the flame

[10] whereas others observed particles only downstream of the flame [11] or did not

comment on the location of the particles [5, 12, 13]. There exist conflicting claims in the

literature as to whether a homogeneous or a heterogeneous mechanism is responsible

for the inhibition. Since particles do not form at all flame conditions, and since the

inhibition action in premixed flames is strong at low mole fraction and levels off with

increasing inhibitor mole fraction [6], it is consistent that the inhibition may be through

homogeneous reactions at low inhibitor mole fractions and heterogeneous reactions at

high inhibitor mole fractions as was suggested by Jost et al. [4]. If particulates play a

key role in the inhibition, then the search for halon alternatives could be directed

toward chemicals that produce similar condensed-phase compounds.

Recently, Babushok and co-workers [14] have modeled the effect of an ideal gasphase flame inhibitor by assigning nearly gas-kinetic rates to a set of plausible radicalscavenging and inhibitor-regenerating reactions. Comparison of the measured burning

velocity of flames containing Fe(CO)5 with the calculated burning velocities of flames

containing an equal mole fraction of the ideal inhibitor showed that Fe(CO)5 (at low

initial mole fraction) performs nearly as well as the ideal inhibitor. Considering this

result, the authors argued that the Fe(CO)5 inhibition mechanism is dominated by

homogeneous gas-phase chemistry.

The goal of the present paper is to examine the extent to which a homogeneous

catalytic radical recombination mechanism for Fe(CO)5 can reproduce experimentallydetermined premixed flame burning velocities and counterflow diffusion flame

extinction strain rates, and to understand the features of such a mechanism. Results of

numerical simulations of one-dimensional premixed flames of various mixtures of

methane, oxygen, nitrogen and iron pentacarbonyl are presented and compared with

experimental measurements. The numerical results are analyzed to provide insight

into the inhibition mechanism. Calculated extinction strain rates of counterflow

diffusion flames are compared with experimental measurements for cases in which the

inhibitor is added to the fuel or air stream. The condensation of iron compounds and

formation of particulates in the flames are briefly discussed.

MODELING APPROACH

One-dimensional freely-propagating premixed flames are simulated using the Sandia

flame code Premix**[15], the Chemkin subroutines [16], and the transport property

 

** Certain commercial equipment, instruments, or materials are identified in this paper to adequately

specify the procedure. Such identification does not imply recommendation or endorsement by the

National Institute of Standards and Technology, nor does it imply that the materials or equipment are

necessarily the best available for the intended use.
4 Printed on 6/9/99

subroutines [17]. The kinetic and thermodynamic data of GRI-Mech 1.2 [18] (32 species

and 177 chemical reactions) serves as a basis for describing the methane combustion,

with iron species and reactions added as described below. Iron pentacarbonyl is added

to the unburned methane/oxygen/nitrogen mixture at mole fractions of up to 500 ppm.

The pressure is one atmosphere.

One-dimensional counterflow diffusion flames are simulated with a numerical

code developed by Smooke [19] and a one-carbon mechanism for methane oxidation

[20] (17 species and 52 chemical reactions). The somewhat smaller methane mechanism

captures the important chemistry of the flame, while reducing the computational time

required for calculating the extinction of the counterflow flames. Iron pentacarbonyl is

added to the unburned fuel or oxidizer stream at mole fractions of up to 500 ppm. The

pressure is one atmosphere.

INHIBITION MECHANISM

Metallic compounds have been studied in high temperature reacting flows for

applications such as flame suppression and materials synthesis [21-28], and rates for

reactions involving gas-phase metallic compounds have appeared in the literature. A

compilation of reactions and rates for a variety of metals in flames can be found in ref.

[29]. In work related to flame inhibition, authors have discussed inhibition

mechanisms that involve catalytic removal of H atoms by metal species (atomic, oxide,

or hydroxide) [23, 25, 30, 31]. The present work is an extension of previous work [32]

in which a gas-phase inhibition mechanism was developed based largely upon the

work of Jensen and Jones [33]. For completeness, the mechanism has now been

expanded to include a more detailed decomposition route for Fe(CO)5, a more

comprehensive set of iron-species reactions, as well as a different route for formation of

FeO from iron atoms.

Using information in the literature and reaction analysis, we compiled a list of

iron-containing species that could exist at significant concentrations in flames. In the

mechanism proposed here, the Fe(CO)5 decomposition products are Fe(CO)4, Fe(CO)3,

Fe(CO)2, FeCO, and Fe [34-37], and the intermediates and product species are Fe, FeO,

FeOH, Fe(OH)2, Fe(O)OH, FeO2, and FeH. There is little evidence that any other iron

compounds can exist at significant concentrations for conditions discussed in this

paper. For the iron-containing compounds, the thermodynamic data are from

references [38-40], and the transport properties are estimated. For the reader’s

reference, these data are listed in Table 1 and Table 2.

Development of Reaction Mechanism

Using the species listed above, over one hundred potential reactions were considered.

Analysis of reaction endothermicities and possible reaction mechanisms led to the

plausible set of reactions listed in Table 3, which shows the kinetic model for gaseous

iron compounds at flame conditions. Rate constants were obtained from the literature

or estimated using empirical procedures and reaction analogy. It was assumed that

iron species are non-reactive with hydrocarbon molecules. It should be emphasized

that the comprehensive set of additional reactions adopted for the present calculations
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should be considered only as a starting point. Numerous changes to both the rates and

the reactions incorporated may be made once a variety of experimental and theoretical

data are available for testing the mechanism. The reaction mechanism consists of four

parts: 1) decomposition of Fe(CO)5, 2) conversion of iron atoms to scavenging species,

3) scavenging of radicals through a homogeneous catalytic reaction cycle [33], and 4)

reactions involving FeO2, Fe(O)OH and FeH. These processes are depicted in Fig. 1.

