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The current study focuses on the radiative characteristics of heavy smoking flames that are formatted by oxygen being added to the fuel stream of a steady laminar ethylene diffusion flame. In such a scenario, based on the experimental local soot temperature and volume fraction results, the soot local radiative fields are obtained by solving the Radiative Transfer Equation (RTE). Along with the diluted range (oxygen volume fraction X O 2 a x from 0 to 32%), the flame undergoes two significant transitions, i.e., smoking and non-smoking; however, only one remarkable radiative intensity transition between 30% and 32% is identified. The radiative loss due to the oxygen chemical effect is further assessed by comparing with that of N2 diluted flames. It is found that the chemical effect on radiative loss promotion is undermined by approximately 25%, compared with that on soot formation promotion. In contrast, the oxygen dilution effect results in a similar reduction level on soot formation propensity and soot radiative loss. Furthermore, the correlations between relative soot consumption rate and cumulated radiative loss below the flame tip region are explored, and the results indicate that the initial relative oxidation rate at the position of z F v , max (the peak location of flame cross-section mean soot volume fraction) and its residence time are two important factors that may affect the final flame smoking/non-smoking transition. Eventually, these local soot radiative intensity fields, together with the local soot temperature and volume fraction fields, provide a more comprehensive benchmark to refine the soot oxidation model as the ISF workshop required and understand the flame transition mechanism in the sooting flames.

International audience; Stressors experience early in life by animals may have carry over impacts on life-traits over the life cycle. Accelerated telomere attrition induced by stress during development and growth could play a role in such delayed effects. Among stressors, exposure to chemicals may modify telomere dynamic but, to date, the trends evidenced between exposure and telomere shortening remains inconsistent. Moreover, the role of corticosterone as a possible mediator of chemical impact on telomere is not yet clearly established. Here, we investigated in wild populations of Red kite whether nestling exposure to metals and pesticides was related to corticosterone concentrations in feathers and telomere length measured in 47 individuals. Lead and mercury concentrations in blood ranged from 2.3 to 59.0 mu g L-1 and to 1.4 to 115.7 mu g L-1 respectively, and were below the toxicity thresholds proposed for wildlife. Rodenticides were detected in 30% of the chicks. Corticosterone increased with mercury and lead in interaction, showing a synergistic effect of these 2 non-essential metals on this stress hormone. Telomere length was not linked to metals and/or rodenticide exposure while it was related negatively to corticosterone. The relationship between telomere and corticosterone was modulated by nestling's age, which suggests that the rate of telomere shortening is higher when corticosterone increases. Our findings propose an effect of low exposure of Red Kite nestlings to mercury and lead mixture to raise baseline corticosterone in feathers. The relationships established suggest the hypothesis that telomere attrition could be an indirect consequence of metal exposure mediated by corticosterone.

International audience; This paper addresses the discrimination of dilution and thermal effects on soot production when nitrogen is added to the fuel stream of a steady laminar ethylene diffusion flame. In this context, the mixture-strength flame height Hf and visible flame height Hv are unambiguously documented and exhibit opposite trends within experimental diluted range (N2 volume fraction from 0 to 0.56). Simultaneous soot volume fraction and temperature fields are mapped for different N2 volume fractions by the Modulated Absorption/Emission (MAE) technique. Two characteristic flame heights and the temperature at the peak soot volume fraction are shown to be crucial parameters to assess the extent of the dilution and thermal effects together with their impact on soot formation in the flame. As a result, the mixture-strength flame height is proposed to characterize the soot formation rate within soot growth region, while the visible flame height is more appropriate to describe the overall sooting propensity in the flame. The probed soot temperature is recommended as the characteristic temperature to assess N2 dilution and thermal effects since the thermal effect is overestimated when using the adiabatic temperature.

