Intermediate Species Profiles in Low-Pressure

Methane/Oxygen Flames Inhibited by 2-H

Heptafluoropropane: Comparison of Experimental Data with

Kinetic Modeling

BRADLEY A. WILLIAMS*, DREW M. L’ ESPE ́RANCE, AND JAMES W. FLEMING

Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5342

Experimental profiles of the intermediate species H, OH, CH, CF, CF2, and CHF are obtained in a 10 torr

premixed flat flame of methane/oxygen in a 1:2 molar ratio, inhibited by a 4% mole fraction of 2-H

heptafluoropropane (HFP, CF3CHFCF3). These data are compared to calculations using a recently published

kinetic mechanism describing the consumption of this fire suppression agent. The profiles in the flame inhibited

by HFP are compared to previously published data for flames containing CHF3 and CH2F2 under the same

conditions of stoichiometry and flux of fluorine atoms. The species profiles relative to the flames containing the

fluoromethanes are accurately predicted and atmospheric pressure flame speeds are fairly well predicted by the

kinetic mechanism. Under equal fluorine loadings, profiles of temperature and of H and OH mole fraction are

virtually identical between the flames containing HFP and CHF3. The flame inhibited by HFP, however, has

approximately twice as much CH* emission as the flame containing CHF3. The kinetic model predicts that

thermal decomposition, rather than H atom abstraction, is the primary destruction mechanism for HFP under

the conditions studied. © 1999 by The Combustion Institute

INTRODUCTION

2-H heptafluoropropane (commonly referred to

as HFP or HFC-227ea, having the chemical

structure CF3CHFCF3) is coming into widespread use as a fire suppressant [1] to replace

brominated compounds such as Halon 1301

(CF3Br), whose production has been prohibited

in developed countries by international treaty.

Due to the practical importance of HFP in fire

protection, as well as its somewhat lower suppression efficiency than Halon 1301 [2, 3], it is

desirable to understand this compound’s chemical behavior in flame environments. Of particular importance is the origin and magnitude of

suppression due to fluorine chemistry, as opposed to physical effects of the agent’s addition

to the flame.

A kinetic mechanism for combustion chemistry of C1 and C2 fluorocarbons has been developed at the U.S. National Institute of Standards

and Technology (NIST) [4–6]. Recently, our

laboratory has published profiles of several intermediate species in premixed low-pressure

methane/oxygen flames containing the series of

fluoromethanes [7]. These experimental data

were compared to predictions of the NIST

fluorocarbon mechanism, and some refinements

to the kinetics were suggested on the basis of

validation against both low-pressure species

profile data and atmospheric pressure flame

speed measurements.

During our experimental study of fluoromethane-inhibited low-pressure flames, we also

recorded profiles in a flame inhibited by HFP.

These data were not included in Ref. 7, because

the fluorine kinetic mechanism did not include

any C3 compounds, and only qualitative interpretations of the flame structure could be made.

More recently, Hynes et al. proposed a kinetic

mechanism [8] for the initial breakdown of HFP

into C1 and C2 fragments. When added to the

NIST HFC mechanism, the HFP submechanism

permits kinetic modeling of HFP-inhibited

flames. In Ref. 8, kinetic model predictions

were compared to experimental measurements

of postflame concentrations of stable species in

lean atmospheric pressure hydrogen/air flames

inhibited by HFP. Hynes et al. have since published a shock tube study of HFP decomposition

in a pyrolytic environment [9], and reported

stable species profiles in an atmospheric pressure hydrogen/ethane/air/HFP flame [10].

*Corresponding author. E-mail: brad@code6185.nrl.navy.mil

National Research Council Postdoctoral Associate 1995–97.

Present address: Department of Physical Sciences, New

Mexico Highlands University, Las Vegas, NM 87701-4073.

COMBUSTION AND FLAME 120:160–172 (2000)

0010-2180/00/$–see front matter © 1999 by The Combustion Institute

PII S0010-2180(99)00081-4 Published by Elsevier Science Inc.
Profiles of several stable and intermediate

species in low-pressure methane/oxygen/argon

flames inhibited by HFP were presented by

Battin-Leclerc et al., in Ref. 1. The profiles were

compared to those of an uninhibited flame, and

a flame inhibited by C2F6. The experimental

flame structure gave reasonable agreement with

predictions of a kinetic model taken from the

work of Westbrook [11] on inhibition by CF3Br,

with some additional steps added to account for

the initial breakdown of the inhibitor into C1

fluorinated species. In the present study we use

the NIST kinetic mechanism as a starting point,

since it is more comprehensive than the mechanism of Westbrook especially in terms of C2

chemistry. Hynes et al. developed their HFP

submechanism using the NIST mechanism to

describe the C1 and C2 fluorocarbon kinetics.

