The RELIEF flow tagging technique and its application in engine testing facilities and for helium-air mixing studies

Raman excitation plus laser-induced electronic fluorescence (RELIEF) images the motion of oxygen molecules in air and other gas mixtures. This is acco mplished by tagging oxygen molecules through vibrational excitation and ima ging them after a short period of time by laser-induced electronic fluoresc ence. The vibrational lifetime of oxygen is sufficiently long and the signa l sufficiently strong to allow this technique to be used over a wide range of how conditions, from low subsonic to hypersonic, and in a variety of gas mixtures including high humidity environments. The utilization of a molecu lar tagging technique such as this is critical for environments in which se eding is impossible or unreliable and for measurements in which a wide rang e of scales needs to be observed simultaneously. Two experiments which have been conducted at national laboratories in medium- to large-scale faciliti es are reported. At the Arnold Engineering Development Center, RELIEF was u sed to examine velocity in a 1 m diameter tunnel for applications in the ar ea of engine testing. At the NASA Langley Research Center, RELIEF is being used to examine supersonic mixing of helium in air in a coaxial jet in asso ciation with studies of fuel-air mixing in hypersonic engines. These applic ations are two examples of the wide range of practical uses for this new te chnology.