WILLIAM G. AGNEW Research Laboratories Division, General MDtors Corp., Detroit, Mich. JOHN T. AGNEW Purdue University, West Lafayette, Ind.
Visible Emission Spectr Diethyl Ether Solutions to many engineering problems-whether or not gas will knock, Diesel engine roughness and cold starting, undesirable odors, and the contribution of Diesel engine exhaust to production of atmospheric smog-are aided by an understanding of fundamental reaction kinetics of low-temperature Combustion. Spectroscopic studies of two-stage flames in a flat-flame burner, presented here, are one means of studying these reactions.
MANY
of the current combustion problems of the automotive engine industry are associated with low- temperature combustion reactions such as those giving rise to cool flames. I t has been fairly well established (4, 6 , 24, 29, 28) that in spark-ignition gasoline engines the lowtemperature reactions which occur in the end gas prior to normal flame combustion play an important role in determining whether or not knock occurs. I t is probable that preignition due to the presence of particles or hot surfaces in the engine combustion chamber also occurs via the low-temperature mechanism. In the Diesel engine, the normal ignition process is one of autoignition, and, with the wide variations in mixture ratio and temperature throughout the combustion chamber, there is little doubt that low-temperature combustion reactions play an important, if not dominant, part in the over-all ignition process. The practical problems of Diesel engine roughness and cold starting are directly dependent on the characteristics of this ignition process. Even under the bzst of operating conditions, a small portion of the fuel supplied to either spark-ignition or Diesel engines appears unburned or only partially burned in the engine exhaust (27, 29). These unburned products of combustion are thought to produce the undesirable odors of Diesel engine exhaust, and may also contribute to the production of atmospheric smog. I t appears that the odorous products are produced 2224
in the engines in regions of low temperature or poor mixture composition where combustion is incomplete, and that the reactions are again of the cool flame type. The importance of these engineering problems ensures the usefulness of any information which might be gathered concerning low-temperature combustion reactions. Spectroscopic techniques, one means of studying these reactions, were applied in the work described here. The visible emission spectrum of cool flames was first reported by EmelCus ( 9 ) in 1926. The spectrum, consisting of a series of diffuse bands between 3000 and 5000 A , . was the same for a number of different fuels. In 1935 and 1936, the emitter of EmelCus’ band spectrum was identified by Fowler and Pearse [see Ubbelohde (33)]and Kondratjew (78) as excited formaldehyde. Topps and Townend ( 3 2 ) in 1946 stabilized two-stage flames of ether and acetaldehyde in a flow system. The first stage appeared identical with the cool flames observed previously. The secondstage flame exhibited the same excited formaldehyde spectrum as the first stage but was more intense, particularly in the wave length region near 4700 A. If the term cool flame is considered to define a flame of relatively low temperature (compared to normal flame temperatures of 3000 or 4000’ F.) and exhibiting a visible spectrum of excited formaldehyde. then both stages in the two-stage flame could be considered as cool flames. Topps and Townend, however, preferred to label the second stage as a “blue
INDUSTRIAL AND ENGINEERING CHEMISTRY
flame.” In any case, the second-stage flame could hardly be associated with a normal flame, Visible cool flame spectra were reported in 1948 by Mullins ( 2 3 ) , when liquid kerosine containing 57, ethyl nitrate was injected into a stream of hot combustion products. Three stages of combustion were distinguished. At the lowest temperature the spectrum appeared to consist only of emission from excited formaldehyde; a t intermediate temperature CH, OH, and strong HCO bands appeared; and a t the highest temperatures the normal flame spectrum, CZ, CH, and O H , appeared. In engine studies the first spectral evidence of preflame reactions was the detection by Withrow and Rassweiler ( 3 6 ) of formaldehyde absorption in the unburned mixture in a fired engine. More recently Levedahl and Broida ( 7 9 ) and Downs, Street, and Wheeler (6) have published visible emission spectra of radiation from the preflame reactions in motored engines. Levedahl and Broida used n-heptane and Downs, Street. and Wheeler used diethyl ether as a fuel. Both groups reported the spectrum to be that of excited formaldehyde. For spectroscopic studies it is desirable to obtain a rather large depth of radiating gases in order to have sufficient intensity available for recording a spectrum. At the same time it is desirable for all of the gases within view of the spectrometer to be in the same state of reaction. These conditions have not been fulfilled in previous work by virtue of the
--
. c30-MESW
SCREEN
whether or not these flames were similar to those studied previously in tubes, burners, and engines. I n addition, it was felt that the identification of the secondstage spectrum of Topps and Townend was not entirely convincing, since in their arrangement the first and second-stage spectra were actually superimposed. Finally, in view of the new apparatus and the possibility of varying fuel-air ratio over a large range, it appeared possible that new information might be gained concerning emitters in the low-temperature multistage reactions. All of this information could, of course, contribute to a better understanding of the preflame reactions leading to autoignition of fuel-air mixtures and a closer approach to solutions of such engine problems as knock, preignition, particle ignition, Diesel combustion roughness, and incomplete burning.
