Shock tube investigation of ignition inethane-oxygen-argon mixtures

COMBUST/ON AND FLAME 18, 115-123 (1972)

115

Shock Tube Investigation of Ignition in Ethane-Oxygen-Argon Mixtures* BURCAT,'t"ROBERT W. CROSSLEY,:~ and KARL SCHELLER Aerost~ce Research Laboratories, fOUght.PattersonAir Force Base, Ohio

and

GORDON B. SKINNER Chemistry Department, Wright State Univerrity, Dayton, Ohio

A detailed study was made of ignition delay t ~ e s in ethane-oxygen-argon mixtuies behind a w.fiected shock wave as indicated by pressure and light emission obsexvations. Measured induction times from 20 to 600/~sec over a temperature interval of 1235-1660°K and associated pressures of frcrm 2--~ atm. To elucidate composition effects, the equivalence ratio of the test mixtures was varied from 0.5-2.0 in a p p . . ~ a t e l y 95~ dilution with argon. The influences of composition and tempexature were experimentally determined separately by the systematic test technklue employed. The composition power dependencies were found to be best conelated by the expression r = /~[Ar]°[C2H6]°'46[02]

and

"L26

sec

/3= 2.,.'.'~× 10-J'*[exp (34.2_'0.5) × RTJ 10'l (m°le/cc)°'7

:

in which ~ is the ignition delay time, and the concentrations are in moles per cubic centimeter. The influence of NO z additive on the ignition delay times of ethane was investigated. The decrease in ignition delay times is discussed. Shocked samples of ethane-oxygen mixtures were quenched before i ~ t l o n . Analyticzl investigation o( prodnct distn'bution revealed substantial prelgnition pyrolysis and that CO was not consume0 in combustion. The consistency of the data with previous fuel ignition experiments is discussed.

I. Intmdection In a recent shock tube investigation [1] of the

comparative ignition delay times of the alkanes methane through pentane, ethane.oxygen mixtures were found to have the shortest induction periods *Presented in part at the Western State Section of Combn.~on Institute Meeth~_= Denver, 1971, m paper 71-16. "['Ohio State University Research Foundation Visiting Research Associate under Contract F33615 67C 1758. Presem Add~ss: Department of Aeroneut~ca~ Engineering, Israel Institute of Technology, Haifa, Israel :[:On leave of absence from Air Force Institute of Technology, W r i ~ h t - P a ~ n Air Force Base, Ohio.

of any of the fuel~ examined. Their behavior ap~ed anomalous, in view of the monoton/c decrease in ignition delay time with increasing number_ of carbon atoms extu~oited by the other hydrocmbons, and merited further examination. A review of the literature revealed very little infommtion on the oxidation of ethane in shock tubes. Only two recent studies [2, 3] came to the author's attention. Both related the oxidation of ethane closely to that of m e t h ~ e aod were designed to elucidate the combustion medmni.qus of the latter fuel. The present study was undertaken to ol:tain more information on the ignition delay times of the ethane-oxygen system over a wider range .)f Copyright Q 1972 by The C o m b u s t ~ Institute ~.~bl~h~l by Ameriam Elsevier Put.shins Company, Inc.

116

~f EKANDERBURCAT,ROBERTW. CROSSLEYand KARLSClfl~_tFR TABLE 1 Composit~ of gixtares Used

m m m m mtlm mmmmm mmmmm) mmmm mmw. mmm|mmmmm i _m ___ammmmm D

~C= H.

