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Spherical detonations of acetylene-oxygen-nitrogen mixtures as a function of nature and strength of initiation
SPHERICAL DETONATIONS OF ACETYLENE-OXYGEN-NITROGEN MIXTURES AS A FUNCTION OF NATURE AND STRENGTH OF INITIATION H. FREIWAL]) AND tI. W. KOCH The spherical detonation of mixtures of acetylene-oxygen-nitrogen was investigated by means of self-luminosity photography and by pressure-time measurements near the detonation. The gaseous mixtures were contained in transparent rubber balloons of different sizes. The detonation was initiated in the balloon center by flame, electric spark, hot wire, exploding wire, or the detonation of a small quantity of lead azide, or mercury fulminate-Tetryl with or without a PETN b(mster. In addition a "linear" gaseous detonation in a tube was transformed to a spherical one and was studied as a function of tube diameter and mixture composition. Generally the detonation region increa~d with increasing ignition energy. With 7 gm PETN the spherical detonations of near-stoichiometric rich mixtures were initiated even with 75% nitrogen, e.g., with more nitrogen than is contained in the corresponding fu~l-air mixture. With spark ignition one always observed an ignition delay and a deflagration preceding the spherical detonation. Tl~e consecutive shock waves produced by the spherical detonation and by reflections were also investigated. ent sizes (15-500 liters). The ovcrpressure in the inflated balloons was 3-4 cm of water. The ratio of fuel to oxygen is expressed by X = (ratio of actual quantity of oxygen in the mixture/stoichiometric quantity of oxygen) ; the "atmosphere" of the mixture is expressed as 02/02 -}- N2 without including the fuel. In the investigation of spherical detonations and its composition limits self-luminosity photography was used employing streak camera, Kerr cell shutter, and high-speed camera. We also investigated the consecutive shock waves produced by the spherical detonation by means of a condenser microphone placed near the detonation. The detonations were initiated in the balloon center by flame, red-hot wire, exploding wire, electric spark, impact of a flying projectile, by the detonation of lead azide, electric detonators and boosters, and by a "linear" detonating mixture in a tube.
Introduction Although detonations of gaseous mixtures in tubes have been known for a long time the existence of self-sustained spherical detonations in the gaseous phase was experimentally proved about 10 years ago by Manson and Ferric, ~ Freiwald and Ude, 2 and Zeldovich, Kogarko, and Simonov3 In 1923 Laffitte 4 observed spherical detonation waves in mixtures of CS2 + 302 but recorded these waves over rather short distances (10 to 13 cm) and initiated the detonati@~ by relatively powerful detonators of I gm of morcury fulminate. There was some doubt regardin~ the character of a self-sustained detonation wave (see Jost~). While composition ranges of flammability of mixtures of many hydrocarbons with air and with oxygen are well known 6 these limits are completely lacking for spherical detonations, with exception of some data in references 1, 2, and 3.
Examples of Spherical Gaseous Detonations and Related Shock Waves
Experimental The gases used in this investigation were of normal purity and used without further purification. The composition was generally known to
-r The gases were filled at normal pressure into thin-walled, transparent rubber balloons of differ-
The spherical detonation of an acetyleneoxygen mixture in a rubber balloon, initiated by a pressed pellet of 0.2 gm of lead azide, and taken with a Kerr cell shutter, i8 shown in Fig. 1. The photograph was taken 36 p,see after the initiation
275
276
D E T O N A T I O N A N D T R A N S I T I O N TO D E T O N A T I O N
FIG. 1. Kerr cell shutter photograph of spherical detonation of 21.6 liters C2H2 ~ 02 in a rubber ba'loon. Initiation: 0.2 gm of Pb N~; 36 ~ec after initiation. and shows clearly the spherical detonation wave and its propagation through the gas mixture. A typical streak camera record of the spherical detonation of C2H2 ~- 2.502 -}- 2.5N2 in a rubber
B
A
d= 32cm
C
balloon is shown in Fig. 2. The gaseous detonation was initiated at time A by the detonation of 0.2 gm of lead azide. The detonation shock wave is partially reflected on the balloon envelope and runs concentrically towards the balloon center where it is reflected at time B. The air.and the lead vapor in the balloon center are heated by the converging shock waves. One can see the converging and diverging shock wave as a luminous cross on the film at time B. At time C one sees the originating second shock wave (combustion products shock wave). In detonations of spherical solid explosives the detonation center is normally covered by nontransparent combustion products. ~ The originating second shock wave is--perhaps for the first time easily recorded by streak camera records in spherical gaseous det0nations.2,s The second shock wave is partially reflected on the spherical boundary of combustion products. I t moves back to the center and reaches it at the time D. The velocities of the waves are: detonation velocity from A 2075 m/sec, reflected shock wave at B 1370 m/see, combustion products shock wave originating in C 930 m/sec, reflected shock wave at D 1010 m/sec. The scatter in these results is about -4-30-50 m/sec, depending on the sharpness of the wave record. This sharpness is decreased by the thin wall of the rubber balloon. The velocity of the reflected shock wave at B depends on the relatively high detonation velocity. The velocity of the wave at D as a converging and diverging wave has a distinctly higher velocity than the products shock wave at C.
