1010
INDUSTRIAL AND ENGINEERING CHEMISTRY LITERATURE CITED
ddams, E . W., and Adkins, H., J . Am. C h e n ~ .Soe., 47, 1358 (1925).
Bell, E. R., Raley, J. H., Rust, F. F.,Seubold, F.H.. Jr., and Vaughan, K.E., Discussions Faradav Snc., 1951, No. 10, 242. Burke, 0 . W., Starr. C. E., and Tueniniler. F. D., “Light Hydrocarbon .Inalysis.” pp. 134, 223-31. Yew Tork, Reinhold Publishing Corp.. 1981. Chernyak, S . Ya., and Shtern. T-. Ya., Doklady A k a d . S a u k S.S.S.R., i 8 , 9 1 (1951). Cook, G. A. (to Linde Air Pioducts Co.), U.5. Patent 2,416,156 (Feb. 18. 1947). Cobk, G. i. (to Union Carbide & Carbon Carp.), I b i d . , 2,614,907 (Oct. 21, 1952). Cooper, D. O., 8.31.thesis, llassachusetts Institute of Tcchnology, 1962. Dickey, F. H., et al., IND.ENG.CHXM., 41, 1673 (1949). Duke, F. R., IXD.ESG. C m s f . , A 1 y . k ~ .ED.,17, 572 (1948!. Dunics, B. L., Perrin. D. D., and Style, D. IT7.. G., Trans. Faraday Soc., 47,1210 (1951). Evans, M. G., Hush, S . S., and r r i , S., Quart. Rev. (London), 6,186 (1952). Fischer, E., and Giebe, G . , Ber., 30, 305 (1897). Geib, K. H., and Harteck, P., 2. p h y s i k . Chem., AIi0, 1 (1934). Harris, C . R. (to E. I. du Pont de Semours 8; Co.), U. S. I’atent’2,533,581(Dee. 12, 1950). Harris, E. J., Trans. Faladall Soc., 44,764 (1948). Harris, E. J., and Egerton, A. C.. Chem. Rers., 21, 287 (1937). Hinshelwood, C. N.,“The Kinetics of Chemical Change,” p. 97, London, Oxford University Press, 1947. Jost, W., tr. by Croft, H. O., “Explosion and Combustion Processes in Gases,” pp. 317, 326, New Tork, RIcGraw-Hill Book Co., 1946. Kahler, E. J.,et al., IKD.Ex.CHEM.,43, 2777 (1951). Kooijman, P. L., Rec. tias. chin.,66, 5 , 217 (1947). Kooijman, P. L., and Ghijsen, W. L., Ibid., 66, 205 (1947). Lacomble, A. E. (to Shell Development Co.), U. 9. Patent 2,376,257 (May 15, 1945).
Vol. 46, No. 5
Lewis, B., and \‘on Elbe, G., “Combustion, Flames, and Ex-. plosion of Gases,” pp. 28, 30. 101, 119, 121, 130, New York, Academic Press, Inc., 1961. AIcDonald, G. E., and Schalla, R. L., Satl. Advisory Comm. Aeronaut,., NACA RM E52G17 (Aug. 28, 1952) hlclane, C. K., J . Chern. Phus., 17,379 (1Y49). and Thornes, L. S.,J . Chem. Soc., 1937, 1656. Newitt, D. M., Overhoff, J. (to N. V. de Bataafsche Petroleum Riaatschappij), Dutch Patent 61.465 (dug. 16, 1948). Paneth, F., and Lautsch, TTr,, Ber., 64, 2708 (1931). Pease, R. N., J . A m . Chem. Soc., 51, 1839 (1929). Ibid.,57,229G (1935). Pease, R. IT,, and LIunro. W. P., Ibid., 56, 2034 (1934). Rice, F. O., and Evering, B. L., Ibid.. 55,3898 (1933). cKee, F. S., and Quagliano, Rodebush, W.H., Iieizer, C . R J. V., J . Am. Chem. Soc., 69, 538 (1947). Satterfield, C. S., and Case, L. C., ISD.EXG.CHEM.,in press. Shtern. V. Ya, and Antonovski, P. L., Doklady &ad. A97acde S.S.S.R., 78,303 (1951). Steacie, E.W. R., ”Atomic and Free Radical Reactions,” A. C.S. Monograph 102, pp. 284. 318, 339-44, 369, New York, Reinhold Publishing Corp., 1946. Stein, T., S.M. thesis, Massachusetts Institute of Technology, 1952. Sewarc. h i . , Discussions Faraday Soc., S o . 10, 143 (1961). Walker, 3. F., “Formaldehyde,” 2nd ed., A.C.S. Monograph 120, New Tork. Reinhold Publishing Corp., 1953. Walsh, A. D., J . Chem. Soc., 1948,331.
