A.S ET AL.

)

122

01

0]

0

11

2]

000

0] Intermediate Species Profiles in Low Pressure Premixed

Flames Inhibited by Fluoromethanes

DREW L'ESPERANCE,t BRADLEY A. WILLIAMS,* and JAMES W. FLEMING

Combustion Dynamics Section, Code 6185, Navy Technology Center for Safety and Survivability, Naval

Research Laboratory, Washington, DC 20375-5342

We have investigated premixed 10 torr methane/oxygen flames containing CH3F, CH2F2, CHF3, and CF4,

Profiles of temperature and CH* chemi-luminescence were acquired, and laser-induced fluorescence (LIF) was

used to obtain profiles of the intermediate species H, OH, CH, CF, CHF, CF2, and CF20. The fluoromethanes

were added in amounts such that each flame had the same flux of fluorine atoms. In the flames containing

CH3F, CH2F2, and CHF3, the methane flow was adjusted to give an equivalence ratio of 1.07 for all three

inhibited flames. The experimental intermediate profiles were compared to predicted profiles calculated from

a hydrofluorocarbon kinetic mechanism recently developed at NIST. No fluorinated intermediates were

detectable in the CF4 inhibited flame, indicating that this agent does not react significantly under the flame

conditions studied. The temperature profiles, H atom profiles, and OH profiles for the other three

fluoromethane inhibited flames are nearly identical, indicating that flames containing different fluorocarbon

compounds, but identical proportions of fluorine atoms, have similar structures. The kinetic model correctly

predicts the location of the reaction zone in the flames containing CH2F2 and CH3F. In the CHF3 flame,

however, the location of the reaction zone is predicted to be too far above the burner surface, and

concentrations of Hand OH are too low. The discrepancy appears to be due to pressure dependence and third

body efficiencies of the agent thermal decomposition. Furthermore, relative amounts of CF, CH, CF2, and CHF

in the different flames are not very well predicted. In general, partially fluorinated methyl and methylene

radicals appear to have a greater than predicted propensity to lose hydrogen atoms rather than fluorine. We

propose modifications to the fluorine mechanism to correct the discrepancies observed in the low pressure

experiments, while simultaneously achieving good agreement with atmospheric pressure flame speed data in

CHJair/CHF3 flames, and, except in rich conditions (<jJ > 1.25) CHJair/CH2F2 flames. © 1999 by The

Combustion Institute

1

1

I

,

•

I

I

,

INTRODUCTION

The global ban on production of stratospheric

ozone-depleting halons (bromine-containing fluorocarbons) has necessitated a search for alternative fire suppressants [1]. Hydrofluorocarbons

(HFCs) are currently used in many instances

where fire protection was previously provided by

halons [2]. Since hydrofluorocarbons lack bromine, primarily responsible for both the fire suppression [3] and ozone depletion properties [1] of

halons, HFCs must be employed in larger quantities to provide protection for an equivalent fire

threat [4, 5]. Owing to space and weight limitations in many applications, HFC-based fire protection systems often must operate with smaller

safety factors than halon systems. Safe and

efficient use of HFCs thus requires greater

tNational Research Council Postdoctoral Associate 1995-

97. Present address: Dept. of Physical Sciences, New Mexico

Highlands University, Las Vegas, NM 87701-4073.

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

COMBUSTION AND FLAME 117:709-731 (1999)

© 1999 by The Combustion Institute

Published by Elsevier Science Inc.

understanding of their flame chemistry than was

needed for the bromine based fire suppressants.

Recently, a chemical kinetic mechanism has

been developed at the National Institute of

Standards and Technology (NIST) to describe

the relevant flame chemistry of C1 and C2 HFCs

[6-9]. Studies of HFC kinetics are relatively

sparse in comparison to the analogous hydrocarbon reactions, especially in view of the large

(""'600 reactions involving fluorine species) reaction set. Therefore, kinetic mechanism development has had to rely extensively on computational modeling of reaction dynamics, and on

analogies with equivalent hydrocarbon reactions [7]. The mechanism has been used to

predict the behavior of hydrofluorocarbon inhibited flames in a variety of laboratory scale

experiments [7, 10-13] with the eventual goal of

modeling suppression of larger scale fires.

Comparison of model predictions with experimental data is necessary to validate and refine

the current understanding of HFC combustion

chemistry. To date, the effect of HFCs on

0010-2180/99/$-see front matter

PH S0010-2180(98)00126-6
710

flames has been investigated primarily by measuring global parameters such as burning velocity, HF and CFzO production, extinction strain

rate, and flame temperature [11-18]. The detailed kinetics of hydrofluorocarbons in combustion environments, by contrast, has not been

subject to extensive experimental investigation.

There have been two studies that have reported

species profiles in fluorocarbon inhibited flames

by mass spectrometry [19, 20]. Vandooren et al.

[19] compared several species in a COlHz/Oz/Ar

flame inhibited by CHF3 to those in an inhibited

flame and one inhibited by CF3Br. The profiles

of the stable hydrocarbon species as well as the

radical species H, 0, and OH, and the fluorinated species CHF3, HF, and CFzO were reported. Kinetic modeling was not performed;

however, the authors concluded that the reactions

I

I

I

I

I

cals CF3, CHF2, and CH2F produced, respecl~

tively, from CHF3, CHzFz, and CH3F, can react

with H atoms, causing removal of either an H or

an F atom to produce the methylene radicals

'CHz, CHF, and CF2.

Alternately, the CHjF3_j radicals can react

with 0 or OH to form the carbonyl compounds

CHzO, CHFO, and CF20. CFzO, carbonyl fluoride, is of particular interest because it is a

relatively stable compound and is the only fluorine product besides HF likely to exit the

combustion zone [7]. The kinetics of the CF20

removal reactions assumed by the NIST mechanism, by reactions with H atoms and HzO [5],

have not been thoroughly characterized experimentally.

Here we present experimental data on intermediate species profiles in 10 torr methane/

oxygen flames containing the series of fluoromethanes. It should be noted in making

comparisons between experimental data and

modeling predictions that the model was constructed primarily for use at atmospheric pressure [8], and most fluorocarbon association reactions included in the NIST mechanism are

assumed to be in the high pressure limit. Intermediate species profiles in premixed flames are

one of the best ways of verifying proposed

kinetic pathways in a chemical mechanism but

are only practical at reduced pressures. The

situation is thus not ideal in that these experimental tests of the mechanism can only be

performed outside its stated range of applicability. Nevertheless, many important features of

the kinetic pathways (for instance, branching

ratios of bimolecular reactions) should not be

pressure dependent.

Because much of the development of the

HFC kinetic mechanism has relied on analogies

with the much better understood hydrocarbon

flame chemistry, comparisons between molecules with differing degrees of fluorine substitution are a good way to investigate the fluorine

kinetics. We therefore study a complete series

of fluoromethanes. CHF3 and CF4 are used as

fire suppressants, while CHzFz is being considered as a replacement for chlorofluorocarbons

in refrigeration applications [21]. CF4 is thought

to be inert in most flame environments because

its only possible reaction pathway is the removal

of a fluorine atom either by thermal dissociation

D. L'ESPERANCE ET AL.

(Rl)

(R2)

CHF3 + H ¢:> CF3 + Hz

CF3 + OH ¢:> CFzO + HF

are primarily responsible for the reduction in

concentration of the radicals H, 0, and OB. It

was also observed that CFzO was a relatively

stable product under the flame conditions used.

Since the fluorine/hydrogen ratio was 1.5, the

authors concluded that the low concentration of

atomic hydrogen present in their flame was

responsible for the persistence of carbonyl fluoride.

Sanogo et al. [20] studied a 31.5 torr (4.2 kPa)

stoichiometric CH,JOz/Ar flame inhibited with

1% CzF6' Profiles were recorded for several

fluorinated species, including the radical species

CF3 and CFz. Kinetic modeling was performed,

using a different reaction set than the NIST

mechanism. Agreement between the model and

experiment was good for most species, but poor

for CFz and CF3, possibly due to the necessity of

using a low electron impact energy in the mass

spectrometer to avoid ion fragmentation [20].

. Previous modeling studies have predicted the

reaction pathways of fluoromethanes in low

pressure flames [7]. Fluoromethanes are

thought to react in flames through hydrogen

atom abstraction by either OH radical or H

atom attack. Since CF4 does not have this

pathway available to it, it is nearly inert in most

flame environments [7]. The fluoromethyl radi-
* The CHF3, CH2F2, and CH3F flames contain equal amounts of H, F, C, and O. The mass flow of CF4 was chosen such

that the F flux was equivalent to the other fluorine containing flames. All flames were operated at 10 ± 0.05 torr. The

uncertainty of the flows is ±1% of the flow.

a Thermal equilibrium calculation [Gordon, S. and McBride, B., NASA Lewis Report NASA SP-273 (1976)].

b CF4 only fluorine product considered in equilibrium calculation.

C Equilibrium distribution of fluorine products (predominantly HF).

TABLE 1

Flow r(!tesFlame (cm3jmin conditions @ DoC)used in experiment*

Adiabatic flame temperature

Mass flow

Agent O2(g/cm2 CH4 -s) cp(KY

800 0.86400 0 X 10-325711.00

8001.08 350167X 10-3 2572 1.07

8001.08 175342X 10-3 2557 1.0

800 1.08 117400X 10-3 2548 1.07

80088400 1.072538 X 10-3(CF4 inertt 1.00

711

uninhibited flame was a stoichiometric methane/oxygen flame without any diluent gas. The

four other flames each contained one of the

compounds CH3F, CHzFz, CHF3, and CF4, The

mole fraction of CHF3 added to the methane/

oxygen mixture was about one third of the

amount needed to blow the flame off the burner

under the flow and pressure conditions used in

our experiment. The relative amounts of the

fluoromethanes were chosen such that all four

inhibited flames contained equal numbers of

fluorine atoms. The flow rates of methane and

oxygen in the CHF3 and CF4 inhibited flames

were the same as in the uninhibited flame. For

the flames containing CH3F and CH2Fz, the

methane flow rate was reduced to maintain the

same equivalence ratio and concentrations of

fluorine, carbon, and hydrogen as the flame

containing CHF3. These three flames (CH3F,

CH2Fz, and CHF3) are predicted to produce

nearly identical concentrations of all final products, and the heat releases are nearly the same.

The equivalence ratios for all the flames with

added agent were slightly fuel rich (<!> = 1.07)

with the exception of the flame containing CF4,

which had an equivalence ratio of unity.

The premixed flames were stabilized on a

water-cooled burner (McKenna Products) enclosed in a vacuum chamber [23]. Flows were

controlled using mass flow controllers (Sierra

Instruments), with an accuracy of 1% of full

scale. The premixed fuel, oxygen, and fluorocarbons flowed through a 6 em diameter sintered

SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES

or by abstraction, both of which have exceedingly slow kinetics [22]. Comparison of the CF4

inhibited flame with other fluoromethane inhibited flames highlights the changes to the flame

structure resulting from chemical activity as

opposed to physical effects (oxygen dilution and

increased heat capacity) of the agent [14].

The fluorinated intermediates CF, CFz, CHF,

and CFzO were monitored by laser-induced

fluQrescence (LIF), as were the species H, OH,

and CH. The profiles and relative concentrations of these intermediates can be used to infer

the reaction pathways followed by the different

fluorocarbons, along with their effect on the

overall structure of the methane flame. Comparison of experimental radical profiles and

kinetic modeling calculations in premixed

flames can provide a more definitive indication

of areas of kinetic mechanisms which need

revision than can global parameters such as

flame speed or extinction conditions.

