INDUSTRIAL AND ENGINEERING CHEMISTRY
2558
T o transfer this quantity of heat by convection would require a film coefficient of a t least 5 2 [B.t.u./(hour) (sq. foot) ( " F.)]. If the flame and shell diameters are increased to 8 inches, the emissivity of the flame will be approximately 0.68 (Equation 3al. Then p =
0.68 8 - (11.4 X 10') 0.50 6
~
=
Over the observed range of operating conditions, measurablc radiant energy was observed only in the primary zone; in this region the greater part of the total energy transfer to the liner may consist of radiation from the flame. Flame emissivities of 0.09 to 0.79 were observed.
18.2 X lo4 B.t,.u,i(hour) (foot) ACKNOWLEDGMENT
Since the flame volume has increased as
(8)'
=
1.78
The experimental work reported here was complcted while t h c author xyas associat,ed with the Lewis Laboratory of the Sational .4dvisory Committee for Aeronautics.
the radiant heat loss per unit volume is 907, of the former value. S U V M A R Y OF R E S U L T S
The folloiiing results were obtained from an investigation of the effects of combustor operating variables on the thermal radiation from the flame of a turbojet combustor. The intensity of radiation from the flame increased rapidly with an increase in combustor inlet pressure and was affected to a lesser degree by variations in fuel-air ratio and air mass flow. The total radiation of the luminous flames (containing incandescent soot particles) was much grcater (4 to 21 times) than the niolccular nonluminouq radiation due to carbon dioxide and water vapor.
Natura
Vol. 46, No. 12
as
LITERATURE CITED
(1) Foot,e, P. D., Fairchild, C. O., a n d Harrison, T. R., Natl. I3ur. Standards, T e c h . Paper 170 (Feb. 16, 1 9 2 1 ) . ( 2 ) Hottel, H. C., and Broughton, F. P., IKD, E s c . CHEM.. AS.^,. ED.,4, 1 W - 7 5 (1932). ( 3 ) Humble, L. V., Lowderniilk, W.H., and Desmon. L. G.. S a t l . Advisory Cornm. Aeronaut., R e p t . 1020 (1951). ( 4 ) Kaplan, L. D . , J . M e t e o r o l . , 9, 1-12 (1952).
W.H., "Heat Transmission," 2nd ed., S e l r Y o r k , McGraw-Hill Book Co.. 1.942. (6) Mtttossi, Frank, and Rauacher, Emma, 2. P / I ~ .125, \ . , S o s . 7-10, 418-22 (1949). (7) Uyehara, 0. h.,and as?ociates, Tmns. A m . S o c . X ~ c h E. t i g i s . , (5) McAdams.
68, 17-30 (1946). 1 2 r . c e i v t ~ior review- J a n u a r y 2 9 , 19:4.
A c c r ~ . i i r September , 17, 1P531.
osions a
DETONATION VELOCITIES AND PRESSURES MELVIN GERSTEIN, EDWARD R . CARLSON, ~ N D FRANCIS U. HILL iVational Adcisory Committee for Aeronautics, Lewis F l i g h t Propulsion Laborutory, Clezeland, Ohio
XP1,OSIOhX in a given system ma?- lead to two different combustion processes: Under some conditions, the explosion results in a flame or combustion wave which mag travel at several hundred feet per second; under other conditions, a detonation wave may result which travels a t several thousand feet per second ( 2 ) . Higher pressures and greater destructiveness are associated with the detonation wave. .llthough many systems that may contain unburned combustible mixtures-Le., exhaust ducts and mixture feed pipes-are designed to withstand the sloiver and lower pressure rise of a combust,ion wave, these systems are often not designed t o ITithstand the greater pressures and deetructiveness of a detonation vave. I t is therefore desirable to knoJT the conditions under which an explosion may develop t,he characterist,ics of a detonation. Much work has been done on the occurrence of detonations in fuel-oxygen mixtures and in some fuel-air mixtures a t atmospheric or elevated pressures ( 2 ) ,but little work has been reported 011 the possibilities of detonation in hydrocarbon-air mixtures a t reduced pressures. This report presents the results of an investigation a t the SXCA Lewis Laboratory to determine whether a stoichiometric natural gas-air mixture a t pressures from 0.4 to 0.2 atmosphere may give rise to explosions with velocities and pressures characteristic of detonation. Natural gas mas chosen as the fuel because of its wide use and availability. I n addition, the relat,ively low flame velocity of methane ( I ) , a major constituent of natural gaE, suggested that it might be less likely t o form detonable mixtures, so that the existence of a detonation hazard in natural gas-air mixtures might imply the existence of such a hazard in other hydrocarbon-air mixtures. The experiments \Yere performed in a pipe 2 feet in diameter and approximately 300 feet
long. The great length was used to ensure suEcient distancc: for build-up of the detonat,ion, and the large diameter was U P C t o minimize wall effects a t the low pressures. The velocity arid pressure of the explosion were measured to determine the nature of the wave, but no attempt' was made to obtain precise research data. Additional experiments were performed to determine thquantity of water required t o extinguish the explo3ion.
