MASON AND WHEELER THE PROPAGATION OF FLAME ETC. 1227 CXXXII1.-The Propagation of Flame in Mixtures of Methane and Air. Part II. Vertical Pro-pagation. Part 1'1. Propagation in Currents of the Mixtures. By WALTER MASON a.nd RICHARD VERNON WHEELER. PART II.-VERTI CAL PROPAGATION. THE olb'je.ctt off the1 eixpelrimeInts. herein described on the velrt.lc.al propagation od flame in mixtures of melthane a,nd air was to! obtain informamtion as to the1 magnitude od t,he ejffe.ct8 olf conve,ctioa curre'nts on the s p e d of t'he flme. Exc,eptl pelrhaqs during the '' unif o m movement," * the trans-f elrelncel of heat by coavection ourrente clelarly plays aln important part in the transmission of fla,mel a.nd is accountlabla for many of ite phenomenat. Folr example the1 well-known fact tha't the limits of inflammabilit'y of gaseous mixtures vary with the posittio,n of the1 paint of ignition a.acoIrding as t.hei flame has to pass upwards or downwards fhrough the1 mixture morel combastiblef gas b,eiing requireid tol f olm a lowe:r-limitl mixtlurei undelr the1 la.t'telr colnditdons, is elxplicab,le on the assumptioln thatl during the do'wnwasd pro.pagatlion of flame colnvelctioa curre'nts do1 not ma,te,ria,lly a,ffeIc& the tra(nsfe1rence of he1a.t tlo unburnt la,yersl 0.f the mixtlure. A like explanat,ioa can be given f o r tlhel fa.& tha,t when mixtures of methane and air aontaining lees than 7.5 and momre than 12.5 pelr cent'. of methane are ignitod a t the cent'rel olf a clolse'd spherical vessel flame reach= the1 bolttom of the v e s d later than itl reaches the1 t'op by a,n interval of time1 which varies with thei meltdhanet-matent of the mixture (T.1918 113 S45). I n general it would seem probable1 thatl flame sho,uld travel vertioally in a given mixture molre rapidly wheln i t is ignited from b l o w t'han whe'n it ,is ignite'd from a,b,ove if only by virtue of the ourrent produced by the1 heiateld gases. Undelr the spelciad c:ondi-tions olf ignitlioln a t the1 centre of a closeld vessel howelver any differenw there may be1 bet$welen the spele8ds o:f upwa,rd and down-wa.rd propagation of flame! is inappreciable with mixtares in which * The " uniform movement " is the term generally applied to the initial slow propagation of flame a t 8 uniform speed that occurs when an inflammable mixture is ignited at the open end of a horizontal tube closed at the other end.Under such conditions the propagation of flame is assumed by Le Chatelier to be '' normal " and to be effected mainly by the conduction of heat from the burning to the unburnt grtses 1228 msoN AND WHEELER THE PROPAGATION OF the s p d of flame is comparativelly rapid (mixtures coataining betwwn 7.5 and 12.5 pen cent. of methane). As belaring on this point mrtaia observations by Schloesing a,nd dO Mondesir on the combustion of mixtmrea of carbon monoxide and air should be noted. Having remarked that one ra,relly olbsetmes the ‘‘ no’rmal ” propaga,ti,oi.oln of flame b,eIca,use of t,ha development of moivemelnts in the gaseloas mixtures duel to the combustion itself they suggest tha,t one of the prinaipal causes of moIvment o r agitation of the burning mixture is the diffelrencm in density between the helabed a8nd the aoild gases “ Un m6lange gazeux a h m 6 pa,r la partie inf6rieturq b’dle toujoars plus vite que lolrsqu’il est allurn6 par 1s partie sup6rieare.Lea gaz ahaads s’618vent en effet a travers leis gax froids aveo une vitlease qui vient s’ajouter B la vitesse normale de propagahion. Mais l’a.oc616ratdon ainsi produita w t toujours tres faib3ei; ella n’elst sensibsle qu’avetc lea d l a n g e s gamux t&s lent6 et renferm6s dans dec; ttubw de grand diamBtm” ( A m . des Mimm, 1883 [viii] 4 298). As regards the (‘ unifolrm movement” (thak is to say “ normal” propagatiom of flame) we drew the colndusion (T. 1917 111, 1053) that if Le Cha,tetlietr’s definition olf it as “la mode de propaga-tion pa8r oolnduatibilit4” be acoepted it is a strictly