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Burning velocities of fluorocarbon-oxygen mixtures
Combustionand Flame
97
Burning Velocities of Fluorocarbon-Oxygen Mixtures R. A. Matula, D. I. Orloff, and J. T. Agnew Combustion KineticsLaboratory. Thermal and FluidSciences. Drexel University,Philadelphia,Pcnnsylwmia. U,S.A.
The burningvelocitiesof mixturesof perfluoropropene(C:)Fo),pcrfluorocyclobutcne(c-C2F,~),and perfluorocyclobutane (c-C~FH)with oxygenat atmosphericpressure are reported. The peak burningvelocityof fluorocarbon-oxygenmixtures cannot be correlated withthe peak calculatedadiabatic flame temperature, but it is shown that the peak burning velocitycan be correlated with maximumequilibriumlluorincatom concentration. These results indicatethat the flame propagation mechanismin fluorocarbon-oxygeBsystemsis dominated by diffusionof reactivefluorineatoms fromthe hot products into the unburned reactants rather than by thermal effects.
Introduction Even though fluorocarbons are known to participate in a number of combustion reactions, only limited data pertainiug to the combustion characteristics of various fluorocarbon-oxidizer systems are available. Fletcher and co-workers studied the combustion of a few fluorocarbons with chlorine trifluoride [1], fluorine [2], and oxygen [3-4]. Colwell, Wachi. and Green [5] reported the stability limits (i.e., the critical boundary velocity gradients for blowoff and flashback) of laminar premixed perfluoroethylene (CaF~)-oxygen flames burning at I atm on small cylindrical tubes. Recently. Matula and Agnew [6] reported the burning velocities of C : F 4 - O z flames at atmospheric pressure as a function of fuel mole fraction. Tbe results reported in Refs. 4 and 6 indicate that there are two significant differences between the flame propagation characteristics of fluorocarbonoxygen and hydrocarbon-oxygen mixtures. First,
the maximum burning velocity of IZuorocarbonoxygen mixtures is considerably lower than that of the corresponding hydrocarbon-oxygen mixture. As an example, the peak burning velocities of C2F4-O ", and CzH4-O ", at atmospheric pressure are approximately 60 cm/sec and 600 cm/sec, respectively [6-7]. Second, the maximum burning velocity of fluorocarbon-oxygen mixtures, unlike that of hydrocarbon-oxygen mixtures, cannot be correlated with maximum adiabatic flame temperature. An example of this phenomenon occurs in the C,.F~-O2 system, whose peak burning velocity and adiabatic flame temperature correspond to fuel mole lYaetions of 0.33 and 0.65, respectively. The present study had two objectives. The first aim was to measure the burning velocity of any of the following fluorocarbons that support stable flames in the presence of oxygen at atmospheric pressure: perfluoromethane (CF.0~ perflooroethane (C2F~), perfluoropropane (CaFs), Comhu~tzcm & Flame, 14,97-102(1970) Copyright41;,19711byTheCombustionInstttute Publishedby AmericanElsevierPubllshjngCompany,Inc.
R. A. Matula, D. I. Orloff. and J. T. Agnew t~
98 perfluorobutane (C,,Fto), perfluoropropene (CaF6), perfluorocyclobutene (c-CgF6), perfluorobutyne-2 (C,,F6), perfluorobutadiene-l.3 (C4F6), and perfluorocyelobutane (¢-C,Fs). The second objective was to find a correlation between peak burning velocity and stoichiometry for fluorocarbon-oxygen systems.
/
Thermochemistry The adiabatic flame temperatures and corresponding equilibrium compositions for the fluorocarbon-oxygen systems of interest were calculated with the aid of a slighty modified F O R T R A N IV computer program obtained from R. Steffensen [8]. Literature values for the heat of formation of C2F.t [9] ( - 1 5 5 . 0 kca[/ mole). CaF 6 [10] ( - 2 6 3 . 4 kcal/mole), c-CgFo I'll] ( - 2 8 9 . 9 kcal/mole), and c-C,,F8 I l l ] (-365.2 kcal/mole) at 298.15°K were used. The following 24 species were considered as possible equilibrium products: C~° C. C:, Ca, C.t, C s, CF. CF2, CF 3, CF,~., CO, CO 2. C302. CF20, CzF :, C,F,. F. CFO, F 2, FO, F~O. O. 02, and O 3. It is well known that the validity of thermoehemical calculations is dependent. on the availability of accurate thermodynamic data for all important quilibrium products. and ~hereforo the accuracy of these calcula-
Figure .'. Equilibrium composition and adiabatic flamc tempcraturc for C~F6-O: mixtures (P = 1.0 atm).