~o0=

%AR

2.1

1 2 3

0.6 2.0 !.06

7

97.3 91

1.85

97.09

4

1.0

7.0

92

5

3.44

6.02

90.54

X...--..4~

Fig. 1. Oscfllogntmof shock no. 66. Sweep velocity, 50 ;~ec/cra; Ts = 1370. The upper bcam records total emission (photomulti~.,e~ 1P28). The lower beam records pressure ~ e r transducer603). composition and temperature in an attempt to delineate its pattern of oxidation and rationalize its singular ignition characteristics. n. A. Aplmams aad T e c t ~ e e A stainless steel sin~e-pulse shock tube of 1.5-in. Ld. ~ used. "[he shock tube and its use have been previously reported [4]. The shock speed was measured over -- 15-in. interval with two Kistler piezoelectric transducers (603A) connected to a digital counter (Hewlett.-P~ckard 5325B). The shock speeds were detern-~ed to within -+1.0 ~ e c , a time corresponding to -+I5°K in our range of temperatures. The actual range of times counted was 450-560 psec. The ignition delay times were followed with a pressure transducer (Kistler 603A) and a photomultiplier (IP28) behind a cylindrical quartz window mounted flush with the wall. A metal plug was inserted in order to locate the window and the pressure transducer 2 mm from the end wall. The photomultiplier measured total emission from the r e a ~ n g mixture. Both traces were fed into a Tektronix type 551 dual beam oscilloscope and the oscillograms photographed on 3000 ASA Polaroid film. In Fig. 1 we present a typical oscilloganL The upper beam. records the total e w i ~ o n and the lower beam the pressure. It will be noted that the light emission traces indicate slightly shorter ignition delay times than the pressure. This condition prevailed in all of our tests. However, in order to be consistent with the

reported data of pre~ous inve~igations [4, 5], we elected to present only the pressure measurements. The light e m i ~ o n records, though providing slightly lower ignition delay ~,.-:~ exlu-bited the same variation with composition (A logr values) as those derived from pressure histories. The igni~on delay times were determined with an accuracy of 5-10%. Before each shock, the shock tube was filled with argon and then evacuated to a few microns, the residual gas being argon. B. Materials Five test mixtures were prepared manometrically and stored at 150 psi in stainless steel cylinders. For each of them, the lines and containers were flushed several times with argon and pumped down to a few microns prior to mixing. The ethane was Phmips 66 "research grade," nominally 99.9% pure. The oxygen and argon were Matheson, gold label and prepufified grade, listed as 99.95% and 99.998% pure, respectively. The driver gas was Airco pure grade helium. Compositions of all the mixtures used are listed in Table 1. Each test grade was allowed to mix for se-,'eral days to attain uniformity in composition before use for experimenL

Lqthn~ ~ ~ e r e the d;d;d;d~n~utionproducts of ignition were investigated, reacted gas samples were withdrawn from the downstream section of the shock tube into ~ preevacuated glass bulb and analyzed on a Barber Coh~an 5000 gas chromatograph. A 6-ft Porapak N column at 50CC was used, connected to an ionization detector for the analysis of CH4, C2H2, C2I'I4, and C=H6. CO was analyzed on a 6-ft activated charcoal column at room temperature with a thermal conductivity

IGNrrlON IN ~

X

YGEN-ARGON MIXTURES

I17

TABLE 2 Experimental Conditions of Six Groups of Shocks Composition, % Group

C 2H a

O,

Eq~walence Ratio

P ! '~', tort

Ts,°K

IA IB IC 2A 2B 2C

0.6 2.0 2.0 1.06 1.0 3.44

2.1 7.0 7.0 !.85 7.0 6.02

I i 1 2 0.5 2

200 $0 200 200 200 200

1268-1610 1350--1652 1235--1510 1372-1642 1258-!$20 1298-1548

detector. Concentrations were determined from the peak areas relative to a cab'bration mixture. Reflected shock temperatures were computed from the conservation equations and the ideal equation of state, assuming thermal equih'bfium and no reaction. The enthalpies for ethane ~---e~ taken from NASA report CR-72570 [6] and for oxygen from JANAF tables of thermochemic~d

properties [7]. i11. Results

A. De~m~ttion of Coace~ation Dcpend~nce For the p~pose of determining the dependency of the ignition delay time on the concentration of the components of the comb~tible mixture, ":t is convenient to write the general relationship

r

=

A exPl /~" ~[Ar]a[C2H6]b[(~2] ~"

