Development of Naphthalene PLIF for Visualizing Ablation Products from a Space Capsule Heat Shield

Abstract

The Orion Multi-Purpose Crew Vehicle (MPCV) will use an ablative heat shield. To better design this heat shield and others that will undergo planetary entry, an improved understanding of the ablation process would be beneficial. 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 visualization of the ablation products in a hypersonic flow is applied. 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-product transport in a hypersonic flow-field. In the current work, a subscale capsule reentry vehicle model with a solid naphthalene heat shield has been tested in a Mach 5 wind tunnel. The PLIF technique provides images of the spatial distribution of sublimated naphthalene in the heat-shield boundary layer, separated shear layer, and backshell recirculation region. Visualizations of the capsule shear layer using both naphthalene PLIF and Schlieren imaging compared favorably. PLIF images have shown high concentrations of naphthalene in the capsule separated flow region, intermittent turbulent structures on the heat shield surface, and interesting details of the capsule shear layer structure. It was shown that, in general, the capsule shear layer appears to be more unsteady at lower angels of attack. The PLIF images demonstrated that during a wind tunnel run, as the model heated up, the rate of naphthalene ablation increased, since the PLIF signal increased steadily over the course of a run. Additionally, the shear layer became increasingly unsteady over the course of a wind tunnel run, likely because of increased surface roughness but also possibly because of the increased blowing. Regions with a relatively low concentration of naphthalene were also identified in the capsule backshell recirculation region and are most likely the result of cross-flow-induced vortices on the capsule afterbody.

Investigation of Gas Seeding for Planar Laser-Induced Fluorescence in Hypersonic Boundary Layers

“…Whereas the standard rhoCentralFoam solver computes the transport of total energy, the modified version solves the sensible enthalpy equation [Eq. (5)] in order to easily include the chemical reaction and species transport terms from the reactingFoam solver [37]. The conservation of mass, momentum (neglecting body forces), and energy used by the rhoCentralBinaryFoam and ANSYS Fluent solvers are defined as…”

“…However, in many situations, a seed gas is required. Examples of when seeding is required for PLIF flow visualization include the study of fuel-air mixing for supersonic combustion ramjet engines [2] and the study of reaction control system jet interactions with hypersonic crossflows associated with planetary entry vehicles [3][4][5].…”

Temperature and number density measurement in non-uniform supersonic flowfields undergoing mixing using toluene PLIF thermometry

“…PLIF imaging based on a variety of tracers has been considered through the literature [1]. Historically, acetone and 3-pentanone [2, 3] have been widely used for scalar and temperature imaging, and more recently, naphthalene [4,5], krypton [6], 1-methylnaphthalene [7], and anisole [8] have been utilized in a variety of applications.Toluene has been identified as a versatile candidate for PLIF studies owing to its large fluorescence quantum yield and strong temperature sensitivity [9][10][11][12][13]. Toluene PLIF has found many applications for the study of mixing, especially in internal combustion engine studies [10,12,[14][15][16][17] where the strong oxygen quenching [18] has been used to obtain a direct measurement of air/fuel ratio [1,12,19].…”

“…PLIF imaging based on a variety of tracers has been considered through the literature [1]. Historically, acetone and 3-pentanone [2, 3] have been widely used for scalar and temperature imaging, and more recently, naphthalene [4,5], krypton [6], 1-methylnaphthalene [7], and anisole [8] have been utilized in a variety of applications.…”