Here we compare the predictions of the HFP

kinetic mechanism (the C3 submechanism of

Hynes et al. [8–10] added to the HFC mechanism revised according to Ref. 7), with experimental species profiles. The flame conditions of

our study are quite different from those in Ref.

8, having considerably higher peak temperatures, slightly rich stoichiometry, and hydrocarbon fuel, as well as being at reduced pressure.

We validate the combined mechanism against

the atmospheric pressure flame speed data of

Linteris et al. [12], who investigated premixed

methane/air flames inhibited by several fluoroethanes and fluoropropanes, including HFP.

Kinetic modeling predictions of adiabatic flame

speeds were reported in Ref. 12 for the mixtures

containing fluoroethanes but not fluoropropanes. We compare modeling predictions of the

present mechanism with those of Hynes et al.

presented in Ref. 10. Based on comparison of

the model predictions to the experimental data

sets, we suggest some refinements to the C2

chemistry in the NIST HFC mechanism.

EXPERIMENTAL

The experimental apparatus and procedures

were identical to those used in our study on

flames containing fluoromethanes [7]. The

HFP-inhibited flame maintained at 10 torr consisted of a stoichiometric methane/oxygen mixture with 4% mole fraction of HPF added. This

flame contained an equal number of fluorine

atoms as the flame inhibited by 8.9% mole

fraction of trifluoromethane, the standard condition for this agent used in Ref. 7. The overall

equivalence ratio (taking into account the

agent) was also nearly identical, f 5 1.08 compared to 1.07 for the flame containing 8.9%

CHF3. Flow conditions of the HFP-inhibited

flame, as well as the uninhibited flame and

flames inhibited by CHF3 and CH2F2 used for

comparison of intermediate species profiles, are

listed in Table 1.

The flame species H, OH, CH, CF, CHF, and

CF2 were monitored by laser-induced fluorescence; the detection schemes for these species

are listed in Table 2, and have been more

completely described previously [7, 13]. Flame

temperature profiles were determined from laser-induced fluorescence (LIF) spectra of the

OH radical. Profiles of CH* chemiluminescence

at 430 nm were also recorded. All species

profiles were measured on a relative basis.

Profiles from an appropriate reference flame

(uninhibited, CHF3-inhibited, or CH2F2-inhibited) were recorded in immediate succession to

those in the flame inhibited by HFP, so that the

relative concentrations of the radical species for

the different flame conditions can be directly

compared. The structure and chemistry of the

flames inhibited by fluoromethanes is discussed

elsewhere [7]; the present study is focused on

the structure of the HFP-inhibited flame in

TABLE 1

Flame Conditions

Agent

Flow Rates (cm3

/min@0°C) Adiabatic Flame

Temperature (K)a f

Mass Flow

(g/cm2 CH -s) 4 O2 Agent

none 400 800 0 2571 1.00 0.86 3 1023

CH2F2 342 800 175 2557 1.07 1.08 3 1023

CHF3 400 800 117 2548 1.07 1.08 3 1023

C3HF7 400 800 50 2560 1.08 1.09 3 1023

INTERMEDIATE SPECIES PROFILES 161
comparison to those containing the C1 compounds.

LIF profiles were obtained by correcting the

measured LIF signal for changes in the intensity

of the laser used to excite fluorescence. The

corrected LIF profiles have been converted to

species mole fraction by correcting for gas density changes and the temperature dependence

of the ro-vibronic level probed. For the species

CHF the correction is very uncertain because

the rotational structure has not been completely

analyzed, and interference is present in the

region between 5–10 mm above the burner.

The LIF signals have not been corrected for

quenching variations between flames. The fluorescence signal was integrated over an interval

of approximately 20 ns immediately following

the laser pulse to minimize the influence of

quenching. For most species probed, fluorescence lifetimes were on the order of 150–200 ns

for all flame conditions. The only exceptions are

CF and CF2, which have short fluorescence

lifetimes (,50 ns) due to radiative decay and

predissociation, and hence are relatively unaffected by quenching. We estimate the experimental uncertainties in the ratios of LIF signals

between different flames to be 610%.

KINETIC MECHANISM AND MODELING

The chemical structures of the inhibited flames

were modeled using the Sandia PREMIX and

related codes [14–16]. The kinetic mechanism

and thermodynamic database used for the

H/C/O chemistry for one-carbon and two-carbon species was the Gas Research Institute

GRI-Mech 2.11 [17], with nitrogen chemistry

deleted. The NIST HFC mechanism, as refined

in Ref. 7, was used for the thermodynamics and

reactions of the C1 and C2 fluorinated species.

This mechanism gave good agreement with both

flame speed measurements and species profiles of

methane flames containing fluoromethanes [7].

