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gas two-phase detonations
Twenty-Fifth Symposium (International) on Combustion/The Combustion Institute, 1994/pp. 79-87
PHENOMENOLOGICAL ASPECTS IN EXPLOSIVE POWDER/GAS TWO-PHASE DETONATIONS ALLEN J. TULIS, WILLIAM K. SUMIDA AND RICHARD P. JOYCE HT Research Institute 10 West 35th Street, Chicago, IL 60616-3799, USA AND
DAVID C. HEBERLEIN AND DIVYAKANT L. PATEL U.S. Army, Mine and Countermine, NVESD Fort Belvoir, VA 22060, USA
Attempts were made to detonate a special ball-milled TNT powder of about 30-,am particle size that was dispersed in air, oxygen, and nitrogen at nominally 1 kg/m3 in a highly instrumented 152-mm-diameter 7.31-m-long horizontal detonation tube. Results were successful in both air and oxygen, but not in nitrogen. New modifications to the detonation tube for this work included implementationof two-color temperaturemeasuring instrumentation, and the repositioning of some piezoelectric pressure transducers and fiberoptic light-detector probes so that phenomena such as spinningdetonation and multiple-front detonations could be better identified and quantified. TIGER Code Chapman-Jouguet (CJ) computations were made for TNT, RDX, and mixtures of TNT and RDX in air, oxygen, and nitrogen at concentrations up to 100 kg/m3. The computed results for TNT in air and oxygen at a concentration of 1 kg/m3 for the detonation velocities, pressures, and temperatures were 2.00 km/s, 4.00 MPa, 3250 K in air and 2.05 kin/s, 4.40 MPa, 3740 K in oxygen. Experimental results for TNT powder were 1.82 kin/s, 4.06 MPa, 3245 K in air and 1.90 kin/s, 4.61 MPa, 4240 K in oxygen; the experimental concentrations, which were monitored with laser optics, were determined to be 1 kg/m3 with an estimated error of + 10%. Subsequent experiments were conducted with RDX, alone and in mixtures with TNT, in attempts to achieve detonation in nitrogen. These were successful except when the RDX percentage fell below 20% by weight. Results of successful detonations of RDX/TNT mixtures indicated multiple-front detonations, suggesting that the TNT was reacting behind the RDX detonation front in such a manner that it supported the overall detonation.
Introduction
cal]y dependent upon stoichiometry, homogeneity, and particle size. Hence, fuel powders dispersed in air are very difficult to detonate, even in one-dimensional detonation tube studies, in which case they generally manifest spinning detonation [4]. However, about two decades ago, it was discovered that molecular explosive powders such as PETN, RDX, and TNT could be dispersed in air and readily detonated, yielding stable detonation characteristics that correlated quite well with analytical thermochemical CJ computations [5]. Because stoichiometry in molecular explosives is essentially intrinsic, particle size and homogeneity would appear to be inconsequential, except for minimum concentrations required to sustain propagation of detonation. Subsequent studies demonstrated that the severe criteria for achieving stable detonation of A1 particles dispersed in air are considerably relaxed by sensitization with small amounts of molecular explosive powders such as RDX [6]. Computations and experiments conducted with A1 and carbohydrate powders
The achievement and phenomenology of heterogeneous two-phase solid particulate/gas detonations have become subjects of considerable interest and investigation in recent years, involving fuel, energetic material, and molecular explosive powders dispersed in air (as well as in other gaseous media). However, the phenomenological aspects of these heterogeneous detonations remain as causes of considerable controversy with regard to classical homogeneous Chapman-Jouguet (cJ) detonation theory. Confined detonations of fuel powders dispersed in air have been documented over a decade ago, e.g., detonation of aluminum (A1) powder [1] and grain dust [2], achieved in one-dimensional detonation/shock tubes. (Although not an issue in this paper, the unconfined detonation of A1 powder dispersed in air has also been unequivocally demonstrated [3].) The criteria, however, are very severe. The solid fuel particles must burn sufficiently fast and effectively to support propagation of detonation, and are therefore criti79
80
DETONATIONS AND EXPLOSIONS 4,000
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3,500
. . . . . . . . . .
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E 2,000 1,500 1,000
500
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TNT Ga
i
y:
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10 Density, kg/m3
to which molecular explosives were added demonstrated extensive mitigation of the severe stoichiometry and detonability requirements, respectively [7], both for dispersions in air and in bulk formulations. In other experimental investigations conducted to gain better insight into the mechanisms of heterogeneous two-phase solid powder/gas detonations, the molecular oxidizers ammonium perchlorate and potassimn perchlorate were dispersed and detonated in a gaseous fuel of ethylene [8]. The achievement of detonation in the case of potassium perchlorate was especially significant, in that it demonstrated detonation could proceed by the sequential mechanisms of (1) decomposition of this solid molecular compound to potassium chloride with the attendant release of gaseous oxygen (net heat of reaction about zero), and (2) subsequent combustion of gaseous ethylene in the gaseous oxygen, within the time frame of a stable, propagating detonation. Organic molecular explosives such as TNT, RDX, and PETN can respond by alternate mechanisms under different stimuli, dependent upon their particular chemical structure. For instance, TNT will readily burn in air, usually uneventfully without transition to detonation, whereas both RDX and PETN will not burn per se but will immediately transition to detonation if exposed to flame or sufficient heat. Hence, TNT is highly fuel rich and, if combusted in oxygen, will release almost three times as much heat of reaction as it will in detonation; RDX is essentially fuel oxidizer balanced, and PETN is somewhat oxidizer rich. In this paper, results of recent studies of confined, one-dimensional detonation tube experiments to assess and characterize the detonation of TNT and RDX in air, oxygen, and nitrogen are presented. Fur-
100
FIG. 1. TIGER Code CJ detonation temperature computations for TNT/air, TNT/oxygen, and TNT/nitrogen systems.
ther improvements in the detonation tube facility used in these experiments have been implemented, including provision to monitor detonation temperatures as well as detonation pressures, velocities, and induction times. Modifications have also been made to allow better assessment of phenomena such as spinning detonation and multiple-front detonations. Analytical TIGER Code CJ computations, using the most recent upgraded version of this code for CHNO and other species [9], were also conducted to assess the detonation characteristics of the various heterogeneous systems investigated and for comparative purposes in regard to experimental results that have been obtained.