EXPERIMENTAL

Flame Conditions

Table 1shows the gas flow conditions of the five

flames studied in this experiment. The following

gases were used as received from the suppliers:

CH4, Air Products, 99.99%; Oz, MG Industries,

99.9%; Ar, MG Industries, 99.9%; CH3F, PCR,

98%; CHzFz, PCR, 98%; CHF3, Dupont,

99.9%; and CF4, Air Products, 99.97%. The

'AL. r

I

react H or jicals

react

y )unds 'It

:FzO llech- );pen- flu- ISflu- [5], thea

interl1ane/ : flu- :tking con- pres- In1nter- and are re-

~pen- IyJlica- ~es :s)osed nThe but arebeof :::hing at be

f the

ogles 

Irbon nole- stitu ! ieries !

II

I 

;ause f 

ation
Emission Excitation Excitation

wavelengthenergy

(nm)(pJ) Transition(nm)

Is-2s-4p (3 photon) 243.1 (2-photon) 486 150

10

R1(8) 311 A-X30(1,0)281.27

01(6) 390 B-X200(0,0)389.98

Oz(21.5) 255 ± 15200 + PI (26.5) 223.30+

A-x334 ± 2060 250

A-x>515(0,3,0)-(0,0,0) nm300 492.42

1T*.;.-n 320-40080 211

The I

were

relatt

Mor

was u

IDcm

fittinl

point

specr

ture I

r SPEC

I

tored

UV-1( tensit) byof profil{ sured mole densit noted uncer tions indece tens atl< nelb. meaSl onscans energ ties K. bymole tween 281 avoid (Corn tralhe moco and the OHFla Buanbofl; irIiiJIer;n

T(x)

Fluorescence was collected at right angles to

the laser beam using a two lens collection

system, which imaged a 0.25-1 cm section of the

probe volume near the center of the burner.

Filters were used to isolate the spectral region

of interest, and the fluorescence was detected by

a photomultiplier (Hamamatsu R1477). Resolved emission from the inhibited flames was

characterized by replacing the emission filters

with a 0.5 m monochromator (Jarrell-Ash).

The fluorescence signal was analyzed by a

boxcar integrator, with a short gate (25 ns) set to

the peak of the fluorescence decay. Where laser

scatter interfered with the fluorescence signal,

the boxcar gate was delayed some 30 ns from

the peak PMT signal. Collision-induced

quenching is minimized by these experimental

protocols but is not removed entirely. Since the

primary objective in the present study is to mak~

relative measurements of species mole fraction

between different inhibited flames, quenching

only affects the overall result if the quenching

environment is appreciably different between

the various flames, or changes dramatically with

location in a particular flame. The quenching

environment should be similar for the flames

studied, since they have similar concentrations

of most major flame species. The quenching

rates of the fluorinated radical species monitored in this study have not been extensively

investigated. CF, CF2, and CF20 all have fluorescence lifetimes (radiative + predissociation)

of 60 ns or less, so quenching should have little

effect on the fluorescence signals of these species.

Changes in the excitation energy were moniD. L'ESPERANCE ET AL.

TABLE 2

Optical Diagnostics Used for Detection of Flame Species

712

OPTICAL DIAGNOSTICS

LIF diagnostics for the monitored species are

summarized in Table 2. The detection schemes

for the flame species have been discussed in

greater detail elsewhere [24, 25]. Details relevant to individual molecules are noted in the

results section. The fluorescence excitation

source was a XeCI excimer-pumped dye laser

(Lambda Physik EMG101E, FL2002). Excitation wavelengths below 300 nm were provided

by frequency doubling the dye laser output

using a BBO (I3-BaB204) crystal. The probe

beam entered the chamber parallel to the

burner surface. The spatial resolution of the

LIF profiles was limited by the 0.5 mm diameter

of the unfocused probe beam. All windows on

the chamber were made of either MgF2 or CaF2

to avoid chemical attack from HF.

stainless steel plate in the central portion of the

burner. A concentric shroud flow was provided

by flowing argon through a sintered brass ring

that surrounded the burner, giving a mass flux

per unit area at the burner surface equal to that

of the flame reactants. The pressure inside the

vacuum chamber was maintained at 10.00 ±

0.05 torr. The flame exhaust was passed through

a soda-lime filter, then through a dry ice trap to

remove water and hydrogen fluoride before

entering the vacuum pump. Temperature and

fluorescence profiles were recorded by translating the burner vertically inside the chamber

using a stepper motor (Parker) while keeping

the laser beam fixed.

'IS:

I

'i

ilr'

Iii:

11
SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES

MODELING

The chemical structures of the inhibited flames

were modeled using the Sandia PREMIX and

related codes [26-28]. The kinetic mechanism

was used, wherex is the height above the burner

in cm. The coefficientsA-E were determined by

fitting the function to the experimental data

points .. These fitted functions were used as

specified temperature inputs to the flame structure calculations discussed below.

1I

I

I

713

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 [29], with nitrogen chemistry

deleted. The NIST HFC mechanism is used for

the thermodynamics and reactions of the fluorinated species [6-9]. The NIST mechanism has

received some updates from that given in ref.

[8]; the changes are listed in Table 3. This

mechanism is referred to below as the base

mechanism. Further adjustments to the mechanism were performed as discussed below.

The NIST mechanism includes nearly all oneand two-carbon species containing hydrogen,

fluorine, and oxygen. The total reaction set

includes 81 species and 759 reactions. Calculations of the experimental burner-stabilized

flames were performed using the experimental

temperature profiles as input. Multicomponent

viscosities were used for species transport, and

thermal effects on diffusivities were considered

for the species H and Hz. The computational

domain extended from the burner surface to 5

cm. Catalytic recombination of atomic hydrogen

on the burner surface was not considered in the

calculation. In both the experimental and calculated profiles, the H atom concentration is

greatly reduced near the burner in the inhibited

flames relative to the uninhibited methane/oxygen flame. Therefore the influence of burner

surface chemistry on the structure of the inhibited flames will be less than on the uninhibited

flame, where it has been found to have a rather

minor influence [30]. Mesh refinement tolerances (local/global variation) for the species

profiles were set to 0.1 for the species concentrations (the GRAD parameter in the PREMIX

code) and to 0.25 for the concentration gradients (CURV parameter). The final grids for the

uninhibited and CF4 inhibited flames contained

approximately 80 mesh points, while the solutions for the other three inhibited flames contained approximately 110 mesh points.

RESULTS

Figure 1 shows the measured temperatures and

the fitted functions. The flame containing CF4

Temperature Profiles

(1)

tored by reflecting some of the incident excitation beam into a reference photodiode (UDT

UV-100). The relationship between signal intensity and reference intensity was determined

by attenuating the excitation beam with a series

of neutral density filters. Most optical transitions were found to be slightly saturated. LIF

profiles were obtained by correcting the measured LIF signal for changes in the excitation

intensity. We have converted the LIF profiles to

mole fraction profiles by correcting for gas

density changes and the temperature dependence of the ro-vibronic level probed, except as

noted below. We estimate the experimental

uncertainties in the ratios of LIF signals between different flames to be ±10%.

Flame temperatures were determined by

measuring 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 =2 0 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 use in the premix calculations

and in the conversion of the LIF data to species

mole fractions, a temperature profile of the form

T(x) = A

1 + B exp (- (x;- C)) - Ex

r

I

~mission

~velength

(nm) .

;6

4 ± 20

515 nm

0-400

1

'0

5 ::!:: 15

ET AL.

19les to

llection

n of the

burner.

. regIOn

:cted by

7). Reles was

1 filters

sh).

d by a

5) set to

re laser

signal,

IS from

nduced

imental

nce the

o make

'raction

~nching

~nching

'etween

lly with

~nching

flames

trations

~nching

; moni-

~nsively

Ie fluo-

;iation)

ire little

species.

~moni-
Changes in Base Fluorine Mechanism From That Oiven in [8]

TABLE 3

ane

dueI

SP

Fig.

ual

stat

tion

calc

mol

'?

Q)

:;

1iJ

Q; 0- f

Q)

I--

adc

tiOJ

me

cas

3-<

Fig

for

by

witl

tha

can

tha

ind

flar

am

ma

bur

Th(

CH

Cz]

has

CH

[32

O.

O.

O.

2000.

O.

1000.

3000.

O.

E"

(cal/mol)

9800.

9000.

5500.

10,000.

9300.

5700.

8000.

O.

318.

5900.

5900.

5900.

5900.

5900.

5900.

5900.

5900.

5900.

5900.

5900.

O.

O.

O.

O.

O.

b

3.1

3.1

3.1

3.2

3.2

3.2

0.0

0.0

0.0

2.8

2.8

2.8

2.8

2.8

2.8

2.8

2.8

2.8

2.8

2.8

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

Arrhenius parameters k =

AT' exp( - E)R T)

D. L'ESPERANCE ET AL.

1.44E+01

1.44E+01

1.44E+01

3.20E+01

3.20E+01

3.20E+01

1.00E+13

1.00E+ 13

2.23E + 13

5.50E+03

5.50E+03

5.50E+03

5.50E+03

5.50E+03

5.50E+03

5.50E+03

5.50E+03

5.50E+03

5.50E+03

5.50E+03

3.00E+ 13

3.00E+ 13

2.00E+ 13

1.00E+ 13

2.00E+13

3.00E+ 13

3.00E+ 13

3.00E+13

5.00E+ 12

9.00E+13

3.00E+13

6.60E+ 13

4.00E+ 11

A

(em, mol, s)

was of the same magnitude as the uncertainty in

the burner position (0.2 mm reproducibility

uncertainty and 0.5 mm beam diameter).

Flame Emission Profiles

The uninhibited flame becomes noticeably

brighter upon addition of CHF3, CH2F2, or

CH3F. In the uninhibited flame, all of the

structured emission in the 300-600 nm spectral

region is due to OH*, CH*, and C~ [31]. No

CFO + 02 = C02 + 0 + F

HCO + CH2F = CH2CO + HF

HCO + CHF2 = CHFCO + HF

HCO + CF3 = CF2CO + HF

CF30 + HCO = >CF2:0 + HF + CO

CHF + 0 = CO + HF

CF + OH = CO + HF

CH3 - CF2 + OH = CH2:CF2 + H20

CH2F - CHF + H02 = CHF:CHF[Z] + H202

714

Reaction

added:

1. CH30H + CH2F = CH30 + CH3F

2. CH30H + CHF2 = CH30 + CH2F2

3. CH30H + CF3 = CH30 + CHF3

4. CH30H + CH2F = CH20H + CH3F

5. CH30H + CHF2 = CH20H + CH2F2

6. CH30H + CF3 = CH20H + CHF3

7. CF + CH20 = >CHF + HCO

8. CF + HCO = >CHF + CO

9. CF:O + CF:O = CO + CF2:0

10. CH3 - CHF + CH20 = CH3 - CH2F + HCO

11. CH3 - CF2 + CH20 = CH3 - CHF2 + HCO

12. CH2F - CH2 + CH20 = CH3 - CH2F + HCO

13. CH2F - CHF + CH20 = CH2F - CH2F + HCO

14. CH2F - CF2 + CH20 = CH2F - CHF2 + HCO

15. CHF2 - CH2 + CH20 = CH3 - CHF2 + HCO

16. CHF2 - CHF + CH20 = CH2F - CHF2 + HCO

17. CHF2 - CF2 + CH20 = CHF2 - CHF2 + HCO

18. CF3 - CH2 + CH20 = CH3 - CF3 + HCO

19. CF3 - CHF + CH20 = CH2F - CF3 + HCO

20. CF3 - CF2 + CH20 = CHF2 - CF3 + HCO

21. HCCO + F = CHF + CO

22. CFCO + F = CF2 + CO

23. C2H3 + F = C2H2 + HF

24. CHF:CF[Z] + F = CHF + CF2

25. CF2:CF + F = CF2 + CF2

Reaction numbers below refer to [8].