Several experiments have shown that decomposition of Fe(CO)5 to Fe and CO

proceeds rapidly at elevated temperatures [36,41]. In our reaction mechanism,

decomposition of Fe(CO)5 involves sequential breaking of the Fe-CO bonds. Bond

energies are used as the activation energies for the decomposition reactions. Rate

constants of reverse reactions, independent of temperature, are taken from the

literature [34,37]. It was found that the reverse reactions proceed at a high pressure

limit at one atmosphere and room temperature, and that the activation energy does not

exceed 10.5 kJ/mol [37].

An important consumption reaction for iron atoms is Fe+O2, and experiments

have shown that the primary product of the reaction is FeO2 [42,43] (in previous work

[32], the primary products were FeO + O [44]). The species FeO2 is converted to FeO

primarily through the reaction FeO2 + O ↔ FeO + O2, which provides one of the

catalysts for the H-atom scavenging cycle

As will be described below, the inhibition mechanism is dominated by the

catalytic cycle for H-atom recombination

FeOH + H ↔ FeO + H2 (35)

FeO + H2O ↔ Fe(OH)2 (20)

 Fe(OH)2 + H ↔ FeOH + H2O (46)

 (net: H + H ↔ H2).

The cycle was developed by Jensen and Jones [33] to account for increased rates of

hydrogen atom recombination in the products of rich hydrogen/oxygen/nitrogen

flames with addition of Fe(CO)5. Rate constants were obtained by fitting calculated Hatom concentration profiles to the experimental measurements. Although the rates for

the catalytic cycle were derived from a hydrogen-oxygen flame, the sequence is likely

to be applicable to a hydrocarbon flame because of the importance of hydrogen-oxygen

chemistry in those flames; also, additional reactions of iron-containing species with

CH3 have been added.

RESULTS AND DISCUSSION

Premixed Flames

The decrease in the laminar burning velocity is used as a measure of the inhibition

action of iron pentacarbonyl. Measurements of burning velocity are from [6], in which

the average burning velocity was measured using the total area method with an

accuracy of approximately ±5%, as described in detail in [6]. Because the relative
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change in the burning velocity could be measured with more confidence than its

absolute value, the premixed flame results are presented in terms of the normalized

burning velocity, which is defined as the burning velocity of the inhibited flame

divided by the burning velocity of the uninhibited flame. For the reader’s reference,

the absolute calculated and measured burning velocities of the uninhibited flames are

listed in Table 4 for each flame condition. The uncertainty of the equivalence ratio

measurement is ±1.4%, the uncertainty of the oxygen mole fraction measurement is

±1.1%, and the uncertainty of the Fe(CO)5 mole fraction measurement is ±4%.

Use of the original rates recommended by Jensen and Jones for reactions 20, 35

and 46 [33] results in qualitative agreement with the measurements, but the predicted

inhibition is weaker than that observed in the experiments (shown by the dotted line in

Fig. 2). For example, for an initial Fe(CO)5 mole fraction of Xin = 100 ppm, the model

predicts a normalized burning velocity of 0.84, while the measured value is 0.63 ±

0.044. Sensitivity analysis [15, 45] is used to examine the relative importance of the

inhibition reactions for burning velocity reduction. Table 5 lists the maximum

sensitivity of the burning velocity to the rate constants of the iron reactions for a

stoichiometric flame with initial Fe(CO)5 mole fractions of 100 ppm and 500 ppm. (The

sensitivity coefficients have been normalized by the highest sensitivity coefficient,

which corresponds to the reaction H+O2 ↔ OH+O.) The results show that the burning

rate is insensitive to the decomposition rate of Fe(CO)5 and sensitive to each reaction in

the catalytic cycle (reactions 20, 35, and 46) roughly equally. Numerical experiments

using different rates for a one-step decomposition reaction for Fe(CO)5 also indicate

that the inhibition is relatively insensitive to the decomposition reaction. Likewise, the

inhibition is also relatively insensitive to the reaction pathway for FeO formation from

Fe.

The magnitude of the inhibition by Fe(CO)5 is sensitive to reactions 20, 35, and

46. Increasing the pre-exponential term of the specific reaction rate constant by the

reported uncertainties (3X, 5X, and 3X for the three reactions, respectively) increases the

inhibition effect and leads to better quantitative agreement with the measurements at

φ=1.0 and XO2

,ox=0.21 (XO2,ox refers to the oxygen mole fraction in the oxidizer prior to

mixing with the fuel). Such modifications to the mechanism lead to reaction rates that

are nearly gas kinetic. For the remainder of the present analyses and figures, these

higher values of the pre-exponential term are used (Table 3 shows the modified

values). Some justification for the use of higher rates exists because of the possibility of

condensation of iron species in the experiments of ref. [33] (condensation will be

discussed in more detail below). The temperature in the Jensen and Jones test burner

(1800 K to 2150 K) was such that iron species were probably supersaturated, creating

the possibility of condensation; this loss of active gas-phase iron-containing molecules

would have led to an underestimation of the reaction rates. Nonetheless, while use of

rate expressions within the reported uncertainty is valid, it should be emphasized

Jensen and Jones’ rate constants were deduced based on measurements in the

recombination region of hydrogen-oxygen-nitrogen flames, while the present flames

are methane-oxygen-nitrogen. Differences in the overall catalytic recombination rates
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caused by the iron species may be due to additional reactions in the present

hydrocarbon system. 

Fig. 2 shows the calculated and measured normalized burning velocity as a

function of initial Fe(CO)5 mole fraction (Xin) for premixed flames with equivalence

ratios of φ=0.9, 1.0 and 1.1 and XO2

,ox=0.21. The experimental results show strong

inhibition at low Xin and a rather sudden leveling off of the inhibition as Xin increases.

The numerical results are qualitatively similar to the measurements; however, the

inhibition effect at low Xin is not as strong as that observed in the experiments

(especially at φ=0.9) and at high Xin the inhibition effect does not diminish as

dramatically as in the experiments. A complete leveling-off of the burning velocity

does not occur in the simulations, even at initial Fe(CO)5 mole fractions of up to 1000

ppm. Numerical tests show that better agreement at different values of φ can be

obtained by adjustment of reaction rates, but such fine tuning seems premature at this

early stage of model development.

