POST-DETONATION ENERGY RELEASE FROM TNT-ALUMINUMEXPLOSIVESFan Zhang, John Anderson, and Akio Yoshinaka

Citation: AIP Conference Proceedings 955, 885 (2007); doi: 10.1063/1.2833268View online: http://dx.doi.org/10.1063/1.2833268View Table of Contents: http://aip.scitation.org/toc/apc/955/1Published by the American Institute of Physics

POST-DETONATION ENERGY RELEASE FROM TNT-ALUMINUM EXPLOSIVES

Fan Zhang, John Anderson, and Akio Yoshinaka

DRDC - Suffield, PO Box 4000, Stn Main, Medicine Hat, Alberta, Canada T1A 8K6

Abstract. TNT and TNT-aluminum composites were experimentally studied in an air-filled 26 m3 chamber for charge masses ranging from 1.1 to 4 kg. Large aluminum mass fractions (35 to 50%wt.) and particle sizes (36 μm) were combined with TNT in two configurations, whereby the aluminum particles were uniformly mixed in cast TNT or arranged into a shell surrounding a cast TNT cylinder. The results show that improved performance is achieved for the shell configuration versus the mixed version during the early afterburning phase (10-40 ms), while both approach the same quasi-static explosion overpressure (QSP) after a long duration. The QSP ratios with respect to TNT in nitrogen are in good agreement with equilibrium predictions. Thus, the large aluminum mass fraction improves spatial mixing of hot fuels with oxidizing gases in the detonation products and chamber air, resulting in more efficient afterburning energy release.

Keywords: Aluminum, TNT, thermobaric explosive, explosion, afterburning. PACS: 47.40.-x, 82.40.Fp, 47.50.Gj, 47.55.Kf.

INTRODUCTION TNT (C7H5N3O6) is a well known explosive with a high oxygen deficiency (-740 g per kg explosive). Detonation of TNT therefore generates hot fuel rich in carbon, carbon monoxide, methane and hydrogen. When mixed with oxidizers, their combustion releases significant energy in addition to that from TNT detonation. In a calorimetric bomb, Ornellas measured an explosion energy of 4573 and 4669 J/g for TNT in vacuum and carbon dioxide, but 14958 J/g in oxygen [1]. Cudzilo et al. observed 4017, 15096 and 17138 J/g for TNT in argon, air and oxygen respectively [2]. These calorimetric data are in agreement with the equilibrium constant volume explosion theory (e.g., Cheetah code [3]) that predicts an explosion energy of TNT in air 3.2 times larger than the detonation energy of TNT alone [4].

In practice, explosion performance of a charge has been evaluated using a closed chamber that also allows dynamic mixing, local expansion

quenching and non-adiabatic conditions. In a 0.15 m3 explosion chamber, Cudzilo et al. obtained a factor of 1.9 in mean explosion overpressure (so called quasi-static overpressure, or QSP) in air or oxygen-enriched air versus argon with 50 g pressed TNT [2]. The converted explosion energy indicates a 25% deficit compared with the calorimetric data, probably due to the wall heat loss inherent in a small chamber. Kuhl et al. reported a QSP ratio of 3 for air versus nitrogen in a 16.6 m3 chamber using a 0.875 kg pressed TNT charge with a length to diameter ratio of 5.5 [5]. Zhang et al. observed a QSP of TNT in air with a 13-15% deficit to the theoretical values for 1.1-4 kg cast TNT with a length-diameter ratio close to unity in a 26 m3 chamber [6]. Figure 1 shows that the QSP of TNT is close to that of C-4 which only has an oxygen deficiency of -300 g per kg explosive. They attributed this energy deficit to the non-uniform mixing between the detonation products and air.

To improve mixing and further increase afterburning energy, large mass fractions of large

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CP955, Shock Compression of Condensed Matter - 2007, edited by M. Elert, M. D. Furnish, R. Chau, N. Holmes, and J. Nguyen 2007 American Institute of Physics 978-0-7354-0469-4/07/$23.00

aluminum (Al) particles are combined with TNT and experimental results are reported in this paper.

Figure 1. Overpressure histories at position P2 from 1.1 kg TNT and C-4 charges in air in a 26 m3 chamber.