Deleted:

PP28.

New parameters:

MA32.

MA33.

MA34.

PPlO.

NN20.

NN42.

0050.

0067.

had nearly the same temperature profile as the

uninhibited flame; the flames containing the other

fluoromethanes all had higher peak temperatures

(attained slightly further above the burner surface) than the uninhibited flame, with the monoand di-fluoromethane having slightly higher peak

temperatures than the trifluoromethane flame.

The temperature rise in the flames containing

CF4, CHF3, and CH2F2 appeared to be pushed

slightly (=0.5 mm) further away from the

burner surface, although the size of this effect
ETAL. SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES 715

ment for the different flames, the CH* emission

profile can be expressed as

(2)

Agent: • none

o CH3F (xO.5)

6 CH2F2 (xO.5)

o CHF3 (xO.5)

• CF4

Agent:

• none, phi = 1.0

o none, phi = 1.1

o CH3F (xO.5)

6 CH2F2 (xO.5)

o CHF3 (x 0.5)

CF4

1.0 1.5 2.0 2.5

Height above Burner (em)

Fig. 2. CH* emission profiles (bottom) at 430± 10 nm for

the uninhibited flame under standard flow conditions

(equivalence ratio of 1.0), an uninhibited flame with an

equivalence ratio of 1.1, and the flames inhibited by fluoromethanes. Profile for the flames containing CH3F,

CHzFz, and CHF3 have been reduced by a factor of two.

Emission profiles (top) estimated from the PREMIX calculation using the revised mechanism by taking the product of

the 0 atom and CzH concentrations.

The overall reaction C2H + 0 ~ products is

written in the GRI mechanism as being temperature independent. The rate of the CH* channel has only been measured at room temperature [32]. If we assume that the branching ratio

is temperature independent, the CH* emission

intensity is then given by the product of the C2H

and 0 atom concentration profiles. The product

of the calculated profiles for the various flame

conditions is plotted in the top panel of Fig. 2.

~7

'1ij

.S!c 6 ..5g 5 '1ij

1/1 'Ew 4

-g 3 -

u:a

c..e 2 ,~ 1 -

co

&! a

4 ~ 1/1

CI) £ 3 c

'1ij 0•S!! E

2

W J:0CI) .~-

co

&!

a ..-r:- 0.5

(R3)

2200

2000 168

g ~ 1400 i1200 lI Agent: •

none

0 CH3F

/:>

CH2F2

0 CHF3

•

CF4

0

24351

Height above Bumer (em)

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 (see text), which were used as inputs to PREMIX

calculations and in converting LIF profiles into relative

mole fractions.

C2H + 0 ¢:} CH* + CO

additional spectral features appear upon addition of CHF3, but the existing band systems

increase in intensity by a factor of 1.25 in the

case of OH*, and by a factor of approximately

3-4 for both CH* and C;. The bottom panel of

Fig. 2 shows CH* emission profiles at 430 nm

for the uninhibited flame, the flames inhibited

by the fluoromethanes, and an uninhibited flame

with an equivalence ratio of 1.1, slightly greater

than that of the inhibited flames. The flames

containing CH3F, CH2F2, and CHF3 are all more

than twice as luminous as the <p = 1.1 flame,

indicating that increased emission in the inhibited

flames is not primarily an effect of richer stoichiometry. The luminous zone in these three flames

moves approximately 2 mm further from the

burner than its location in the uninhibited flame.

The flame containing CH3F produced the most

CH* luminescence, followed by the CH2F2 flame,

and then the CHF3 flame. CF4, in contrast, produces little change in the CH* emission profile.

Recently, the reaction

has been identified as the primary source of

CH* chemiluminescence in hydrocarbon flames

[32]. Assuming a similar quenching environo.

o.

O.

2000.

O.

1000.

3000.

O.

Ea

(cal/mol)

9800.

9000.

5500.

10,000.

9300.

5700.

8000.

O.

318.

5900.

5900.

5900.

5900.

5900.

5900.

5900.

5900.

5900.

5900.

5900.

O.

O.

o.

O.

O.

ticeably

'2F2' or

of the

;pectral

31]. No

:ainty in

ucibility

).
D. L'ESPERANCE ET AL.

s,

21

la

n

h

tl

f]

C

sl

o

n

Ie

tt

pc

th

th

w

I ~

I b

I ti

I ~

I

3

A~~nL__ -- none

~ CH3F

-b- CH2F2

-e-- CHF3

__ CF4

Agent:

• none

o CH3F

A CH2F 2

o CHF3

• CF4

///\

//// CHF3 base

__ // mechanism

"I

.•...• 0

~ 4.0 !::

0

~ 3.0·- n!

La. •... (5<II 2.0 :1!: :I: "C

<II -U 1.0 :ca:<II 0.0

1.0

!:: 0~n! 0.8 La. •...

~

0 0.6 :i:

:I: 0.4 <II Q) +:: >n! It:

o.o~ 0

2

Height above Burner (em)

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

profiles. Predictions using the revised mechanism ([8] plus

Tables 3 and 4) are indicated by solid lines, the prediction

using the base mechanism ([8] plus Table 3) for the CHF3

flame is shown in dashed lines. Predictions for the other

inhibited flames using the base mechanism were similar to

those using the modified mechanism.

OH

Calculated and measured OH profiles are

shown in Fig. 4. LIF profiles of the hydroxyl

radical were recorded by exciting the R] (8) line

of the A-X (1,0) band. Fluorescence was detected both on the (1,1) and (0,0) bands [35].

Typical laser energies were 30 0 for recording

profiles. For temperature measurements, the

probe energy was attenuated to 2 0 in an unfocused beam of approximately 0.01 cm2 cross

section to avoid saturation of the transition.

The OH profiles have many features in common with the H atom profiles, which is to be

expected given the fast equilibrium between the

two species [36]. As with the H atoms, the OR

profiles are almost identical for the uninhibited

and the CF4 inhibited flames. The peak OH

mole fractions of the CHF3, CH2F2, and CH3F

inhibited flames were similar to that of the uninH

CH3 + CHjP3-i -'? CHz=CHjFz_j + HF.

(R4)

The CH* emission profiles give an indication

(albei t an indirect one) of the importance of the

Cz chemistry in the inhibited flames. Evidently

the recombination of fluorinated methyl radicals with CH3 is more facile than the recombination of two methyl radicals.

Calculated and measured H profiles are shown

in Fig. 3. Hydrogen atoms were detected by a

two-step excitation process involving a two photon absprption at 243 nm (150 0 focused with

a 35 cm focal length lens) from the Is to 2s state,

followed by a single photon excitation at 486 nm

(5-10 0, unfocused) to the 4p state [33, 34].

Fluorescence was detected at 486 nm. The

intensity of the 486 nm beam was adjusted to be

near the saturation intensity, while the LIF

signal exhibited a quadratic dependence on the

243 nm laser energy. Interference presumably

from CFz was noted in the flames inhibited by

CHzF2 and CHF3; the interference signal depended only on the presence of the 243 nm

light. There was also a small amount of laser

scatter at 486 nm. To correct for these complications, the fluorescence signal was recorded

with each of the two beams blocked, and these

profiles were subtracted off the profile recorded

with both beams present.

The peak H mole fractions in all the flames

are nearly identical, although the CHF3, CHzFz,

and CH3F inhibited flames have higher peak

temperatures, which would normally tend to

increase the radical's mole fraction. The position of the H concentration rise in the CHF3,

CH2F2, and CH3P inhibited' flames is moved

further away from the burner.

The predicted 0 ato,? concentrations in the

various flames vary by less than a factor of two,

so the CH* emission intensity will be governed

chiefly by the CzH profile. The formation of Cz

species in the flames containing fluoromethanes

is predicted to be dominated by methyl +

fluoromethyl recombination reactions:

716

Intermediate Species Profiles
E ET AL. SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES 717

1.5

Height above Burner (em)

Fig. 5. Calculated (top) and measured (bottom) CH profiles. Calculations using the revised mechanism are shown in

solid lines, those using the base mechanism are shown in

dashed lines. For the CF4 inhibited flame, the two calculations are indistinguishable.

2.0

Agent:

• none

o CH3F

'" CH2F 2

o CHF3

• CF4

'" - ~ ~ Meohao;sm,

2.5 .•.. ~ ~ 4.0 -- Base

5 -- Revised

:;::;

~ 3.0 

•.. /" •..

1.5 / Agent:

LL

/ Q)

/ -- none ~ 2.0 

1.0

/ -e- CH3F

/

:I: 

-A- CH2F2 

/ U

/ -e- CHF3 1.0 'C

0.5 / Q)

/ -+- CF4

- 

/ ()

yo

/"

---. CHF3 base mechanism

:c

0.0 e 0.0

a

1.0

:;::; 5 2.5

:;::;

()

~ 

LL 2.0

Agent: Q)

0.6 '0

none • 1.5 :E

0 CH3F :I: 

0.4 CH2F2

U 

1.0

'"

Q)CHF3 0 >

:;::;

0.2 ~ 0.5 CF4 

• 

It:

0.0

231 0.0 0.5

Height above Burner (em)

Fig. 4. Calculated (top) and measured (bottom) OH profiles. Predictions using the revised mechanism are indicated

by solid lines, the prediction using the base mechanism for

the CHF3 flame is shown in dashed lines. Predictions for the

other inhibited flames using the base mechanism were

similar to those using the modified mechanism.

3

om) H atom

!ism ([8] plus

he prediction

for the CHF3

for the other

~re similar to

ent:

none

CH3F

CH2F 2

CHF3

CF4

~~e:nL_-

-- none

-&- CH3F

--8- CH2F2

-e- CHF3

__ CF4

roflles are

e hydroxyl

.R1 (8) line

::e was deJands [35].

r recording

ments, the

in an unfocm2 cross

msition.

res III comch is to be

etween the

ns, the OH

Jninhibited

peak OH

and CH3F

)f the uninhibited flame, but as with the H atom profiles,

the OH concentration rise occurred further

from the burner than predicted by the model.

CH

Calculated and measured CH profiles are

shown in Fig. 5. The CH radical was monitored

on the Q1(6) line of the B-X (0,0) band near 390

nm [37]. Fluorescence was monitored at the

same wavelength. Typical pulse energies were

200 ILJ in an unfocused beam. Due to scattered

laser light, the boxcar gate was delayed by 30 ns

relative to the laser pulse.

The profiles of ground state CH show much

less sensitivity to the addition of the agents

than do the profiles of CH* emission. The

peak CH mole fraction is increased in all of

the inhibited flames; the effect is greatest in

the flame inhibited by methyl fluoride, in

which the peak CH mole fraction increases by

some 70%. The CH mole fraction profile is

slightly increased (by about 10%) in the flame

inhibited by CF4,

CF2

Calculated and measured CF2 profiles are

shown in Fig. 6. The CF2 radical was detected by

exciting at a wavelength of 250 nm, roughly in

the middle of the A-x transition, which at room

temperature extends from 270 to 220 nm [25].

Typical pulse energies were 60 ILJ, unfocused.

Fluorescence was collected at 334 nm, using a

20 nm bandpass filter. The fluorescence signal

was extremely strong, and the light had to be

attenuated so that the PMT could operate in a

region of linear response.