The decrease in the calculated inhibition effect as Xin increases is due to the

decrease in the quantity of superequilibrium H atoms (defined as the difference between

the peak XH in the flame and the equilibrium XH at the flame temperature) [6,24,32,46].

This decrease can be seen in Fig. 3, where XH,peak - XH,eq is plotted for varying Xin for the

φ=1.0 flame (the XO2

,ox =0.20 and 0.24 flames are discussed below). Since the inhibition

cycle (reactions 20, 35, and 46) is based on the scavenging of H radicals, the chemical

effect of the inhibitor decreases with decreasing quantity of superequilibrium H atoms

and a saturation effect is observed. Interestingly, halogenated flame inhibitors also

show a saturation effect [47, 48], and the cause of the reduced effectiveness, reduction

in radical superequilibrium, may be the same.

The effect of oxygen mole fraction on the burning velocity is presented in Fig. 4a,

Fig. 4b, and Fig. 4c for φ=1.0, 1.1, and 0.9. Fig. 4a shows the normalized burning

velocity as a function of Xin for φ=1.0 flames with three oxidizer compositions: XO2

,ox =

0.20, 0.21, and 0.24. The experimental results show that as XO2

,ox increases, the

inhibition effect decreases. As Fig. 4a-c show, the numerical model qualitatively

predicts this behavior with respect to XO2

,ox for φ=1.0, φ=1.1 and φ=0.9. Further, an

analysis of the numerical results shows that for stoichiometric flames with 50ppm

Fe(CO)5 in the reactants, the catalytic recombination mechanism accounts for 20, 19, and

15% of the total H-atom reaction flux for consumption of H, for XO2

,ox=0.20, 0.21, and

0.24. That is, at higher oxygen mole fractions, the creation and destruction fluxes for

hydrogen become larger, and the iron-species reactions become a smaller fraction of the

total flux (for a fixed initial inhibitor mole fraction).

Fig. 5a shows calculated mole fraction profiles of the iron compounds, H-atom

and temperature through the stoichiometric premixed flame with XO2

,ox = 0.21 and

Xin=100 ppm. The zero point of the x-axis is defined as the position where 50% of the

CH4 has been consumed. The mole fraction profile of Fe(OH)2 exhibits a local

minimum in the region of maximum H-atom mole fraction. At that location, the mole

fractions of H and OH are sufficiently large so that the reactions of Fe(OH)2 with H and
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OH proceed at a rate faster than the Fe(OH)2 production reactions. Fig. 5b. shows

similar species and temperature profiles but with Xin = 500 ppm. The flame thickness

is increased as a result of the lower burning rate. At this higher inhibitor loading, the

profile for Fe(OH)2 does not show a local minimum, since with higher mole fractions of

the inhibiting species, H is the limiting species in the catalytic scavenging cycle (recall

from Fig. 3 the sharp decrease in XH as Xin increases).

The rapid decomposition of Fe(CO)5 creates the possibility of condensation of

iron particles in the pre-heat zone of the flame. To investigate this possibility, an

abbreviated kinetic nucleation model for iron atoms [8] was included in the chemical

mechanism (with iron clusters up to Fe9). The kinetic nucleation model consisted of

reactions describing the growth and coagulation of iron particles. For a stoichiometric

flame with Xin = 100 ppm, the calculations showed that some condensation occurs

between the Fe(CO)5-decomposition zone and the flame reaction zone, but there is not

enough time for significant amounts of nucleation before the Fe reacts to form other

species or the gas temperature rises to slow the nucleation rate. Not more than 5 - 10%

of the Fe condenses and participates in nucleation reactions in the pre-reaction zone,

which has a relatively small effect on the burning rate. This conclusion that

condensation has an insignificant effect on the burning rate, however, only applies to

condensation of iron atoms in the pre-heat region, and not to other species or other

regions of the flame. For example, some of the iron particles that form in the pre-heat

zone may vaporize, or continue to grow as they pass through the flame front. More

importantly, other iron compounds may condense later in the flame.

Counterflow Diffusion Flames

Counterflow diffusion flames provide additional opportunities to study the behavior of

Fe(CO)5. The inhibitor can be subjected to different chemical and thermal histories by

varying the reactants, the location of the inhibitor addition, and the flame location. The

reduction in the extinction strain rate (aext, defined as the axial velocity gradient in the

oxidizer stream at extinction) is used as a measure of the inhibition action of iron

pentacarbonyl.

The counterflow diffusion flame results are presented in terms of a normalized

extinction strain rate, which is defined as the ratio of the extinction strain rate of an

inhibited flame to that of an uninhibited flame. The normalized value is used since

Chelliah et al. [49] have shown that the absolute values of aext can depend on

experimental burner design and the numerical description of the flow field, whereas

the trends in aext are independent of the flow-field characteristics in the experiment or

model.

Fig. 6 shows the measured and calculated normalized extinction strain rates for a

methane/air flame with varying Fe(CO)5 input. The flame is located on the oxidizer

side of the stagnation plane, and the maximum temperature is approximately 1800 K at

extinction. For the uninhibited flame, the measured aext is 610 ± 30 s-1 and the calculated

aext is 520 s-1. Experimental results [6] show that when Fe(CO)5 is added to the oxidizer

stream a significant decrease in aext results. In contrast, when the Fe(CO)5 is added to
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the fuel stream, little change in aext results. The numerical simulations qualitatively

reproduce the significant dependence of inhibition on the location of Fe(CO)5 addition.

The effect of Fe(CO)5 on the extinction of two other counterflow flame

configurations was described in ref. [6]: (1) a flame located on the fuel side of the

stagnation plane (13% CH4/87% N2 vs. 45% O2/55% N2) and (2) a flame of diluted

methane located on the oxidizer side of the stagnation plane (20% CH4/80% N2 vs. 45%

O2/55% N2). In these two flames, addition of Fe(CO)5 to either the fuel or the oxidizer

stream resulted in little reduction in aext. Calculations of aext, using the mechanism in

Table 3, however, showed a significant decrease in aext as the inhibitor concentration

increased for the inhibitor in either stream. The next section discusses some possible

reasons for the difference.