Analytical Results Figures 1 through 3 present the analytical TIGER Code computational CJ results for detonation temperatures, pressures, and velocities for TNT in air, oxygen, and nitrogen, as a function of concentration. Similar computations were also conducted for various mixtures of TNT and RDX in nitrogen, and are presented in Figs. 4 through 6. In Fig. 1, it is evident that, in the case of TNT, the gaseous phase is quite influential in regard to detonation temperature, up to concentrations of about 10 and 50 kg/ma in the case of air and oxygen, respectively, as would be expected. However, there is extensive variability in the lower concentrations; the influence of the amount of oxygen is paramount. Similar observations are noted in Figs. 2 and 3 for the case of detonation pressures and velocities, but to a much lesser degree. Enhancement of detonation characteristics by the improvement of stoichiometry
PHENOMENOLOGICAL ASPECTS IN EXPLOSIVE POWDER/GAS TWO-PHASE DETONATIONS
81
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FIG. 2. TIGER Code c j detonation pressure computations for TNT/ air, TNT/oxygen, and TNT/nitrogen systems.
100
Fro. 3. TIGER Code CJ detonation velocity computations for TNT/ air, TNT/oxygen, and TNT/nitrogen systems.
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available from oxygen is manifested mainly in detonation temperatures. Figures 4 through 6 illustrate that detonation characteristics of RDX are considerably better than those of TNT when detonated in nitrogen, as would be expected based on the fact that TNT is so fuel rich and that RDX is essentially stoichiometrically balanced in regard to its fuel/oxidizer structure.
D e t o n a t i o n T u b e Facility
Figure 7 illustrates the latest modified and instrumentally improved IITRI detonation tube Facility. The instrumented section is constructed of stainless
steel and, as illustrated, contains one set each of diametrically opposed piezoelectric pressure transducers and fiber-optic light-detector probes a t each of the 11 stations. However, to better assess phenomena such as spinning detonation and multiple-front detonations, the five sensor locations at four of the 1i stations were altered so that Station Nos. 2 and 8 consisted of five pressure sensors for each, and similarly, Station Nos. 3 and 9 consisted of five light sensors for each. Ratio two-color radiometry was selected for providing the basis for measurement of two-phase detonation temperatures. This technique for temperature measurements is based on the relative relationship of incident radiation at one wavelength
82
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Fro. 4. TIGER Code CJ detonation temperature computations for various TNT/RDX mixtures in nitrogen.
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to that at another wavelength, i.e., Plank's law for spectral emissive power of a blackbody radiator at relatively short wavelengths. Fiber optics were used to receive and transmit the radiation from the detonation front to appropriate detectors and signal conditioning circuitry. For more comprehensive assessment of this methodology, the temperaturemeasuring instrumentation (TMI) involved three color wavelengths to provide three independent measurements and to cover a larger portion of the radiation spectrum. The TMI system consists of a quartz lens, a trifurcated bundle made from glass fibers, bandpass filters, and silicon photodiodes. The fiber optics, in conjunction with the bandpass filters, are used to provide the three different color outputs from the temperature source. The fiber bundles used are comb ran-
iii
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100
FIG. 5. TIGER Code CJ detonation pressure computations for various TNT/RDX mixtures in nitrogen.
domized to produce evenly divided outputs. When light is uniformly incident onto the large unified input, the trifurcated bundle transmits 30% of the total to each leg. Approximately 4% is reflected at the input and output. The transmittance of glass fiber over a 1-m length is about 48%, and remains relatively constant over the wavelength range 400-1300 nm. The numerical aperture is 0.56, thus providing an acceptance cone angle of 68~ The bandpass filters are three-cavity interference filters, pass a narrow band of visible light, and reject out-of-band wavelengths in the UV and IR range. The center wavelengths of 500, 600, 700, and 800 nm were selected. Two units were fabricated and implemented into the instrumented section of the detonation tube. Unit No. 1 has center wavelengths of 500, 600, and 700 nm, and Unit No. 2 has center wavelengths of 600,
PHENOMENOLOGICAL ASPECTS IN EXPLOSIVE POWDER/GAS TWO-PHASE DETONATIONS
83
3o00 i. i i!i 2500 E 9
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FIG. 6. TIGER Code cJ detonation velocity computations for various TNT/RDX mixtures in nitrogen.
100
Density, kg/m3
POWDERDISSEMINATIONSYSTEM
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FIG. 7. Schematic of the detonation tube. 700, and 800 nm. All filters have a bandwidth of 70 nm. The transmittance for these filters is in the range of 50-60%. The filters were selected to cover the visible and near-IR raaage and have one color pmr in common, 600 and 700 nm. This selection was made to provide information on response as a function of location as well as on the consistency of the measurements. The photodiodes have a radiant sensitivity of about 0.35 A/W in the range of interest. The radiant sensitivity is nil in the UV and IR range and somewhat wavelength dependent in the band of interest. The photodiodes are operated in the photoelectric mode and provide for a rise time (10-90%) of 0.25 #s with a linear response range extending over five orders of magnitude. Results of pretest calibrations and the data from
the first detonation tube experiment showed that the use of a collimating beam probe was a desirable feature; hence, all three channels of each unit were equipped with colhmating probes for all subsequent experiments. In addition, the line amplifiers used for the initial experiments were removed from the system after it was determined that the amplitude of the signals generated by the photodiodes was of a suitable level for recording. The radiated energies are not measured directly; they are conducted through the optical fibers and, in turn, through the filters to the sensing elements of the photodiodes. The radiated energies cause current flows in the photodiode circuits, which are detected by measuring the voltages developed across appropriate load resistors. These voltages are linearly related to the radiated energies, but their amplitude is
DETONATIONS AND EXPLOSIONS
84
TABLE 1 Experimental detonation characteristics of TNT powder dispersed in various gases
Gas
Velocity (km/s)
Pressure (MPa)
Temperature (K)
Induction Time ~us)
Mr Oxygen Nitrogen
1.82 1.90 .
4.06 4.61
3245 4240
18 15
.
influenced by the overall system characteristics. This TMI system is explicitly described elsewhere [10].
Experimental Procedure The experimental procedures in these studies were essentially identical to those previously reported [5,8], with some modifications/improvements. Two lasers were used as depicted in Fig. 7 for monitoring concentration of the powders as well as establishing the timing for the initiation of the explosive charge used to initiate detonation in these heterogeneous systems. The reason for the extension of the detonation tube is that, although in most experiments the initiation charge consisted of a detonator and a small 2.8-g booster pellet (A4), the initiation of detonation in many experiments required substantially larger initiation charges, e.g., 10 to over 30 g of total explosive charge. Calibration studies indicated that the shorter detonation tube did not allow sufficient attenuation of the blast output to preclude interference to the pressure and light sensors; i.e., in some experiments, blast enhancements were obtained that could have been misconstrued to be nonideal detonations. Calibration experiments with the present extended detonation tube using explosive initiation charges up to 30 g C4 indicated complete attenuation of light output signals and decaying blast pressures below 0.5 MPa in the instrumented section. The most difficult aspect of these dispersed powder experiments was, and remains, the achievement of homogeneous concentrations throughout the detonation tube. Calibration experiments were always conducted prior to detonation testing in order to obtain concentration information as well as timing requirements.
