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Compréhension approfondie de la combustion de particules d’aluminium par diagnostics optiques avancés et simulation numérique
No full textIn the development of new solid propulsion engines, aluminum particles are incorporated into the propellant to act as an energetic additive. This process significantly enhances the overall performance of the engine, but it also generates a two-phase flow inside the combustion chamber. Such a flow can lead to performance losses and instabilities, sometimes giving rise to destructive vibratory phenomena. To mitigate these drawbacks, it is essential to gain a better understanding of the mechanisms involved in aluminum combustion, as well as the characteristics of the resulting combustion products. With this objective, the combustion of a single aluminum particle was investigated using an electrodynamic levitator. This device makes it possible to isolate, stabilize, and ignite an individual particle in a perfectly controlled environment. To characterize the process, several optical diagnostics were developed. A light extinction method, based on white LED backlighting, provides a multispectral measurement of the optical extinction of the alumina cloud formed around the burning particle. In parallel,the implementation of a numerical method combined with precise camera calibration makes it possible to distinguish gaseous emissions from emissions arising from condensed phases. The coupling of these two approaches enables the measurement of local quantities in the immediate vicinity of the burning particle, such as temperature, species concentration, and the size of the alumina particles produced. This methodology therefore provides novel and crucial insights into the combustion process of aluminum. The study was conducted under various environments to identify the factors influencing combustion mechanisms.Diluents such as nitrogen, helium, or argon were used, with adjusted oxygen proportions, increasing pressures, and, in some cases, carbon dioxide as an oxidizer. These experimental variations highlight key parameters that drive combustion and govern the characteristics of the products formed. The data obtained provide valuable targets for improving numerical models. The model developed at ArianeGroup was therefore compared with the experimental results. Significant discrepancies were observed, mainly attributed to an overly simplistic representation of the condensation phenomenon of combustion products. To address thislimitation, a more advanced model based on nucleation theory was implemented. In this new framework, condensation is no longer described by a simple Arrhenius law, but by an approach combining a nucleation rate and a growth rate, both calculated from heterogeneous condensation theory. Although discrepancies between experiments and simulations remain, the results confirm the importance of accurately modeling the processes involved in alumina formation. This work thus emphasizes both the intrinsic complexity of aluminum combustion and the essential contribution of a close coupling between advanced experiments and modeling in order to progress toward a complete understanding and control of this phenomenon.Dans le cadre du développement de nouveaux moteurs à propulsion solide, des particules d’aluminium sont intégrées au propergol afin d’agir comme additif énergétique. Ce procédé améliore significativement les performances globales du moteur, mais il engendre également un écoulement diphasique à l’intérieur de la chambre de combustion. Cet écoulement peut être source de pertes de rendement et d’instabilités, parfois à l’origine de phénomènes vibratoires destructifs. Pour limiter ces inconvénients, il est essentiel de mieux comprendre les mécanismes mis en jeu lors de la combustion de l’aluminium, ainsi que les caractéristiques des produits de combustion. Dans cette optique, l’étude de la combustion d’une particule unique d’aluminium a été entreprise à l’aide d’un lévitateur électrodynamique. Ce dispositif permet d’isoler, de stabiliser et d’allumer une seule particule dans un environnement parfaitement contrôlé. Afin de caractériser le processus, plusieurs diagnostics optiques ont été développés. Une méthode d’extinction de la lumière, basée sur un rétro-éclairage par LED blanche, offre une mesure multispectrale de l’extinction optique du nuage d’alumine formé autour de la particule. En parallèle, la mise en place d’une méthode numérique associée à une calibration précise de la caméra permet de distinguer les émissions gazeuses de celles issues des phases condensées. Le couplage de ces deux approches rend possible la mesure de grandeurs locales au voisinage immédiat de la particule en combustion, telles que la température, la concentration en espèceset la taille des particules d’alumine produites. Cette méthodologie fournit ainsi des informations inédites et cruciales sur le déroulement de la combustion d’aluminium. L’étude a été conduite dans des environnements variés afin d’identifier les facteurs influençant les mécanismes de combustion. Des diluants tels que l’azote, l’hélium ou l’argon ont été utilisés, avec des proportions d’oxygène ajustées, des pressions croissantes, et du dioxyde de carbone comme oxydant. Ces variations expérimentales permettent de mettre en évidence des paramètres clés qui pilotent la combustion et conditionnent les caractéristiques des produits formés. Les données obtenues offrent des cibles précieuses pour l’amélioration des modèles numériques. Le modèle développé par ArianeGroup a ainsi été confronté aux résultats expérimentaux. Des écarts notables ont été