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Effect of dilution and Lewis number of the fuel in the fuel-air mixture on the heat flux during flame wall interaction (FWI)
No full textInternational audienceMost combustion systems where flame is bounded by walls encounter Flame-wall interactions (FWI). Heat losses during FWI are in the magnitude of MW/m2 leading to concern about inefficiency, and thermal stresses. Hence, FWI is a topic of significant interest. With the goals of climate change pushing us toward renewable fuels with least carbon emissions, it is imperative to understand the FWI of renewable fuels like hydrogen-air mixture. In this study, FWI in a head-on-quenching configuration is carried out in a constant volume chamber. High-speed surface temperature measurement is carried out to compute instantaneous wall heat flux. Adding inert diluents to methane-air mixtures, the effect of dilution on the peak of heat flux during FWI is studied. Dilution affects the peak of heat flux predominantly by changing the adiabatic flame power of the fuel-air mixture. Furthermore, a large variation in flame power is carried out by changing the dilution fraction in the methane-air mixture to quantify its effect. It is found that the peak of heat flux and non-dimensional heat flux follows a nonlinear decrease with an increase in flame power. Knowing this effect, FWI of three different fuels, that is, methane, hydrogen, and a mixture of acetylene and hydrogen, having different Lewis numbers (Le) of fuel in a fuel-oxidizer mixture are studied at equivalent flame power. This comparison shows that FWI of hydrogen-air mixtures in HOQ configuration have a lower peak of heat flux compared to other fuels which may be due to the preferential diffusion of hydrogen-air mixture
Repenser la formation des enseignant·es à l'aune des nouveaux enjeux de l'enseignement de la phonologie
No full textInternational audienceDans la continuité des précédentes journées d’étude de l’ALOES, destinées aux enseignant·es d’anglais et qui visent à explorer les nouveaux enjeux de l’enseignement de la phonologie, nous, un collectif de spécialistes de phonétique et phonologie de l’anglais, proposons de nouvelles pistes de réflexions concrètes afin de repenser la formation des enseignant·es en phonologie.Ainsi, nous prenons comme point de départ les objectifs phonologiques de l’enseignement de l’anglais et faisons des propositions pour les concours qui intègrent une question de phonologie (Capes et Agrégation) et pour les cursus universitaires, notamment en licence. Il est attendu des élèves qu’ils et elles soient familiarisé·es aux schémas intonatifs de l’anglais, à l’accentuation tonique, aux relations entre écrit et oral ainsi qu’aux variations phonétiques et phonologiques dans les usages de l’anglais. Les enseignant·es doivent donc maîtriser ces différents aspects de la phonétique et phonologie de l’anglais (sans pour autant en être des spécialistes) et être en capacité de les enseigner, ce qui implique que les questions de phonologie au Capes et à l’Agrégation doivent refléter ces attendus. Actuellement, c’est en grande partie le cas, et on peut se féliciter de la diversification des questions qui a été opérée ces dernières années, de sorte que l’ensemble de ces sujets soient couverts.Toutefois, dans le cadre de cette communication, nous revenons sur quelques points concernant les questions de phonologie aux concours et proposons des pistes de réflexion visant à renforcer l’adéquation entre les objectifs phonologiques du secondaire et la formation des enseignants. Tout d’abord, les questions de phonologie aux concours sont présentées comme des questions « sans programme », mais il existe en réalité un ensemble de thématiques et de savoir-faire bien identifiables. Nous proposons de les expliciter et d’en délimiter plus clairement les contours. Par ailleurs, certaines questions (actuellement présentes dans les sujets ou non) gagneraient à être réévaluées au regard des attendus et de leur capacité à discriminer les candidat·es aux concours. Par exemple, nous suggérons de :Revoir le système d’évaluation de la transcription phonémique de phrase, exercice dont les rapports soulignent le faible taux de réussite chaque année ;Définir clairement le périmètre des attendus pour l’accentuation lexicale et les relations écrit-oral afin de ne conserver que les règles les mieux établies et les plus pertinentes pédagogiquement ;Restructurer le format des questions relatives à l’intonation, et de ne conserver que des questions portant sur l’un des aspects de l’intonation (groupes intonatifs, toniques ou tons) nécessitant une justification.Ces pistes de réflexion sont applicables à la formation des étudiant·es du supérieur, en particulier celles et ceux qui se destinent à l’enseignement. Par ailleurs, il nous semble également souhaitable d’explorer la possibilité d’intégrer d’autres variétés, notamment dans les programmes universitaires
Affirmative Resolution of Bourgain's Slicing Problem using Guan's Bound
