An experimental study of the afterburning region of lean tetrafluoroethyleneoxygen flames

C O M B U S T I O N A N D F L A M E 26, 141-150 (1976)

141

An Experimental Study of the Afterburning Region of Lean Tetrafluoroethylene-Oxygen Flames DAVID I. ORLOFF Assistant Professor, College of Engineering, University of South Carolina, Columbia, S.C. 29208

and

RICHARD A. MATULA Professor and Chairman, Mechanical Engineering and Mechanics Department, Drexel University, Philadelphia, Pennsylvania 19104

The afterburning region of premixed, lean, one dimensional tetrafluoroethylene-oxygen flames has been studied. Composition and temperature profiles were measured for flames burning near 80 torr on a water cooled porous disk burner. Measured compositions and temperatures were compared to those predicted by thermodynamic equilibrium, and the species CF4, COFz, Oz, and CO were found to be in superequilibrium while COs and estimated atomic fluorine were lower than equilibrium predictions. Temperatures in the afterburning region of these flames were found to be between 300-500 °K higher than the predicted adiabatic flame temperatures. However, the measured flame temperatures were found to compare favorably with flame temperatures derived from the measured compositions and the first law of thermodynamics. Composition and temperature profiles were used in conjunction with the quasi-one dimensional flame equations to calculate net reaction rate profiles of stable species in the afterburning region of the flames studied. The calculated net reaction rate profiles were found to be independent of uncertainties in temperature measurement and in the case of COF2 and CF4 to show a trend away from the calculated equilibrium. These results indicate errors in the thermochemical data associated with species in this system.

Introduction

In recent years, fluorocarbons have been utilized in a number of high temperature applications and hence an understanding of the combustion of fluorocarbon-oxygen systems is of general interest. Polytetrafluoroethylene (Teflon), (C2F4)n,has been utilized as a subliming ablation material, the principal product of Teflon pyrolysis is the tetrafluoroethylene (C2F4) monomer, hence an understanding of the Cz F4-Oz combustion system is of particular interest. The spectral emission and absorption of radiant energy by the high temperature combustion products of CzF4-Oz mixtures is strongly dependent on both the temperature

and chemical composition of the products. Studies in our laboratory and others [1,2] have indicated that both the temperature and composition of CzF4-O2 combustion systems is considerably different than that predicted by standard equilibrium calculation techniques. Since major differences between the computed and the actual state of the gas may lead to significant variations in the predicted and true radiant signature of Teflon ablating systems, it is important to determine the extent and possible reasons for these apparent variations from thermochemical predictions. In light of these considerations the present experimental program was designed to measure the temperature and stable species concen-

Copyright ~1976 by The Combustion Institute Published by American Elsevier Publishing Company, Inc.

142

D A V I D I. O R L O F F and R I C H A R D A. M A T U L A

trations in a C2F4-O~ system where one would anticipate that thermodynamic equilibrium would be favored.

the burner was monitored both photographically and by means of a slide wire resistor electrical system. Additional details concerning the experimental apparatus are available in Ref.

Experimental

[3].

A schematic diagram of the burner facility is shown in Fig. 1. The 2 in. diameter burner was housed in a 6" x 4" x 2"pyrex cross so that the flame could be operated at subatmospheric pressures. The entire burner configuration was located in a fume hood as a safety precaution. The burner housing incorporated two 4" optical quality glass viewing ports and three probe ports which could be employed to insert an igniter, temperature probe, and a gas sampling probe into the flame. The burner entered the housing through a sliding "O-ring" seal, its position being varied by a rack and pinion traversing mechanism. The relative position of

The fuel, C2F4 had a purity of 99%, while the oxidizer was ultra purity, 99.99%, oxygen, both of which were purchased from Matheson Gas Co. Both the C2F4 and 02 were analyzed by mass spectrometer and found to be within the manufacturer's specifications. All gases were used as received from the supplier. Mole fraction profiles of stable species in the afterburning region of the flame were obtained by withdrawing quenched samples through a fused silica microprobe, and analyzing the samples with the aid of a Bendix MA-1 time of flight mass spectrometer (TOFMS). The sampling manifold was constructed from Teflon

.oo°-/

°

TO FORE PUMP

IEF VALVE

THERMOCOUPLE PROBE

IGNITION

Fig. 1. Schematic diagram of burner facility.

o-,o

voLTs NCTION

!