GLASS TUBE
SCRE ENS
.ICES;
I +
1/8 IN.
BURNER TUBE
Equipment and Procedure
Figure 1.
Fiat-flame burner
particular equipment with which cool flames have been formed. However, Powling (26) and Egerton and Thabet (8) have developed a flat-flame burner which appears nearly ideal for spectroscopic studies. In addition, these authors have demonstrated that a variety of different types of flames, including cool and
two-stage flames, can be stabilized in the flat-flame burner. The cool and two-stage flames formed in flat-flame burners have been studied in detail by infrared radiation techniques a t the General Motors Research Laboratories and at Purdue University (7, 5, 35). I t was of interest to know
i PER CENT Figure 2.
ETHER
BY WEIGHT
Types of flame formed at various fuel-air ratios
Burner. Figure 1 shows a cross-sectional view of the flat-flame burner. The burner mouth had an inside diameter of 2 inches, and the inside diameter of the enclosing glass tube was 23/4 inches. The fuel-air mixture supplied to the burner was prepared as follows: Compressed air from the building supply lines was passed in two streams through pressure regulators and a pair of rotameters. One metered air stream was then bubbled through two containers of diethyl ether in series. This primary stream became essentially saturated with diethyl ether a t room temperature, and was then mixed with the secondary air stream in proportions to give the desired fuel-air ratio. The air flow rotameters were calibrated using a wet test meter, and the ether consumption was determined by weighing. Ether concentration, expressed as weight per cent ether in the mixture, could be determined by this means within about f2 or 301,. Flames were ignited in the burner with the stabilizing screen removed. The screen was then replaced and the desired type of flame obtained by adjusting the flow rate and fuel-air ratio to the proper values. As an aid to the understanding of the spectra presented in this report, it is well to review the flame types produced at different fuel-air ratios with diethyl ether. The situation is described in Figure 2, where the fuel concentrations are intended only as approximations of the values a t which flame characteristics change. Starting at the lean limit of inflammability for normal flames (Figure 2), it is possible to stabilize normal flames over a narrow range of fuel-air ratios (Region I). As the mixture is made richer, the flame speed increases and higher flow rates are required for stability, until eventually a point is reached where the flame can no VOL. 48, NO. 12
DECEMBER 1956
2225
longer be kept OK the burner matrix (Region 11). Operation cannot be maintained for long with the flame seated on the matrix, since the burner is excessively heated under these conditions. However, on the rich side of stoichiometric the flame speed decreases to the point where stable normal flames can again usually be maintained (Region 111). Further enrichment causes a luminous yellow column to appear behind the flame front (Region IV), and a t slightly richer mixtures a faint cool flame appears below the (previously) normal flame front. In this instance the two flame fronts constitute a two-stage flame (Regions V and VI). The yellow column disappears with further enrichment (Region V to Region VI) as the cool flame becomes brighter and the second stage becomes more diffuse and separates further from the cool flame. Finally, a t a considerably richer fuel-air ratio, the second-stage flame disappears through the stabilizing screen, leaving only a bright cool flame in the burner (Region VII). The demarcation between Regions I V and V is not a t all distinct, and it is difficult to determine at just what fuel-air ratio the cool flame appears. The faintness of the cool flame, its close proximity to the second stage, and the contrasting brightness of the yellow column all contribute to the difficulty. In addition, there is no discernible change in the rich normal flame front as it takes on the character of the second-stage flame. (This situation is dealt with spectroscopically below.) The temperature of the cool flame is known to be some 300' F. lower that that of the second-stage flame in Region V I (5). Spectrometer. The spectrometer used for these studies was a commercial double-pass monochromator with a lithium fluoride prism and a photomultiplier detector. The monochromator was arranged with respect to the flat-flame burner as shown in Figure 3. An image of the flame was focused on the monochromator slit by means of a spherical mirror and two flat mirrors. The two flat mirrors served to rotate the flame image 90' and thus make the flat-flame front parallel to the vertical monochromator slit. An enlargement of the flame image occurred in the optical system, so that the 12-mm. long slit actually covered a horizontal "slice" of the flame only about '/4 inch in length through the central portion of the flame. Space resolution in the flame was sufficient to distinguish clearly the different zones of reaction as defined in Figure 2. A screw adjustment on the burner was used to change the elevation for observing any particular position in the flames. The amplified output of the photomultiplier detector was recorded with a commercial strip chart recorder. A motor-driven wave length drive rotated
2226
INDUSTRIAL AND ENGINEERING CHEMISTRY
-
S P H E R I C ~ L MIRROR
SLIT
WAVE LENGTH, DRIVE
IMAGE
SIDE VIEW
Figure 3.