limited phew-manan obtainable mly in t u b a within a certain range of diametm, hrge etnoagh tlo prewelnt appreciable cololling by the walls but narrow enolugh to suppress the influence of convection ourrents so that the diameter of tthe tube in which the flames travel sholuld be spelcrified wheln speeds purporting t,a be those of the1 ‘(uniform movement ” ojr ‘( nolrmad ” propaga’tion olf flame a8re reoolrded.Alterna,tiveily the initial slow movementl olf flame a.t a unifolrm spmd sh.ould be regasded simply as a past,iculas phase in t,he propaga,tioln of flame thamtl omurs when ignit’ioln is etffeded (in a quies,cmt mixture) a.t the open elnd of a straight horizontal tube (of any diameter) dosed a t the olther end and notl as reeulting from a pa,rtdcular m,olde of hela$ tra,nsfeIreaxe.* As.a result olf the present investigation i t is shown tha.t the1 lahter is the prelferablei if not the only co.rrectl way oif regarding the uniform movemeat; for tbe idea tha,t this “noirmal ” prolpagation of flame is soletly by con-duct<ion of heah frolm layelr to layer of the mixt;uret is untena>ble. Furtheir it is shown tha,t there is a uniform r6gime in the propaga-tion of flame from the open to1 the closed end of a straight tube * The influence of convection currents is noticeable with the fastest flames in mixtures of methane and air in tubes 10 cm. in diameter the visible effect being a turbulence of the flame-front; whilst in tubes of smaller diameter (5 to 9 cm.) although there is no turbulent appearance the shape of the flame-front shows that there is a definite movement of the hotter gases t;owards the upper part of the tube (T.1914 105 2609) FLAME t& MIXTURES OF ME~HANE AND AIR. P A B ~ II. 122 when the tube is vertiml a'nd the1 direction of travel of the flm0 is either upwards or downwards. I. lgniti,om ait the Open End of a VmticaJ Tuba Closed at the oltlter End.-Under them ooinditions t'he first phase in the propa-gation of flame in a horizotntlal tuba is tlhe uniform move8ment. If this uniform moveunat of flame in a holrizontd tube represents welntially t\he t,ransfelrence of heat by conduction from layer to layer od thei mixtarel simi1a.r conditions off ignitioin witb al vertioal tubs should produce a similar uniform movelment when the open end of the tube is above so that t'he flame! tra,vells downwards; for convectioa currmtls oannoit tlhetn b'e helld to platy a greater part in the transfe,rence of hetat toi t,he unbu,rnt mixture than wheln the flame is tlra,vedling horiz,ont'ally.Moreover tlhet initlal speieds oif the flames in like1 mixhures dul-ing theiir downwa,rd propagahioln in a vertical tube and during the uniform molveimetnt in a horizontal t,ube should be a,pproxima,tdy the same at all eve1nt.e in narrow tubes in which the aation of convelatiotn ourrent8 during horizonta.1 propagatioa may be1 assumed to be at a minimum. During upward prolpagation on the contrary the initial speelds of the flmm should be faster than during the '' normal " uniform movetment in a horizontlad tube aad i t would seem unlikelly that any phase in tqhe propagation s f flame corresponding with the uniform movement would be of long duration.A glass t ' u h 5 am. in diamet'elr and 5 m. long was employeld for ttha fir& series of experiments with screen-wires " folr recording tlhe speeds of flames stlretcheld a80roIss a,t intelrvals of 50 cm. Ignition was at a 5 nun. spark-gap 4 an. from the open end 04 the tube, and the first surelea-wire was str&ched a.t a distance of 20 crn. from the point of ignition. The moluth of the tuba wm provided with a flange ground t a relceive an end-pi- which was helld in position by metal clips whilstl t,he tubs wa,s being filleld witb the mixt\ure.Before ignitJag the mixture this end-piece wa)s slid gently ts onel side in such a maanelr as t a avoid caasing dist,urb,-anm of the gaaea within the t4ube. The methold of reicolrding the t,imes of fusion of the sclreen-wires