',

!l

i

'BOTTOM IDISK ,TEMP.~

' t DIGITAL 1 ~'--M.V. I

METER DISK TEMP.

CH.NO. I RECOROER H N'

.~ . . . . .. . . . . -- VOLTS

,

L._

COOLING WATER DRAIN

t PREMIXED GAS INLET

"COOLING WATER

AFTERBURNING REGION OF LEAN CzF4--O2 FLAMES

143

~,e-~TO T.O.F.MS. T

® ®

GAUGE

B

"~r'J_..J

"

IOl ILl

GAUGE BYPASSTO INLET MANIFOLD

VALVE

TO

CO

r.~lr

C02

$ERV.

CZF6

W ARGON SIF4 CF4

FORE PUMP _ _ t ~

TO HOOD

Fig. 2. Flame sampling manifold.

and stainless steel tubing, see Fig. 2. Quenched samples of combustion products were dynamically sampled by the TOFMS across a 0.0005" diameter gold-foil molecular leak. A typical flame spectrum consisted of between 12 and 17 peaks (See Fig. 3). The species identified in the combustion products and their major m/e peaks were: CO (28), C2F4 (31), O2 (32), COz (44), COFz (47), CF4 (69), SiF4 (85). The very small 18 peak was neglected since it was of the order of the H20 background in the spectrometer. The fact that H F (20) was absent in the flame spectra indicates that hydrogen impurities were not significant. It was assumed that the small concentration of SiF4, found in the quenched samples, was due to reaction of atomic fluorine with the fused silica microprobes. Flame spectras were analyzed with the aid of a standard computer program. A direct flame photographic technique was used to record the location of the microprobe

from the luminous edge of the flame. A Calumet bellows camera, Ilex-Calumet 165 mm f:6.3 lens, and polaroid high speed (type 57) film were employed. Exposure times of 1/30 sec were used to obtain a single photograph which could be used to locate the microprobe tip, the luminous edge of the flame and the bottom disk of the burner, which was used as a reference scale. Flame photographs, (See Fig. 4) showed curvature of the luminous edge of the flame due to uneven cooling at the bottom disk. The flame curvature, as determined photographically, was modeled in later calculations by writing the flame equations in terms of spherical geometry. Preliminary temperature measurement experiments on the C2F4-O2 flame s y s t e m showed that fine 0.005-0.001" diameter, silica coated platinum vs. platinum 10% rhodium thermocouples were consumed rapidly by the flame. In the present study, temperature pro-

144

D A V I D I. O R L O F F and R I C H A R D A. M A T U L A

I0 VOLT SCALE 32

85

69 66

~ 0 4 7 4-4

34 31 28

19 18 16

12

A

MASS TO CHARGE RATIO ,

role

Fig. 3. Typical mass spectra of quenched flame products from the afterburning region of C2F4-O 2 flames.

files were measured with large diameter (0.050.07 cm) platinum vs. platinum 10% rhodium thermocouple probes which were corrected for large radiation losses [3]. The experimentally determined mole fraction and corrected gas temperature profiles in the afterburning region for three lean C~F4-O2 flames are presented in Figs. 5-7. The initial conditions, see Table 1, were maintained relatively uniform for the three flames studied. The luminous edge of flames A and B were stabilized close to the bottom porous disk, 1.7 and 1.4 cm respectively, while flame C was stabilized at a distance of 2.3 cm above the bottom disk. In all of the experiments, the distance between the top and bottom porous disks was maintained constant at 5.72 cm. A pronounced drop in temperature may be observed in the afterburning region of flame C, which was due to the quenching effect of the top porous disk. This same phenomena has been observed in C~H4-air flames [5]. Discussion State of the Combustion Products

