This article was downloaded by: [Harvard College] On: 29 April 2013, At: 04:30 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Journal of Energetic Materials Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/uegm20 Chemical Kinetic Modeling of HMX and TATB Laser Ignition Tests CRAIG M. TARVER a a Energetic Materials Center, CA, USA Published online: 17 Aug 2010. To cite this article: CRAIG M. TARVER (2004): Chemical Kinetic Modeling of HMX and TATB Laser Ignition Tests, Journal of Energetic Materials, 22:2, 93-107 To link to this article: http://dx.doi.org/10.1080/07370650490492842 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
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This article was downloaded by: [Harvard College]On: 29 April 2013, At: 04:30Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Energetic MaterialsPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/uegm20
Chemical Kinetic Modeling of HMX andTATB Laser Ignition TestsCRAIG M. TARVER aa Energetic Materials Center, CA, USAPublished online: 17 Aug 2010.
To cite this article: CRAIG M. TARVER (2004): Chemical Kinetic Modeling of HMX and TATB LaserIgnition Tests, Journal of Energetic Materials, 22:2, 93-107
To link to this article: http://dx.doi.org/10.1080/07370650490492842
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representationthat the contents will be complete or accurate or up to date. The accuracy of anyinstructions, formulae, and drug doses should be independently verified with primarysources. The publisher shall not be liable for any loss, actions, claims, proceedings,demand, or costs or damages whatsoever or howsoever caused arising directly orindirectly in connection with or arising out of the use of this material.
Chemical Kinetic Modeling of HMX andTATB Laser Ignition Tests
CRAIG M. TARVER
Energetic Materials Center, Lawrence LivermoreNational Laboratory Livermore, CA, USA
Recent high-power laser deposition experiments on octa-hydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and1,3,5-triamino-2,4,6-trinitrobenzene (TATB) producedignition times from milliseconds to seconds. Global chemi-cal kinetic thermal decomposition models for HMX andTATB developed to predict thermal explosion experimentslasting seconds to days are applied to these laser ignitionexperimental data. Excellent agreement was obtained forTATB, while the calculated ignition times were longerthan experiment for HMX at lower laser fluxes. Inclusionof HMX melting and faster reaction for liquid HMX inthe HMX decomposition model improved the agreementwith experiment at lower laser energies.
Keywords: HMX, TATB, decomposition, thermal
Introduction
Accident scenarios involving pressed solid high explosives includeheating to thermal explosion, impact formation of localized ‘‘hotspots,’’ and shock compression of voids creating very hot regions[1]. Global chemical decomposition models have been developedto predict times to explosion and the location with in the explo-sive charge where runaway reaction first occurs [2{4]. These
Address correspondence to C. M. Tarver, Energetic MaterialsCenter, Lawrence Livermore National Laboratory, Livermore, CA94551. E-mail: [email protected]
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Energetic Materials, 22: 93{107, 2004Copyright # Taylor & Francis Inc.ISSN: 0737-0652 printDOI: 10.1080/07370650490492842
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calculations are used as the bases for estimations of the violence ofthermal explosions as functions of heating rate, confinement,damage, and porosity [5]. Because of the absence of experimentalchemical kinetic data at higher temperatures, these models arealso being used to estimate the critical conditions for ‘‘hotspot’’ ignition during impact and shock compression scenarios[6] and the growth rates of shock-induced hot spots duringshock-to-detonation transition (SDT) processes [7]. They are cur-rently being used to model shock initiation and detonation wavepropagation in a grain-scale model [8] and in a statistical hot spotreactive flow model being developed in the thermal-mechanical-hydrodynamic coupled computer code ALE3D [9].
Recently Ali et al. [10] heated octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and 1,3,5-triamino-2,4,6-trinitro-benzene (TATB) pellets to thermal ignition using twoCO2 lasers.The measured times to explosion are in the second-to-milli-second regime, and the measured average ignition temperatureswere 650K for HMX and 708K for TATB. These times toexplosion are shorter than those that can be measured in mostthermal explosion experiments, such as the One-DimensionalTime to Explosion (ODTX) apparatus [11]. Thus these rapidlyheated laser ignition experiments represent an excellent test ofthe HMX and TATB global chemical decomposition models.This test helps determine whether these models, normalized toexperimental data in the 453{593K range, can be used forpredictions of thermal response of HMX and TATB at highertemperatures, such as those occurring in impact and shock-induced hot spots. In this paper, the laser ignition experimentsare briefly discussed. Then the global HMX and TATB chemicaldecomposition models are presented. The calculated times toignition are then compared to the experimental measurementsin the Results section. Finally, some conclusions are drawn, andsome future research areas are identified.