For the initial breakdown of HFP, the reaction set

from Ref. 8 was used as the starting point.

This mechanism detailed above was modified

in the following ways (Table 3). First, the original

NIST HFC mechanism did not include reactions

of the CF3-CHF radical (a major thermal decomposition product of HFP) with atomic hydrogen.

In Ref. 12 kinetics for two product channels of this

reaction were proposed, as were revised kinetics

for a few other reactions. We adopted four of the

reactions proposed in Ref. 12, including the two

CF3-CHF 1 H reactions. The rate expressions

from Ref. 9 were used for the HFP thermal

decomposition reactions (numbers refer to Table

3)

CF3CHFCF3 f CF3CHF 1 CF3 , (R3-1)

CF3CHFCF3 f CF3CF5CF2 1 HF. (R3-2)

Thermal decomposition for CF3CHF,

CF3CHF f CF25CHF 1 F, (R3-19)

was added to the mechanism, with parameters

from Ref. 9. Additional reactions suggested by

Hynes et al. [10, 18] for HFP and products

(mostly reactions with hydrocarbon radicals)

have been added to the reaction set. Finally,

additional product channels (discussed in more

detail below) were added for the reactions of O

and OH with CF25CHF.

TABLE 2

Optical Diagnostics Used for Detection of Flame Species

Species Transition

Excitation

Wavelength

(nm)

Excitation

Energy

(mJ)

Emission

Wavelength

(nm)

H 1s-2s–4p (3 photon) 243.1(2-photon)

1486.1

150

10

486

OH R1(8)A–X (1,0) 281.27 30 311

CH Q1(6)B–X(0,0) 389.98 200 390

CF Q2(21.5) 1 P1(26.5)1

Q1(19.5)A–A(1,0)

223.30 200 255 6 15

CF2 A–X 250 60 334 6 20

CHF A–X(0,3,0)-(0,0,0) 492.42 300 .515 nm

162 B. A. WILLIAMS ET AL.
Falloff behavior for the thermal decomposition reactions in the HFP mechanism was estimated, since low-pressure kinetic studies were

not always available for analogous hydrocarbon

reactions. For the low-pressure Arrhenius parameters, the activation energy was assumed to

be identical to that in the high-pressure limit.

The preexponential factor was chosen such that

TABLE 3

Kinetic Mechanism

Mechanism consists of GRI-Mech 2.11, NIST HFC mechanism as revised in Ref. [7], HFP submechanism of Ref. [8],

with the following changes:

Arrhenius Parameters k(T) 5 ATb

exp(2Ea/RT)

Reaction A(mol, cm, s) b Ea (cal/mol) Ref.

1. C3F7H(1M) 5 CF3 1 CF32CHF(1M)

high-pressure limit 7.9E 1 15 0.0 85,000 [9]

low-pressure limit 3.46E 1 21 0.0 85,000 a,b

2. C3F7H(1M) 5 C3F6 1 HF(1M)

high-pressure limit 7.9E 1 12 0.0 69,600 [9]

low-pressure limit 3.46E 1 18 0.0 69,600 a,b

3. C3F7H 1 CH3 5 C3F7CH4 5.7E 1 10 0.0 9,500 [10]

4. C3F7H 1 C2H3 5 C3F7 1 C2H4 6.0E 1 10 0.0 7,000 [18]

5. C3F7(1M) 5 CF2:CF2CF3(1M)

high-pressure limit 1.0E 1 14 0.0 76,000 [8]

low-pressure limit 4.38E 1 19 0.0 76,000 a,b

6. C3F7(1M) 5 C3F6 1F(1M)

high-pressure limit 2.0E 1 14 0.0 69,000 [8]

low-pressure limit 8.75E 1 19 0.0 69,000 a,b

7. CF32CHF 1 CF3 5 C3F6 1 HF 7.0E 1 16 21.17 4,330 [18]

8. C3F6 1 O 5 CF3COF 1 CF2 5.8E 1 12 0.0 2,500 [10]

9. C3F6(1M) 5 CF3 1 CF2:CF(1M)

high-pressure limit 3.2E 1 13 0.0 105,600 [8]

low-pressure limit 1.4E 1 19 0.0 105,600 a,b

10. C3F6(1M) 5 CF2:CF2 1 CF2(1M)

high-pressure limit 3.2E 1 13 0.0 94,000 [8]

low-pressure limit 1.4E 1 19 0.0 94,000 a,b

11. CF3CHO 1 CH3 5 CF3CO 1 CH4 1.0E 1 11 0.0 7,400 [18]

12. CF3CHO(1M) 5 CF2CO 1 HF(1M)

high-pressure limit 1.0E 1 13 0.0 30,000 [8]