RECEIVED for review April 27, 1953. ACCEPTEDJanuary 29, 1954. IIaterial supplementary t o this article has been deposited as Document No. 4198 with tlie AD1 duxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured by citing the document number and by remitting $1.25 for photoprints or $1.25 for 35-mm1.microfilm. Advance payment is required. Make checks or money orders payable to Chief, Photoduplication Service, 1.ibrary of Congress.
Pressure Limits of Flame Propagation Qf Propane-Air Mixtures INFLUENCE OF WALL QUENCHING FRANK E. BELLES AND DOROTHY AI. SIMOX Sational Advisory Committee f o r Aeronautics, Cleveland, Ohio
ROBERT C. WEAST Case InstitzLte of Technology, Cleveland, Ohio
T
HE accelerated pace of combustion research in recent years
has produced a large amount of data on the so-called fundamental properties of combustion. The interpretation of these results, however, has been made more difficult by their very quantity and by the fact that, until recently (fO), there has been little understanding of tlie possible relat,ions of the combustion phenomena to one another. This difficulty is quite apart from the basic one-that, owing to lack of knowledge of the most intimate, molecular-scale processes of combustion, no universally applicable theory of combustion has as yet appeared. It seemed t o the authors t,hat one way in vhich combustion knowledge could be partly systematized x-as through a more careful study of the pressure limits of flame propagation. It has long been knoxyn t h a t there are concentration limits of flammability for the process of combustion (9)-that is, the mixture of fuel and oxidant may contain too much (rich limit) or too little (lean limit’) fuel to burn. Many workers have investigated the concentration limits at atmospheric pressure (2); a few have determined the
effect of reduced pressure on these limits and have found that the concentration range of flammability narrows as the pressure is reduced, until at some critical pressure the rich and lean limits converge (3, 4, I f , 12). The reeult is the familiar U-shapcd curve of pressure limit against fuel concentration? in which the uprights of the U correspond to the concentration limits of flammability. These pressure limits have been measured and reported as a fundamental property of combustion, b u t scattered references in the literature, summarized by Friedman and Johnston (Or), present some evidence t h a t the limit,s are possibly governed by quenching effects. From this point of view, then, t,he pressure limit is not a property of the combustible mixturc alone, b u t is due t o effects of t,he confining walls on the propagation of the flame. The aim of the work described herein, therefore, was to systematize part of the combustion data by making a quantitntivc? connection between preesure limits and wall quenching. Tile method of at,tack \\-as to measure the pressure limits of flame
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1954
propagation for propane-air mixtures in flame tubes of several different diameters. A rather limited study of this type has recently been carried out (8), b u t it was subject t o difficulties in connection with ignition, The technique of the present work avoided these difficulties, and the results are believed to be more precise. The investigation was limited to propane because of its ease of handling, and because its combustion properties are representative of saturated hydrocarbons in general.
1011
cision absolute manometcr. Dry air, with a water content less than 0.03% by volume, was then admitted t o bring the carboy t o atmospheric pressure. Thorough mixing of the propane and air was assured by stirring with motor-driven vanes sealed into the carboy through a metal bellows.
APPARATUS AND PROCEDURE
APPARATUS.The apparatus for the determination of the pressure limits of flame propagation is sketched in Figure 1. It comprised a test section, a fuel mixing and metering system, and an evacuation system. The test section consisted of a series of interchangeable glass flame tubes combining an ignition section and a propagation aection smoothly sealed together t o form a one-piece unit. The propagation sections were 120 cm. long and had inside diameters of 6.6, 4.7, 3.8 2.8, 2.2, and 1.6 cm. Each 120-cm. tube was sealed t o an ignition section 9.0 cm. in diameter and 25 cm. long, with a 65/40 spherical ground-glass inner joint sealed t o the lower end of the section. The ground joints of the ignition sections fitted into a n outer joint on the stem of a 35-mm.-bore stopcock, which provided a connection between the flame tube and a 47-liter carboy. 20 2.0
I
I
3.0
I
3.5
II
I
I
4.0 4.5 5.0 PROPANE IN AIR, percent by volume
Figure 2.
