Understanding the precursor to detonation: Probing high-pressure deflagration with laser ignition experiments

Suddenly, there’s a flash of intense light and heat, followed by a rapidly expanding fireball. Combustion of high explosives is everywhere in popular culture, and it’s also critical for ensuring the safety and reliability of the U.S. stockpile.

While detonations often get all the credit for combustion, deflagrations—their subsonic, less famous precursors—are also fundamental to understanding the safety and sensitivity of high explosives.

In a new study, researchers at Lawrence Livermore National Laboratory (LLNL) have conducted laser ignition experiments in a diamond anvil cell and employed large-scale quantum molecular dynamics (QMD) simulations to investigate the products of deflagration at high pressures. The results could improve models of deflagration and high explosives overall. The work is published in the journal Combustion and Flame.

“A deflagration will generally precede a detonation, so understanding deflagration chemistry is important for understanding the necessary processes that are required for a detonation,” said LLNL scientist and first author Brad Steele.

These experiments and models aim to determine the products (resulting materials) of a deflagration. The composition of deflagration products, especially the solids, influences the amount of energy and pressure released in the reaction and whether it transitions to a detonation.

Typically, deflagration is studied at relatively low pressures. But by using laser ignition in a diamond anvil cell, the team was able to acquire data at high pressures that are comparable to the detonation pressure of high explosive LLM-105.

“The experimental approach is a modernized version of the technique first developed at LLNL in the 1990s,” said co-author and project principal investigator Jonathan Crowhurst. “It allows us to probe burn dynamics and chemistry in microscopic samples of high explosives at very high pressures.”

At these high pressures, the deflagration products of the experiment were transparent. However, the team’s experiment only detected molecular nitrogen, which did not account for the additional elements thought to be present, like carbon, hydrogen and oxygen. To better understand this, they looked to simulations.

The researchers used large-scale QMD simulations to investigate the pressure dependence of the product chemistry. They found reaction mechanisms that produce extended disordered clusters containing nitrogen and the additional elements.

“The condensed-phase chemistry of energetic materials has typically been simulated using potentials that do not model reaction kinetics accurately. Here we get qualitative agreement with experiment by more accurately modeling reaction kinetics with QMD,” said Steele. “The main drawback is that the method is extremely computationally expensive, so it requires the high-performance computing power available here at LLNL.”

In both the experiments and models, the authors found evidence of pressure reduction during deflagration. The predicted presence of nitrogen and oxygen in the disordered clusters is consistent with a delay in the formation of gaseous products, a result that could prevent a deflagration from transitioning into a full detonation.

Future work will focus on confirming these findings, applying the techniques to other energetic materials, and incorporating both into practical, macroscopic models that could help guide the design of better high explosives.