36 MASON AND WHEELER THE PROPAGATION OF V.-The Propagation of Flame in Mixtures of Methane and Air. Part I. Horizontal Yropaga tion. By WALTER MASON and RICHARD VERNON WHEELER. IN previous communications to this Society a study of the initial “ uniform movement” of flame in gaseous mixtures has been pre-sented (T. 1917 111 267 1044; 1919 115 578). The majority of the experiments of which an account has been given have been with methane as the combustible gas. The general conclusions drawn as to the character and ratisonale of the uniform movement are however applicable to all inflammable mixtures. The uniform movement is one phase in the propagation of flame, and is of comparatively short duration. The speeds attained by th FLAME IN MIXTURES OF METHANE AND AIR. PART I.37 flames during its regime are comparatively slow; very slow com-pared with that of the detonation-wave but slow also compared with the speeds during other phases in the propagation of flame in mixtures wherein the detonation-wave normally does not develop. The value of determinations of the speeds of flames during the uniform movement lies in the measure thereby afforded of the general behaviour of a given iiiflanimable mixture or range of mix-tures immediately after ignition the measurements when made under standard conditions being physical constants. Knowledge is however odten necessary of the maximum speed attainable a t any time during the course of the propagation of flame in mixtures of a given comb’ustible gas with air or oxygen. Such knowledge is of prime importance for example in respect of mixtures of methane and air in connexion with the safe wolrking of coal mines.I n the present paper a description is given of the phases other than the uniform movement during the horizontal propagation of flame in mixtures of methane and air. The several series of experiments were carried out in tubes of different dimensions and materials. It is important when compar-ing one series of experiments with another that due regard should be paid to the details given respecting the tubes employed. I n the majority of the experiments measurements of speeds were made by tho “ screen-wire ” method of which full details have been given in earlier papers (T. 1914 105 2610; 1917 111 1053). Supple-mentary information was obtained by photographic analysis of the flames.I n order to obtain the1 photographs the flames were caused to travel along a tube of brass 5 cm. in diameter furnished with a window of quartz which was focussed on a rapidly revolving film by means of a quartz lens. The use of quartz enabled the light falling on the film to be sufficiently actinic to record the movements of the flames with con~iderab~le detail. (I) Ignition at the Open End of a Tube Closed at the Other End. The initial phase of propagation of flame when the mixture is contained in a horizontal tube closed a t one end and open a t the other and ignition is a t the open end constitutes the “uniform movement .” The linear duration of the uniform movement is controlled by the s p e d of the flame (and thus by the composition of the inflam-mable mixture); by the length diameter and unifmormity of bore of the tube; in short by such factors as influenoe the establishment of resonance in the colrimn of g a w in the tube.Eventually as a dirmt outcome of the establishment of resonance the flame-froa 38 MASON AND WHEELER THE PROPAQATION OF acquires a periodic undulatory motion (sele T. 1919 116 584) leading sooner or later to violent vibrations which vary considerably in amplitude but remain periodic. This phase in the propagation of flame was discovered by Schloesing and de MondBsir and was termed ‘( le mouvement vibra-toire” by Mallard and Lei Chatelier (Ann. des Mines 1883 [viii], 4 331). Although accurate record can be obtained of the develop-ment of the ‘( vibratory movement ” under chosen conditions the measuremonts-of the mean speed of the flame for example-are not of much theoretical significance or practical value for the speed of the flame during any one vibration and the amplitude of the vibrations is very susceptible of changes designed or inadvertent, in the experimental conditions.Sol far as mixtures of methane and air are concerned it is perhaps sufficient to record a few of the data obtained as indicative of the general character of this phase in the propagation of flame for com-parison with the uniform movement which precedes it. Thus with mixtures containing betwelen 10 and 10.5 per cent. of methane and with a tube of brass 240 cm. long and 5 cm. in diameter the signi-ficant measurements obtained by photographic means are as f OllO~W : Speed of ff ame during unif o m movement Linear duration of uniform movement ...... 80 om. . . . 