,t
,, !
Figure 3. Equilibrium composition and adiabatic flame temperature for c-C,F6-O= mixtures (P= 1.0 arm),
? ~, •
.i
C~4 m ~
,Mtcr,0N
Figure I, Equilibviurn composition and adiabatic flame temperature for C=Fg-O2 mixtures(P = i .0 atm).
Figure g. Equilibrium composition and adiabatic flame temperature for c-C=Fu-O2 mixtures(P= 1.0 arm),
99
Burning Velocities of Fluorocarbon-Oxygen Mixtures tions depends on the validity of the J A N A F [9] data which were employed. The numerical results for mixtures of C2F~-O2, CaF6-Oz, c-C4F6-Oz, and c-C,~Fs-O 2 are graphically represented in Figs. 1--4. These results are for isobaric combustion at 1 atm, with the reactants initially at 298.15°K.
Experimental
Procedures
and
l~',esults
The premixed fluorocarbon-oxygen mixtures were burned in a bunsen-type microburner apparatus which has been described in a previous publication [6]. Both the oxidizer and fuel flow rates were determined with the aid ofrotameters calibrated against either a wet-test ~r~eter or a bubble flow meter. The extra-dry-grade oxygen, minimum purity of 99.6 per cent, and the fluorocarbons used in these experiments were purchased from the Matheson Company and Penninsular Chemresearch Inc., respectively. Penninsular Chemreseareh specified that the minimum purities of all of the fluorocarbons were 98 per cent, and all of the gases were taken directly from the cylinders and used without further purification. The burning velocities were determined by applying the bunsen cone method. The surface area of the conical flame was determined by measuring the height and the diameter of the base of the visible luminous cone. The cone height was measured with the aid ol a cathetometer capable of resolving 0.05 mm, and the diameter of the base was equated to the diameter of the burner tip. From previous experience in this laboratory and others, we estimate that this technique is accurate to within __. 10-20 percent. The experimentally measured burning velocities of CaF6.-O 2, c-C,~F(,-O,, c-C,~F8-O 2, and CzF.~-O2 as a function of fuel mole fracfon are shown in Figs. 5 and 6. The burning velocities of the C2F,~-O 2 system were taken from Re£ 6. The smoothed c-C,F~-O= burning velocities reported by Fuller E4] are also shown in Fig. 6.
i oi t21
~u~c ~ o ~ FaAcrlos
Figure 5. Burningvelocity and adiabatic flame temperature for C~F~-Oz and e-C~Fo-O2 mixtures (P= 1.0 ~,tm).
C4~0
~Ar~FRO~E~e
'°i '°i
FUrL ~OL~ r~Acrron
Figure 6. Burning velocityand adiabatic flame tel'apevature Ibr c-CaF.-O: and C,,Fa-O= mixtures(P= 1.0 arm),
100 Two additional C,,F 6 compounds, perfluorobutyne-2 and perfluorobutadiene-l,3, were also found to burn in the presence of oxygen. However, since only limited supplies of these two compounds were available, burning velocities of these fluorocarbons were not determined. Four of the saturated fluoroearbons--CF4, CzF6, CaF8, and C4F~o---could not be ignited on the microburner apparatus used. A constantvolume combustion bomb, utilizing a spark plug as the ignir.ion source, was also employed in an unsuccessful attempt to ignite a number of C3Fs-Oz mixtures with initial pressures in the range 0.1-1.0 atm. Possible ignition was determined by monitoring the pressure in the bomb as a function of time, with the aid of a sensitive pressure transducer and subsequent gas chromatographic analysis of the gases remaining in the bomb after the spark had been fired.