~
EXPERX4IEXThL
While t,he velocity and pressure of a fully developed detountion are relatively insensitive to the apparatus and conditions oi the test, the likelihood that detonation with result from a flame is very dependent on the experimental variables ( 2 ) . For this reason, a rakher complete description is given of those parts of the experiment which could influence the occurrence of a detonation. PIPISGSYSTELI.A photograph of the pipe used in this study is shown in Figure 1, and a diagram showing ewential feat>uresof the apparatus in Figure 2 . The pipe consisted of sections of 3i8-inch seamless and '/l-inch spiral-weld pipc 2 feet in diarn welded together to form the desired length. The longest run of pipe was 305 feet, followed by a %-foot run a t right. angles. A. tee was used to change the direction of trawl, so that an aluminum rupture disk could be placed a t the end of the long run. -4 neoprene rupture disk closed the inlet of the 305-foot section. The 25-foot section was followed by another tee and a section leading to a large plenum. This tee held an aluminum rupture disk, and the plenum shown in Figure 3 contained two ncoprenc rupture disks. The short length of pipe leading to the plenum, and the plenum itself, which was 11 feet long and 7 feet
December 1954
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2559
Figure 1. Three-Hundred-Foot Pipe Used in Detonation Study
in diameter, contained water spray bars. The water was added t o prevent the explosion from entering the exhauster located in the building behind the plenum in Figure 3. FUEL A N D AIR SYSTEM.B sketch of the upstream end of the pipe, where fuel and air were admitted, is shown in Figure 4. Air entered through a vertical section of pipe containing an orifice; the orifice opening was chosen t o obtain critical flow under the conditions of the tests. As the external pressure was always atmospheric, the mass flow was determined by the size of the orifice. The pressure within the pipe was set by a throttling valve ahead of the exhauster. Fuel entered through the manifold shown in Figure 4, and the fuel flow was controlled by adjusting the feed pressure. A near-stoichiometric mixture was maintained for all the tests. T h e natural gas consisted of approximately 90 mole % methane, 8 mole % ethane, and 2 mole % nitrogen by volume. I n a typical run, the air flow and pressure were adjusted, fuel was admitted t o the system, and, after a 1-minute delay, the mixture was ignited by lighting a magnesium flare 10 feet from the end of the pipe. The igniting flare, Thich consisted of an illuminant charge containing magnesium, was 8 inches in size and burned for 45 seconds. WATERSYSTEM.T o study the nater flows necessary to extinguish the explosion, spray rings were located around the pipe approximately 50 feet from the ignition source (Figure 5 ) . I n the first tests a single ring containing low-pressure swirl-type nozzles was used. I n later tests two rings were used, separated first by 1 foot and later by 5 feet. INSTRUMENTATIOS. Detonation velocity was determined in several ways. The simplest method consisted of photographing jets of flame issuing from 1-inch holes drilled in the pipe a t various locations, as shown in Figure 2. The holes were capped by soft rubber pads, which were held in place by the difference in pressure between the outside atmosphere and the partial vacuum within the pipe. The pads were
blown off as the explosion wave passed, and a small jet of flame could be observed and recorded on a motion-picture film. From timing marks on the film and the distance between the openings, average explosion velocities for the region between the jets of flame could be calculated. Additional velocity measurements were made with ionization gaps located a t 20-foot intervals. The impulse formed by completing the circuit across the gap was recorded on an oscillograph. From the time between impulses, average explosion velocities between the gaps could be determined. Additional velocity data were also obtained from the pressuke records. EXPLOSION DISK
0
*
IGNITION
L
l
l
,
*
FLAME HOLES PRESSURE PICKUPS WATER SPRAYS
I l l l l l l l l l l l l l l l l l l
50
100
150
200
I l l
I 1
250
I l l
300
LENGTH, f t
Figure 2.