90 cm. per second. Faint undulations of the flame-front. appear after the flame has travelled 32 cm. The mean speed of the flame is not affected by these undulations; their amplitude is small,. and their period is that of the resonating column of gases in the tube. The amplitude of the undulations increases gradually from 1.7 cm. over the distance 32-50 cm. to1 1.9 cm. over the distance 50-60 cm. and 2.2 cm. over the distance 60-80 cni. It then begins t o increase rapidly, becoming 3.6 over the distance 80-90 cm. During this period o€ rapid increase in amplitude of the undulations the mean speed of the flame falls to 64 cm.per second. Eventually the “vibratory movement,” which owes its origin to1 an undulation of abnormal amplitude is established. During the vibratory movement the oscillations of the flame are of wide amplitude-25 cm. or more-and the mean speed of trans-lation of flame is considerably enhanoed. It will be seen on exam-ination of Plate I Fig. 1 that the change of speed from that of the uniform movement (90 cm. per sec.) t o that of the vibratory move-ment (278 cm. per sec.) is fairly abrupt and that the latter speed is maintained a t a constant mean value over a considerable dis-tance. Finally as the flame approaches the closed end of the tube FLAME IN MIXTURES OF METHANE A N D AIR. PART I. 39 its mean speed decreasw although it still continues to vibrate to the end.In the table that follows data are given respecting successive portions of the vibratory movement each portion being specified by the distance along the tube over which the flame travelled. Vibratory Movement of Flame. 10-10.5 per cent. in air.) (Tube of brass 240 cm. long and 5 cm. in diameter. Methane Distance along tube. 0-80 cm. 80-90 90-107 107-170 170-200 200-220 220-240 Mean speed of flame. Cm. per sec. 64 278 62 38 --Maximum speed during forward motion of flame. Cm. per see. Unif o m movement 292 Gradual change 2,430 Gradual change 416 -Amplitude of vi br ati om. Cm. 3.6 26.0 1.4 ----Frequency of vibrations. Mean values. It may be noted that as indicated by the frequency of the vibra-tions the resonating oolumn of gases is that lying between the closd 0nd of the tube and the flame-front at any given moment.Thus the calculated mean value for the frequency of vibration of a column of gases in an unflanged tube of brass 5 cm. in diameter is 48 if the tube is 130-140 cm. long and 74.5 if it is 50-93 cm. long. These measurements bear reference only to the particular condi-tions of experiment specified but they could be reproduced with an exactness which must be considered remarkable when the compli-cated character of the1 phenomena is borne in mind. In this respect better fortune has attended the experiments than that which befel Mallard and Le Chatelier who have stated “ En rep6tant plusieurs fois la meme experience dans dehj conditions identiques B elles-mGmes, le mouvernent vibratoire ne se reproduit jamais deux fois de la m@me f a p n ” (Zoc.c i t . p. 333). No doubt the rapid speed of flame i n the mixture (CS + 6NO) employed by Mallard and Le Chatelier for their experiments would tend to emphasise irregularities in the results. It has already been stated that the vibratory movement is the direct result of’ the resonance of the oolumn of gases in the tube. It was shown in connexion with experiments on the propagation of flame in mixtures of acetylene and air that resonance by whatever means induced can be made manifest by the undulatory motion of flame as it travels along tubes the1 periods of the undulations agree-ing closely with the periods calculated for organ-pipes of the dimen 40 MASON AND WHEELER THE PROPAGATION OF sions of the t u b employed.As the resonance becomes stronger, the amplitude of the undulations of the flame front perform increases since the flame acquires its motiojn from the vibrating column of gases. There is thus produced an agitation of the gaseous mixture which eventually becomes of sufficient importance to affect appreciably the speed of a flame travelling through it. (In this connexion see T. 1919 115 81.) The vibratory movement is indeed an excellent example of the effect of agitation or turbulence in accelerating the translation s f flame through a gaseous mixture. The effect is a mechanical one. During each forward impulse the flame is drawn rapidly through previously unburnt mixture by reason of the motioa acquired by the resonating column of gases.I n a certain degree also the forward motion of the flame is assisted by the expansion in volume of the burning gases especially when the flame is a t some distance from the open end of the tube so that escape of the expanded gases there is retarded. The latter effect is more pronounced when the mixture is ignited a t the clomd end of a tube open a t the other end conditions which will be considered in the succeeding section of this paper. (11) Ignition at the Closed End of a Tube Open at the Other End. The two phases in the prolpagation of flame the '' uniform move-ment " and the " vibratory movement," are characteristic of what occurs with .mixtures of a oombustible gas and air when ignition is at the open end of a tube closed a t the other end.Under such conditions with some combustible gases (for example hydrogen) when mixed with air and with all when mixed with pure