low-pressure limit 8.75E 1 18 0.0 30,000 a,b

13. CF3CHO(1M) 5 CF3 1 HCO(1M)

high-pressure limit 4.0E 1 16 0.0 80,000 [8]

low-pressure limit 3.5E 1 21 0.0 80,000 a,b

14. CF3COF(1M) 5 CF3 1 CF:O(1M)

high-pressure limit 4.3E 1 16 0.0 90,000 [8]

low-pressure limit 3.76E 1 21 0.0 90,000 a,b

15. CF:O 1 CHF2 5 CF2CO 1 HF 2.7E 1 13 0.0 20,000 [12]

16. CF32CHF 1 H 5 CHF:CF2 1 HF 6.56E 1 24 23.57 4,225 [12]

17. CF32CHF 1 H 5 CH2F 1 CF3 4.28E 1 16 20.74 4,750 [12]

18. HCO 1 CF3 5 CF2CO 1 HF 2.7E 1 13 0.0 0 [12]

19. CF32CHF(1M) 5 CHF:CF2 1 F(1M)

high-pressure limit 6.3E 1 14 0.0 62,000 [9]

low-pressure limit 5.51E 1 19 0.0 62,000 a,b

20. CHF:CF2 1 O 5 CF2 1 CHF:O 3.21E 1 06 2.0 0 a

21. CHF:CF2 1 O 5 CHF 1 CF2:O 3.2E 1 06 2.0 2,000 a

22. CHF:CF2 1 OH 5 CF2:O 1 CH2F 2.0E 1 06 2.0 2,850 a

23. CHF:CF2 1 OH 5 CHF:O 1 CHF2 4.0E 1 06 2.0 2,850 a

a

Estimated, see text.

b

Third-body efficiencies: H2O:9.0, CH4:2.0, CO:1.5, CO2:2.0, C2H6:3.0, CH3F:6.0, CH2F2:6.0, CHF3:6.0, C3F7H:12.0,

HF:2.0.

INTERMEDIATE SPECIES PROFILES 163
at a temperature of 1600 K, the transition

between low-pressure and high-pressure behavior occurs at 1.5 atm for C2 species (in nitrogen)

and 0.3 atm for C3 species. Kinetics in the

transition region were assumed to follow the

Lindemann form. Third-body efficiencies relative to nitrogen were the same used for the

fluoromethane decomposition reactions in Ref.

7. For HFP itself, a third-body efficiency of 12

was used.

Calculations of the experimental burner-stabilized flames were performed using the experimental temperature profiles as input. Multicomponent viscosities (the keyword MULT in

the PREMIX code) and thermal diffusivities for

H and H2 (TDIF keyword) were used to calculate species transport. The computational domain extended from the burner surface to 5 cm.

Mesh refinement tolerances (local/global variation) for the species profiles were set to 0.1 for

the species concentrations (GRAD parameter

in PREMIX) and to 0.25 for the concentration

gradients (CURV parameter). The final grid for

the uninhibited flame contained approximately

80 mesh points, while the solutions for the

inhibited flames contained approximately 110

mesh points.

TEMPERATURE PROFILES

Flame temperatures were determined from rotational energy distributions of the OH radical

by exciting the A-X (1,0) band near 281 nm. For

the temperature measurements, the laser energy was attenuated to ,1 mJ to avoid saturation. A broadband UV Filter (Corning 7-54)

was used to collect fluorescence on both the

(1,1) and (0,0) transitions. Spectral scans comprising typically between 10–40 spectral lines

covering a broad range of rotational energies

were performed at 15 points in each of the

flame conditions used. Statistical uncertainties

in the temperature fits ranged from 20–45 K.

Burner surface temperatures were measured by

an uncoated type K (Chromel–Alumel) thermocouple. For the PREMIX calculations and in

conversion of LIF data to species mole fractions, flame temperature profiles were represented by the (empirical) functional form

T~ x! 5 A

1 1 B expS

x 2 C

D D

2 Ex (1)

where x is the height above the burner in cm.

The coefficients A–E were fit to the experimental data points. The fitted functions were used as

temperature inputs to the PREMIX calculations, and for converting fluorescence intensity

profiles to species mole fractions.

Temperature measurements and fitted functions are shown in Fig. 1 for the HFP-inhibited

flame and, for comparison, the uninhibited

flame as well as the flames inhibited by CHF3

and CH2F2. The temperature profile of the

flame containing HFP is nearly indistinguishable from that containing trifluoromethane at

an equal loading of fluorine atoms.