Diagram of Long R u n of Pipe Showing Linear Location of Test Elements
Static pressure during the passage of the explosion wave was estimated in two ways. Catenary diaphragm pressure pickups, two-arm, strain-gage type (Control Engineering Corp. ), were located in the wall of the pipe a t three positions, as shown in Figure 2. The current from the transducer was recorded on an oscillograph. Calibration lines were placed on the oscillograph film before and after each run. The diaphragms had a natural frequency of 40,000 cycles per second. Timing marks on the film made it possible to determine the time elapsed between pressure pulses from the three elements Additional information
2560
INDUSTRIAL AND ENGINEERING CHEMISTRY
concerning the explosion pressure was obtained by the use of bursting disks made from materials of predetermined bursting pressure
Vol. 46, No. 12
The method of calculating the detonation properties was taken from Jost and Lewis and von Elbe (9,3). A temperature T ?is assumed and Equation 3 is solved for v1/v2 (3)
THEORETICAL DETONATION PROPERTIES
I n order to determine whether the explosion had the characteristics of a detonation, a comparison was made between the measured explosion velocity and pressure and the theoretical properties calculated for a detonation wave a t the test conditions.
Figure 3.
+ 0.08CzHs + 2.0802 + (7.83 + 0 02)Kz 1.06Coz + 2 OlHzO + 7 85Sq
C , ( T , - TI) - AE - R
2 2 ,
-; 2 ( ) 1
+ T I 3) = 0 V1
(4)
Exhaust System
The folloning equation R as used in the calculation t o ieprewit the chemical reaction; dissociation XTas neglected, eo the result gives only an upper limit to the detonation pressure and velocity
O.9CHd
where L equals molar volume, cc. per mole. This value of V I / V , is used in Equation 4
+
(1)
For this reaction n,/n, = 1.004 = 1
to obtain a second approximation of 7'2. The procedu 2 is repented until 1'2 is determined to sufficient accuracy. A value of 3200" K. was found satisfactory for an initial temperature of 300" E(. (v,/v, = 1.74). The high final temperature indicates that considerable dissociation would occur, but the experimental accuracy did not warrant the more extensive calculations required t o include dissociation. The static-pressure ratio is calculated from the equation
where
n = moles of gas
1 = initial condition of unburned gas 2 = final condition of buined gas
The heat of reaction based on heats of combustion all gaseous products is
(4)to
give
AH = 18,260 calories pel mole = A E y n-here AH = enthalpy change in chemical reaction LE-, = internal energy change in chemical reaction
The detonation velocity is calculated from the equation
D = % d Z 02
= 1 86 X 105 cm. per second
= 6000 feet per second
The impact or total pressure is given by The energy content of the product molecules was taken froni was calculated from the equation Lewis and von Elbe (3).