EXPERIMENTAL PROCEDURE

The 26 m3 chamber is 3 m I.D. and 4 m long, designed to sustain 1500 psi hydrostatic pressure (Fig. 2). This chamber can withstand an 8 kg TNT explosion and is able to accommodate a range of loading densities up to 0.31 kg/m3. The explosion chamber can be vacuum-sealed and filled with any gases at any desired initial pressure. The charge is suspended in the center of the chamber using a breech system on the top of the chamber. Instrumentation included Endevco piezoresistive pressure transducers, a local pyrometric sensor and high speed video as shown in Fig. 3, where gauge P9 is located on the opposite side.

Figure 2. 26 m3 Explosion chamber and gauge locations.

Figure 3. TNT/Al mixed and shelled charges

Experiments were conducted for 1.1, 2.2 and 4 kg nominal charge masses corresponding to loading densities of 0.042, 0.084 and 0.152 kg/m3 (see Table 1). Two types of Al particles were used: Valimet atomized H-30 Al with a mean diameter of 36 μm and Silberline DF-1667 flaked Al. The effect of particle distribution was investigated in two configurations as shown in Fig. 3, whereby the aluminum particles were uniformly mixed in cast TNT or arranged into a shell surrounding a cast TNT cylinder. The charge density was 1.59 g/cm3 for pure cast TNT and 1.9 g/cm3 for the TNT/Al (35%wt.) mixture. The mixed charge was bare with a length-diameter ratio L/D = 0.83, while the shelled charge was contained in a 2.2 mm thick polyethylene casing with L/D ~ 1. The charge was detonated from the top using a detonator (0.08 g PETN plus 1.031 g RDX) and a 50 mm diameter C-4 booster. The chamber air pressure was brought to one standard atmosphere before each test.

RESULTS AND DISCUSSION

Figure 4 displays an early phase of explosion from a 4 kg TNT/Al (35%wt.) shelled charge. The C-4 booster and TNT detonation products are identified at the top of the expanding cylinder, while the bright light of Al combustion can be seen first below the cylinder, then on and above the cylinder during the expansion, before the shock front reaches the chamber wall at about 900 μs. Thus,

Ignition Point

TNT/Al Mix TNT/Al Shell C-4 Booster

P1 P2 (P9) P3

P5

Local pyrometric sensor

Video

Time, msec

Ove

rpre

ssur

e, p

si

-5 0 5 10 15 20 25 30 35 40 45 50-5

0

5

10

15

20

25

30

35

40

45

50U05-095A - P2 - C4 - 1.1kgU05-319A - P2 - TNT - 1.1kgU05-095A - P2 - C4 - 1.1kg smoothedU05-319A - P2 - TNT - 1.1kg smoothed

Time (msec)

Ove

rpre

ssur

e (p

si)

0.6 0.8 1 1.2 1.4 1.6 1.8-50

50

150

250

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the reactive dispersal of Al particles enhances the mixing even before the reflected shock-induced Richtmyer-Meshkov instability occurs.

FIGURE 4. Explosion process of a 4 kg TNT/Al H-30 (35%wt.) shelled charge in the 26 m3 chamber (#7162A).

Figure 5 shows wall pressure records of various

4 kg charges. The shock front from TNT arrives first followed by that from the TNT/Al flake (35%wt.) shell, TNT/Al H-30 (35%wt.) shell, TNT/Al H-30 (50%wt.) shell and TNT/Al H-30 (35%wt.) mixture. A double reflected shock front structure is identified for the shelled charges. Due to the afterburning, the subsequent reverberating waves from TNT/Al charges overtake those of TNT. Figure 6 displays the long-time pressure histories. During the first 40 ms, the mean pressure levels indicate that the pressure increase rate, in decreasing order, is TNT/Al H-30 35%wt. shell followed by the Al H-30 50%wt. shell, Al flake 35%wt. shell and the mixture. Thereafter all TNT/Al charges approach approximately the same QSP, a value that is about 25% higher than the QSP of TNT in air. It is noticeable that for the

2.2 kg charge mass, the mean pressure for the shelled charges is higher during the first 10 ms, but lower thereafter and reaches a smaller QSP than that of the mixture charges (see Table 1). The lower QSP may be partially due to energy loss to the poly-ethylene casing, particularly for smaller charges.

Figure 5. Overpressure histories at position P9 from 4 kg TNT and TNT/Al H-30 charges in the 26 m3 chamber.

Figure 6. Overpressure histories at position P1 from 4 kg TNT and TNT/Al H-30 charges in the 26 m3 chamber.