D
~,,
// DETONATION 1
i
REFLECTION
WAVE( PRODUCTSBOUNDARY)
i
DETONATIONEND
I
~, :'RODUCTS SHOCKWAVE i PRODUCTS
REFLECTIONWAVE (BALLOONENVELOPE)
FIG. 2. Streak camera record of spherical detonation of 16 liters of C2H2 -t2.502 -{- 2.5N2; X = 1; 50% 02 in "at,ldsphere." Initiation: 0.2 gm of Pb Ne.
SPHERICAL
277
DETONATIONS
REFLECTION WAVE (BALLOON ENVELOPE) FIRST SHOCKWAVE
CiMBUSTION PRODUCTS SHOCK WAVE REFLECTION WAVE ( PRODUCTS BOUNDARY)
p [atm]
IJA~ ~t:5 ~W
uJU ~w o_.-I ~LL 6~JW
0
I
2
3
TIME [ms ] DISTANCE BALLOON CENTER- MICROPHONE: 1 m
FIG. 3. Pressure-time record near to detonation of 184 liters of acetylene-air mixture (25% C2H2). Distance balloon center to microphone: 1 m. On high-speed camera records one sees similar consecutive changes of brightness in the balloon center due to converging and diverging waves. The same sequence of shock waves was observed by recording the pressure as a function of time 9 (Fig. 3). The "reflected" pressure was recorded 12.5
INITIATION BY DETONATOR XDETONATOR + 7 gm PETN IIELECTRIC
10
2.5
LU'
by a condenser microphone~ situated I m distance from the balloon center. Behind the first airshock wave one sees the shock Wave reflected on the balloon envelope and at the center (point B in Fig. 2) and the wave reflected on the boundary of combustion products and in the center (point D in Fig. 2). The sequence of shock waves is independent of the nature of the gaseous mixture and was observed with mixtures of hydrogen, ethylene, and propane with oxygen and nitrogen. The time of the occurrence of shock waves and of luminous phenomena produced by them depends only on the geometry of the balloon. The maximum reflected pressure of the first shock wave as a function of the acetylene concentration in mixtures of 51 gmof acetylene with air are shown in Fig. 4.
> O
Ccmposition R e g i o n s of Spherical Detonation
-.s: 2.5
0
tt
5 , i
1 DETONATION Ii LIMITS: ~
,
~j
10
II
% C2H2 IN AIR
I J'~
15
20
I
',I
DETONATOR =; DETONATOR+ 7gin PETN
25 I I *, ~ i =i
FIG. 4. Maximum (reflected) pressure of the shock wave of spherical detonation of C~H2-air mixtures as function of C~H2 percentage for different initiation. Fifty-one grams of (46 liters) C2H2. Distance balloon center to microphone: 1 m.
Ignition by Flame. With a commercial igniter producing a flame jet of 10 to 15 cm length one obtains a spherical detonation of C2H~-02 mixtures in the rangc from 16% (v/v) C:H2 to 54% C2H2 corresponding to X between 2.3 and 0.34 (Fig. 5). In the C2H2-O2-N2 mixture a spherical detonation occurs up to a maximum nitrogen content of 33% in approximately stoichiometrie mixtures. Mar/son1 found narrower limits in C~H2-02 mixtures (25-50% C2H2) probably due to a weaker igniter flame. -!
Ignition by Electric Spark. An electric spark was used between tungsten electrodes with a gap dis-
278
DETONATION AND TRANSITION TO DETONATION 100% 0 2
lO0~t FI2
80%
00~
20%
40%
~
N2
C2H 2 ~
Fro. 5. C o m p o s i t i o n regions of spheriesJ d e t o n a t i o ~
20%
100% C i H 2
80%
of C 2 H 2 ~ O ~ N 2 mixtures.
Ignition: (a) by igniter flame jet; (b) by electric spark, energy: 4.5 joules. Results of Mansonl: O to curve (a); 9 to curve (b); energy 12 joules.
t
2
...i ;, = I ( i )
~e
DETONATION /
J i//
EXPERIMENTS WITH GAP DISTANCE : 2.5 - 5 mm
- ~ ~ 0.S9 (x) ,x
O!
ioo o
; 80 20
AIR
I 60 40
I
I
40 ~ 02 | IN THE 60 ~; N 2 j~"ATMOSPHERE"
FIG. 6. Delay of spherical detonation of CII-II-O2-Ni mixtures after ignition by electric spark (4.5 joules) as a function of the composition of the "atmosphere."
