QUANTITATIVE MEASUREMENTS OF ABLATION-PRODUCTS TRANSPORT IN A SUPERSONIC TURBULENT BOUNDARY LAYER USING SIMULTANEOUS PIV AND NAPHTHALENE PLIF

Abstract

The dispersion of a passive scalar in a Mach 5 turbulent boundary layer is investigated using a lowtemperature sublimating ablator (naphthalene). Twodimensional fields of naphthalene mole fraction and velocity are obtained by using simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF). The images show large-scale naphthalene vapor structures that coincide with regions of relatively low streamwise velocity. Additionally, the covariance of naphthalene mole fraction with velocity indicates that an ejection mechanism is transporting low-momentum, highscalar-concentration fluid away from the wall, resulting in the protrusions of naphthalene vapor evident in the instantaneous PLIF images. Mean profiles of streamwise velocity and naphthalene mole fraction are also presented, with the mean mole fraction profile having the expected shape of a classical scalar boundary layer profile. INTRODUCTION There remains continued interest in the study of ablation owing to the need to develop suitable thermal protection systems (TPS) for spacecraft that undergo planetary entry. Charring ablation experienced during atmospheric entry is a complex process involving heat and mass transfer, and codes that predict it require a number of coupled submodels, each of which requires validation (Smits et al., 2009). For example, Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulation (LES) codes require models of the turbulent transport of ablation products; however, suitable scalar-velocity data under relevant conditions are very rare (Ho et al., 2007). Additioanlly, Stogner et al. (2011) conducted an uncertainty analysis of ablation calculations and concluded that turbulence models are one of the largest contributors to the uncertainty of ablation predictions for re-entry flows. A technique has been under development at The University of Texas at Austin that uses planar laser-induced fluorescence (PLIF) of a low-temperature sublimating ablator (naphthalene) to enable visualization of the ablation products in a hypersonic flow (Lochman, 2010; Buxton et al., 2012; Combs et al., 2014a; Combs et al., 2014b; Combs and Clemens, 2015). Lochman (2010) first used naphthalene PLIF to study the transport of ablation products in a Mach 5 turbulent boundary layer. In his work, the technique provided images of naphthalene vapor in the turbulent boundary layer with excellent signal-to-noise ratio (SNR), visualizing both large and small-scale turbulent structures. However, while some spectroscopic measurements were made, a temperature correction was only applied to the profile of the naphthalene boundary layer to yield a “corrected” profile and the images presented are qualitative visualizations. Additionally, no velocity data was collected. In the work by Buxton et al. (2012), naphthalene PLIF was performed simultaneously with particle image velocimetry (PIV) in a Mach 5 turbulent boundary layer using the same insert design as Lochman, providing scalar-velocity data of ablation-products transport in the boundary layer. Mean and instantaneous boundary layer profiles of velocity, RMS velocity, and fluorescence signal were determined. The results indicated that there was a strong correlation of high fluorescence signal with negative streamwise and positive spanwise velocity fluctuations away from the wall. However, naphthalene concentration was not determined from the PLIF images since the PLIF signals were not corrected for temperature effects and the PLIF images had a low SNR. Combs et al. (2014a, 2014b) used naphthalene PLIF to image ablation-products transport from the heat shield of an Orion Multi-Purpose Crew Vehicle (MPCV) model in a Mach 5 flow. Still, these images were purely qualitative flow visualizations with no attempt made to correct the PLIF signal for temperature and pressure effects and no velocity data was collected. Most recently, Combs and June 30 July 3, 2015 Melbourne, Australia 9 5C-2