The flame excitation spectrum of CF2 is essentially a continuum due to substantial popu- lations in numerous excited ro-vibrationallevels

at combustion temperatures. Since the rotational transition(s) probed are unknown, we
at I,

reI,

can

in 1

not

10

ule

sta

bo

pe

lar

Fu

fla

so

ba

lin

la1

ab

nc

ta:

rea

drc

wit

LlI

the

flaI

f

for

LlJ

spe

ble

Spe

ha'

the

rot

109

roc

pn

the

SPI

th

m

re

of

FJ

Agent:

o CH3F

'" CH2F2

o CHF3

• CF4

/ - - Base Mechanism

CH2F~ \ -- Revised Mechanism

\

\

'"

'"

..,~0 2.0 .•... ~r::

U .Q 1.5 t': (5u.GI

1.0 ~ u. J:() "'C UGI as :cE

0.0 a..

r::

1.0

0

ttI +; u. •...(J 0.8 (5~u. J:GI 0.6

0.4 () ~GI ttI 0.2 Qj 0: 0.0

0.0

r

,

D. L'ESPERANCE ET AL. t

I

I

~

CHF

Q5 1.0 1.5 2~

Height above Burner (em)

Fig. 7. Calculated (top) and measured (bottom) profiles of

CHF. Calculations using the revised mechanism are shown

in solid lines, those using the base mechanism are shown in

dashed lines. Fluorescence signal from CHF is not observed

in the flame containing CF4, Near 1.0 em the profiles may

have been affected by saturation of the detector due to

flame emission (see text).

CHF3 ~ CF3 ~ CF2 occurs more slowly than

the analogous pathway CH2F2 ~ CHF2 ~ CF2.

The CF2 concentrations in the CH3F and CF4

inhibited flames are less than 1% that of the

CHF3 flame. No CF2 signal was observed in the

CF4 inhibited flame even when the CF4 flow was

quadrupled.

Calculated and measured profiles of CHF are

shown in Fig. 7. CHF was excited on the most)

intense bandhead of the (0,3,0)-(0,0,0) band oft

the A-x transition near 492 nm [25, 38]. CHF!

represents an intermediate case between thel• strongly fluorescent CF2 molecule, and lCHz, which can only be detected in flames with

extreme (0,0,0) band difficulty to attempt [39]. toWedetect choseCHFthe becausel (0,3,0)-1

it has a shorter radiative lifetime than the bands

Agent:

o CH3F

'" CH2F2

o CHF 3

- - Base Mechanism

-- Revised Mechanism

718

..,~

0 7.0 .•...

~r::

0 6.0 +; ~ (J 5.0 U.GI

4.0

(5 ~N 3.0 u.

() 2.0 "'C -GI (J 1.0 :c E 0.0 a..

1.0

r:: +;0(J 0.8 ~ u.GI 0.6 (5 ~u.N () 0.4 .2:CD 1U 0.2

Qj 0: 0.0 L 0.0

0.5 1.0 1.5 2.0

Height above Burner (em)

Fig. 6. Calculated (top) and measured (bottom) CFz profiles. Calculations using the revised mechanism are shown in

solid lines, those using the base mechanism are shown in

dashed lines. No CFz fluoresence signal was detected in

flames inhibited by CF4 or CH3F.

have corrected the LIF profiles only for density

variations in the flame. The lack of distinct

spectral features makes it difficult to prove that

CF2 is indeed the source of the LIF signal. The

observed fluorescence decay time of =60 ns,

however, matches the reported value for CF2.

Also, the signal is present in the flames containing CHF3 and CH2F2>but not CH3F, strongly

suggesting that the source must have two fluorine atoms. The intense fluorescence from CF2

interferes with the detection of other species

such as CF, OH, H, and CF20. The structureless interference due to CF2 may be easily

subtracted off from profiles of OH, CF, and H

atoms, which have well structured spectra. For

larger molecules such as CF20, interference

from CF2 inherently limits the detection sensitivity.

CF2 is slightly more abundant in the CHF3

flame than in the CH2F2 flame. CF2 appears

later in the CHF3 flame than in the CH2F2

flame, indicating that the reaction pathway
:::E ET AL. SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES 719

Height above Burner (em)

Fig. 8. Calculated (top) and measured (bottom) CF profiles. CF was produced in detectable amounts from all

agents except CF4, Calculations using the revised mechanism are shown in solid lines, those using the base mechanism are shown in dashed lines.

CF20

Carbonyl fluoride is detectable by LIF but has a

rather weak signal and a broad featureless spectrum at flame temperatures. We attempted to

record profiles of CFzO in the inhibited flames'

but were unsuccessful due to very weak signals,

1.5 2.0

Agent:·

o CH3F

'" CH2F2

o CHF3

• CF4

/--\ \

1\

CHFJ \

\

\

\

"--.

0.5. 1.0

CH2F 2 Mechanism: .

- - Base

-- Revised

1.01-

"to

E 1.5 c:

o

••

C,)

E

u.

Q)

(5

:E

u.

o 0.5

"C

-

Q)

C,)

:c

~ 0.0 a.

c: •• 0C,) 1.5 (5u.EQ)

1.0

:E u. 0Q)

> 0.5

•• a;co

e::: 0.0 0.0 L

(1,5) bands [41] using a 15 nm bandpass filter

centered at 255 nm. Typical pulse energies were

200 0in an unfocused beam. Since CF is one of

the few fluorine species which has a. well-defined spectrum at flame temperatures, interference from CF2 was corrected for by tuning the

laser just off the CF line and subtracting this

background signal from the profiles. The fluorescence lifetime was approximately 20 ns, consistent with previous determinations [41). There

was essentially no scattered light .transmitted by

the filter, so the boxcar gate was set to the peak

of the laser pulse. CF was produced in detectable amounts from all the agents except for CF4,

CF

at longer wavelengths and fluoresces in a region

relatively free from interference [38]. A signifi-

'cant fluorescence signal from CHF is observed

in the flames containing CH3F and CHzFz but

not CHF3 or CF4, Since CHF3 is thought to

react predominantly by abstraction of the hydrogen atom, the observed pattern is consistent

with the assignment of CHF as the source of the

LlF signal and also the model's prediction of

the reaction pathways of the fluoromethanes in

flames.

A rotational analysis has not been carried out

for this band, making it difficult to convert the

LlF profiles into mole fractions; Unlike the CFz

spectrum, the CHF spectrum displays appreciable structure, so temperature corrections to the

spectrum are probably more significant. We

have estimated the temperature influence on

the LIF signal by assuming a ground state

rotational energy of 300 em -1. The rotational

energy of the spectral feature used for monitoring is unknown but is probably fairly low since

room temperature spectra of CHF also display

prominent band heads. The major influence on

the temperature correction is the rapid increase

in the molecular partition function due to population of higher rotational and vibrational

states. The converted profiles are shown in the

bottom panel of Fig. 7. The exact shape and

peak location is only approximate due to the

large and uncertain temperature correction.

Furthermore, in the luminous zones of the

flames (>8 mm above the burner), there was

some saturation of the photomultiplier due to

background luminescence, resulting in a baseline drift. Although the locations of the calculated peak CHF concentrations disagree by

about 3 mm with the experimental values, this is

not necessarily significant in light of the uncertainties in the data analysis. The data does

indicate that CHF peaks slightly earlier and at a

=30% lower mole fraction in the CH3F flame

than in the CH2Fz flame.

Calculated and measured CF profiles are shown

in Fig. 8. CF was excited on the overlapping

resonance lines Q"z(21.5), P1(26.5), and Q1(19.5)

of the A-X (1,0) band at 223.299 nm [40).

Fluorescence was detected on the (1,4) and

~chanism

Mechanism

gent:

CH3F

CH2F2

CHF3

CF4

;lowly than

[Fz::} CFz·

F and CF4

that of the

wed in the

F4 flow was

5 2.0

I)

m) profiles of

sm are shown

I are shown in

; not obselVed

: profiles may

tector due to

f CHF are

n the most

,0) band of

, 38]. CHF

~tween the

and lCHz,

ames with

he (0,3,0)-

IF because

1the bands
012 3

Height above Burner (em)

Fig. 9. LIF Profiles of CF20. A profile of CFzO was

obtained from a CzFJOz flame [25,42]. The signal obtained

from fluoromethane inhibited CHJOz flames is many times

weaker and may be complicated due to interference from

CFz and vibrationally excited Oz.

as well as interference from other species, including CF2 and vibration ally excited O2, We

were, however, able to set an upper bound on

the amount of carbonyl fluoride which could be

present in the inhibited flames, based on comparison with the CzF JOz flame where CF20 is a

stable product and is produced in abundance

[42]. Figure 9 shows profiles recorded at an

excitation wavelength of 211.03 nm in the HFC

inhibited CHJOz flames, as well as in a 20 torr

C2F JOz flame identical to the 35% fuel case of

Douglass et al. [42]. The signal in the CZF4

flame is primarily due to CFzO, especially in the

region above 1.5 cm. Time-resolved fluorescence decays can distinguish fluorescence contributions from CF2 and oxygen observed closer

to the burner; the lifetime of CF2 is 60 ns, the

lifetime of O2 is less than 15 ns (limited by the

laser pulse width), while that of CF20 is 30 ns

[25].

The signal level in the inhibited methane

flames is many times weaker than that of the

C2F J02 flame. In the C2F J02 flame studied for

comparison, the partial pressure of CF20 at 20

torr is predicted to be 6 torr in the post-flame

zone [42]. The LIF signal is at least 80 times

weaker in the post-flame zone (x = 3.0 cm) of

..,..,

••••••••••

...•.... ~ •

•

•

to

tio

In

CI

CI

we

ity

ger

frel

d1I(

SIne

the

a (I

flav

too

has

ane

the

nate

whi:

alw,

SF

mc

sta

sllr

pr(

For

The

mg

crei:

nea

in t

fOf\!

CHJ

had

foIle

ahei:

pror

base

are 1

to th

fOlln

In

ad or

Neve

sligh

the r

Analysis and Mechanism Refinement

the inhibited methane/oxygen flames under the

same excitation and detection conditions. This

indicates that CF20 has a mole fraction of less

than 0.01 in the inhibited methane/oxygen

flames; less than 10% of the F atoms in the

flames studied exit the reaction zone as CF20.

This upper bound to the CFzO mole fraction is

consistent with the prediction of the chemical

kinetic mechanism for these flames. Evidently

interferant species contribute to the fluorescence as an appreciable LIF signal is observed

in the reaction zone in the uninhibited as well as

the inhibited flames. It is not clear how much of

the fluorescence is due to CFzO.

D. L'ESPERANCE ET AL.

Calculations using the base mechanism are

shown in dashed lines in Figs. 3-8. For the

CHF3 flame the predicted location of the reaction zone (as indicated by any of the intermediate species monitored here) is several mm too

far from the burner. Furthermore, the relative

concentrations of the intermediate species for

the different agents are not always well predicted. The concentrations of the fluorinated

intermediates CF and CF2 are overpredicted in

the trifluoromethane flame relative to the

flames containing the other agents. CHF is

underpredicted in the CH3F flame relative to

the CH2F2 flame. The CH mole fraction is

overpredicted in the CH2F2 flame compared to

all the other flames. CH* emission based on the

product of the CzH and 0 atom profiles is

overpredicted in the CH3F and CH2F2 flames.

