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Vol. 17, No. 3
Radiation from Nonluminous Flames’ By R. T. Haslam, W. G. Lovell, and R. D. Hunneman MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS,
From a consideration of the data presented herein, and from a critical interpretation of other data, it is seen that the radiation which accompanies a nonluminous flame, although of definite amounts, varies greatly with the condition of the flame. The radiation drops off as the amount of primary air is increased beyond that for complete combustion, and with less air the radiation is also decreased. For methane, with theoretical air, the radiation is 30.3 kg. calories per mol of gas burned, and with 140 per cent of theoretical air, 27.2 kg. calories. For carbon monoxide the radiations for these flame compositions are 6.8 and 6.3 kg. calories and for illuminating gas the values are 15.7 and 15.0 kg. calories. These do not check with the values predicted by the theory propounded by von Helmholtz. The radiation from flames of different gases burned with air varies within wide limits with the gases used; and the radiation with theoretical air, expressed as per cent of the latent heat radiated, has the values of 14.9 per cent
for methane, 10.4 per cent for carbon monoxide, and 13.8 per cent for illuminating gas. There is no apparent simple relationship between the amount of radiation and the composition of the gas or the products of combustion-in contradiction to previously held beliefs. The radiation per unit area for a flame is found to increase with the depth of flame according to an exponential law of the form R = K (1-e-kz), where K and k are constants depending upon the composition of the flame and the temperature of the gases before combustion. The radiation from a flame under conditions of pre-mixing is decreased as the air for combustion is preheated. The view is put forth that radiation from a flame is a matter intimately connected with the chemical reactions which are taking_place there rather than with temperature, also that in the spectroscopic study of flame radiation there exists a powerful tool for the investigation of complex chemical reactions.
KNOWLEDGE of the quantitative laws governing the radiation of energy from a flame should prove of
will radiate an amount of energy equal to the radiation for the combustion of a mol of carbon monoxide plus that for 2 mols of hydrogen. Comparison of such calculated values with experimental data should check the validity of the theory, and the experimental basis upon which Helmho!tz’s theory rests is the agreement between his calculated and observed values, as shown in Table I.
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inestimable value from a theoretical as well as a practical standpoint. Such knowledge should furnish a method of attaek for the investigation of the mechanism of many combustion reactions. Radiation from flames comes directly into account in many industrial heating operations, such as in billet heating, in open-hearth furnaces, and in the internal combustion engine. Work of Previous Investigators Up t o the present time but little work has been done on the subject of flame radiation, and the information a t hand is both fragmentary and largely qualitative. The first recorded measurements of radiation from flames were mzde by von Helmholtz* in 1890. H e determined the absolute values of radiation from flames of various gases when burning in air. His apparatus consisted of a short upright tube into the base of which flowed measured amounts of air and the gas to be burned, and the flame a t the top of the tube burned freely in the atmosphere. The energy radiated was measured by means of a bolometer situated a t some distance from the flame. Helmholtz found that the amount of radiation per unit of gas burned depended upon the kind of burner used, the size of the flame, the amount of air mixed with the gas before burning, the temperature of the gases before combustion, and the kind of gas used. His values for the amount of energy radiated, as per cent of the latent heat in the gas, ranged from 5 to 15 per cent. The amount of radiation was found to decrease with the aeration, especially with luminous flames, and also to decrease with increased temperature of the mixture before combustion. The amount of radiation from different gases varied over rather wide limits. Upon these experimental results Helmholtz based his theory of flame radiation, according t o which the radiation comes only from the products of combustion, Helmholtz postulated that the formation of a mol of water vapor in a flame will radiate a definite amount of energy, which may be determined by measurements of the radiation of a hydrogen flame. Furthermore, the formation of a mol of carbon dioxide was considered as resulting in the radiation of a definite amount of energy, which may be determined by measuring the radiation from a carbon monoxide flame. On this basis a flame of methane, for instance, which burns with the formation of 1 mol of carbon dioxide and 2 mols of water per mol of methane, ~
1 Presented before the Section of Gas and Fuel Chemistry a t the 68th Meeting of the American Chemical Society, Ithaca, N. Y.,September 8
to 13. 1924. 2
“Die Licht und Warmestrahlung verbrannter Gase.” Berlin, 1890.