Arrangement of burner and monochromator
(a) RICH NORMAL FLAME 15 % ETHER
0 "
----*_.Lll-ILl'
8
0
7000
5500
4000
4500
l_lLlLIIL
IP28 GAIN 4
3600
3300
3100
2900
WAVE LENGTH. ANGSTROMS
(b) LEAN NORMAL FLAME 7 % ETHER
8
-c.
I
I 8 I I
7000
5500
4500
4000
3600
3300
WAVELENGTH -ANGSTROMS
Figure 4.
Normal flame spectra
3100
2900
1P28 2000 to 6250 A,, peak 3400 A. 1P21 3250 to 6600 A., peak 4000 A.
the monochromator wave length drum a t a constant speed, and it was possible to sweep the wave length region from 3000 to 4000 A. in approximately 7 minutes. The wave length drum was calibrated using 24 emission lines of mercury, cadmium, and sodium in the range from 2900 to 7100 A. The calibration curve was estimated to be accurate to within about 5 A., although the accuracy with which drum revolutions were marked on the recorder chart would seem to limit the final accuracy of wave length determinations on the spectra to about f10 A. at 4000 A. Two different photomultiplier tubes were used for this work, with wave length sensitivities (as indicated by the manufacturer) as follows :
In operation these two detectors appeared to give nearly identical spectra, which is not entirely surprising in view of the fact that most of the spectra studied were contained almost entirely in the region between 3300 and GOO0 A. where the response curves were not radically different. The 1P21 tube, however, was somewhat more sensitive than the 1P28. Results and Discussion Spectra for the various flames are presented in Figures 4 through 7. Accompanying each spectrum is a small diagram of the burner and flame. The
( a ) REGION
XE
- - - - - --
HCHO
I
> ' . cn
z
W
I-
z
Linear Wave Length, A.
LIF Prism, A./Mm.
3000
29
6000
190
4500
4000
3600
3300
3100
Wave Length, A. Intensity HCHO 2 4 10 3 9 10 5 8 9 8
7 7 5 3 1 0
5890 . 5896
2900
WAVE LENGTH, ANGSTROMS
9 5
HCO (b) REGION X t
3 5 % ETHER
IP21 GAIN 2 SLIT 4 0 0 , ~ HCHO
1
I-'
- *-
I
i 7000
5500
4000 3600 3300 WAVE LENGTH, ANGSTROMS
4500
(c) REGION
3100
P 30% ETHER
I
,
I
I
1
5500
4500
4000
3600
3300
WAVE LENGTH, ANGSTROMS
Figure 5.
Cool flame spectra
I
3100
Width, A.
100 400 100 400
2.9 11.6 19 76
to
Slit
2858 2946 3014 3046 3115 3186 3220 3298 3377 3417 3502 3588 3636 3730 3824 4092
6
Wave Length, A.
Intensity CH
3871 3889 4313 4315 4384 4890
5 4 10 3 3 2
cz 4382 4737 5165 5636 6191
2 9 10 8 3
OH 2811 3064 3067 3078 3089 3428
4 10 10 5 10 2
7
8 3 10 10 4 10 10 3 8 8 3 5 4 3 Taken from (2, 11,17,25).
2900
IP21 GAIN 2 SLIT 250w
7000
Slit Width, Microns
Table II. W a v e Lengths and Relative intensities of Principal Emission Bands"
Na 5500
Wave Length Range Corresponding
Dispersion for
I
7000
Monochromator Resolution
3410 3540 3706 , 3767 3856 3960 4053 4129 4243 4360 4448 4567 4707 4821 4942 5107
50% ETHER
IP21 GAIN 2 SLIT 3 5 0 , ~
Table 1.
