吸气模式激光推进空气等离子体光谱

利用实验室自行研制的紫外预电离CO2激光器进行了激光等离子体实验,实验采用底面直径均为60mm,焦距分别为5 mm和10 mm的两种抛物面光船。介绍了激光等离子体光谱和明显的特征谱线,以及激光等离子体温度估计模型;分析了两种光船对激光等离子体的影响。实验表明:相同底面直径10 mm焦距光船产生的等离子体信号峰值和持续时间均略大于5 mm焦距光船;10 mm焦距光船产生的等离子体温度明显高于5 mm的,最大温度产生时间比5 mm光船要晚一些。     

管中激光加速器的推力形成过程

管中激光加速器是一种新型的激光推力器,弹射体在管道中被加速。利用光线追踪法,根据光线在网格中的传播路径信息,运用辐射输运方程,得到流场的能量源项;数值求解含能量源项的流体控制方程,对管道中等离子体流场激波的形成和演化过程进行了研究。模型能够模拟空气光学击穿、空气对激光能量的吸收、等离子体对激光的屏蔽作用和推力形成过程,通过计算得到冲量耦合系数为3.61×10-4N/MW,等离子体内能转化为流场动能的比例约为9%。     

Laser jet propulsion based on the continuous optical discharge

Problems of working process organization in the absorption chamber of a laser jet propulsion are considered. A method for plasma stabilization in the near-axial region of the absorption chamber is proposed; the method is based on the axisymmetric swirled working gas counterflow around a continuous optical discharge. The conditions ensuring continuous stable plasma “combustion” depending on the geometric chamber sizes, flow temperature, working gas type are determined. Possible deviations of a real working process from the theoretical statements are analyzed.     

激光推进辐射击穿温度特性的数值研究

在9种等离子体击穿温度下,数值模拟了二次反射式聚焦系统聚焦情况下激光推力器内流场的演化过程,得到了不同击穿温度对应的能量沉积率、推力峰值、冲量和冲量耦合系数,能量沉积率和推进性能参数在某个等离子体击穿温度值处发生突变。根据空气对激光的逆韧致吸收系数公式,计算了CO2激光辐照下不同等离子体击穿温度对应的空气辐射自由程,发现当等离子体击穿温度为14000K时,辐射自由程为1.4mm,与计算网格的典型尺寸相当,此时入射激光能量在一个网格内以一定效率被吸收,由此确定了基于逆韧致吸收的激光等离子体的击穿温度。     

透射式激光等离子体微烧蚀推力性能研究

激光等离子体微推进技术经过近10年的发展,越来越受到业内人士的广泛关注。针对透射式激光等离子体微烧蚀物理过程,构建了理论分析模型,并进行了仿真。结合仿真结果,分析了激光脉宽和靶材的选取对烧蚀产生的推力和冲量的影响,提出了激光微烧蚀推进参数设计过程中,需要注意的几个问题。采用冲击摆系统测量激光微烧蚀靶材推力特性,对理论模型进行修正,与实验结果对比,模型与实验测量结果符合良好。     

激光等离子体减阻技术研究进展

利用激光等离子体减小高超声速飞行器的气动阻力对节能和最大限度地提高飞行器性能具有重要的意义。简单概述了高超声速飞行器的军事需求及等离子体减阻技术的分类;总结了激光等离子体减阻机理的实验研究和数值模拟情况;讨论了等离子体能量、点火位置和形状等参数对减阻性能的影响,目前利用激光等离子体技术可以减小70%的波阻。针对中国的激光等离子体减阻研究工作,分析了目前国内外研究中存在的问题,提出了深入开展激光等离子体减阻机理研究等建议。     

激光光船周围连续/稀薄混合流场的数值模拟

当火箭模式激光推进器在高空稀薄大气中飞行时,需要在激光聚焦区附近喷射稠密气体工质,从而利用激光聚焦击穿气体工质形成等离子体,为光船提供推力。为了分析该光船的推力产生过程,采用连续流体力学计算方法(CFD)和直接数值模拟蒙特卡洛方法(DSMC)耦合的混合算法,数值模拟飞行高度为80 km飞行马赫数为5的激光光船周围高温连续气体和稀薄环境大气耦合的非定常混合流场,并得到推力随时间的演化曲线。结果表明,稀薄效应明显增加了激波厚度,高温效应延迟了推力的产生,并导致冲量耦合系数和比冲的降低。     

