Analysis of thermal safety impact of airflow on the process of femtosecond laser processing explosive charge
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摘要: 由于炸药具有热传导系数小、对温度极其敏感的特点，在使用多脉冲飞秒激光对其进行持续加工时，极有可能在炸药内形成热累积，从而导致点火、燃烧等危险事件的发生。为了降低激光加工材料过程中的热效应，人们普遍采取在材料加工表面施加气流的方法。为了研究加载气流条件下，炸药装药在飞秒激光作用下产生的烧蚀产物的运动规律以及炸药装药内部的温度变化，建立了加载气流条件下飞秒激光加工炸药装药过程的二维流固耦合计算模型，对在单侧、双侧不同入射角度的亚音速气流作用下，飞秒激光加工奥克托今（HMX）炸药装药的过程进行了数值模拟计算。计算结果表明：单侧气流会在炸药加工表面形成漩涡流，导致烧蚀气体产物在炸药表面做旋转运动，加重了烧蚀产物对炸药的热影响；双侧气流会在远离炸药加工表面的地方形成较大的漩涡流，从而使烧蚀气体产物迅速离开炸药加工表面，有效降低了炸药的温度，提高了飞秒激光加工炸药装药过程的安全性。Abstract: Due to the low thermal conductivity and high sensitivity to temperature of explosive, it is highly possible to form heat accumulation inside the explosive during the continuous processing of multi-pulse femtosecond lasers, leading to dangerous events such as ignition and combustion. To reduce the thermal effects during laser processing of materials, applying airflow is a common choice. To study the motion law of ablation products generated by explosive under the action of femtosecond laser and the temperature changes in explosive under the action of airflow, a two-dimensional fluid-solid coupling calculation model of femtosecond laser processing explosive under the action of airflow is established. Numerical simulation calculations are conducted on the process of processing cyclotetramethylene-tetranitramine (HMX) explosive using femtosecond laser under the action of subsonic airflow with different incidence angles on one or both sides. The calculation results show that a unilateral airflow will form a vortex flow on the surface of the explosive, causing the ablation products to rotate on the surface of the explosive, exacerbating the thermal effect of the ablation products on the explosive; The bilateral airflow will form a large vortex flow far from the surface of the explosive, causing the ablation products to leave the surface of the explosive quickly, effectively reducing the temperature of the explosive and improving the safety in the process of femtosecond laser processing explosive.
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Key words:
- femtosecond laser /
- explosive /
- numerical simulation /
- airflow /
- thermal safety
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图 1 切向气流作用下，飞秒激光烧蚀炸药装药过程的物理模型示意图
Figure 1. Schematic diagram of the physical model of the ablation of explosive charge by femtosecond laser under the action of tangential airflow
图 2 切向气流作用下，飞秒激光烧蚀炸药装药的计算模型示意图
Figure 2. Schematic diagram of calculation model of the ablation of explosive charge by femtosecond laser under the action of tangential airflow
图 3 不同入射角度的单侧气流作用下，空气域中流场速度的分布
Figure 3. Velocity distribution of flow field in air under the action of unilateral airflow at different incidence angles
图 4 45°单侧气流作用下，炸药表面监测点处的温度-时间变化曲线
Figure 4. Temperature-time curves at the monitoring points on the surface of the explosive under the action of 45° unilateral airflow
图 5 不同入射角度的双侧气流作用下，空气域中流场速度的分布
Figure 5. Velocity distribution of flow field in the air domain under the action of bilateral airflow at different incidence angles
图 6 无外加气流时流场的分布
Figure 6. Distribution of flow field without the action of external airflow
图 7 45°双侧气流作用下，空气域中烧蚀气体产物的分布
Figure 7. Distribution of ablative gas products in the air domain under the action of 45° bilateral airflow
图 8 45°双侧气流作用下，空气域中流场温度的分布
Figure 8. Temperature distribution of flow field in air under the action of 45° bilateral airflow
图 9 45°双侧气流作用下，HMX炸药装药未烧蚀区域内的温度分布
Figure 9. Temperature distribution in the non-ablated area of HMX under the action of 45° bilateral airflow
图 10 不同入射角度的双侧气流作用下，监测点温度-时间变化的曲线
Figure 10. Temperature-time curves of monitoring points under the action of bilateral airflow at different incidence angles
表 1 HMX的多步热分解反应动力学参数
Table 1. Kinetic parameters of multi-step thermal decomposition reaction of HMX
explosive reaction step E/(kJ·mol−1) Z/s−1 Q/(kJ·kg−1) HMX 1 204 7.99×1020 −42 2 221 1.41×1021 −252 3 186 2.61×1016 559 4 143 1.60×1012 5620 
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表 2 HMX的物性参数
Table 2. Physical properties of HMX
density/
(kg·m−3)specific thermal
capacity/(J·kg−1·K−1)thermal conductivity/
(W·m−1·K−1)absorption
coefficient/m−1reflection
coefficientthermal diffusion
coefficient/(m2·s−1)1 905 1050 0.345 1.434×106 0.090 1.72×10−7
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