It might be expected that the state of the post flame gases studied in these experiments would be near thermodynamic equilibrium. There-

fore, a comparison of the predicted equilibrium state to the measured state was undertaken. The "NASA Complex Chemical Equilibrium Composition Program" [4] was used to predict the equilibrium composition and adiabatic flame temperature for the three flames reported in this study. The results of these computations are listed in Table 2. In order to test the sensitivity of the predicted equilibrium compositions to temperature, additional computations utilizing the initial conditions corresponding to flame B were carried out. These equilibrium calculations (See Fig. 8) were performed at a constant pressure of 80 torr for various temperatures bracketing the range of the predicted adiabatic flame temperature of Table 2. The equilibrium predictions (See Fig. 8) indicate that a considerable amount of atomic fluorine, F, may be expected in the equilibrium products of combustion of the C2F4-O2 flames studied. Since the sampling procedure used in this work was designed only to sample stable flame species, F atoms as well as other atomic and radical species were not experimentally measured. The existence of significant quantities of unmeasured F atoms in the reaction products may have introduced potential errors

AFTERBURNING REGION OF LEAN C~F4--O2 FLAMES

145

Fig. 4. Samplingof lean C2F4-02 flame(FlameA). TABLE 1 Initial Conditionsfor FlameExperiments Flame A B C

Xf

0.172 0.165 0.168

TB P° V2 (°K) (gm/cm sec)

P (Torr)

333 321 334

80.0 80.0 85.0

.00317 .00337 .00341

in the measured stable species mole fractions as analyzed by the TOFMS. Therefore, a procedure was devised which allowed the estimation of potential errors in measured stable species mole fractions, due to the existence of F atoms in the combustion products. This procedure was based on the hypothesis that highly reactive F atoms did not pass through the microprobe and sampling manifold without either reacting with the fused silica to form SiF4, or being absorbed somewhere in the gas sampling system (prior to introduction into the TOFMS). A simple mathematical relation-

ship between the measured species mole fractions and the actual species mole fractions in the combustion products can be readily developed in terms of the actual F atom mole fraction in the combustion products and the measured mole fraction of SiF4. This relationship is based on the premise that the mole ratio of carbon to oxygen in the flame and in the spectrometer are identical. The following correction equation was derived [3]:

X i actual =

fl

-

-

XF actual

~

XSi F4 measured J

x, measured

(1)

where X F actual

= mole fraction of fluorine atoms in the combustion products.

XSiF4 measured measured mole fraction of SiF4 in the quenched flame sample. =

DAVID I. ORLOFF and RICHARD A. MATULA

146

ZlO0

OAO

0.I0

.~'300 TEMP I100 .EIO0 0.011 1900

O.Ol

17oo ,Ik+

.

02XlO.i Iv

o

.

O

o

-"

~

ool

1 5 0 0 ..2. i-

OzXK) " l

.ISO0

!

hTO0

'"

~, o.o+ i,i.

.?. 0

8

0.04,

g

¢,

0.0~'

~

e,

COF! x I 0 " I

°" ~

~ COFjIXI

v

•

•

SIF4

$1F.,

9

I

i

2

OISTANCI[ FROM LUMINOUS E O e [ ,

o

DCSTANCE FROM LUMINOUS F.DGE , CId+

Fig. 7. Composition and corrected temperature profiles (Flame C).

Cll.

Fig. 5. Composition and corrected temperature profiles (Flame A).

,

MOLE

o

FRACTION

~

o

o

|

o

p

b

•

'

o.

I

++

n

N

~

n

b

_x

_x o

o CORRI~CTF'O TEMPERATURE ,

o

o

eK

Fig. 6. Composition and corrected temperature profiles (Flame B).

Xi actual

Xi measured

= mole fraction of stable species in the combustion products. = measured mole fraction of stable species in the quenched flame sample.

Since the mole fraction of SiF4 was experimentally determined, the application of the correction equation requires an estimate of the

t400

teO0

2tO0

TEMPERATURE , eK

Fig. 8. Equilibrium composition at assigned temperature and pressure.