Condensation of Iron Compounds

Fig. 2 and Fig. 4 show the experimentally measured normalized burning velocity

decreasing sharply at low inhibitor mole fractions but eventually leveling off at high

inhibitor mole fractions. Additionally, the addition of Fe(CO)5 to a counterflow

diffusion flame had the strongest relative effect at low Xin. An explanation for the

decrease in inhibition as Xin increases is that gaseous iron compounds—which are the

primary inhibiting species—condense at higher inhibitor mole fractions, thus reducing

the concentration of inhibiting species. Such behavior was discussed by Jensen and

Webb [7] in numerical studies of suppression of afterburning in rocket plumes. Their

calculations showed that the formation of particles in the reaction zone lowered the

radical removal rate (i.e. the suppression) and led to less effective prevention of

afterburning in a rocket plume as compared to the pure gas-phase effect. Further

evidence of the importance of condensation can be found in the experimental data of

Fig. 4a-c, which shows that as XO2

,ox increases for constant φ, the normalized burning

velocity levels off at a higher value of Xin. Higher values of XO2

,ox result in higher flame

temperature and higher saturation vapor pressures for the iron compounds, so that

higher amounts of inhibitor are required for condensation to begin.

Fig. 7 shows that there exists a potential for condensation of iron species in the

Fe(CO)5-inhibited premixed flame described in this paper. The figure displays the

calculated supersaturation ratio (the ratio of the local mole fraction to the saturation

mole fraction, S = X / Xsat) of Fe and FeO through the flame front. The saturation mole

fraction was calculated from the gas temperature at each location in the flame using

data from ref. [38]. In the preheat zone of the flame, the supersaturation ratios for Fe

and FeO are very high, but as the temperature increases, the saturation mole fraction

increases more rapidly than the actual mole fraction, thus lowering the supersaturation

ratio. Also, as Xin increases, the supersaturation ratio increases at each location. Note

that the horizontal line at S=1.0 marks the saturation point, which is not necessarily the

condensation point.

The critical supersaturation ratio (Sc) can be used to describe the likelihood of

condensation of a supersaturated vapor. At values of S above Sc, rates of nucleation

and particle growth are high, whereas below Sc, rates of nucleation and particle growth

are relatively small. Frurip and Bauer [9] measured the Sc of iron vapor between 1600
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K and 2200 K and found Sc values of 10 and 1000, demonstrating that supersaturation

ratios greater than unity do not necessarily lead to condensation of iron vapor.

CONCLUSIONS

While the highly efficient inhibition action of metallic compounds in hydrocarbon-air

flames has been known for some time, there has existed controversy in the literature as

to whether the mechanism involves gas-phase or heterogeneous chemistry. This paper

presents the first numerical modeling of iron pentacarbonyl's extremely strong

inhibition action in Bunsen-type premixed and counterflow diffusion flames, and

provides evidence that inhibition occurs primarily by homogeneous gas-phase

chemistry at low initial Fe(CO)5 mole fraction. While we do not believe that the present

calculations explicitly rule out heterogeneous chemical effects, we believe that the

proposed mechanism, based on homogeneous chemistry, can explain much—but not

all—of our measurements.

Calculations using the rates constants for the catalytic cycle reactions (20, 35, and

46) suggested in ref. [33] yield normalized burning velocities in qualitative agreement

with experimental measurements; however, they predict less inhibition than was

measured. Analysis of the numerical results confirms that the primary inhibition

occurs through the catalytic cycle of reactions (20, 35, and 46). An increase in the rate

constants (within experimental uncertainty) of reactions (20, 35, and 46) leads to

improved agreement between experiments and calculations at low Fe(CO)5 mole

fractions for several equivalence ratios and oxygen concentrations. At higher initial

Fe(CO)5 mole fractions, however, the calculations predict a stronger effect than

measured and do not predict the leveling off in burning velocity that was measured.

Likewise, for a counterflow diffusion flame of methane flowing against air, calculations

of the extinction strain rate agree with experimental measurements at low values of Xin,

but at higher Xin the simulations predict a stronger effect on aext than was measured. In

both cases, the reduction in Fe(CO)5 effectiveness is suggested to be caused by

condensation of iron-containing species when higher initial mole fractions of inhibitor

are added.

Several results reported in this paper support the hypothesis that the flame

inhibition effect of Fe(CO)5 is primarily a result of gas-phase scavenging reactions: 1)

failure of the gas-phase model to predict the leveling off of burning velocity at high Xin,

2) the shift of the leveling-off point (in experimental results) of normalized burning

velocity to higher Xin as XO2

,ox increases (and flame temperature increases), and 3)

calculated supersaturation ratios of iron compounds that are significantly higher than

unity in some regions of the flames.

The performance of the present gas-phase mechanism is considered very good;

nonetheless, certain experimental observations are not fully accounted for. In

particular, the predicted inhibition for lean premixed flames is not strong enough, and

for diffusion flames with diluted methane the calculations significantly overpredict the

inhibition. The range of flame temperature studied here is fairly narrow: about 2100 K

to 2400 K in the premixed flames and about 1800 K to 2000 K in the diffusion flames.
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Future research will examine a wider variety of counterflow diffusion flames—which

will allow greater variation in temperature and gas composition—and will measure

particulate properties to elucidate the role of condensed iron compounds. More

research is desirable to test the validity of the higher rates for reactions 20, 35, and 46

indicated here, and to determine if additional inhibition reactions are important in

hydrocarbon flames.

Acknowledgments

The financial support of the Alexander von Humboldt Foundation for D.R. and of the

Army Research Laboratory (project leader K. McNesby) for V.B. is gratefully

acknowledged. The helpful suggestions of Professor Harsha Chelliah concerning the

counterflow diffusion flame calculations contributed greatly to this research, as did the

generosity of Professor Mitchell Smooke for the use of his diffusion flame code. The

authors also thank Mr. Newton Liu for helping with the numerical simulations and Dr.

Wing Tsang for helpful discussions and suggestions.

REFERENCES

[1] Lask, G., and Wagner, H.G., Eighth Symposium (International) on Combustion,

Williams and Wilkins Co., Baltimore, 1962, p. 432.