tance from 2.2 to 5 mm. t~ The electrodes were mounted in the balloon center. The stored energy of the 1 ~t F-condenser charged to 6 kV was 18 joules, but only 25% of the spark energy was effective in the spark as determined by calorimetry, n The limits of spherical detonations initiated by a spark of 4.5 joules energy were from approximately 18 to 57% C~H~ in the CaH~)~. mixture (Fig. 5). Curve b in Fig. 5 shows the spark ignition results with a maximum of 40.5 % N2 at )t = 0.8 in the three-component system. In the case of initiation by electric spark a deflagration always precedes the spherical detonation propagating with a velocity of about 100300 m/see. The time delay from the spark to beginning of detonation depends on the gas mixture composition (Fig. 6). In a fuel-rich mixture ()~ = 0.59) the delays are shorter than in the stoichiometric mixture. The delay of 0.1 m/see with stoichiometric CaHr-O~ mixture is in reasonable agreement with the results of Bollinger, Fong, and Edse = who determined a detonation induction distance of 2 cm for the same mixture in a 15-ram I.D. tube ignited by a melting 0.005inch copper wire. The results of Kistiakowsky and Kydd 13 of the detonation delay of a mixture of 40% C2H2, 40% 0~, and 20% Ar at the head of a 10-cm diameter tube agree well with our results.
SPHERICAL DETONATIONS
279
100~ 02
A//.
100% H2 i
~
/
80~ 20~
.
-
~
~
~
60% 40~
".,
-
~ C2H2 ~
N2
~
20~ 80%
~
10~ C2H2
FIO. 7. Composition regions of spherical detonations of C2Hr-Or-N~ mixtures. Initiation: by (a) electric detonator; (b) electric detonator and 7 gm PETN booster; (c) 50, 100, 200 mg Pb Ne. Results of Mausont: O, electric detonator. this initial detonation. We used small pressed pellets of lead azide from 50 to 200 mg, detonator No. 8 containing 0.4 gm of mercury fulminate and 0.8 gm of tetryl in a thin copper cap, and a booster of 7 gm of P E T N (Fig. 7).
The electrode gap length has no influence on the time delay (Fig. 6).
Initiation by Detonation. The ranges of spherical detonation of ternary mixtures initiated by a detonation depend strongly on the strength of 100~02
t. tUBES
!
l
28 mm ~6
h ~]20 mm 4) mm
100N H 2
100% C2H2
80%
60%
20%
40%
~
C2H 2 ~
H2
2(~o 80%*
FIo. 8. Composition regions of spherical detonation of C2Hr-OrN2 mixtures. Ignition by transformation of linear gaseous detonation in a spherical one. O, result of ZeldoviehS; X = 1; critical diameter, 28 ram.
280
D E T O N A T I O N A N D T R A N S I T I O N TO D E T O N A T I O N
Initiation by "Linear" Gas Detonation. "Linear"
Ignition by Exploding Wire. An "exploding" wire
detonations of gaseous mixtures propagating in a tube are able to transform into spherical detonations. 1~There is a critical transformation diameter for each mixture. For a linear gas detonation in a 28-ram diameter tube ending in the balloon center, the composition region for transformation in spherical detonation (Fig. 8) is nearly the same as that of the 0.2 gm PbN6 initiation (Fig. 7). Our results agree very well with those of Zeldovich3 who determined the critical diameter for stoichiometric mixtures of acetylene and oxygen with increasing addition of nitrogen. If the tube with an inside diameter of 36 mm is enlarged at the tube end in the balloon center, to a diameter of 120 mm the domain of spherical detonation of C~H2-O2-N2 mixtures is expanded (Fig. 8) so t h a t it is nearly identical with that of the detonator initiation.
produces a local increase of temperature and a shock wave, similar to an electric spark. A 40 g diameter copper wire of 10-mm length (0.1 ohm) was "exploded" by discharging a 64 #F-condenser charged to 450 V. The composition region of spherical detonation of C2H~-O~-N2 mixtures ignited by the 6.5 joules exploded wire is somewhat larger than the domain for the 4.5 joule spark (Fig. 5 curve b). Generally an exploding wire seems to be more effective in ignition of spherical gaseous detonations than an electric spark of the same energy.
Ignition by Flying Projectile. The impact of a rifle bullet flying at 300-800 m/see against the thin wall of the inflated rubber balloon is able to produce a spherical detonation. A bullet piercing transparent paper of 0.03 ram (800 m/sec) or an aluminum foil of 0.05 mm thickness ( 3 0 0 m/see) initiates a spherical detonation of a fuelrich (k = 0.59) C2H2-O~-N2 mixture with 20.3% N2 after a very short delay.
Ignition by Red-Hot Wire. For a thermal ignition source we used a 0.13 mm diameter red-hot resistance wire (1 cm length; 0.8 ohm). Contrary to results of other authors ~ we succeeded in igniting a spherical detonation of 350 liters of C2H2-O2 from 15 to more than 35% C2H2 without much preceding deflagration.
