Paul A. Urtiew

Chemical kinetic prediction of critical parameters in gaseous detonations

A theoretical model including a detailed chemical kinetic reaction mechanism, for hydrogen and hydrocarbon oxidation is used to examine the effects of variations in initial pressure and temperature on the kinetic induction properties of gaseous fuel-oxidizer mixtures. Fuels considered include hydrogen, methane, ethane, ethylene, and acetylene. Induction lengths are computed for initial pressures between 0.01 and 10.0 atmospheres and initial temperatures between 200 K and 500 K. These induction lengths are then compared with available experimental data for critical energy and critical tube diameter for initiation of spherical detonation, as well as detonation limits in linear tubes. Combined with earlier studies concerning variations in fuel-oxidizer equivalence ratio and degree of dilution with N2, the model provides a unified treatment of fuel oxidation kinetics in detonations.

Thermal relaxation at interfaces following shock compression

Thermal conduction processes at material interfaces representative of those produced by shock compression are studied in order to understand the temperature history at the interface. The transient effects due to small gaps and thin layers are calculated, as well as the modifications due to a melting transition in one of the materials.

On the inadequacy of gasdynamic processes for triggering the transition to detonation

A complete sequence of stroboscopic laser-schlieren records of the nonsteady flow field ahead of an accelerating turbulent flame, in a stoichiometric hydrogen-oxygen mixture contained in a tube, permits an accurate determination of pressure and temperature profiles in time along the particle path leading to the center of the “explosion in the explosion” that promotes the transition to detonation. On the basis of known correlations for the steady-state induction time, the upper bound for the fraction of the induction process that has occurred as a consequence of just the gasdynamic processes of wave compression up to the onset of detonation is then computed, yielding a surprisingly low value of not more than 4 per cent. These results are offered as a clear demonstration that the gasdynamic processes per se are insufficient to bring about the transition to detonation if it takes place in a close vicinity of the flame front. For this purpose, direct effects of the flame must be taken into account. Their evaluation is left, however, for further study.

Temperature deposition caused by shock interactions with material interfaces

In shock‐wave compression experiments in which the interface temperature between two different materials is important, it is necessary to take account of more than the usual ideal shock impedance matching conditions. Several examples are discussed by means of hydrodynamic analysis of the interaction of strong shock waves with nonideal interfaces. It is demonstrated that significant residual thermal inhomogeneities exist near the interface which may be of use in understanding and measuring thermal properties of shock‐compressed materials.

Effect of shock loading on transparency of sapphire crystals

Single‐crystal sapphire was found to lose some of its original transparency in the near infrared (0.9 μm) when subjected to strong dynamic compression in the pressure range between 100–130 GPa (1–1.3 Mbar). Experimental evidence of this phenomenon is presented and discussed in relation to some of the other known properties of the crystal.

Sensitivity of 2,6-Diamino-3,5-Dinitropyrazine-1-Oxide

ABSTRACT The thermal and shock sensitivities of plastic bonded explosive formations based on 2,6-diamino-3,5-dinitropyrazine-1-oxide (commonly called LLM-105 for Lawrence Livermore Molecule #105) are reported. The One-Dimensional Time to Explosion (ODTX) apparatus was used to generate times to thermal explosion at various initial temperatures. A four-reaction chemical decomposition model was developed to calculate the time to thermal explosion versus inverse temperature curve. Three embedded manganin pressure gauge experiments were fired at different initial pressures to measure the pressure buildup and the distance required for transition to detonation. An Ignition and Growth reactive model was calibrated to this shock initiation data. LLM-105 exhibited thermal and shock sensitivities intermediate between those of triaminotrinitrobenzene (TATB) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazine (HMX).

On the violence of thermal explosion in solid explosives

Abstract Twenty large scale experiments were conducted to determine the levels of violence of thermal explosions produced by various confinement and heat flow conditions. Heavily confined cylinders of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and triaminotrinitrobenzene (TATB) were heated at rates varying from 2°C/min to 3.3°C/h. Fourteen of the cylinders were hollow, and inner metallic liners with small heaters attached were used to produce uniform temperatures just prior to explosion. A complex thermocouple pattern was used to measure the temperature history throughout the charge and to determine the approximate location where the runaway exothermic reaction first occurred. The violence of the resulting explosion was measured using velocity pin arrays placed inside and outside of the metal confinement cylinders, flash x-rays, overpressure gauges, and fragment collection techniques. Five cylinders were intentionally detonated for violence comparisons. The measured temperature histories, times to explosion, and the locations of first reaction agreed closely with those calculated by a two-dimensional heat transfer code using multistep chemical decomposition models. The acceleration of the confining metal cylinders by the explosion process was accurately simulated using a two-dimensional pressure dependent deflagration reactive flow hydrodynamic model. The most violent HMX thermal explosions gradually accelerated their outer cases to velocities approaching those of intentional detonations approximately 120 μs after the onset of explosion. The measured inner cylinder collapse velocities from thermal explosions were considerably lower than those produced by detonations. In contrast to the HMX thermal reactions, no violent thermal explosions were produced by the TATB-based explosive LX-17. A heavily confined, slowly heated LX-17 test produced sufficient pressure to cause a 0.1 cm bend in a 2 cm thick steel plate.

Use of chemical kinetics to predict critical parameters of gaseous detonations

Computational modeling of chemical kinetics of fuel oxidation under conditions similar to those encountered in detonation waves is described. Sample applications to predicting critical detonation parameters are described, including lean and rich limits to detonation in linear tubes, direct initiation of unconfined spherical detonations by means of high explosive charges or by transition from a linear tube, and kinetic inhibition of detonation.

The melting temperature of magnesium under shock loading

Temperature data from optical intensity measurements through anvils at high shock pressures are reported for magnesium samples. When account is taken for imperfect gaps at the anvil interface, the temperature measurements are shown to determine the melting line of magnesium at pressures from 40 to 50 GPa (400 to 500 kbar). The resulting high‐pressure melting line for magnesium is in good agreement with the Lindemann law, with reasonable high pressure values for the Gruneisen coefficient γG.

Effect of confinement and thermal cycling on the shock initiation of LX-17

Abstract The shock initiation of the insensitive high explosive LX-17, which contains 92.5% triaminotrinitrobenzene (TATB) and 7.5% Kel-F binder, was studied under two simulated accident conditions: initially confined charges were heated to 250°C and shocked; and unconfined charges were thermally cycled between 25° and 250°C and shocked. Previous research on unconfined TATB-based explosives heated to 250°C revealed increased shock sensitivity. This increase was attributed to both the increased porosity caused by the unsymmetrical thermal expansion of TATB, which resulted in more hot spot ignition sites, and the faster growth of hot spot reactions due to the increased surrounding temperature. In this study, aluminum confinement was used to decrease the thermal expansion of LX-17. The shock sensitivity of confined LX-17 at 250°C was observed to be less than that of unconfined charges at 250°C but greater than that of unconfined, ambient temperature LX-17. The thermal cycling results showed that the LX-17 heated to 250°C and then shocked at 25°C was more sensitive than pristine LX-17, because irreversible growth had produced more ignition sites. LX-17 held at 250°C for an hour or fired at 250°C after two thermal cycles did not appear to be significantly more shock sensitive than LX-17 heated to 250°C and shocked immediately. Therefore it is unlikely that TATB is thermally decomposing into less stable intermediate species at 250°C. The Ignition and Growth reactive flow model for shock initiation of LX-17 was normalized to these experimental results to provide a predictive capability for other accident scenarios that cannot be tested directly.