To remedy these deficiencies, we carried out

a sensitivity and reaction path analysis and

adjusted selected rates of reactions involving

fluorinated species in an iterative process. Our

goal was to improve predictions of our low

pressure data, while simultaneously achieving

good agreement with published data on atmospheric pressure flame speeds for methane/air

flames containing CHF3 and CH2F2• In addition

to constructing a better validated model, we also

hoped to identify individual reactions which

may require further experimental or theoretical

investigation.

Recently, Saso et al. [12] have recommended

adjustments to the reactions of CHF3 and FCa

with H, as well as FCO thermal decomposition,

Flame:

o CHiO;CH3F

'" CHiO;CH2F 2

o CHiO;CHF3

• C2Fi02

•

•

•

•

•

•

••• •

•

•

1.0

0.0

0.2

1.2

n;

5 0.8

en

II.

::i 0.6 (I)

>

;;III

Qi 0.4 It:

720
ET AL. SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES

Ider the

ns. This

1 of less

:/oxygen

; in the

5 CF20.

lction is

hemical

vidently

flu ores-

,bserved

s well as

much of

lsm are

For the

he reacermedimm too

relative

:cies for

'ell pre-

)finated

licted in

to the

CHF is

ative to

ction is

Jared to

d on the

ofiles is

flames.

ried out

'sis and

,lVolving

~ss. Our

Jur low

chieving

n atmohane/air

3.ddition

,we also

s which

~oretical

mended

nd FCO

Josition,

to better fit the atmospheric pressure propagation speed of CHF3 inhibited flames. Included

in [12] was modeling of our preliminary data for

CH, CF, CF2, and OH in the uninhibited and

CHF3 inhibited flames. Even with the refinements, the model still predicted too great a

standoff distance (distance from the burner

surface to the maximum of the intermediate

profiles) for our CHF3 flame conditions.

To investigate the cause of this discrepancy,

we calculated sensitivities of the burning velocity of low pressure freely propagating flames. In

general, a higher adiabatic burning velocity in a

freely propagating flame is equivalent to a reduced standoff in a burner stabilized flame,

since the closer the flame front is to the burner,

the greater the heat loss while still maintaining

a (nonadiabatic) flame speed equal to the gas

flow velocity. Thus, a predicted standoff which is

too large generally means that the flame speed

has been underpredicted. For all the fluoromethanes which contain hydrogen, a rate increase in

the thermal decomposition of the agent to eliminate HF is predicted 1'0 increase the flame speed,

while hydrogen abstraction by 0, H, or OH nearly

always has a negative sensitivity coefficient, meaning that an increase in the reaction rate decreases flame speed. In the HFC mechanism

nearly all reactions are reversible, so an increase

in the rate coefficient will increase both the

forward and backward rates commensurately.

Thermal Decomposition Reactions

For the CHF3 flame in particular, the reaction

(R5)

had the second highest sensitivity coefficient,

following the· reaction of CHF3 with OH, and

ahead of the three reactions to which Saso et al.

proposed modifications. Furthermore, in the

base mechanism, the Hand OH mole fractions

are underpredicted in the CHF3 flame relative

to the uninhibited case; these profiles also were

found to have high sensitivities to reaction (R5).

In refining the HFC mechanism, we have

adopted the recommendations of Saso et al.

Nevertheless, these adjustments yielded only a

slight improvement in the predicted position of

the reaction zone, similar to the finding in [12].

721

It appears that reaction (R5) is the culprit in the

inaccurate prediction of the position of the

intermediate profiles.

Since the thermal decomposition reactions of

the fluoromethanes are important in predicting

the flame speed and structure, it is necessary to

consider their pressure dependence for our

conditions. In the case of CHF3, the reaction is

written in the falloff regime in the base mechanism, following the study of Hidaka et al. [43].

The reactions of CH3F and CH2F2> by contrast,

are written as being unimolecular in the HFC

mechanism. Although the prediction of the

flame standoff distance for the base mechanism

is reasonable for mono- and di-fluoromethane,

the assumption of the decomposition reactions

being pressure independent is unreasonable at

10 torr. The kinetics of the CH3F decomposition reaction are based on the study of Schug

and Wagner [44] (but written in the opposite

direction), who derived both unimolecular and

bimolecular kinetics parameters. We have used

the parameters of [44], assuming a Lindemann

falloff behavior. At atmospheric pressure, the

reaction rate is in the low-pressure limit above

about 1600K due to the reduction of the molecular density as temperature increases at a fixed

pressure. At 10 torr, the reaction is in falloff

over the entire temperature range of interest.

The decomposition kinetics of CH2F2 are

based on a RRKM calculation performed for

the HFC mechanism development. As with the

CHF3 and CH3F decompositions, this rate is

likely to also be in falloff at atmospheric pressure and below. Therefore, we have converted the

kinetic parameters to a bimolecular rate while

keeping the rate at atmospheric pressure unchanged. Several other decomposition pathways

exist for the fluoromethanes, but the HF elimination channel is, according to sensitivity and reaction pathway analysis, by far the dominant route

for the hydrofluoromethanes. We have not put in

falloff behavior for the other channels, since they

are unlikely to have a significant effect on the

mechanism's overall performance.

We have added falloff behavior to all other

decomposition reactions, which, according to

the base mechanism, were predicted to be important for our conditions. The reactions CF4 ¢;>

CF3 + F and CF2 ¢;> CF + F were converted to

bimolecular rates. For CF4, the decomposition
D. L'ESPERANCE ET AL.

H2 vs. HF Elimination in Chemically

Activated Decompositions

Reaction pathway analysis indicates that the

greatly overpredicted peak CH mole fraction in

the CHzFz flame is due to the reaction

(

f

r

r

n

I(

a

f

p

fr

m

te

pI

C

he

CJ

an

an

all it ~

be

Fu

the

(RIO)

(R 11)

(R12)

HF + CH ---? CF + Hz,

CHF + H ¢:> CF + Hz, t1Hf = -92 kJ

(R9)

is lower in energy, but was not included in the

HFC mechanism. Tsai and McFadden detected

both CF and CH products but were unable to

determine a branching ratio due to secondary

reactions and uncertainties in detection efficiency. The lack of a significant increase in the

CH concentration in the difluoromethane flame

relative to the other inhibited flames indicates

that the CF channel is the dominant one. We

have put in a branching ratio of approximately

70% for Reaction (R9) while keeping the total

rate of reactions (R8) and (R9) fixed to the

measured value.

The reaction

CHF + H ¢:> CH + HF. t1Hf = -21 kJ

(R8)

The kinetic parameters for this reaction in the

HFC mechanism are based on the study of Tsai

and McFadden [47], which measured the total

disappearance rate of CHF. The alternate channel,

which according to the base mechanism was

predicted to be quite imp9rtant, is essentially

equivalent to a redistribution of the branching

ratio between reactions (R8) and (R9). The rate

in the base mechanism was set to the upper limit

suggested by RRKM calculations. Adding reaction (R9) to the mechanism allowed this rate to

be lowered by one third to better match the

peak CH concentrations in the flames inhibited

by CH3F, CHzFz, and CHF3 relative to the

uninhibited flame.

The product channels of the fluoromethyl

reactions with atomic hydrogen

CHiF3-j + H ---? CHjFZ_i + HF

CHjF3-i + H -> CHj-1F3-i + Hz

reaction, along with F atom abstraction by H, is

the only possible destruction mechanism. Sensitivity analysis showed that the CFz thermal

decomposition reaction was predicted to have a

significant influence on the propagation speed

of the trifluoromethane flame, at least when it

was written as being unimolecular. After being

converted to a bimolecular rate, the influence of

this reaction was greatly reduced. Also, the

decomposition reactions of fluoroethylenes to

eliminate HF were predicted to dominate destruction of CFz=CHz and CHF=CHz, which

are formed in the CHF3 and CHzFz flames,

respectively, by the reactions:

722

CF3 + CH3 ---? CHz=CHz + HF, (R6)

CHFz + CH3 ---? CHF=CHz + HF. (R7)

Reactions (R6) and (R7) are predicted to be the

dominant route to formation of Cz species

under our conditions. The thermal decomposition of the resulting fluoroethylenes has only

been investigated above 4 atmospheres, where it

appears to be in the high pressure limit [45, 46].

We have added falloff behavior to these reactions, assuming that the transition between high

pressure and falloff occurs somewhere in the

vicinity of atmospheric pressure.

Another issue is the third body efficiencies of

flame species for these decomposition reactions.

The NIST mechanism includes no third body

efficiencies for the CHF3 decomposition reaction;

all the shock tube studies on CH3F and CHF3

decomposition [43, 44] were performed in argon.

We have assumed third body efficienciesfor these

reactions equal for most species to those given in

the GRI mechanism for methane dissociation,

along with efficiencies of 2 for HF, 9 for water,

and 6 for the fluoromethanes. The low pressure A

factors in the decomposition reactions have been

divided by 0.7 (the relative third body efficiency

used in GRI-mech for Ar for most decomposition reactions). The net result is that the overall

third body efficiency in the flame reaction zones

is increased by a factor of six relative to argon.

In methane/air flames, the effect is much less

significant since Nz, which by definition is assigned a relative third body efficiency of unity,

has a mole fraction of approximately 70%

throughout the flame.
=:EET AL. SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES 723

y

s that the

fraction in

on

- 21 kJ

(R8)

tion in the

Idy of Tsai

j the total

nate chan-

-92 kJ

(R9)

:led in the

1detected

unable to

secondary

;tion effiase in the

ane flame

indicates

one. We

oximately

: the total

~d to the

(RIO)

nism was

ssentially

>ranching

The rate

Jper limit

ling reac-

.israte to

latch the

inhibited

e to the

romethyl

(Rll)

(R12)

is another area of the mechanism to which the

experimental data suggests some modifications.

The base mechanism predicts that the HF channel overwhelmingly dominates (>96% branching ratio for both mono- and di-fluoromethyl

radicals), largely on the basis of a lower energy

barrier assuming the reaction proceeds through

a chemically activated fluoromethane intermediate. The Hz channel could also proceed via a

direct abstraction as well as by elimination from

a complex, whereas abstraction of a fluorine

atom would occur at an extremely slow rate.

In the methyl fluoride flame, the base mechanism predicts that CHzF radicals are nearly all

converted to lCHz by the reaction by atomic

hydrogen. Very little CHF is predicted to be

formed in this flame relative to the CHzFz flame.

Formation of CF in the CH3F flame is predicted

to occur almost exclusively via the CH + HF

reaction. In the difluoromethane flame, formation

of CFz is predicted to occur primarily through the

disproportionation reaction with methyl radicals:

(R13)

This reaction causes a slight overprediction of

CFz in the CHzFz flame relative to the CHF3

flame when kinetics of the fluoromethane decomposition reactions are placed in the falloff regime.

The rate of (R13) has been reduced to correctly

predict the ratio of CFz between the difluoromethane and trifluoromethane flames. The kinetics of the CHFz + H reactions have not been

altered in our modification. It is conceivable that

H atoms, rather than CH3 radicals, constitute the

primary reaction partner leading to CFz formation

from CHFz, although our data provide no information on this issue. One study [48] at room

temperature reported that CFz and CHF were

produced in comparable amounts from the H +

CHFz reaction. For the CHzF + H reaction, we

have increased the branching ratio of the

CHF + Hz channel to better predict the CHF

and CF profiles. In the reactions of CHF, CHzF,

and CHFz with H atoms, formation of Hz is in

all cases the lowest energy product channel, and

it seems plausible that the branching ratios may

be higher than the base mechanism predicts.

Further investigation of the product channels of

these reactions would be extremely useful.