Discussion of the Results. Influence of Ignition Energy on Detonation Limits The results demonstrate the influence of ignition energy on the detonation limits, as shown distinctly in the ternary mixtures by the maximum nitrogen content. The regions of spherical detonation characterized by this maximum nitrogen content in near-stoichiometric acetylene rich mixtures (k = 0.8) as a function of ignition energy are shown in Fig. 9. The ignition values from detonation yield a continuous curve, tend: ing with increasing ignition energy asymptotically to a nitrogen content of about 77 % at 10~ joules. The results of ignition by "linear" gas detonation are inserted in the curve. In this way we can determine the ignition energy of gaseous detonations propagating in tubes in the absence of deflagration (the ignition delay being less than 10 #sec and most probably only a few microseconds.) 3 The curve was extrapolated to a value of 10
X 50, 100, 200 m9 PbN6
u ESTIMATED FROM RESULTS OF LITCHFIELD
A DETONATOR I) DETONATOR + 7gin PETN
O TUBE, DIAMETER/vIM
80
,.. ~ 70
~r
"'AI~"
[I
,L
I IJ
X
1
~4o
28
U
'~
3O
f PARK
~'~ 1o I1,1 IL o
]~/
J
L" / /
1 ~'
10
/
55~s DEFLAGRATION
I
| I Ill
I
102
1 I I 103
, 1 I I I[ H 104
r rI
l0 s
IGNITION ENERGY [joules]
FIG. 9. Maximum nitrogen content in spherical detonations of C2Hz-O2-N2 mixtures as a function of ignition energy; ), = 0.8.
SPHERICAL DETONATIONS joules and 30 joules at N2 = 0. The 10 joules value is estimated from results ~4 with exploding wire ignition in H2-O2 mixtures (13 joules) and ethylene-oxygen (8.7 joules). The 30 joules value is derived from a fit to the curve in Fig. 9.
(
y
\ p ~ -- p l
lax
2.
3.
In x0
where p = % N~, Poe = 77%, x = energy in joules. Plotting In [PoJ(Poo -- P)3 versus in lnx we find n = 3, with p = 0, x0 is about 30 joules. For spark ignition one finds ignition delays of at least 50 ~sec, even for C2H2-02 mixtures with a spark energy of about 5 joules. This ignition delay probably decreases with increasing ignition energy. I n Fig. 9 we inserted the spark ignition data of a ternary mixture with 14% N~ and a spark energy of 4.5 joules. This value differs from the extrapolated curves. The spark is followed by deflagration for 55 ~sec with a velocity of about 300 m/sec. The combustion energy of the acetylene burned during the delay time is a b o u t 310 joules. This energy, will initiate ternary mixtures with 60% N2 in direct initiation by detonation. This result indicates t h a t only those ignition methods are comparable which produce a detonation without an intermediate state of deflagration.
4. 5. 6.
7. 8. 9. 10. 11. 12. 13.
REFERENCES 1. MANSON,N. and FERRIE, F.: Fourth Symposium (International) on Combustion, p. 486. Williams
14.
281
and Wilkins, 1953; MANSON, N.: Rev. Inst. Fran~. P~trole. Ann. Comb. Liquid. 9, 133 (1954). FREIWALD, H. and UDE, H.: Compt. rend. 236, 1741 (1953); FREIWALD, H. and UDE, H.: Z. Elektrochem. 61, 663 (1957). ZELDOVICH,Y. B., KOGARHKO,S. M., SIMONOV~ N. N.: Zhurnal Tekhnickeskoi Fiziki 26, 1744 (1956); translated in Soviet P h y s . - - J E T P . 1, 1689 (1957). LAFFITTE~P.: Compt. rend. 177, 178 (1923); Ann. Phys. 10, Ser. IV, 645 (1925). JOST~ W.: Explosions-und Verbreanungsvorgdnge in Gasen, p. 185, Springer, 1939. COWARD, H. F. and JONES, G. W.: Limits of Flammability ~of Gases and Vapors. Bur. of Mines, Bull. 503, Washington, 1952. SCHARDtN, H.: Communs. Pure and Appl. Math. VII, 223 (1954). FREIWALD, Hi: Z. Elektrochem. 65, 711 (1961). FREIWALD, H. and UDE, H.: Explosivstoffe 7, 251 (1959). FREIWALD, H. and UDE, H.: Compt. rend. 2~1, 736 (1955); Z. Elektrochem. 59, 910 (1955). BOULAY, J. and FREIWALD, H.: LRSL Note Technique 18a/57. BOLLINGER, L. E., FONG, M. C. and EDSE, R.: ARS Journal 31,588 (1961). i KISTIAKOWSKY, G. B. and KYDD, P. H . : J. Chem. Phys. 23, 271 (1955). LITCHFIELD~E. L.: Phys. Fluids 5, 114 (1962).
Discussion PROF. 1~. MANSON (University of Poitiers, France) : I think that it might be of interest to bring to your attention the fact that Professor Laffitte performed some experiments on spherical detonations as early as 1924 (Annales de Chemic, Paris). I would like to ask Dr. Freiwald what is the accuracy of his measurements of detonation velocity?
"linear" detonation we found the delay time to be less than the limit of time resolution of our smear camera (10 msec). Zeldovich mentioned delay times of 1 to 2 ~sec for the C~H2-O2-N2 mixtures used in our experiments.