Discussion The experimentally determined burning velo. cities of C3F6-O 2, c-C,,F~-O:, and c-C;,Fs-O: are shown in Figs. 5 and 6. Burning velocities of neither C3F6-O 2 nor c-C,,F6-O z mixtures have been reported previously: however, the c-C.,F8-O z results can h,e compared with the values given by Fuller 1"4]. Hiz data were obtained from a flat flame deflagration tube, while the present data were determined by applying the bunsen cone method to small conical flames. Although the two sets of data are in general agreement (see Fig. 6), the results reported in this paper are approximately 10-20 per cent higher than those of Fuller. Discrepancies of this order of magnitude in burning velocity determinations, based on different experimental techniques, are r~ot unusual. Mix~.ures of CF4, C2F~,, C3F s, or C~.Ft o a~d oxygen could not be ignited at STP. Croomes [12]. using a hot-wire ignition source, could not ignite mixtures of either CzF6-O 2 or CzF6-H z in a calorimeter bomb. Our C3Fs-O 2 mixtures could not even be spark-ignited in a bomb. The
R. A. Matula, D. I. Orloff, and J. T. Agnew
low reactivity of these saturated fluorocarbonoxygen mixtures is very different from that of the analogous hydrocarbon-oxygen mixtures. The results of this investigation, along with the results previously reported by Fuller [4] and Matula and Agnew [6], clearly indicate that the maximum burning velocity of fluorocarbonoxygen flames cannot be correlated with the maximum adiabatic flame temperature. However, inspection of the data shown in Figs. 1-6 indicates that peak burning velocities for fluorocarbon-oxygen mixtures can be correlated with the fuel mole fraction corresponding to the maximum equilibrium fluorine atom concentration. A similar correlation between spatial velocities :rod equilibrium fluorine atom concentration in Fz-CI z flames has beer reported by Fletcher and Arabs [13]. The ~'esults of the present study indicate that the mechanism for flame propagation [,,t fluorocarbon-oxygen systems is dominated by the diffusion of reactive fluorine atoms from the hot products in~o the unburned reactants rather than by thermal considerations.
This research was sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant AF-AFOSR-68-1606.
References I. FLt~i~('HE~t.E. A.. and A~,II~s.L, L., Cambu,~i~m& Flame. 8, 275 (1965), 2, FLETCHER,E. A., and KI'FI'L~ON. D. B,, C'omhtlslioll & Flame, 12, 119 (1968). 3. FLE'ICHER,E. A,, and KITI'ELSON,D. B.. Combustion & Flame, 12. 164 (1968). 4, FULLI~R, L. E . "Simple Fundamontal Flame Speed Measurements--A Flat Flame D¢flagration Tube," M. S. Thesis, University of Minnesota (July 1968). 5. COI.WELt.,J. E,, WaCHI. F. M.. and GaE~,t~E.S, A., "'An investigation of the tetralfluoroethylene.oxygen flame." Aerospace Corp. Rept, TDR-469-5250-40-16 (1965), 6, MATUL^. R. A., and AGNEW,J. T . Combustion & Flame. 13. 101 (1969). 7, AGNEW. J. T.. and GRAIFF. L, B.. Combustion & Flame, 5. 209 (1961).
Burning Vdoclties of Fluorocarbon-Oxygen Mixtures 8. S'rEI~NSEN,R., "A FORTRAN IV Program for Thermoehemie',d Calculations Involving the Elements AI. B, Be, C, F, H~Li, Mg, N. and O and Their Compounds." Ph, D. Thesis, Purdue University (January 1966), 9. STtrLL.D, R., Project Director, JANAF Thermochemical Tables, U. S. Govt. Rept. PB-168-370and Addenda (1965), 10. DutJs, H. C.. Ind. Eng. Chem,. 47, 1445(1955). l 1. STULL.D. R,* WESTRUM.JR.. E, F,. and SINKE.G. C,,
101
The Chemical Thermadyanamic,~"ql"Orqanic Comptlund~, Johr~ Wiley: New York (in pcess). 12. CROOM~S.E, F.. Comhu,stion& Fl¢~me.10. 71 (1966). 13, FLETCHER,E. A,, and AMBS,L, L., C(l#lhti.~l[ott& F/gillie, 12, 115 (196,q),
(Received M a y 27, 1969; revised July 15, 1969)
97
Burning Velocities of Fluorocarbon-Oxygen Mixtures R. A. Matula, D. I. Orloff, and J. T. Agnew Combustion KineticsLaboratory. Thermal and FluidSciences. Drexel University,Philadelphia,Pcnnsylwmia. U,S.A.