Experimental Genesis and Results It is known that PETN and RDX, which are not combustible as is TNT, detonate individually as discrete particles when dispersed in a gaseous medium such as air [11]. However, it is also known that the
.
.
addition of flaked M powder to PETN and RDX dispersions in air provides considerably improved detonation characteristics [12]. Hence, it became evident that the M is reacting within the detonation zone in such a manner that the overall detonation and blast output are enhanced. In other words, the M participates in the detonation mechanism. Because TNT is an excellent fuel as well as an explosive, studies were conducted with specially ballmilled TNT of about 30-/tm average particle size, dispersed in air, oxygen, and nitrogen in the detonation tube. The concentrations were about i kg/ma. The TNT readily detonated in air and in oxygen, but it did not detonate in nitrogen. Certainly TNT powder dispersed in nitrogen, if of sufficient concentration and suitable size and confinement, will detonate. Numerous additional experiments were conducted, i.e., with initiation explosive charges up to 30 g C4 or Detasheet, and at concentrations in excess of 2 kg/m3. All failed to detonate. Evidently, the presence of oxygen was requisite to propagate detonation under the experimental conditions imposed. Results of representative experiments, i.e., those that had the most uniform concentration as well as incremental pressures and velocities throughout the instrumented section of the detonation tube, are ineluded in Table 1. These values can be compared to the TIGER Code analytical computations at the approximately equivalent concentration of 1 kg/m 3, as presented in Table 2. To further investigate this phenomenology, experiments were conducted with about 22/~m RDX alone and as mixtures of RDX and TNT dispersed in nitrogen. The RDX alone, as expected, readily detonated in nitrogen. Mixtures of RDX and TNT of 75:25, 50 : 50, 25 : 75, and 10 : 90 by weight RDX: TNT were also tested. Detonations were achieved in all of these ratios except the 10 : 90 ratio; subsequently, ratios of 15 : 85 and 20 : 80 were also tested; the 15 : 85 ratio failed, but the 20 : 80 ratio detonated. Figure 8 illustrates the TMI responses at the three wavelengths monitored at Station No. 8, along with a piezoelectric pressure transducer response at the same location, for RDX dispersed and detonated in nitrogen gas. Figure 9 illustrates the same responses,
PHENOMENOLOGICAL ASPECTS IN EXPLOSIVE POWDER/GAS TWO-PHASE DETONATIONS
85
TABLE 2 Analytical TIGER code detonation characteristics of TNT powder dispersed in various gases at 1 kg/m
Gas
Velocity (krrds)
Pressure (MPa)
Temperature (K)
Induction Time ,us
Air Oxygen Nitrogen
2.00 2.05 1.50
4.00 4.40 2.10
3250 3740 1775
NA~ NA NA
~NA = not applicable.
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i .........
; .... i .......
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i
.... i ......
m
I
t
l
CONSTANTTIME BASE, 100 #s/div
FIG. 8. TMI responses at three wavelengths of 600, 700, and 800 nm in the detonation of RDX powder dispersed in nitrogen along with a piezoelectric pressure transducer response (top trace), all at Station No. 8 and on the same time base.
i]~ !
...... i ........
~s
.............
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i
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FIG. 10. Temperature, pressure, and light sensor responses in the detonation of 30-,um TNT powder dispersed in oxygen in the detonation tube. First (top) trace: TMI Unit No. 1 response; 2nd trace: piezoelectric pressure transducer response; 3rd trace: fiber-optic light-detector response; and 4th (bottom) trace: TMI Unit No. 2 response. Top three responses at Station No. 6; bottom trace at Station No. 8, with all traces on the same time base.
:::::: is at Station No. 8. All responses are on the same time scale.
i--
6
Conclusions ~ i
:
i
i
CONSTANTTIME BASE, 100 ys/div
FIG. 9. TMI responses at three wavelengths of 600, 700, and 800 nm in the detonation of 50 : 50 weight ratio mixtures of RDX and TNT powders dispersed in nitrogen along with a piezoelectric pressure transducer response (top trace), all at Station No. 8 and on the same time base.
at the same location, for a mixture of 5 0 : 5 0 R D X : T N T by weight also dispersed and detonated in nitrogen. For comparative purposes, Fig. 10 is included here to illustrate the m u c h "cleaner" and essentially singular responses of temperature, pressure, and light sensors for the denotation of TNT powder in oxygen. The T M I Unit 1, pressure, and light responses are at Station No. 6; the T M I Unit 2 response
It is premature to make definitive conclusions regarding this work, but the following observations will suffice for now. U n d e r the constraints of the experimental conditions imposed by the detonation tube size, concentrations investigated, and initiation techniques used, the nominal 30-#m average particle size T N T does not appear to be detonable in nitrogen. T h e r e can be a n u m b e r of implications regarding this. First, from Table 2 it is observed that, at a TNT eoneentration of 1 kg/m 3 in nitrogen, the detonation temperature and pressure are about 50% less, and the detonation velocity is about 25% less, than that in air or oxygen. Hence, energetically this concentration may be too low to propagate detonation in nitrogen; below certain m i n i m u m concentrations, even R D X and PETN will fail to detonate whether dispersed in air or any other gaseous medium.
86
DETONATIONS AND EXPLOSIONS
Second, this would suggest that TNT detonation in a gaseous medium, at very low concentrations, is dependent on a burning mechanism such as is the case for fuels, e.g., A1. Both analytical and experimental results confirm that TNT dispersions in air provide superior detonation characteristics to those of RDX or PETN at low concentrations wherein the oxygen from the air enhances the TNT energy release [4]. Third, based on the detonation of RDX/TNT mixtures in nitrogen, as readily indicated in the sensor responses of pressure, light, and TMI records, it would appear that the RDX is detonating and that the TNT is "reacting" behind the RDX detonation front so as to extend the reaction zone, i.e., a "second front" detonation. Finally, in all of these experiments involving TNT and RDX in various gaseous media, detonations that were achieved did not involve spinning detonation. The detonation fronts were generally classical, such as for homogeneous fuel/oxidizer gaseous systems, with the exception noted above regarding mixtures of RDX and TNT. Regarding the particle size of the TNT, in previous detonation tube experiments involvingTNT particles dispersed and detonated in air [4], smaller, average 30-/~m particle size TNT detonation characteristic results correlated very well with analytically predicted values, whereas for larger, average 200-/zm particle size TNT, the results were considerably lower than those of the smaller particles. This was attributed to the fact that when the smaller particles detonated they were much more readily enhanced by oxygen from the air. However, in a nitrogen atmosphere, it is quite possible that larger particles would be more readily detonated.
REFERENCES 1. Tulis, A. J., and Selman, J. R., Nineteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1982, pp. 655-663.
