Femtosecond laser electronic excitation tagging (FLEET) velocimetry is characterized for the first time at high-pressure, low-temperature conditions. FLEET signal intensity and signal lifetime data are examined for their thermodynamic dependences; temperatures range from 89 K to 275 K while pressures are varied from 85 kPa to 400 kPa. The FLEET signal intensity is found to scale linearly with the flow density. An inverse density dependence is observed in the FLEET signal lifetime data, with little independent sensitivity to the other thermodynamic conditions apparent. FLEET velocimetry is demonstrated in the NASA Langley 0.3 m Transonic Cryogenic Tunnel. Velocity measurements are made over the entire operational envelope: Mach numbers from 0.2 to 0.75, total (stagnation) temperatures from 100 K to 280 K, and total pressures from 100 kPa to 400 kPa. The velocity measurement accuracy is assessed over this domain of conditions. Measurement errors below 1.15 % are typical, with slightly decreasing accuracy as temperatures are decreased. Assessment of the measurement precision finds a zero-velocity precision of 0.4 m s−1. The precision is observed to have a weak temperature dependence as well, likely a result of the shorter lifetimes experienced at higher densities. The velocity dynamic range is found to have a nominal value of 650. Finally the spatial resolution of the measurements is found to be dominated by the physical size of the FLEET signal and advective motion. The transverse spatial resolution is found to be 1 mm, the spanwise spatial resolution to be 2–3 mm, while the streamwise spatial resolution is dependent on velocity with a minimum of 2 mm and a maximum of 3.3 mm.

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An experimental study is performed to investigate the feasibility of conducting simultaneous mixture fraction, soot volume fraction and velocity imaging in sooting jet flames. The measurements are performed in the soot-inception region of ethylene jet flames, where the yellow luminous region first appears in the flame. Three-component velocity and soot volume fraction are measured by stereoscopic particle image velocimetry and laser-induced incandescence, respectively. The mixture fraction is inferred from laser-induced fluorescence of krypton gas seeded into the fuel stream. To obtain mixture fraction from the fluorescence signal, the signal must be corrected for density and fluorescence quenching effects. This correction is accomplished by invoking an assumed state relationship that is derived from a laminar strained-flame calculation. Once properly calibrated, the krypton planar laserinduced fluorescence data give the mixture fraction, temperature and major species near the regions of soot formation. The krypton is seeded into the fuel jet at a mole fraction of approximately 4%. The fluorescence of krypton is achieved by two-photon absorption at 214.7 nm and the resulting fluorescence is collected at 760.2 nm. The krypton fluorescence signal is rather weak, particularly near the reaction zones where density is lowest, and so adequate signal-to-noise ratios could be achieved in a sheet only about 1 mm in height, which effectively limited this study to a line measurement of mixture fraction. The temperature field derived from the mixture fraction field was compared to temperatures obtained from thermocouple measurements. The mean radial temperature profiles using the different techniques show excellent agreement and this serves to validate the methodology used to map from fluorescence signal to mixture fraction and temperature. The resulting data are of high enough quality as to allow the investigation of the kinematics, thermo-chemical state and even the dissipation fields near regions of soot formation.

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Hydroxyl tagging velocimetry (HTV) is characterized for the first time at extended pressures (1 and 3 atm) and temperatures (295 K to 673 K) in an attempt to improve measurement precision for low speed flows. While previous investigations have focused on ambient and flame temperatures (1400 K), the present study investigates between these two extreme conditions, wherein both the local chemistry and thermodynamic state of the gas may hinder or aid the functionality of the technique. Effects of temperature and pressure on the hydroxyl (OH) excitation spectrum are assessed and compared to simulations to determine the optimal laser frequency, and OH species lifetime and tracer line diffusion are examined to determine the relative efficiency of the photo-dissociation process and the quality of the resulting signal. A two-fold increase in the photo-dissociation efficiency is observed at elevated temperatures. Tag line spread was found to be dominated by shear rather than molecular diffusion. Velocity measurement precision was characterized for time delays ranging from 5 μs to 3.2 ms and was found to be inversely proportional to the time delay selected, supporting the need for the extended tracer lifetimes observed at higher temperatures when used for low-velocity applications. Velocity profiles measured in heated jets of nitrogen and air indicate measurement uncertainties as low as 0.1 m (at confidence level), while comparison with particle image velocimetry (PIV) measurements showed peak deviations in the observed velocity profiles to be less than . The results suggest the high utility of HTV at making measurements in low-velocity flows at moderate temperatures.