F atom mole fraction in the combustion products. An estimate of the F atom mole fraction may be obtained by employing conservation of atomic mass in conjunction with the actual

AFTERBURNING REGION OF LEAN C2F4--O2 FLAMES mole fractions of stable species in the combustion products. The atomic molar ratio, R, of fluorine to carbon in the premixture was 2.0, and hence, R must also be equal to 2.0 in the combustion

147

products. The numerical value of R at any position in a flame may be readily computed, in terms of the measured mole fractions of stable fluorine and carbon containing species and the unknown F atom mole fraction, by use of Eq. (1),

I

1 - XSiF4 measured ] 2XcoF2 measured + 4XCF4 measured + " )(Factual 1- XFactua I R= XCOF measured + XCF4 measured + )(CO measured + measured

XC02

2.0al'

.t

Figure 9 shows a plot of R vs. various assumed values of Xractual, for the three flames studied in this work. Also shown is the probable error associated with the calculated ratio, R, due to probable errors in the experimentally measured species mole fractions. For flames A, B, and C the estimated F atom mole fractions are in the ranges 0.05-0.07, 0.00-0.02, and 0.007-0.02 respectively. Substitution of these ranges of F atom mole fraction along with experimentally measured mole fractions of SiF4 into Eq. (1) indicates that the correction to the experimentally measured stable species mole fractions may be neglected when computing the actual mole fraction profiles and in subsequent net reaction rate calculations. However, the estimated F atom mole fractions were employed in the First Law analysis which is discussed later in this paper. Comparison of the predicted equilibrium mole fractions at the computed adiabatic flame temperature, see Table 2, to the measured mole fractions of COFz, CF4, 02, and CO (See Fig. 5-7) indicates that these species are in superequilibrium amounts. However, CO2 and estimated F are below the equilibrium predictions in the afterburning region of the flames studies. The maximum measured flame temperatures in flames A, B, and C were in excess of the computed adiabatic flame temperatures, see Table 2 and Figures 5-7. Comparison of the predicted equilibrium compositions at the measured flame temperatures (See Fig. 8) and measured mole fractions result in an even more significant deviation from equilibrium. Reaction rate calculations which are accurate to

j

/

(2)

s s

S

FLAME C 1,951~ s •

N

.2.0S-

~,

.0

J

1.95

.

s

.->>--..

-~+~'

FLAME B

J

Z

I

s

j

1,951

O+O!

•

i

0.04

0.06

0,08

Fig. 9. Atomic molar ratio vs. estimated atomic fluorine mole fraction.

--- 15%, see Table 3, indicate that the afterburning region of the flames studied are relatively inert except for the oxidation of CO and production of CO2. In fact the concentration of superequilibrium species COFz and CF4 were observed to be increasing slowly, that is, shifting even further away from the predicted equilibrium. Superequilibrium amounts of COFz and/or CF4 have been observed in other CzF4-Oz combustion systems studied at the Combustion Kinetics Laboratory at Drexel University and at other laboratories [1,2]. Fenimore [1], in a preliminary study of the post flame gases of a premixed C2 F4-O2 flame burning at one atmosphere and at various initial stoichiometries, has measured ratios of CF+ to COF2 which are clearly above equilibrium predictions. Feni-

148

DAVID I. O R L O F F and RICHARD A. M A T U L A TABLE 2

Predicted Equilibrium and Adiabatic Flame TemperatureC2F4-O 2 System

XC2F4

Flame

A 0.172

B 0.165

C 0.168

Tg, °K

333

321

334

P, atm

0.1053

0.1053

0.1118

Adiab. fl. Temp.,°K

"~ •

I

....

1709

1719 --

XCF 2

1.02 X 10"10 <1.00 X 10"10 <1.00 X 10q °

XCF 3

3.35 X 10"s

2.84 X 10-8

P,

F(NIMORE (196B)

-

I

ATM.

THIS WORK eO TORR

EQUILIBRIUM CALC. I ATM. J.E.¢OLWI[LL

_z 1720

C.

