Status of the CNRS-LCSR program on high pressure droplet vaporization and burning

Depending on the surrounding flow and thermodynamic conditions, a single droplet may experience several gasification regimes, ranging from the envelope flame regime to pure vaporization. In practical situations, such as rocket propulsion or diesel combustion, the size distribution of droplets is, at best, bimodal, so that the possibility exists for the simultaneous presence of various regimes. For example, very small droplets are transported by the gas phase with zero relative velocity. This picture validates then the spherical symmetry hypothesis applied to the droplet and to the diffusion flame enveloping it. On the other hand, for larger droplets, a relative velocity exists due to drag forces. The most important influence of forced convection on droplet burning is the possibility to extinguish globally the envelope flame, or to establish a flame stabilized in the wake region. The burning rates of these regimes differ strongly. The characteristic time of droplet gasification is also influenced by the surrounding pressure and temperature. A parametric investigation of single droplet burning regimes is then helpful in providing the necessary physical ideas for sub-grid models used in spray combustion numerical prediction codes. The CNRS-LCSR experimental program on droplet vaporization and burning deals with these various regimes: stagnant and convective monocomponent droplet burning convective mono and bicomponent droplet vaporization; high temperature convective mono and biocomponent droplet vaporization; burning regimes of hydrazine and hydroxyl-ammonium-nitrate based monopropellant droplets and the vaporization regimes of liquid oxygen droplets. Studies on interacting droplets and on liquid aluminum droplets will start in the near future. The influence of high pressure is a common feature of all these studies. This paper summarizes the status of the CNRS-LCSR program on the effects of high pressure on monocomponent single droplet burning and vaporization, and some recent results obtained under normal and reduced gravity conditions with suspended droplets are presented. In the work described here, parabolic flights of an aircraft is used to create a reduced gravity environment of the order of 10(exp -2) g

PIV Measurements in an Underexpanded Hot Free Jet

International audienceThe objective of the work reported herein is to demonstrate the ability of the PIV technique to provide correct measurements of the velocity field in steady underexpanded hot free jets. A hot-gas generator capable to create jets with an initial diameter of 25 mm, total pressure up to 4 MPa, and total temperature up to 2200 K is presented together with the associated technique of stagnation conditions determination. The implementation of the PIV method is described paying attention to the choice of the PIV system components and to the seeding technique. Experimental results are presented on the observed jet structure and directly measured geometry of the Mach disk. The results on the velocity field are analyzed considering the effect of the kind of seeding particles as well as the jet stagnation conditions. Results from the experiment and numerical simulation are compared to validate the measurements of the velocity field

Auto-ignition of near-ambient temperature H2/air mixtures during flame-vortex interaction

International audienceThis paper demonstrates auto-ignition in reactants at approximately 350 K, upstream of curved H 2 /air flame surfaces during flame/vortex interaction. Temperature fields were measured using laser Rayleigh scattering during head-on interactions of toroidal-vortices with stagnation flames. Repeatable ignition occurred along the ring of the vortex-slightly towards the center-when it was approximately 1 mm upstream of the wrinkled flame surface. The resultant outwardly propagating toroidal flame led to approximately twice the volumetric heat release rate over the duration of the interaction. The ignition occurred in a region of low fluid dynamic strain rate that was farther from the flame than the region of maximum vorticity. Evidence of additional ignition pockets was found upstream of other flame wrinkles, preferentially near the highest magnitude flame curvatures. Different hypotheses for explaining this observation are discussed. The possibility of substantial heat release driven by auto-ignition and complicated diffusion has implications for reaction rate closure models and transport models used in turbulent combustion simulations

Los arenales costeros del litoral catalán (la bahía de Rosas)

[ES] Se distinguen dos fuentes para los minerales pesados que se encuentran en las playas de la bahía de Rosas: los basaltos de Olot, para la augita, olivino e hiperstena y las rocas metamórficas del macizo de los Alberes y Cabo de Creus de donde proceden la andalucita, silimanita y distena. La distribución de los minerales, se explica por el transporte efectuado por las corrientes de deriva, los temporales y el viento. Las anomaiías en la distribucidn de algunas especies se deben a los accicentes del terreno y a las condiciones dinámicas muy activas de la bahía, que afectan a la seleccidn de minerales.[EN] The heavy minerals of Gulf of Rosas coastal sand have two different sources. The more frequent heavy minerals are augite and olivine which come from Olot basalts. The metamorfic association is presented by andalusite, siliimanite and kyanite and they come from the metamorfic rocks of the Pyrenees and Cap of Creus massif. Homblende can have two origins: the metamorfic and the granitic rocks of river Muga basin.Peer reviewe

Nutrition in hemodialysis patients previously on a supplemented very low protein diet