Experimental
Ali et al. [10] used two different CO2 lasers to heat 1 cm diameterby 6.4mm thick pressed pellets of HMX and TATB to thermal
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ignition. The Edinburgh Instruments Ltd. PL-6 180 Watt laserhas a rise time of approximately 2ms. The PRC Corp. SL 10001000 Watt laser has a rise time of 0.6ms. The laser beam profilesand the diagnostics used are fully discussed. For irradiances lessthan 60W=cm2, no differences were observed in the measuredignition delays using the two lasers under atmospheric condi-tions. TATB exhibited a single ignition time dependence over theentire range of laser fluxes, while the HMX data exhibited twodistinct trends with a transition region of high scatter in the50{100W=cm2 range. The ignition temperatures measured usingfast response thermocouples did not change significantly for therange of irradiances used. Ali et al. [10] also included HMX laserignition time data from other experimental studies that agreedwell with their measurements. Various experimental details,uncertainties, and future experimental research were discussed.
Chemical Kinetic Decomposition Modelsfor HMX and TATB
Three-to-five-step global chemical decomposition models havebeen developed for several solid high explosives. The HMX che-mical decomposition model consists of four reactions and fivechemical species. The reaction sequence is [4]
The major pathways for HMX decomposition have beenreviewed by Behrens et al. [12]. The solid-solid beta-to-delta
Calculated HMX and TATB Laser Ignition Times 95
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phase transition is treated as a separate reaction in Equation (1)[4], whereas previously it had been included with Equation (2)as one overall endothermic process [2]. Equation (2) describesthe initial ring and bond breaking endothermic step(s). HMXdecomposition is known to produce as the main intermediateproducts CH2O plus N2O under some temperature and pressureconditions and HCN plus HNO2 under other conditions [12].Equation (3) is slightly exothermic, and thus most of HMX’schemical energy is released during the gas phase formation ofthe final stable reaction products by second-order gas phasereactions in Equation (4).
The TATB model is based on less chemical kinetic data thanthe HMX model and has not changed recently [5]. It consists ofthree reactions and four species. The TATB sequence is
TATB�!Solid Intermediate A+H2O; ð5Þ
Solid Intermediate A�!Solid Intermediate B + Gases, ð6Þ
Solid Intermediate B�!Final Gaseous Products. ð7Þ
It is known that TATB reacts mainly in the condensed phaseand that all of the possible H2O molecules can be formedduring extremely slow heating leaving C6H6O3 behind [5]. Atfaster heating rates, the first two reactions, Equations (5) and(6), are assumed to be endothermic steps that eliminate H2Oand then other intermediate gaseous products, such as NO. Thethird reaction is assumed to be an exothermic, second-orderreaction. Some kinetic data are available and used for the ratesof the three reactions [5]. In the ODTX apparatus, TATB-basedexplosives exhibit essentially the same times to explosion underheavy confinement and with no confinement [2, 5]. Thus it islikely that gas phase reactions do not dominate the chemicalenergy release process in TATB as they do in HMX.
Table 1 lists the thermal property and reaction rate para-meters for the HMX decomposition model, and Table 2 liststhose for TATB. Both models have been used to calculate
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times to and locations of thermal explosion for the ODTX andother thermal experiments with various heating rates, degreesof confinement, and geometries [2{5]. These models are useddirectly to model the laser ignition experiments of Ali et al.[10] by placing a time-dependent heat flux boundary conditionon the top of the pellet. The experimental uncertainties in thelaser flux reaching the solid explosive surface discussed by Aliet al. [10] are not considered in these two-dimensional calcula-tions using the Chemical TOPAZ heat transfer code [13]. Thecalculated times to thermal ignition for HMX and TATB arecompared to experiment in the next section.
Results
The times to ignition for laser-heated HMX and TATB are cal-culated using the Chemical TOPAZ code with very fine zoningto ensure that the calculations have converged to a consistentanswer. For these 0.64 cm thick pellets, 320 zones are sufficient.After the appropriate laser pulse rise time, a constant fluxboundary condition is applied to the pellet surface. The HMXand TATB chemical decomposition models in Tables 1 and 2agree well with slower heating rate and longer time-to-ignitionthermal explosion experiments. Figure 1 shows the comparisonbetween the measured and calculated times to ignition as var-ious laser irradiances for HMX. Figure 2 shows a similar com-parison for TATB. Both thermal decomposition models agreewell with the laser ignition data. The HMX model overpredictsthe times to ignition at lower laser fluxes and underpredictsthose for high laser fluxes. The TATB model predicts the mea-sured times to ignition very well over the entire range of irra-diances. The TATB model is based on less experimentalchemical kinetic and thermal explosion data but appears toagree with this high-temperature, short-time-duration, laser-driven time-to-ignition data better than the HMX model does.