- - EQUILIBRIUM

ET ALT(196~)

¢AL¢.

80 8 1'60 TORR

3.21 X 10-8

XCF 4

3.86 X 10-2

3.91 X 10-2

3.87 )( 10-2

XCO

4.71 X 10"s

3.86 X 10"5

4.33 X 10-5

XCO F

1.85 X 10-8

1.51 X 10-8

1.74 X 10-8

XCOFz

1.31 X 10q

1.27 X 10q

1.29 X 10"1

XCO 2

1.29 X 10-1

1.23 X 10q

1.25 X 10"1

XF

1.80 X 10"l

1.67 X 10"I

1.73 X 10"l

XFO

1.58 X 10-5

1.48 X 10"5

1.58 X 10"5

XFO 2

2.35 X 10-6

2.35 X 10 -6

2.46 X 10-6

XF2

7.93 X 10"s

7.37 X 10-5

7.80 X 10"5

XF20

3.26 X 10-9

3.06 X 10-9

3.34 X 10-9

X0

1.22 X 10-4

1.11 X 10-4

1.18 X 10-4

XO2

5.21 X 10"1

5.44 X 10-1

5.34 X 10-1

Note: X/= mole fraction of species i.

k Xoz/Xe2F4 ,N RatmxToR(

Fig. 10. Comparison of CF4/COF2 in products to equihbrium predictions. tency of the mole fraction and temperature profile d a t a o b t a i n e d in this s t u d y is d i s c u s s e d . T h e energy equation for a quasi-one dimensional, s p h e r i c a l f l a m e m a y b e w r i t t e n [8] A r 2 [pHv + ~ . N J l i V i - • d T / d r l

m o r e ' s d a t a a r e r e p r o d u c e d in Fig. 10 w h e r e d a t a f r o m f l a m e s A , B, a n d C o f this s t u d y a r e i n c l u d e d f o r c o m p a r i s o n . It m a y b e o b s e r v e d that the results of the present study and those o f F e n i m o r e a r e in e x c e l l e n t a g r e e m e n t . A l s o p l o t t e d in Fig. 10 a r e t h e e q u i l i b r i u m c a l c u l a t i o n s p e r f o r m e d in this s t u d y at 0.1 a n d 1.0 atmospheres and the earlier calculations of C o l w e l l et al. [6] at o n e a t m o s p h e r e . T h e p r e d i c t i o n s at 0.1 a n d 1.0 a t m o s p h e r e s a r e i d e n t i cal, w h i l e t h e s m a l l d i s c r e p a n c y b e t w e e n Colw e l l ' s c a l c u l a t i o n a n d t h e p r e s e n t c a l c u l a t i o n is d u e t o m o r e c u r r e n t J A N A F [7] d a t a w h i c h w a s u s e d in t h e c a l c u l a t i o n s o f this s t u d y . C o m p a r i son of the measured ratios of either Fenimore o r this w o r k to t h e e q u i l i b r i u m c a l c u l a t i o n s s h o w s t h a t t h e e x p e r i m e n t a l f l a m e w o r k imp l i e s s u p e r e q u i l i b r i u m r a t i o s o f CF4 to C O F 2 .

Comparison of Predicted and Measured Flame Temperatures In this section a method of checking the consis-

= constant.

(3)

!

T h e t h r e e t e r m s o n t h e left h a n d side o f E q . (3) r e p r e s e n t c o n v e c t i o n , diffusion, a n d c o n d u c t i o n o f e n e r g y at a n y p o s i t i o n in t h e f l a m e . T h e terms due to diffusion and conduction may be s h o w n t o b e n e g l i g i b l e in t h e a f t e r b u r n i n g a n d p r e h e a t r e g i o n s o f t h e f l a m e s s t u d i e d , so t h a t E q . (3) is s i m p l i f i e d to

afterburning

(4)

where X i = actual mole fraction of species i in flame, H i = molar enthalpy o f species i, cal-gm mole "1 , M i = molecular weight of species i, gm-gm mole "l .

