火星环绕器羽流效应仿真研究

火星探测器发动机羽流效应影响飞行任务的安全,采用差分求解NS方程与直接模拟蒙特卡洛耦合的方法对火星环绕器姿轨控发动机羽流力、热以及污染效应进行了仿真研究。结果表明:发动机羽流对火星环绕器气动力、热及污染效应影响主要在发动机储箱上,最大热流值可以达到1789 W/m^2,需要注意;发动机羽流对展开太阳能电池翼的气动力和气动力矩相对于发动机推力和推力力矩来说,小3~4个量级,影响较小;已分离着陆器在距发动机出口300 m以内时,羽流对已分离着陆器的影响较大,气动热流在100 W/m^2以上;在100 m以内时,气动热流在1000 W/m^2以上,需要对敏感设备采取一定的防护措施。不同热适应系数气动热流仿真对比计算结果表明,随着热适应系数的增加,热流密度值呈线性增加。数值模拟结果可以为火星环绕器工程设计提供参考。

月球探测器着陆过程羽流热效应数值模拟研究

考虑到月球探测器着陆过程变推力发动机工作可能引发羽流热效应,通过差分求解N-S方程与直接模拟蒙特卡洛(DSMC)耦合的方法,针对探测器变推力发动机羽流的连续流区与稀薄流区进行了数值模拟,获得了着陆过程不同状态探测器表面的羽流气动热流密度分布,分析了探测器在不同高度及坡度受到的羽流热效应,研究了探测器因落月惯性与发动机延时拖尾等特殊工况羽流气动热的影响程度。数值模拟结果表明,探测器受到的羽流气动热影响总体随着发动机出口与月面间距离减小而急剧增强,随着距离减小至0.434 m,热流密度最大值为429 kW/m^(2),而且月面坡度对羽流热效应有减弱作用。研究结果可为探测器关机策略的制定提供支撑,为探测器关键部位的热防护提供输入,为探测器整体的设计优化提供服务。

Experimental and numerical analysis of the heat flux characteristic of the plume of a 120-N thruster

The convective heat transfer of the plume of a 120-N thruster is investigated experimentally and numerically. Numerical results agree well with experimental results in that there is a nonlinear decrease in convective heat transfer with an increasing cone angle. It is also found that convective heat transfer decreases with increasing distance from the thruster outlet. Furthermore, the convective heat transfer of the plume mainly concentrates within a 35° cone angle and the heat flux decreases to the same order as solar radiation at the Earth’s surface when the cone angle exceeds 60°. The results of the study will help improve spacecraft design.

真空羽流智能化计算

真空环境中,姿轨控发动机工作产生的高温高速真空羽流会对航天器产生气动力、气动热、污染、电磁干扰和视场干扰等效应,影响航天器正常工作甚至任务成败。因此,真空羽流及其效应评估和防护是航天领域的关键科学和工程问题。直接模拟蒙特卡洛(DSMC)方法是真空羽流数值模拟的主流方法,但DSMC是一种粒子模拟方法,非常耗时,严重制约了真空羽流及其效应的评估效率。提出了一种基于卷积神经网络的直接模拟蒙特卡洛(CNN-DSMC)方法。CNN-DSMC的训练集包括两个部分:将DSMC羽流数值模型的几何拓扑与边界条件信息作为训练集的输入,将DSMC数值模拟得到的羽流流场数据作为训练集的输出。将该训练集输入卷积神经网络进行训练,可得到高精度、高效率的真空羽流智能化计算模型,以此预测不同条件下的真空羽流流场。以月球探测器月面着陆过程中的真空羽流场为例,分别使用CNN-DSMC和DSMC数值模拟了在不同着陆高度条件下的真空羽流流场流速和密度。结果显示,两种方法结果基本一致,流场流速和密度的平均相对误差分别小于6.0%和8.8%。但与传统的DSMC方法相比,CNN-DSMC方法的计算速度提升至少4个量级,最大可达6个量级。因此,本文提出的CNN-DSMC方法在真空羽流数值模拟方面具有较强的应用潜力。

Maxwell模型中keV能量的Xe粒子对铜表面的适应系数

使用热流传感器对氙离子推力器轴向距离为500、700 mm和900 mm,径向角度为0°~15°(推力器出口平面中心为圆心,推力器出口轴线为0°)范围内羽流热流密度的分布进行了实验研究,获得了热流随角度和半径变化的实验数据。采用PIC-DSMC (particle in cell direct simulation of Monte Carlo)算法在不同适应系数下对实验条件进行仿真分析,对比仿真结果和实验结果得到适应系数。结果表明:keV能量的Xe粒子对热流传感器表面(铜)的适应系数接近1。

Plume aerodynamic effects of cushion engine in lunar landing

During the second period of China ＂Tanyue＂ Project, the explorer will softland on the moon. The cushion engines are used to decelerate the explorer and reduce the impact on the lunar ground. It is necessary to study its plume effects on the explorer component. The self-developed PWS （Plume WorkStation） software based on direct simulation Monte Carlo （DSMC） method is used to simulate the plume effects of two 150 N engines. Due to the complex structure of the explorer, PWS uses a decoupling method to treat the boundary mesh, which mainly interacts with simulation particles, and has no relation with the computational grids. After the analytical expressions of plane surfaces and curved surfaces of each boundary block are given, the particle position within or without the boundary blocks can be easily determined. Finally the 3D plume field of two 150 N engines is simulated. The pressure, temperature and velocity distributions of plume field are clearly presented by three characteristic slices. The aerodynamic effects on the explorer bottom, the landfall legs and antenna are separately shown. The compression influence on the plume flow of four landfall legs can be observed.