No full text20 pagesWe provide the final step in the resolution of Bourgain's slicing problem in the affirmative. Thus we establish the following theorem: for any convex body of volume one, there exists a hyperplane such that the -dimensional volume of is lower bounded by a universal constant. Our proof combines Milman's theory of -ellipsoids, stochastic localization with a recent bound by Guan, and stability estimates for the Shannon-Stam inequality by Eldan and Mikulincer
Experimental Investigations on Carbon Segmented Seals With Smooth and Pocketed Pads
No full textInternational audienceAbstract The present paper introduces experimental results for radial segmented seals operating at high rotating speed. These seals are made of three or more interlocking, carbon-graphite segments, in permanent contact with the rotor while also accommodating its dynamic displacements. Currently, many efforts are made to model these behaviors and for obtaining accurate leakage flowrate and friction torque predictions. A novel test rig fitted with a casing for measuring leakage and friction torque produced by two carbon segmented seals was developed. The tested seals were composed of three segments. Two types of segmented seals were tested: two seals with flat pads and one seal with pocketed pads. The seals were installed two by two in a casing and were simultaneously tested in a back to back arrangement. The process fluid was air, with a maximum inlet pressure of 0.3 MPa, while it was set to ambient at the outlet. The rotor diameter was 100 mm with a maximum rotating speed of 24,000 rpm. The main measurements were the inlet mass flowrate and friction torque produced by the two seals. Results show that the seal with pocketed pads produced higher leakage rates but lower friction torque compared to the smooth pads. This suggests that the seal with pocketed pads operated with segments completely lifted from the rotor surface. Mass flow rates measurements during run-up/coast-down tests revealed a hysteresis effect, which could be the consequence of segment tilt. The experimental results could also be useful to validate theoretical models
Thermal behaviour and combustion characteristics of JET A-1, HEFA-derived SAF and their blends for aeronautical applications
No full textInternational audienceThermal and combustion characteristics of aviation fuels are useful in system design forcombustors and modelling for engine design and their performance. This study investigates thethermal behaviour and combustion properties of pure JET A-1 (J100), HEFA-derived SAF(HS100) and its blends (JxHSy) using thermogravimetric analysis (TGA), and differentialscanning calorimetry (DSC) techniques. The analyses were supported by compositionalevaluation with gas chromatography-mass spectrometry (GC-MS) and the fuel properties werecompared according to ASTM standards. Thermal stability and mass loss rates showed gradualdegradation patterns for HS100, while J100 revealed sharper, more rapid degradation. J100degraded between 85°C-168°C, and HS100 degraded between 100°C-205°C, indicatingimproved thermal stability for HS100. HS100 displayed higher energy density, lower viscosity,and a broader exothermic peak, enabling cleaner combustion and extended engine life. DSCanalysis revealed distinct thermal peaks: J100 has an endothermic and exothermic peak at218°C and 234°C, respectively, while HS100 showed a broader endothermic and exothermicpeak at 231°C and 248°C, respectively. J30HS70 blend exhibited intermediate peak values,combining the thermal advantages of both components. The enthalpy values are J100 (−90.53J/g), HS100 (−69.68 J/g) and J30HS70 (−167.90 J/g). For combustion indices, J100 has the highest comprehensive performance index (2.10 × 10⁻⁶ mass²/min² °C³) and ignition index(3.50 × 10⁻⁴ mass/min °C²). The lowest values for both indices were found for HS-rich blends,attributed to their lower aromatic and higher paraffinic compositions. From GC-MS analysis,J100 showed 45% low-molecular-weight alkanes C8–C15 while HS100 had only 21%,contributing to HS100’s cleaner combustion characteristics. The findings confirm that HEFASAFand its blends are promising sustainable alternatives for aviation, offering thermalstability, cleaner combustion, and environmental benefits. Future studies on modelling combustion dynamics and emissions in real-world jet engines would provide deeper insights
Electrocatalysis of organics for electrolysis and/or fuel cells: Some thoughts on using the ratio of forward to backward peak current as a measure of electrocatalyst efficiency and/or poisoning
No full textInternational audienceIn the current context of sustainability, the selective electrocatalytic transformation of biomassderived organic substances into value-added products should offer vast design possibilities for power generation or the electrosynthesis of fuels and commodity chemicals. In this contribution, we have examined a number of concepts concerning the electrocatalysis of organic molecules for which noble metals cannot be excluded from the electrocatalyst composition without compromising the significant energy savings promised in electrolyzers (up to 50% for H2 co-production compared to conventional water electrolysis). The widespread practice of using the ratio of forward peak current to backward peak current as a measure of activity, anti-poison capacity or removal of adsorbed poisons or intermediates is unsuitable based voltammetry and spectroelectrochemical analysis