One reason for the higher calculated times to explosion forHMXmay be that HMXmelts at 558K, and the average ignitionin these experiments is 650K. HMX melting was not included inthe HMX decomposition model for slower thermal events,
Calculated HMX and TATB Laser Ignition Times 97
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Table
1Thermal
andreaction
rate
param
etersforHMX
mod
el
BetaHMX
Delta
HMX
Solid
interm
ediate
Interm
ediate
gases
Final
gases
1.Initialdensity
(g=cm
3)
1.85
1.70
2.Heatcapacity
(cal=g-K)at:
298K
0.24
0.24
0.22
0.24
0.27
373
0.30
0.30
0.27
0.26
0.28
433
0.34
0.34
0.31
0.27
0.28
563
0.40
0.40
0.36
0.29
0.29
623
0.46
0.46
0.42
0.31
0.30
773
0.55
0.55
0.50
0.35
0.31
>1273
0.55
0.55
0.50
0.42
0.35
3.Thermal
conductivity
(cal=cm
-g-K
)at:
298
1.28
�10
73
1.18
�10
73
1.08
�10
73
9.80�107
41.0�107
4
373
1.09
�10
73
1.00
�10
73
9.20
�10
74
8.8�107
41.0�107
4
433
1.02
�10
73
9.20
�10
74
8.30
�10
74
8.3�107
41.0�107
4
563
8.15
�10
74
8.15
�10
74
8.15
�10
74
8.15�107
41.0�107
4
623
7.50
�10
74
7.50
�10
74
7.50
�10
74
7.5�107
41.0�107
4
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773
1.00
�10
74
1.00
�10
74
1.00
�10
74
1.0�107
41.0�107
4
>1273
1.00
�10
74
1.00
�10
74
1.00
�10
74
1.0�107
41.0�107
4
4.Heatof
form
ation
(cal=g)
þ61.0
þ71.0
þ131.0
72.0
71339.0
5.Reactionrate
para
-metersNaxqZe7
E=RT
(whereNais
massfraction
)
Reaction
lnZ
E(kcal=mol)
Reactionorder�
Heatofreaction
q(cal=g)
148.13
48.47
1þ10.0
248.7
52.70
1þ60.0
337.8
44.30
17
133.0
428.1
34.10
27
1337.0
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Table
2Thermal
andreaction
rate
param
etersfortheTATB
mod
el
TATB
Solid
interm
ediate
ASolid
interm
ediate
BGaseous
products
1.Initialdensity
(g=cm
3)¼
1.835g=cm
3
2.Heatcapacity(cal=g-K)at:
298K
0.26
0.24
0.26
0.26
433
0.36
0.33
0.28
0.28
573
0.45
0.39
0.29
0.29
623
0.47
0.41
0.30
0.30
673
0.49
0.42
0.30
0.30
773
0.54
0.48
0.35
0.35
>1273
0.54
0.48
0.35
0.35
3.Thermalconductivity
(cal=cm
-g-K
)at:
298K
2.10
�10
73
1.05
�10
73
5.00
�10
74
1.00
�10
74
433
1.56
�10
73
7.80
�10
74
3.90
�10
74
1.00
�10
74
573
1.10
�10
73
5.50
�10
74
2.70
�10
74
1.00
�10
74
623
1.00
�10
73
5.00
�10
74
2.50
�10
74
1.00
�10
74
673
1.00
�10
73
1.00
�10
74
1.00
�10
74
1.00
�10
74
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773
1.00
�10
74
1.00
�10
74
1.00
�10
74
1.00
�10
74
>1273
1.00
�10
74
1.00
�10
74
1.00
�10
74
1.00
�10
74
4.Heatofform
ation(cal=g)
7142.7
792.7
þ42.7
7742.7
5.Reactionrate
param
eters
NaxqZe7
ERT(w
here
Naismassfraction
)
Reaction
lnZ
E(kcal=mol)
Reactionorder�
Heatof
reaction
q(cal=g)
148.0
60.0
1þ50.0
229.8
42.0
1þ50.0
326.8
33.8
27700.0
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because almost all of the data are taken at temperatures belowthe melting point [4]. Inclusion of the endothermic melting pro-cess increases the HMX times to explosion slightly, but it isknown that organic solid explosives generally react faster in theliquid phase than in the solid phase, because of the greater mobi-lity of liquids and intermediate products [12]. ThemodernODTXapparatus [11] has been used to measure three times to thermalexplosion for HMX at temperatures exceeding the meltingpoint. These three times to thermal explosion at the ODTX con-stant temperatures are listed in Table 3. The new ODTX closes
Figure 1. Experimental and calculated ignition times vs. laserirradiance for HMX.