Relative Importance of H, 0, and OH

Reaction Partners With Hydrofluorocarbon

Radicals

The fluoromethyl and fluoromethylene radicals

can react with atomic hydrogen as described

above, or with 0 or. OH to form the oxidized

species CFzO, CHFO, FCO, and HCO. In the

base mechanism, reaction with H is predicted to

be the dominant path in most cases. Since the 0

and OH reactions produce different products

than the H reactions, the relative reaction rates

with the different partners will influence the net

yield of, for instance, CF from CFz. These reactions will thus affect the profiles of the species we

have detected, although unfortunately none of the

oxygen containing products are sufficiently amenable to LIF diagnostics to obtain a direct measurement of their profiles. CHFO and HCO both

have transitions in the vicinity of 250 nm, but

interference from CFz would preclude their detection at these wavelengths for our conditions.

Some kinetic studies have been performed on 0

atom reactions with the hydrofluorocarbon radicals, but the OH reaction rates have, in most

cases, never been directly measured. In the

HFC mechanism, the OH kinetics are largely

based on the analogous 0 atom reactions.

Kinetic rates of the reactions of CFz with H

and 0 atoms have been measured at room

temperature by Tsai and McFadden [49, 50],

while the reaction rate with OH has only been

inferred from kinetics modeling of flames inhibited by CF3Br performed by Biordi et a1.[51, 52].

The base mechanism assumes that the reaction

rates with 0 and particularly with H increase with

temperature, the result being that at flame temperatures, CFz is almost quantitatively converted

to CF by the reaction with H atoms. The base

mechanism greatly overpredicts the amount of

CF in the trifluoromethane flame relative to the

mono- and di-fluoromethane flames. The only

way to reasonably bring the predicted CF concentrations into agreement with the experiment

is to assume that CFz has reaction rates with 0

and/or OH comparable to those with H under

flame conditions. We have made the rates temperature independent and set the rate with OH

to four times the value for O.

The rate of the reaction CHF2 + OH -?

CHFO + HF was reduced by a factor of two to
724

better fit the CHF and CF profiles in the

difluoromethane flame. The reaction CF + OH

~ CO + HF has been increased to move the

CF concentration peak closer to the burner.

Even in the revised mechanism, the CF profiles

in all the flames remain about 2 mm too far

from the burner, even though the H, OH, CH,

and CF2 profile maxima are all predicted to

within 1 mm of the experimental value for all

the flames. Since the reaction zone as a whole is

predicted to occur in the right place, the problem seems to be that either the CF formation or

removal kinetics are too slow. Since CF is

produced primarily from CHF in the methyl

fluoride flame (in the revised mechanism) and

from CF2 in the trifluoromethane flame, there is

not a single CF formation reaction that can be

responsible for the late CF peak in all three

flames. Concerning the removal kinetics, the

reaction rates of CF with Hand 0 have been

measured at room temperature only [49, 50, 53],

while the rate with OH has not been measured.

The recent measurement of Van Hoeymissen et

al. [53] for the CF + 0 reaction measured a

value three times larger than did the study of

Tsai and McFadden [50], from which the kinetic

parameters in the base mechanism have been

taken. We have increased the rates by a factor

of two at high temperatures for the Hand 0

reactions while maintaining the same rates as in

the base mechanism at 300K and increased the

rate with OH. These changes moved the CF

profile slightly toward the burner but not

enough to obtain good agreement with the

experimental profiles. It is also conceivable that

the rate of CF + O2 should be increased at high

temperature, since O2 is the most important

reaction partner in CF removal. The preexponential factor assumed in the base mechanism,

2 X 1013 cm3jmol-s, however, is already fairly

high for a radical reaction with a closed shell

species, and approaches that of the analogous

reaction for CH, which is generally observed to

be more reactive than CF [53].

The recombination rates of the fluoromethyl

radicals CH2F and CHF2 with CH3

Fi

C

(R16) A

Reaction Pathways and Profile Comparisons

With Experiment

D. L'ESPERANCE ET AL.

have been lowered by 30% from the base mechanism. The base mechanism predicts that the

C2H concentration (and by extension the

amount of CH* emission) should be three times

as high in the mono- and di-fluoromethane

flames as in the trifluoromethane flame,

whereas the experimental ratio of the CH*

emission profiles is slightly less than a factor of

2. Sensitivity analysis for C2H concentrations

found that the reactions above had the highest

sensitivities of any reactions involving fluorine

because they constitute the principal route to

formation of C2 species in these flames. Even

with the revisions, the ratio of predicted CH*

emission is still too high. Further adjustment of

the rates is probably not warranted based on the

emission profiles. Both the temperature dependence of the C2H + 0 ~ CH*(A2~) + CO

reaction and detailed validation of the C2 hydrocarbon and fluorocarbon kinetics leading to

C2H formation require further investigation.

Reaction pathways of the fluorinated agents in

the burner stabilized flames were determined by

integrating the chemical production rates across

the flame zone and summing individual rates

with all possible reaction partners. The results

for the base mechanism are shown in Figs. 10,

12, and 14, for the flames containing CH3F,

CH2F2, and CHF3, respectively. Predictions for

the revised mechanism are shown in Figs. 11, 13,

and 15. Arrow thicknesses are proportional to

the flux of carbon atoms integrated over the

flame zone. Next to each reaction path, the

reaction partner(s) are written in decreasing

order of importance.

For the small fraction of agent which is

predicted to react in the flame containing CF-t'

the reaction pathways of the CF3 radical are

similar to those in the CHF3 inhibited flame in

most respects. The one exception was that very

small amounts of C2 hydrofluorocarbons were

produced, due to most of the agent consumption taking place after the methyl radical had

(R14) disappeared, giving the reaction

(R15)

CH2F + CH3 = C2H4 + HF,

CHF2 + CH3 = CHFCH2 + HF
ET AL. SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES 725

~mechhat the

)n the·

:e times

lethane

flame,

e CH*

ictor of

trations

highest

:luorine

Dute to

s. Even

~dCH*

nent of

Ion the

depen-

+ CO

Cz hyding to

.tion.

isons

~ents in

ined by

s across

il rates

results

'igs. 10,

CH3F,

.ons for

.11,13,

onal to

ver the

.th, the

Teasmg

hich is

19 CF~,

cal are

lame in

lat very

IS were

nsump-

::al had

(R16)

Fig. 10. Reaction pathways of fluorocarbon species in the

CH,JOzlCH3F flame according to the base mechanism.

Arrow thicknesses are proportional to the carbon atom flux.

The most important reaction partner(s) are written next to

each arrow in decreasing order of importance.

a very low flux due to the lack of spatial overlap

in the profiles of the two reactants.

The modifications made to the mechanism

are summarized in Table 4. The revised mechanism gives very good predictions of profiles

and relative concentrations for most species.

The profiles of CHF appear about 3 mm later

than the experiment indicates; however, the

experimental difficulties of spectral interference

and temperature correction make this disagreeFig. 11. Reaction pathways of fluorocarbon species in the

CH,JOzlCH3F flame according to the revised mechanism.

Arrow thicknesses are proportional to the carbon atom flux,

Fig. 12. Reaction pathways of fluorocarbon species in the

CH,JOz/CHzFz flame according to the base mechanism.

Arrow thicknesses are proportional to the carbon atom flux.

ment of marginal significance. The calculated

CF profiles are about 2 mm too far from the

burner. Since the CF spectrum is well characterized and has little interference, this discrepancy is statistically significant. Additional data

on the formation and removal kinetics of CF at

high temperatures are probably needed to correct this disagreement.

The H atom profiles deviate from the prediction in the postflame zone (>2 cm above the

burner). Specifically, the experimental H atom

mole fraction begins to decrease while the calculation predicts an increase over the entire compuFig. 13. Reaction pathways of fluorocarbon species in the

CH,JOz/CHzFz flame according to the revised mechanism.

Arrow thicknesses are proportional to the carbon atom flux.
726

Fig. 14. Reaction pathways of fluorocarbon species in the

CH,JOz/CHF3 flame according to the base mechanism.

Arrow thicknesses are proportional to the carbon atom flux.

tational domain. This behavior is generally seen in

this type of burner geometry [33, 54, 55] in all

types of flame systems and therefore does not

depend on the fluorine chemistry. It may be a

consequence of preferential lateral diffusion

depleting the H atom mole fraction when the

height above the burner becomes comparable to

the burner radius. This phenomenon is not likely

due to radial spread of the flame with height

Fig. 15. Reaction pathways of fluorocarbon species in the

CH,JOz}CHF3 flame according to the revised mechanism.

Arrow thicknesses are proportional to the carbon atom flux.

D. L'ESPERANCE ET AL.

above the burner, since the mole fraction of OH

(which has a much lower diffusivity) is quite

accurately predicted by the model up to 5 cm.

Flame Speed Validation of Revised

Mechanism

The HFC mechanism has already been shown to

accurately predict the propagation speed of

atmospheric pressure methane/air flames containing CHzFz under most conditions [11]. Furthermore, the modifications of Saso et al. produced good agreement with experimental data

for methane/air flames containing CHF3. Since

our proposed changes to the kinetics have been

dictated by low pressure experiments, it is necessary to make sure that these changes maintain

agreement with the atmospheric pressure flame

speed data. Several of the reactions we have

adjusted were identified in the study of Linteris

and Truett as having significant sensitivities for

flame speeds.

We performed calculations to compute burning velocities of freely propagating atmospheric

pressure flames containing di- or tri-fluoromethane for eight representative conditions

for which experimental data are available. The

reactant mixtures were composed of methane,

air, and inhibitor with ratios of methane/oxygen

of 0.45, 0.5, and 0.55, corresponding to equivalence ratios of 0.9, 1.0, and 1.1 for the uninhibited flames. The mixtures contained the agents

CHzFz or CHF3 at up to 7% mole fraction.

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 diffusivities for H and Hz, and

windward differencing on the convective term.

The initial temperature of the fresh gases was

set to 298.2K. Meshes were refined until tolerances of 0.1 and 0.2, respectively, for GRAD

and CURV, were satisfied, or until a maximum

of 150 grid points was reached. The final meshes

contained approximately 120 grid points for the

uninhibited methane/air flames and> 140 grid

points for the inhibited flames.

Results of the atmospheric pressure flame

speed calculations are summarized in Table 5.

In Figs. 16 and 17, the calculated flame speeds

for CH,JOz/CHF3 and CH,JOz/CHzF2 mixtures,

SPEC

Reacti

MDl.

MD4.

MD7.

MD8.

MD9.

MDlO

MAJ.

NN8.

NN2l.

NN23.

NN25.

NN30.

NN30c

NN3l.

NN40.

NN4l.

NN43.

NN44.

NN5l.

PP27.

PP29.

ED45.

ED47.

EC3.

EC5.

JDl.

JD2.
:<:ETAL.

on of OH

I is quite

) 5 cm.

SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES

TABLE 4

Adjustments to Base Fluorine Mechanism'

Arrhenius Parameters k = AT" exp( - Ea/RT)

727

Reaction

A

(em, mol, s) b

Ea

(cal/mol) Remarks

shown to

speed of

mes con-

[11). Fur-

:t al. promtal data

:F3• Since

lave been

it is necmaintain

ure flame

we have

f Linteris

ivities for

ute burn10spheric

r tri-fluonditions

1ble. The

methane,

Ie/oxygen

o eqUlvauninhibhe agents

Lction.

erformed

flame on

)t boundment visi Hz, and

ive term.

~ases was

1til tolerr GRAD

naxlmum

11 meshes

ts for the

> 140 grid

.Ie flame

Table 5.

Ie speeds

mixtures,

MDI.

MD4.

MD?

MD8.