激光聚焦点火区域流场演化的数值研究

激光聚焦点火区域形成的低密度球腔会制约吸气式高重频脉冲激光推进发动机的稳定工作。基于球形点火模型,用数值计算方法研究了等离子体的初始温度对球腔大小的影响以及球腔的演化特点。研究表明,当等离子体的能量较大时,它的初始温度对球腔大小的影响较小;球腔的密度不足环境气体密度的0.1倍,温度高达几千K,压强接近环境气体压强;球腔半径与等离子体动力学半径的比值稳定在0.29左右。通过对球腔演化规律的分析,为激光器参数的控制和聚焦系统的设计提供了一定的参考依据。     

A method for calculating the laser rocket engine thermal state based on the optical discharge with numerous plasma formations

A method is presented for calculating the thermal state of the laser rocket engine (LRE) structure with numerous plasma formations in the absorption chamber. The results of evaluating radiant heat fluxes and the temperature of the LRE structure walls with regard for convective and conductive heat exchange are given; the actions of a heat flux with the specified distribution density on the structure surface are considered as functions of time and thermal radiation, that is due to the temperature and optical properties of a radiating body. It is shown that working process organization in the absorption chamber on the basis of numerous plasma formations makes it possible to produce a uniform profile of the heat flux distribution along the absorption chamber axis and thus reduce a possibility of separate structure sections overheating.     

激光参数对光船性能影响分析

采用三温度11组元热化学非平衡空气模型计算了激光能量在等离子体中的沉积过程,并在激光脉冲作用结束8 μs后采用平衡空气模型,完成激光推进光船工作过程的数值模拟,研究了不同入射激光能量和脉冲宽度对光船推进性能的影响。结果表明:当脉冲宽度相同时,入射激光能量越大,所得冲量耦合系数越大;当入射激光脉冲能量相同时,脉冲宽度越小,所得冲量耦合系数越大。将计算所得冲量耦合系数与Schall实验所得结果进行比较,结果非常吻合。     

激光等离子体点源减阻技术中入射能量对气动阻力的影响

数值模拟了来流马赫数为5时的圆球绕流冷流场,以及来流与头部驻点附近注入激光能量而产生的等离子体相互耦合形成的流场的演化过程。计算结果表明:施加局部能量点源能够改变原有的弓形激波结构,使其变为斜激波,从而减小气动阻力;气动阻力随入射激光能量的增大而逐渐减小,当入射激光能量为1.1 J时,气动阻力减小的百分比高达40%。     

激光重熔等离子喷涂纳米氧化锆热障涂层组织与性能

采用等离子喷涂纳米氧化锆(ZrO_2-8%Y_2O_3)团聚粉末制备了纳米氧化锆热障涂层,利用连续CO_2激光对其进行重熔处理.以常规热障涂层作为比较对象,研究了纳米氧化锆热障涂层和激光重熔涂层的组织结构、硬度、抗热冲击性能.结果表明:纳米氧化锆热障涂层组织结构为独特的纳米-微米复合结构,主要有柱状晶和未熔融或部分熔融纳米颗粒组成;激光重熔热障涂层的组织结构为表面等轴晶+断面柱状晶.硬度试验和抗热冲击性能试验综合比较结果显示:相对于常规氧化锆热障涂层,纳米氧化锆热障涂层和激光重熔热障涂层拥有更好的性能.因此将纳米技术和激光重熔表面处理技术与等离子喷涂技术结合起来制备热障涂层是提高热障涂层性能的非常有前景的工艺方法.     

透射式水工质的高耦合效率激光推进模式

为了提高激光推进的耦合效率,防止光学器件污染,提出了透射式液体工质的激光推进模式。在这种推进模式中,Nd:YAG固体激光器中产生的450 mJ,7 ns激光穿过透明的玻璃基底后,与燃烧室中的液体推进剂相互作用,产生的等离子体在玻璃基底、燃烧室和液体推进剂的约束下膨胀,产生很高效率的动量转换,使靶获得初速度。纯水和黑墨水分别被用作推进剂,通过比较实验结果发现,在这种模式中墨水比水更适合做推进剂。通过改变燃烧室的长度和孔径,得到的耦合系数的最大值为17 858.3 N/MW。     

激光深熔焊熔池动力学特性理论分析

探讨了激光深熔焊过程中焊接熔池流体流动的各种驱动力,分析了熔池流动热边界层和固液界面粘性边界层的作用,研究了激光小孔出口的等离子体焰流对熔池Marangoni流的影响.     