DR. H. FREIWALD (German-French Research Institute, France): We know very well the experiments of Professor P. Laffitte [Cempt. rend. 177, 178 (1923); Ann. Phys. 10, Set. IV, 645 (1925)]
waves appearing in the microphone-pressure records in Dr. Freiwald's paper, I draw attention to the work of Brode [BRODE, H. L., Phys. Fluids 2, 217 (1959)] on the origin of such a sequence of shocks. Brode's integration of the hydrodynamic equations shows that expansion of an initially uniform sphere of compressed gas must give rise, in addition to an outward-facing shock wave, to an inward-facing shock. The latter implodes on the center, and subsequently is reflected, after outward movement, at the contact surface. As a result there is an indefinite number of pulsations of decreasing amplitude, as the pressure records in this case demonstrate.
with mixtures of CS2 -{- 302 in a glass balloon of 21 cm diameter. The mixture was initiated in the balloon center by 1 gm of mercury fulminate. Because of the effect of the relatively strong shock waves from the initiator there was some doubt about the eXistence of a really self-sustained spherical detonation in the mixture. Also from the theoretical point of view a spherical detonation in gases was believed to be impossible at that time. In the case of initiation of spherical detonation by
DR. W. E. GORDON (Combustion and Explosives Research Inc.): Concerning the secondary shock
Introduction Although detonations of gaseous mixtures in tubes have been known for a long time the existence of self-sustained spherical detonations in the gaseous phase was experimentally proved about 10 years ago by Manson and Ferric, ~ Freiwald and Ude, 2 and Zeldovich, Kogarko, and Simonov3 In 1923 Laffitte 4 observed spherical detonation waves in mixtures of CS2 + 302 but recorded these waves over rather short distances (10 to 13 cm) and initiated the detonati@~ by relatively powerful detonators of I gm of morcury fulminate. There was some doubt regardin~ the character of a self-sustained detonation wave (see Jost~). While composition ranges of flammability of mixtures of many hydrocarbons with air and with oxygen are well known 6 these limits are completely lacking for spherical detonations, with exception of some data in references 1, 2, and 3.
Examples of Spherical Gaseous Detonations and Related Shock Waves
Experimental The gases used in this investigation were of normal purity and used without further purification. The composition was generally known to
-r The gases were filled at normal pressure into thin-walled, transparent rubber balloons of differ-
The spherical detonation of an acetyleneoxygen mixture in a rubber balloon, initiated by a pressed pellet of 0.2 gm of lead azide, and taken with a Kerr cell shutter, i8 shown in Fig. 1. The photograph was taken 36 p,see after the initiation
275
276
D E T O N A T I O N A N D T R A N S I T I O N TO D E T O N A T I O N
FIG. 1. Kerr cell shutter photograph of spherical detonation of 21.6 liters C2H2 ~ 02 in a rubber ba'loon. Initiation: 0.2 gm of Pb N~; 36 ~ec after initiation. and shows clearly the spherical detonation wave and its propagation through the gas mixture. A typical streak camera record of the spherical detonation of C2H2 ~- 2.502 -}- 2.5N2 in a rubber
B
A
d= 32cm
C
balloon is shown in Fig. 2. The gaseous detonation was initiated at time A by the detonation of 0.2 gm of lead azide. The detonation shock wave is partially reflected on the balloon envelope and runs concentrically towards the balloon center where it is reflected at time B. The air.and the lead vapor in the balloon center are heated by the converging shock waves. One can see the converging and diverging shock wave as a luminous cross on the film at time B. At time C one sees the originating second shock wave (combustion products shock wave). In detonations of spherical solid explosives the detonation center is normally covered by nontransparent combustion products. ~ The originating second shock wave is--perhaps for the first time easily recorded by streak camera records in spherical gaseous det0nations.2,s The second shock wave is partially reflected on the spherical boundary of combustion products. I t moves back to the center and reaches it at the time D. The velocities of the waves are: detonation velocity from A 2075 m/sec, reflected shock wave at B 1370 m/see, combustion products shock wave originating in C 930 m/sec, reflected shock wave at D 1010 m/sec. The scatter in these results is about -4-30-50 m/sec, depending on the sharpness of the wave record. This sharpness is decreased by the thin wall of the rubber balloon. The velocity of the reflected shock wave at B depends on the relatively high detonation velocity. The velocity of the wave at D as a converging and diverging wave has a distinctly higher velocity than the products shock wave at C.
D
~,,
// DETONATION 1
i
REFLECTION
WAVE( PRODUCTSBOUNDARY)
i
DETONATIONEND
I
~, :'RODUCTS SHOCKWAVE i PRODUCTS
REFLECTIONWAVE (BALLOONENVELOPE)
FIG. 2. Streak camera record of spherical detonation of 16 liters of C2H2 -t2.502 -{- 2.5N2; X = 1; 50% 02 in "at,ldsphere." Initiation: 0.2 gm of Pb Ne.