149

AFTERBURNING REGION OF LEAN C2F4--O2 FLAMES TABLE 3 Tabulation of Reaction Rate Data Measured net reaction rates (gin mole-cm'3-sec "1 X 106) Flame

A A C A B B C B B

Temp. °K

2266 2193 2039 1984 1969 1919 1878 1748 1619

RCO

RC02 RO2

-.612 -.465 -.555 -.370 -.448 -.361 -.529 -.310 -.291

+.314 +.238 +.284 +.189 +.253 +.204 +.261 +.174 +.163

Employing the species molar enthalpies from the J A N A F tabulation [7], Eq. (4) was solved for the flame temperatures of flames A, B, and C consistent with the measured mole fractions of stable species while also including the estimated fluorine atom mole fractions. The results of these computations are presented in Table 4 for comparison to the measured peak temperatures and the calculated adiabatic flame temperatures. Since the F atom mole fractions were given in terms of an estimated range, the first law analysis did not yield a single value for the expected peak temperature. Inspection of these results indicates that the measured and calculated peak temperatures are in good agreement, and are in all cases considerably higher than the computed adiabatic flame temperatures. Conclusions

The afterburning region of lean C2F4-O2 flames at fuel mole fractions of 0.172, 0.165, and 0.168 and pressures near 80 torr have been investigated, and the temperature and mole fraction profiles of quenched stable species CO, COs,

-1.414 -1.069 -0.250 -0.839 -0.234 -0.184 -0.267 -0.149 -0.133

RCOF2

RCI,' 4

+.568 +.431 +.136 +.343 +.097 +.078 +.118 +.068 +.064

+0.093 +0.071 +0.060 +0.057 +0.010 +0.009 +0.052 +0.009 +0.008

and COF2 have been determined. The measured concentration of CO2 and estimated concentration of F atoms have been shown to be b e l o w p r e d i c t e d equilibrium concentrations, while the concentration of C F 4 , COF~, 02, and CO have been measured to be in superequilibrium amounts. Peak temperatures that are well above the predicted adiabatic flame temperatures have been compared to temperatures computed from the measured compositions, employing the First Law of thermodynamics. The measured and First Law temperatures were found to be in good agreement. The observed variations from predicted thermodynamic equilibrium suggest that either equilibrium is reached very slowly in the afterburning region of lean C2F4-O2 flames or some errors may be present in the thermochemical data of various fluorocarbon species. The second interpretation is favored by the authors since reaction rate data in the afterburning region indicate shifts away from the computed thermodynamic equilibrium. 02, CF4,

TABLE 4 Computed adiabatic Measured peak temperature flame temperature, (corrected for radiation heat OK loss), OK Flame 1720 1709 1719

2266 1991 2095

Temperature from first law, OK 1960-2160 2300-2470 1900-2040

150

DAVID I. ORLOFF and RICHARD A. MATULA

Support frorn the Air Force Office o f Scientific Research under grant AF-AFOSR-71-2125 is gratefully acknowledged.

5.

References

6.

1. Fenimore, C. P., General Electric Report No. 68-C231 (1968). 2. Modica, A. P., J. Phys. Chem. 74, 1194 (1970). 3. Orloff, D. I., An Experimental Study of the Afterburning Region of Lean Tetrafluoroethylene-Oxygen Flames, Ph.D. Thesis, Drexel University (1973). 4. Gordon, S. and McBridge, B. J., Computer Program for Calculation of Complex Chemical Equilibrium

7. 8.

Compositions, Rocket Performance Incident and Reflected Shocks, and Chapman-Jougurt Detonations, NASA-SP-273. Dixon-Lewis, G. and Isles, G. L., 7th Combustion Symposium (1959). Colwell, J. E., Wachi, F. M., and Greene, S. A., Aerospace Corp. Report No. TDR-469 (5250-40)-16, (1965). Stull, D. R. and Prophet, H. ,JANAF Thermochemical Tables, NSRDS-NBS 37, (1971). Fristrom, R. M. and Westenberg, A. A., Flame Structure, McGraw-Hill (1965).

Received 5 June 1975; revised 8 December 1975