\RT/

(1)

in which the exponents a, b, and c express the p~v,er dependence of the ignition delay time on argon, ethane, and oxygen, respectively. A is a consie.m, characteristic of the system; E is a parameter equivalent to an overall activation enerFo' for the ignition process which defines its temperature dependence. The concen~tion factors in Eq. (I) are those be~ind the reflected shock and are calculated from relations of the type

[C2H6] 5

_

XC2He Pl (~lt) RT l x

(II)

~.z~etex Evaluated a

a, a + b + ¢ a +b +¢ c c, b b

where Xc2H6 is the mole fraction of ethane in the test nfixture and the subscripts i and 5 refer to initial and final conditions, respectiwdy. To evaluate the composition depmden'cie~ (a, b , and c ) and the temperature coefficient E, two sets of experiments cow, sting of three groups each were conducted over suitable ranges of initial prepare, equivalence ratio, argon dilution, and final te~epemture. These are s u m m a ~ e d in Table 2. DetaiL~ of some representative shocks are compiled m Table 3. Comparison of the observed induction times among the three groups in Set 1 (see Table 2), all of which consisted of stoichiometdc mixtures of ethane and oxygen, enabled both the argon dependence a and the overall pressure dependence a + b + c to be determined. To illustrate the procedure, consider the comparison of measured ignition delays between Groups IA and 1B to evaluate ~ e argcn power dependence. For both series of experiments, the concentr~tion~ of oxygen and propane behind the reflected shock axe identical, but their argon concentratio~t~ differ by a factor of 3pproximately four. Thus, if the induction times for Group 1B are given by ~ :

r(iB) =

A exp(~,~)[Ar]a[C2H~]b[02]© (HI)

they can be expressed fcr Group iA as

rct~ = A exP(R-~)14 x

(iv)

ALEXANDER BURCAT, ROBERT W. CROSSLEY and KARL SCI1EIJ.ER

118

TABLE 3 Expaimcntal Condieons and lgnitieu lkiay Tunes/or R e p a f f i ~ i v e ~ t s Compositio-~ % Se1"leS

Shock No.

Call ,

O:

Ts, °1[

pt, tort-

p$" It[l~l

P'[Pl

IA

1 5 8 11 12 15 16 17 18 19 25 27 31 33 35 40 42 44 46 50 53 56 59 61 62 63 66 68 70 72

0.6

2.1

2.0

7.0

2.0

7.0

1.06

1.85

1.0

7.0

3.44

6.02

1502 1398 1438 1610 1310 1606 1445 1555 1410 1350 1432 1388 1484 1220 1322 1560 1442 1498 1692 1370 1330 134~ 1258 1434 1520 1298 1370 1420 1460 1548

195 210 205 180 215 49 64 44 49 52 190 194 182 209 209 182 195 198 179 188 204 194 202 190 170 194 199 185 179 177

7.67 7.37 7.52 7.94 6.78 2.52 2.76 2.15 2.03 2.00 8.07 7.83 8.21 6.74 7.76 7.85 7.41 8.01 8.31 6.$7 7.15 6.94 6.44 7.56 7.43 7.58 8.$5 8.47 8.61 9.41

5.97 5.72 S.t~2 6.25 5.49 7.31 6.81 7.16 6.70 6.50 6.76 6.63 6.93 6.03 6.40 6.30 6.01 6.16 6.45 5.82 6.01 6.06 5.78 6.33 6.56 6.86 7.15 7.35 7.51 7.83

1B

IC

2A

21]

2C

from which it follows that a=

[logrl~, - 1ogrlB]

-

log4

IogriA - le

Or)

log4

Similady, for the pressure dependency: a+b+c

-

log t i c - IB

(Vl)

log4

The experimental results foz the three groups in ~ t 1 are plotted in appropriate coordinates in Vtg. 2, logr versus 1/T. It is apparent that within the experimental scatter, the data for Groups I A and

1B can be r e p r e ~ t e d by the same straight line, indicaVmg no effect of atom, a = 0 • 0.05. The