Turbulent boundary layer separation control using plasma actuator at Reynolds number 2000000

An experimental investigation was conducted to evaluate the effect of symmetrical plasma actuators on turbulent boundary layer separation control at high Reynolds number. Compared with the traditional control method of plasma actuator, the whole test model was made of aluminum and acted as a covered electrode of the symmetrical plasma actuator. The experimental study of plasma actuators' effect on surrounding air, a canonical zero-pressure gradient turbulent boundary, was carried out using particle image velocimetry(PIV) and laser Doppler velocimetry(LDV) in the 0.75 m × 0.75 m low speed wind tunnel to reveal the symmetrical plasma actuator characterization in an external flow. A half model of wing-body configuration was experimentally investigated in the  3.2 m low speed wind tunnel with a six-component strain gauge balance and PIV. The results show that the turbulent boundary layer separation of wing can be obviously suppressed and the maximum lift coefficient is improved at high Reynolds number with the symmetrical plasma actuator. It turns out that the maximum lift coefficient increased by approximately 8.98% and the stall angle of attack was delayed by approximately 2° at Reynolds number2 ×10~6. The effective mechanism for the turbulent separation control by the symmetrical plasma actuators is to induce the vortex near the wing surface which could create the relatively largescale disturbance and promote momentum mixing between low speed flow and main flow regions.     

MACH2 simulations of a micro laser ablation plasma thruster

In this work the detailed physical processes occurring in the high density plasma that is ejected from the solid propellant surface in a small laser ablation thruster are simulated using MACH2. Qualitative results of the laser ablation process that leads to propellant erupting from the surface and leaving behind a crater in a solid Teflon® propellant are presented. Simulations were conducted for a 0.5 μs laser pulse (FWHM) at 935 nm with laser pulse energy ranging from 20 μJ to 2 mJ. Simulation results indicate that crater diameter and depth increase with pulse energy. The impulse bit also increases with pulse energy. Specific impulse follows the opposite trend and decreases with laser pulse energy. The simulated impulse bit for a 2 mJ, 0.5 μs laser pulse over-predicts that reported in the literature for a 2 mJ, 2 ms laser ablation thruster pulse by approximately one order of magnitude and under-predicts the specific impulse by approximately one order of magnitude.     

Relation between laser flux,temperature and ionisation equilibrium in laser produced plasmas

Laser produced plasmas are a useful source of highly stripped ions for XUV spectroscopy and it is therefore important to understand the relation between the incident laser flux, plasma temperature and ionisation equilibrium.     

不同辅助启动材料对激光推进器冲量的影响

为了获得不同辅助材料对激光推进器工作性能的影响,利用高速运动分析仪测量冲击摆的最大摆角,根据摆角确定激光推进器的冲量,测量了不同辅助启动材料下激光推进器冲量。在压强为0.1 MPa时,POM辅助下激光推进器冲量为4.6 mN.s,是未辅助时的2.496倍。试验结果表明:大部分辅助启动能够明显地增强等离子体的能量吸收率,高分子材料POM的辅助启动效果最佳;随着环境压强的下降,辅助启动的作用更加重要。     

激光与空气作用过程中辐射能量损失的研究

激光与空气相互过程中,与辐射相关的各种过程起着特别重要的作用。研究了高温气体辐射对气体流体力学过程的影响。辐射能量密度和压力对气体状态和运动的影响完全可以忽略不计,而流场的辐射能量损失不能忽略。利用光线追踪法,运用辐射输运方程,根据光线在网格中的传播路径,计算空气吸收的激光能量;根据网格厚度,确定辐射能量损失。数值求解含能量源项的流体控制方程,对激光击穿空气的等离子流场演化过程进行了模拟,得到了辐射能量损失随入射激光能量的变化规律。     

TiAl合金表面激光重熔Al_2O_3-13wt%TiO_2复合陶瓷涂层组织结构

采用等离子喷涂和激光重熔复合工艺在TiAl合金表面制备了Al2O3-13wt%TiO2复合陶瓷涂层.为了减少重熔层裂纹等缺陷,采用较低的激光功率和能最密度进行重熔.用扫描电镜(SEM),能谱仪(EDS)和X射线衍射仪(XRD)分析了涂层形貌、微观结构和相组成.结果表明,经过激光重熔处理后,陶瓷涂层颗粒细化,片层状组织得以消失,致密性提高,获得了基本没有裂纹等缺陷的重熔层;激光重熔亚稳相γ-Al2O3转变为稳定相α-Al2O3;由于陶瓷材料导热系数较低的影响,激光重熔时无法使整个陶瓷层实现重熔,重熔后的陶瓷涂层形成了晶粒细小的等轴品重熔区、烧结区以及片层状残余等离子喷涂区.