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Combined Schlieren and Rayleigh Measurements to investigate Radiation Effects on Laminar Flame Speed and Flame Temperature in Methane-Air Spherical Flames
No full textInternational audienceIn combustion process simulations, detailed models consider radiative emission and absorption effects of gases. In order to evaluate the performance of these models, spherical expanding flames (SEF) can be analysed with regard to differences in laminar flame speeds and flame temperature profiles of the burned gases. For this purpose, a combined high-speed Schlieren and time-resolved Laser Rayleigh Scattering (LRS) measurement technique is introduced in this work to simultaneously capture flame propagation speed and time resolved flame temperature profiles, respectively. Spherically expanding stoichiometric methane-air flames were measured in an optically accessible constant volume chamber at ambient temperature and an elevated pressure of 5 bar. A pulsed high-power LED emitting red light was directed through an optical lens system before being captured by a high-speed CCD camera to image the evolution of the flame radius. A focused continuous-wave laser beam and a high-speed CMOS camera were employed to capture Rayleigh scattering images. Spherically expanding flames were simulated for adiabatic and radiation affected cases. Radiation was considered using an optically thin model (OTM) and a fitted statistical narrow-band correlated-k model (SNB). It is demonstrated that the Rayleigh scattering technique can quantitatively resolve temperature profiles in the burned regions of spherically expanding flames. The sensitivity of the LRS results to minor pressure increase during the combustion process is demonstrated. A comparison between the simulated and experimental temperature profiles indicates good agreement in peak temperature values. Notably, a strong correlation in the temperature gradient was observed for the SNB model, while the OTM model overpredicted the temperature decrease in the burned gas. Future work will aim to reduce uncertainty byminimizing background noise and improving the setup for further microgravity applications
Combined Stretch, Radiation, and Low-Temperature Chemistry Effects on Flame Propagation of Nitrogen-Diluted Mixtures
No full textInternational audienceLow-temperature chemistry (LTC) significantly affects ignition processes of various diesel-like fuels. However, the LTC effect on flame propagation is often considered negligible. This assumption is challenged under conditions close to the flame propagation limits. Time scales are increased under these conditions, making the flames more affected by the radiative heat losses. These losses lead to a temperature decrease in the burned gas, decreasing the importance of high-temperature pathways and benefiting LTC pathways. Moreover, radiation effect is known to affect planar and spherically expanding flames differently, which can lead to a difference in LTC behavior. The combined effect of stretch and radiation on LTC has not yet been studied in detail. This numerical study shows theimportance of radiation and stretch effects on combustion dynamics of premixed LTC-fuel flames under nitrogen dilution. Unstretched premixed simulations were conducted for diethoxymethane (DEM) and n-heptane as exemplary diesel-like LTC-fuels. DEM is a bio-hybrid fuel suitable for use in compression-ignition (CI) engines, while n-heptane serves as a well-established reference component in CI mixtures. The simulation results indicate an increased role of LTC with nitrogen dilution. Planar flame simulations have revealed similar numerical convergence limits for DEM and n-heptane once radiation is applied. However, radiation is found to alter temperature and LTC radical distribution across the flame differently, increasing the latter for DEM and decreasing it for n-heptane. Nonlinear LTC radical mole fraction evolution and more pronounced temperature decrease due to radiation effect are observed for transient flames for both fuels. Further analysis of the coupled buoyancy- and radiation-induced changes in the flame structure and their effects on LTC behavior under the studied conditions is required to gainmore insights into the LTC-fuel combustion dynamics
Investigating the role of NH<sub>2</sub>* as an indicator of preferential diffusion effects in premixed NH3/air flames
No full textInternational audienceThe objective of this work is to use an economical diagnostic tool – chemiluminescence of NH2* – to give direct information on the H2 preferential diffusion effects in premixed ammonia-air flames. Abundant quantities of NH2* are produced in ammonia-air flames which makes it an interesting tracer of heat release, as stated in a previous study. Evidence of preferential diffusion was observed using a Bunsen-type burner and attributed to ammonia decomposition into hydrogen leading to preferential diffusion. In this study, new analyses based on additional experiments and simulations are proposed to support and quantify this hypothesis. Mixtures of equivalence ratios 0.9–1.4 have been investigated. With a Bunsen burner, we were unable to stabilize flames leaner than 0.9. The flow velocities required to stabilize these flames are very low (<0.2 m/s) due to the low flame speed of ammonia/air mixtures. Spectroscopic and local chemiluminescence measurements of the flames were calibrated against a quartz-tungsten halogen lamp. The unique spectrum of each equivalence ratio depicting the reactivity of the flame was studied, and the ratio of two excited species: NH2*/OH* was used to provide a rough estimate of the equivalence ratio. This NH2*/OH* ratio was later applied to estimate the local equivalence ratio along the flame contour. Analysis of the NH2* profiles along the flame contour for various cases revealed changes in intensity and profile thickness, indicating the effects of H2 preferential diffusion. Relative lower intensities and thicker profiles of NH2* indicated a low reactivity and vice-versa. Since NH2* can be used as a good indicator, a sub-mechanism to predict these profiles has been proposed. The rate constants of pure collision reactions were determined using the collision theory, and the rate constant of the chemiluminescence reaction was determined experimentally. Other involved reactions and species were identified using a sensitivity analysis on NH2 and NH2*. The proposed sub-mechanism was compared with the experimental profiles, and the impact of the base mechanism has been stated
Entropy Production in General Balance Laws