2. Kauffman, C. W., Wolansld, P., Arisoy, A., Adams, P. R., Maker, B. N., and Nicholls, J. A., Dynamics of Shock Waves, Explosives, and Detonations (J. R. Bowen, N. Manson, A. K. Oppenheim, and R. I. Soloukhin, Eds.), AIAA, New York, 1984, Vol. 94, p. 221. 3. Tulis, A. J., First (International) Colloquium on Explosivity of Industrial Dusts, Polish Academy of Sciences, Warsaw, 1984, pp. 178-186. 4. Tulis, A. J., Sumida, W. K., Heberlein, D. C., Patel, D. L., and Egghart, H., Fifth (International) Colloquium on Dust Explosions, Oficyna Wydawnicza Politechniki Warszawskiej, Warsaw, 1993, pp. 391-400. 5. Tulis, A. J., Austing, J. L., and Heberlein, D. C., Fourteenth (International) Symposium on Shock Tubes and Waves, New South Wales University Press, Kingston, New South Wales, Australia, 1983, pp. 447-454. 6. Tulis, A. J., International Symposium on Intense Dynamic Loading and Its Effects, Beijing Science Press, Beijing, P. R. China, 1986, pp. 213-218. 7. Tulis, A. J., Austing, J. L., Sumida, W. K., Baker, D. E., and Hrdina, D. J., Ninth Symposium (International) on Detonation, Office of the Chief of Naval Research, Arlington, Virginia OCNR-113291-7, 1989, pp. 972-983. 8. Egghart, H., Heberlein, D. C., Tulis, A. J., and Sumida, W. K., Eighteenth (International) Pyrotechnics Seminar, liT Research Institute, Chicago, 1992, pp. 1083-1107. 9. Hobbs, M. L., and Baer, M. R., EUROPYRO 93---5th Congres (International) de Pyrotechnie du Groupe de Travail, Association Francaise de Pyrotechnie, Vairessur-Marne, France, 1993, pp. 53--59. 10. Tulis, A. J., Sumida, W. K., and Joyce, R. P., Advanced Detonation Tube Studies of Hybrid Solid-Gas Dispersions, liT Research Institute ReportNo. E06661-17, October 1993. 11. Tulis, A. J., Advanced Concepts for Chemical Energetics Manifestations, IIT Research Institute Report No. H01105-F; unclassified volume, 1988. 12. Tulis, A. J., Development and Demonstration of a Bimedal Mine Clearing Munition, U.S. Army BRDEC, liT Research Institute Final Report No. CO6705, March 1993.
COMMENTS Martin Sichel, The University of Michigan, USA. Professor Hertzberg has commented on the influence of particle size on detonability. We observed this effect in the case of RDX dust about 10 years ago. For a given loading ratio, the finer dust particles were more difficult to detonate or, in fact, transition to detonation failed to occur. Modeling suggested that the reason for this behavior is that the finer dust absorbs heat more rapidly so that the temperature of the gas around the particles is lower. This resuits in an increase in ignition delay or a failure of ignition altogether.
Author's Reply. Your previous findings regarding the influence of particle size, as well as the comments made by Professor Hertzberg, are relevant and have also been observed by us in other studies in which particle size was specifically investigated. Your suggestion based on your modeling work is very reasonable; however, we believe that mass transport also plays an important role; i.e., finer particles are readily accelerated into the high-temperature convective gas flow behind the incident shock wave whereas more massive particles will "resist" and induce considerable convective, viscous heating of the surface of
PHENOMENOLOGICAL ASPECTS IN EXPLOSIVE POWDER/GAS TWO-PHASE DETONATIONS
87
these larger particles. In the case of monopropellants such as RDX, this may be sufficient to induce ignition, or possibly direct detonation, of such larger particles much more quickly than the smaller particles that are entrained in the gas flow and depend on slower, mainly conductive heating. The influence of particle size is a complex matter, involving factors such as particle geometry, chemical kinetics, and the type of particles; i.e., ignition, and particularly detonation, of starch, aluminum, and RDX particles could be expected to involve differing phenomena.
ture, pressure, and velocity, than RDX or HMX, or PETN for that matter, since the presence of oxygen from air contributes substantially to the overall energy release at this near-stoichiometric concentration for TNT in air. The induction time was obtained by measuring the time difference between the arrival of the shock front (pressure response signal) relative to the flame front (light response signal) at each monitoring station. Since 11 stations were used in each experiment, both stability and accuracy of induction times as well as the detonation were determined.
A. A. Borisov, Institute of Chemical Physics, Russia.
Herman Krier, University of Illinois., USA. I'm not so sure that if you could have suspended greater concentrations of TNT in nitrogen (i.e., greater than 1-2 kg/m 3) that you would have seen a detonation. Clearly, a richer concentration of your 30-,um particles if promptly shock ignited would detonate, even in nitrogen.
What was the minimum TNT concentration in detonable mixtures with air? We performed similar experiments with TNT, RDX, and HMX suspensions in air and found that TNT mixtures detonated at 0.25 kg/m 3, whereas HMX and RDX mixtures detonated starting with 1 kg/m 3. We found that the TNT mixtures were more detonable than HMX and RDX. How did you define the induction time?
Author's" Reply. We did not specifically investigate the lower concentration limits for detonation of TNT, RDX, and PETN in air but did achieve detonations of all three at concentrations in the range of 0.5 kg/m 3. Your finding that TNT would detonate in air down to 0.25 kg/m 3, in particular at a concentration considerably lower than those for RDX and HMX, is not surprising; analytical computations indicate that TNT has much greater detonation performance at 0.25 kg/m 3 in terms of detonation tempera-
Author's Reply. You are absolutely correct; and we did state in the paper: "Certainly TNT powder dispersed in nitrogen, if of sufficient concentration and suitable size and confinement, will detonate . . . . In fact, we have achieved the detonation of TNT powder dispersed in air, in an unconfined state, at concentrations in excess of 20 and perhaps as high as 50 kg/m 3. Obviously, at these concentrations the influence of oxygen from the air is inconsequential; hence, it can be inferred that detonation would have been achieved in the absence of oxygen altogether.