The burningvelocitiesof mixturesof perfluoropropene(C:)Fo),pcrfluorocyclobutcne(c-C2F,~),and perfluorocyclobutane (c-C~FH)with oxygenat atmosphericpressure are reported. The peak burningvelocityof fluorocarbon-oxygenmixtures cannot be correlated withthe peak calculatedadiabatic flame temperature, but it is shown that the peak burning velocitycan be correlated with maximumequilibriumlluorincatom concentration. These results indicatethat the flame propagation mechanismin fluorocarbon-oxygeBsystemsis dominated by diffusionof reactivefluorineatoms fromthe hot products into the unburned reactants rather than by thermal effects.
Introduction Even though fluorocarbons are known to participate in a number of combustion reactions, only limited data pertainiug to the combustion characteristics of various fluorocarbon-oxidizer systems are available. Fletcher and co-workers studied the combustion of a few fluorocarbons with chlorine trifluoride [1], fluorine [2], and oxygen [3-4]. Colwell, Wachi. and Green [5] reported the stability limits (i.e., the critical boundary velocity gradients for blowoff and flashback) of laminar premixed perfluoroethylene (CaF~)-oxygen flames burning at I atm on small cylindrical tubes. Recently. Matula and Agnew [6] reported the burning velocities of C : F 4 - O z flames at atmospheric pressure as a function of fuel mole fraction. Tbe results reported in Refs. 4 and 6 indicate that there are two significant differences between the flame propagation characteristics of fluorocarbonoxygen and hydrocarbon-oxygen mixtures. First,
the maximum burning velocity of IZuorocarbonoxygen mixtures is considerably lower than that of the corresponding hydrocarbon-oxygen mixture. As an example, the peak burning velocities of C2F4-O ", and CzH4-O ", at atmospheric pressure are approximately 60 cm/sec and 600 cm/sec, respectively [6-7]. Second, the maximum burning velocity of fluorocarbon-oxygen mixtures, unlike that of hydrocarbon-oxygen mixtures, cannot be correlated with maximum adiabatic flame temperature. An example of this phenomenon occurs in the C,.F~-O2 system, whose peak burning velocity and adiabatic flame temperature correspond to fuel mole lYaetions of 0.33 and 0.65, respectively. The present study had two objectives. The first aim was to measure the burning velocity of any of the following fluorocarbons that support stable flames in the presence of oxygen at atmospheric pressure: perfluoromethane (CF.0~ perflooroethane (C2F~), perfluoropropane (CaFs), Comhu~tzcm & Flame, 14,97-102(1970) Copyright41;,19711byTheCombustionInstttute Publishedby AmericanElsevierPubllshjngCompany,Inc.
R. A. Matula, D. I. Orloff. and J. T. Agnew t~
98 perfluorobutane (C,,Fto), perfluoropropene (CaF6), perfluorocyclobutene (c-CgF6), perfluorobutyne-2 (C,,F6), perfluorobutadiene-l.3 (C4F6), and perfluorocyelobutane (¢-C,Fs). The second objective was to find a correlation between peak burning velocity and stoichiometry for fluorocarbon-oxygen systems.
/
Thermochemistry The adiabatic flame temperatures and corresponding equilibrium compositions for the fluorocarbon-oxygen systems of interest were calculated with the aid of a slighty modified F O R T R A N IV computer program obtained from R. Steffensen [8]. Literature values for the heat of formation of C2F.t [9] ( - 1 5 5 . 0 kca[/ mole). CaF 6 [10] ( - 2 6 3 . 4 kcal/mole), c-CgFo I'll] ( - 2 8 9 . 9 kcal/mole), and c-C,,F8 I l l ] (-365.2 kcal/mole) at 298.15°K were used. The following 24 species were considered as possible equilibrium products: C~° C. C:, Ca, C.t, C s, CF. CF2, CF 3, CF,~., CO, CO 2. C302. CF20, CzF :, C,F,. F. CFO, F 2, FO, F~O. O. 02, and O 3. It is well known that the validity of thermoehemical calculations is dependent. on the availability of accurate thermodynamic data for all important quilibrium products. and ~hereforo the accuracy of these calcula-
Figure .'. Equilibrium composition and adiabatic flamc tempcraturc for C~F6-O: mixtures (P = 1.0 atm).
