thermochemical_qsp.py – A fast-running thermochemical model for predictions of quasi-static pressure for confined detonations of plasticised high explosives

<h2>Introduction</h2><p dir="ltr">When explosives detonate in a confined space, repeated reflections of the initial shockwave lead to the development of a long-term quasi-static pressure (QSP), which can cause severe structural damage and loss of life. Mixing of the detonation products with an oxygen-rich atmosphere results in additional energy release through ‘afterburn’ reactions, which further increase this pressure. Government agencies tasked with protecting the public from explosive threats require tools that can quickly assess the risk posed by QSP for a range of possible scenarios. Existing empirical relationships for TNT offer rapid calculation, but have not been validated against other explosive types. Conversely, specialised CFD and thermochemical codes can accurately predict QSP development for many explosives, but are computationally expensive. Here we develop a fast-running thermochemical model for the confined detonation of RDX- and PETN-based plastic explosives. Using simplified detonation and combustion reactions for the explosive and binder, we calculate the resulting internal energy change in the chamber atmosphere, and hence the QSP. The resulting predictions are within 3% of experimental values for both oxygenated and inert atmospheres, indicating that the contributions of detonation and afterburn to the energy release are reliably modelled. The thermochemical model is provided as a Python script which can be readily adapted for other explosives and atmospheric conditions.</p><h2>Example usage</h2><p dir="ltr">An immediate prediction of QSP can be made using the Python function <i>thermochemical_qsp()</i>.</p><p dir="ltr">The function syntax is <i>thermochemical_qsp(mass, explosive_type, volume, atmosphere)</i>, where <i>mass</i> defines the explosive mass in kg, <i>explosive_type</i> is currently defined for 'PE4', 'PE8' and 'PE10', <i>volume</i> defines the confined volume in m³, and <i>atmosphere</i> is the initial gas mixture, currently defined for 'air', 'nitrogen' and 'argon'. The initial pressure and temperature of the atmosphere may also be specified inside the function.</p><p dir="ltr">For example, a prediction for 50g of PE10 in a 0.275m³ air-filled space could be made using the command:<br><i>thermochemical_qsp(mass=0.05, explosive_type='PE10', volume=0.275, atmosphere='air')</i></p><h2>Further information</h2><p dir="ltr">The atmospheric gases listed in gas_data.json use thermochemical data from the NIST Chemistry WebBook, https://doi.org/10.18434/T4D303.</p><p dir="ltr">For more information on model assumptions and validation, see Barr et al. 'Fast-running thermochemical predictions of quasi-static pressure for confined detonations of plasticised high explosives'</p>