PHENOMENOLOGICAL ASPECTS IN EXPLOSIVE POWDER/GAS TWO-PHASE DETONATIONS ALLEN J. TULIS, WILLIAM K. SUMIDA AND RICHARD P. JOYCE HT Research Institute 10 West 35th Street, Chicago, IL 60616-3799, USA AND
DAVID C. HEBERLEIN AND DIVYAKANT L. PATEL U.S. Army, Mine and Countermine, NVESD Fort Belvoir, VA 22060, USA
Attempts were made to detonate a special ball-milled TNT powder of about 30-,am particle size that was dispersed in air, oxygen, and nitrogen at nominally 1 kg/m3 in a highly instrumented 152-mm-diameter 7.31-m-long horizontal detonation tube. Results were successful in both air and oxygen, but not in nitrogen. New modifications to the detonation tube for this work included implementationof two-color temperaturemeasuring instrumentation, and the repositioning of some piezoelectric pressure transducers and fiberoptic light-detector probes so that phenomena such as spinningdetonation and multiple-front detonations could be better identified and quantified. TIGER Code Chapman-Jouguet (CJ) computations were made for TNT, RDX, and mixtures of TNT and RDX in air, oxygen, and nitrogen at concentrations up to 100 kg/m3. The computed results for TNT in air and oxygen at a concentration of 1 kg/m3 for the detonation velocities, pressures, and temperatures were 2.00 km/s, 4.00 MPa, 3250 K in air and 2.05 kin/s, 4.40 MPa, 3740 K in oxygen. Experimental results for TNT powder were 1.82 kin/s, 4.06 MPa, 3245 K in air and 1.90 kin/s, 4.61 MPa, 4240 K in oxygen; the experimental concentrations, which were monitored with laser optics, were determined to be 1 kg/m3 with an estimated error of + 10%. Subsequent experiments were conducted with RDX, alone and in mixtures with TNT, in attempts to achieve detonation in nitrogen. These were successful except when the RDX percentage fell below 20% by weight. Results of successful detonations of RDX/TNT mixtures indicated multiple-front detonations, suggesting that the TNT was reacting behind the RDX detonation front in such a manner that it supported the overall detonation.
Introduction
cal]y dependent upon stoichiometry, homogeneity, and particle size. Hence, fuel powders dispersed in air are very difficult to detonate, even in one-dimensional detonation tube studies, in which case they generally manifest spinning detonation [4]. However, about two decades ago, it was discovered that molecular explosive powders such as PETN, RDX, and TNT could be dispersed in air and readily detonated, yielding stable detonation characteristics that correlated quite well with analytical thermochemical CJ computations [5]. Because stoichiometry in molecular explosives is essentially intrinsic, particle size and homogeneity would appear to be inconsequential, except for minimum concentrations required to sustain propagation of detonation. Subsequent studies demonstrated that the severe criteria for achieving stable detonation of A1 particles dispersed in air are considerably relaxed by sensitization with small amounts of molecular explosive powders such as RDX [6]. Computations and experiments conducted with A1 and carbohydrate powders
The achievement and phenomenology of heterogeneous two-phase solid particulate/gas detonations have become subjects of considerable interest and investigation in recent years, involving fuel, energetic material, and molecular explosive powders dispersed in air (as well as in other gaseous media). However, the phenomenological aspects of these heterogeneous detonations remain as causes of considerable controversy with regard to classical homogeneous Chapman-Jouguet (cJ) detonation theory. Confined detonations of fuel powders dispersed in air have been documented over a decade ago, e.g., detonation of aluminum (A1) powder [1] and grain dust [2], achieved in one-dimensional detonation/shock tubes. (Although not an issue in this paper, the unconfined detonation of A1 powder dispersed in air has also been unequivocally demonstrated [3].) The criteria, however, are very severe. The solid fuel particles must burn sufficiently fast and effectively to support propagation of detonation, and are therefore criti79
80
DETONATIONS AND EXPLOSIONS 4,000
I;ttt[
I
3,500
. . . . . . . . . .
I
,~ 3 , 0 0 0
i
~= 2 , 5 0 0
!ii
E 2,000 1,500 1,000
500
0.1
TNT Ga
i
y:
~i~ + Nitre gen 1
10 Density, kg/m3
to which molecular explosives were added demonstrated extensive mitigation of the severe stoichiometry and detonability requirements, respectively [7], both for dispersions in air and in bulk formulations. In other experimental investigations conducted to gain better insight into the mechanisms of heterogeneous two-phase solid powder/gas detonations, the molecular oxidizers ammonium perchlorate and potassimn perchlorate were dispersed and detonated in a gaseous fuel of ethylene [8]. The achievement of detonation in the case of potassium perchlorate was especially significant, in that it demonstrated detonation could proceed by the sequential mechanisms of (1) decomposition of this solid molecular compound to potassium chloride with the attendant release of gaseous oxygen (net heat of reaction about zero), and (2) subsequent combustion of gaseous ethylene in the gaseous oxygen, within the time frame of a stable, propagating detonation. Organic molecular explosives such as TNT, RDX, and PETN can respond by alternate mechanisms under different stimuli, dependent upon their particular chemical structure. For instance, TNT will readily burn in air, usually uneventfully without transition to detonation, whereas both RDX and PETN will not burn per se but will immediately transition to detonation if exposed to flame or sufficient heat. Hence, TNT is highly fuel rich and, if combusted in oxygen, will release almost three times as much heat of reaction as it will in detonation; RDX is essentially fuel oxidizer balanced, and PETN is somewhat oxidizer rich. In this paper, results of recent studies of confined, one-dimensional detonation tube experiments to assess and characterize the detonation of TNT and RDX in air, oxygen, and nitrogen are presented. Fur-
100
FIG. 1. TIGER Code CJ detonation temperature computations for TNT/air, TNT/oxygen, and TNT/nitrogen systems.
ther improvements in the detonation tube facility used in these experiments have been implemented, including provision to monitor detonation temperatures as well as detonation pressures, velocities, and induction times. Modifications have also been made to allow better assessment of phenomena such as spinning detonation and multiple-front detonations. Analytical TIGER Code CJ computations, using the most recent upgraded version of this code for CHNO and other species [9], were also conducted to assess the detonation characteristics of the various heterogeneous systems investigated and for comparative purposes in regard to experimental results that have been obtained.
Analytical Results Figures 1 through 3 present the analytical TIGER Code computational CJ results for detonation temperatures, pressures, and velocities for TNT in air, oxygen, and nitrogen, as a function of concentration. Similar computations were also conducted for various mixtures of TNT and RDX in nitrogen, and are presented in Figs. 4 through 6. In Fig. 1, it is evident that, in the case of TNT, the gaseous phase is quite influential in regard to detonation temperature, up to concentrations of about 10 and 50 kg/ma in the case of air and oxygen, respectively, as would be expected. However, there is extensive variability in the lower concentrations; the influence of the amount of oxygen is paramount. Similar observations are noted in Figs. 2 and 3 for the case of detonation pressures and velocities, but to a much lesser degree. Enhancement of detonation characteristics by the improvement of stoichiometry
PHENOMENOLOGICAL ASPECTS IN EXPLOSIVE POWDER/GAS TWO-PHASE DETONATIONS
81
iiiii!!iiiill iiii
'11 t:
100 '
ii iii
10 II', "'-~. . . .
I IIIII
III
l
i
0.1 ....
:
i iiiii
IIIIll
1
0.1
100
FIG. 2. TIGER Code c j detonation pressure computations for TNT/ air, TNT/oxygen, and TNT/nitrogen systems.
100
Fro. 3. TIGER Code CJ detonation velocity computations for TNT/ air, TNT/oxygen, and TNT/nitrogen systems.