[2] Vanpee, M., and Shirodkar, P., Seventeenth Symposium (International) on

Combustion, The Combustion Institute, Pittsburgh, 1979, p. 787.

[3] Miller, D.R., Evers, R.L., and Skinner, G.B., Combust. Flame 7: 137 (1963).

[4] Jost, W., Bonne, U., and Wagner, H.G., Chem. Eng. News 39: 76 (1961).

[5] Bonne, U., Jost, W., and Wagner, H.G., Fire Research Abstracts and Reviews 4: 6

(1962).

[6] Reinelt, D., and Linteris, G.T., 26th Symposium (International) on Combustion, The

Combustion Institute, Pittsburgh, 1996, p. 1421.

[7] Jensen, D.E. and Webb, B.C., AIAA J. 14: 947 (1976)

[8] Krestinin, A.V., Smirnov, V.N., Zaslonko, I.S., Khimicheskaya Fizika (Chemical

Physics) 9(3): 418 (1990).

[9] Frurip, D.J. and Bauer, S.H., J. Phys. Chem. 81(10): 1001 (1977).

[10] Vree, P.H., and Miller, W.J., Fire Research Abstracts and Reviews 10: 121 (1968).

[11] McMillin, B.K., Biswas, P., and Zachariah, M.R., J. Mater. Res. 11(6): 1552 (1996).

[12] Egerton, A., and Rudrakanchana, S., Proc. R. Soc. London A225: 427 (1954).

[13] Linevsky, M.J., Metal Oxide Studies - Iron Oxidation, Technical Report RADC-TR71-259, October 1971.

[14] Babushok, V., Tsang, W., Linteris, G., Reinelt, D., “Chemical Limits to Flame

Inhibition”, accepted for publication in Combust. Flame, 1998.

[15] Kee, R.J., Grcar, J.F., Smooke, M.D., and Miller, J.A., A Fortran Computer Program

for Modeling Steady Laminar One-dimensional Premixed Flames, Sandia National

Laboratories Report SAND85-8240, 1991.
12 Printed on 6/9/99

[16] Kee, R.J. Rupley, F.M., and Miller, J.A., CHEMKIN-II: A Fortran Chemical Kinetics

Package for the Analysis of Gas Phase Chemical Kinetics, Sandia National Laboratory,

SAND89-8009B, 1989.

[17] Kee, R.J., Dixon-Lewis, G., Warnatz, J., Coltrin, R.E, and Miller, J.A., A Fortran

Computer Package for the Evaluation of Gas-Phase, Multicomponent Transport Properties,

Sandia National Laboratory, SAND86-8246, 1986.

[18] Frenklach, M., Wang, H., Yu, C.-L., Goldenberg, M., Bowman, C.T., Hanson, R.K.,

Davidson, D.F., Chang, E.J., Smith, G.P., Golden, D.M., Gardiner, W.C., and

Lissianski, V., GRI-Mech 1.2, http://www.me.berkeley.edu/gri_mech.

[19] Smooke, M.D., Puri, I.K., and Seshadri, K., Twenty-First Symposium (International)

on Combustion, The Combustion Institute, Pittsburgh, 1986, p. 1783.

[20] Peters, N. in Reduced Kinetic Mechanisms for Applications in Combustion Systems (N.

Peters and B. Rogg, Eds.), Springer-Verlag, New York, 1993, p. 3.

[21] Erhard, K.H.L., and Norrish, R.G.W., Proc. R. Soc. London A234: 178-191 (1956).

[22] Norrish, R.G.W., Seventh Symposium (International) on Combustion, Butterworths,

London, 1959, p. 203.

[23] Rosser, W.A., Inami, S.H., and Wise, H., Combust. Flame 7: 107 (1963).

[24] Hastie, J.W., High Temperature Vapors, Academic Press, New York, 1975.

[25] Friedman, R., and Levy, J.B., Combust. Flame 7: 195 (1963).

[26] Bulewicz, E.M., and Padley, P.J., Thirteenth Symposium (International) on

Combustion, The Combustion Institute, Pittsburgh, 1971, p. 73.

[27] Jensen, D.E., J. Chem. Soc. Faraday Trans. II 76: 1494 (1980).

[28] Cotton, D.H., and Jenkins, D.R., Trans. Faraday Soc. 67: 730 (1971).

[29] Jensen, D.E. and Jones, G.A., Combust. Flame 32: 1 (1978).

[30] Iya, K.S., Wollowitz, S., and Kaskan, W.E., Fifteenth Symposium (International) on

Combustion, The Combustion Institute, Pittsburgh, 1975, p. 329.

[31] Birchall, J.D., Combust. Flame 14: 85 (1970).

[32] Reinelt, D., Babushok, V., and Linteris, G.T., “Numerical Study of the Inhibition of

Premixed and Diffusion Flames by Iron Pentacarbonyl”, Eastern States Section

Meeting of The Combustion Institute, Hilton Head, SC, December 1996.

[33] Jensen, D.E. and Jones, G.A., J. Chem. Phys. 60: 3421 (1974).

[34] Seder, T.A., Ouderkirk, A.J., Weitz, E., J. Chem. Phys. 85(4): 1977 (1986).

[35] Engelking, P.C., and Lineberger, W.C., J. Am. Chem. Soc. 101(19): 5569 (1979).

[36] Lewis, K.E., Golden, D.M., and Smith, G.P., J. Am. Chem. Soc. 106: 3905 (1984).

[37] Weitz, E., J.Phys.Chem. 91: 3945 (1987).

[38] Gmelin Handbook of Inorganic and Organometallic Chemistry (8th Edition). Fe.

Supplement Vol. B1, Springer-Verlag, New York, 1991, p. 46.

[39] Gurvich, L.V., Veyts, I.V., Alcock, C.B. (Eds.), Thermodynamic Properties of

Individual Substances, 3rd Edition, Nauka, Moscow, 1978-1982. (NIST Special

Database 5. IVTANTERMO-PC, NIST, Gaithersburg, MD 20899).
13 Printed on 6/9/99

[40] Chase, M.W., Jr.; Davies, C.A.; Downey, J.R., Jr.; Frurip, D.J.; McDonald, R.A.;

Syverud, A.N., JANAF Thermochemical Tables (Third Edition), J. Phys. Chem. Ref.