oxygen, the vibratory movement is succeeded by the detonation-wave, provided that the combustible gas and oxygen are in suitable proportions. With no mixture of methane and air (at atmospheric temperature and pressure) is the detonation-wave thus developed but the vibra-t o y m'ovement continues until the flame is extinguished either on reaching the closed end of the tube or occasionally during an abnormally extensive backward movejment before the end is xeached. When ignition of a mixture of methane and air is a t the closed end of a tube open a t the other no uniform movement takw place, but the speed of the flame increases rapidly as it travels towards the open end.For comparison with the uniform and vibratory movements, experiments were made with a sesies of mixtures in a horizontal tube of glass 5 cm. in diameter and 600 cm. long. Fine screen FLAME IN MIXTURES OF METHANE A?XD AIR. PART I. 41 wires of copper were stretched across the tube a t half-metre dis-t a n m and the times taken for the flames to travel between these screen-wires me'asured by the method described in previous com-munications. The mixtures were ignited a t a spark-gap 3 cm. from the closed end. The speed of the flame in some of the mixtures reached 29 m. per second over the last half-metre length of the tube and so far as could be judged was nearly uniform!y accelerated from the begin-ning.It seemed possible therefore that with a tube of greater length and larger diameter a permanent maximum velocity of flame, such as is characteristic of the detonation-wave might eventually be attained. A steel tube 30.5 cm. in diameter and 90 m. long was used to test this suppition. It was found that flame did not continue t o propagate in any mixture beyond a distance of 15 m. from the closed end a t which ignition was effected. Violent vibrations were developed after the flame had travelled 10 m. in the course of which the flame was extinguished. The same result was obtained when the mixtures were ignited a few cm. (ten t o twenty) from, instead of at the closed end a condition which would have the e'ffect of imparting an impetus to the flame a t the beginning, thereby hastening the development of the detonation-wave (com-pare Dixon Ph& Trans.1903 A. 200 345). The extinction of the flame after travelling such a short distance in a long tube under the conditions of these experiments is no doubt caused by the products of combustion when cooling tending to produce a partial vacuum behind the flame (the end of the tube from which the flame started being sealed) which is therefore dragged back over part of the path it has already travelled. This may occur several times the flame alternately leaping forward and being drawn back but eventually a sufficient proportion of the burnt mingles with the unburnt gases t o prevent further propaga-tion of flame.(111) Ignition at One End of a Tube open at Both Ends. I f the reason assigned f'or the extinction of the flame when travel-ling from the closed to the open end of a long tube is correct-a reason intended to apply only to such comparatively slowly-moving fla.mes as are obtained with mixtures of methane and air *-extinc-* With mixtures of coal-gas and air for example in which the flames are initially more rapid than with methane and air the vibratory movement continues (in a steel tube 30.5 cm. in diameter) until the detonation-wave is developed. The speed of the wave in a mixture containing 17 per cent. of coal-gaa is 1760 m. per second. 0 42 MASON AND WHEELER THE PROPAOATION OF tion should not occur when both ends are open (so that the cooling of the products of combustion cannot create a partial vacuum behind the flame) and the speed a t which the flame travels should be rapid.Several series of experiments were made to test this point it being important to determine the conditions under which the most rapidly moving flames are obtained in mixtures of methane and air, and the order of magnitude of the speeds. FIG. 1. Time seconds. me first series of experiments was in a tube of glass 5 cm. in diameter and 500 cm. long for comparison with the series carried out in the same tube in which ignition was a t a clmed end. The results are shown graphically in Fig. 1 in which distance along the tube is plotted against time zero time being the moment of fusion of the first screen-wire which was 10 cm. from the point of ignition.Wit,h all but the lower-limit mixture (5.40 per cent. methane), in which the speed of flame is uniform there is a gradual and s FLAME MlXTUBES OF METHANE AND AIR. PART I. 43 far as the records can indicate regular acceleration of speed as the flame travels from end to end of the tube. In no instance did extinction of the flame occur although in most of the experiments slight vibrations were noticed a t different stages in the develop-ment of the propagation the incidence of these vibrations being earlier the more rapid was the flame. There was also with the mixtures richer in methanel a noticeable check in the progress of €he flame as it