High-flux sublimation of a 3D carbon/carbon composite: surface roughness patterns

3D carbon-fibre reinforced carbon composites (3D ) are used as thermal protection systems for atmospheric re-entry, where they are exposed to strong ablation. Particularly, sublimation of the carbonaceous material plays an important role during the re entry. To study this, an arc image furnace under controlled Argon flow is used, with heat fluxes of 8 MWm−2 to 10 MWm−2. The furnace and the sample thermal response have been numerically simulated prior to the experiments and match in-situ temperature measurements. Scanning electron microscopy and 3D profilometry with digital optical microscopy were used in order to characterise the epi-macro-structural and the epi-micro-structural roughness of the composite surface, evidencing a faster recession of the fibres as compared to the matrix. Carbon nanotextures have been assessed by using High-Resolution Transmission Electron Microscopy and Polarised Light Optical Microscopy, showing that the matrix is more organised than the fibre. This can explain the “inverse” behaviour under sublimation as compared to oxidation. The results have been qualitatively interpreted using numerical simulation of differential surface recession

Prévision de transition sur la trajectoire du vol Reentry-F avec les PSE à l'équilibre chimique

International audienceThe laminar-turbulent transition is a key feature of hypersonic flows on reentry vehicles. For a smooth wall, its position can be estimated by a stability analysis of the boundary-layer, using the Local Stability Theory (LST) or the Parabolized Stability Equations (PSE). These techniques have been applied to the trajectory of Reentry-F flight experiment, assuming an axisymmetric flow and air at thermochemical equilibrium. The results have been compared with experimental data and results from previous published works. The transition is found to move gradually closer to the nose when the altitude decreases and the Reynolds number increases

A propos du rôle de la correction d'Hanratty dans la réponse linéaire d'un écoulement turbulent contraint par une paroi ondulée

International audienceScallop patterns forming on erodible surfaces were studied historically using a linear analysis of the inner region of a turbulent boundary layer growing on a corrugated wall. Experimental observations show a phase shift between the shear stress at the wall and the wall oscillation that depends on the wavenumber. An ad hoc correction applied to the turbulent closure and due to Hanratty et al. (Thorsness et al. , Chem. Engng Sci. , vol. 33, issue 5, 1978, pp. 579–592; Abrams & Hanratty, J. Fluid Mech. , vol. 151, issue 1, 1985, p. 443; Frederick & Hanratty, Exp. Fluids , vol. 6, issue 7, 1988, pp. 477–486) was systematically used to recover the reference experimental results. In this study, Reynolds-averaged Navier–Stokes (RANS) and direct numerical simulations (DNS) were performed and revealed the role of the Boussinesq assumption in the results obtained. We show that the Hanratty correction acts as a palliative to the misrepresentation of Reynolds stresses due to the use of the Boussinesq hypothesis. The RANS calculations based on a turbulence model using a second-order moment closure recovered the expected results obtained in the reference DNS calculations, in particular with respect to wall heat transfer. The analysis of these results highlights the critical importance of the anisotropy of the diagonal Reynolds stresses on the prediction of wall transfer under these conditions and their implication in the occurrence of scalloping.Les motifs de coups de gouges se formant sur des surfaces érodables ont été étudiés historiquement à l'aide d'une analyse linéaire de la région interne d'une couche limite turbulente se développant sur une paroi ondulée. Les observations expérimentales montrent un déphasage entre la contrainte de cisaillement à la paroi et l'oscillation de la paroi qui dépend du nombre d'ondes. Une correction ad hoc appliquée à la fermeture turbulente et due à Hanratty et al. (Thorsness et al., Chem. Engng Sci., vol. 33, numéro 5, 1978, pp. 579-592 ; Abrams & Hanratty, J. Fluid Mech., vol. 151, numéro 1, 1985, p. 443 ; Frederick & Hanratty, Exp. Fluids, vol. 6, numéro 7, 1988, p. 477-486) a toujours dû être utilisée pour récupérer les résultats expérimentaux de référence. Dans cette étude, des simulations Navier-Stokes à moyenne de Reynolds (RANS) et des simulations numériques directes (DNS) ont été réalisées et ont révélé le rôle de l'hypothèse de Boussinesq dans les résultats obtenus. Nous montrons que la correction de Hanratty agit comme un palliatif à la mauvaise représentation des contraintes de Reynolds due à l'utilisation de l'hypothèse de Boussinesq. Les calculs RANS basés sur un modèle de turbulence utilisant une fermeture au second ordre ont retrouvé les résultats attendus obtenus dans les calculs DNS de référence, en particulier en ce qui concerne le transfert de chaleur à la paroi. L'analyse de ces résultats met en évidence le rôle critique de l'anisotropie des contraintes de Reynolds diagonales sur la prévision du transfert à la paroi dans ces conditions et leur implication dans l'apparition des coups de gouges