relevés, principalement attribués à une représentation trop simpliste du phénomène de condensation des produits de combustion. Pour remédier à cette limite, un modèle plus élaboré a été implémenté, reposant sur la théorie de la nucléation. Dansce nouveau cadre, la condensation n’est plus décrite par une simple loi d’Arrhenius, mais par une approche combinant un taux de nucléation et un taux d’accroissement, calculés à partir des lois de la condensation hétérogène. Si des divergences subsistent entre expériences et simulations, les résultats obtenus confirment l’importance d’une modélisation fine des processus de formation de l’alumine. Ce travail souligne ainsi à la fois la complexité intrinsèquede la combustion de l’aluminium et l’apport essentiel d’un couplage étroit entre expériences de pointe et modélisations avancées pour progresser vers une compréhension et une maîtrise complètes de ce phénomène
Visualization of Ammonia engine by pre-chamber igniter
No full textInternational audienceAmmonia is considered more and more as a promising carbon-free fuel for internal combustion engines to contribute to the decarbonization of several sectors where replacing conventional engines with batteries or fuel cells remains unsuitable. However, ammonia properties can induce some challenges for efficient and stable combustion. This study investigates the use of an active pre-chamber ignition system fueled with hydrogen and compares it to conventional spark ignition, with a focus on lean limit operation and early flame development. Experiments were conducted on a single cylinder optical engine with a compression ratio of 9.5, equipped with a quartz window in the piston for natural flame luminosity imaging using a high-speed camera. The engine was fueled with a mixture of 95% ammonia and 5% hydrogen by volume. Ammonia was injected and mixed with air in the intake port while hydrogen was directly injected into the prechamber. As a function of the intake pressure (1.0, 0.9, 0.8, and 0.7 bar), the lean operating limit was determined as 0.7, 0.8, 0.85, 0.9 for both active pre-chamber and spark ignition strategies. In fact, with hydrogen prechamber ignition, significant faster combustion was achieved which reduced by 30% the overall combustion duration. The visualization of natural emission of flame revealed a more distributed and faster flame propagation with the active prechamber, indicating a higher ignition performance and faster combustion under lean ammonia-hydrogen conditions, highlighting its potential for extending operating limits and improving combustion robustness
hughes2d
No full textA numerical scheme to approximate the solutions of the Hughes model using a finite volume scheme for the scalar conservation law and a fast marching algorithm for the Eikonal equation
Victoire Girard discussion of: Climate, natural resources, and conflict
No full textInternational audienc
Un monnayage d’imitation au type Élaia (Éolide d’Asie) ? L’apport des analyses élémentaires dans le classement des monnaies d’argent de la cité
No full textInternational audienc
Persistent Luminescence in Highly Nonstoichiometric GAGG Garnets Gd 3+x [Al 2 Ga 3 ] 1‐x/5 O 12 (0 ≤ x ≤ 0.6) Doped with Ce 3+ /Cr 3+
No full textInternational audienceThe gadolinium garnet Gd 3 Al 2 Ga 3 O 12 co‐doped with Ce 3+ and Cr 3+ (GAGG‐Ce,Cr) has been widely studied due to its unusual bright yellow long‐lasting persistent luminescence properties. Here, rapid containerless melt‐quenching is used as part of a two‐step glass‐crystallisation synthesis process to obtain a new highly nonstoichiometric form of this garnet, of composition Gd 3+x [Al 2 Ga 3 ] 1‐x/5 O 12 with 0 ≤ x ≤ 0.6 (ns‐GAGG). For compositions x > 0, powder X‐ray diffraction analysis confirms that excess Gd 3+ is accommodated at the Al 3+ /Ga 3+ sublattice in octahedral coordination, by substituting up to 30% of these sites. This mode of substitution complexifies the local structure of the garnet host, which is shown to influence certain luminescence properties in the analogous highly nonstoichiometric Y 3+x Al 5‐x O 12 (0 < x < 0.4) and Gd 3+x Al 5‐x O 12 (0 < x < 0.6) systems. Co‐doping ns‐GAGG with Ce 3+ and Cr 3+ produces green‐yellow persistent luminescence when x = 0, which undergoes a redshift to yellow‐orange as the Gd 3+ content increases to x = 0.4. However, this radical modification of the host composition does not strongly affect the afterglow kinetics. These results demonstrate an effective and high‐precision way of decoupling color and kinetics in persistent luminescent garnets, which is usually hard to achieve using the standard stoichiometric material engineering approach
A disaggregated analysis of the impact of corruption on tax revenue mobilization in developing countries : Evidence of a nonlinear relationship
No full textInternational audienceOver the past two decades, increasing attention has been given to the mobilization of tax revenues in developing countries. Numerous empirical studies have investigated the impact of economic, structural, institutional, and social factors on public revenues, with a strong focus on corruption. This article contributes to the literature by distinguishing between various types of corruption and examining their nonlinear relationships with tax revenue. By utilizing disaggregated V-Dem indicators for corruption, and applying a dynamic GMM approach to address the endogeneity of corruption, the article also examines macroeconomic determinants of tax revenue mobilization in 122 middle-and lowincome countries from 1990 to 2017. The findings demonstrate that corruption has a nonlinear relationship, shaped by both its scale and nature