approached a particular point succeeded by a spurt forward after that point had been passed. This effect seemed trace-able to a slight ridge around one of the small holes with which the tube had been pierced (by means of a blow-pipe flame) to receive the screen-wires used to record the time of passage of the flame.This ridge projected less than 1.5 mm. within the tube; the fact that it could markedly affect the progress of a flame in a tube 50 mm. in diameter is a striking example of the sensitiveness of flames to turbulence in the mixture even such slight turbulence as the small projecting ridge would cause. This subject will be dealt with in a subsequent. communication. Another series of experiments covered the whole range of inflam-mable mixtures of methane and air and was made in a glass tube 9 cm. in diameter and 620 cm. long. This series is of value for comparison of the mean speeds of the flame over measured distances with the speeds of the uniform movement in a tube of the same diameter.Such a comparison is made diagrammatically in Fig. 2, which records speed-percentage curves ( A ) for the distance (measured from the point of ignition) 50-100 cm. and ( B ) for the distance 407-467 cm. in the tube open a t both ends the speeds being the mean speeds of the flames over those distances; and (C) for the1 uniform movement. In addition a curve (D) is given showing the mean speed of flame over ths distance 20-120 cm. in a tube 5 cm. in diameter closed a t one end and open a t the other, ignition being a t the closed end. The mean speed over the distance 407-467 is wen to be greatest in the mixture containing 10 per cent.of methane and to be about four times the speed of the uniform movement in that mixture in a tube of the same diameter. The speed of the flame in all mix-tures (except the limit mixtures) was found as with the tube of 5 cm. in diameter to increase continuously over the whole distance travelled and as when ignition was a t the closed end of a similar tube it seemed possible that the detonation-wave might be developed if the flame could travel far enough. I f not it was necessary to know what change in the character of the propagation would interpose to prevent it. The steel tube 30.5 cm. in diameter was brought into requisition C" 44 MASON 'AND WHEELER THE PROPAGATION OF to test this. The length of the tube in the first instance was 15-25 m.and records were obtained of the times taken for the flame to travel measured distances from the point of ignition in different mixtures. As usual the fastest speed of flame was obtained with mixtures containing between 9.5 and 10.6 per cent. FIG. 2. 1800 1600 1400 2 1200 0 % 3 CJ F4 1000 f ii 800 ps % ' 600 & 400 200 4 6 8 1 I I 1 12 14 1 Methane per cent. of methane but no speed approaching that of the detonation-wave was recorded the maximum being 917 cm. per second attained after travelling 14 m. in a mixture containing 10.25 per cent. of methane. It appeared from the records that the flame which could not b directly oberved had acquired a vibratory character after travel-ling half the length of the tube. No indication of this was gven by the sound of the flames as they travelled and the vibrations were presumably of small amplitude.Vibratory propagation in the steel tube such as was obtained when osne end of the tube was closed had hitherto been accompanied by a staccato note but the flames now produced seemed to the ear to travel unhaltingly from one end of the tube to the other issuing into the air with a sharp report. The length of the tube was therefore increased ta 90 m. in the expectation that if the flame had indeed become vibratory in character after travelling 6 or 7 m. a greatly increased distance of travel would produce readily recognisable vibrations of large amplitude. Such was in fact the result; the propagation ulti-mately became strongly vibratory but the early stages of the propagation were profoundly modified by the increased length given to the tube.Instead of increasing rapidly in speed from the beginning as when the tube was 15-25 m. in length the flames now travelled from the point of ignition a t a constant and com-paratively slow speed over a distance of between 12 and 15 m. (dependent ‘on the composition of ths mixture) and then began to vibrate. The vibrations acquired their greatest amplituGe about half-way along the tube and continued throughout the remaining distance. I n mixtures containing between 9.5 and 10.5 per cent. of methane the speed of the flame over the first 12-15 m. averaged 200 cm. per second. Thus the records obtained over this range of mixtures were : Methane Initial speed of flame.per cent. Cm. per second. 