SPHERICAL
277
DETONATIONS
REFLECTION WAVE (BALLOON ENVELOPE) FIRST SHOCKWAVE
CiMBUSTION PRODUCTS SHOCK WAVE REFLECTION WAVE ( PRODUCTS BOUNDARY)
p [atm]
IJA~ ~t:5 ~W
uJU ~w o_.-I ~LL 6~JW
0
I
2
3
TIME [ms ] DISTANCE BALLOON CENTER- MICROPHONE: 1 m
FIG. 3. Pressure-time record near to detonation of 184 liters of acetylene-air mixture (25% C2H2). Distance balloon center to microphone: 1 m. On high-speed camera records one sees similar consecutive changes of brightness in the balloon center due to converging and diverging waves. The same sequence of shock waves was observed by recording the pressure as a function of time 9 (Fig. 3). The "reflected" pressure was recorded 12.5
INITIATION BY DETONATOR XDETONATOR + 7 gm PETN IIELECTRIC
10
2.5
LU'
by a condenser microphone~ situated I m distance from the balloon center. Behind the first airshock wave one sees the shock Wave reflected on the balloon envelope and at the center (point B in Fig. 2) and the wave reflected on the boundary of combustion products and in the center (point D in Fig. 2). The sequence of shock waves is independent of the nature of the gaseous mixture and was observed with mixtures of hydrogen, ethylene, and propane with oxygen and nitrogen. The time of the occurrence of shock waves and of luminous phenomena produced by them depends only on the geometry of the balloon. The maximum reflected pressure of the first shock wave as a function of the acetylene concentration in mixtures of 51 gmof acetylene with air are shown in Fig. 4.
> O
Ccmposition R e g i o n s of Spherical Detonation
-.s: 2.5
0
tt
5 , i
1 DETONATION Ii LIMITS: ~
,
~j
10
II
% C2H2 IN AIR
I J'~
15
20
I
',I
DETONATOR =; DETONATOR+ 7gin PETN
25 I I *, ~ i =i
FIG. 4. Maximum (reflected) pressure of the shock wave of spherical detonation of C~H2-air mixtures as function of C~H2 percentage for different initiation. Fifty-one grams of (46 liters) C2H2. Distance balloon center to microphone: 1 m.
Ignition by Flame. With a commercial igniter producing a flame jet of 10 to 15 cm length one obtains a spherical detonation of C2H~-02 mixtures in the rangc from 16% (v/v) C:H2 to 54% C2H2 corresponding to X between 2.3 and 0.34 (Fig. 5). In the C2H2-O2-N2 mixture a spherical detonation occurs up to a maximum nitrogen content of 33% in approximately stoichiometrie mixtures. Mar/son1 found narrower limits in C~H2-02 mixtures (25-50% C2H2) probably due to a weaker igniter flame. -!
Ignition by Electric Spark. An electric spark was used between tungsten electrodes with a gap dis-
278
DETONATION AND TRANSITION TO DETONATION 100% 0 2
lO0~t FI2
80%
00~
20%
40%
~
N2
C2H 2 ~
Fro. 5. C o m p o s i t i o n regions of spheriesJ d e t o n a t i o ~
20%
100% C i H 2
80%
of C 2 H 2 ~ O ~ N 2 mixtures.
Ignition: (a) by igniter flame jet; (b) by electric spark, energy: 4.5 joules. Results of Mansonl: O to curve (a); 9 to curve (b); energy 12 joules.
t
2
...i ;, = I ( i )
~e
DETONATION /
J i//
EXPERIMENTS WITH GAP DISTANCE : 2.5 - 5 mm
- ~ ~ 0.S9 (x) ,x
O!
ioo o
; 80 20
AIR
I 60 40
I
I
40 ~ 02 | IN THE 60 ~; N 2 j~"ATMOSPHERE"
FIG. 6. Delay of spherical detonation of CII-II-O2-Ni mixtures after ignition by electric spark (4.5 joules) as a function of the composition of the "atmosphere."
tance from 2.2 to 5 mm. t~ The electrodes were mounted in the balloon center. The stored energy of the 1 ~t F-condenser charged to 6 kV was 18 joules, but only 25% of the spark energy was effective in the spark as determined by calorimetry, n The limits of spherical detonations initiated by a spark of 4.5 joules energy were from approximately 18 to 57% C~H~ in the CaH~)~. mixture (Fig. 5). Curve b in Fig. 5 shows the spark ignition results with a maximum of 40.5 % N2 at )t = 0.8 in the three-component system. In the case of initiation by electric spark a deflagration always precedes the spherical detonation propagating with a velocity of about 100300 m/see. The time delay from the spark to beginning of detonation depends on the gas mixture composition (Fig. 6). In a fuel-rich mixture ()~ = 0.59) the delays are shorter than in the stoichiometric mixture. The delay of 0.1 m/see with stoichiometric CaHr-O~ mixture is in reasonable agreement with the results of Bollinger, Fong, and Edse = who determined a detonation induction distance of 2 cm for the same mixture in a 15-ram I.D. tube ignited by a melting 0.005inch copper wire. The results of Kistiakowsky and Kydd 13 of the detonation delay of a mixture of 40% C2H2, 40% 0~, and 20% Ar at the head of a 10-cm diameter tube agree well with our results.
SPHERICAL DETONATIONS
279
100~ 02
A//.
100% H2 i
~
/
80~ 20~
.
