Two-Photon Laser-Induced Fluorescence Measurements of Atomic Oxygen Density in Hypersonic Plasma Flow

Abstract

An experimental investigation of high-enthalpy air flow generated by L2K arc-jet facility of DLR is carried out by means of two-photon absorption laser-induced fluorescence (TALIF) measurements of atomic oxygen at O×3p×3P1;2;0 transition. The experiment has been conducted with an arc current of 600 A, a mass flow rate of 36 g∕s, and a total pressure of 105 Pa, resulting in both mass-specific enthalpy of ∼8.4 MJ∕kg and a reservoir temperature of 4200 K. Atomic oxygen has been characterized in terms of translational temperature and absolute number density either in the freestream or behind a shock wave upstream a blunt body. A calibration method based on TALIF measurements of xenon at 6p'[3∕2]2 transition has been used. The results highlight that in the freestream the fluorescence signal exhibits a peak activity lower with respect to that inspected in the shock layer. The measured lifetimes 25.8 ns and 11.4 ns indicate that, in the shock layer, the depletion of the atomic Oxygen excited state occurs mainly via quenching. The analysis of the line broadening of the spectral profile of atomic oxygen reveals an increment of the translational temperature in the shock layer with respect to that measured in the freestream, resulting in 2500 K and 1400 K, respectively. It is found that in the freestream the absolute number density attains ∼5.55E23 m-3 and the presence of the shock wave induces an increment of the number density of ∼26.9E23 m−3, that is, 4.85 times higher than that detected in the freestream.