1", ~t~ec

130 290 235 80 625 54 190 95 270 440 85 120 60 550 230 100 320 160 SO 400 140 95 275 40 20 370 197 115 76 38

difference in ordinal values between Groups I C and 1B (-0.42) represents an overall or pressure dependency a + b + c = - 0 . 7 0 4- 0.05. It should be noted that in order to obtain the same final concentrations o f oxygen and ethane in mixtures o f Groups I A and 1B while maintaining a fourfold difference in argon concentration, the following constraint must be satisfied:

(~.~2H~ x ps/pt)ls (?~2H6 x

ps/p1) 1A

~(L4J

"4 e%e

pI(1B)

Since the specific heats of the mb:ture IB are higher than those o f 1A, their density ratios are approximately 15% greater. For thi~; reason, the reactants for Gro~tps I A and I B were mixed in the ratio o f percentages o f 3 3 3 to ob~m postshock

IGNITION IN ErHANF.~XYGEN-ARGON MIXTURES

119 1600

°/

500

emo

t

~

I

'

t~oo [ OQ

I /

2

/ 2~

~mo "~

I

a~

g

•

gO

IC

I

i

I

t

/

a6

,

. I

I

a7

o8

K~/ T5 "K

oflogrvermsllTfor~xics 1. Thetmrameters evaluated are the argon power dependence and the overall ~ ) dependence; logr = 0 for argon and 0.42 for overall dependence. Fig. 2. A plot

concentration ratios of 4. The same considerations were observed in preparing the mixtures for Set 2. Intercomparison of the induction times for the groups in Set 2 provided values for the oxygen and ethane dependencies. The data are "shown in Fig. 3. The oxygen influence is represented by the ordinal difference o f 0 ~ between Groups 2A and 2B, equ~v-~ent to a value o f c = -1.33 + 0.15. Oomparing the values for 2B and 2C, it is seen that an increase in ethane concentration increases the ignition delay logr(2B-2C) = 0.34 and b =0_~7 +0.1. To reduce all experimental data to a s~ngte correlating line, a normalizing parameter /3, defined as

F~z. 3. A plot of log 1" versus lIT for series 2. The parameters evabaated are the ethane and ~xygen power dependence; log T = 0.8 for oxygen and 0.34 for ethane.

/ ca

c~ _~

[

,~..aA /~

I o I

was introduced and subjected to a least squares analysis to produce the best fit with observed values [8]. The exponents determined in this manner were b = 0A6, c = - 1.26, which differed only slightly from the dependencies obtained directly from experiment. All of the experimental results are plotted in Fig. 4 in the form log versm 1 / T utili~ng the least squares fit. The

~-Ic " o

m" ~--~8• ~ - @ ' . ~ .

,

.

I

* ©3',n'],,~"*,~

°7

I

F~, 4. A plot of log ~ versus l / T for se~es ! and 2. The

solid line represents the best least squares fit to the experimental data. scatter is agreeablY small, indicating a very satisfactory correlation of the data. The slope of the line (i.e., the temperature dependence of the ignition delay time) co;~esponds t~ an ac~vation energy of 34.2 ± 0.5 kcal/mole.

120

ALEXANDF~ BURCAT,ROBERTW. CROSSLEYand KARL SCEI~_LER ___

16130

WOO

TABLE4 Calculated Rein.letof NO,

GOO'K

T,°K 1200 1350 1500

NO, ~

IO=1 mo~e/cc

0.04 0.21 0.63

0.6 x I 0 "z* 3.0 x lff"re 9.0x 10-is

a 021, =/h[Ar]t [NO

~!

¢~5

where [NO2]t is the amount o f NO2 decomposed. For temperatures under 2000°K, k2 is given by the authors as i

t

0.7

!