MD9.

MDlO.

MAJ.

NN8.

NN21.

NN23.

NN25.

NN30.

NN30a.

NN31.

NN40.

NN41.

NN43.

NN44.

NN51.

PP27.

PP29.

ED45.

ED47.

EC3.

EC5.

JD1.

JD2.

CH3F (+M) = lCHz + HF

(+M)

High-pressure limit: 1.00E+ 14 0.0

Low-pressure limit: 1.50E+ 16 0.0

Third body efficiencies: Hz:2.0 HzO:9.0 CH4:2.0 CO:1.5 COz:2.0 CZH6:3.0

CH3F:6.0 CHzFz:6.0 CHF3:6.0 HF:2.0

CHF + HF + M = CHzFz + M 3.04E+26 -3.26

Third body efficiencies: Hz:2.0 HzO:9.0 CH4:2.0 CO:l.5 COz:2.0 CZH6:3.0

CH3F:6.0 CHzFz:6.0 CHF3:6.0 HF:2.0

CHF3 + M = CFz + HF + M 3.41E+30 -4.00

Third body efficiencies: Hz:2.0 HzO:9.0 CH4:2.0 CO:1.5 COz:2.0 CZH6:3.0

CH3F:6.0 CHzFz:6.0 CHF3:6.0 HF:2.0

CF4 + M = CF3 + F + M 9.00E+34 -4.64

Third body efficiencies: Hz:2.0 HzO:9.0 CH4:2.0 CO:1.5 COz:2.0 CZH6:3.0

CH3F:6.0 CHzFz:6.0 CHF3:6.0 CF4:6.0

CHzF + H = lCHz + HF 1.10E+14 0.0

CHzF + H = CHF + Hz 5.00E+13 0.0

CHF3 + H = CF3 + Hz 3.76E+ 13 0.0

CHFz + OH = CHFO + HF 1.00E+ 13 0.0

CFz + 0 = FCO + F 1.20E+ 13 0.0

CFz + OH = CF:O + HF 1.00E+ 13 0.0

CFz + OH = CFzO + H 4.00E+13 0.0

CHF + H = CH + HF 6.50E+13 0.0

CHF + H = CF + Hz 2.30E+14 0.0

CFz + H = CF + HF 2.35E+ 13 0.0

CF + 0 = CO + F 8.00E+13 0.0

CF + OH = CO + HF 8.00E+ 13 0.0

CH + HF = CF + Hz 2.00E+13 0.0

CF + H = C + HF 8.00E+13 0.0

CFz + M = CF + F + M 6.00E+26 -2.85

Third body efficiencies: HzO:6.0 CH4:2.0 CO:1.5 COz:2.0 CZH6:3.0

CH3F:6.0 CHzFz:6.0 CHF3:6.0 CF4:6.0 HF:2.0

CO + F + M = FCO + M 3.09E+19 -1.40

Third body efficiencies: Hz:2.0 HzO:18.0 CH4:2.0 CO:1.5 COz:2.0 CZH6:3.0

FCO + H = CO + HF 2.50E+ 13 0.0

CH3 + CHzF = CZH4 + HF 1.85E+19 -1.86

CH3 + CHFz = CHzCHF + HF 1.30E+15 -0.586

CHFz + CHzF = CHzFz + CHF 1.00E+ 13 0.0

CH3 + CHFz = CH4 + CFz 2.50E+13 0.0

CHzCHF ( + M) = CzHz + HF

(+M)

High-pressure limit: 1.00E + 14 0.0

Low-pressure limit: 4.20E + 15 1.0

Third body efficiencies: Hz:2.0 HzO:6.0 CH4:2.0 CO:1.5 COz:2.0 CZH6:3.0

CH3F:6.0 CHzFz:6.0 CHF3:6.0 HF:2.0

CHzCFz (+ M) = CzHF + HF

(+M)

High-pressure limit: 2.50E+ 14 0.0

Low-pressure limit: . 9.00E+ 15 1.0

Third body efficiencies: Hz:2.0 HzO:6.0 CH4:2.0 CO:1.5 COz:2.0 CzH6:3.0

CH3F:6.0 CHzFz:6.0 CHF3:6.0 HF:2.0

85,000.

67,499.

4060.

69,050.

122,400

o

o

13,100

o

o

o

o

o

o

o

1410

1410.

o

o

106,000

-487

o

1870.

634.

4400.

800.

70,800.

70,800.

86,000.

86,000 .

[44]

[44]

[56]

[12]

Added channel

[57]

[12]

[12]
D. L'ESPERANCE ET AL.

CD

E 1.0 co

u::

"

2

:.0

:r

c 0.8 'c:J

.9

CD

>

~ 0.6 CD

a:::

-0

<lJ

<lJ

e>-

rn

<lJ 0.4 E

ro

u::

Fig. 17

ments I

the re~

O.Z

SPEC

flame

SInce

the \

foune

measl

hibite

Ou

the n

flame

ett,v.

tions

tivity

with

CHzI

ing t

flame

sition

for IE

fluor<

come

had ~

lowel

0.05 j

the H

of O.~

as th,

mg a

spee(

rate

10

o

Remarks

o

78,000.

78,000.

100,000.

100,000.

Ea

(cal/mol)

o Calculation, CH4/0Z = 0.50

• Calculation, CH4/0Z = 0.45

o Saso, et al. CH40z = 0.50

• Saso, et al. CH4/0Z = 0.45

o Linteris and Truett, CH40z = 0.50

• Linteris and Truett, CH4/0Z = 0.45

b

•

o Calculation, CH40z = 0.55

<$> Linteris and Truett, CH40z = 0.55

o

1.0 I- •

CD

E

ro

u:

"

~ 0.8 :E

<::

'c

::J

.2

CD

.~ 0.6 m

Qj

a:::

"

CD

CDa.

rn 0.4 CD

E

ro

u:

were in good agreement with the measurements

of Linteris and Truett.

The two conditions for which the mechanism

gave poor predictions of the flame speed were

both substantially fuel rich, having equivalence

ratios of approximately 1.3 (taking into account

the effect of the agent on the flame stoichiometry). By contrast, the CHF3 inhibited flames for

which burning velocity data is available all have

equivalence ratios less than 1.2. The source of

the difficulty in predicting the difluoromethane

O.Z

Z 4 6 8

CHF3 Mole % in Fuel/Air Mixture

Fig. 16. Comparison of experimental flame speed measurements [11, 12J for CHJair/CHF3 flames with calculations

using the revised mechanism.

A

(em, mol, s)

Arrhenius Parameters k = AT' exp(-E)RT)

TABLE 4

Continued

CHFCHF[Z]

(+M) = C2HF + HF (+M)

High-pressure limit: 2.50E+ 14 0.0

Low-pressure limit: 9.00E+ 15 1.0

Third body efficiencies: H2:2.0 H20:6.0 CH4:2.0 CO:1.5 CO2:2.0 C2H6:3.0

CH3F:6.0 CH2F2:6.0 CHF3:6.0 HF:2.0

CHF = CF2 (+M) = C2F2 + HF

(+M)

High-pressure limit: 2.50E14 0.0

Low-pressure limit: 9.00E15 1.0

Third body efficiencies: H2:2.0 H20:6.0CH4:2.0 CO:1.5 CO2:2.0 C2H6:3.0

CH3F:6.0 CH2F2:6.0 CHF3:6.0 HF:2.0

Reaction

JD3.

728

JD4.

TABLE 5

* Reaction numbers refer to [8J.

Test Cases for Flame Speed Calculations Using Revised

Mechanism ([8] modified according to Tables 3 and 4)

respectively, are plotted normalized by the uninhibited flame speed at the same CH,JOz ratio.

The experimental data of Linteris and Truett

[11] and of Saso et al. [12] are plotted for

comparison. The agreement of the calculations

with the experimental data is very good for

CHF3. The agreement is less good for the

flames containing CHzFz under some conditions. At 6% CHzFz with a methane/oxygen

ratio of 0.50, and 5% CHzFz with a methane/

oxygen ratio of 0.55, flame speeds were underpredicted by about 25%. In contrast, the calculated speeds of flames with CH,JOz = 0.50 with

5% CHzFz, and CH,JOz = 0.45 with 5% CHzFz,

CHJ02 Agent mole Equivalence Flame speed

Agent

ratio fractionratio (cm/s)

0.45-0.90 35.25

0.501.00 0.55-39.37 1.10 40.11

0.45 404%21.57 1.010.50 50%19.20 1.117.11%3.85 1.150.55 519%16.27 1.200.45 50%22.37 1.160.50 50%18.13 1.24

0.55 6.00% 500%11.8628 1.2933
ET AL. SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES 729

246 8

CH2F2 Mole % in Fuel/Air Mixture

Fig. 17. Comparison of experimental flame speed measurements [11] for CH,Jair/CHzFz flames with calculations using

the revised mechanism.

flame speeds must involve fluorine chemistry,

since both the study of Linteris and Truett and

the validation tests of. the GRI mechanism

found good agreement with experimentally

measured flame speeds for methane/air (uninhibited) flames at these equivalence ratios.

Our modifications did not substantially alter

the mechanism's performance for the CH2F2

flames from that reported by Linteris and Truett, who also observed substantial underpredictions of flame speeds for rich conditions. Sensitivity analysis of the flame speed for the flame

with the methane/oxygen ratio of 0.55 and 5%

CH2F2 found that the fluorinated reactions having the highest sensitivity coefficients for the

flame speed were the agent thermal decomposition and reactions with Hand OH, the same as

for leaner stoichiometries. Reactions involving

fluoromethyl + (fluoro )methyl including recombination and hydrogen disproportionation

had sensitivities about an order of magnitude

lower (absolute values of 0.005 compared to

0.05 for the difluoromethane removal reactions;

the H + O2 = OH + 0 reaction has a sensitivity

of 0.5, the highest value in the inhibited as well

as the uninhibited flames). For a reaction having a sensitivity coefficient of X for the flame

speed, a 1% increase in the reaction's kinetic

rate will cause an X% change in the flame

OJ

~ 1.0 r + e + -0 + 2

1) <1> 0

~ 0.8 'c

::>

.8

OJ

.?: ]i 0.6 OJ

a::

-0

OJ

OJ

C.

en

OJ 0.4 E

ro

u:

:marks

rements

;hanism

;d were

valence

account

ichiom-

.mesfor

all have

!Urce of

lethane

50

45

50

t5

)2 = 0.50

)2 = 0.45

o

10

measurelculations

0.2 o

• Calculation, CH4/02 = 0.45

o Calculation, CH4/02 =0.50

o Calculation, CH402 = 0.55

+ Linteris and Truett, CH4/02 = 0.45

o Linteris and Truett, CH4/02 = 0.50

o Linteris and Truett, CH4/02 = 0.55 +

0 0

+

0

0

+.

0

+ 0 <1>

0 +

0

+

0 <1> <1>

0

0

0

0

0 <1> 0 0

10

speed, a positive sensitivity coefficient corresponding to an increased flame speed. The

recombination of two (fluoro )methyl radicals to

form an ethylene with HF elimination had a

positive sensitivity coefficient, while hydrogen

disproportionation to form a (fluoro )methane

and a (fluoro )carbene had a negative sensitivity

coefficient. Alterations to the kinetics of the C2

reactions are probably needed to correctly predict flame speeds .under rich conditions, but

such changes cannot be validated without additional data. Species profiles for rich stoichiometries (ideally comprising some C2 species as

well as those monitored here) are needed to

ascertain which aspects of the kinetics need

attention.

