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

Abstract

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 vib-rotational 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 <i>T</i>_v and rotational temperature <i>T</i>_r from the equilibrium temperature <i>T</i>_eq 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.