Michael J. Kaneshige

Modeling TNT ignition.

A 2,4,6-trinitrotoluene (TNT) ignition model was developed using data from multiple sources. The one-step, first-order, pressure-dependent mechanism was used to predict ignition behavior from small- and large-scale experiments involving significant fluid motion. Bubbles created from decomposition gases were shown to cause vigorous boiling. The forced mixing caused by these bubbles was not modeled adequately using only free liquid convection. Thorough mixing and ample contact of the reactive species indicated that the TNT decomposition products were in equilibrium. The effect of impurities on the reaction rate was the primary uncertainty in the decomposition model.

PETN Ignition Experiments and Models

Ignition experiments from various sources, including our own laboratory, have been used to develop a simple ignition model for pentaerythritol tetranitrate (PETN). The experiments consist of differential thermal analysis, thermogravimetric analysis, differential scanning calorimetry, beaker tests, one-dimensional time to explosion tests, Sandias instrumented thermal ignition tests (SITI), and thermal ignition of nonelectrical detonators. The model developed using this data consists of a one-step, first-order, pressure-independent mechanism used to predict pressure, temperature, and time to ignition for various configurations. The model was used to assess the state of the degraded PETN at the onset of ignition. We propose that cookoff violence for PETN can be correlated with the extent of reaction at the onset of ignition. This hypothesis was tested by evaluating metal deformation produced from detonators encased in copper as well as comparing postignition photos of the SITI experiments.

Development of scalable cook-off models using real-time in situ measurements.

Scalable thermal runaway models for cook‐off of energetic materials (EMs) require realistic temperature‐ and pressure‐dependent chemical reaction rates. The Sandia Instrumented Thermal Ignition apparatus was developed to provide in situ small‐scale test data that address this model requirement. Spatially and temporally resolved internal temperature measurements have provided new insight into the energetic reactions occurring in PBX 9501, LX‐10‐2, and PBXN‐109. The data have shown previously postulated reaction steps to be incorrect and suggest previously unknown reaction steps. Model adjustments based on these data have resulted in better predictions at a range of scales.

Radiant heating cookoff experiments and predictive simulations for fast cookoff.

Thermal initiation (cookoff) of energetic material-laden devices (rocket motors and munitions) during accidental fires is an important safety concern. An article within a pool fire is an example of potential fast cookoff scenerio that has significant potential for a catastrophic result. Beginning in 2007, a collaborative experimental and model development research program under the Joint Munitions Program (JMP) was initiated at SNL/NM to address energetic material response to fast cookoff. The efforts expanded our ongoing research program studying slow cookoff phenomena and sought to answer the key question of ”Can our kinetics models derived under slow cookoff conditions be applied to accurately represent energetic material behavior at fast cookoff conditions?” The simplest categorization of slow cookoff is material centered within the energetic material whereas a fast cookoff event is material ignition that occurs at a heated boundary. The external heating rate related to the heat conduction within the energetic material determines the ignition location and thus, the cookoff charaterization of slow or fast. We have developed a benchtop experiment that confines an energetic material sample and exposes it to constant incident heat fluxes common in fire in a controlled and reproducible fashion. Temperatures within the sample near the heated surface are measured using thermocouples and the time-to-event is determined

Small-scale cook-off experiments and models of ammonium nitrate

ABSTRACT We have completed a series of small-scale cook-off experiments of ammonium nitrate (AN) prills in our Sandia Instrumented Thermal Ignition test at nominal packing densities of about 0.8 g/cm3. We increased the boundary temperature of our aluminum confinement cylinder from room temperature to a prescribed set-point temperature in 10 min. Our set-point temperature ranged from 508 to 538 K. The external temperature of the confining cylinder was held at the set-point temperature until ignition. We used type K thermocouples to measure temperatures associated with several polymorphic phase changes as well as melting and boiling. As the AN boiled, our thermocouples were destroyed by corrosion, which may have been caused by reaction of hot nitric acid (HNO3) with nickel to form nickel nitrate, Ni(NO3)2. Videos of the corroding thermocouples showed a green solution that was similar to the color of Ni(NO3)2. We found that ignition was imminent as the AN boiling point was exceeded. Ignition of the AN prills was modeled by solving the energy equation with an energy source due to desorption of moisture and decomposition of AN to form equilibrium products. A Boussinesq approximation was used in conjunction with the momentum equation to model flow of the liquid AN. We found that the prediction of ignition was not sensitive to small perturbations in the latent enthalpies.

Prediction of Spatial Distributions of Equilibrium Product Species from High Explosive Blasts in Air.

Blast waves from an e xplosion in air can cause si gnificant structural damage. As an example, cylindrically-shaped charges have been used for over a century as dynamite sticks for mining, excavation, and demolition. Near the charge, the effects of geometry, standoff from the ground, the proximity to other objects, confinement (tamping), and location of the detonator can significantly affect blast wave characteristics. Furthermore, nonuniformity in the surface characteristics and the density of the charge can affect fireball and shockwave structure. Currently, the best method for predicting the shock structure near a charge and the dynamic loading on nearby structures is to use a multidimensional, multimaterial shock physics code. However, no single numerical technique curren tly exists for predicting secondary combustion, especially when particulates from the charge are propelled through the fireball and ahead of the leading shock lens. Furthermore, the air within the thin shocked layer can dissociate and ionize. Hence, an appropriate equation of state for air is needed in these extreme environments. As a step towar ds predicting this complex phenomenon, a technique was developed to provide the equilibrium species composition at every computational cell in a n air blast simulation as an initial condition for hand-off to other analysis codes for combusti on fluid dynamics or radiation transport. Her e, a bare cylindrical charge of TNT detonated in air is simulated using CTH, an Eulerian, finite volume, shock propagation code developed and maintained at Sandia National Laboratories. The shock front propagation is computed at early times, including the detonation wave structure in the explosi ve and the subs equent air shock up to 100 microseconds, where ambient air entr ainment is not sig nificant. At each computational cell, which could have TNT detonation products, air, or both TNT and air, the equilibrium species concentration at the density-energy state is computed using the JCZS2i database in the thermochemical code TIGER. Thi s extensive database of 1267 gas (including 189 ioniz ed species) an d 490 condensed species can predict thermodynamic states up to 20,000 K. The results of these calculations provide the detailed three-dimensional structure of a thin shock front, and spatial species concentrations including free radicals and ions. Further more, air shock predictions are compared with experi mental pressure gage data from a right circul ar cylinder of pressed TNT, detonated at one end. These complime ntary predictions show excellent agreement with the data for the primary wave structure.