Introduction
Understanding the thermal response of energetic materials exposed to accidental heating is essential for ensuring the safety of storage, transportation, and practical applications. Although hexanitrostilbene (HNS) is one of the most thermally stable explosives, the physical mechanisms governing its ignition during programmed-heating cook-off have remained unclear.
In a recent study published in Energetic Materials Frontiers , a team of researchers combined laboratory cook-off experiments with numerical simulations to establish a heat transfer–reaction coupling framework that quantitatively explains the thermal response of millimeter-scale HNS charges under different heating conditions.
"The thermal response of HNS small-scale charges under programmed heating is governed by coupled heat transfer and chemical reaction,” explains corresponding author Peng Zhu. “The apparent ignition temperature should be understood as a macroscopic manifestation of heat transfer–reaction coupling rather than a direct indicator of intrinsic decomposition kinetics alone."
Heat Transfer and Chemical Reactions Jointly Govern Ignition
The researchers conducted programmed-heating cook-off experiments using HNS charges with different dimensions while systematically varying heating rate and packing density. Numerical simulations were then performed to visualize temperature evolution and reaction progression inside the charges throughout the heating process.
“We found that ignition behavior is determined by the coupling between heat transfer and chemical reactions,” shares Zhu. “Increasing the heating rate shortens the residence time within the intermediate-temperature region while enlarging transient internal temperature gradients, resulting in higher apparent ignition temperatures.”
Similarly, increasing charge size enhances internal thermal lag and raises the pre-ignition central temperature, ultimately producing higher measured ignition temperatures.
Charge Size and Density Cooperatively Regulate Thermal Response
The researchers further demonstrated that charge geometry significantly affects thermal response characteristics. “For the smallest specimens (Φ4 × 4 mm), the measured ignition temperature is only weakly influenced by internal heat-transfer resistance and therefore better represents the intrinsic thermal decomposition behavior of HNS,” says Zhu. “In contrast, for larger charges (Φ5 × 4 mm and Φ5 × 5 mm), internal heat transfer increasingly dominates the ignition process, leading to stronger thermal lag, larger temperature gradients, and higher apparent kinetic parameters.”
Packing density also played a dual role during cook-off. “Increasing density improves effective thermal conductivity, allowing heat to penetrate more efficiently and reducing thermal lag during the early heating stage,” adds Zhu.
However, the accompanying increase in volumetric heat capacity and enhanced heat redistribution required a higher central temperature before thermal runaway can occur. “Under the experimental conditions investigated, this latter effect dominates, resulting in a slight increase in the measured ignition temperature as density increases,” says Zhu.
These findings offer valuable guidance for thermal hazard evaluation, safety design, and predictive modeling of advanced energetic materials subjected to accidental thermal environments.
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Contact author:
School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China b Micro-Nano Energetic Devices Key Laboratory, Ministry of Industry and Information Technology, Nanjing, 210094, China. E-mail address: zhupeng@njust.edu.cn (P. Zhu)
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Energetic Materials Frontiers
Thermal response and heat Transfer–Reaction coupling of HNS small-scale charges during programmed-heating cook-off.
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