O.8

O.9

s Hg. 5. A #ot of logr ~ liT for a mixtme of 2% C: H,, 7~ O:, and ~ NO, in argon. The reference line reprrs~__ _u the ignition delay times of the same mixture not ¢x~*'~ininE~NO: (Group IC). For the conditions covered by these experiments, the ind~tion 2ime for ignition may be

exp, es~d as r

= 2.35x

10 -14 exp[ R ~

]

[Ar]°[C2H6]°-a6 [02] -L26

see

(vii) where concentrations are in moges per cubic centimeter. iL l p ~ o a ofF_.llumelnPreseace of NO= Additive To the mixtme of 2% C=He and 7% 02 in argon we have added 0.2% NO2 (10% of the fuel content). A series of shocks was run with this mixture. In Fig. 5, we present a comparison of the ignition delay times obtained with this mixture and the ignitior, delay times of the original mixture 1C (not containing NO2). The sho:-tening is o f a factor of 1 . 7 . A reasonable assumption may be that the shortening of the ignition delay times may be caused by oxygen atoms generated by the decomposition of NO2. The decomposition of NO2 has been studied by Huffman and Davidson [9] and tftraoka and Hardwick [I0]. They found

k2=

10 ~6-°72 exp ~ T )

ec mole-~

$4~ -1

T~king t as the ignition delay time at the given temperature, we calculated the amount of NO2 decomposed and the res~!ting concentration of oxygen atoms. The results appear in Table 4. We can therefore assume that an average of 3-5 x 10 - t ° mole/cc of oxygen atoms may cause a shortening of the ignition delay by 40%. C. Pmdact D~nlmtim Seeking further insight into the ignition process, we ran a series of shocks on stoichiometric n ~ t u r e s (2% C2H6~_7% 02--91% At)in which the reaction was quenched shortly before ignition, and the product gases were extracted from the endwall region of the driven section for analysis. These tests were conducted at sufficiently low temperatures for file ignition delay time to exceed experimental dwell time ("-700 vsec). Product distributions obtained in this manner are presented in Fig. 6 as a function of temperature. Compositions after ignition are shown at the higher temperatures. One of the most striking facts to be gleaned from these results is that very substantial pyroIysis of the fuel precedes its ignition. Roughly 50% of the ethane decomposes before ignition, with the generation of large quantities of ethylene and minor amounts of methane and acetylene. CO makes its first appearance at a rather late stage in the "--tuition process and increases steadily with temperature to a plat~u well beyond the ignition

IGNITION IN ETRKNE-OX'YGEN-ARGONMIXTURES poinL R is not consumed during combustion. The concentration; o f other components decrease abruptly after "z~nition in the expected manner. Approxhteately 95% of the ethane initially present disappears in the shock-initiated combustion; 30% is partially oxidized to CO. A few percent appear in the form of unburned p y r o l y ~ products and the remainder is presumably converted to CO2, for which no analysis could be conveniently made. I V . ~ The two investigations of ignition in ethaneoxygen mixtures previously mentioned [2, 3] are not very detailed and, in both cases, they are related to methane ignition delay data. Cooke and W~Jliarn~ [2] report a set o f 27 shocks run in three series. No general correlation is deduced, and the main aim is to devise a kinetic model for the reactions oc~,rring. The same p~ogram used for ethane is used for methane except for changing the first reaction of d~o,,-Lvosition. Comparing with c~r data, we Y.ad that the activation energy is h~e ou~. 33 + 3 k cal/mole. However, Cooke and ~atiams do not state their experimental conditions with sufficient precision, nor do they relate their ashamed reaction scheme to their experimental dat.~_. Bowman also presents a methane-related in~stigation [3] based on a fair m o u n t ofexl~rimental data. He shows that his data correlat,~ with almost the same parameters as the methane data. However, he imroduces into the correlation a factor G, which masks the normal inconsistency of the data and makes it posm'ble to relate them to two quite different power dependencies:

,[021Ls [Ca i.~1o.2a t

[021 [C2~]

G t -- 1 + - -

for rich mixtures

,{O210.3[C2H61L7 0 [C2H6] G2 -- 1 + - -

for lean mixtures

[o21 Bowman also uses the methane oxidation kinetic model for ethane by changing the fLrst reaction for decomposition of fuel only. Unlike W'flliams [2],