and t>he general mode of pro+ mdure far similar expesiments are described in T. 1914 105 2609. As mt,iaipated t'ha speleds ab whioh the fla'mee traveilled down-wards were with all the1 mixtures employed unifoirm over a con-sidetrable dist'anae from t1he pointl of ignitioln. A viblra8tolry mow-ment bega,n in genera,l solmewhat ea(r1ier than during holrizontal propaga,tioin in a tube of the same1 diameter but witb no mixture was the distanw traveilled by t'hel flame1 a't a unifotrm speed lelss tha,n 100 om.and with t'hose containing thei lowelr pelrcentages o t metthanel it somettimes exceeded 200 0121. z z" 1230 MASON AND WHEELER THE PROPAGATION OB As regards t'he magnitudes off the speeids however our expelotea-tion (based o1n the assumptbm thab the propagation of flame during the uniform movemelnt in a horizointal tubs of small diatmetler is mainly by colnduot,ion ob heat) was falsified. I n Fig. 1 the results otbt'aineld for dolwnward propagation of fla'mel acre recorded as a speed-pelrcelntage ourvei for ootmpwisoln witlh the curve for the uniform movement in a holrizont8al tube of the same diameter. Fxa. 1. Bolth mvw are 04 the same type but the speeds of the flames during the initial &age of downward propagation are over the whollei range of inflaznmab'ls mixtluree but- t'wwol-thirds the speeds during the unif olrm movement.Prelsumab~ly t>hpn the1 t-raasfelrenm of helat by convect-ion pla#ys a aonsidelra,blel pa,r2 in aiding the propamgation of flaime during the (ho'rizontlal) unifolrm movemelnt wein in fairly nasrow tubes. This being so i t seemeld probable that contmry to our first assumption, the speled of propaga,tdon of flame upwa'rds would be not muuh i FLAME IN MIXTURES OF METHANE AND AIR. PART 11. 1231 a.ny faster ttha*n during horizoiita'l pro!paga,tion unless furt*heIr belnelfit from convection currents is obtained during the fo'rmer condi tions. Measure,ments of the speeds during t'he upward propagatlon od fla8me were notl easily made b.y the! met,hod od screlein-wires except at or nelar the limits of inflammabilitly.Over moat of the1 range of inflammable1 mixt,ures t,hhs flames somelt,imes began ta vibrate violentdy within 50 or 60 cm. of t,he open end of the tube. From t3he moment of ignition hoiwevetr the flames even those that tsave1le:d fastest seetmed to mo've a,t unifolrm or but slightly accelesating speeds. Attempts welrel theselfore madel to1 measure, t,he,se speelds the first screen-wire being placed 10 cm. from the point of ignition and the succeeding wires at distances of 25 m. from eaclh other. The1 speed of t'he flame between the first and selcrond serelea-wires was sometimes enhanced by the impettus given by the source8 of ignitiolii a distance of travel1 otf 10 cm. beling baseJy su.fficientl to allow t'his impeltus t.o die amway and a vibratory movemeat often began a t or before.the third screeln-wire. Howelver t,het measuremelnts were consistent enolugh to show t.ha,t otver tlhe cmnparafivelly short period during which the speed olf the flame travelling upwards could be reigarded as oonstant that1 speeid for a,ny mixtnrel of met,hanel and air was not grelater than du.ring t,he uniform molvvemetnt in a horizontal tube; it was someltimm less'. The reaults are inserted in Fig. 1 as approximate po,iiit,s markeld by oirclee. With the rnixtsres near the limits of inflammability, such 8,s those ccmt'aining 5.45 5-55 a.nd 13.4 pesr celnt'. of rmtha.net, the flame6 travelled over a distanwl of 200-300 cm. a.t uniform spwds which could be deltermineld a,ccura,teIly and were distinctly slower t'han thoisel of the1 unif olrm mo'vvemelnt in corresponding mixtures in a horizontal tabel.To confirm the'w reaults two1 selriea of de;telrmina,tions of speeds of flamee in a numbelr of mixturels of melthanel a.nd air were1 made by photographing on a rapidly revolving film the illuminatioln of a quartz window in a brass tubel 5 an. in diameter along which the fla;mea t'ravelleld (1) ho,rizolntally and (2) ve'rtioally upwa,rds (see t,his vol. p. 37). As t'hus melasureid the speeds ovelr the1 who3e range od mixturw welrel from 3 to 5 am. per second less whetn the flames trawlled upwasds than when t,heiy tsave1lle:d horizont,ally . Probably this slight diffelreaoe in the! speelds a,rises frofm t.he differ-ence there is in the shajpe od t,hel flame-front,; wheln travelling horizontally the flame-front has an ellipttical cross-section (see T., 1914 105 2609 Fig.2) whelrela8s when tra.vedling upwards it is cllroular so tha<t iii a t,ube8 04 give'n diamelter t'hhs circumfere,nce of The remsultls were ra,thelr irregula'r. z z" 1832 MASON AND WHEELER THE PROPAGATlON OF t-he flams-front relahive tlo its arela is greatetr with an upward-than with a hosizontally-moving flame and the uoo'ling elffeat of the walls olf the tube is correepondingly grea,teir. Sinm with flames tlrawlling horizontally thel lelngt'h of the1 major axis of the elliptioal flamefront is a t its maximum with mixtlurels nelar tlhel limit's 09 inflammability the maximum diffelreinoe in speed betwen upward-and horizontallly-moving flames in a tubel od given diameter should be found with such mixtures.This is so. Di agramm atio r e p eaeii t.a t i oins of Fra. 2. 2 Distance along the tube measured from the point of ignition. Cm. the comphtel progrem of the flames travellling in a 10 per wlnt. mixture of methane and air from elnd to elnd (240 an.) of the brass tubel with a quartz windoiw are1 giveln in Fig. 2. These diagrams have been reduced in size from tracings taken from the actual photographs which measured 60 x 40 am. and welre olbtained in the manner derscribed in Part I of this research. For the purpose of cornpaxison the flames should be regarded as travelling from right t-o left of the diagram and the photographia film as moving vextically upwards ; aatu-ally the flame was moving vectieally upwards far A , vertically dolwnwards for B, and horizontally forC.The last-named is a traoing of the pholtoigraph repro-duced as Fig. 1 Plate I oln p. 66 olf this volume,. - The rate of revolution olf the film was not the same f o r eaoh selt of photographs, so that direct comparison olf the spwlds of the fla;ntes oannot be made from the diagrams whioh however &ow clearly that the general behaviour of a fiame travelling from tlhe open to the dosed end of ik tube is not dependent on the position of the tube. WB hafve alrelady commented on the irregularity of some of the resulk obtained wheln attempting to1 mewwe the initial speeds of the flames by the smeen-wire method during upward propagation C D [To face 3 ~ ~ 6 1233 Trans FLAME m MIXTURES OF METHANE AND AIR.PART II. 1933 The reason foir the irregularity under such conditions od experiment hluame apparelnt when t.he molvemelnt of the flamee w a photo-graphed. For it was foand that with mixturm containing beltween 9.0 and 11.0 per cent. of melthane the flames were very sansitive t o the efiect olf resonannoel of the tuba and could acquire either of two speleids (but not speelds intelrmeldiata beltweem the two). If rwolnancsel was set up in the usuarl way by the movement of the flame along the tube the speeld of the flame was normal but if by some action on the part of the1 eixperimelntelr the tube was caused t a rmonate at o r very shortly after the momentl of ignition, the melan speed of the flcwnei was 20 cm.per selcolnd slower than normal. It has already been olbselrved and is in fact apparent from the photographs and diagrams thatl the1 incidence of rewnmce retards the general folrward movement od the flame until suoh time rn a wtgain delgres of turbulence is imparted to the mixture. This reltarding effect is well illustrated by the photographs reproduced in the Plate which show the progress of the flame in a 9.7 per mnt. mixture od methane and air in tlhe b r w tube over the distance 25-55 an. from the point of ignition ( A ) when the tube was remnating and ( B ) when it was not. The photographs were taken within a felw minute8 off each othelr on the same film, which was revolving at the1 same speed in each instance. Further infolrma,tiola as to the diffelream in ahara,ak between the two flamm was obtlaineld from photographs sholwing the shape of the flame-front (see Plate).A quartz tlube 5 m. in dimelm, was fixeid v e r t i d l y and mvelreld with black paper except for a slit 2.5 mm. broad a t right a8ngles tlol the axis of the tuba and 25 am. from t<he open end (the lolwer elnd) a t which ignition was &mteii. Flames travelling up the tlube in a mixture containing 8.3 per cent. of methane were photographed as they passed the slit oln a film which also travelled upwards. As each portioa of the flame-frojnt appeared a t the slit it was thus pholtographed on a fresh portion of the film and its shape was relcordeld. The arrange m a t of the quaztz lens used to1 focus the slit on the photograiphia film was suoh that an image onequarter the size of the object was obtained; the speed olf the film was therefore regulated to be as nearly 8,s possible onequarter that of the flames so as tol preeerve their relative propotiims.It will be seen that in C when the t u b wa9 raonating the undulahions olf the flame-froint are well defined aind it is unsymmetrical; in D when the tube was not resonating the flame-front is quite symmebrioal. The important point t o have established is that a phase in the propagat?ion of flame during whioh the speed is uniform is obtained when the flame trs~vels from the open to the c1-d end of a tub 1234 MASON AND WREELER THE PROPAGATION or whetheir the direction of t'ravel is horizoatlal velrt'ioally upwa,rds or vert<ioally downwards and that t,his unifolrrn speed for any given mixture is sloweat during downwasd propagation This means thab it is not permissiblei t o regard the '' uniform movement " (that is tfol say the phase of uniform speed of flame during horizontal propagation) as repremnt'ing " la viteese normals d s propagation de la fla,mme pas cofnductibilit6." I f propagation od flame solefly by conductioa of hea.t olbtains at a,ll our enrperimenh show that it is mast likely to olcmr when the1 flame travells downwards the effect of oonveatioln currelntls being assumed t,heIn t'o be negligible.When travelling dmvnwa,rds in a' glass tube 5 am. in diameter, the flame-front during the period olf uniform speed of prodpagation of flame is with all mixturw olf rmthane and air a steiady olr but gentdy undula.ting disk.There is no suspicion olf turbulence suah as mahm i& appeia,ranoe with the more rapid fla'mes wheln they travel holrizontally in such it tube. Itl seemeld possible therefore, that if convectioa currents doc nolt ma,terially assist the flame in i h propagation downwa,rds a considerable increase in t-ha diameter of t'he t,ub.e do'wn whiclh &he flamea tram1 could be made withoutl tlhelir speeds beling augmented. On trial however u&ng a met'al tube 23 an. in diameter a,nd 7 m. long i t was found thaft with mixtura aontaining mare than 7 and less than 12 per cent. of met'hane the speleds of tqhe flamea welre considerably grelater than in a glass tubme 5 am. in diamekelr a5 the curve in dotted line i n Fig. 1 shows.Wit$ the mixtures containing bet,ween 9 and 11 per cent'. of mstha.ne the1 ratio between the speed of the flame a.nd the diameter o'f the t,ube wa,s nelarly tlhe game a's when the flamels t'ravell horimntaJly (see Fig. 3 T. 1917 11.1 1052). HO~W-ever it may be during the downwasd propagation of flame in mixture6 oontaining lem than 7 olr moire than 1 2 per mnt. of methane it is obvious thelrelf o,re tha.t somet'hing othelr than con-duction of he1a.t is responsible1 f olr tlhel delvdopmelnt of tlhe molrei rapidly moving flamels in the1 tube olf lasger diametelr. The enhanced speeds ON€ t8hhel flamels as they travel downwards in the larger tubel are1 no1 dolubt. duel to turbulelnce of the flamekfront eingendemd