Research on vacuum plume and its effects

In vacuum environment, the exhaust flow of attitude control thrusters would expand freely and produce the plume, which possibly causes undesirable contamination, aerodynamic force and heating effects to the spacecraft. Plume work station （PWS） is developed by Beihang University （BUAA） for numerically simulating the vacuum plume and its effects. An approach which combines the direct simulation Monte Carlo （DSMC） method and difference solution of Navier-Stokes （N-S） equations is applied. The internal flows in nozzles are simulated by solving the NS equations. The flow parameters at nozzle exit are used as the inlet boundary condition for the DSMC calculation. Experimental studies are carried out in a supersonic low density wind tunnel which could simulate the 60-80 km altitude environment to investigate the plume and its effects. To demonstrate the capability of PWS, numerical simulations are performed for the vacuum plume of several typical attitude control thrusters. The research results are of great help for the engineering design.

Design of double-layer anti-sputtering targets for plume effects experimental system

The back-sputtering effect of an electric thruster used in a vacuum chamber has an adverse effect on the thruster, vacuum chamber and experimental equipment. An anti-sputtering target （AST） can extend the operating life of the vacuum chamber by reducing the back-sputtering deposition effect. Vacuum chambers used in international facilities have successfully incorpo- rated ASTs. To improve an AST＇s performance, a double layer structure AST based on the plume effects experimental system （PES） was designed. The structure of primary-end of the AST was designed as a flat while the secondary end was designed as a shutter to resolve factors such as service life, back flow rate of the sputtered material, economy and system security. To veri- fy the effects of the double layer AST, the sputtering effects of the AST with and without the secondary-end were evaluated by a DSMC-PIC algorithm. Additionally, the experimental verifications of the bare chamber and the target were performed. The results indicate that the secondary end graphite sheets can reduce the back flow yield approximately 50%. The design of the double layer AST can be introduced as a reference for the design of an anti-sputtering target.

Simulation of two-phase plume field of liquid thruster

When the liquid propellant thruster works,its plume field would contain many propellant liquid droplets,especially at pulse state.Liquid droplets may move along with the gas flow and deposit on the components of spacecraft as contamination.The simulation of the plume field involving the gas molecules and liquid droplets is an important part in contamination studies of thruster plume.Based on the PWS software developed by Beihang University(BUAA),axial-symmetric two-phase direct simulation Monte Carlo(DSMC) method is used with the liquid droplet taken as a kind of solid particle.The computation of gas-to-particle effect and gas reflection on the particle surface are decoupled.The inter-particle collision is also considered.The gas parameters at nozzle exit of 120N engine after 20 ms pulse work are taken as the entrance condition of the numerical simulation.Four test cases are conducted for comparison of different collision modules.Simulation results show that the effects of liquid propellant droplets mainly concentrate near the axis line of engine.The particle-to-gas collision would cause evident differences in the gas field and subtle differences in the particle phase.The liquid droplets in the plume field are generally accelerated and convected by the gas molecules.The DSMC method is proved to be a feasible solver to numerically simulate the two-phase flow involving solid phase and rarefied gas flow.

Investigation on aerodynamic force effect of vacuum plumes using pressure-sensitive paint technique and CFD-DSMC solution

Pressure-sensitive paint(PSP) technique was employed to experimentally investigate the aerodynamic force effect of vacuum plume in this study. The characterization and comparison for two types of PSP were firstly conducted in an air pressure range from0.05 to 5000 Pa. The PSPs were prepared using PtTFPP as the active dye and different binders, i.e., polymer-ceramic(PC) and poly(1-trimethylsilyl-1-propyne) [poly(TMSP)]. The static calibrations showed that PtTFPP/poly(TMSP) had a higher pressure sensitivity and a lower temperature dependency compared to PtTFPP/PC in this pressure range. The pressure distributions of a single and two interacting plumes impinging onto a flat plate model were measured using PSP technique. The experimental data were compared to numerical solutions that combined both the computed fluid dynamics(CFD) and direct simulation Monte Carlo(DSMC) methods. Remarkable agreements were achieved, demonstrating the feasibility and accuracy of the numerical approach.Finally, the aerodynamic force effect of interacting plumes at different separation distances was investigated numerically.

Experiment of Hall thruster plume effects on different ground experimental conditions

Reducing the influence of back-sputtering effect can effectively improve the accuracy of the measurement of the Hall thruster plume effect.Quartz crystal microbalance(QCM)was used to measure the deposition and sputtering distribution of HET-40 thruster plume on two different experimental conditions:case1,using liquid nitrogen heat sinks and case 2,without using liquid nitrogen heat sinks.Meanwhile,X-ray photoelectron spectroscopy(XPS)was used to analyze the composition of the QCM surface after two experiments.The results of the two experiments showed that the sputtering rate under the condition of case 1 was slightly higher than case 2.Especially within the range of 90°to 110°relative to the thruster axis,case 1 experiment result showed sputtering effect,while case 2 experiment showed deposition effect.Through analysis of the experimental results,it can be found that using liquid nitrogen heat sink to reduce the temperature of the inner wall surface of vacuum chamber can effectively adsorb the particles sputtered by the plume and reduce the concentration of back-sputtering particles,leading to the above phenomenon.