Caractérisation de la propagation d'une flamme dans un milieu diphasique (brouillards) en microgravité

La combustion des sprays compte pour 25% de la consommation mondiale d’énergie. Les carburants liquides sont exploités sous forme d’aérosol (spray) afin de bénéficier de leur propriété fortement exothermique, offerte par la combustion de leur phase gazeuse. La combustion des aérosols touchent de nombreuses applications industrielles, incluant les moteurs à combustion interne, les turbines à gaz et la production d’électricité. Or, la combustion diphasique telle qu’elle existe en pratique résulte d’un couplage complexe faisant intervenir de nombreux processus comme : l’atomisation, la vaporisation, des écoulements turbulents, la cinétique chimique. Une description complète est possible uniquement par le biais de simulations numériques, mais les investigations expérimentales sont essentielles pour fournir des données quantitatives dans des configurations contrôlées afin d’alimenter les modèles. Une configuration expérimentale simplifiée a été reproduite avec un brouillard monodisperse en taille de gouttes et la propagation d’une flamme laminaire pour mettre en lumière les phénomènes clés de l’interaction spray/combustion.Initié dans le cadre du groupement de recherche Micropesanteur Fondamentale et Appliquée (MFA) du CNRS et du CNES en 2008, ce projet expérimental s’inscrit dans la continuité des études de vaporisation et de combustion de carburant liquide menées à l’ICARE depuis plusieurs années. Les expériences ont bénéficié de conditions de gravité réduite à 10-2 g obtenues lors de vols paraboliques à bord de l’avion ZERO-G du CNES. Ces conditions uniques de travail offrent un environnement propice à une étude paramétrique avec un brouillard statique et une répartition spatiale homogène (annulation de la sédimentation des gouttes).À l’issue des trois ans, une base de données importante a été constituée pour valider ou bâtir des modèles de combustion diphasique, aussi bien en mélange non réactif (création des gouttes, granulométrie et vaporisation) qu’en mélange réactif (vitesse de flamme, domaine de stabilité, présence d’imbrûlés).Spray combustion accounts for 25% of the world’s energy use. Liquid fuels are used through aerosol (spray) in order to benefit to their high exothermicity, released by the combustion of the gaseous phase. Two-phase combustion is of practical importance in a wide range of technological applications that include automotive engines, gas turbines and power generation. However, realistic spray combustion lies in complex interactions with: atomization, vaporization, turbulent flows, chemical kinetic. An entire description of spray combustion is only possible by a numerical way with a theoretical approach. In another hand, experimental investigations are necessary to provide quantitative data with idealized spray configuration. A simplified spray was reproduced with a monodisperse aerosol and a laminar flame in order to highlight the predominant phenomena in spray/combustion interaction.Experiments were conducted under reduced gravity conditions to 10-2 g obtained by parabolic flights with the Airbus ZEROG from the CNES. Microgravity offers a good parametric study with a quiescent aerosol and homogeneously distributed (without settling effect on droplets).A strong and exhaustive database has been built to validate or establish a comprehensive two-phase model of combustion, on the spray itself (droplet formation, size and droplet concentration, vaporization rate) and on the reactive mixture (flame velocity, stability regime, presence of liquid droplets in the burned gases)

Experimental Investigation of the Mechanisms of Cellular Instabilities Developing on Spherical Two-Phase Flames

International audienceThe presence of liquid fuel droplets in a flammable mixture causes cellular instabilities on the flame surface, which significantly enhances the flame speed when compared to the fully vaporized case. The prediction of the mechanisms responsible for the onset of cellularity for two-phase mixtures is essential to better understand spray combustion. The present study considers an innovative experimental strategy to isolate and investigate any potential mechanisms. The fuel droplets were replaced by inert water droplets in order to amplify the thermal sink effect, characterized by the absorption of part of the heat released by the flame, and to suppress the local enrichment of fuel formed around droplets. Spherical expandingflames with narrow-size distribution droplets were used and qualitative comparisons of the flame structure were performed with a shadowgraph system. The results have shown that the heat sink has no significant effect, whereas the local enrichment of fuel appears as a key phenomenon, which suggests that in the case of fuel droplet aerosols the onset of cellularities is triggered in the inhomogeneous part of the gaseous phase