2.5
Effect of Tube Diameter on Pressure Limits of Flame Propagation
PR
F U E L MIXING AND STORAGE
his
CHAMBER
Figure 1. Apparatus for Determining Pressure Limits
The entire pressure-limit apparatus was made of glass in order t o eliminate leaks and t o prevent absorption of propane by rubber connections. The evacuation system consisted of a mercury diffusion pump backed by a mechanical pump. A cold trap waa provided to freeze water vapor out of the evacuated gases. With this arrangement, it was possible t o evacuate the entire system t o a pressure of 1 micron of mercury. The ignition system was composed of a pair of spark electrodes entering the ignition section from the stem of the stopcock below, and a power supply capable of delivering a rapid capacitance spark. A capacitance spark was chosen as the ignition source in order t o eliminate the convection currents arising from heated coils or from the prolonged passing of an inductance spark, as i t was believed that these currents would tend t o disturb the flame as it passed into the propagation section of the flame tube. The purpose of the large-diameter ignition section was to avoid the effects of tube diameter on the ignition of the mixtures, and t o provide sufficient volume for dissipation of any excess ignition energy. Thus, it was not necessary t o go through the tedious procedure of optimizing the spark energy and spark gap for each new flame tube and for each change in mixture composition. All the experiments were run a t one setting of the spark energy and gaptp. The 47-liter carboy acted as a plenum t o absorb the pressure rise due t o combustion in the flame tube. Thus, it also served t o minimize pressure disturbances on the propagating flame front. PREPARATION OF PROPANE-AIR MIXTURES. Propane-air mixtures were prepared and stored in a 47-liter carboy (see Figure 1). The carboy was evacuated t o 0.3 mm. of mercury or less, as read on the McLeod gage, and propane of 99 mole % minimum purity was admitted until its pressure corresponded t o the desired percentage of atmospheric pressure (assuming ideal gas behavior). The propane pressure was read with a cathetometer on the pre-
After the pressure limit of a mixture prepared in this manner had been determined, the pressure within the storage carboy was. of course, less than atmospheric. Therefore, leaner mixtures could be prepared by subsequent dilutions with room air. These dilutions were measured on the precision manometer. This procedure gave satisfactory results, as shown by the fact t h a t the pressure limits of corresponding freshly prepared mixtures and those made by dilution were the same, within the experimental error of the limit determinations.
For each run, the flame tube and all of the sysPROCEDURE. tem up t o the mixture-storage carboy were evacuated t o a pressure of less than 0.1 mm. of mercury. The pumping system was then closed off and the fuel-air mixture was admitted t o the flame tube t o the desired test pressure. S e x t , the pressure in the plenum was equalized t o the mixture pressure b y adding or pumping out room air. The large stopcock between the flame tube and the plenum was slowly opened and the mixture was ignited. Depending upon the test pressure, the flame would either propagate upward the entire length of the flame tube or be extinguished a t its mouth-that is, a t the place a h e r e the ignition section joined the propagation section. The pressure limits for most of the propane-air mixtures were established t o within il mm. of mercury by repeated trials a t various pressures-that is, two pressures were found t h a t differed by 2 mm., the higher of which permitted flame propagation throughout the length of the flame tube, whereas the lower caused flame extinction at the tube mouth, and the limit recorded was the average of the two pressures. In mixtures the compositions of m-hich were only slightly richer than the lean concentration limit of flammability, flames were occasionally observed that were extinguished somewhere between the mouth and the upper end of the narrow tube. For the sake of consistency, however, the pressure limit in these cases n'as taken as the pressure a t which extinction occurred at the mouth of the tube I n addition, erratic flame behavior was encountered in some mixtures richer than 5.0% in propane; these flames were rough and oscillatory in their propagation, and the pressure limits were not so well reproducible in this rich region. D a t a are therefore reported from the lean limit of flammability u p t o 5.0% propane, although limits were measured throughout the flammable range.