In this study, quantitative model of two-photon excitation and fluorescence spectra of carbon monoxide based on up-to-date spectroscopic constants collected during an extensive literature survey was developed. This semi-classical model takes into account Hönl–London factors, quenching effects (collisional broadening and shift), ionization and stark effect (broadening and shift), whereas predissociation is neglected. It was specifically developed to first reproduce with a high confidence level the behavior of our experimental spectra obtained from laser-induced fluorescence (LIF) measurements, and then to allow us to extrapolate the fluorescence signal amplitude in other conditions than those used in these experiments. Synthetic two-photon excitation and fluorescence spectra of CO were calculated to predict the fluorescence signal at high pressures and temperatures, which are representative of gas turbine operating conditions. Comparison between experimental and calculated spectra is presented. Influence of temperature on both excitation and fluorescence spectra shapes and amplitudes is well reproduced by the simulated ones. It is then possible to estimate flame temperature from the comparison between experimental and calculated shapes of numerical excitation spectra. Influence of pressure on both excitation and fluorescence spectra was also investigated. Results show that for temperature below 600 K and pressure above 0.1 MPa, the usual Voigt profile is not suitable to reproduce the shape of the excitation spectrum. We found that the Lindholm profile is well suited to reproduce the pressure-dependence of the spectrum in the range 0.1 to 0.5 MPa at 300 K, and 0.1 to 0.7 MPa at 860 K. Beyond 0.7 MPa, in this temperature range, it is shown that the Lindholm profile does no longer match the spectral profiles, in particularly the red wing. Further analyses taking into account the line mixing phenomenon at higher pressure are thus discussed.

Article number: 199; International audience; Aromatic molecules are promising candidates for using as a fluorescent tracer for gas-phase scalar parameter diagnostics in a drastic environment like engines. Along with anisole turning out an excellent temperature tracer by Planar Laser-Induced Fluorescence (PLIF) diagnostics in Rapid Compression Machine (RCM), its fluorescence signal evolution versus pressure and temperature variation in a high-pressure and high-temperature cell have been reported in our recent paper on Applied Phys. B by Tran et al. Parallel to this experimental study, a photophysical model to determine anisole Fluorescence Quantum Yield (FQY) is delivered in this paper. The key to development of the model is the identification of pressure, temperature, and ambient gases, where the FQY is dominated by certain processes of the model (quenching effect, vibrational relaxation, etc.). In addition to optimization of the vibrational relaxation energy cascade coefficient and the collision probability with oxygen, the non-radiative pathways are mainly discussed. The common non-radiative rate (intersystem crossing and internal conversion) is simulated in parametric form as a function of excess vibrational energy, derived from the data acquired at different pressures and temperatures from the literature. A new non-radiative rate, namely, the equivalent Intramolecular Vibrational Redistribution or Randomization (IVR) rate, is proposed to characterize anisole deactivated processes. The new model exhibits satisfactory results which are validated against experimental measurements of fluorescence signal induced at a wavelength of 266 nm in a cell with different bath gases (N2, CO2, Ar and O2), a pressure range from 0.2 to 4 MPa, and a temperature range from 473 to 873 K.

This paper explores the influence of oxygen dilution on soot production when oxygen is added to the fuel stream of a steady laminar ethylene flame established over the Santoro axisymmetric coflow burner. Interestingly, at an oxygen mole fraction ( X O 2 a x ) t r a n s located between 30 and 32%, a transition occurs as the influence of X O 2 a x is suddenly reversed. While the peak mean soot volume fraction increases with increasing X O 2 a x within the range below ( X O 2 a x ) t r a n s , it is reduced with increasing X O 2 a x beyond ( X O 2 a x ) t r a n s . To help understand this transition, soot temperature and volume fraction fields are measured by the two-dimensional Modulated Absorption/Emission technique. To assess the sensitivity of the transition conditions, carbon dioxide is added to the coflowing oxidizer stream. Increasing CO2 mole fraction in the coflow, as replacement of N2 in the air, significantly mitigates soot formation in the flame but does not influence the transitional oxygen concentration ( X O 2 a x ) t r a n s within the resolution of the measurements. Due to the persistence of the transitional oxygen concentration ( X O 2 a x ) t r a n s over a wide range of CO2 replacement of N2 in the coflowing oxidizer stream, it can be considered a distinct characteristic for the assessment of numerical simulations incorporating soot formation and oxidation models. As an original database, the concomitantly measured soot temperature and volume fraction distributions are attached to the present paper as supplemental materials, thus documenting the aforementioned transitions for the whole range of CO2 content of the coflow investigated.

