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International audience; This work studies the influence of phoretic motion (thermophoresis and diffusiophoresis) of fine alumina particles ("smoke") produced during the combustion of aluminum. Direct numerical simulations on a single aluminum droplet burning in a quiescent environment suggest that thermophoresis is the main mechanism driving smoke back to the aluminum surface, hence a major contributor to the oxide lobe development. The presence of this lobe is found to distort the flowfield, which favors hot and smoke-rich regions closer to the lobe, thereby enhancing thermophoresis. This combination of aerodynamic and thermophoretic effects leads to a mass rate of deposited smoke which is consistent with experimental data. A simplified model, deduced from simulation results, is able to predict the size of the final oxide residue in good agreement with measurements. This study supports that aluminum oxide present on the burning aluminum particle is largely due to material formed in the flame, subsequently desposited by thermophoresis.

International audience; The existence of different distinct phases during the aluminum combustion process was unambiguously proved and widely discussed in several studies on single burning aluminum particles. Indeed, in the case of a combustion in diffusion vapor phase, the formation of growing aluminum oxide caps on the droplet surface is suspected to drive the transition from a symmetric to an asymmetric regime. Especially based on diverse observations for aluminum particles burning in O2 /N2 , a hypothesis was proposed on the effect of N2 which probably contributes to the regime transition occurrence. To extend research efforts and get a further understanding of the asymmetric transition development, and, more generally on aluminum combustion, an analysis in O2 /N2 , O2 /Ar and O2 /He oxidizing mixtures is presented. This work is based on an advanced experimental technique allowing to levitate a single particle and visualize its self-sustained combustion with high temporal and spatial resolution. Combustion parameters of ignited micron-sized aluminum particles (30 to 100 µm) at atmospheric pressure were accurately determined using optical diagnostics. Results provide a new quantified dataset and allow to compare the relative effect of N2 , Ar and He as diluent gases in the aluminum combustion process. Burning times are found to be similar, regardless of the considered diluent gas, as well as the aluminum consumption rate in symmetric regime. However, estimations on the relative duration of the symmetric phase demonstrate a potential influence of N2 which promotes the regime transition in comparison with Ar and He for low molar fraction of oxidizer. The ratio between flame and droplet diameters is also modified depending on the inert gas and is determined. Observed trends are discussed and compared to available data to improve the description and quantification of the relative influence of the inert gases.

International audience; Flame speed is extremely important as it affects the performances of many industrial systems. More- over, its significance makes it a major target for the validation of kinetic mechanisms, which explains the necessity to provide ever more accurate data. Flame speed dependence on pressure and temperature conditions is interestingly assessed using, among others, spherically expanding flame in constant volume chambers. In these conditions, the flame speed derivation, based solely on the pressure evolution in the chamber, requires empirical models. The current study describes a perfectly spherical chamber with full optical access allowing simultaneous recording of the pressure inside the chamber and, fully innovative, of the flame radius evolution until the flame vanishes at wall. A careful description of the new set-up and of the accuracy of the measurements, in particular of the flame radius, is presented here. In parallel with experiments, one-dimensional transient simulations were carried out to identify the limits of the proposed new method. Then, the simultaneous use of pressure and flame radius information is compared to the traditional constant volume method based on empirical models. A first advantage relies in the direct detection of the development of instabilities during the flame propagation. In addition, although the flame speed is extremely sensitive to the flame radius determination, the actual experimental accuracy allows significant improvements in terms of accuracy, notably as initial pressure and temperature are elevated. This new set-up will allow major advances in the measurement of laminar flame velocity under extreme thermodynamic conditions.