Nutrition in hemodialysis patients previously on a supplemented very low protein diet.BackgroundNutritional safety of protein-restricted diets in patients with chronic renal failure is controversial. In the present study, we have assessed the evolution of nutritional status after initiation of hemodialysis in patients previously treated by a supplemented very low protein diet (SVLPD).MethodsNutritional data were prospectively collected during the first year of hemodialysis from 15 consecutive patients treated with a SVLPD (0.3 g protein/kg/day supplemented with essential amino acids, calcium, iron, and vitamins) and compared to 15 age- and gender-matched end-stage renal disease (ESRD) patients previously on a less-restricted diet (0.90 ± 0.21 g protein/kg/day) who started hemodialysis during the same period. Dual-energy x-ray absorptiometry (DEXA) was used to assess body composition at 0, 6, and 12 months. Hemodialysis prescriptions, biologic data and 3-day food records were collected every 3 months.ResultsProtein intake was higher than 1.2 g/kg/day in both groups as soon as 3 months after the start of hemodialysis. Albumin and prealbumin increased significantly during the first 6 months in all patients. Body mass index (BMI) increased in all patients (+0.97 ± 1.31 kg/m2; P < 0.001) reflecting a gain in fat mass in the overall population (+2.36 ± 2.94 kg/m2; P < 0.001) while lean body mass remained stable overall.ConclusionOnce on hemodialysis, SVLPD patients rapidly increased protein intake. Nutritional status improved in all patients, with a gain in fat mass in all, and a gain in lean body mass in SVLPD men only. These data indicate that treatment with a SVLPD prior to hemodialysis initiation is nutritionally safe

Experimental study on curvature effects and preferential diffusion for perturbed laminar premixed ammonia-air flames

Ammonia-air flames are known for low reactivity and have been posing as a huge hindrance in employing the chemical as a sustainable fuel of tomorrow. Curvature is a parameter that could influence the flame structure and so the position of the maximum heat release rate. Flame-acoustic interactions on a Bunsen burner are performed to study the local flame response to highly perturbed flows. NH2* chemiluminescence is used to study the reactivity of these flames. Non-perturbed flames are used as a reference to understand the inherent behaviour of Bunsen ammonia flames. A case study has been chosen for an equivalence ratio ranging between 1.0 and 1.4 at atmospheric conditions to study perturbed flames. The objective is to study the effect of curvature induced by the perturbations on the reactivity of the flame. It was seen that this given case study was quite complex as the flame response was to multiple factors like the effect of Lewis number, convective-diffusion velocities, decomposition of ammonia into hydrogen, thereby, promoting preferential diffusion of hydrogen in both large-scale and locally for certain cases apart from the generated acoustic perturbation which itself dictates the flow regime of the fresh gases, etc. Since the Damköhler number was around 1, the perturbation time scales and the reactivity time scales were comparable and so none of the effects could be ignored. It was concluded that for richer flames where Le>1, the negative curvature promoted the production of hydrogen leading to local enhancement in reactivity. A change in the local thickness due to the induced curvature was seen for all conditions

Effets de l'enrichissement en oxygène sur une flamme turbulente non-prémélangée, méthane-air, stabilisée par un swirl

Ce travail présente les effets de l’enrichissement en oxygène sur le comportement d’une flamme non-prémélangée méthane air dans un brûleur co-axial à swirl. L’étude porte plus particulièrement sur la stabilité de la flamme et les émissions polluantes telles que les NOx, CO2, CO et CH4. Les expériences sont menées dans une chambre de combustion cylindrique de 25 kW refroidie par une circulation d’eau. Le brûleur est constitué de deux tubes concentriques avec un swirl placé dans la partie annulaire afin de mettre en rotation l’oxydant. Le tube central achemine le méthane jusqu’à un injecteur radial qui comporte huit trous uniformément répartis situés juste en dessous du plan de sortie du brûleur. Les mesures des gaz brûlés sont effectuées par des analyseurs multi-gaz à l’aide d’une sonde de prélèvement en sortie de la chambre de combustion. Des expériences de chimiluminescence du radical OH* sont menées pour décrire la structure et la stabilité de la flamme dans les cas air et air enrichi en oxygène. Les hauteurs d’accrochage, les fluctuations de la base de la flamme, et les longueurs de flamme sont déterminées. Les mesures sont effectuées pour une concentration en oxygène qui varie de 0 à 40 % en volume, un nombre de swirl de 0,8 à 1,4 et une richesse globale de 0,7 à 1. Les résultats montrent que l’ajout de l’oxygène à l’air améliorent la stabilité de la flamme en étendant les limites de soufflage. L’augmentation de la concentration en oxygène conduit à une diminution des hauteurs de décrochage et une réduction des fluctuations de la base de la flamme. Les mesures ont montré que l’augmentation du nombre de swirl améliore significativement la stabilité de la flamme. L’analyse des gaz brûlés a révélé que les émissions de CO2 augmentent linéairement avec la concentration en oxygène. Les émissions de CO décroissent exponentiellement tandis que les émissions de NOx, augmentent exponentiellement avec l’enrichissement en oxygène