EMISSION PROFILES

The methane/oxygen flame becomes markedly

brighter upon addition of HFP. Profiles of CH*

emission at 430 nm are shown in the bottom

panel of Fig. 2. The flame inhibited by HFP has

twice the luminescence of the flame inhibited by

CHF3, which in turn is slightly more than three

times as luminous as the uninhibited flame. In

Fig. 1. Temperature profiles of the flames studied. Individual data points were determined from LIF spectra of OH;

statistical uncertainties are shown. Solid lines indicate functional fits (Eq. 1) which were used as inputs to PREMIX

calculations and in converting LIF profiles into relative

mole fractions.

164 B. A. WILLIAMS ET AL.
the top panel of Fig. 2, CH* emission is estimated from the kinetics calculations by taking

the product of the C2H and O atom concentrations [7], since the C2H 1 O reaction is thought

to be primarily responsible for CH chemiluminescence. The peak value of this product compared to the uninhibited flame is quite close to

the experimental ratio of the emission intensities. The kinetic model predicts that formation

of C2H in the HFP-inhibited flame occurs, as it

does in flames containing fluoromethanes,

through recombination of fluorinated methyl

radicals with CH3. The fact that HFP forms C2

species in its initial decomposition steps does

not account for the greater luminescence than

in the CHF3-inhibited flame. Rather, the fraction of CF3 radicals that react with CH3 is

greater in the HFP flame because the agent’s

decomposition proceeds more quickly, leading

to a better spatial overlap between the profiles

of the two radicals. Also, HFP produces significant quantities of the partially fluorinated

methyl radicals CH2F and CHF2, whose recombination with CH3 in flames inhibited by CH3F

and CH2F2 leads to greater CH* chemiluminescence (at constant equivalence ratio and F:H

ratio) than in the CHF3-inhibited flame.

LIF PROFILES OF INTERMEDIATE

SPECIES

Calculated and experimental profiles of H atom

and OH are shown in Figs. 3 and 4, respectively.

As with the temperature profile, the profiles of

both these species are virtually identical between the flame inhibited by HFP and that

inhibited by CHF3 at an equal loading of fluorine atoms. The profiles of these two radicals

are also nearly identical for the flames containFig. 2. CH* emission profiles (bottom) at 430 6 10 nm for

the uninhibited flame and the flames inhibited by CHF3 and

HFP. Profiles for the inhibited flames have been reduced by

a factor of 2. Emission profiles (top) estimated from the

PREMIX calculation by taking the product of the O atom

and C2H concentrations.

Fig. 3. Calculated (top) and measured (bottom) H atom

profiles for the uninhibited flame, and the flames inhibited

by CHF3 and HFP.

INTERMEDIATE SPECIES PROFILES 165
ing mono- and difluoromethane as for trifluoromethane [7]. The kinetic modeling also predicts the close similarity of the H and OH

profiles between the flames inhibited by CHF3

and HFP.

Calculated and experimental profiles of CH

are shown in Fig. 5. The location of the CH

mole fraction peak is nearly identical between

the flames inhibited by HFP and by CHF3, but

the flame containing HFP has a slightly higher

peak CH concentration. The calculation predicts that the CH peak occurs slightly earlier in

the HFP-inhibited flame than in the CHF3-

inhibited flame. The CH concentrations in the

two inhibited flames, both relative to each other

and to the uninhibited flame, are predicted

quite well by the kinetic mechanism. The additional CH predicted to be formed from HFP is

produced primarily from 1CH2 and CHF, which

are produced in turn from CH2F and CHF2.

Profiles of CHF are shown in Fig. 6. The CHF

profile in the CH2F2-inhibited flame is used as a

reference for the HFP-inhibited flame, because

no detectable LIF signal for this species was

observed in the CHF3-inhibited flame. The LIF

profile for this species suffers from interference

in the luminous zone of the flame, as well as a

large and ill-defined temperature correction [7].

For these reasons, there is a great deal of

uncertainty in the location of the maximum

CHF concentration. The experimental data do

indicate a significant production of CHF in the

HFP-inhibited flame, roughly 20–30% that of

the CH2F2-inhibited flame. The kinetic model

predicts significant CHF production, in agreement with the experimental data. Analysis of

the kinetic pathways (see below) indicates that

CHF2 and CH2F are predicted to be the primary precursors to CHF.

Figures 7 and 8 show profiles of CF2 and CF,

respectively. The concentrations of these species in the HFP-inhibited flame relative to the

CHF3-inhibited flame are predicted fairly well.

The CF2 peak occurs earlier in the HFP-inhibFig. 4. Calculated (top) and measured (bottom) OH profiles for the uninhibited flame, and the flames inhibited by

CHF3 and HFP.

Fig. 5. Calculated (top) and measured (bottom) CH profiles for the uninhibited flame, and the flames inhibited by

CHF3 and HFP.