Data, Suppl. 1, 1985, 14, 1-1856.

[41] Freund, H.J., and Bauer, S.H., J. Phys. Chem. 81(10): 994 (1977).

[42] Akhamadov, U.S., Zaslonko, I.S., Smirnov, V.N., Kinet. Catal. 29(2): 291.

(Engl.transl. p. 251) (1988).

[43] Helmer, M., Plane, J.M.C., J. Chem. Soc. Faraday Trans., 90(3): 395 (1994).

[44] Fontijn, A., Kurzius, S.C., and Houghton, J.J., Fourteenth Symposium (International)

on Combustion, The Combustion Institute, Pittsburgh, 1973, p. 167.

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295 (1988).

[46] Biordi, J.C., Lazzara, C.P., Papp, J.F., in Halogenated Fire Suppressants (R.G. Gann,

Ed.), American Chemical Society, Washington, D.C., 1975, p. 256.

[47] Linteris, G.T., and Truett, L.F., Combust. Flame 101:15 (1996).

[48] Linteris, G.T., Burgess, D.R., Babushok, V., Zachariah, M., Westmoreland, P., and

Tsang, W., Combust. Flame 113(1-2): 164 (1998).

[49] Chelliah, H.K., Law, C.K., Ueda, T., Smooke, M.D., and Williams, F.A., TwentyThird Symposium (International) on Combustion, The Combustion Institute,

Pittsburgh, 1991, p. 503.
14 Printed on 6/9/99

Table 1: Thermodynamic Properties of Iron-Containing Species

cp(T) [J/mol-K]

Species

ΔHf

°(298)

[kJ/mol]

S°(298)

[J/mol-K]

300 400 500 600 700 800 1000 1200 1500 2000 2500 3000

Fe(CO)5 [40] -727.9 439.3 171.1 189 200.8 209.82 217.1 223.11 228.09 238.49 244.63 250.25 252.22 255.01

Fe [39] 415.3 180.4 25.69 25.52 24.89 24.23 23.6 23.14 22.51 22.18 22.22 23.22 24.69 26.19

FeO [39] 261.2 235.6 37.36 37.66 37.78 37.82 37.91 38.07 38.45 38.91 39.75 41.34 43.26 45.35

FeO2 [39] 79.6 266.4 42.55 46.86 49.87 51.97 53.39 54.35 55.65 56.4 57.03 57.53 57.78 57.91

FeOH [39] 105.6 250.9 51.84 53.26 53.81 54.1 54.43 54.85 56.07 57.7 60.25 63.76 65.98 67.24

Fe(OH)2 [39] -303.8 286.8 84.56 89.45 91.84 93.22 94.27 95.27 97.32 99.58 102.97 108.11 112.42 116.02

Fe(O)OH)[39] 17.8 279 54.1 59.71 63.64 66.4 68.45 70.04 72.47 73.47 76.36 78.66 80 80.83

FeH [38] 510.4 225.1 29.2 29.7 31.3 32.9 34.1 36.4 38.1
15 Printed on 6/9/99

Table 2: Estimated Transport Properties for Iron-Containing Species. ε/kb, LennardJones potential well depth (K); σ, Lennard-Jones collision diameter (Å)

Species ε/kb (K) σ (Å)

Fe(CO)5 530 6.0

Fe 3000 4.3

FeH 3000 4.3

FeO 3000 4.3

FeO2 400 4.4

FeOH 400 4.4

Fe(OH)2 600 4.4

Fe(O)OH 500 4.4
16 Printed on 6/9/99

Table 3: Chemical kinetic mechanism for gaseous iron species in the flame. k = A Tb

exp(-Ea / (R T)). Units for k are cm, mole, s. The iron-containing species in the

