亚大气压六相交流电弧放电等离子体射流特性数值模拟

Numerical modelling for characteristics of the meso-pressure six-phase alternative current arc discharge plasma jet

以临近空间高超声速飞行器和航天器再入大气环境飞行过程中其表面产生的高密度非平衡态等离子体为研究对象,基于本研究组所建立的多相交流电弧放电等离子体实验平台(MPX-2015),开展了非平衡态氩等离子体射流特性的二维数值模拟研究.在亚音速条件下二维、非平衡数值模拟所得到的计算结果与实验测量结果符合良好.超音速条件下的数值模拟结果表明,随着真空腔压强的降低,等离子体射流流速明显增大,覆盖钝体头部的等离子体鞘套的厚度先减小,而后又增加,鞘套的空间均匀性以及等离子体向钝体表面的总传热量均显著降低,而钝体头部的局部电子数密度则增大.数值模拟结果为在MPX-2015上开展超音速条件下的实验研究提供了理论指导.

In the re-entry process of hypersonic vehicle in near space, the violent interaction between the vehicle and the surrounding air will ionize the air and leaves a complex environment in the vicinity of the vehicle surface. Both the flow field and the communication between the vehicle and the controlling center on the earth are significantly affected by the generated plasma layers. This will result in serious system operation problems such as the communication blackout or radio blackout. Numerical modelling is one of the most widely used methods to investigate such complicated physical-chemical processes involving coupled magneto-hydrodynamics, heat transfer, dissociation, ionization, excitation and their reverse processes. Due to the strong collision, non-uniform and non-equilibrium characteristics of the plasma layers formed in the vicinity of the vehicle surface, a self-consistent physical-mathematical model, as well as a database for the transport properties of non-equilibrium plasmas, describing the non-equilibrium features of plasmas is one of the pre-requisites for numerical simulations. This paper focuses on the non-equilibrium plasmas produced near the bluff body surface in the re-entry process of hypersonic vehicles in near space, and a new non-equilibrium plasma model which has been developed previously by our group is employed for conducting two-dimensional（2 D） simulations on the characteristics of the non-equilibrium argon plasma jets based on the multiphase gas discharge plasma experimental platform-2015（MPX-2015） established in our laboratory. The modelling is conducted under two different flow conditions,i.e., the sub-sonic flow condition and the super-sonic flow condition. Under the sub-sonic flow condition, the 2 D nonequilibrium modeling results are consistent well with the experimental measurements which validates the reliability of the non-equilibrium physical-mathematical model, as well as the developed computer codes in this study. The modeling results under the super-sonic flow conditions show that the spatial uniformity of the plasma layer surrounding the bluff body, as well as the total heat flux to the bluff body surface from plasmas, decreases significantly with the increase of the plasma jet velocity; while the local electron number density increases in the vicinity of the head of the bluff body,the thickness of the plasma layer surrounding the bluff body first decreases, and then increases. These modelling results provide a theoretical guidance for conducting experimental studies under a super-sonic flow condition on MPX-2015.In the future research, we will extend the physical-mathematical model to investigate of the transient, non-equilibrium features of the air discharge plasmas, and the complicated interactions between the plasma jet and the surrounding air,and/or the downstream bluff body under different operating conditions. Simultaneously, we will also try to develop the in-situ experimental methods to obtain the spatiotemporal distributions of the temperature, velocity and species concentrations in the plasma layer, and conduct a comparison between modelling results and measured data.