166 B. A. WILLIAMS ET AL.
ited flame, which the model predicts although

the profile shapes do not match exactly. The

location of the CF mole fraction peak is slightly

too far from the burner; this also occurs in the

model prediction of all the flames inhibited by

C1 fluorocarbons.

When reaction (R19) was added to the mechanism, the peak mole fraction of CF2 was overpredicted in the HFP-inhibited flame. Analysis

of the reaction pathways indicated that CHF:

CF2 produced by (R19) should be destroyed

almost exclusively by the reaction

CHF:CF2 1 H f CH2F 1 CF2. (R1)

It is likely that attack by O and OH contribute

more to this species’ removal than the mechanism predicts. We added additional channels for

these reactions (R3-20–R3-23), significantly improving agreement of the model with the experimental CF2 profiles.

KINETIC PATHWAYS AND MECHANISM

REFINEMENT

Kinetic pathways are shown in Fig. 9. Arrow

thicknesses between the different fluorocarbon

species are proportional to the molar flux integrated across the flame. In some cases, incoming and outgoing arrow thicknesses do not sum

to zero when, for instance, a C3 species breaks

up into smaller fragments. The predicted reaction pathways in our flame conditions are considerably different than those presented by

Hynes et al. in Ref. 8. In our CH4/O2 flames,

thermal decomposition dominates agent destruction, even after falloff kinetics are added to

the decomposition reactions. This is in contrast

to the finding of Hynes et al., where hydrogen

abstraction was predicted to account for the

majority of agent destruction. The flame studied

in Ref. 8 (hydrogen/air with equivalence ratios

between 0.3 and 0.5) had low peak temperatures

but high radical concentrations. In the present

conditions, H atom abstraction from HFP (primarily by OH) accounts for only about 5% of

agent destruction. We initially modeled the

Fig. 6. Calculated (top) and measured (bottom) profiles of

CHF for the flames inhibited by CH2F2 and HFP. The flame

inhibited by CH2F2 was used for reference since no CHF

signal was observed in the flame inhibited by CHF3. Near

1.0 cm the profiles may have been affected by saturation of

the detector due to flame emission.

Fig. 7. Calculated (top) and measured (bottom) CF2 profiles for the flames inhibited by CHF3 and HFP.

INTERMEDIATE SPECIES PROFILES 167
flame using the HFP mechanism of Ref. 8,

which led to the prediction that HF elimination

(R3-2) predominates over C-C scission (R3-1)

in destroying HFP. When the kinetic expressions from Ref. 9 are used for (R3-1) and

(R3-2), the calculation now shows that C-C

dissociation accounts for almost 90% of agent

removal.

The CF3-CHF radical produced by (R3-1) is

predicted to be removed primarily by two reactions, neither of which was included in the

original HFC mechanism or in the kinetic

model used in Ref. 8. Thermal dissociation to

eliminate an F atom from the CF3 group (R3–

19) is the dominant removal mechanism. For

this reaction we use the parameters from Ref. 9

with estimated falloff behavior. The HFC mechanism includes thermal decomposition reactions for fluorinated ethyl radicals involving

C-H bond dissociation, but not C-F dissociation,

the most likely process if the molecule contains

a CF3 group. It seems likely that analogous

reactions should be added to the HFC mechanism for other fluoroethyl radicals. Reaction

with H atom (R3-17) is predicted to be a

secondary process in CF3-CHF destruction. The

reaction of CF3-CHF with OH to produce

CF2CO, which Ref. 8 predicted to be the dominant removal reaction, is of minor importance

under our conditions when (R3-17) and (R3-19)

are considered.

The removal reactions of CHF:CF2 are another area of the mechanism which appears to

require attention. The inclusion of (R3-19)

greatly increases the importance of this species

in the kinetic pathways of HFP. When this

reaction was added, predictions of flame speeds

(discussed below) were greatly improved, but

the concentration of CF2 was overpredicted.

The removal of CHF:CF2 was predicted to

almost exclusively by reaction (R1), which accounts for the overprediction of CF2. This reaction is endothermic by about 10 kcal/mol.; the

alternative product channel, CHF2 1 CHF, is

too endothermic to be significant.