Table I-Data
T a k e n f r o m H e l m h o l t z “Licht u n d W a r m e s t r a h l u n g v e r b r a n n t e r Gase” Mols product RADIATION, formed per IN RELATIVE UNITS Per cent of mol gas burned Luminous Nonluminous theoretical GAS Hi0 COz Obsd. Obsd. Calcd. air“ H2 1 0 74 0 CO 0 1 177 0 2 1 391 327 325 55 CHI C2H1 2 2 1140) 510 502 141 Illuminating 1.2 0.5 310 181 179 30 Not given in a Calculated from data as given for nonluminous flame. original table. cit., p. 63. b Given elsewhere as 1200. LOG.
This table, copied from Helmholtz’s paper, gives, in the first three columns, the gas used and the products of combustion. The fourth and fifth columns give the observed radiation, in relative units. The sixth column gives the radiation, also in the same relative units, calculated for the gas in question according to the theory. The last column, which does not appear in t h e original work, gives the per cent of theoretically required air which is pre-mixed with the gas before burning in the free flame. These values were calculated from information given elsewhere in the published account. From the data given, it is impossible to find the radiation corresponding to any one given aeration, with the exception of zero primary air (which corresponds in many cases to a luminous flame, of no interest here). Complete aeration data are given only for illuminating gas, and in this case the relative radiation for a 6-mm.flame varies from 100 to 30 over a range of primary aeration of from about 30 to 150 per cent. Hence the selection of any particular value must of necessity be decidedly arbitrary. On the other hand, the amount of primary aeration is so important a factor that all comparisons should be based upon a comparable aeration. However, in Helmholtz’s experiments, as is evident from the table, the per cent of primary aeration varies from 0 to 141 per cent. There is also a n unknown amount of secondary aeration. The criterion of aeration upon which the values chosen were determined seems to be the production of a flame with just enough air so that it is free from incandescent carbon particles. Evidently, this value will vary from zero primary air, in the case of hydrogen and carbon monoxide, to a very high value for acetylene, for instance. In addition, this criterion will depend upon the type of burner used and the air conditions and currents, as well as the kind of gas. Such a criterion of comparable aeration, therefore, seems t o be without
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any basis whatsoever. If, on the other hand, the comparison of calculated and observed values as above is based on aeration as per cent of theoretical air, the only reasonable basis, it is evident that the fortuitous agreement observed by Helmholtz. will no longer be obtained. Working a t the same time as Helmholtz, Julius2 measured the wave leng‘th and intensity of the radiation emitted from various gases when burned in a flame. He found, for nonluminous flames, that the intensity of radiation showed very pronounced maxima for wave lengths corresponding to the absorption spectra of the products of combustion, whatever the gases used, while wave lengths corresponding to the absorption spectra of the unburned gases or of inert gases were not observed. This observation tends t o verify the theory of Helmholtz that the radiation comes only from the products of combustion. However, too much weight should not be placed on the spectral determinations of Julius, as the resolving power of his spectroscope was rather low, and the spectra are only in approximate agreement with recent measurements of gas spectra, since in his curves many of the maxima now known do not appear a t all. Callendar8 measured radiation from flames of coal gas burning in a modified Meker burner. He found that large flames radiated more per unit area than small ones, and that the radiation increased with the depth of flame behind the unit area according t o an exponential law. Callendar also found that in the Mkker burner flames the radiation dropped off, as more primary air was admitted to the base of the burner. His values of radiation were about 15 per cent of the latent heat in the gas burned. Paschen4 tried to measure radiation from gases heated in a closed tube in an attempt to discover if there is any such thing as purely thermal radiation from a heated gas. Owing to the difficulty of separating the radiation from the gas and that from the tube containing the gas, however, his results are not of great significance. David5 measured the radiation from explosions of coal gas and air in a closed cylinder. He found that the time of maximum radiation occurred just before the time of maximum pressure, which would indicate that the radiation is intimately connected with the chemical reaction taking place.