by convelation or eddy currelnts. We can t'hhen fully elndorse Schloesing a,nd de MondBsir's st.ate melnt that the1 no1rma.l propaga,tion 0.f flame is ra8rely oibserveld wheln the word " normal" is tlakeln to imply "by wnduct,ion of h a t ." It is highly a.bnorma,l t,ha.tl such a model off helat t,rans-ference should alone1 be operativet during t'hhet prolpaga.tdo;ll of flame. In so far t.here,forel as t,he te,rm '' unifolrm movement" of flame has bwn helld to be1 the1 no,rmad propaga;t,ian of flame by conduction of heat it ought tol be discasded. The1 t,elrm is howewr a usefu FLAME IN MIXTURES OF METHANE AND AIR. PART II. 1235 one and fittingly describes a phase in the propagation of flame ( obtainalble under a variet,y olf conditions which should be specified) t,he identifioatioln and measurement of which is of considerable FIU.3. 0 0.6 1.0 1.5 2.0 C Time. Seconds. 5 value. As a name withouh implying a moldel olf heatl transfelrence during the phenomena itl deearibee there is no1 reason why it should not be retained a8nd it may be conveaielnt to do so. 11. Ignition at Ona End o f a Vertical Tubs Opem at bofth Ends. -Under thew coinditlims the tube in which the' flame travel 1236 MUOPT AND WHEELER THE PROPAGATION OF besom- a ('chimney,'' and t'here is added tot the speed oC a flame travelling up the tube the speed of the draught produced by the chimney. I n addition t'h mechaaical effeat of this draught in promolting tnrbulemm of the! mixtlure proba,b3y aagmeats tshs speed olf the flame. Flame doles not pass downwards if the mixture is ignited a t the t o p of the tube but aolnttinues to1 burn a t the1 mouth until the whole of the mixture has been drawn up.FIQ. 4. Methane per cent. I n Fig 3 are shown timedist<ancei curves for the prolpagatim of flame upwards in several mixture8 oif me\thane and air in a velrtiaal glass tube 5 m. in diaslelteir and 5 m. loag open a t both ends. Them curve8 should ba compared with those obtained wheln the tube was horizontal (Fig. I this voll. p. 42). The clharahr of bokh sets orf ourves is similar and indioatles a gradual and regular amdelratioa of speled as the flme travels from elnd to end of the tuba. This acaderation mmrs with the lowar-limit mixture (5. FLAME IN MIXTURES OE' METHANE AND AIR. PART 111. 1237 per mnt. of methane) when the flame travels upwards a result which displays dearly tlhe chimney e$e& of the tube for t3he speed of the flame in the lowelr-limit mixture is quite uniform when i t travels horizontally.The spwds of the flames a t all stage3 of their propagattion are wnsidmably faster during upward than during horizontal propa-gation for the reasons already given. Fig. 4 records the mean speeds over measureld distaaoes f o r the whole range of inflammable mixtwm ( A ) for the distance 50-100 an. measured from the fir& smmn-wire whiah was 10 an. from the point olf ignition and ( B ) for the distanaer 200-300 cm. A mrve (C) shoiwing the mean s p e d over the distanaa 50-100 cm. during holrizolntal propagatioi in the same tube is given for croimparison. Curve B presents a characteristio novel to thew invatigations on the propagation of flame in mixtures of methane and air, inasmuah as the maximum speed of flame is foiund not L ~ B is usual, with mixtures containing between 9.5 and 10.5 per cent.of methwe but with thoee containing about 12 pelr cent. This result which is of course due t o the chimney &eat of the tube, is probably q u i b adventitioas ; with. the particular diameter and length of tube employed the chimney effect assumed a maxirnum with mixtures colntaining about 12 per mt. of methane perhaps by reason of the ompoeition of the products of combustion of those zllixturm. 111. ZgnitioYn at the Closed E d of a Vertical Tube Open at the olther Ertd.-When the mixture is ignited a t the aloaed end of a tube olpen at the other end the positlion of the tube does not affeot the resulk.The rwulh obtained when the tube is holrizontal have been d e m i b d in Part I. PART III.