,t
,, !
Figure 3. Equilibrium composition and adiabatic flame temperature for c-C,F6-O= mixtures (P= 1.0 arm),
? ~, •
.i
C~4 m ~
,Mtcr,0N
Figure I, Equilibviurn composition and adiabatic flame temperature for C=Fg-O2 mixtures(P = i .0 atm).
Figure g. Equilibrium composition and adiabatic flame temperature for c-C=Fu-O2 mixtures(P= 1.0 arm),
99
Burning Velocities of Fluorocarbon-Oxygen Mixtures tions depends on the validity of the J A N A F [9] data which were employed. The numerical results for mixtures of C2F~-O2, CaF6-Oz, c-C4F6-Oz, and c-C,~Fs-O 2 are graphically represented in Figs. 1--4. These results are for isobaric combustion at 1 atm, with the reactants initially at 298.15°K.
Experimental
Procedures
and
l~',esults
The premixed fluorocarbon-oxygen mixtures were burned in a bunsen-type microburner apparatus which has been described in a previous publication [6]. Both the oxidizer and fuel flow rates were determined with the aid ofrotameters calibrated against either a wet-test ~r~eter or a bubble flow meter. The extra-dry-grade oxygen, minimum purity of 99.6 per cent, and the fluorocarbons used in these experiments were purchased from the Matheson Company and Penninsular Chemresearch Inc., respectively. Penninsular Chemreseareh specified that the minimum purities of all of the fluorocarbons were 98 per cent, and all of the gases were taken directly from the cylinders and used without further purification. The burning velocities were determined by applying the bunsen cone method. The surface area of the conical flame was determined by measuring the height and the diameter of the base of the visible luminous cone. The cone height was measured with the aid ol a cathetometer capable of resolving 0.05 mm, and the diameter of the base was equated to the diameter of the burner tip. From previous experience in this laboratory and others, we estimate that this technique is accurate to within __. 10-20 percent. The experimentally measured burning velocities of CaF6.-O 2, c-C,~F(,-O,, c-C,~F8-O 2, and CzF.~-O2 as a function of fuel mole fracfon are shown in Figs. 5 and 6. The burning velocities of the C2F,~-O 2 system were taken from Re£ 6. The smoothed c-C,F~-O= burning velocities reported by Fuller E4] are also shown in Fig. 6.
i oi t21
~u~c ~ o ~ FaAcrlos
Figure 5. Burningvelocity and adiabatic flame temperature for C~F~-Oz and e-C~Fo-O2 mixtures (P= 1.0 ~,tm).
C4~0
~Ar~FRO~E~e
'°i '°i
FUrL ~OL~ r~Acrron
Figure 6. Burning velocityand adiabatic flame tel'apevature Ibr c-CaF.-O: and C,,Fa-O= mixtures(P= 1.0 arm),
100 Two additional C,,F 6 compounds, perfluorobutyne-2 and perfluorobutadiene-l,3, were also found to burn in the presence of oxygen. However, since only limited supplies of these two compounds were available, burning velocities of these fluorocarbons were not determined. Four of the saturated fluoroearbons--CF4, CzF6, CaF8, and C4F~o---could not be ignited on the microburner apparatus used. A constantvolume combustion bomb, utilizing a spark plug as the ignir.ion source, was also employed in an unsuccessful attempt to ignite a number of C3Fs-Oz mixtures with initial pressures in the range 0.1-1.0 atm. Possible ignition was determined by monitoring the pressure in the bomb as a function of time, with the aid of a sensitive pressure transducer and subsequent gas chromatographic analysis of the gases remaining in the bomb after the spark had been fired.
