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

Abstract

Raman excitation plus laser-induced electronic fluorescence (RELIEF) images the motion of oxygen molecules in air and other gas mixtures. This is accomplished by tagging oxygen molecules through vibrational excitation and imaging them after a short period of time by laser-induced electronic fluorescence. The vibrational lifetime of oxygen is sufficiently long and the signal sufficiently strong to allow this technique to be used over a wide range of flow conditions, from low subsonic to hypersonic, and in a variety of gas mixtures including high humidity environments. The utilization of a molecular tagging technique such as this is critical for environments in which seeding is impossible or unreliable and for measurements in which a wide range of scales needs to be observed simultaneously. Two experiments which have been conducted at national laboratories in medium- to large-scale facilities are reported. At the Arnold Engineering Development Center, RELIEF was used to examine velocity in a 1 m diameter tunnel for applications in the area 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 association with studies of fuel-air mixing in hypersonic engines. These applications are two examples of the wide range of practical uses for this new technology.

Microscale flow visualization

“…Typical radiative emission times are on the order of 0.1-1 ms (Lempert et al 2002). Reviews of gas phase flow-tagging velocimetry were provided by Miles et al (2000), and Pitz et al (2000). More recently, Lempert et al (2002) performed MTV measurements using acetone (instead of biacetyl) in sonic and supersonic microjets produced by a 1 mm diameter nozzle.…”

High-speed imaging in fluids

“…Laser-induced fluorescence (LIF) is frequently used in fluids research, such as the in flow visualization of turbulent mixing [117][118][119] and combustion processes [120][121][122], in (micro) particle image velocimetry (µPIV) [123,74], and in molecular tagging [124][125][126][127]. Molecular fluorescence imaging uses pulsed dye laser tuned for resonant spectroscopic excitation, while for the measurement of velocity fields pulsed Nd:YAG lasers provide the nonspecific fluorescence excitation of micron-sized or nanometer-sized tracer particles.…”

Methods for Prediction of High-Speed Reacting Flows in Aerospace Propulsion

“…The center jet flow was either a mixture of 95% helium and 5% oxygen (by volume) or pure argon. A small amount of oxygen was added to the helium to allow the streamwise component of velocity to be measured using the RELIEF technique [132]. Details concerning the geometry of the rig and the methodology used for its design and instrumentation can be found in [130].…”