Single Pulse Broadband Nitrogen CARS Measurements on a High Enthalpy, Hypersonic Flow Field at Temperatures up to 5800 K

Abstract

Single pulse broadband Coherent Anti-Stokes Raman Scattering (CARS) was used to measure the thermal non-equilibrium flow properties in the high enthalpy hypersonic flow field (11.8 MJ/kg, 5800 K) of the arc heated facility L3K in Cologne. Nitrogen CARS was applied in the free stream flow at 64 mbar and in the vicinity front of a probe body at app. 1 bar. At both conditions a distinct thermodynamic non-equilibrium was found, but vice versa for both positions. In the free stream the vibrational temperature was about 920 K higher than the rotational temperature, which should be in equilibrium with the translational movement within a few collisions. Downstream the shock front (e.g. between shock and body) a rotational temperature exceeded the vibrational temperature by about 1100 K. The CARS measurements on nitrogen were chosen because of the high thermal stability of molecular nitrogen, resulting in a mass fraction of still 76 %. Additionally, the spectral simulation of nitrogen CARS spectra is well developed. The aim of this study was to test the feasibility to perform time resolved CARS temperature measurements on nitrogen which is the main molecular component of the flow. Details of the experimental set up and results are presented.