-THE PROPAGATION OF FLAME IN CURRENTS OF MIXTURES OF METHANE. AND AIR. The object) olf these experiments was (i) ta determine whether the f a a t that the g a m were moving as a ourrent would simply add to the speed of the flame the s p e d of that o u r r a t or whether an effaat of turbulence would be introduced and (ii) tol s w if a maller quantity of methane than that which f m s the lower-limit mixture (5.4 per cent. for horizmtal or upward propagation of flame) in al sttill atmoaphere woluld be oapable of aonveying flame from one place ta another if the mixture itedf were moving. (i) The experiments with a vertiaal tuber open at1 bath ends, deaaribed in Part I1 of this rewarah had shown that the speed o 1238 MASON AND WHEELER THE PROPAGATION OF the current induced in such a tube acting as a chimney was added to the speed of a flame travelling upwards in it- and it was assumed thah turbulence caussd by the current further increased the speed of thet flame though no proof of this oould be given sinm the velocity of tthe current was indeterminate.A tube 2.5 om. in diameter and 220 can. long was fixed hori-zontally and conneloteid by rubber tubing ta it large metal gas-holder al short brass tube with a disk of metal gaum at either end being intelrposed to pre,vent flame travelling baok into the holder. Strelams of a mixture of methane and air could be caused to pass at differentl speelds along the tube both ends of whiah were open, by varying the counter-weight8 on the bell of the holder and open-ing tlhe wide-bore meitlal tap thereloa to ita full extent The mixtures+ were ignited electrioally at a point 26 am.from the colnnexion la the brass t u b and the speeds of the flames were measured by means of smeen-wirea the firstr soreen-wire being 50 am. from tlhe point of ignition. A mixture oontaining 6-35 per cent<. of methane was uwd as being one in which the speteld of flame in a still atmosphere under the conditions of the1 expelrimeats would not be excessive in corn-parison with thatt of the currents that could be p r o d u d . The rwults giving the mean sp*lds of the flames over 100 cm. were as follow : . Speed of current.Speed of flame. Difference. Cm. per second. Cm. per second. Cm. per second. Nil 287 287 23 682 559 43 952 909 75 1527 3462 Itl is thus apparent tlhat the major effeut of the ourrent is that of turbulence. The fact that suah a cmparatively gentle mover ment o0f the mixture as is represented by a speed of 23 m. per s e d sholuld nearly double the speed of tlhe flame is very striking and emphasim the important part that oonvwtioln ourrents must play in the transmission of flame. (ii) When a homogelne,ous mixture of methane and air aantaining b&ween 4 and 5 per cent. of methane is caused to1 travel1 slowly Q V ~ T a small flame such its that of a. candle flares of flame off burning methane are produced which follow the direc-tion of the current# and may eixtend for a oolnsiderable distance-a distance greater than the length of the “cap” p r o d u d by a similar mixture around such a flame in a still atmosphere.These flares of burning methane may eveln bwome detaahed from the sourcw of heah around which they form and float away for a short, distance in the stream of mixture) (T. 1914 105 2594) FLAME IN MIXTURES OF METHANE AND AIR. PART 1x1. 1239 We had also obtlained evidence that if a sufficient degree of turbulence is prolduced in a closed vessel by means of a rapidly revodving fan in a mixture od methane and air colntaining not less than 5.0 per cent. of methane flame will travel throughout the mixture following the current induced by the fan (T. 1914 105, 2595). Since the elffelct of causing SL mixture af methane and air ta travel along a tube at a fairly slow speed is tol impart turbulence ta the mixtum and sinm tghe degree of turbulence thus imparted is sufiioient greatly to increase the speed of propagation of flame in (that is t o say the1 rats of combustion of) the mixture it semned probable that tlhe loiwer limit of inflammability which is partly dependent on t<he rate of combustiion of the gas might be 10% with a slow stream of methane and air than with a quiescent mixture, just as the lolwelr limit is less for upward propagation of flame (5.4 per cent.) wheln the1 combustioln is aided by conveotion current8 than for downward (6.0 per cent.).A horizontlal glass t l u h 5 cm. in diameter and 320 cm. long was used for the elxperimelnts which were conducted in a similar manner to thow already described.The point of ignition was 17 am. from the brass t’ube ooanelotYing the1 glass tubel to1 the gas-holdelr and the first smwn-wire was 35 cm. from the point! of ignition. The elxperimenta were as fallow : (a) CH, 5.15 per cent.-Speled of current 65 om. pelr second. A flame aboutl 2.5 am. long tlravelled the whole1 length of the t u b moupying the upper half af it. The speed of the flame inmemeld gradually over the last 200 am. from 75 ta 85 m. per s e n d . ( 6 ) CH, 5.10 per cemt.-Spmd of current 65 am. per second. Observations as in previous experiment the speed of the flame being about 70 om. per seioond throughout. (c) CH, 5-05 per cent.-Spd of current 65 m. per second. A more powerful secondary disoharge had to be used to ignite this mixture.The flame produced was only 1.2 m. long but it travelled the whole length of the tqube at1 a speed of 65 om. per second-that is to say at the speed of the mrrent of mixture. (d) CH, 5.02 per cent.-The intense flaming ” discharge obtained when a Wehnelt ekmtrolytid interrupter is used in the primary cimuit of an induction coil had ~ Q I be used to ignite this mixture. Whein ignited a small flame tlravelled throughout the lengt-h od the tubel a t the sped od the current of mixture namely, 65 50 and 35 cm. per second in successive trials. With a faster s p d of curreat (about 80 cm. per second) or a slower s p d (abut 30 am. per seoond) the flame1 travelled from 200 t o 250 cm, and then did orutr 1940 MUON AND WHEELER THE PROPAGATION OF FLAME1 ETC.(e) CH, 5.00 per cemt.-With no speed of aurrent would flame travel mare than about 50 caa. from tlhe point of ignition. This was so whether the Wehnellt disaharge used to ignite the mixture WM maintained or passed mo~melntlarily, Experimenb (d) and (e) were repeated many times with the same remits a qixture containing 5-02 oc 5.03 per celnt. of methane dolea not enable flame to brave1 in it when it is a6 rest,, but when it i5 moving i~19 a crumelntl along a hbe flame travels in it at tlhe spwd of the current. A mixture containing 5.00 per cent. od methane oould not be a a w d to propagate flame under the wnditions of the exp&nente. Mixtures containing 4.95 4-90 cx 4.85 per m t . of methane gave flaresl of flame extending a few am. only along the tube. The scale on which these expmimelnlis were conduded is perhaps wasdy suffioient tor warrant the conclusiozl that flame would be aarried indeifinitely in a homogeneous mixture contlaining 5-02 pelr wnk. of methane along the roadway of a mine folr example so l a g as a aurrelnt of from 35 to 65 am. per waond was maintained. The general charadm 09 the expelriments howelver pointed to a sharp distindion between a mixture1 (5.00 per cent. methane) obviously inaapable 04 maintaining a flame apart from tihe source of heat which oiriginateld it and one (5.02 pelr clelnt. methane) in which flame was maintaineld afteir the1 ignitling souroe had been withdrawn. The flame would travel alolng tihe rood of the roadway. The waaluding parts of $,his research will dsal gelnerally with the e$eate of turbulelnoel oln the propagation olf flame in mixtura of methane and air aa prolducrd by tlhe passage of the mixtures as mrrenb almg a tube or gdlery and as induced loloally by the presence of ratridions in the gallery. Horn OFP~UB EXPERIMENTAL STATION, ESKMTALS CUMBERLAND. [Received August Qlst 1930.