Measurements of Ablation-Products Transport in a Mach 5 Turbulent Boundary Layer using Naphthalene PLIF

Abstract

Ablation is a multi-physics process involving heat and mass transfer and codes aiming to predict ablation are in need of experimental data pertaining to the turbulent transport of ablation products for validation. Here, a technique developed at The University of Texas at Austin that uses planar laser-induced fluorescence (PLIF) of a low-temperature sublimating ablator (naphthalene) to enable quantitative visualization of the ablation products in a hypersonic flow is employed. Although high-temperature ablation is difficult and expensive to recreate in a laboratory environment, low-temperature sublimation creates a limited physics problem that can be used to explore ablation-products transport in a hypersonic flow-field. In the current work, the transport of ablation products in a Mach 5 turbulent boundary layer is investigated using the PLIF technique. Naphthalene vapor is introduced into the flow by ablation of a solid naphthalene plug located upstream of the imaging field of view and mounted flush with the wind tunnel floor. Additionally, necessary spectroscopic measurements were made to enable the conversion of naphthalene fluorescence signal into naphthalene mole fraction. The spectroscopic measurements included making flowing test cell measurements of the naphthalene fluorescence lifetime and integrated fluorescence signal over a range of 100-525 K and 1-100 kPa in air. The fluorescence lifetime and signal were both observed to increase monotonically up to approximately 400 K. Additionally, the effect of oxygen quenching was evident in the fluorescence-lifetime data as the measured lifetimes exhibited the expected Stern-Volmer behavior, decreasing with increasing air pressure. These results were then used to develop empirical relationships for naphthalene fluorescence yield and absorption cross section. Using these spectroscopic data and a first order approximation for the mean temperature profile across the boundary layer, the naphthalene PLIF images were converted into twodimensional fields of naphthalene mole fraction with an uncertainty of ± 20%. The images revealed large-scale naphthalene vapor structures in the turbulent boundary layer out to wall distances of approximately y/δ = 0.6. Naphthalene mole fraction in these structures was calculated to be approximately 1% of the saturation mole fraction at the temperature and pressure conditions present in the boundary layer. The mean profile of mole fraction in the boundary layer was similar to a classic scalar profile with high concentration at the wall decaying to a constant near-zero value in the freestream. Additionally, converting the PLIF images to naphthalene mole fraction showed that a high signal region near the wall (y/δ < 0.05)—previously discounted as either laser scatter or fluorescence from deposition of solid-phase naphthalene on the wall—could potentially be signal from a high-concentration naphthalene-vapor layer, since the calculated mole fraction is on the order of 5% of the saturation mole fraction.