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radiation is intimately connected with the chemical reaction taking place and is not a purely thermal phenomenon, and that the radiation comes from the vibrations of the molecules of the products of combustion at the instant of their formation. However, discrepancies in experiments and theories and the lack of quantitative data obviously demand further investigations of the entire problem. Experimental Methods The purpose of the present experiments was (1) to investigate the absolute amount of radiation from flames of different gases, and the effect of raried aeration and preheating; and (2) to study the effect of flame size and depth of flame upon the radiation per unit area-that is, to see if a flame mas transparent to its own radiation. (1) For a study of the first part it is necessary to have a small flame of known composition, which means that the total aeration of the flame must he measured, and to have a means for measuring the radiation from the flame. To meet these requirements there were utilized small inclosed flames with solely primary aeration, since secondary aeration is difficult of measurement; and as a means of measuring the radiation a thermopile was situated a t a distance from the flame. The apparatus constructed is shown in Figure 1. The thermopile was a twelve-junction copper-constantan pile constructed by W. W. Coblentz of the Bureau of Standards, and in its protective case was set up as shown, with the leads passing out through insulated air-tight binding posts to a “High Sensitivity” galvanometer with special mounting to protect it from stray vibrations. The galvanometer WB$ read from a distance by means of a telescope and scale. &--
explosion
ifneeded
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0
fer
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air inlet
9as lnlef
Fiaure 1
A summary of previous experimental work, therefore, indicates in a general way that radiation from a nonluminous flame for a given gas varies with the form and kind of the flame; the amount of aeration, radiation decreasing with increasing aeration; the amount of preheating, radiation decreasing with the amount of preheating; and the size and the depth of the flame. The view has been held that the
* J. Gas Lighting, 111, 644 (1910). 4 5
W i c d . A n n . , SO, 409; 61, 1 (1894); 62, 209 (1895). Proc.:Roy. Sor. (London),BSA, 537 (1911).
The distance from the flame to the thermopile varied in different determinations from 60 to 70 em., but was measured accurately a t the time of each experiment. With flames whose maximum size did not exceed that of a sphere 0.5 cm. in diameter, the ratio of the diameter of the flame to the distance from the flame to the thermopile was of the order of 0.01. An approximate calculation showed that considering such a flame as a point source would not introduce an error of over 0.5 per cent, which is within the experimental error.
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The narrow dead air space between the pile and the flame prevented convection currents, as was proved by the ease with which determinations were checked at different distances and times. The absence of windows made a correction for selective absorption unnecessary. To avoid temperature changes in the thermopile itself, it was insulated from the rest of the apparatus, and the entire apparatus inclosed in an outside protective case of corrugated pasteboard. To eliminate the effect of stray and reflected radiation falling upon the pile, the entire interior was painted with a fine gas black suspended in turpentine. Such surfaces are ordinarily considered as “95 per cent black;’’ and although calculations were based upon the assumption that the surfaces were 100 per cent black-that is, that there was no reflection from the wall in back of the flame-this error is not over 2 per cent in absolute value and should affect all determinations proportionally. The flame was produced a t a burner consisting of a short tube with a piece of fine gauze over the tip so as to produce a modified MBker burner. This was necessary (as the flames were produced solely with primary air) to prevent striking back and blowing out of the flame as the air-gas ratio was varied in different experiments. A number of different types of burner were tried, but only one was found which would produce a small uniform flame under the conditions of the experiments. The flame appeared as a faint bluish, bulging cone about 0.5 em. high. Different burner tips with different areas were used with different gases on account of the varying rates of flame propagation which the gases possess. Under the experimental conditions the burner did not get hot. The flame was inclosed in the water-jacketed inclosure kept at a temperature constant within 0.1O C., which quite sufficed to give reproducible results. The hot products of combustion passed out a t the top of the flame chamber; outside air had no access to the chamber, so that there was no secondary aeration. The gases used were methane, taken from a cylinder of special natural gas which analyzed 95 per cent methane, pure carbon monoxide generated by the action of sulfuric acid upon formic acid, and illuminating gas taken from the Cambridge city mains. COz 0 . 0 per cent; CnHzn, 0.0 per cent; 0 2 , 0 . 0 per