QSP performance can be dictated by the adiabatic constant volume explosion theory:

Vhmpex ΔΔ = , (1)

or in a simple form for an ideal gas:

Vqm)1(pex −= γΔ . (2)

where γ is the isentropic exponent. Here, Δpex is the explosion overpressure (QSP), which scales with

26 μs 47 μs

150 μs

67 μs 88 μs

275 μs

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the change of enthalpy per kg charge in air before and after explosion or explosion energy, q, per kg charge in air and loading density, m/V, indicating air stoichiometry (m - charge mass, V - chamber volume). The results from equation (1) using the Cheetah equilibrium code are shown in Fig. 7, indicating that an increase in aluminum mass fraction results in an increase from 2.8 to 3.7 in QSP ratio of TNT-Al in air w.r.t. TNT in nitrogen at a fuel-lean or near-stoichiometric loading ratio.

Figure 7. Equilibrium chamber QSP ratio w.r.t. TNT-N2 (Cheetah BKWS).

Table 1 summarizes the QSP and explosion

temperatures obtained by a local pyrometric sensor based on two wavelengths (690 and 568 nm). It is shown that the QSP of TNT-Al composites outperforms that of pure TNT by a factor of 1.25 to 1.35. At the loading density of 0.152 kg/m3, the ratio of experimental QSP w.r.t. theoretical value of TNT in nitrogen achieves 2.8-2.9, in agreement by 5-9% with the equilibrium values predicted in Fig. 7. Thus, the addition of large Al mass improves spatial mixing of hot fuels with oxidizing gases in the detonation products and chamber air, resulting in a more efficient afterburning energy release.

ACKNOWLEDGEMENTS

The authors would like to thank P. Lambert, B. Eichelbaum, K. Mudri, L. Légaré, M. Churcher and the Field Operations Section for their experimental support. This work was partially funded by the Advanced Energetics Program of DTRA.

TABLE 1. Experimental QSP and temperature.

Test# Charge TNT/Al

Mass1 kg

T K

QSP2

psig QSP3 Ratio

5319A 100/0 1.014 2100 28.09 2.12 5325A 100/0 1.054 1900 26.55 2.01 5320A 100/0 2.012 2500 52.47 2.11 5326A 100/0 2.112 2500 51.23 2.07 5321A 100/0 3.903 2600 85.45 2.33 7142A 65/35 H-

30 Mix 2.150 2500 70.89 2.86

7148A 65/35 H-30 Mix

2.104 2550 67.19 2.71

7149A 65/35 H-30 Mix

3.90 2600 107.0 2.92

7172A 65/35 H-30 Mix

3.914 103.3 2.81

7158A 65/35 H-30 Shell

1.310/ 0.770

2600 64.3 2.60

7170A 65/35 H-30 Shell

1.332/ 0.770

65.3 2.64

7162A 65/35 H-30 Shell

2.568/ 1.40

2800 105.2 2.87

7169A 65/35 Fl. Shell

2.534/ 1.40

103.8 2.83

7179A 65/35 Fl. Shell

2.478/1.40

100.9 2.75

7157A 50/50 H-30 Shell

1.922/ 2.002

2400 104.5 2.85

7171A 50/50 H-30 Shell

1.902/2.0

101.8 2.77

1) Plus C-4 booster mass: 86g for 5319A, 46g for 5325A, 188g for 5320A, 88g for 5326A, 97g for 5321A and 100g for the rest. 2) Average over 5 transducers. 3) w.r.t. Cheetah TNT-N2.

REFERENCES

1. Ornellas, D. L., Lawrence Livermore National Laboratory Report UCRL-52821, 1982.

2. Cudzilo, S., Paszula, J., Trebinski, R., Trzcinski, W. And Walanski. P., Proc. Intl. Symp. on Hazards, Prevention, Mitigation of Industrial Explosions. Shaumburgh, IL, pp.50-67, 1998.

3. Fried, L. E., Howard, W. M. and Souers, P. C., UCRL-MA-117541 Rev. 5, 1998.

4. Ripley, R. C., Donahue, L., Dunbar, T. E., and Zhang, F., Proc. 19th Military Aspects of Blast and Shock, Calgary, Canada, 2006.

5. Kuhl A. L, Forbes J, Chaudler J, Oppenheim A. K, Ferguson R. E., Proc. Intl. Symp. on Hazards, Prevention, Mitigation of Industrial Explosions. Shaumburgh, IL, pp.1-49, 1998.

6. Zhang, F., Yoshinaka, A., Anderson, J., Proc. 19th Military Aspects of Blast and Shock, Calgary, Canada, 2006.

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