121 IOO

IIOO

Fig. 6. Product distribution of ethane-oxygen-argon mixture as a function of the reflected shock temperature. Reactants were present in stoichiometric quantities (2% C:H+ + 7%02). however, Bowman correlates his calculations to his experimental data. His activation energy is much higher than ours, 40-54 kcal/mole. His overall dependence (-2.) is also very different from ores (--0.7). An approximate comparison of our data with one set taken from each of the foregoing Fapers (made by recalculation of data read from their graphs) indicates that for comparable conditions, their ignition delay times are about a factor of 3 lower than ours. We must stress, however, that Williams [2], measured the igiition delay times behind the incident shock waves, while Bowman [3] measured reaction times; therefore, the comparison with our ignition dehty times has no firm basis. Our investigation was undertaken in an effort to explain the anomalously short ignition delay times o+S~-e,rved in a previous study [.1]. This anomaly is demonstrated in Fig. 7, in which values of log 7, calculated from the experimentally derived correlations, are plotted againt I/T for equal standard concentrations of 10 -s mole/cc of f u e l and oxygen diluted with approximately 70% argon: The temperature dependence of the ignition delay rime is very nearly the same for methane and propane, 46.5 kcal/mole [5] for the former and 42.$ kcal/mole [4] for the latter, whereas that for ethane is appreciably lower, 34.2 kcal/mole. This

122

ALEXANDER BURCAT, ROBERT W. CROSSLEY and KARL SCHELLER

differences, it appears that the initiation step is not the only rate-det~,,,,:,ing reaction in the ignition process. The composition dependencies, not evident from this particular g r J ~ . are for ¢aeh of the three fuels: %

r c , , - [Ar]°[CH4] °'33 [02]- L03 [5] rC2 H6 "[AI]°[C2H6]°'46[02] - L26

/

rcs ns - [Ar]°[C3Hs]°'57[O..] -L22 [4]

/" / i

!

I

1

I

FW,. 7. ~ p ~ t ¢.,flog r venm$ If/" fmmethane. e0,~ae. ~md pmpaae. The three l i t ~ w ~ e cakulated by ~ e re,owing empirical t ~ a t i m ~ t l n at ~ar.dani concew tralions of lff"s moklcc for fuel and oxygen.

r

= 3.62 ×

= = o35

~" =

~-4

10-t4 e z p [ ( 4 6 . 5

x '10 - z 4

x 10b/RT]

e3p[(34.2

x 10-Z4e~p[(42.2

x

x

IArPIC~P -33[0~]- Lo3sec. MP)IRT] [Ado[C~]o.~[Oz]- L ~ see. [Ob/RT] [...~de[C~H,]e.~Od- L,~ see.

state o f afl'a~s is somewhat $t~.,-pr~-i~. Since the phases o f the ignitiea process have been shown from the a n ~ data to involve decomposition of the fuel, we would have anticipated that there would have been a closer correspondence between ethane and propane than is actually found. In both these fuels, the decomposition undoubtedly proceeds through rupture on a C - C bond:

C 2 ~ -. 2CtI3 C~He ~ CH5 + C2H5 whereas methane requires a ruptare of a C - H bond: CH4 -~Ctt 3 + H The activation energy for the former process is much lower than for the latter. Since the observed temperature dependencies do not reflect these