-
~
~
~
60% 40~
".,
-
~ C2H2 ~
N2
~
20~ 80%
~
10~ C2H2
FIO. 7. Composition regions of spherical detonations of C2Hr-Or-N~ mixtures. Initiation: by (a) electric detonator; (b) electric detonator and 7 gm PETN booster; (c) 50, 100, 200 mg Pb Ne. Results of Mausont: O, electric detonator. this initial detonation. We used small pressed pellets of lead azide from 50 to 200 mg, detonator No. 8 containing 0.4 gm of mercury fulminate and 0.8 gm of tetryl in a thin copper cap, and a booster of 7 gm of P E T N (Fig. 7).
The electrode gap length has no influence on the time delay (Fig. 6).
Initiation by Detonation. The ranges of spherical detonation of ternary mixtures initiated by a detonation depend strongly on the strength of 100~02
t. tUBES
!
l
28 mm ~6
h ~]20 mm 4) mm
100N H 2
100% C2H2
80%
60%
20%
40%
~
C2H 2 ~
H2
2(~o 80%*
FIo. 8. Composition regions of spherical detonation of C2Hr-OrN2 mixtures. Ignition by transformation of linear gaseous detonation in a spherical one. O, result of ZeldoviehS; X = 1; critical diameter, 28 ram.
280
D E T O N A T I O N A N D T R A N S I T I O N TO D E T O N A T I O N
Initiation by "Linear" Gas Detonation. "Linear"
Ignition by Exploding Wire. An "exploding" wire
detonations of gaseous mixtures propagating in a tube are able to transform into spherical detonations. 1~There is a critical transformation diameter for each mixture. For a linear gas detonation in a 28-ram diameter tube ending in the balloon center, the composition region for transformation in spherical detonation (Fig. 8) is nearly the same as that of the 0.2 gm PbN6 initiation (Fig. 7). Our results agree very well with those of Zeldovich3 who determined the critical diameter for stoichiometric mixtures of acetylene and oxygen with increasing addition of nitrogen. If the tube with an inside diameter of 36 mm is enlarged at the tube end in the balloon center, to a diameter of 120 mm the domain of spherical detonation of C~H2-O2-N2 mixtures is expanded (Fig. 8) so t h a t it is nearly identical with that of the detonator initiation.
produces a local increase of temperature and a shock wave, similar to an electric spark. A 40 g diameter copper wire of 10-mm length (0.1 ohm) was "exploded" by discharging a 64 #F-condenser charged to 450 V. The composition region of spherical detonation of C2H~-O~-N2 mixtures ignited by the 6.5 joules exploded wire is somewhat larger than the domain for the 4.5 joule spark (Fig. 5 curve b). Generally an exploding wire seems to be more effective in ignition of spherical gaseous detonations than an electric spark of the same energy.
Ignition by Flying Projectile. The impact of a rifle bullet flying at 300-800 m/see against the thin wall of the inflated rubber balloon is able to produce a spherical detonation. A bullet piercing transparent paper of 0.03 ram (800 m/sec) or an aluminum foil of 0.05 mm thickness ( 3 0 0 m/see) initiates a spherical detonation of a fuelrich (k = 0.59) C2H2-O~-N2 mixture with 20.3% N2 after a very short delay.
Ignition by Red-Hot Wire. For a thermal ignition source we used a 0.13 mm diameter red-hot resistance wire (1 cm length; 0.8 ohm). Contrary to results of other authors ~ we succeeded in igniting a spherical detonation of 350 liters of C2H2-O2 from 15 to more than 35% C2H2 without much preceding deflagration.
Discussion of the Results. Influence of Ignition Energy on Detonation Limits The results demonstrate the influence of ignition energy on the detonation limits, as shown distinctly in the ternary mixtures by the maximum nitrogen content. The regions of spherical detonation characterized by this maximum nitrogen content in near-stoichiometric acetylene rich mixtures (k = 0.8) as a function of ignition energy are shown in Fig. 9. The ignition values from detonation yield a continuous curve, tend: ing with increasing ignition energy asymptotically to a nitrogen content of about 77 % at 10~ joules. The results of ignition by "linear" gas detonation are inserted in the curve. In this way we can determine the ignition energy of gaseous detonations propagating in tubes in the absence of deflagration (the ignition delay being less than 10 #sec and most probably only a few microseconds.) 3 The curve was extrapolated to a value of 10
X 50, 100, 200 m9 PbN6
u ESTIMATED FROM RESULTS OF LITCHFIELD
A DETONATOR I) DETONATOR + 7gin PETN
O TUBE, DIAMETER/vIM
80
,.. ~ 70
~r
"'AI~"
[I
,L
I IJ
X
1
~4o
28
U
'~
3O
f PARK
~'~ 1o I1,1 IL o
]~/
J
L" / /
1 ~'
10
/
55~s DEFLAGRATION
I
| I Ill
I
102
1 I I 103
, 1 I I I[ H 104
r rI
l0 s
IGNITION ENERGY [joules]
FIG. 9. Maximum nitrogen content in spherical detonations of C2Hz-O2-N2 mixtures as a function of ignition energy; ), = 0.8.