I
2900
I
location in the flame configuration a t which the slit image was placed is indicated by the arrow. The fuel-air" ratio region refers to the region as defined in Figure 2; again, the fuel concentrations shown in the figures are only approximations. The gain figures refer to settings on the amplifier, in which the smaller numbers indicate increased gain. The slit width settings varied between 100 and 500 microns, and some indication of the spectral resolution is given in Table I. Markers on the spectra indicate the wave lengths of particular emission bands of known emitters which are identified on the spectra and in Table 11. The markers were located by converting the wave length values in Table I1 to VOL. 48, NO. 12
DECEMBER 1956
2227
monochromator drum readings with the monochromator calibration curve and plotting these values directly on the spectra. The coincidence of the markers with particular features in the spectra is the basis for the interpretation of the spectra. Normal Flames. Although normal flame spectra have been recorded by a number of investigators and nothing new was found in the present work, two normal flame spectra, recorded with the same equipment as the cool and two-stage flames, are presented here to provide a familiar basis with which to evaluate the other spectra. Both of the normal flame spectra (Figure 4) were recorded with the flame stabilized on top of the stabilizing screen. This was primarily a matter of convenience, in that this position gave a very stable flame. The lean flame fluttered considerably when stabilized inside the burner, and a t this particular time, for some reason, the rich normal flame could
not be stabilized in the burner (although it had been done before). Nevertheless, these flames are representative of the fuel-air ratio ranges I and 111in Figure 2. The rich normal flame spectrum in Figure 4,a, showed a moderately intense OH band with its sharp head at 3064 A. ; C Zstood out strongly a t 4737, 5165, and 5635 A . ; CH appeared strongly a t 4313 A. and perhaps weakly a t 3871, 3889, and 4890 A.; both C H and Cz probably contribute a t 4383 A.; and there was possibly some indication of HCO bands, although the signal-to-noise ratio was so low for these that there is considerable uncertainty. The unresolved yellow sodium D lines appeared very strongly near 5892 A. This radiation apparently arose from an impurity on the stabilizing screen (the yellow emission could be wen with the naked eye and was localized at one point in the flame). In the lean normal flame (Figure 4,b) the OH emission was relatively stronger, and the C Zwas undetectable. The CH
(0)REGION P[
4313 A. band remained, and the HCO spectrum became rather prominent. The HCO bands are relatively weak in most hydrocarbon-air normal flames. They appear most strongly in the ethylene flame and are also usually fairly strong with diethyl ether ( 7 7 ) . The bands, which are often called the hydrocarbon flame bands or the Vaidya bands, have been attributed to the HCO radical, and this identification has been generally accepted, although it has not been conclusively proved. Background radiation was strong in both the rich and lean flames (the slit was twice as w-ide for the lean flame as for the rich flame). Gaydon ( 7 7 ) has listed a number of possible sources for continuous emission from flames, including the recombination of positive ions with an electron and reassociation of free atoms and radicals. I n the present case the association reaction given by Gaydon CO 0 -+ COZ hv
+-
45% ETHER
+
(b) REGION YL 35% ETHER lP28 GAIN 3
SLIT 400)
HCHO
SLIT 5 0 0 )
__--___
> k m z
!w
z
5500
7000
4500
4000
3600
3300
2900
3100
WAVE LENGTH, ANGSTROMS
g
(c) REGION P 30% ETHER IPZl GAIN 5 SLIT25OJJ
(d) REGION P 30% ETHER IP28 GAIN 4
L 0 I
l=-
>
t
m Y z
E -+---J 1
7000
5500
4500
4000
3600
3300
3100
2900
WAVE LENGTH, ANGSTROMS
I (e)
0 z
"/"I'
(
4
I
N
\-.-.-
7000
5500
4500
4000
3600
3300
3100
2900
7000
5500
WAVE LENGTH, ANGSTROMS
4500
4000
3600
3300
WAVE LENGTH, ANGSTROMS
Figure 6.
2228
I
25% ETHER
IP,
1 I I
HCHO
(f) REGION P o r E
REGION P o r E 30% ETHER IP21 GAIN 5 SLIT 400) 1-7~7,JjL-u - *= HCO
hs,qq& 1
3
INDUSTRIAL AND ENGINEERING CHEMISTRY
Second-stage flame spectra
3100
2900