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and seals in 1{2 s [11], so these measured times to explosion aremeaningful. The three calculated times to explosion using theHMX decomposition parameters listed in Table 1 are shown inTable 3 and are considerably higher than experiment. Incorporat-ing HMXmelting as a 56.4 cal=g endotherm [14] between 557 and559K and increasing the natural logs of the frequency factors by 4and decreasing the activation energies by 4 kcal=m for reactions2, 3, and 4 results in the shorter ODTX times to explosion listedin Table 3. Applying these faster reaction rates to the measuredHMX laser ignition times at various irradiation levels yields
Figure 2. Experimental and calculated ignition times vs. laserirradiance for TATB.
Calculated HMX and TATB Laser Ignition Times 103
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the time-to-explosion curve shown in Figure 3. The resulting cal-culated ignition times are closer to the experimental times atlower laser fluxes but farther from experiment at high fluxes.
Liau and Lyman [15] calculated the times to ignition for theHMX laser experiments using a chemical kinetics scheme invol-ving 45 species and 232 gas phase reactions. Their calculatedHMX ignition times agreed well with experiment, except inthe lower flux range, where their model also gave a steeperslope than observed experimentally. When the gas plumeproduced at the HMX surface was allowed to expand radiallyto eight times the original area 2 cm from the HMX surface,their calculated ignition times at lower laser fluxes agreedmore closely with experiment.
Conclusions
HMX and TATB global chemical decomposition mechanismsderived from thermal explosion data in the seconds-to-days
Table 3HMX model including melting and faster rates
time frame are applied to recent high-power laser ignitionexperiments that produced times to ignition in the millise-cond-to-second time frame. The overall agreement betweenthe measured and calculated times to ignition is good, espe-cially for TATB. This agreement is certainly reasonableenough for these models to be used to estimate critical tem-peratures for various hot spot sizes formed during impactand shock initiation processes until such hot spot temperaturesand dimensions can be measured experimentally [6{9]. The
Figure 3. HMX times to ignition: experimental and calculatedwith regular and faster reaction rates.
Calculated HMX and TATB Laser Ignition Times 105
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close agreement with the TATB laser ignition times may implythat gas phase absorption of laser energy is not important forTATB, which agrees with previous conclusions that most ofthe preexplosion chemistry of TATB occurs in the condensedphase. On the other hand, HMX decomposition is believed tobe very dependent on gas phase and HMX surface reactions[12, 15]. Therefore gaseous products of HMX decompositionleaving the irradiated surface may absorb significant amountsof laser energy, as discussed by Liau and Lyman [15]. Ali et al.[10] plan to expose HMX and TATB to high laser powers,resulting in faster ignitions that will further test the decompo-sition models. Chemical kinetic reaction rate experiments onliquid HMX are needed to shed identify increases in reactionrates upon melting.
Acknowledgments
The author would like to thank Arif Ali of the Los AlamosNational Laboratory for providing a printout of his experimen-tal data. This work was performed under the auspices ofthe United States Department of Energy by the LawrenceLivermore National Laboratory under contract no. W-7405-ENG-48.
References
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[2] McGuire, R. R. and C. M. Tarver. 1981. Seventh Symposium(International ) on Detonation, Naval Surface Weapons CenterNSWC MP 82-334, Annapolis, MD, pp. 56{65.
[3] Tarver, C. M., T. D. Tran, and R. E. Whipple. 2003. Propellants,Explosives, Pyrotechnics, 28: 189{193.
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[7] Tarver, C. M., and A. L. Nichols III. 1998. Eleventh InternationalDetonation Symposium, Office of Naval Research ONR 33300-5,Aspen, CO, pp. 599{609.
[8] Reaugh, J. E. 2002. Grain Scale Dynamics in Explosives. Lawr-ence Livermore Laboratory Report UCRL-ID-150388, October.
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[11] Tran, T. D., L. R. Simpson, J. Maienschein, and C. Tarver. 2001.32nd International Annual Conference of Fraunhofer-Institut f €uurChemische Technologie (ICT), DWS Werbeagentur und VerlagGmbH, Karlsruhe, Germany, 25: 1{25.
[12] Behrens, R., Jr., S. B. Margolis, and M. L. Hobbs. 1998. EleventhInternational Detonation Symposium, Office of Naval ResearchONR 33300-5, Aspen, CO, pp. 533{542.
[13] Nichols, A. L., III, and K. W. Westerberg. 1993. Numerical HeatTransfer, Part B, 24: 489{499.
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