No full textInternational audienceGiven a general scalar balance law, i.e., in several space dimensions and with flux and source both space and time dependent, we focus on the functional properties of the entropy production. We apply this operator to entropy solutions, to distributional solutions or to merely L ∞ functions. Proving its analytical properties naturally leads to the projective tensor product of C 1 spaces and to further natural extensions to space and time dependent or complex valued "entropies". Besides various qualitative properties, this extended framework allows to obtain new quantitative formulae -also by means of Fourier transforms -that provide different representations of the entropy production. Remarkably, this operator also furnishes a faithful representation of any L ∞ function
Revealed Preferential Short-Range Anion Ordering in Disordered RbM<sub>2</sub>O<sub>5</sub>F (M = Nb, Ta) Pyrochlore-Type Oxyfluorides
No full textInternational audienceDescribing the crystal structure of disordered materials with mixed-occupancy crystallographic sites is essential for understanding their physicochemical properties and designing new materials tuned to targeted functionalities. Here, we investigate the structure of RbM2O5F (M = Nb, Ta) pyrochlore-type oxyfluorides using a multimodal approach that combines experimental and computational techniques. Rietveld structural refinement of PXRD data confirmed that these oxyfluorides are isostructural and their average crystal structure is disordered. The anionic site, 48f, is co-occupied by O and F, while the Rb site, 32e, is occupied at 25%. The shapes of the high-field solid-state 19F MAS, and 87Rb and 93Nb (CT)MAS and 3QMAS NMR spectra, indicate that the local environment of these nuclei is distributed. Using the "supercell" approach, models incorporating different anion arrangements and Rb atoms distributed in their crystallographic site, were built and relaxed using DFT, and NMR parameters for 19F, 87Rb, and 93Nb, were computed using the PAW and GIPAW approaches. The models showing the best agreement between computed and experimental NMR parameters are made up exclusively of [MO5F]6- octahedra, [RbO15F3]32-, [RbO16F2]33-, and [RbO14F4]31- cages, indicating the existence of a preferential short range anion ordering in these pyrochlores, instead of the expected random distribution
Existe-t-il une correspondance entre la priorisation des attentes des patients atteints de troubles musculosquelettiques et les représentations de ces attentes par les kinésithérapeutes ?
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Hormonal and metabolic responses across phases of combined oral contraceptive use and menstrual cycle in young elite female athletes
No full textInternational audiencePurpose Despite the significant number of female athletes using combined oral contraceptives (COCs), there is scant literature on their hormonal and metabolic effects across different phases. Methods In order to contribute to a wider knowledge of COC-action mechanisms involved in athletes’ performance and health, we therefore examined the effects of low-dose monophasic COC (ethinylestradiol/levonorgestrel) intake on sex hormones (estradiol, progesterone, sex hormone binding protein (SHBG)) as well as on a large number of pituitary (LH, TSH, prolactin) and peripheral (triiodothyronine, cortisol, DHEA, DHEA-S, aldosterone, osteocalcin, 25(OH)D) basal hormone levels in nine young elite female athletes, across COC administration (first and second half of active hormone intake, washout phases), compared to eleven female athletes without hormonal contraception across their normal menstrual cycle (NMC, i.e., early follicular, end follicular/peri-ovulatory, mid-luteal phases). Results COC vs. NMC increased SHBG (p < 0.01), TSH, cortisol and 25(OH)D (p < 0.05), and decreased DHEA and DHEA-S (p < 0.05) concentrations. Across COC and NMC phases, higher estradiol and aldosterone concentrations (p < 0.05) were observed during the washout and mid-luteal phases, respectively. Conclusion In highly trained female athletes, COC vs. NMC induced several hormonal alterations, irrespective of the phases, leading to potential ergogenic and clinical repercussions that merit clarification. In NMC athletes, the impact of endogenous sex hormone fluctuations on the parameters studied appeared limited, perhaps mitigated by intense physical training, with only aldosterone change. Given the high prevalence of vitamin D insufficiency, it seems warranted to monitor this parameter, not yet routinely considered in female athletes, taking into account COC intake
Résurrections, le passé entre histoire et fiction à l'époque romantique
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Early Classification of Human Motion Intent for Exoskeleton Assistance Using Kinematics
No full textExoskeletons can reduce physical effort and the risk of work-related musculoskeletal disorders by providing robotic assistance in repetitive tasks. Designing exoskeletons that seamlessly assist human users without disrupting their natural movements poses a significant challenge, necessitating accurate prediction and adaptation to their motion intent. Particularly, an erroneous detection of motion intent could result in large adversarial effects where the exoskeleton resists the user's desired movement. We propose to analyze the possibility of early and accurate intent detection using only kinematic information and quantify the disturbance that a failed intention detection could entail. We evaluate different classification methods with voluntarily ambiguous experimental data on the intention underlying reaching movements towards four targets in a parasagittal plane. We show that, while recent advances in time series classification -namely using a convolutional and residual neural network-can enable earlier and more accurate intent detection, an informed adaptation of the assistance according to the classification's confidence level is necessary
APOBEC cellular enzymes as ‘Court Jesters’ in SARS-CoV-2 evolution
No full textHuman cellular APOBEC enzymes were largely reported as involved in innate antiviral defenses. We previously highlighted that in SARS-CoV-2 genomes obtained in our center, nearly half of ‘hyperfertile’ or ‘fertile’ mutations while 23% of neutral/weakly deleterious mutations had APOBEC signatures. Here we determined that 29% of mutations we named ‘killers’ as detected in quasispecies but not in consensus genomes, have APOBEC signatures. Overall, these results do not suggest that human APOBEC act as a defense agent against SARS-CoV-2 but as ‘Court Jesters’, being neither friends nor foes but only enzymes whose activity can either favor or hamper viral fitness