e%
RESCLTS AND DISCUSSION
and y, was calculated by assuniing
c, - c, = R where -&E = ET - E", C , = mean heat capacity a t constant volunie of ploclucts, calories per mole-degree C , = mean heat capacity a t constant pressure of products, calories per mole-degree y = specific heat ratio, C,/C, R = gas constant 2' = absolute temperature
-
Detonation occurred in all runs in which a successful ignition was made and no extinguishing system was used. The spatial velocities ranged from 1000 t o 2000 feet per second about 100 feet from the ignition source to 5600 to 7200 feet per second about 300 feet from the ignition source. Velocities of about 6000 feet per second were also observed in the %foot section of pipe folloning the first bend. Bursting of the diaphragm and turning of the flow appeared to have no effect on the progress of the detonation. Some typical velocity patterns are shown in Figure 6. Several of the curves suggest that the propagation velocity passes through a maximum. Although this was originally believed to
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1954
greater than the theoretical. The calibration was made by static loading of the pickup with suitable weights. The detonation wave is essentially a sharp pulse and the sudden pressure rise may have induced vibrations in the pickup, the diaphragm oscillating about the actual pressure. The traces of Figure 7 suggest such behavior. Rupture disks made of material t h a t burst a t 245 pounds per square inch absolute in static tests were also ruptured during the tests. As these disks are made of thin metal, they may a1so be set into vibration and thus fail. These results indicate that some materials, tested by static loading, may rupture, although their theoreticai bursting pressure is well above the theoretical steady-state detonation pressure. If their failure is indeed due t o vibration, more massive materials of the same bursting strengths may not rupture. However, this fact was not determined.
__ 12 GATE VALVES
VALVE SHUT-OF NATURAL CAS
PLENUM CHAMBER
Figure 4.
Fuel-Air System
be the result of the experimental error in the fairly crude oxperiment, such maxima have been reported in other more precise investigations (6). The final velocities obtained near the end of the straight run of pipe are given in Table I. The average value of 6300 feet per second ie in reasonable agreement with the theoretical velocity of 600 feet per second.
TABLE 11. DETOSATION PRESSURES Initial pressure, Atm.
Initial Velocity, Feet/Sec. 117 100 76 67 67
Initial ~ ~ l ~ ~ Static i t Pressure ~ , Rise, Lb./Sq. Inch Abs. Feet/Sec. 130 ft. 230 it. 305 ft. 7a 32a ... 70 216b 274 b 270 b
0.40
TABLE I. FINAL DETONATIOK VELOCITIES Initial Pressure, Atm. 0.20 0.23 0.30 0.33 0.33 0.37 0.37 0.40 0.40
2561
Detonation Velocity, Feet/Sec.
0.37
70
0.37 0.23
32a 198b
2676
70
36a 234 b
43a 2746
281 6
70
...
269 b
iiib
18a
54 b
Initial pressure step. b Maximum pressure rise.
Av. velocity
6300
?c
900
gOOOL
R
0000
Two typical pressure traces obtained with the catenary pickups are shown in Figure '7. The first type of record shows a n initial pressure step, followed by a rapid pressure rise. Such traces are typical of most of those obtained near the center of the pipe and, according t o Turin and Huebler ( 6 ) , are characteristic of the region in which the explosion is changing from a combustion wave t o a detonation wave. The Recond type of record shows no step but only the steep pressure rise and is typical of most of the traces obtained near the end of the pipe. T h e heights of the two traces are not comparable, since the records represent two different runs. The static pressures at various points in the pipe are compared in Table 11; the peak pressures are several times
76 0
I TOWER WATER
...
a
66 65 170 176
COOLING
...
1886
- 14"
0
I
1
I
100
*
I
200
70
70
3.5 5.5 I
I
70
300
DISTANCE, f t
HOSE
Figure 6.
GATE VALVE
Typical Detonation Velocity Patterns
SOLENOIO VALVE ,,-PRESSURE
GAGE
I--
-
FLOW ---L
1 I
L
i"
F I R S T STAGE 8 EOUALLY SPACED SPRAY N O Z Z L E S
Figure 5.
SECOND STAGE 8 EOUALLY SPACED SPRAY N O Z Z L E S
Single-Ring Water-Spray System
Only one attempt was made t o gage the total or impact pressure. In this test, a disk designed for a pressure of 465 pounds per square inch absolute was placed at the end of the 300-foot straight run of pipe. The disk was ruptured, although its bursting pressure is well above the theoretical total pressure of 160 pounds per square inch absolute. I n all previous tests, the diaphragm at the end of the pipe was made of thin sheet aluminum designed to burst at about 45 pounds per square inch absolute. When the rupture disk rated a t 465 pounds per square inch absolute was used, the noise created on venting the detonation was considerably greater than in the other runs, and only in this run were windows broken in adjacent buildings. The strength of the rupture disks appeared to have no effect on the detonation velocity.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2562
PREYENTIOK OF DETONATIOS
After it ha.d been established that a detonation could occur, the nest problem was to determine whet,lier the detonation could be prevented. It is significant that the combination of water sprays plus a sudden expansion arrested bot’h detonation and flame in the
Vol. 46, No. 12
a large amount of wat,er was required. Rest results were obtained when the two rings were separated by about 5 feet; water flow rates of 6.8 pounds per pound of fuel were effective. The single failure occurred after a dry run during which the spray ring was badly distorted.
