Characterization of Complex Porous Structures for Reusable Thermal Protection Systems: Effective-Permeability Measurements

Abstract

Reusable thermal protection systems are one of the key technologies that have to be improved to enable long-duration hypersonic flights. Transpiration cooling has been demonstrated to be one of the most promising active cooling techniques in terms of coolant mass requirements and disturbance of the external flow. Previous numerical studies, conducted by the authors on the conjugate boundary-layer/material-response analysis, along with the current manufacturing capability of manipulating the natural properties of porous materials (e.g., porosity, permeability, and thermal conductivity) have demonstrated the cooling potential when variable transpiration is considered. In this work, a methodology for the nonintrusive characterization of the local effective permeability of a complex carbon–carbon porous structure is proposed. The concept of effective permeability, conceived as the local blowing capability of a porous structure with respect to a selected coolant fluid, is also discussed. Specifically, the coolant (air) mass flux blown from a conical porous surface has been measured by a hot-film probe at a distance specified by an appropriate reference elementary area and the Reynolds number based on the channels’ diameter. These measurements have then been related to the pressure gradient across the local material’s thickness by using Darcy’s law. Measurements have revealed a higher effective permeability near the nose of the cone where two longitudinal delaminations have been identified. The asymmetric blowing capability of the cone highlights the importance of characterizing the entire thermal protection system instead of defining the overall properties of the material, which can be drastically different at the full-scale level due to the geometry, the system integration (i.e. structural constraints), and the intrinsic defectology coming from the manufacturing process. Additionally, the mass fluxes measured on the external porous surface will support the numerical aerothermal rebuilding of the wind-tunnel experiment on the transpiration cooling.

Characterization of Complex Porous Structures for Reusable Thermal Protection Systems: Porosity Measurements

“…The correct probing distance is fundamental to obtain meaningful velocity measurements in terms of minimum fluctuations with respect to the mean velocity field ( Fig. 11 in [6]). In fact, in the near-wall region, single jets are discernable with a pattern that mimics the perforated arrangement of the porous surface, whereas, at the merging distance (L m ), the jets coalesce together [7].…”

“…The influence of the probe distance from the wall has been analyzed by applying the theory of fluid flow through perforated plates/screens that allowed identifying the range of optimal distances for the mass flux measurements [6][7][8]. The correct probing distance is fundamental to obtain meaningful velocity measurements in terms of minimum fluctuations with respect to the mean velocity field ( Fig.…”

“…The 2-D segmentation of several planes, longitudinal and perpendicular to the centerline of the sample (Figs. 2 and 3, respectively), has been used for defining two fundamental parameters needed to correctly perform the permeability tests by hot-film anemometry [6].…”

“…10 is referred (CS9), corresponds to the region above the internal plenum that has variable thickness (Figs. 2a and 8b in [6]). …”

“…The output data from the CT scan allows defining the most important guidelines needed to correctly perform the local effectivepermeability measurements [6]. In particular, the two-dimensional (2-D) sectioning and the three-dimensional (3-D) digital reconstruction of the specimen have been used to calculate, first, the internal porosity of the prototype specimen and, then, to define the minimum dimension of the characteristic area, or control surface (CS), to be probed for the mass flux measurement (Sec.…”