非平衡等离子体流场相干反斯托克斯拉曼散射光谱计算及振转温度测量

Coherent anti-Stokes Raman scattering spectral calculation and vibrational-rotational temperature measurement of nonequilibrium plasma flow field

高温非平衡问题是高超声速流动中最基本的科学问题之一,而热力学非平衡特性的准确表征是理解高温非平衡问题和高超声速空气动力学的基础,如何准确可靠地表征流场的热力学非平衡特性是解决高超声速飞行器在稀薄流域高温非平衡问题的关键.本文基于相干反斯托克斯拉曼散射基本原理,开发了面向非平衡流场的振转温度反演算法,并在宽温度范围静态环境开展验证.搭建了非平衡等离子体流场相干反斯托克斯拉曼散射测温实验平台并开展实验验证,结果表明微波等离子体处于热力学非平衡状态,并且振动温度和转动温度与微波功率成正比,而热力学非平衡度与微波功率成反比,当微波功率从80W增加至180 W时,等离子体电子数密度增加,中性粒子通过与电子碰撞获得能量使振动温度从(2201±43)K增加至(2452±56)K、转动温度从(382±20)K增加至(535±49)K;而处于振动激发态的分子通过振动-平动弛豫过程(对于N2分子弛豫速率与温度成正比)将部分振动能转化为平动能,导致振动温度与转动温度的差异降低,等离子体热力学非平衡度从0.83降低至0.78.

How to characterize thermodynamic non-equilibrium characteristics of flow field accurately and reliably is the key to solving the thermal and chemical non-equilibrium problem,which is one of the most basic scientific problems in hypersonic aerodynamcis.Based on the principles of coherent anti-Stokes Raman scattering(CARS)and modified exponential gap(MEG)Raman linewidth model,a CARS spectral computation and vibrotational temperature inversion program is proposed for characterizing the thermodynamic non-equilibrium properties of high-temperature gas flow field.The influence of vibrational temperature and rotational temperature on Raman linewidth and CARS spectral characteristics are studied theoretically.A CARS system is built and the corresponding accuracy in a wide temperature range is verified in a static environment that is established by using a high-temperature tube furnace and a McKenna burner.The results show that the average relative deviation of the vibration temperature Tv and rotational temperature Tr from the equilibrium temperature Teq are 4.28%and 3.34%respectively in a range of 1000 to 2300 K,and the corresponding average repeatability is 1.95%and 3.03%respectively.These results indicate that the vibrational temperature and rotational temperature obtained by the non-equilibrium program are in good agreement with those obtained from the thermal equilibrium program.Finally,a non-equilibrium microwave plasma flow is built and its vibrational temperature and rotational temperature are obtained by using the developed program.The results show that the microwave plasma is in thermodynamic non-equilibrium,and the vibrational temperature and rotational temperature are proportional to microwave power,while the thermodynamic non-equilibrium degree exhibits an opposite trend.With microwave power increasing from 80 to 180 W,the vibrational temperature of plasma increases from(2201±43)K to(2452±56)K,the rotational temperature increases from(382±20)K to(535±49)K,for which the principal reasons are that the increase in microwave power leads to an increase in electron number density,and neutral particles obtain energy through collision with electrons,resulting in the increase of vibrational temperature,rotational temperature,and translational temperature.The thermodynamic non-equilibrium degree decreases from 0.83 to 0.78 with the microwave power increasing,which is due to the V-T relaxation rate increasing.The molecules in the excited vibrational states lose energy through collision with ground state molecules(i.e.V-T relaxation process),leading the vibrational energy to be converted into translational energy.For N2 molecules,the V-T relaxation rate is directly proportional to the temperature,which causes the difference between vibrational temperature and rotational temperature to decrease with microwave power increasing,and non-equilibrium degree to decrease with microwave power increasing as well.