International audience; Two-photon excitation laser-induced fluorescence of carbon monoxide (CO-LIF) is investigated experimentally in order to determine the applicability of this technique for imaging CO concentration in aeronautical combustors. Experiments are carried out in a high temperature, high-pressure test cell, and in a laminar premixed CH4/air flame. Influence of temperature and pressure on CO-LIF spectra intensity and shape is reported. The experimental results show that as pressure increases, the CO-LIF excitation spectrum becomes asymmetric. Additionally, the spectrum strongly shifts to the red with a quadratic dependence of the collisional shift upon pressure, which is different from the classical behavior where the collisional shift is proportional to pressure. Moreover, pressure line broadening cannot be reproduced by a Lorenztian profile in the temperature range investigated here (300–1750 K) and, therefore, an alternative line shape is suggested.; Une étude expérimentale de spectroscopie LIF à deux photons de la molécule CO (CO-LIF) est menée afin d'évaluer sa faisabilité en mode imagerie dans des moteurs aéronautiques. Les expériences sont réalisées d'une part dans une cellule à hautes température et pression et dans une flamme de prémélange CH4/air. Nous présentons l'influence de la température et de la pression sur l'intensité et la forme des spectres LIF de CO. Le spectre d'excitation devient asymétrique quand la pression augmente. Il est fortement décalé vers le rouge avec une dépendance quadratique du décalage collisionnel avec la pression, ce qui diffère de la classique dépendance linéaire. L'élargissement par pression du spectre n'est pas reproduit avec la fonction Lorentzienne et un profil collisionnel de Lindholm est utilisé pour simuler les spectres dans une certaine gamme de pression et températures.

The objective of this study is to develop a quantitative measurement of temperature from fluoranthene fluorescence under conditions with elevated temperatures and pressures. Absorption and fluorescence of fluoranthene are studied in a pressure and temperature range of 0.1–4 MPa and 473–873 K for the two excitation wavelengths of 266 and 355 nm. The influence of thermodynamic parameters such as pressure, temperature and gas composition is characterized in a high-pressure and high-temperature facility. When increasing the pressure, the vibrational relaxation mechanisms are favored, which involves an increase of the fluorescence quantum yield. Concerning the temperature effect, the fluorescence decreases with increasing temperature, this is explained by efficient radiative mechanisms of intersystem crossing. The quenching effect is strongly efficient in air, with a fluorescence evolution described by the Stern–Volmer relation. Over the temperature range studied, the fluorescence quantum yield decreases exponentially by two orders of magnitude. From the temperature dependence of fluoranthene fluorescence in different ambient gases for 266 and 355 nm excitation, the potential temperature measurements in homogeneously and inhomogeneously seeded systems are suggested: the single-color detection technique, the single-excitation two-color detection technique and the dual-excitation wavelength technique.

A certain possible approach for the control of HCCI chemistry is to use kinetic chemistry mechanisms. This opens a field of interest that lead to the composition of a validated reduced PRF chemistry mechanism. For this purpose a skeletal chemical reaction mechanism for n-heptane and for iso-octane are constructed from a detailed n-heptane and iso-octane mechanism of the Chalmers University of Technology. Subsequently these two mechanisms are forged into one reduced chemical reaction mechanism for mixtures of n-heptane and iso-octane (39 species and 47 reactions). This mechanism is numerically validated against the Chalmers mechanisms, respecting the HCCI application range. The reduced mechanism is also successfully numerically validated against another more detailed mechanism provided by LLNL. Engine experiments are performed validating this mixture mechanism with respect to the fuel composition containing n-heptane and iso-octane. The influence of the compression ratio and the equivalence ratio ...