Reactions of CHF:CF2 with O and OH are

assumed in the HFC model to proceed by

analogous pathways to those for ethylene [19],

CHF:CF2 1 O f CFO 1 CHF2 (R2)

CHF:CF2 1 OH f CF2:CF 1 H2O. (R3)

It seems plausible that other channels may

become more important as the ethylene becomes progressively more fluorinated. For the

reaction of O atom with C2F4, the products are

COF2 1 CF2. We have postulated (R3-20) and

(R3-21) as analogous product channels for the

O atom reaction with trifluoroethylene. For the

reaction of OH with ethylenes, H atom abstraction was the only product channel considered

[19]. Reaction (R3) is essentially isothermic,

and it appears that addition may be a more

likely mechanism for OH attack on a heavily

fluorinated ethylene. This possibility was not

included in the HFC mechanism, partly because

one possible set of products (following HF

elimination) would be fluorinated vinoxy radicals, which are currently not included in the

mechanism [19]. In (R3-22) and (R3-23) products are assumed to be a fluorinated methyl

radical and a fluorocarbonyl (which could result

from H atom migration followed by C-C bond

rupture after the initial addition). This choice of

products is motivated in part to avoid introducFig. 8. Calculated (top) and measured (bottom) CF profiles

for the flames inhibited by CHF3 and HFP.

168 B. A. WILLIAMS ET AL.
ing additional species into the mechanism. Both

reactions are significantly exothermic.

Addition of (R3-20)–(R3-23) corrected the

discrepancy in the CF2 profile predictions

caused by the addition of (R3-19). The flame

profile data indicates that CHF:CF2 reacts significantly with some species besides H, but it

does not prove that the kinetics and products we

have postulated are correct. In Ref. 19 it was

remarked that consideration of additional product channels for the O and OH reactions with

fluoroethylenes would be desirable at a subsequent stage of validation. We concur with this

view in light of the importance of fluoroethylenes in the kinetic pathways of HFP and probably other fluoroethanes and fluoropropanes as

well.

ATMOSPHERIC PRESSURE FLAME

SPEED CALCULATIONS

Linteris et al. [12] published flame speed measurements of atmospheric pressure methane/air

mixtures inhibited by HFP as well as other

fluorinated ethanes and fluorinated propanes.

Comparisons with calculations using the NIST

HFC mechanism were reported for the fluorinated ethanes. No comparisons with model

predictions were reported for HFP.

As an additional test of the HFP submechanism, we performed flame speed calculations in

comparison to the data of Linteris et al. [12].

Calculations were performed for flames containing mole fractions of approximately 3%

HFP in methane/air mixtures with CH4/O2 ratios of 0.45, 0.50, and 0.55. The flame speed

calculations were performed on a domain extending 25 cm from the flame on the cold

boundary, and 60 cm on the hot boundary. The

calculations used multicomponent viscosities,

thermal diffusion for H and H2, and windward

differencing on the convective term. The initial

temperature of the fresh gases was set to 298.2

K. The solutions were obtained on meshes

having 150 grid points. Previously, we used the

same kinetic mechanism employed here (without the HFP submechanism) to calculate flame

speeds of CH4/air/CHF3 and CH4/air/CH2F2

mixtures [7]; good agreement with experimental

data was obtained for all conditions except for

difluoromethane-inhibited flames at high equivalence ratios (f . 1.25).

In initial calculations, predicted flame speeds

in the three HFP-inhibited mixtures were far

too low, ranging from 40% to 70% of the

experimental values. Sensitivity and reaction

pathway analysis indicated that thermal decomposition should dominate HFP removal, and

that predominance of C-C bond dissociation as

opposed to HF elimination should increase

Fig. 9. Reaction pathways of

fluorocarbon species in the

CH4/O2/C3HF7 flame according to the kinetic mechanism. Arrow thicknesses are

proportional to the molar

flux of carbon-containing

species. The most important

reaction partner(s) are written

next to each arrow in decreasing order of importance.

INTERMEDIATE SPECIES PROFILES 169
flame speed. This effect is presumably due to

the creation of two radicals by (R3-1), as opposed to two stable species by (R3-2). In Ref. 9

Hynes et al. derived new rate expressions for the

HFP decomposition reactions, and added (R3-

19) for CF3-CHF removal. The revised kinetic

parameters for (R3-1) and (R3-2) greatly increased the predicted predominance of C-C

bond rupture in HFP removal, and led to flame

speed predictions 1–2 cm/s higher.

The addition of (R3-19) caused a much larger

increase in the predicted flame speed for all

stoichiometries. The F atom generated by this

reaction reacts predominantly with water to

yield HF 1 OH, generating one of the important flame radicals. Addition of Reactions (R3-

20)–(R3-23) did not significantly affect the predicted flame speed. Using the final mechanism

as detailed in Table 1, the calculated flame

speed results are given in Table 4. The predictions agree with the experimental data within

30% for all three test cases. The prediction is

least accurate for the rich stoichiometry, where

formation of larger HFC species not included in

the present mechanism is most likely to be most

important.

One unusual aspect of the experimental flame

speed measurements reported by Linteris et al.

was that at a fixed agent concentration, HFP

caused a greater reduction in flame speed for

the 0.50 methane/oxygen ratio than for the 0.55

ratio. By contrast, all other HFCs agent tested

caused a progressively greater reduction in

burning velocity as the methane/oxygen ratio

was increased between 0.45 and 0.55. Our calculation does not predict the behavior observed

experimentally for HFP; rather, it predicts that

the inhibition effectiveness becomes progressively greater as the stoichiometry becomes

richer.