mechanism are Fe, FeO, FeOH, Fe(OH)2, FeO2, FeH, Fe(O)OH, FeCO, Fe(CO)2,

Fe(CO)3, Fe(CO)4, Fe(CO)5

Reaction A b Ea/R Reference or note

1 Fe(CO)5 => Fe(CO)4+CO 2.00E+15 0 20131 [36]

2 Fe(CO)4+CO => Fe(CO)5 3.50E+10 0 0 [34], [37]

3 Fe(CO)4 => Fe(CO)3+CO 3.00E+15 0 2516 e, [35]

4 Fe(CO)3+CO => Fe(CO)4 1.30E+13 0 0 [34], [37]

5 Fe(CO)3 => Fe(CO)2+CO 3.00E+15 0 16105 e, [35]

6 Fe(CO)2+CO => Fe(CO)3 1.80E+13 0 0 [34], [37]

7 Fe(CO)2 => FeCO+CO 3.00E+15 0 11575 e, [35]

8 FeCO+CO => Fe(CO)2 1.50E+13 0 0 e

9 FeCO+M => Fe+CO+M 6.00E+14 0 10317 [8]

10 Fe+CO+M => FeCO+M 5.00E+14 0 0 [8]

11 FeCO+O => Fe+CO2 1.00E+14 0 0 e

12 Fe+O+M = FeO+M 1.00E+17 0 0 e

13 Fe+OH+M = FeOH+M 1.00E+17 0 0 e

14 Fe+H+M = FeH+M 1.00E+15 0 0 e

15 Fe+O2 = FeO+O 1.20E+14 0 10065 [44]

16 Fe+O2(+M) = FeO2(+M) 2.00E+13 0 0 [42], [43]

LOW 1.50E+18 0 2013

17 FeO+O+M = FeO2+M 1.00E+16 0 0 e

18 FeO+H+M = FeOH+M 1.00E+17 0 0 e

19 FeO+OH+M = Fe(O)OH+M 5.00E+17 0 0 e

20 FeO+H2O = Fe(OH)2 1.62E+13 0 0 [33]

 *

21 FeO+H = Fe+OH 1.00E+14 0 3020 e

22 FeO+CH3 = Fe+CH3O 1.00E+14 0 6039 e

23 FeO+H2 = Fe+H2O 1.00E+13 0 2516 [27], [33]

24 FeO2+H+M = Fe(O)OH+M 1.00E+17 0 0 e

25 FeO2+H = FeO+OH 1.00E+14 0 7549 e

26 FeO2+OH = FeOH+O2 1.00E+13 0 6039 e

27 FeO2+O = FeO+O2 1.50E+14 0 755 e

28 FeOH+O+M = Fe(O)OH+M 1.00E+18 0 0 e

29 FeOH+OH = Fe(OH)2 6.00E+11 0 0 e

30 FeOH+OH = FeO+H2O 3.00E+12 0 1007 [27]

31 FeOH+O = Fe+HO2 3.00E+13 0 10569 e

32 FeOH+O = FeO+OH 5.00E+13 0 755 e

33 FeOH+CH3 = FeO+CH4 5.00E+13 0 755 e

34 FeOH+H = Fe+H2O 1.20E+12 0 604 e

35 FeOH+H = FeO+H2 1.50E+14 0 805 [33]

*

36 Fe(O)OH+H+M = Fe(OH)2+M 1.00E+16 0 0 e

37 Fe(O)OH+CH3 = FeO+CH3OH 2.00E+13 0 2013 e

38 Fe(O)OH+H = FeO+H2O 2.00E+13 0 0 e

39 Fe(O)OH+H = FeO2+H2 5.00E+13 0 503 e

40 Fe(O)OH+H = FeOH+OH 4.00E+13 0 1007 e

41 Fe(O)OH+OH = FeOH+HO2 3.00E+13 0 10065 e

42 Fe(O)OH+OH = FeO2+H2O 5.00E+13 0 0 e

43 Fe(O)OH+O = FeOH+O2 5.00E+13 0 0 e
17 Printed on 6/9/99

44 Fe(O)OH+O = FeO+HO2 1.00E+13 0 7046 e

45 Fe(O)OH+O = FeO2+OH 5.00E+13 0 0 e

46 Fe(OH)2+H = FeOH+H2O 1.98E+14 0 302 [33]

*

47 Fe(OH)2+OH = Fe(O)OH+H2O 1.00E+13 0 9059 e

48 Fe(OH)2+CH3 = FeOH+CH3OH 1.00E+13 0 11575 e

49 FeH+O+M = FeOH+M 1.00E+15 0 0 e

50 FeH+O2+M = Fe(O)OH+M 1.00E+15 0 0 e

51 FeH+O2 = FeOH+O 1.00E+14 0 5033 e

52 FeH+H = Fe+H2 5.00E+13 0 0 e

53 FeH+O = Fe+OH 1.00E+14 0 0 e

54 FeH+OH = Fe+H2O 1.00E+14 0 0 e

55 FeH+CH3 = CH4+Fe 1.00E+14 0 0 e

e - estimation

*

 original recommended pre-exponentials for reactions 20, 35, and 46 are 5.40E+12,

3.00E+13, and 6.60E+13, respectively [33].
18 Printed on 6/9/99

Table 4: Calculated (vo,num) burning velocities, measured (vo,exp, [6]) burning

velocities, and calculated maximum temperatures for the uninhibited methaneoxygen-nitrogen premixed flames. The uncertainty in equivalence ratio is 1.4%. The

uncertainty in the XO2

,ox is 1.1%.

φ XO2,ox vo,num

(cm/s)

vo,exp

(cm/s)

Tmax, num

(K)

0.9 0.21 36.5 37.1 ± 1.9 2135

0.9 0.24 51.1 51.4 ± 2.6 2278

1.0 0.20 35.8 33.2 ± 1.7 2178

1.0 0.21 40.6 40.6 ± 2.0 2227

1.0 0.24 55.6 59.2 ± 3.0 2353

1.1 0.20 35.9 33.8 ± 1.7 2149

1.1 0.21 40.7 39.3 ± 2.0 2207

1.1 0.24 55.4 53.4 ± 2.7 2353
19 Printed on 6/9/99

Table 5: First-order sensitivity coefficient of the burning velocity with respect to

reaction rate for reactions with iron-containing species (v0 is the burning velocity, Ai

is the pre-exponential of reaction i) for two values of Xin, normalized by the peak

value for all reactions at all flame locations ( which corresponds to H+O2 ↔ OH+O ),

and ordered by sensitivity coefficient at 100 ppm. Reactions with sensitivity

coefficients with absolute values below 10-4 at 500 ppm are not listed.

