Quantitative measurements of ablation-products transport in supersonic turbulent flows using planar laser-induced fluorescence
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A recently-developed experimental technique based on the sublimation of naphthalene, which enables imaging of the dispersion of a passive scalar using planar laser-induced fluorescence (PLIF), is applied to a Mach 5 turbulent boundary layer and a NASA Orion capsule flowfield. To enable the quantification of naphthalene PLIF images, quantitative fluorescence and quenching measurements were made in a temperature- and pressure-regulated test cell. The test cell measurements were of the naphthalene fluorescence lifetime and integrated fluorescence signal over the temperature range of 100 K to 525 K and pressure range of 1 kPa to 40 kPa in air. These data enabled the calculation of naphthalene fluorescence yield and absorption cross section over the range of temperatures and pressures tested, which were then fit to simple functional forms for use in the calibration of the PLIF images. Quantitative naphthalene PLIF images in the Mach 5 boundary layer revealed large-scale naphthalene vapor structures that were regularly ejected out to wall distances of approximately y/δ = 0.6 for a field of view that spanned 3δ to 5δ downstream of the trailing edge of the naphthalene insert. The magnitude of the calculated naphthalene mole fraction in these structures at y/δ = 0.2 ranged from approximately 1-6% of the saturation mole fraction at the wind tunnel recovery temperature and static pressure. An uncertainty analysis showed that the uncertainty in the inferred naphthalene mole fraction measurements was ± 20%. Mean mole fraction profiles collected at different streamwise locations were normalized by the mole fraction measured at the wall and a characteristic height of the scalar boundary layer, causing the profiles to collapse into one “universal” mole fraction profile. Two-dimensional fields of naphthalene mole fraction were also obtained simultaneously with velocity by using particle image velocimetry (PIV) and PLIF. The images show large-scale naphthalene vapor structures that coincide with regions of relatively low streamwise velocity. The covariance of naphthalene mole fraction with velocity indicates that an ejection mechanism is transporting low-momentum, high-scalar-concentration fluid away from the wall, resulting in the protrusions of naphthalene vapor evident in the instantaneous PLIF images. Lastly, naphthalene PLIF was used to visualize the dispersion of gas-phase ablation products on a scaled Orion capsule model at four different angles of attack at Mach 5. High concentrations of scalar were imaged in the capsule recirculation region. Additionally, intermittent turbulent structures were visualized on the heat shield surface, particularly for the 12° and 52° AoA cases.