International audience; Advanced combustion processes induced by self-ignition mechanisms in piston engines, such as Homogeneous Charge Compression Ignition (HCCI), especially need an accurate spatio-temporal temperature information because their phenomenology therefore their performances highly depend on the thermal conditions. The objective of this study is to measure the temperature distribution inside a rapid compression machine (RCM) using anisole planar laser induced fluroescence. The present paper gives a new insight into the interpretation of RCM autoignition data. It also brings useful information for kinetics-related combustion research and combustion modeling. The anisole planar laser-induced fluorescence (PLIF) technique is applied, in order to observe the formation and the evolution of temperature heterogeneities in the combustion chamber prior to the auto-ignition process. Anisole is used as a novel tracer species, and the fluorescence signal's dependence on parameters, such as temperature, pressure and bath gas composition, is quantified in a high-pressure, high-temperature facility. Calibration of the fluorescence signal is defined under RCM-related temperature and pressure conditions and a protocol is proposed for post-processing of the PLIF image sequences, allowing the quantitative field temperatures to be determined at successive instants following compression. In order to gain a better understanding of the mixture process, Particle Image Velocimetry (PIV) measurements are analysed under the same conditions. The correlation between thermal and aerodynamic phenomena is determined. The temperature field is found to be non-uniform, with hot and cold centre positions resulting from recirculation inside the chamber, combined with heat transfer effects from the chamber wall.

International audience; Vaporization and oxidation of liquid fuel droplets are basic mechanisms in spray combustion for various industrial applications. In this work, coupledeffects of temperature and pressure on n-alkane droplet vaporization are investigated in experiments conducted in a high-pressure and high-temperature gasification facility equipped with the fiber-suspended droplet technique. The influence of temperature and pressure on n-heptane and n-decane vaporization rates is particularly emphasized with regard to critical conditions of the liquid fuel. Characteristics of n-alkane droplet vaporization, and heating and vaporization times are compared with those of vegetable oil methyl esters—biofuels used in diesel engines. Finally, chemical species formed during the oxidation of these biofuels are identified by gas chromatography in oxidation experiments performed in a jet-stirred reactor. The influence of temperature, pressure, and equivalence ratio on regulated and unregulated pollutants is discussed. Emissions of pollutants obtained during biofuels oxidation are compared with those identified during the oxidation of a synthetic diesel fuel.

International audience; Laser-induced fluorescence of anisole as tracer of isooctane at an excitation wavelength of 266 nm was investigated for conditions relevant to rapid compression machine studies and for more general application of internal combustion engines regarding temperature, pressure, and ambient gas composition. An optically accessible high pressure and high temperature chamber was operated by using different ambient gases (Ar, N2, CO2, air, and gas mixtures). Fluorescence experiments were investigated at a large range of pressure and temperature (0.2–4 MPa and 473–823 K). Anisole fluorescence quantum yield decreases strongly with temperature for every considered ambient gas, due to efficient radiative mechanisms of intersystem crossing. Concerning the pressure effect, the fluorescence signal decreases with increasing pressure, because increasing the collisional rate leads to more important non-radiative collisional relaxation. The quenching effect is strongly efficient in oxygen, with a fluorescence evolution described by Stern–Volmer relation. The dependence of anisole fluorescence versus thermodynamic parameters suggests the use of this tracer for temperature imaging in specific conditions detailed in this paper. The calibration procedure for temperature measurements is established for the single-excitation wavelength and two-color detection technique.