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
1012
The esperimeiit~stheniwlvcs iiiatle it clear that the mouth of the narrow tube a-as acting as a czritical diameter. For the apparatus used, the critical diameter was therefore defined as the minimum diameter of a circular opening through which a flame n-ould propagate in a mixture o€ a given composition, a t a givrn pressure. GENEK.AL TRESDS
Curves of prrssui~climit plotted against volume per cent propane in air for tubes 6.6, 4.T, 3.8, 2.8, 2.2, and 1.6 em. in inside diameter are presented in Figure 2. For the reasons described above, the dat,a a r e not, reported for mixtures containing more than 5.0’% propane. At any given concentration, the minimum pressure for propagation is increased as tube diameter is decreased; and, a t a given pressure, the concentration range of flammability narrows as the tubes become smaller. Previous work has shown t h a t the concentration limits of flammability a t at,mospheric pressure are nearly independent of the diameter of the tube used in the determination, provided the tube is large, and that the limits are usually unaffected by the first reduction in pressure bclow 1 atmosphere ( 2 ) . The natural extensions of the Faired curves of Figure 2 show that the present data are consistent ivith t,hese observations: The curves for the five largest tubes merge a t a propanp concentration close t o the reported lean conc~mtmtionlimit a t atmospheric prcssurc, 2.12% (IO). IDENTIFICATION OF I’RESSUllE LIMITS WITH W A L L QUENCHING
The relation of critical diameter for flame propagation and pressure limit is shown in Figure 3, a log-log plot. Separate straight lines ir-ere obtained for each of the four concentrations: 2.50, 3.00, 3.50, and 4.03 (stoichiometric) volume % propane in air. The slope of this log-log dependence shows that, the crit,ical diameter is proportional to the pressure raised to a ncgativc rxponent, the value of which is equal t,o the slope of t h r mperimental line. The slopes of the lineP of Figurr 3 were calculated by the method of least squares. The pressure dependelice of the mitical diameter for propagation decreases with decreasing propane concentration. The range of exponents on prrssure is -0.97 for the stoichiometric mixture (4.03yG)t o -0.7G for the 2.50% misturt,. P R O P A N E IN AIR ( P E R C E N T BY
E 100
-0.97 -.92
cr‘ 6-
w
t
: 4a -
-.a5
3.00
0
-
-.76
0
2 u
2-
u
I-
t a
t
.02
.04 .06 .08 .I
\
.2
, , .6 .0 I , ,
.4
PRESSURE, otrn.
Figure 3.
miniinum slit widths for fliiph back, :is I ured by Ii’rieclman :ind Johnston, and the critical tubc diameters determinrd in the present, investigation. Log-log plots of minimum slit nidth against presdurt: tor t lie data of Friedman and Johnstoil :trc given in Figure 1 for prolxrne coiieeiitrations of 3.00, 3.50, and 4.03 (stoichiometric) % i i y volume. The slopes o l t hr Ptraiyht line3 obtained werv also (8x1ciilated by the met>hod of h r t squares, and agree well w i t h t h e c PROPANE IN AIR (
SLOPE
% by vol.)
I.2..
.IO .2 ’ .4 .6 l . 2 0 PRESSURE, atrn.
’203.04 .06
Figure 4.. Kelatioii of Minimum Slit Width to Pressure (5) slopes calculated for the present data.
In addition, the slop(‘ o f the t x o sets of lines (Figuies 3 and 4) vary in the same mttnncm n ilh propane concentrations. The experimental pressure drpcntiriicies of critical diameter and minimum .lit width are comp,ilcti in Tablc I.
Xcpat1r.e Power of
-
l’mpane in Air, Yoi. cc
4 03 3 50 3 00 2 :o
(‘ritical diameter
Pressure Minimum slit width 0.88 0.88
0.83
S LOPE
VOLUME)
0-
Vol. 46, No. 5
Relation of Critical Diameter to Presslire
The data presented in Figure 2 give the pressure-concentration boundary for flame passage through a tube, with tube diameter as a parameter. The same type of information was obtained in a completely different manner by Friedman and Johnston (6), who measured propane-air quenching distances by determining the narrowest slit through which a Bunsen flame will flash back when mixture flow is reduced. Thus, if quenching is a major factor in controlling pressure limits, there should he a relation between
The critical tube diameter in centimeters for flame propagation at a given concentration and pressure is riot numerically the same as the minimum slit width for flash back of a Bunsen flame. Ilow?ver, if critical diameter is governed by a quenching process, as is minimum slit width for flash back, the two distance6 iiiight be expected t o be related by a constant factor which depends on thr geometry of the apparatus. The ratios of critical diameters arid minimum slit widths for three propauc-air mixtures are given in Table 11. The minimum slit widths were taken from the data of Friedman and Johnston as given in Figure 4; extrapolations \Tere made for mixture pressures below 0.0832 atmosphcrc. Inspection of the data in Table I1 shows that the ratio of critical diameter t o minimum slit width may vary slightly r i t h propanc concentration; however, t,he over-all average is 1.43, &