Illlll
III
10 Density, kg/m3
2,500
E
2,000
r
/
> 1,500 *,
1,ooo TNT Ga:
50C
0.1
;y;tal
-}- Nitro ;]e n " 1
/gen 10
Density, kg/m3
available from oxygen is manifested mainly in detonation temperatures. Figures 4 through 6 illustrate that detonation characteristics of RDX are considerably better than those of TNT when detonated in nitrogen, as would be expected based on the fact that TNT is so fuel rich and that RDX is essentially stoichiometrically balanced in regard to its fuel/oxidizer structure.
D e t o n a t i o n T u b e Facility
Figure 7 illustrates the latest modified and instrumentally improved IITRI detonation tube Facility. The instrumented section is constructed of stainless
steel and, as illustrated, contains one set each of diametrically opposed piezoelectric pressure transducers and fiber-optic light-detector probes a t each of the 11 stations. However, to better assess phenomena such as spinning detonation and multiple-front detonations, the five sensor locations at four of the 1i stations were altered so that Station Nos. 2 and 8 consisted of five pressure sensors for each, and similarly, Station Nos. 3 and 9 consisted of five light sensors for each. Ratio two-color radiometry was selected for providing the basis for measurement of two-phase detonation temperatures. This technique for temperature measurements is based on the relative relationship of incident radiation at one wavelength
82
DETONATIONS AND EXPLOSIONS !~l 84184184 84 i iiiili! ~i
e-
4 iiiiiiii!i 2
~a I-
,....--
II E ~a
11"NT/RDXICo~pffs~ti):
itio
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0
0.1
,5 + , 0 O , , o R ~ ,
1
100
10 Density, kg/m3
1,000 . . . . . . . . . . ] '~[l
100
II',I , , ,~
~ ,,J,F
[Jlr
,
,
I
I
~JJJ~
IIII
Ilia
~
,
frr
, ,,
Fro. 4. TIGER Code CJ detonation temperature computations for various TNT/RDX mixtures in nitrogen.
.....
,,
'
~-: :::::
r
O..
I
I I I I I
I
dL,, '"
Ill! r
0.1 0.1
~100~/o
iiLLJ
'5/25 I - ~ 3 /
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Density, k g / m 3
to that at another wavelength, i.e., Plank's law for spectral emissive power of a blackbody radiator at relatively short wavelengths. Fiber optics were used to receive and transmit the radiation from the detonation front to appropriate detectors and signal conditioning circuitry. For more comprehensive assessment of this methodology, the temperaturemeasuring instrumentation (TMI) involved three color wavelengths to provide three independent measurements and to cover a larger portion of the radiation spectrum. The TMI system consists of a quartz lens, a trifurcated bundle made from glass fibers, bandpass filters, and silicon photodiodes. The fiber optics, in conjunction with the bandpass filters, are used to provide the three different color outputs from the temperature source. The fiber bundles used are comb ran-
iii
10:)o,i D
100
FIG. 5. TIGER Code CJ detonation pressure computations for various TNT/RDX mixtures in nitrogen.
domized to produce evenly divided outputs. When light is uniformly incident onto the large unified input, the trifurcated bundle transmits 30% of the total to each leg. Approximately 4% is reflected at the input and output. The transmittance of glass fiber over a 1-m length is about 48%, and remains relatively constant over the wavelength range 400-1300 nm. The numerical aperture is 0.56, thus providing an acceptance cone angle of 68~ The bandpass filters are three-cavity interference filters, pass a narrow band of visible light, and reject out-of-band wavelengths in the UV and IR range. The center wavelengths of 500, 600, 700, and 800 nm were selected. Two units were fabricated and implemented into the instrumented section of the detonation tube. Unit No. 1 has center wavelengths of 500, 600, and 700 nm, and Unit No. 2 has center wavelengths of 600,
PHENOMENOLOGICAL ASPECTS IN EXPLOSIVE POWDER/GAS TWO-PHASE DETONATIONS
83
3o00 i. i i!i 2500 E 9
2000
= 1500
10001
50O 0.1
L: i t:ttHtLt,,oIo, : J.II 1
10
FIG. 6. TIGER Code cJ detonation velocity computations for various TNT/RDX mixtures in nitrogen.
100
Density, kg/m3
POWDERDISSEMINATIONSYSTEM
INSTRUMENT
/
CAMERA(2) 'i INITIATOR
AREA
t
I
..
LASER(2) I
7.31m
-
.
.
.
.
.
FIG. 7. Schematic of the detonation tube. 700, and 800 nm. All filters have a bandwidth of 70 nm. The transmittance for these filters is in the range of 50-60%. The filters were selected to cover the visible and near-IR raaage and have one color pmr in common, 600 and 700 nm. This selection was made to provide information on response as a function of location as well as on the consistency of the measurements. The photodiodes have a radiant sensitivity of about 0.35 A/W in the range of interest. The radiant sensitivity is nil in the UV and IR range and somewhat wavelength dependent in the band of interest. The photodiodes are operated in the photoelectric mode and provide for a rise time (10-90%) of 0.25 #s with a linear response range extending over five orders of magnitude. Results of pretest calibrations and the data from
the first detonation tube experiment showed that the use of a collimating beam probe was a desirable feature; hence, all three channels of each unit were equipped with colhmating probes for all subsequent experiments. In addition, the line amplifiers used for the initial experiments were removed from the system after it was determined that the amplitude of the signals generated by the photodiodes was of a suitable level for recording. The radiated energies are not measured directly; they are conducted through the optical fibers and, in turn, through the filters to the sensing elements of the photodiodes. The radiated energies cause current flows in the photodiode circuits, which are detected by measuring the voltages developed across appropriate load resistors. These voltages are linearly related to the radiated energies, but their amplitude is
DETONATIONS AND EXPLOSIONS
84
TABLE 1 Experimental detonation characteristics of TNT powder dispersed in various gases
Gas
Velocity (km/s)
Pressure (MPa)
Temperature (K)
Induction Time ~us)
Mr Oxygen Nitrogen
1.82 1.90 .
4.06 4.61
3245 4240
18 15
.
influenced by the overall system characteristics. This TMI system is explicitly described elsewhere [10].
Experimental Procedure The experimental procedures in these studies were essentially identical to those previously reported [5,8], with some modifications/improvements. Two lasers were used as depicted in Fig. 7 for monitoring concentration of the powders as well as establishing the timing for the initiation of the explosive charge used to initiate detonation in these heterogeneous systems. The reason for the extension of the detonation tube is that, although in most experiments the initiation charge consisted of a detonator and a small 2.8-g booster pellet (A4), the initiation of detonation in many experiments required substantially larger initiation charges, e.g., 10 to over 30 g of total explosive charge. Calibration studies indicated that the shorter detonation tube did not allow sufficient attenuation of the blast output to preclude interference to the pressure and light sensors; i.e., in some experiments, blast enhancements were obtained that could have been misconstrued to be nonideal detonations. Calibration experiments with the present extended detonation tube using explosive initiation charges up to 30 g C4 indicated complete attenuation of light output signals and decaying blast pressures below 0.5 MPa in the instrumented section. The most difficult aspect of these dispersed powder experiments was, and remains, the achievement of homogeneous concentrations throughout the detonation tube. Calibration experiments were always conducted prior to detonation testing in order to obtain concentration information as well as timing requirements.