SPHERICAL DETONATIONS joules and 30 joules at N2 = 0. The 10 joules value is estimated from results ~4 with exploding wire ignition in H2-O2 mixtures (13 joules) and ethylene-oxygen (8.7 joules). The 30 joules value is derived from a fit to the curve in Fig. 9.
(
y
\ p ~ -- p l
lax
2.
3.
In x0
where p = % N~, Poe = 77%, x = energy in joules. Plotting In [PoJ(Poo -- P)3 versus in lnx we find n = 3, with p = 0, x0 is about 30 joules. For spark ignition one finds ignition delays of at least 50 ~sec, even for C2H2-02 mixtures with a spark energy of about 5 joules. This ignition delay probably decreases with increasing ignition energy. I n Fig. 9 we inserted the spark ignition data of a ternary mixture with 14% N~ and a spark energy of 4.5 joules. This value differs from the extrapolated curves. The spark is followed by deflagration for 55 ~sec with a velocity of about 300 m/sec. The combustion energy of the acetylene burned during the delay time is a b o u t 310 joules. This energy, will initiate ternary mixtures with 60% N2 in direct initiation by detonation. This result indicates t h a t only those ignition methods are comparable which produce a detonation without an intermediate state of deflagration.
4. 5. 6.
7. 8. 9. 10. 11. 12. 13.
REFERENCES 1. MANSON,N. and FERRIE, F.: Fourth Symposium (International) on Combustion, p. 486. Williams
14.
281
and Wilkins, 1953; MANSON, N.: Rev. Inst. Fran~. P~trole. Ann. Comb. Liquid. 9, 133 (1954). FREIWALD, H. and UDE, H.: Compt. rend. 236, 1741 (1953); FREIWALD, H. and UDE, H.: Z. Elektrochem. 61, 663 (1957). ZELDOVICH,Y. B., KOGARHKO,S. M., SIMONOV~ N. N.: Zhurnal Tekhnickeskoi Fiziki 26, 1744 (1956); translated in Soviet P h y s . - - J E T P . 1, 1689 (1957). LAFFITTE~P.: Compt. rend. 177, 178 (1923); Ann. Phys. 10, Ser. IV, 645 (1925). JOST~ W.: Explosions-und Verbreanungsvorgdnge in Gasen, p. 185, Springer, 1939. COWARD, H. F. and JONES, G. W.: Limits of Flammability ~of Gases and Vapors. Bur. of Mines, Bull. 503, Washington, 1952. SCHARDtN, H.: Communs. Pure and Appl. Math. VII, 223 (1954). FREIWALD, Hi: Z. Elektrochem. 65, 711 (1961). FREIWALD, H. and UDE, H.: Explosivstoffe 7, 251 (1959). FREIWALD, H. and UDE, H.: Compt. rend. 2~1, 736 (1955); Z. Elektrochem. 59, 910 (1955). BOULAY, J. and FREIWALD, H.: LRSL Note Technique 18a/57. BOLLINGER, L. E., FONG, M. C. and EDSE, R.: ARS Journal 31,588 (1961). i KISTIAKOWSKY, G. B. and KYDD, P. H . : J. Chem. Phys. 23, 271 (1955). LITCHFIELD~E. L.: Phys. Fluids 5, 114 (1962).
Discussion PROF. 1~. MANSON (University of Poitiers, France) : I think that it might be of interest to bring to your attention the fact that Professor Laffitte performed some experiments on spherical detonations as early as 1924 (Annales de Chemic, Paris). I would like to ask Dr. Freiwald what is the accuracy of his measurements of detonation velocity?
"linear" detonation we found the delay time to be less than the limit of time resolution of our smear camera (10 msec). Zeldovich mentioned delay times of 1 to 2 ~sec for the C~H2-O2-N2 mixtures used in our experiments.
DR. H. FREIWALD (German-French Research Institute, France): We know very well the experiments of Professor P. Laffitte [Cempt. rend. 177, 178 (1923); Ann. Phys. 10, Set. IV, 645 (1925)]
waves appearing in the microphone-pressure records in Dr. Freiwald's paper, I draw attention to the work of Brode [BRODE, H. L., Phys. Fluids 2, 217 (1959)] on the origin of such a sequence of shocks. Brode's integration of the hydrodynamic equations shows that expansion of an initially uniform sphere of compressed gas must give rise, in addition to an outward-facing shock wave, to an inward-facing shock. The latter implodes on the center, and subsequently is reflected, after outward movement, at the contact surface. As a result there is an indefinite number of pulsations of decreasing amplitude, as the pressure records in this case demonstrate.
with mixtures of CS2 -{- 302 in a glass balloon of 21 cm diameter. The mixture was initiated in the balloon center by 1 gm of mercury fulminate. Because of the effect of the relatively strong shock waves from the initiator there was some doubt about the eXistence of a really self-sustained spherical detonation in the mixture. Also from the theoretical point of view a spherical detonation in gases was believed to be impossible at that time. In the case of initiation of spherical detonation by
DR. W. E. GORDON (Combustion and Explosives Research Inc.): Concerning the secondary shock