International audience; A detailed description of the vaporization of an isolated droplet has been carried out in this experimental study aimed at investigating ethanol and another aliphatic alcohol, 1-propanol. The characterization of the vaporization phenomenon is necessary for this liquid fuel to develop efficient design of injection systems for propulsion and power generation. Particularly, the vaporization rates and their dependency on temperature, important features for modeling and design, are explored for both ethanol and 1-propanol at intermediate to high temperatures. The experimental setup consists of a pressure chamber in which the furnace, the droplet formation, the droplet support, and motion devices are located. An alcohol droplet is located at the intersection of the cross quartz fiber (diameter of 14 µm) with a controlled initial diameter (range of 300–600 µm). Ambient temperature is varied from 298 to 973 K, at atmospheric pressure. The “quasi-steady” theory has been used to compare and to explain all experimental results. The results show that the d2 law is obeyed and an average vaporization rate is achieved in the case of 1-propanol vaporization. On the other hand, the real impact of the water concentration on the vaporization rate of an ethanol droplet in a large range of temperature is also examined, where two “quasi-steady” periods are observed on the d2 curves, clearly showing that the vaporization of an ethanol droplet is accompanied by the simultaneous condensation of water vapor on the droplet surface and thus the temporal evolution of the droplet squared diameter exhibits an unsteady behavior. The histories of the instantaneous vaporization rates calculated from the d2(t) curves of both 1-propanol and ethanol droplets confirm this phenomenon.

The study of the gasification of a droplet via vaporization, which involves heat, mass and momentum transfer processes in gas and liquid phases, and their coupling at the droplet interface, is necessary for better understanding and modeling of complex spray and mixture formation issues. A detailed description of the vaporization of an isolated droplet has been realized in this experimental study aimed at investigating the impact of the water vapor contained in the surrounding gas on the evaporation of an ethanol droplet. The experimental set-up consists of a heated chamber with a cross quartz fibers configuration as droplet support. An ethanol droplet is located at the intersection of the cross with a controlled initial diameter (300–550μm). Ambient temperature is varied from 350 to 850 K. The real impact of the water concentration on the vaporization rate of an ethanol droplet in a large range of temperature is examined, showing that the vaporization of an ethanol droplet is accompanied by the simultaneous condensation of water vapour on the droplet surface and thus the temporal evolution of the droplet squared diameter exhibits an unsteady behaviour. The histories of the instantaneous vaporisation rates calculated from the d2 (t) curves confirms this non-stationary aspect of the phenomenon.Copyright © 2011 by ASME

International audience; The purpose of this work is to get fundamental knowledge and to understand experimentally the fluorescence phenomenon usually used in laser-based imaging diagnostics. A complete review of thermodynamic effects (pressure, temperature, oxygen and tracer concentration) on biacetyl fluorescence is presented for a wide range of temperature and pressure. Biacetyl fluorescence increases with pressure and then levels off at high pressure when the relaxation is complete. The influence of the temperature is explained by a competition between the intersystem crossing and fluorescence. The effect of oxygen fluorescence quenching is important at high pressure and is discussed.

International audience; To characterise the mechanisms occurring in the deposit formation during the combustion of vegetable oils used as biofuels in Diesel engines, it is necessary to investigate the vaporisation of vegetable oil droplets under various ¯ow, pressure and temperature conditions. In the current work, experimental results about the vaporisation of rapeseed and sun¯ower oil methyl ester droplets at high temperatures are presented. The ®bre-suspended droplet technique is used and the time evolution of droplet diameter during vaporisation is observed by imaging technique. Average and instantaneous vaporisation rates have been estimated from the d 2-curves at temperatures between 473 and 1020 K and at atmospheric pressure. The droplets of vegetable oil methyl esters evaporate like mono-component droplets with a very significant heating phase. A comparison with experimental results obtained with n-alkanes droplets (from n-pentane to n-decane) and prediction of the quasi-steady theory is also presented and discussed.