Experimental Genesis and Results It is known that PETN and RDX, which are not combustible as is TNT, detonate individually as discrete particles when dispersed in a gaseous medium such as air [11]. However, it is also known that the
.
.
addition of flaked M powder to PETN and RDX dispersions in air provides considerably improved detonation characteristics [12]. Hence, it became evident that the M is reacting within the detonation zone in such a manner that the overall detonation and blast output are enhanced. In other words, the M participates in the detonation mechanism. Because TNT is an excellent fuel as well as an explosive, studies were conducted with specially ballmilled TNT of about 30-/tm average particle size, dispersed in air, oxygen, and nitrogen in the detonation tube. The concentrations were about i kg/ma. The TNT readily detonated in air and in oxygen, but it did not detonate in nitrogen. Certainly TNT powder dispersed in nitrogen, if of sufficient concentration and suitable size and confinement, will detonate. Numerous additional experiments were conducted, i.e., with initiation explosive charges up to 30 g C4 or Detasheet, and at concentrations in excess of 2 kg/m3. All failed to detonate. Evidently, the presence of oxygen was requisite to propagate detonation under the experimental conditions imposed. Results of representative experiments, i.e., those that had the most uniform concentration as well as incremental pressures and velocities throughout the instrumented section of the detonation tube, are ineluded in Table 1. These values can be compared to the TIGER Code analytical computations at the approximately equivalent concentration of 1 kg/m 3, as presented in Table 2. To further investigate this phenomenology, experiments were conducted with about 22/~m RDX alone and as mixtures of RDX and TNT dispersed in nitrogen. The RDX alone, as expected, readily detonated in nitrogen. Mixtures of RDX and TNT of 75:25, 50 : 50, 25 : 75, and 10 : 90 by weight RDX: TNT were also tested. Detonations were achieved in all of these ratios except the 10 : 90 ratio; subsequently, ratios of 15 : 85 and 20 : 80 were also tested; the 15 : 85 ratio failed, but the 20 : 80 ratio detonated. Figure 8 illustrates the TMI responses at the three wavelengths monitored at Station No. 8, along with a piezoelectric pressure transducer response at the same location, for RDX dispersed and detonated in nitrogen gas. Figure 9 illustrates the same responses,
PHENOMENOLOGICAL ASPECTS IN EXPLOSIVE POWDER/GAS TWO-PHASE DETONATIONS
85
TABLE 2 Analytical TIGER code detonation characteristics of TNT powder dispersed in various gases at 1 kg/m
Gas
Velocity (krrds)
Pressure (MPa)
Temperature (K)
Induction Time ,us
Air Oxygen Nitrogen
2.00 2.05 1.50
4.00 4.40 2.10
3250 3740 1775
NA~ NA NA
~NA = not applicable.
d I--
".............. i ............... i ............... i ............... i............... i ............... i ............... i ............. .......
i .........
; .... i .......
i .......
i .......
i
.... i ......
m
I
t
l
CONSTANTTIME BASE, 100 #s/div
FIG. 8. TMI responses at three wavelengths of 600, 700, and 800 nm in the detonation of RDX powder dispersed in nitrogen along with a piezoelectric pressure transducer response (top trace), all at Station No. 8 and on the same time base.
i]~ !
...... i ........
~s
.............
i
i
9~ .......
i i ....~".....~.~q . . ~.......... ~_d " ' ~ , ...... i ................ CONSTANTTIME BASE, 200 ~s/div
FIG. 10. Temperature, pressure, and light sensor responses in the detonation of 30-,um TNT powder dispersed in oxygen in the detonation tube. First (top) trace: TMI Unit No. 1 response; 2nd trace: piezoelectric pressure transducer response; 3rd trace: fiber-optic light-detector response; and 4th (bottom) trace: TMI Unit No. 2 response. Top three responses at Station No. 6; bottom trace at Station No. 8, with all traces on the same time base.
:::::: is at Station No. 8. All responses are on the same time scale.
i--
6
Conclusions ~ i
:
i
i
CONSTANTTIME BASE, 100 ys/div
FIG. 9. TMI responses at three wavelengths of 600, 700, and 800 nm in the detonation of 50 : 50 weight ratio mixtures of RDX and TNT powders dispersed in nitrogen along with a piezoelectric pressure transducer response (top trace), all at Station No. 8 and on the same time base.
at the same location, for a mixture of 5 0 : 5 0 R D X : T N T by weight also dispersed and detonated in nitrogen. For comparative purposes, Fig. 10 is included here to illustrate the m u c h "cleaner" and essentially singular responses of temperature, pressure, and light sensors for the denotation of TNT powder in oxygen. The T M I Unit 1, pressure, and light responses are at Station No. 6; the T M I Unit 2 response
It is premature to make definitive conclusions regarding this work, but the following observations will suffice for now. U n d e r the constraints of the experimental conditions imposed by the detonation tube size, concentrations investigated, and initiation techniques used, the nominal 30-#m average particle size T N T does not appear to be detonable in nitrogen. T h e r e can be a n u m b e r of implications regarding this. First, from Table 2 it is observed that, at a TNT eoneentration of 1 kg/m 3 in nitrogen, the detonation temperature and pressure are about 50% less, and the detonation velocity is about 25% less, than that in air or oxygen. Hence, energetically this concentration may be too low to propagate detonation in nitrogen; below certain m i n i m u m concentrations, even R D X and PETN will fail to detonate whether dispersed in air or any other gaseous medium.
86
DETONATIONS AND EXPLOSIONS
Second, this would suggest that TNT detonation in a gaseous medium, at very low concentrations, is dependent on a burning mechanism such as is the case for fuels, e.g., A1. Both analytical and experimental results confirm that TNT dispersions in air provide superior detonation characteristics to those of RDX or PETN at low concentrations wherein the oxygen from the air enhances the TNT energy release [4]. Third, based on the detonation of RDX/TNT mixtures in nitrogen, as readily indicated in the sensor responses of pressure, light, and TMI records, it would appear that the RDX is detonating and that the TNT is "reacting" behind the RDX detonation front so as to extend the reaction zone, i.e., a "second front" detonation. Finally, in all of these experiments involving TNT and RDX in various gaseous media, detonations that were achieved did not involve spinning detonation. The detonation fronts were generally classical, such as for homogeneous fuel/oxidizer gaseous systems, with the exception noted above regarding mixtures of RDX and TNT. Regarding the particle size of the TNT, in previous detonation tube experiments involvingTNT particles dispersed and detonated in air [4], smaller, average 30-/~m particle size TNT detonation characteristic results correlated very well with analytically predicted values, whereas for larger, average 200-/zm particle size TNT, the results were considerably lower than those of the smaller particles. This was attributed to the fact that when the smaller particles detonated they were much more readily enhanced by oxygen from the air. However, in a nitrogen atmosphere, it is quite possible that larger particles would be more readily detonated.
