Abstract:When the hypersonic vehicle re-enters the atmosphere, it is subjected to the compression of the shock wave and the viscous blocking effect in the shock layer. The air temperature in the surrounding flow field is between 4 000~15 000 K, which causes oxygen and nitrogen molecules in the air to dissociate, resulting in high-temperature gas effect and the formation of high enthalpy chemical nonequilibrium flow. There are a large number of carbon elements in thermal protection materials on the surface of aircraft. Generally, the reaction between oxygen and carbon is the main reaction at high enthalpy. But when the enthalpy is greater than 18 mJ·kg-1,the dimensionless mass ablation factor for the reaction of nitrogen atoms with carbon on the aircraft surface is BCN>0.172 5,and at the time, the dimensionless mass ablation factor of carbon in high enthalpy air is BCair>0.345; as a result, the nitriding ablation of carbon becomes very intense, which is equivalent to the oxidation ablation. Meanwhile, the dissociated nitrogen atoms also produce a large amount of heat in the catalytic reaction on the surface of the aircraft, which makes the aircraft surface withstand more thermodynamic impact. Therefore, the analysis of nitrogen atoms in high enthalpy chemical nonequilibrium flow field is of great practical significance. A high enthalpy chemical nonequilibrium flow field is established in ground simulation equipment, and nitrogen atoms can be well studied by measurement. Two-photon absorption laser-induced fluorescence (TALIF) technology, as a non-contact measurement, can directly obtain the concentration distribution without disturbing the flow field. Nitrogen atoms in the flow field are excited by a pulse laser, and two-dimensional nitrogen atom fluorescence signals are obtained through ICCD arranged outside a wind tunnel test section in a direction perpendicular to a plane formed by the flow field and the laser. In order to ensure the fluorescence image is clear and the field of view is appropriate, the Nikon f=50 mm F/1.4 lens is selected as the front stage light receiving device. Experimental imaging is the cumulative result of 50 exposures to eliminate the uncertainty caused by turbulence and laser energy jitter. By testing around the theoretical excitation wavelength, 206.717 nm is optimized as the best excitation wavelength in the formal experiment. At the condition of the optimal laser wavelength, the laser energy is adjusted from small to large, and the unsaturated linear region for the nitrogen atom in this environment is less than 1.8 mJ. In the formal experiment, the laser energy is 1.6 mJ, which is in the linear region. Based on the analysis of the fluorescence intensity extracted along the laser centerline obtained from the fluorescence image, it was found that both the subsonic flow and the supersonic flow presented a hump-shaped distribution along the radial direction. Compared with the previous work of oxygen atoms, it was found that the nitrogen molecules in the flow field had not been completely dissociated, which was consistent with the flow field characteristics of the experimental wind tunnel.
Key words:High-enthalpy chemical non-equilibrium flow;Nitrogen atom;Laser-induced fluorescence;Optimization
罗 杰,马昊军,王国林,肖学仁. 激光诱导荧光技术在高焓空气氮原子测量中的应用[J]. 光谱学与光谱分析, 2021, 41(07): 2135-2141.
LUO Jie,MA Hao-jun,WANG Guo-lin,XIAO Xue-ren. Preliminary Study on Atom N in High-Enthalpy Flow Field. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2021, 41(07): 2135-2141.
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