9.60 198 9.70 188 9.75 203 10.10 213 This speed is a little faster than that of the uniform movement in similar mixtures in the sam0 tube (170 crn. per second). The important point is however that the speed should remain constant over so great a distance. Although open a t both ends a long tubs is thus found t o impress upon a flame started a t one end oonditions similar to those obtaining with a shorter t u b c l o d a t the distal end. The resistance to the expansive force of the burning gases afforded by the long column of unburnt mixture in advance of the flame corresponds (nearly) in effect with the resistance) of a closed end; so close is the correspondence that the flame is caused to proceed a t the outset with a (( uniform movement,” but little faste 46 MASON AND WHEELER THE PROPAGATION OF than the uniform movement as ordinarily developed in mixtures of the same methane-content in a tube of the same diameter.Photographic Analysis of the Flames. I n Plate 1 are shown time-distance curves obtained photographi-cally for the propagation of flame in a 10 per cent. mixture of methane and air in a tube of brass 5 cm. in diameter and 240 cm. long. The flames travelled horizontally from right to left and the photographic film can be regarded as moving vertically upwards its speed of travel being 30 cm. per second. The full length of the tube 240 cm. is shown in the photographs each of which is com-posite being obtained by joining together photographs of successive sections of the tube 30 cm.in length. For Fig. 1 the tube was closed a t the left-hand end and ignition was a t the right-hand open end; folr Fig. 2 the tubfe was open a t both ends and ignition was a t the right-hand end; and for Fig. 3 the right-hand end of the tube was closed and ignition was effected there the left-hand end being open. The relative speeds at. which the flame traversed the full length of the tube are! readily deduced from these photographs which also illustrate the' general behaviour !of the flames under ths different con-ditions of ignition of the mixtures and require no. description. It should be noted however that Fig. 3 discloses the presence of rapid vibrations during the progress of the flame which as stated earlier in this paper was judged by visual observation to travel unchecked through the tube a t a speed which according to determinati'ons by the screen-wire method seemed to be nearly uniformly accelerated.I n Plates 2 and 3 details of the flames as they passed through a section of the tube 30 cm. in length are shown the section chosen being that indicated in Plate 1 by the vertical white lines. To obtain these photographs the speed of the film was increased to 90 cm. per second. Calculations made from them are as follow : PLATE 2. T u b closed a t one end; ignition a t o p n end. Mean speed of flame . . . . . . . . . 246 cm. per sec. Maximum speed during forward rnove-ment of vibration . . . . . . . . . 1,990 cm. per sec. Average frequency of vibrations ... 76 The calculated frequency f o r the fundamental tone of the tube during the longitudinal vibration of air within i t is 68 i f ths lengt PLATE 1 PLATE 2 FIG.1. FIG. 2. PLATE 3 FLAME IN MIXTURES OF METHANE AND AIR. PART I. 47 of the vibrating column be assumed to be 125 cm. and 88 if 95 cm. These are the distances of the flame-front from the closed end of the tube a t the beginning and end of the photograph respectively. The mean value is 78. PLATE 3 FIG. 1. Tube open a t both ends; ignition a t one end. Mean speed of flame . . . . . . . . . 480 cm. per sec. Maximum speed during forward rnove-ment of vibration . . . . . . . . . 3,730 cm. per Bec. PLATE 3 FIG. 2. Tub,e closed a t one end; ignition at closed end. Mean speed of flame . . . . . . . . . 1,050 cm. per sec.ment of vibration . . . . . . . . . 5,760 cm. per sec. Maximum speed during forward move-Amplitude of vibrations . . . . . . 30 cm. Of the three conditions under which the ignition of mixtures Qf methane and air has been effected in these experiments that whlch would lead to the most disastrous results in industry is the third-ignition a t one end of a tube or gallery open a t both ends. For although the initial speed of the flame is not then so great as when ignition is a t a closed end continued propagation is assured and there may be developed momentarily during the vibratory motion velocities and pressures as great as any produced throughout the life of a flame started a t a closed end. The fastest speed of flame recorded in any experiment was about 60 m. per second and was of short duration. This is not of the same order of magnitude as the speed of the detonation-wave in gaseous mixtures. It would not be wise t o conclude however that the detonation-wave cannot in any circumstances be developed in mixtures of methane and air a t normal temperature and pressure. On the contrary in several experiments in the steel tube 90 m. lsong and open a t both ends in which restrictions were introduced a t two points (consisting of steel rings which reduced the diameter of the tube to 28.6 cm. a t those points) the development of the detonation-wave seemed imminent. Further description of these experiments which are being continued is reserved until the subject of the effects of turbulence on the propagation of flame in gaseous mixtures is discussed. ESKMEALB, CWBERLAND. [Received December 4th 1919.