REFERENCES 1. Tulis, A. J., and Selman, J. R., Nineteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1982, pp. 655-663.
2. Kauffman, C. W., Wolansld, P., Arisoy, A., Adams, P. R., Maker, B. N., and Nicholls, J. A., Dynamics of Shock Waves, Explosives, and Detonations (J. R. Bowen, N. Manson, A. K. Oppenheim, and R. I. Soloukhin, Eds.), AIAA, New York, 1984, Vol. 94, p. 221. 3. Tulis, A. J., First (International) Colloquium on Explosivity of Industrial Dusts, Polish Academy of Sciences, Warsaw, 1984, pp. 178-186. 4. Tulis, A. J., Sumida, W. K., Heberlein, D. C., Patel, D. L., and Egghart, H., Fifth (International) Colloquium on Dust Explosions, Oficyna Wydawnicza Politechniki Warszawskiej, Warsaw, 1993, pp. 391-400. 5. Tulis, A. J., Austing, J. L., and Heberlein, D. C., Fourteenth (International) Symposium on Shock Tubes and Waves, New South Wales University Press, Kingston, New South Wales, Australia, 1983, pp. 447-454. 6. Tulis, A. J., International Symposium on Intense Dynamic Loading and Its Effects, Beijing Science Press, Beijing, P. R. China, 1986, pp. 213-218. 7. Tulis, A. J., Austing, J. L., Sumida, W. K., Baker, D. E., and Hrdina, D. J., Ninth Symposium (International) on Detonation, Office of the Chief of Naval Research, Arlington, Virginia OCNR-113291-7, 1989, pp. 972-983. 8. Egghart, H., Heberlein, D. C., Tulis, A. J., and Sumida, W. K., Eighteenth (International) Pyrotechnics Seminar, liT Research Institute, Chicago, 1992, pp. 1083-1107. 9. Hobbs, M. L., and Baer, M. R., EUROPYRO 93---5th Congres (International) de Pyrotechnie du Groupe de Travail, Association Francaise de Pyrotechnie, Vairessur-Marne, France, 1993, pp. 53--59. 10. Tulis, A. J., Sumida, W. K., and Joyce, R. P., Advanced Detonation Tube Studies of Hybrid Solid-Gas Dispersions, liT Research Institute ReportNo. E06661-17, October 1993. 11. Tulis, A. J., Advanced Concepts for Chemical Energetics Manifestations, IIT Research Institute Report No. H01105-F; unclassified volume, 1988. 12. Tulis, A. J., Development and Demonstration of a Bimedal Mine Clearing Munition, U.S. Army BRDEC, liT Research Institute Final Report No. CO6705, March 1993.
COMMENTS Martin Sichel, The University of Michigan, USA. Professor Hertzberg has commented on the influence of particle size on detonability. We observed this effect in the case of RDX dust about 10 years ago. For a given loading ratio, the finer dust particles were more difficult to detonate or, in fact, transition to detonation failed to occur. Modeling suggested that the reason for this behavior is that the finer dust absorbs heat more rapidly so that the temperature of the gas around the particles is lower. This resuits in an increase in ignition delay or a failure of ignition altogether.
Author's Reply. Your previous findings regarding the influence of particle size, as well as the comments made by Professor Hertzberg, are relevant and have also been observed by us in other studies in which particle size was specifically investigated. Your suggestion based on your modeling work is very reasonable; however, we believe that mass transport also plays an important role; i.e., finer particles are readily accelerated into the high-temperature convective gas flow behind the incident shock wave whereas more massive particles will "resist" and induce considerable convective, viscous heating of the surface of
PHENOMENOLOGICAL ASPECTS IN EXPLOSIVE POWDER/GAS TWO-PHASE DETONATIONS
87
these larger particles. In the case of monopropellants such as RDX, this may be sufficient to induce ignition, or possibly direct detonation, of such larger particles much more quickly than the smaller particles that are entrained in the gas flow and depend on slower, mainly conductive heating. The influence of particle size is a complex matter, involving factors such as particle geometry, chemical kinetics, and the type of particles; i.e., ignition, and particularly detonation, of starch, aluminum, and RDX particles could be expected to involve differing phenomena.
ture, pressure, and velocity, than RDX or HMX, or PETN for that matter, since the presence of oxygen from air contributes substantially to the overall energy release at this near-stoichiometric concentration for TNT in air. The induction time was obtained by measuring the time difference between the arrival of the shock front (pressure response signal) relative to the flame front (light response signal) at each monitoring station. Since 11 stations were used in each experiment, both stability and accuracy of induction times as well as the detonation were determined.
A. A. Borisov, Institute of Chemical Physics, Russia.
Herman Krier, University of Illinois., USA. I'm not so sure that if you could have suspended greater concentrations of TNT in nitrogen (i.e., greater than 1-2 kg/m 3) that you would have seen a detonation. Clearly, a richer concentration of your 30-,um particles if promptly shock ignited would detonate, even in nitrogen.
What was the minimum TNT concentration in detonable mixtures with air? We performed similar experiments with TNT, RDX, and HMX suspensions in air and found that TNT mixtures detonated at 0.25 kg/m 3, whereas HMX and RDX mixtures detonated starting with 1 kg/m 3. We found that the TNT mixtures were more detonable than HMX and RDX. How did you define the induction time?
Author's" Reply. We did not specifically investigate the lower concentration limits for detonation of TNT, RDX, and PETN in air but did achieve detonations of all three at concentrations in the range of 0.5 kg/m 3. Your finding that TNT would detonate in air down to 0.25 kg/m 3, in particular at a concentration considerably lower than those for RDX and HMX, is not surprising; analytical computations indicate that TNT has much greater detonation performance at 0.25 kg/m 3 in terms of detonation tempera-
Author's Reply. You are absolutely correct; and we did state in the paper: "Certainly TNT powder dispersed in nitrogen, if of sufficient concentration and suitable size and confinement, will detonate . . . . In fact, we have achieved the detonation of TNT powder dispersed in air, in an unconfined state, at concentrations in excess of 20 and perhaps as high as 50 kg/m 3. Obviously, at these concentrations the influence of oxygen from the air is inconsequential; hence, it can be inferred that detonation would have been achieved in the absence of oxygen altogether.