R
A
Figure 7.
Detonation Pressure Traces Left. W i t h step Right. Without step
eshaust plenum in every experiment, The neoprene diaphragms on the plenum were never rupt,ured and no damage was done t o the exhaust system. I t has been reported ( 2 ) that a sudden espansion can convert a detonation wave into an ordinary flame. Upon re-entering a contracted section, honever, the det,onat,ion may again be formed. I n an exhaust system, t,herefore, it is also desirable t o quench the flame :md completely eliminate t,he posaiI,le recurrence of det>onation.
Although the flame and detonation were apparently stopped by the proper injection of water, some sort of a disturbance cvidently continued down the pipe. The small hole shown a t the end of the 300-foot length of pipe in Figure 1 is t,ypical of a run in which the flame and detonation vere effectively quenched; hoLwver, the diaphragm n-ae damaged as shown. The damage resulting during runs in which the detonation was prevented is significant if a system contains delicate equipment that may be damaged by t,he disturbance passing through the pipe. CONCLUSION S
TABLE 111. Initial Pressure, htm. n.33
0.30-0.37 0.37
0.40 0.33-0.37
EFFECTIVEXESS 01”\\-ATER
IKJSCTIOS
Initial ~ ’ ~ i ~ ~ i ~Lb. , -Water , reet/Sec. Lb. i;uel
Detonation I-es S O
Single Injection Ring €3; 8.4
1
1
Double Injection Ring, I Foot Apart 66-78 16.7
..
1
..
2
Double Injection Ring, d Feet Apart 65 22.0 7.0 170 00
5.8
2 1
3
Explosions of stoichiometric, natural gas-air mixtures can develop velocities characterist,icof det,onations a t initial pressures as low as 0.2 atmosphere. Measured explosion pressures exceeded the t,heoreticallypredicted pressures of a detonation. The detonation hazard can be reduced by the proper application of water sprays in the region in nhich the detonation is being developed. The combination of water sprays and a large increase in the volume of the system stopped the established detonation. LITERATURE CITED
(1) Gerstein, lI.,Levine, O . , and TVong, E. L., J . Am. Chem. SOC.,
72, 418-22 (1951).
Attempts were also made to determine whether water sprays alone, located in the transition region between flame and detonation, could prevent the detonation. I t is obvious that sufficient water will extinguish the flame; the objective of this phase of the work n-as to determine the approximate minimum water requirements of the system. The results are summarized in Table 111. The single-spray ring gavc crratic results, w e n when approximately 8.4 pounds of water per pound of fuel wcrc addcd. The use of two spray rings about 1 foot apart gave better results, although
(2) Jost. W., “Explosion and Combustion Processes in Gases,” New York, McGraw-Hill Book Co.. 1946. (3) Lewis, B., arid van Elbe, G., “Combustion, Flames and Explosions of Gases.” Kew York, Academic Press, 1951. (4) Natl. Bur. Standards, “Selected Values of Properties of Hydrocarbons,” Circ. C 461 (1947). ( 5 ) Turin, J. J., and Huebler, J.. American Gas Association, Report> t o Committee on Industrial and Commercial Gas Research, Pioject 1 GR 59 (July 1950, April 1951). RECEIVED for review M a y 21, 1954. ACCEPTED August 27, 1951. Presented before t h e Division of Gas and Fuel Chemistry a t t h e 124811 Meeting of the A ~ F F ~ ~CHEMICAL CAN SOCIETY.CHICAGO, 111.
