高层临近空间以接近第一宇宙速度长时间巡航的新型飞行器对我国的战略安全及空间和平利用具有重要的意义。气动特性是进行相关飞行器构型设计以及控制设计的重要参数。高焓空气等离子体流动对地面研究相关新型巡航飞行器气动特性具有重要的意义。因此，开展高焓空气等离子体特性研究具有重要的意义和应用前景。本文以力学所长时间运行稀薄气体风洞建设为背景，对高焓空气等离子体的产生、表征及应用等关键问题进行研究，主要内容如下：

1.   采用二维平衡态流体模型，首先系统且全面地模拟了单高频感应耦合空气等离子体发生器内部流动与传热性能，揭示了放电功率、电源驱动频率、发生器结构参数（包括发生器出口半径、线圈的匝数与放电管间的距离、线圈螺距、工作压力等）对等离子体特性的影响规律：高功率有利于生成更高焓值且稳定均匀空气感应耦合等离子体；发生器半径尺寸过大或过小均不利于产生均匀稳定高温等离子体流动，需要选择最优点；更高的电源驱动频率有助于形成稳定且放电均匀等离子体；调节线圈匝数可以改变等离子体发生器放电线圈电流；线圈螺距增大，发生器内高温区域面积增大，但最高温度降低；工作压力为4 atm时发生器内最高温度相比于工作压力为1 atm时增大，但高温区域面积减小。由于感应耦合等离子体放电受到趋肤效应影响，以上所有计算得到的发生器内温度场分布中，高温区域主要集中在放电管内壁附近，发生器中心区域温度要相对较低。其次计算了直流-射频等离子体发生器的流动与传热特性，考虑了发生器进气口为高温气体（T=10 000 K）时感应耦合发生器内流场与温度场分布，发现此时整个发生器内温度分布均匀，中心区域同样可以获得较高的等离子体温度分布。基于数值模拟结果，提出了等离子体发生器的设计参数选择范围。

2.   根据数值模拟结果，开展了直流-射频耦合放电等离子体发生器的研制和等离子体特性的实验测量。探讨了空气射频放电条件下电源输入功率与气体流量间的耦合特性，计算了发生器热损失，利用发射光谱法诊断距离发生器出口10 mm处等离子体粒子种类。

3.   针对高焓等离子射流开展应用研究。首先针对稀薄超高速来流密度的测量难题，基于流场中悬丝小球的受力分析，实现了对来流稀薄高焓气体密度的估算。其次研究石墨材料的保型烧蚀特性，比较了氮气和氧-氮混合气体放电对材料烧蚀特性的影响。

以上研究丰富了对高焓空气等离子体相关关键问题的认识。

The cruise vehicle of long cruising hours which approaching the first cosmic velocity in upper near space is of great significance to our strategic security and peaceful use of space. The aerodynamic characteristics are important parameters for the relevant vehicle configuration design and control design. High enthalpy air plasma flow plays a crucial role in the ground simulation of the aerodynamic characteristics of new cruise vehicles. Therefore, the study of high enthalpy air plasma characteristics is of great significance and application. In this paper, the key issues of generation, characterization and application of high enthalpy air plasma are studied in the background of the construction of a long-running rarefied gas wind tunnel at the Institute of Mechanics. The main contents are as follows:

1.   A two-dimensional equilibrium fluid model is firstly used to systematically and comprehensively simulate the internal flow and heat transfer performance of a single high frequency inductively coupled air plasma generator for different operating and geometric parameters, revealing the influence of discharge power, power drive frequency, generator structure parameters (including generator outlet radius, number of turns of coils and distance between discharge tubes, coil pitch, operating pressure, etc.) on the plasma characteristics: high power is conducive to the generation of higher enthalpy and stable uniform air inductively coupled plasma; too large or too small generator radius size is not conducive to the generation of uniform and stable high temperature plasma flow, and the optimal point needs to be selected; high frequency helps to form a stable and uniform discharge plasma; adjusting the number of turns of the coil can change the plasma generator discharge coil current; as the coil pitch increases, the area of the high temperature region in the generator increases, but the maximum temperature decreases; the maximum temperature in the generator at an operating pressure of 4 atm increases compared to the operating pressure of 1 atm, but the area of the high temperature region decreases. Since the inductively coupled plasma discharge is affected by the skin effect, the temperature field distribution inside the generator obtained from all the above calculations is mainly concentrated in the high temperature region near the inner wall of the discharge tube, and the temperature in the central region of the generator is relatively low. Next, the flow and heat transfer characteristics of the DC-RF plasma generator when the generator inlet is a high-temperature gas (T=10 000 K) is calculated, and it is found that the temperature distribution of inductively coupled plasma generator is uniform throughout the generator, and a high plasma temperature distribution can be obtained in the central region as well. Based on the numerical simulation results, a range of design parameters for the plasma generator is proposed.

2.   Based on the results of numerical simulations, the development of a DC-RF coupled discharge plasma generator and experimental measurements of plasma characteristics are carried out. The coupling characteristics between power input and gas flow rate under air inductively coupled plasma discharge conditions are investigated, the power dissipation of the generator is explored by calculating the heat loss, and the emission spectroscopy method has been used to diagnose the plasma particle species at a distance of 10 mm from the generator outlet.

3.   Application studies are carried out of high enthalpy plasma jets. Firstly, for the difficult measuring problem of the density of rarefied ultrahigh-speed incoming flow, the density estimation was achieved based on the force analysis of the suspended sphere under the ultrahigh-speed rarefied incoming flow. Next,the conformal ablation characteristics of graphite material in high enthalpy plasma plume are studied, and the effect of nitrogen and oxygen-nitrogen mixed gas discharges on the ablation characteristics of the materials were compared.

The above studies have enriched the understanding of key issues related to high enthalpy air plasma.