Reusable thermal protection systems are one of the key technologies that have to be improved in order to afford long-duration hypersonic flights. Transpiration cooling has been demonstrated to be one of the most promising active cooling techniques in terms of temperature decreasing and coolant mass saving. The coupling of the boundary layer with the thermal response of selected porous materials plays a crucial role in enabling the practical use of the transpiration cooling technique for reusable thermal protection systems. In this work, a novel test-rig for the non-intrusive characterization in terms of local permeability of a customized porous Carbon-Carbon nose-tip is proposed. The new concept of effective permeability, conceived as the local blowing capability of a porous structure with respect to a selected coolant fluid, is also introduced. The coolant (air) mass-fluxes blown from the porous surface of the specimen, and measured by a hot-film probe, are related to the average pressure gradient across the local material thickness by using the Darcy's law on prescribed locations. A parallel work, based on the X-Ray computed tomography scan of the prototype specimen, has been carried out with the purpose of defining the most important guidelines for the effective-permeability tests. Specifically, the calculation of the average porosity is used to define the minimum area to be probed with the hot-wire. The analysis of the statistical distribution of the void structures inside the C/C cone, coupled to the use of the theory of fluid-flow through perforated plates, is performed to determine the correct distance of the hot-wire from the wall. The results show permeability variation among the surveyed locations ranging from 6% to 172%. The effective-permeability map obtained allows classifying the prototype C/C mask as a semi-pervious structure. In particular, the higher effective permeability is located near the stagnation point region where two delaminations are located. 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 and the intrinsic defectology due to the manufacturing process. Nomenclature= slope of the chart pressure gradient vs. Darcy's velocity, ( • / 2 ) = transversal distance from the wall, ( .) ℎ = channels' diameter, ( ) = diameter of the control points, ( .) = local specimen's thickness, ( .) 2 ̿ = permeability tensor, ( 2 ) = permeability for the r-direction, ( ) = length, ( ) = merging or coalescence distance, ( .) ̇ = measured flow rate, ( ) = network-mesh size, ( .) = external pressure, ( ) = internal-plenum pressure, ( ) = external-plenum pressure, ( ) 2 = coefficient of determination, (−) ℎ = Reynolds number based on the channel's diameter, (−) = time, ( ) = calibration temperature, ( ) = hot-film temperature, ( ) = measured temperature during permeability tes...

This work is focused on the nonintrusive characterization of the local and average porosity of a prototype carboncarbon nose, representative of a reusable thermal protection system based on transpiration cooling. A study based on the x-ray computed tomography scan of the specimen has been carried out with the purpose of defining the most important guidelines for the permeability tests, which are the minimum area to be probed with a hot-film anemometer and the correct distance of the mass flux sensor from the wall. The former has been calculated from the average porosity calculation, whereas the latter has been retrieved from the statistical analysis of the dimensions, and the distribution of the void structures inside the porous network coupled to the theory of fluid flow through perforated plates. Several longitudinal and transversal sectioning planes with respect to the symmetry axis of the carbon mask have been analyzed to calculate the internal porosity from the two-dimensional images, whereas the threedimensional reconstruction of the sample has been used to retrieve the average volumetric porosity. Both the nominal values of the two-dimensional porosity and volumetric porosity have provided the same dimension of the characteristic area to be probed with a hot-film sensor for the permeability measurements. Preliminary permeability tests, performed within the predicted dimension of the control surface, have confirmed the uniformity of the mean velocity field and allowed verifying the range of variation of the correct distance of a hot-film sensor from the wall obtained from the statistical analysis of the computed tomography images. Nomenclature D ch = channel diameter, μm D CS = diameter of control points, in. d w = transversal distance from wall, in. H = local specimen thickness, in. l = length, μm L m = merging or coalescence distance, in. M = network-mesh size, in. _ m = measured flow rate, SLPM Re ch = Reynolds number based on channel diameter, -R 2 = coefficient of determination, -u = seepage or interstitial velocity, m∕s ε sup = superficial porosity, -ε 2-D = porosity calculated from 2-D CT images, -ε 3-D = volumetric porosity, -τ = tortuosity, -Subscripts ch = channels eff = effective N 2 = nitrogen REV = reference elementary volume RES = reference elementary surface st = straight channel t = tortuous channel