It is noteworthy that in Ref. 12, burning

velocities were significantly underpredicted for

mixtures inhibited by CF3-CH2F and CF3-

CHF2. CF3-CH2F is largely converted to CF3-

CHF by hydrogen abstraction, while CF3-CHF2

forms CF3-CF2. Based on the current modeling

results for HFP, the addition of Reaction (R3-

19) and an analogous decomposition for CF3-

CF2 should significantly increase the predicted

flame speeds from the values obtained in Ref.

12, improving agreement with the experimental

measurements.

DISCUSSION

The agreement between the experimental intermediate species data and the predictions of the

kinetic model is quite good. There is a slight

discrepancy in the predictions of CF2 profiles,

suggesting that removal kinetics of fluorinated

ethylenes by O and OH require further investigation. The addition of Reactions (R3-20)–(R3-

23) was the only change made to the mechanism

on the basis of improving the fit to the experimental profiles. No changes whatsoever were

made to the C1 fluorocarbon kinetics, because

predictions of fluoromethane-inhibited flame

structures are sensitive to these reactions, and

any alterations would have to be validated

against the data modeled in Ref. 7.

The kinetics of the HFP and CF3-CHF thermal decomposition reactions given in Ref. 9

greatly improve predictions of burning velocities; we have adopted these rate expressions for

this reason. In Ref. 9 additional reactions and

species were included in the kinetic mechanism

which were not included in Ref. 8. We have not

incorporated these into our mechanism since

they are likely to be less important in a nearTABLE 4

Calculated Flame Speeds of CH4/air/CF3CHFCF3 Mixtures

CH4/O2 ratio HFP mole %

Calculation Experimentb

U (cm/s) U/Uo

a U U/Uo (U/Uo)calc/(U/Uo)exp

0.45 2.96 17.76 0.50 22.8 0.61 0.83

0.50 2.93 16.27 0.41 15.4 0.37 1.11

0.55 2.91 12.32 0.31 17.4 0.44 0.70

a

Speed normalized by speed of uninhibited CH4/air flame at same CH4/O2 ratio. b

Ref. 12.

170 B. A. WILLIAMS ET AL.
stoichiometric flame than in a purely pyrolytic

environment. It is possible, however, that for

rich stoichiometries or higher inhibitor concentrations these may need to be considered. The

C3 reaction set used here is minimal in size

compared to the C2 reactions in the NIST HFC

mechanism and it should not be considered as

being comprehensive.

To investigate the applicability of the mechanism given here to the conditions of Hynes et

al., we modeled the conditions of Ref. 10 using

the experimental temperature profile as input.

The model calculations of species profiles were

qualitatively similar to those obtained by Hynes

et al. There were some differences; in particular

the COF2 concentration was calculated to be

about 30% higher as a result of our modifications to the C1 kinetics in Ref. 7. Hynes et al.

measured somewhat lower concentrations of

this species in the postflame zone than were

predicted by their mechanism. Secondary reactions (hydrolysis in particular) of COF2 following probe sampling may, however, account for

the discrepancy [8]. The differences in the

mechanisms between that used here and that of

Ref. 10 had a much smaller effect on the species

profiles than did the perturbations introduced

by the sampling probe, which Hynes et al.

treated phenomenologically. The experimental

data presented in [10] does not appear to unequivocally favor either mechanism over the

other.

The agreement of the modeling predictions

using the current mechanism with the species

profiles is very encouraging since there are

significant differences in stoichiometry, pressure, and temperature between the present experiment and the conditions studied by Hynes

et al. The present data set is only an indirect

measure of the initial HFP decomposition pathways, since all the species monitored are several

reactions removed from the original agent.

Other sets of validation data are desirable to

experimentally verify the kinetic pathways of

HFP in flames.

In terms of refinements to the HFC mechanism, it appears that thermal decomposition

reactions of fluoroethyls to produce atomic fluorine are important and need to be added. This

is likely to have implications for modeling inhibition by C2 as well as C3 agents. Revision of O

and OH reaction kinetics with fluoroethylenes

will impact flame structure calculations, but

appears to have little effect on global parameters such as flame speed.

We thank Robert Hynes for his helpful discussions and for sharing with us the thermodynamic,

transport, and kinetic parameters for the HFP

submechanism. This work was supported by the

U.S. Naval Sea Systems Command.

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Received 1 December 1998, revised 21 May 1999, accepted 8

June 1999

172 B. A. WILLIAMS ET AL.