Conclusions

We have obtained profiles for a number of

intermediate species in methane/oxygen flames

containing C1 hydrofluorocarbons and propose

chemically plausible adjustments to selected reactions in the HFC mechanism. This revised

mechanism provides good agreement with both

our low-pressure profile data and, except for

rich stoichiometries, atmospheric pressure

flame speeds. In making changes to the kinetic

mechanism, it must be emphasized that because

of the large number of kinetic parameters,

which could conceivably be modified and the

. rather limited range of data, there is not a

unique set of adjustments that will adequately

reproduce the experimental results. Having information on oxygen containing fluorine species

and on C2 species would be very useful in

diagnosing further uncertainties in the mechanism, which include the relative reactivities of

the fluorinated radicals with 0, H, and OH, and

the rather poor prediction of flame speeds

under rich stoichiometries. Flame speed data on

methane/air/CH3F mixtures, while not of practical importance for fire suppression, would add

to the data needed to interrogate hydrofluorocarbon kinetics. The profiles obtainable for the

CHF radical could benefit from investigation of

alternative wavelengths for a fluorescence diagnostic and especially from an improved temperature correction. Individual reactions in the

HFC mechanism, which, based on our study,

appear to require attention include the product

I

j

jiil1

!
730

channels of atomic hydrogen reactions with

fluorinated radicals (Hz vs HF product), and the

falloff behavior and particularly the third body

efficiencies of flame species for thermal decomposition reactions.

We thank Phillip Westmoreland and Don Burgessfor helpful discussions. This work wasfunded

by the US. Naval Sea Systems Command.

REFERENCES

1. Scientific Assessment of Ozone Depletion: 1994, WMO

Global Ozone Research and Monitoring Project-Report No. 37, World Meteorological Organization, Geneva, 1995.

2. Miziolek, A. W., and Tsang, W., eds., Halon Replacements: Technology and Science, ACS Symposium Series

611, American Chemical Society, Washington, 1994.

3. Westbrook, C K., Combust. Sci. Technol. 34:201-225

(1983).

4. Sheinson, R. S., Penner-Hahn, J. E., and Indritz, D.,

Fire Safety J. 15:437-450 (1989).

5. Nyden, M. R., Linteris, G. T., Burgess, Jr., D. R. F.,

Westmoreland, P. R., Tsang, W., and Zachariah, M. R.,

in Evaluation of Altemative In-Flight Fire Suppressants for

Full-Scale Testing in Simulated Aircraft Engine Nacelles

and Dry Bays, Grosshandler, W. L., Gann, R. G., and

Pitts, W. M., eds. NIST SP 861 (1994), pp. 467-642.

6. Burgess, Jr., D. R. F., Zachariah, M. R., Tsang, W.,

and Westmoreland, P. R., Thennochemical and Chemical /(jnetic Data for Fluorinated Hydrocarbons, NIST

Technical Note 1412, NIST, Gaithersburg, MD., 1995.

7. Westmoreland, P. R., Burgess, Jr., D. R. F., Zachariah,

M. R., and Tsang, W., Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute,

Pittsburgh, 1994, pp. 1505-1511.

8. Burgess, Jr., D. R. F., Zachariah, M. R., Tsang, W.,

and Westmoreland, P. R., Prog. Energy Combust. Sci.

21:453 (1996).

9. The current NIST HFC mechanism may be downloaded from http://fluid.nist.gov/ckmech.html.

10. Noto, T., Babushok, V., Burgess, Jr., D. R. F., Hamins,

A., Tsang, W., and Miziolek, A., Twenty-Sixth Symposium (International) on Combustion, The Combustion

Institute, Pittsburgh, 1996, pp. 1377-1383.

11. Linteris, G. T., and Truett, L., Combust. Flame 105:

15-27 (1996).

12. Saso, Y., Zhu, D. L., Wang, H., Law, C K, and Saito,

N., Combust. Flame 114:457-468 (1998).

13. Linteris, G. T., Burgess, Jr., D. R., Babushok, V.,

Zachariah, M., Tsang, W., and Westmoreland, P.,

Combust. Flame 113:164-180 (1998).

14. Papas, P., Fleming, J. W., Sheinson, R. S., Twenty-Sixth

Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, pp. 1405-1411.

15. Fallon, G. S., Chelliah, H. K, and Linteris, G. T., TwentyD. L'ESPERANCE ET AL.

Sixth Symposium (Intemational) on Combustion, The

Combustion Institute, Pittsburgh, 1996,pp. 1395-1403.

16. McNesby, K. L., Daniel, R. G., Modiano, S. H., and

Miziolek, A. W., Proceedings of the Halon Options

Technical Working Conference, Albuquerque, NM,

May 7-9, 1996, pp. 295-305.

17. Babushok, V., Burgess, Jr., D. F. R., Linteris, G. T.,

Tsang, W., and Miziolek, A. W., Proceedings of the

Halon Options Technical Working Conference, Albuquerque, NM, May 9-11, 1995, pp. 239-250.

18. Brabson, G. D., Walters, E. A., Schiro, J., Spencer, C,

Tapscott, R. E., and Patterson, R. A., Proceedings of

the Halon Options Technical Working Conference, Albuquerque, NM, May 7-9, 1996, pp. 213-222.

19. Vandooren, J., Nelson da Cruz, F., and Van Tiggelen,

P. J., Twentj-Second Symposium (International) on

Combustion, The Combustion Institute, Pittsburgh,

1988, pp. 1587-1595.

20. Sanogo, 0., Delfau, J.-L., Akrich, R., and Vovelle, C,

Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp.

1489-1496.

21. Womeldorf, C, Grosshandler, W., and King, M., Proceedings of the 1996 Fall Technical Meeting of the

Eastern States Section of the Combustion Institute,

Hilton Head, SC, Dec. 9-11, 1996, pp. 257-260.

22. Berry, R. J., Ehlers, C J., Burgess, D. R., Zachariah,

M. R., and Marshall, P., Chem. Phys. Left. 269:107-116

(1997).

23. Fleming, J. W., Burton, K A., and Ladouceur, H. D.,

Chem. Phys. Left. 175:395-400 (1990).

24. Williams, B. A., and Fleming, J. W., Combust. Flame

98:93-106 (19?4).

25. L'Esperance, D., Williams, B. A., and Fleming, J. W.,

Chem. Phys. Left. 280:113-118 (1997).

26. Kee, R. J., Grear, J. F., Smooke, M. D., and Miller,

J. A., Sandia Report SAND85-8240, 1985.

27. Kee, R. J., Rupley, F. M., and Miller, J. A., Sandia

Report SAND89-8009, 1989.

28. Kee, R. J., Warnatz, J., and Miller, J. A., Sandia Report

SAND83-8209, 1983.

29. Bowman, CT., Hanson, R. K., Gardiner, W. C,

Lissianski, V., Frenklach, M., Goldenberg, M., and

Smith, G. P., GRl-Mech 2.1I-An Optimized Detailed

Chemical Reaction Mechanism for Methane Combustion and NO Fonnation and Rebuming, GRI Report

GRI-97/0020, 1997, Gas Research Institute, Chicago.

http://www.ME.Berkeley.edu/gri _mech/

30. Williams, B. A., and Fleming, J. W., Combust. Flame

110:1-13 (1997).

31. Gaydon, A. G., The Spectroscopy of Flames, Chapman

& Hall, London, 1957.

32. Devriendt, K, Van Look, H., Ceursters, B., Peeters, J.,

Chem. Phys. Left. 261:450-456 (1996).

33. Williams, B. A., and Fleming, J. W., Combust. Flame

100:571-590 (1995).

34. Goldsmith, J. E. M., and Laurendeau, N. M., Opt. Left.

15:576-578 (1990).

35. Dieke, G. H., and Crosswhite, H. M.,J. Quant. Spectry.

Radiative Transfer 2:97 (1962).

36. Griffiths, 1. F., and Barnard, J. A., "Flame and ComSI

37.

38.

39.

40.

41.

42.

43.

~4.

45.

46.

47.
TAL. SPECIES PROFILES IN FLUOROMETHANE-INHIBITED FLAMES

on, The

;-1403.

H., and

Options

e, NM,

;, G. T.,

s of the

?, Albuncer, C.,

'dings of

nce, Al-

'iggelen,

nal) on

tsburgh,

'eIle, c.,

~ombus994, pp.

M., Pro-

~ of the

rnstitute,

50.

chariah,

107-116

., H. D.,

t. Flame

5,J. W.,

. Miller,

Sandia

2 Report

W. c.,

\.1., and

Detailed

~ombusReport

:hicago.

t. Flame

hapman

~ters, J.,

t. Flame

'Pt. Lett.

Spectry.

d Cornbustion, Blackie Academic and Professional, Glasgow,

UK, 1995, pp. 102-109.

37. Rensberger, K. J., Jeffries, J. B., Copeland, R. A.,

Kohse-Hoinghaus, K., Wise, M. L., and Crosley, D. R.,

Applied Optics 28:3556-3566 (1989).

38. Ashfold, M. N. R., Castano, F., Hancock, G., and

Ketley, G. W., Chern. Phys. Lett. 73:421-424 (1980).

39. Sappey, A. D., Crosley, D. R., and Copeland, R. A.,

Appl. Phys. B. 50:463 (1990).

40. Porter, T. L., Mann, D. E., and Acquista, N., J. Mol.

Spectr. 16:228-263 (1965).

41. Booth, J.-P., Hancock, G., Toogood, M. J., and McKendrick, K. G., J. Phys. Chem. 100:47-53 (1996).

42. Douglass, C. H., Williams, B. A., and McDonald, J. R.,

Combust. Flame 107:475-478 (1996).

43. Hidaka, Y., Nakamura, T., and Kawano, H., Chem.

Phys. Letters 187:40-44 (1991).

44. Schug, K. P., Wagner, H. Gg., Z. Chem. Phys. 86:59-66

(1973).

45. Simmie, J. M., Quiring, W. J., and Tschuikow-Roux,

E., J. Phys. Chem. 74:992-994 (1970).

46. Simmie, J. M., and Tschuikow-Roux, E.,J. Phys. Chem.

74:4075-4079 (1970).

47. Tsai, C.-P., and McFadden, D. L., J. Phys. Chem.

94:3298 (1990).

731

48. Clark, D. T., and Tedder, J. M., Trans. Faraday Soc.

62:399 (1966).

49. Tsai, c.-P., and McFadden, D. L., J. Phys. Chem.

93:2471 (1989).

50. Tsai, c.-P., and McFadden, D. L., Chem. Phys. Lett.

173:241-244 (1990).

51. Biordi, J. c., Lazzara, C. P., and Papp, J. F., J. Phys.

Chem. 80:1042-1048 (1976).

52. Biordi, J. C., Lazzara, C. P., and Papp, J. F., 1. Phys.

Chem. 82:125 (1978).

53. Van Hoeymissen, J., De Boelpaep, I., Uten, W., and

Peeters, J., 1. Phys. Chem. 98:3725-3731 (1994).

54. Bernstein, J. S., Fein, A., Choi, J. B., Cool, T. A.,

Sausa, R. c., Howard, S. L., Locke, R. J., and Miziolek,

A. W., Combust. Flame 92:85-105 (1993).

55. Heard, D. E., Jeffries, J. B., Smith, G. P., and Crosley,

D. R., Combust. Flame 88:137-148 (1992).

56. Modica, A. P., and Sillers, S. J., J. Chem. Phys.

48:3283-3289 (1968).

57. Modica, A. P.,1. Chem. Phys. 44:1585-1589 (1966).

Received 2 October ]997; revised 17 August ]998; accepted 27

August ]998