which supposedly gives continuous emission extending from the green (5500 A.) to beyond 2500 A., appears to offer the best explanation of the background radiation. However, in Figure 4,b, the sharp cutoff of radiation in the region from 5000 to 6 0 0 0 A. may very well have been due to decreased sensitivity of the photomultiplier detector, rather than to an actual decrease in radiation intensity. Cool Flames. These spectra at three different air-fuel ratios (corresponding to Regions VII, VI, and V in Figure 2) are shown in Figure 5. These spectra are all very similar, particularly in the range from 3200 to 5100 A. where the spectrum is clearly, and apparently only, that of excited formaldehyde. The emission between 5100 and 6500 A. is unidentified. The resolution and noise level are such that the detection of individual bands is difficult in this region. However, a number of peaks which appear to stand out from the noise are indicated in Figure 5. This region of the spectrum also appeared to be affected somewhat by fuel-air ratio, the intensity . near 5800 A. decreasing in the leaner flame. Downs, Street, and Wheeler (6) also observed some additional emission in the range between 4700 and 5700 A. in engine cool flame spectra. However, a considerable improvement in resolution and signal-to-noise ratio is required before the emitter can be identified. I t is not known whether the considerable background radiation in the cool flame spectra resulted merely from the overlapping of many bands, or whether a continuum of some kind was present, but the overlapping must certainly exist to some extent. Second-Stage Flames. Spectra for second-stage flames for a number of fuelair ratios are shown in Figure 6 . These flames represent fuel-air ratio ranges VI, V, and perhaps IV, as indicated in Figure 2. Proceeding through spectra a, b, c, etc., in Figure 6, the fuel-air ratio was decreasing and the second-stage flame was approaching a transition to a rich normal flame. Figure 6,a, illustrates the spectrum for the richest second-stage flame obtainable in the flat-flame burner. The same formaldehyde-bands observed in the cool game were evidenced in this second-stage flame spectrum. The over-all intensity of the second-stage flame was somewhat greater (the amplifier gain was 4 instead of 2 which was used at the same slit width for the cool flame in Figure 5), and the intensity distribution was also slightly altered. In particular, the long wave length formaldehyde bands appeared stronger relative to the short wave length bands in the second-stage flame, the greatest change occurring in the region from 4600 to 4700 A. I t was also in this region that Topps and Townend (32) reported an increased relative intensity in their second-stage flame. O n the other
hand, the series of small, closely spaced bands observed in the cool flame between 5100 and 5800 A. was not evident in the second-stage flame, and the absence of this emission may have altered the apparent intensities of the long wave length formaldehyde bands if consideraable overlapping existed. When the second-stage flame was leaned slightly (Figure 6) but still kept in fuel-air ratio Region V I (no yellow column), there was no appreciable change in the spectrum. The spectra in Figure 6,a and b, are essentially the same in spite of the fact that a 1P21 photomultiplier was used in one case and a 1P28 in the other. Figure 6,c, shows a still leaner flame, this time with a faint yellow column behind the second-stage flame (Region V). Observations were still made directly in the second-stage flame front, and in this case there was a change in the spectrum. A small C H band a t 4313 A. appeared in the midst of the formaldehyde spectrum; new peaks appeared a t 3588, 3636, 3298, and 3220 A. ; and changes in the shapes of the formaldehyde bands gave evidence of other new bands a t 3730, 3502, and 3377 A. These new bands are attributed to H C O (Table 11), and they appeared to increase in intensity a t the expense of the formaldehyde spectrum. Figure 6,d (in which the mixture was still leaner than in Figure 6,c, even though the absolute ether concentration was not known more accurately than r t l % ) , shows a stronger C H 4313 A. band, and the HCO spectrum took still more from the formaldehyde spectrum. The small formaldehyde bands a t wave lengths greater than 4360 A. were no longer clearly distinguishable. Additional bands appeared a t the short wave length end of the spectrum. A peak near 3115 A. joined the 3220 A. band. The H C O spectrum definitely predominates in Figure 6,e, and becomes still more prominent in Figure 6,f. Figure 6,e and f , also shows some increase in intensity at the long wave length end of the spectrum. No new bands were detected in this region, but the formaldehyde bands were apparently still present a t 4567 and 4707 A. Thus, the formaldehyde is still present even in the leanest second-stage flame. The persistence of the band a t 3960 A. (a formaldehyde band) also lends confirmation to this statement. Finally, with respect to Figure 6, the H C O wave length markers (determined from Table 11) correspond more closely to the band heads than to the band peaks, the bands themselves being degraded toward the red. This agrees with the spectra of Hornbeck and Herman (77). At the time this work was performed great difficulty was experienced in attempting to stabilize a rich normal flame (Regions I11 and IV) in the burner.