All of the ignition delay times are seen to be independent of ~ argon concentra-:ice. In each case, the fuel acts as an inln'oitor for its own over the concenUatimz nmge cowred in these experiments. Of comse, since the fuel serves a dual function in both initiating chain reactions and tempesting chain branching reactions, it would be anticipated that its inhibitory effect would diminish in leaner mixtures. S~ce the combustion of ethane could plau~'bly be assumed to f o ~ w the ~tme kinetic pattern as that of methane ~ -;t ~ , ~ t e d into methyl radicals, its inh~iting effect might be expected to be intennediam between that of met_Lnme and propane. This is confirmed by the experimental results. The reason for the variation in oxygen dependencies among the three fuels is cbscme. The difference between ethane and propane in oxygen dependency can reasonably be attn'buted to experimental scat*.~. Meth~ne's appreciably lower oxygen dependency appears to be zeal. The explanatice of the influence of oxygen on ignition undoubted~y ties in the details of ~ e oxidation scheme and cannot be fruitfully discussed on the basis of the present experimental evidence. That a difference should exist between methane and propane is not ~.ar'din~ However, it had been anticipated ~ priori that ethane's o~ygen dependency would be closer to that of methane rather than propane in view of the assumed similarity in their combustion. Some inferences about the role of the oxygen atoms in the oxidatkm may be drawn from the NO2 additive experiments. If we are correct and the shorterdn~, of the ignition delay times comes

IGNITION IN ETHANE-OXYGEN-ARGONMIXTURES

123

from the addition of oxygen atoms through the decomposition of NO2 (which is questionable in view of file small amounts of oxygen aton~ generated), then we have a direct measurement of the efficiency of oxygen atoms in shortening ignition delay times. The product distribution results indicate that considerable ethane decomposition occurs prior to ignition. Propane-oxygen mixtures exhibited the same behavior. However, they differed in one ~ i f ~ c a n t aspect. For ethane, the CO produced prior to ignition was not consumed during combusaon. In the case of propane, the CO concentratiot- dropped abruptly after i~ition. How this circun~-tance is related to the greater ease of'iL,nition o f ethane is not clarified by the present experiments. We can tentatively suggest that the more rapid ignition of ethane does not allow sufficient time for the more coml~ete oxidation of CO to CO=. Current calcula~ion~ in progress on methane oxidation support this suggestion. Ethane ~iecomposition s;udies [11] revealed an appreciable argon dependence, indicating that this reacti . did not attain its first-order iimit in our ignition experiment;,. The lack of an argon dependence for the ignition delay time offered further evidence for the belief that the probable initiation step was not rate controlling. This fact introduces a problem into the interpretation of the oxidation m e c h a ~ m for ethane. Previous workers [2, 3] had ~ that, save for the initial decomposition step (C=I'I6 ~ 2CH3), e t h ~ c ' s ~cactions were similar to those of methane. In this context, we

might suppose that the decomposition reaction was dominant in the ignition kinetics of ethane and responsible for its shorter ignition delay times. Our f'~dings do not lend support to such a view and throw some doubt ori the adequacy of the s~-nplified proposed scheme. REFERENCES 1. Bmcat, A , Lifslfitz, A., and K. Schellex, Combustion & F/ame 16, 29 (1971). 2. Cooke, D. Fq sad Williams, /~, Thirteenth Symposiwn (International) on Combustion, The Combustion Institute, Pittsburgh (1971), p. 757. 3. Bowman, C. Tq Combustion Sci TecK'ml 2, 161 (1970).

5. 6. 7. 8.

G. 13, Thirteenth Symposium (International)on Combust/on, The CombustionInstitute, Pittsburgh (1971), p. 745. ~ A., Schelk~, K., B~cat, A., and Skinny, G. 13.,C o ~ n & F/ame 16, 311 (1971). Main,R. P., NASARepLCR-72570 (1969). JANAF Themmchemical Dala, Dow Chemical Co., Midland, Mieh"~,~n necsey, J. G., ne~te, L~-*_~ C~man,.R. J , Y. Chent Ed. 45, 728 (1968); ~ , L G., ARL Rept. 68-0111, WPAFB,Ohio (June, 1968).

9. ~

R. E.g and Davidso~ N.J. Am. Chem. ,.gor.

81, 2311 (1959). 10. mrKka, EL, sad llmdwick, R., J. Chert Phy~ 39, 2161 (1963). I I. Bun=t, A~ Crm~ey, R. W. Schellex,K., snd ~ e r , G. B., "The Shock Tube Decomposition of Ethane" (to be published).

[Recvived May 1971; revised version received June 1971)