Microgravity experiments and numerical studies on ethanol/air spray flames

International audienceSpray flames are known to exhibit amazing features in comparison with single-phase flames. The weightless situation offers the conditions in which the spray characteristics can be well controlled before and during combustion. The article reports on a joint experimental/numerical work that concerns ethanol/air spray flames observed in a spherical chamber using the condensation technique of expansion cooling (based on the Wilson cloud chamber principle), under microgravity. We describe the experimental setup and give details on the creation of a homogeneous and nearly monosized aerosol. Different optical diagnostics are employed successfully to measure the relevant parameters of two-phase combustion. A classical shadowgraphy system is used to track the flame speed propagation and allow us to observe the flame front instability. The complete characterization of the aerosol is performed with a laser diffraction particle size analyser by measuring the droplet diameter and the droplet density number, just before ignition. A laser tomography device allows us to measure the temporal evolution of the droplet displacement during flame propagation, as well as to identify the presence of droplets in the burnt gases. The numerical modelling is briefly recalled. In particular, spray-flame propagation is schematized by the combustion spread in a 2-D lattice of fuel droplets surrounded by an initial gaseous mixture of fuel vapour and air. In its spherical expansion, the spray flame presents a corrugated front pattern, while the equivalent single-phase flame does not. From a numerical point of view, the same phenomena of wrinkles are also observed in the simulations. The front pattern pointed out by the numerical approach is identified as of Darrieus–Landau (DL) type. The droplets are found to trigger the instability. Then, we quantitatively compare experimental data with numerical predictions on spray-flame speed. The experimental results show that the spray-flame speed is of the same order of magnitude as that of the single-phase premixed flame. On the other hand, the numerical results exhibit the role played by the droplet radius in spray-flame propagation, and retrieve the experiments only when the droplets are small enough and when the Darrieus–Landau instability is triggered. A final discussion is developed to interpret the various patterns experimentally observed for the spray-flame front

Burning velocities and jet-stirred reactor oxidation of diethyl carbonate

International audienceThere is current interest in utilizing oxygenated biofuels such as carbonates in blends with conventional oilderived liquid fuels. Carbonates, commonly used as electrolyte solvents in Li-ion cells, could ignite after abusive operating conditions. Improving the kinetic modeling of the oxidation of these bio-derived oxygenates requires further investigation under well-controlled conditions. An experimental and detailed chemical kinetic modeling study of diethyl carbonate (DEC) oxidation and combustion was performed. Experiments were carried out in a jet stirred reactor over a wide range of equivalence ratios, temperatures, and pressure. Mole fractions of stable species were measured in the jet stirred reactor at atmospheric pressure. Burning velocities of DEC/air mixtures were determined at elevated temperature over a range of pressures and equivalence ratios. A detailed chemical kinetic modeling was performed using the present experimental results and existing literature data and model. The model represents fairly well the present data. Sensitivity and reaction paths analyses were used to rationalize the results

Women's traditional knowledge, use value, and the contribution of tamarind (Tamarindus indica L.) to rural households' cash income in Benin

Women's Traditional Knowledge, Use Value, and the Contribution of Tamarind ( Tamarindus indica L.) to Rural Households' Cash Income in Benin. This study examined differences in knowledge, use values, and contribution of tamarind (Tamarindus indica) to women's cash income during the dry season, focusing on seven tribal groups in Benin. Data were gathered using semistructured individual interviews and monitoring, and were analyzed using quantitative ethnobotanical methods. Principal component analysis was applied to describe the use value and use forms of tamarind according to different tribes. Tamarind was found to play an important role in local communities' livelihoods. Overall, 26 different uses were mentioned for tamarind products. Most commonly, the fruit (pulp) was used to make beverages, as a laxative and purgative, and it seems to be the only plant part sold commercially. Bark was frequently used as a medicine to treat wounds, and leaves were used to make porridge and as an antibiotic. Medicinal, cultural, and material use categories were correlated best with the Fulani, whereas commerce was most correlated with Gourma tribes (PCA analysis). There were significant differences for tamarind utilization among the tribal groups, with overall ethnobotanical use values (EUVT) ranging from 10 to 14, and contribution to cash income ranging from 8.8% to 56.4%. In view of its domestication potential, it is crucial that traditional tribal knowledge of tamarind be preserved and integrated into management policies. Further development and research needs for utilization and conservation are improvement of commercialization, organization of market channels, and extent of genetic diversity within and among populations

Ethnic differences in use value and use patterns of the threatened multipurpose scrambling shrub (Caesalpinia bonduc L.) in Benin

African communities traditionally use medicinal plants for their primary healthcare. To ensure the sustainable use of these species one of the crucial issues is to document African communities’ indigenous medicinal knowledge. To achieve this goal, the present study evaluated the use frequency and the knowledge of local Beninese communities on the endangered scrambling shrub (Caesalpinia bonduc). Results revealed that local populations use 20 properties from the leaves, roots and seeds of the species to fight against childbirth, to treat burns and for cultural practices like games, weddings and the Fâ ritual. The global credibility level of these properties equalled to 75%, indicating that C. bonduc is perceived as very important for local populations. Roots are more intensively used than leaves and seed respectively. The study clearly showed ethnic and age differences in use value and patterns of the species. For example, Kotafon ethnic group had a fair knowledge on the species while Fon and Bariba ethnic groups hold the lowest number of users