For this reason it was not possible to record spectra during the complete transition of a second-stage flame to a rich normal flame. The spectrum in Figure 6,f, actually represents the leanest second-stage flame obtained, and in this case the cool flame beneath the secondstage flame was so faint and so close to the second-stage flame front that there was considerable doubt as to its existence. Thus, the flame of Figure 6,f, may actually have represented Region I V in Figure 2. These results indicate that, as the second-stage flame is made leaner, a gradual transition occurs from the rich second-stage flame spectrum of excited formaldehyde to the rich normal flame spectrum of CH, C2, OH, and weak H C O (Figure 4,a). During the transition, however, a region is passed through in which the H C O spectrum definitely predominates. In the present case instability prevented determination of the point a t which C2 and OH first appeared, and the experiments were ended in the region where H C O predominates. Under these circumstances the rich limit of inflammability for normal flame is not clearly defined, because the transition from the normal to the second-stage flame is not sharp. HCO Emission in Lean Second-Stage Flames. The transition in the emission spectrum of the second-stage flame from the formaldehyde spectrum to the predominantly HCO spectrum, as the oxygen content of the mixture is increased, may be of considerable significance from a reaction kinetic standpoint. Previous investigators (3, 70, 72, 76, 77, 22,30,34) have recorded H C O emission from normal flames, but usually with lean mixtures. Since these bands are most intense with ethylene as a fuel, they have been referred to as the ethylene flame bands or more generally the hydrocarbon flame bands. Gaydon (70) found the HCO bands in the outer cone of chilled hydrocarbon flames and tried to correlate the band intensity with chemical analyses of the interconal gases. He concluded that “the presence of the hydrocarbon flame bands in the outer cone was always associated with the formation of peroxidic substances in the interconal gases, and that although aldehydes, mostly formaldehyde, were usually present as well, the strength of the aldehyde formation did not follow the strength of the flame bands in the spectrum.” I n spite of this correlation Gaydon and his coworkers (3, 74) later obtained evidence indicating that the H C O radicals did not come directly from the breakup of either the fuel or any fuel-derived peroxide. This conclusion was reached when deuterated acetylene and hydrogen were mixed and burned with oxygen. The expected DCO did not appear. Instead there were H C O bands, indicating that the H VOL. 48, NO. 12
DECEMBER 1956
2229
atom in the radical came from outside the original fuel molecule, perhaps from a general supply of H atoms formed in the reaction. Herman, Hornbeck, and Laidler (76, 77) observed the H C O bands quite strongly with very lean acetylene-oxygen flames stabilized in a special burner. The HCO radical was an early product in their proposed reaction mechanism for acetylene and oxygen. They also suggested that the reason for the H C O bands’ appearing strongest with lean mixtures was that these radicals were deactivated in rich mixtures by collisions with CZ, CH, and O H radicals. However, Gaydon and Wolfhard (75) disagreed with this explanation primarily on the basis of the low population of Cz and C H radicals and the fact that the OH concentration is no greater in rich mixtures than in lean mixtures. Another possible mechanism for the formation of HCO radicals in flames involves the decomposition of formaldehyde or a reaction of formaldehyde with other species in the flame to form HCO. I n support of the formaldehyde decomposition theory is the observation by Dyne and Style (7) that the H C O bands are strongly in evidence in the fluorescence spectrum of formaldehyde excited by ultraviolet radiation. Another series of pertinent experiments was that carried out by Gaydon, Moore, and Simonson (72, 22) using very lean methane-air mixtures in a high-compression motored engine. A4s the methane concentration was gradually increased from zero, there first appeared a blue emission from excited formaldehyde, a cool flame. As the mixture was made richer, the spectrum changed to one of strong H C O and C H and O H . Finally, as the lean limit of inflammability was approached, the H C O became relatively weaker and a yellow luminosity, presumably due to carbon, appeared. These results are remarkable in their similarity to the sequence of spectra recorded in the present work and also that of Mullins ( 2 3 ) . Of course, the experiments of Gaydon, Moore, and Simonson dealt with extremely lean mixtures of methane, and Mullins’ were with kerosine and ethyl nitrate, while thosc described here deal with extremely rich mixtures of ether. The reversal of fuelair ratios from rich to lean when going from tube and burner experiments to engine tests has been noted previously in cool flame work (7, 6). I t is possible that the mechanism responsible for the formation of H C O radicals is entirely different for lean flames of fuels such as ethylene, acetylene, and methane and the relatively low-temperature, second-stage flames discussed here, which are formed in very rich diethyl ether-air mixtures. From the same viewpoint, the formaldehyde evident in the very rich flames may be
2230
(a) BETWEEN SECOND-STAGE FLAME AND YELLOW COLUMN REGION P 30% ETHER
I 0
I
>
t cn z
w
t
z
WHO
i
1
I
!
7000
5500
5500
-+
HCO
+ OH
(1)
is not greater than 8 kcal., and that this reaction is preferred to thp alternate re-
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
2900
3100
2900
Spectra after second-stage flame
formed in a reaction mechanism different from that of formaldehyde in the lean flame. Nevertheless, in view of the facts mentioned above and the general literature covering these low-temperature reactions, it is believed that the data of the present investigators indicate a mechanism of H C O formation from reaction of formaldehyde in the rich second-stage flame of diethyl ether-air mixtures. One simple observation which appeared significant was that the pungent aldehyde odor of the product gases from the two-stage flame became much less pronounced as the flame was leaned through the transition region from 45 to 25% ether. This confirmed the change in the spectrum and perhaps suggests that the decrease in formaldehyde concentration is at a level such that it is significant in the reaction kinetics. It is possible that the following is a reasonable explanation of the increased H C O radiation and decreased formaldehyde radiation as the second-stage flame is made leaner. Steacie (37) has summarized evidence to show that the activation energy for the reaction
+ HCHO
I
3100
THE YELLOW COLUMN REGION P 30% ETHER 1P28 GAIN 4 SLIT 3 5 0 ~ 1
4500 4000 3600 3300 WAVE LENGTH, ANGSTROMS
Figure 7.