# Reaction

dv dA

dv dA

i

i

0

0 max

 100 ppm 500 ppm

46 Fe(OH)2+H=FeOH+ H2O -5.1E-02 -2.1E-01

35 FeOH+H=FeO+H2 -2.8E-02 -1.0E-01

20 FeO+H2O=Fe(OH)2 -2.1E-02 -5.0E-02

32 FeOH+O=FeO+OH -1.6E-02 -5.6E-02

27 FeO2+O=FeO+O2 -1.6E-02 -5.5E-02

16 Fe+O2(+M)=FeO2(+M) -1.4E-02 -1.9E-02

33 FeOH+CH3=FeO+CH4 -1.2E-02 -7.8E-02

28 FeOH+O+M=Fe(O)OH+M -4.6E-03 -1.8E-02

30 FeOH+OH=FeO+H2O -2.5E-03 -1.5E-02

29 FeOH+OH=Fe(OH)2 -1.2E-03 -6.0E-03

13 Fe+OH+M=FeOH+M -6.4E-04 -2.8E-04

24 FeO2+H+M=Fe(O)OH+M -4.9E-04 -1.6E-03

34 FeOH+H=Fe+H2O -4.8E-04 -3.7E-03

12 Fe+O+M=FeO+M -3.6E-04 -3.3E-04

25 FeO2+H=FeO+OH -3.3E-04 -1.3E-03

42 Fe(O)OH+OH=FeO2+H2O -2.7E-04 -4.9E-04

18 FeO+H+M=FeOH+M -1.8E-04 -2.6E-04

39 Fe(O)OH+H=FeO2+H2 -1.7E-04 -2.1E-04

9 FeCO+M=>Fe+CO+M -1.6E-04 -3.1E-04

38 Fe(O)OH+H=FeO+H2O -1.5E-04 -1.8E-04

1 Fe(CO)5=>Fe(CO)4+CO -7.1E-05 -2.0E-04

48 Fe(OH)2+CH3=FeOH+CH3OH -3.2E-05 -1.1E-04

31 FeOH+O=Fe+HO2 8.8E-06 -1.3E-04

19 FeO+OH+M=Fe(O)OH+M 1.0E-04 2.4E-04

22 FeO+CH3=Fe+CH3O 3.4E-04 1.6E-03

26 FeO2+OH=FeOH+O2 7.0E-04 6.1E-03

15 Fe+O2=FeO+O 7.8E-04 -1.4E-03

23 FeO+H2=Fe+H2O 1.6E-03 3.9E-03

21 FeO+H=Fe+OH 2.5E-03 1.6E-03
20 Printed on 6/9/99

Fe(CO)5

Fe FeO2 FeO

FeOH Fe(OH)2

+H

+H

+H2O

+H

+O2 +O

Fig. 1: Schematic diagram of reaction pathways based on the gas-phase mechanism

described in this paper. Reaction partners are listed next to each arrow.
21 Printed on 6/9/99

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Fe(CO)5 Mole Fraction (ppm)

Normalized Burning Velocity

1.1

1.0

0.9

original, φ = 1.0

Fig. 2: Calculated and measured normalized burning velocity of premixed CH4/O2/N2

flames with XO2

,ox = 0.21 and varying amounts of Fe(CO)5. The solid lines are the

calculated normalized burning velocities using the rates in Table 3; the dashed line

is the calculated normalized burning velocity using the mechanism in Table 3 with

the original pre-exponential factors for reactions 20, 35, and 46 from ref. [33].

Symbols are measured normalized burning velocity from ref. [6] for φ=0.9 (triangles),

φ=1.0 (circles) and φ=1.1 (squares).
22 Printed on 6/9/99

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0 100 200 300 400 500

Fe(CO)5 Mole Fraction (ppm)

XH,peak - XH,eq

0.21

X = 0.24 O2,ox

0.20

Fig. 3: Calculated dependence of superequilibrium H-atom mole fraction (XH,peak -

XH,eq) on Fe(CO)5 addition in premixed stoichiometric flames with XO2

,ox = 0.20, 0.21,

and 0.24. The calculated value of XH,eq is approximately 2.8⋅10-4 for XO2

,ox = 0.20,

4.1⋅10-4 for XO2

,ox = 0.21, and 1.0⋅10-3 for XO2,ox = 0.24.
23 Printed on 6/9/99

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Fe(CO)5 Mole Fraction (ppm)

Normalized Burning Velocity

 0.20

X = 0.24 O 2

,ox

0.21

φ = 1.0

(a)

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Fe(CO)5 Mole Fraction (ppm)

Normalized Burning Velocity

 0.20 0.21

φ = 1.1

X = 0.24 O2,ox

(b)

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Fe(CO) 5 Mole Fraction (ppm)

Normalized Burning Velocity

 0.21

φ = 0.9

X = 0.24 O2

,ox

(c)

Fig. 4: Calculations and measurements of normalized burning velocity of premixed

CH4/O2/N2 flames with XO2

,ox = 0.20 (squares), 0.21 (circles), and 0.24 (triangles). (a) φ

= 1.0, (b) φ=1.1, and (c) φ=0.9. Experimental data from ref. [6].
24 Printed on 6/9/99

0E+00

2E-05

4E-05

6E-05

8E-05

1E-04

-2 -1 0 1 2 3

Distance (mm)

Mole Fraction

0 500 1000 1500 2000 2500

Temperature (K)

Fe(OH)2

Fe(CO)5

Fe(OH)

T

H/50

Fe

FeO

FeO2

(a)

0E+00

1E-04

2E-04

3E-04

4E-04

5E-04

-2 -1 0 1 2 3

Distance (mm)

Mole Fraction

0

500

1000

1500

2000

2500

Temperature (K)

Fe(OH) 2

Fe(CO)5

Fe(OH)

T

H/2

Fe

FeO

FeO2

(b)

Fig. 5: Gas temperature, iron compound mole fractions, and H atom mole fraction in

a CH4/O2/N2 flame with φ = 1.0 and XO2

,ox = 0.21. (a) 100ppm Fe(CO)5. (b) 500 ppm

Fe(CO)5. The location of zero corresponds to the point of 50% CH4 consumption.

Note the change in mole fraction scale from (a) to (b).
25 Printed on 6/9/99

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500

Fe(CO)5 mole fraction (ppm)

Normalized Extinction Strain Rate

Fig. 6: Normalized extinction strain rate for counterflow diffusion flame as Fe(CO)5

input varies. Closed symbols: measurements with the Fe(CO)5 in the oxidizer; open

symbols: measurements with Fe(CO)5 in the fuel; solid line: calculations with

Fe(CO)5 in the oxidizer; and dashed line: calculation with Fe(CO)5 in the fuel.

Experimental data from ref. [6].
26 Printed on 6/9/99

1E-04

1E-02

1E+00

1E+02

1E+04

1E+06

1E+08

-2 -1 0 1 2 3

Distance (mm)

Supersaturation Ratio

0

500

1000

1500

2000

2500

Temperature (K)

Fe (100 ppm)

FeO (100 ppm) Fe (500 ppm)

FeO (500 ppm)

T (500 ppm)

T (100 ppm)

Fig. 7: Calculated supersaturation ratios (Si = Xi / Xi,sat) for Fe (solid lines) and FeO

(dashed lines) in a φ = 1.0 and XO2

,ox = 0.21 flame at Fe(CO)5 concentrations of 100

ppm and 500 ppm. Note the horizontal line at S=1.0 to mark the saturation point

(but not necessarily the condensation point). Temperature profiles are also shown.

The location of zero corresponds to the point of 50% CH4 consumption.