Calculation of pressure parameters at ignition moment of HMX-based aluminized pressed explosives under slow cook-off
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摘要: 为了研究HMX基含铝压装炸药在慢烤过程中点火时刻的压力参量，设计了0.1和1℃/min升温速率下的慢烤试验，并对炸药内部进行了多点测温。在此基础上，基于炸药的通用烤燃模型，将HMX的多步分解机制与铝粉反应相结合，并考虑其分解中的相变过程，建立了HMX基含铝压装炸药慢烤反应速率与压力相关的计算模型并进行数值模拟研究。试验结果表明，在0.1℃/min的升温速率下，端盖喷出，壳体沿轴向撕开裂缝，无药粉残留，判定炸药发生爆燃反应；在1℃/min的升温速率下，壳体发生轻微变形，有部分药粉残留，判定炸药发生燃烧反应。数值研究结果表明，随着热刺激强度的提高，炸药的点火温度呈对数上升趋势，而烤燃弹的反应进度和内部压力呈现指数下降趋势，且烤燃弹内部的反应压力在HMX相变前呈缓慢上升趋势，相变后呈快速上升趋势。Abstract: In order to study the pressure parameters of HMX-based aluminized pressed explosives at the ignition moment of slow cook-off, slow cook-off test were designed at 0.1 and 1℃/min heating rates, and an internal multi-point temperature measurements were taken inside explosives. On this foundation, based on the universal cook-off model of explosives, combining the multi-step decomposition reaction mechanism of HMX-based explosives with the reaction of aluminum powder, and considering the phase transition process in the decomposition of HMX-based explosives, a slow cook-off calculation model for pressure-department reaction rate of HMX-based aluminized pressed explosives was established. This study wrote the calculation model as a user defined function and imported it into Ansys Fluent to perform calculations. Slow cook-off tests were conducted on large aspect ratio (5:1) HMX-based aluminized pressed explosive charges with 4 mm shell thickness at heating rates of 0.1℃/min and 1℃/min and compared with simulation results. And then, the numerical simulations of the temperature field and internal pressure changes were performed before ignition of the cook-off bomb at heating rates of 0.055, 0.1, 0.2, 0.3, 0.5, and 1℃/min. It was found that at the heating rate of 0.1℃/min, after the test reaction, the end cover was ejected, the shell was axially cracked, and there was no powder left, which was judged to be a deflagration reaction; at the heating rate of 1℃/min, the shell was slightly deformed, with some powder left, and it was judged that a combustion reaction had occurred. The numerical calculations show that as the heat stimulus increases, the ignition temperature of the explosive tends to increase logarithmically, while the extent of reaction and internal pressure of the cook-off bomb tend to decrease exponentially. Before the HMX phase transition, the internal pressure inside the cook-off bomb grows slowly, after the HMX phase transition rapidly increasing, and finally it rises sharply near the ignition moment.
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图 3 不同升温速率下炸药各测点温度‐时间曲线
Figure 3. Temperature-time curves of explosives at various monitoring points under different heating rates
图 5 不同升温速率下监测点温度及弹体内部压力曲线
Figure 5. Curves of monitoring-point temperature and internal pressure of the bomb at different heating rates
图 8 不同升温速率下点火时刻温度梯度示意图
Figure 8. Temperature gradient diagram of ignition moment at different heating rates
图 9 点火时刻烤燃弹体中轴线上的温度分布
Figure 9. Temperature distribution on the central axis of the cook-off bomb at ignition moment
图 10 不同热刺激强度下烤燃弹点火时刻温度及内部压力曲线
Figure 10. Temperature and internal pressure curves of the cook-off bomb at ignition moment under different thermal stimulation intensities
图 11 不同热刺激强度下烤燃弹点火时刻的温度分布
Figure 11. Temperature distribution in the cook-off bomb at ignition moment under different thermal stimulation intensities
图 12 不同热刺激强度下烤燃弹反应进度曲线
Figure 12. Extents of reaction of the cook-off bomb under different thermal stimulation intensities
表 1 点火时刻及点火时刻不同测点的温度
Table 1. Ignition moments as well as temperatures of different measuring points at ignition moments
升温速率/(℃·min−1) 点火时刻/s T1/℃ T2/℃ T3/℃ 0.1 43920 203.9 220.2 205.3 1 10740 209 221.9 230.4 注：T1、T2和T3分别为点火时刻测点1～3的温度。 
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表 2 不同网格尺寸模型的数值计算结果
Table 2. Numerically-calculated results by the models with different grid sizes
网格尺寸/mm 网格数量 外壁温度/℃ 内部压力/MPa 0.3 2806242 215.26 10.65 0.4 1196698 215.27 10.96 0.5 734650 214.93 11.26
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表 3 两种升温速率下点火时间及监测点温度计算值与试验值的对比
Table 3. Comparison of calculated and experimental values of ignition time and temperature at monitoring points under two different heating rates
升温速率/
(℃·min−1)点火时间 T1 T2 T3 试验值/s 计算值/s 误差/% 试验值/℃ 计算值/℃ 误差/% 试验值/℃ 计算值/℃ 误差/% 试验值/℃ 计算值/℃ 误差/% 0.1 43920 42100 4.14 203.9 200.39 1.72 220.2 218.97 0.56 205.3 213.34 3.92 1 10740 11160 3.91 209 214.93 2.84 221.9 227.26 2.42 230.4 228.4 0.87
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