气固相互作用对吸气式电推进系统进气道性能的影响

Effects of gas-surface interaction on the inlet performance of an atmosphere-breathing electric propulsion system

针对上层大气层吸气式电推进系统进气道设计问题,采用直接模拟Monte Carlo(DSMC)方法对吸气式电推进系统的进气道内流问题进行系统的数值模拟,考虑气固相互作用(GSI)模型对进气道内流特征、进气道收集和压缩性能的影响,并从气体动理论的角度阐明了其作用机理。结果表明:GSI模型对进气道的压缩和收集性能具有巨大的影响,适应系数的降低即GSI中镜面反射比例的升高,能显著提高压缩因子和收集效率,适应系数从1.0降低到0.2使得压缩因子增大7倍多,收集效率增大将近4倍。适应系数的降低能有效增大进气道压缩因子和收集效率的机制是气体分子在内凹型压缩段壁面发生镜面反射之后汇聚通过焦点,进入电离加速段。上层大气层吸气式电推进系统的进气道应该采取几何外形/表面材料相互耦合的设计思路:既采用内凹型压缩段的进气道,更需要通过材料选取或表面加工降低GSI适应系数,使得GSI尽量接近镜面反射。

[Objective] The atmosphere-breathing electric propulsion(ABEP)system has become a highly promising candidate for drag compensation in spacecraft operating in very low Earth orbit.To improve the inlet design of ABEP systems,this study performs a comprehensive numerical investigation of gas flows inside the inlet.The primary objective is to gain insight into the effects of the gas-surface interaction(GSI)model on the flow features,compression,and collection performances.[Methods]This paper explores ABEP inlet flows using the direct simulation Monte Carlo(DSMC)method.A typical altitude of 180 km in the upper atmosphere is considered,and four GSI accommodation coefficients(σ=1,0.8,0.5,and 0.2)are selected.The DSMC method simulates gas flows according to the motion of a cluster of simulation particles,where each particle represents a large number of real gas molecules.In the DSMC method,particle motions are computed deterministically,whereas intermolecular collisions are calculated statistically.Each simulation particle travels at a constant velocity until it collides with another simulation particle or a solid surface.In the event of an intermolecular collision,an appropriate molecular collision model is employed to compute post-collision velocities,and in the event of gas-surface collisions,a suitable GSI model is adopted to calculate the molecular velocity after reflection.In this work,the internal energy exchange is modeled using the Larsen-Borgnakke scheme.Further,the intermolecular collision is handled using the variable hard sphere model and the no time counter-collision sampling technique.The simulation is always evaluated as an unsteady flow,and a steady result is obtained as the large-time state of unsteady simulation.After achieving a steady flow,the simulation particles in each cell are sampled for a sufficient duration to decrease statistical scattering.All macroscopic field quantities(such as mass density,velocity,and temperature)and surface quantities(such as surface pressure,shear stress,and heat flux)are calculated based on these time-averaged data.[Results]Numerical results show that the distributions of gas pressure and mass flux are considerably affected by the GSI models.The lower the GSI accommodation coefficient,the higher the gas pressure and the larger the mass flux.Consequently,the GSI accommodation coefficients play a vital role in the compression factor and collection efficiency of the inlet.Furthermore,the decrease in the GSI accommodation coefficient from 1.0 to 0.2 leads to an increase in the compression factor and collection efficiency by a factor of 7 and 4,respectively.In addition,as the GSI accommodation coefficient decreases,the high-pressure region moves toward the ionization section,facilitating the ionization of neutral gas molecules.The following mechanism underlies this effect:after reflecting in a specular manner from the concave surface,the gas molecules congregate at the focus and enter the ionization section.[Conclusions]To improve the inlet design of an ABEP system,a combination of the geometric design and surface-material design should be adopted.A concave compression section should be employed,and at the same time,the inlet surface should be smoothened to decrease the GSI accommodation coefficient.