Discussion The experimentally determined burning velo. cities of C3F6-O 2, c-C,,F~-O:, and c-C;,Fs-O: are shown in Figs. 5 and 6. Burning velocities of neither C3F6-O 2 nor c-C,,F6-O z mixtures have been reported previously: however, the c-C.,F8-O z results can h,e compared with the values given by Fuller 1"4]. Hiz data were obtained from a flat flame deflagration tube, while the present data were determined by applying the bunsen cone method to small conical flames. Although the two sets of data are in general agreement (see Fig. 6), the results reported in this paper are approximately 10-20 per cent higher than those of Fuller. Discrepancies of this order of magnitude in burning velocity determinations, based on different experimental techniques, are r~ot unusual. Mix~.ures of CF4, C2F~,, C3F s, or C~.Ft o a~d oxygen could not be ignited at STP. Croomes [12]. using a hot-wire ignition source, could not ignite mixtures of either CzF6-O 2 or CzF6-H z in a calorimeter bomb. Our C3Fs-O 2 mixtures could not even be spark-ignited in a bomb. The
R. A. Matula, D. I. Orloff, and J. T. Agnew
low reactivity of these saturated fluorocarbonoxygen mixtures is very different from that of the analogous hydrocarbon-oxygen mixtures. The results of this investigation, along with the results previously reported by Fuller [4] and Matula and Agnew [6], clearly indicate that the maximum burning velocity of fluorocarbonoxygen flames cannot be correlated with the maximum adiabatic flame temperature. However, inspection of the data shown in Figs. 1-6 indicates that peak burning velocities for fluorocarbon-oxygen mixtures can be correlated with the fuel mole fraction corresponding to the maximum equilibrium fluorine atom concentration. A similar correlation between spatial velocities :rod equilibrium fluorine atom concentration in Fz-CI z flames has beer reported by Fletcher and Arabs [13]. The ~'esults of the present study indicate that the mechanism for flame propagation [,,t fluorocarbon-oxygen systems is dominated by the diffusion of reactive fluorine atoms from the hot products in~o the unburned reactants rather than by thermal considerations.
This research was sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant AF-AFOSR-68-1606.
References I. FLt~i~('HE~t.E. A.. and A~,II~s.L, L., Cambu,~i~m& Flame. 8, 275 (1965), 2, FLETCHER,E. A., and KI'FI'L~ON. D. B,, C'omhtlslioll & Flame, 12, 119 (1968). 3. FLE'ICHER,E. A,, and KITI'ELSON,D. B.. Combustion & Flame, 12. 164 (1968). 4, FULLI~R, L. E . "Simple Fundamontal Flame Speed Measurements--A Flat Flame D¢flagration Tube," M. S. Thesis, University of Minnesota (July 1968). 5. COI.WELt.,J. E,, WaCHI. F. M.. and GaE~,t~E.S, A., "'An investigation of the tetralfluoroethylene.oxygen flame." Aerospace Corp. Rept, TDR-469-5250-40-16 (1965), 6, MATUL^. R. A., and AGNEW,J. T . Combustion & Flame. 13. 101 (1969). 7, AGNEW. J. T.. and GRAIFF. L, B.. Combustion & Flame, 5. 209 (1961).
Burning Vdoclties of Fluorocarbon-Oxygen Mixtures 8. S'rEI~NSEN,R., "A FORTRAN IV Program for Thermoehemie',d Calculations Involving the Elements AI. B, Be, C, F, H~Li, Mg, N. and O and Their Compounds." Ph, D. Thesis, Purdue University (January 1966), 9. STtrLL.D, R., Project Director, JANAF Thermochemical Tables, U. S. Govt. Rept. PB-168-370and Addenda (1965), 10. DutJs, H. C.. Ind. Eng. Chem,. 47, 1445(1955). l 1. STULL.D. R,* WESTRUM.JR.. E, F,. and SINKE.G. C,,
101
The Chemical Thermadyanamic,~"ql"Orqanic Comptlund~, Johr~ Wiley: New York (in pcess). 12. CROOM~S.E, F.. Comhu,stion& Fl¢~me.10. 71 (1966). 13, FLETCHER,E. A,, and AMBS,L, L., C(l#lhti.~l[ott& F/gillie, 12, 115 (196,q),
(Received M a y 27, 1969; revised July 15, 1969)