0
,
I
4500 4000 3600 3300 WAVE LENGTH, ANGSTROMS
(b)
7000
l
action 0
+ HCHO
-+
CO
+ H20
(2)
On the assumption that the atomic oxygen concentration is proportional to the molecular oxygen concentration and the temperature, both of which increase as the very rich second-stage flame is made leaner, it would appear that Reaction 1 offers a plausible explanation of the increase in HCO radicals a t the expense of formaldehyde. As the temperature increases still further with the addition of more oxygen, the HCO concentration would be expected to decrease again by the dissociation reaction HCO
+
CO
+H
(3)
with an activation energy of the order of 16 to 26 kcal. (20). This would correspond to the case of a rich normal flame (approached by the lean second-stage flame) for which the H C O emission is shown to be low in Figure 4. Finally, as the normal flame is made quite lean (Figure 4), an entirely different mechanism for the production of HCO radicals may exist. Yellow Column. In the fuel-air ratio Regions IV and V (Figure 2), a column of yellow luminosity appears behind the
normal or second-stage flame front. There is a dark space several millimeters thick between the blue flame front and the yellow column, and the column extends upward to the stabilizing screen. The luminosity appears the same as that obtained in a diffusion (or candle) flame, and it has been attributed to incandescent carbon particles (8). Under some conditions large quantities of carbon are, in fact, deposited on the stabilizing screen when the yellow column is in evidence. However, in none of the present work was there any deposition of carbon. Figure 7 4 , shows a spectrum recorded i n ‘ fuel-air ratio Region V (two-stage flame with yellow column), with the monochromator slit imaged in the dark space between the second-stage flame and the yellow column. Divergence of the light path on either side of the focal point may have permitted some radiation to reach the detector from both the second-stage flame front and the yellow column. The spectrum, in fact, shows remnants of the HCO-CH-HCHO spectrum of the second-stage flame and at the same time shows an apparent continuum of emission in the wave length range from 4500 to 7000 A. The spectrum in Figure 7,b, was recorded with the monochromator slit imaged in the center of the yellow column inch behind the second-stage flame. All vestiges of CH, HCO, and H C H O were absent in this location and only the continuous emission remained. I t is possible that some banded structure existed near the peak of the continuous spectrum, but the noise level makes this uncertain. Gaydon and Wolfhard (73) have stated: ‘‘. the typical emission from the ordinary yellowish luminous flames is due to carbon. The intensity distribution in the continuous spectrum of these flames is near that of a Planckian radiator. The evidence that the emission is due to solid particles of carbon seems quite conclusive.” Figure 7,b, appears to be in general accord with this statement. However, a Planckian (black body) radiator a t the temperatures produced in a second-stage flame should have its maximum emission intensity in the near-infrared region of the spectrum. This was checked using a lead sulfide photoconductive detector, but only bands resulting from water, carbon dioxide, and hydrocarbons were evidenced in the range from 1.0 to 3.0 microns. Perhaps the emission observed in the visible region represents extremely large “carbon molecules” with many carbon atoms, and yet not of such size as to constitute “solid particles.” Such a molecule would be expected to emit a banded spectrum with lines so closely spaced that they could perhaps not be resolved with even the highest resolution spectrometer. There is little doubt that the yellow luminosity is somehow associated with
..
carbon formation in flames, and additional information on this matter might be of great help in many practical problems, such as combustion chamber deposition in internal combustion engines, gas turbines, jet engines, and stationary burners.
r
Summary Emission spectra recorded in the range from 3200 to 6500 A. for cool flames formed in a flat-flame bufner with diethyl ether and air were similar to those reported previously for cool flames formed in other apparatus. I n the range from 3200 to 5100 A. the spectrum consisted of emission from excited formaldehyde superimposed on a strong background. Additional radiation in the range from 5100 to 6500 A. could not be identified. Fuel-air ratio, within the limits in which cool flames could be produced, had little effect on the spectrum. The emission spectrum of the richest second-stage flame obtained in the flatflame burner was similar to that of the cool flames, except that the relative intensity in the region from 4600 to 4700 A. was somewhat greater and the unidentified radiation between 5100 and 6500 A. was not evident. This result is in general agreement with the observations of Topps and Townend (32). As the second-stage flame was made leaner and approached the conditions of a rich normal flame, the formaldehyde spectrum was gradually replaced by an extraordinarily intense spectrum of H C O and CH. The spectrum of the yellow luminous column appearing after the leanest second-stage flames and richest normal flames showed only an apparently continuous emission extending from about 3600 A. to the long wave length cutoff of the photomultiplier detector a t about 6500 A. An infrared spectrum from 1.0 to 3.0 microns for this same yellow column showed no continuous emission. Thus, there seems to be some uncertainty as to whether or not the yellow luminosity represents black body emission from solid carbon particles, as would be assumed from statements by Gaydon and Wolfhard (73) and Eyerton and Thabet (8).
