气液两相介质中脉冲激光诱导爆轰波演化过程的热动力学基础研究

Energy conversion was mainly completed by detonation waves during the process of water-actuating-medium laser propulsion. The sputtering phenomenon of microdroplets was obvious when partial liquid on the water-film surface was breakdown by laser irradiation, which brought the low energy-conversion efficiency of the laser. The research thoughts about detonation waves produced by pulsed-laser-induced water droplets were proposed in view of thermokinetics. The transportation problems of energy, mass and momentum among the incident laser beam, the plasma, the air and the water droplets were analyzed by means of the combination between thermodynamic theory and transport theory, and an universal mathematical model that could reasonably described the coupled solution between irradiation transport and fluid dynamics was presented, those were used to elucidate the influences of droplet-particle characteristics on the generation and propagation mechanisms of detonation waves induced by pulsed laser in gas-liquid two-phase media, which can improve the intensity of detonation waves and reduce the ignition threshold through modulating droplet-particle characteristics including working fluid, particle size and concentration. The rapid-hybrid-particle level-set method was employed to trace and capture the wave front of detonation wave to reveal the variation characteristics of internal fine structures during the evolution process of detonation waves and provide theoretical basis for the research on artificial control of detonation waves. All in all, the contents mentioned above can establish a solid theoretical foundation for the performance optimization of water-droplets laser propulsion, and offer a powerful theoretical guidance for the application of the process of water-droplets-laser breakdown in other fields, such as communication, optical image transmission, particle removal of space debris, and so on.

水工质激光推进的能量转化主要由爆轰波来实现。激光辐照水膜表面，击穿表面部分液体时，会出现明显的微小液体颗粒的溅射现象，使激光能量的转化效率偏低。本项目从热动力学角度，提出脉冲激光诱导雾化水滴产生爆轰波的研究思路：热力学理论和迁移理论相结合，考察入射激光束、等离子体、空气和雾化水滴间的能量、质量、动量输运问题，凝练出一套具有普适性的能够合理描述辐射输运和流体力学耦合求解的数学模型，以阐明气液两相介质中液相颗粒特性对激光诱导爆轰波产生及其传播机理影响，通过调制液相颗粒特性（工质、粒度、浓度等），提高爆轰波强度，降低点燃阈值；利用快速杂交粒子水平集方法追踪和捕捉爆轰波波阵面，揭示爆轰波演化过程中内部微细结构的变化特征，为爆轰波传播时人工控制研究提供理论依据。可为优化水滴激光推进性能研究奠定坚实的理论基础，为水滴激光击穿过程在通信、光学图像传输、空间垃圾颗粒的清除等领域的应用提供有力的理论指导。

本项目研究气液两相介质中脉冲激光诱导爆轰波演化过程的热动力学特性，通过机理分析、数值计算和实验考察激光诱导雾化水滴产生爆轰波时，LSDW内部微细结构的变化规律，掌握激光与液体相互作用过程中LSDW产生及传播的特征，其研究成果为优化水滴激光推进性能奠定理论基础，为水滴激光击穿过程在通信、光学图像的传输等领域的应用提供理论指导，具有较强的前瞻性和探索性。建立单水滴激光击穿模型，研究入射激光功率密度、波长、水滴半径的击穿特性，得到影响介质击穿阈值的因素与发展趋势。结合气体介质激光支持爆轰波模型和云雾爆轰模型，建立气液两相介质的激光支持爆轰波模型，研究爆轰波前后主要参数的变化特征。将气液两相介质的激光支持爆轰波模型应用于激光推进过程，建立气液两相介质激光推进的理想动力循环模型，分析能量转换效率的影响因素，通过对气液两相介质激光作用下能量沉积以及热力循环的分析，得到循环效率的表达式。冲压比和定容增压比是影响能量转化的因素，能量转化效率随着冲压比和定容增压比的增大而增大。在数值模拟方面，分别分析不同液相占比下气液两相介质激光支持爆轰波的流场内部微细结构，结合抛物形光船模型模拟实际情况下气液两相介质激光推进中流场与光船耦合情况。实验方面，对比空气与两相介质、不同液相占比情况下的击穿阈值，发现两相介质能够降低击穿阈值，击穿阈值大小与液相占比无关，但是液相占比越高，击穿概率越大。利用高速相机得到了激光支持爆轰波的等离子体发光图像，对比分析空气和两相介质的击穿情况，得到激光推进中光船所获得的冲量、冲量耦合系数、能量转化效率。研究结果表明，冲压比和定容增压比是影响能量转换效率的主要因素，增大冲压比或定容增压比都能有效提高能量转换效率，增大介质中水滴所占质量比能够有效增大定容增压比，提高能量转换效率。