Laser-Induced Fluorescence Apparatur for Application in Rarefied Hypersonic Flow Fields Part II: Free Jet Experiments with Nitric Oxide (NO)

Kurzfassung

Laser-Induced Fluorescence (LIF) spectroscopy is applied to supersonic free jet expansions to determine rotational and vibrational temperatures in rarefied flow fields such as those obtained during the reentry of HERMES. For an effective LIF process the NO molecule was chosen. Three possible electronic transitions (A-X, B-X, and D-X) were calculated by a computer code which was developed to simulate excitation spectra of NO up to 2000 K, including non-equilibrium of rotational and vibrational energy. A tunable ArF*-excimer laser was used to stimulate the LIF-process in free jet expansions in a vacuum wind tunnel (V2G). Test gases were NO + N2 and NO + He at stagnation temperatures from 1000 K to 1200 K. From the measured excitation spectra the rotational temperatures of the NO molecules can be obtained. A surface model made from copper was installed in the supersonic flow field. Its wall temperature could be varied from 600 K to 1000 K. Temperature measurements near the wall were obtained under near free molecular conditions, which are relevant for the heat transfer and gas-surface interaction of reentry vehicles.