literature Cited (1) Agnew, W. G., Agnew, J. T., Wark, K., Jr., “Fifth Symposium (International) on Combustion,’’ pp. 76678, Reinhold, New York, 1955. ( 2 ) Brode, W. R., “Chemical Spectroscopy,” 2nd ed., Wiley, New York, 1947. (3) Broida, H. P., Gaydon, A. G., Proc. Roy. Soc. (London) A218, 60-9 (1953). (4) Cornelius, W., Caplan, J. D., S.A.E. Quart. Trans. 6, 488-508 (1952). ( 5 ) Donovan, R. E., Agnew, W. G., J. Chem. Phys. 23, 1592-6 (1955). (6) Downs, D., Street, J. C., Wheeler, R. W., Fuel 32, 279-309 (1953).
(7) Dyne, P. G., Style, W. G., Discussions Faraday SOC.2, 159-61 (1947). (8) Egerton, A. C. G., Thabet, S. K., Proc. Roy. Suc. (London) A2i1, 44571 11952). (9) EmelLuus, H. J., J . Chem. SOG.1926, 2948-51; 1929, 1733-.9. (10) Gaydon, A. G., Proc. Roy. Soc. (London) Ai79, 439-50 (1942). (11) Gaydon, A.. G., “Spectroscopy and Combustion Theory,” 2nd ed., Chapman & Hall, London, 1948. (12) Gaydon, A. G., Moore, N. P. W., Simonson, J. R., Proc. Roy. SOC. (London) A230, 1-19 (1955). (13) Gaydon, A. G., Wolfhard, H. G., “Flames : Their Structure, Radiation, and Temperature,” p. 163, Chapman & Hall, London, 1953. (14) Gaydon, A. G., Wolfhard, H. G., “Fourth Symposium (International) on Combustion,” pp. 21118, Williams & Wilkins, Baltimore, 1953. (15) Gaydon, A. G., Wolfhard, H. G., Proc. Phys. Suc. (London) A64,310-11 (1951). (16) Hgrman: R. C., Hornbeck, G. A., Laidler, K. J., Science 112, 497-8 ’ (1950). (17) Hornbeck, G. A., Herman, R. C., Natl. Bur. Standards iU. S.). Circ. 523. DD. 9-18. 1954. (18) Kondratjew, V., Acta Physicochim. U. R. S. S. 4, 556-8 (1936). (19) Levedahl, W. J., Broida, H. P., Anal. Chem. 24, 1776-80 (1952). (20) Lewis, B., Von Elbe, G., “Combustion Flames and Explosions of Gases,” p. 97, Academic Press, New York, 1951. (21) Magill, P. L., Hutchinson, D. H., Stormes, J. H., Pruc. Natl. Air Pollution Symposzum, 2nd Symposium, pp. 71-83, Stanford Research Institute, Stanford, Calif., 1952. (22) Moore, N. P. W., Simonson, R. R., Nature 173, 543-4 (1954). (23) Mullins, B. P., “Third Symposium on Combustion Flame and Explosion Phenomena,” pp. 704-1 3, Williams & Wilkins, Baltimore, 1949. (24) Pastell, D. L., S.A.E. Quart. Trans. 4, 571-87 (1950). (25) Pearse, W. B., Gaydon, A. G., “The Identification of Molecular Spectra,” 2nd ed., Wiley, New York, 1950. (26) Powlin J., Fuel 28, 25-8 (February 194971 Retaill:aau, E. R . , Richards, H. A., Jr., Jones, M. C. K., S.A.E. Quart. Trans. 4, 438-54 (1950). Rifkin, E. B., Walcutt, C., Betker, G. W., Jr., Zbid., 6, 472-87 (1952). Rounds, F. G., Bennett, P. A., Nebel, G. J., S.A.E. Trans. 63, 591-601 1‘1955). (30) Smith, ’E. C. W., Proc. Roy. SOC. (London) A174, 110-25 (1940). (31) Steacie, E. W. R., “Atomic and Free Radical Reactions,” p. 358, Reinhold, New York, 1946. (32).Topps, J. E. C., Townend, D. T. A., Trans. Faraday Sx. 42, 345-53 (1946). (33) Ubbelohde, A. R., Proc. Roy. SOC. ( L o n d m ) A152, 354-78 (1935). (34) Vaidya, W. M., Ibid., A147, 513-21 (1934). (35) Wark. K., Jr.. Ph.D. thesis, Purdue University, 1955. (36) Withrow, L. L., Rassweiler, G. M., IND. ENG. CHEM. 25, 1359-66 (1933). I
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RECEIVED for review October 26, 1955 ACCEPTEDJune 16, 1956 VO1. 48, NO. 12
DECEMBER 1956
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