Structural approach for Ca(PO3)2-NaPO3-B2O3 composition glasses

The structure of glasses in the system Ca(PO3)2– NaPO3–B2O3 has been investigated using Raman and NMR (31P MAS, 11B) measurements. For calcium borophosphates of composition (1-x)Ca(PO3)2.xB2O3 (0£x£0·6), structural models are proposed for glasses of varying x. In the phosphate rich compositional domain (

Structural approach for Ca(PO3)2-NaPO3-B2O3 composition glasses

The structure of glasses in the system Ca(PO3)2– NaPO3–B2O3 has been investigated using Raman and NMR (31P MAS, 11B) measurements. For calcium borophosphates of composition (1-x)Ca(PO3)2.xB2O3 (0£x£0·6), structural models are proposed for glasses of varying x. In the phosphate rich compositional domain (

Erbium luminescence properties of niobium-rich oxide glasses

Erbium luminescence has been investigated in niobium borophosphate glass. A correlation between luminescence intensity at 1532 nm, OH absorption and the structure of the glass network was established as a function of the niobium oxide concentration. An increase of the radiative efficiency corresponding to 4I13/2 → 4I15/2 (4f–4f) transition of Er3+ ions has been observed for niobium richest glasses. The modification of the Er3+ ions site is analyzed as a function of the niobium concentration. A Judd–Ofelt analysis has been applied to emphasize the structural change. An increase of the up conversion was also measured as the niobium oxide content increases

Structural influence of alumina in Zn-Cd-Pb phosphate glasses

A structural investigation of (100-y)[50P2O5-15ZnO-(35−x)PbO-xCdO]-yAl2O3 polyphosphate glasses was carried out using Raman, NMR (31P, 207Pb and 27Al) and XPS spectroscopies. In alumina-free glasses (y=0), the CdO substitution for PbO induces a strengthening of the network formation, up to x=20. For x>20, the network connectivity decreases because less cations are located in network former sites. In aluminophosphate glasses (x=20 and

Structural influence of alumina in Zn-Cd-Pb phosphate glasses

A structural investigation of (100-y)[50P2O5-15ZnO-(35−x)PbO-xCdO]-yAl2O3 polyphosphate glasses was carried out using Raman, NMR (31P, 207Pb and 27Al) and XPS spectroscopies. In alumina-free glasses (y=0), the CdO substitution for PbO induces a strengthening of the network formation, up to x=20. For x>20, the network connectivity decreases because less cations are located in network former sites. In aluminophosphate glasses (x=20 and

Effect of glass composition on the chemical durability of zinc phosphate-based glasses in aqueous solutions

The chemical durability of glasses with the composition 40P2O5-55ZnO-1Ga2O3-4Ag2O and 41P2O5-51ZnO-8Ga2O3 (mol%) was studied by measuring the rates of aqueous dissolution in neutral, acidic and alkaline aqueous solutions and discussed as a function of the glass composition. The change in the pH of the solutions as a function of the immersion time of the samples was used to study the dissolution mechanism. Using XRD and SEM/EDXA, we showed that the dissolution in deionized (DI) water and HCl consists of the leaching of the phosphate chains into the medium along with (i) the formation of a hydrated layer with the composition Zn2P2O7*3H2O and also of AgCl agglomerates when immersed in HCl and (ii) a leaching out of P, Ga and Ag when immersed for more than 180 min in DI water and for more than 60 min in HCl. The dissolution in NaOH-Na2CO3 consists of a net consumption of the OH− along with the formation of layers of Zn3(PO4)2*(H2O)4 and Zn(H2PO2)2*H2O with no apparent diffusion of P, Ga and Ag when immersed for as long as 240 min. Increasing the Ga2O3 concentration in zinc-phosphate glass at the expense of Ag2O lowers the dissolution rate when immersed in DI water, HCl and NaOH-Na2CO3 probably due to a reinforcement of the glass network

Structure and nonlinear optical properties of sodium–niobium phosphate glasses

International audienceThe structural description of xNb2O5–(1 − x)(NaPO3) (0 less-than-or-equals, slant x less-than-or-equals, slant 0.40) glass network is reported through high-field (18.8 T) 17O and 93Nb nuclear magnetic resonance, O1s X-ray photoelectron spectroscopy and Raman spectroscopy. The different oxygen sites (POP, PONa, NbOP and NbONb) are resolved on the spectra, their relative proportion determined and compared to indirect calculations obtained from the decomposition of 31P NMR spectra. A good agreement is obtained, thus confirming the formation of the Nb(OP)6−y(ONb)y units with increasing y leading to the clusterization of NbO6 octahedra, mainly for x > 0.2, which is related to the incomplete dissociation of Nb2O5 in the NaPO3 melt. The formation of these clusters is responsible for the large increase of the nonlinear optical properties, mainly due to the high polarizability of the Nb–O–Nb bond