International audience; The combustion of fossil fuels, which are mostly based on hydrocarbon chains, produces large quanti ties of carbon dioxide which is a major contributor to the global warming. Due to alarming concerns, initiatives are taken to promote the use of ’zero-carbon fuel’. The use of ammonia as an alternative fuel in combustors like gas turbines and spark-ignition engines have already been tested. As high pressures and temperatures are encountered in these combustors, it is essential to find laminar flame speed data for ammonia combustion at these conditions. The present study focuses on performing experiments at elevated conditions for different equivalence ratios in a constant volume spherical chamber. A literature study was performed to select and evaluate the most recent kinetic mechanisms for ammonia combustion under these conditions. A sensitivity analysis highlighting the key reactions for two of the schemes has been performed. A range of exponential factors for temperature and pressure which is used to calculate the flame speeds for a given reference conditions : αand βhave been determined.

International audience; There 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.

Sometimes forest fires have unpredictable and dangerous behaviors for human lives. One of these behaviors leads to accelerating forest fires, characterized by an augmentation of the rate of spread and of the energy released by the fire. The phenomenon mechanisms are not well understood but a thermochemical approach supports that volatile organic compounds coming from heated vegetation and smoke may create a flammable atmosphere near the fire front; according to the literature, α-pinene (C10H16) and benzene (C6H6) were found to be the main compounds. Therefore, we studied α-pinene/benzene/air mixtures by varying the initial temperature from 75°C to 180°C, equivalence ratio from 0.7 to 1.4, and fuel proportion. We focused on two combustion characteristics: laminar burning speeds and Markstein lengths. We observed a maximal laminar burning speed shifted from 1.1 equivalence ratio, when mixtures are rich in α-pinene, to 1.2, when mixtures are rich in benzene. With the increase of benzene percentage, l...

International audience; The 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.

With the different kinds of fuel now available, modern spark ignition engines have to be adapted, owing not only to the difference between the characteristic heat of combustion of the different fuels, but also to the response of the flame to stretch. As the burning rate is a function of the laminar burning speed which is a function of the flame stretch, this parameter has to be taken into account by combustion models. In this study, a zero-dimensional combustion model, based on the Flame Surface Density equation, is enhanced with a model for the stretched laminar burning speed. Numerical results are compared with the experimental results of three lean air-fuel mixtures with isooctane, propane and methane as fuels, that have similar unstretched laminar burning speeds but different Lewis numbers. Laminar burning speed, flame radius, burnt mass fraction, and turbulent flame wrinkling are compared with experimental results. The simulation trends are similar to those of experimental results. Methane and propane (Lewis number 0.99 and 1.80 respectively) show similar wrinkling rates, while isooctane (Lewis number 2.90) has a lower wrinkling rate. The observed difference between computed burnt mass fraction with stretched and unstretched flame speed models reveals that the impact of stretch and Lewis number needs to be taken into account and that a stretched laminar burning speed must be used for modeling, especially for mixtures with a Lewis number much greater than unity.

International audience; A laminar burning velocities database, measured by a shadowgraph method on spherical expanding flames, is still required to improve numerical simulation of kinetic mechanism. Iso-octane/air mixtures at high temperatures (373 and 423 K), high pressure (1, 3, 5, and 10 bar), and high dilution rate were studied over a range of equivalence ratios. CO2, H2O, and exhaust gas recirculation (EGR) were used as diluent over dilution range from 5 to 25 vol%. An explicit correlation giving the laminar burning velocity from the initial pressure, the initial temperature, the dilution rate, and the equivalence ratio is proposed for those flames. Markstein length was also investigated under these conditions. A correlation depending on the same variables was also proposed. Particular attention was given to provide a Markstein length correlation depending on the dilution rate.

International audience; This paper reports experimental studies of cellular flames instabilities in the case of an expanding two-phase spherical flame under microgravity conditions. The presence of liquid fuel droplets leads to the triggering of those instabilities on the flame surface (cracks or cellular) which have an extensive impact on flame behaviour, such as propagation speed and morphology. The main focus is to figure out on the interaction of the droplets with the propagating flame, and specifically on the possible correlation between the aerosol properties (i.e. droplets size and inter-distance) and the flame morphology. A high-speed shadowgraph technique is used along with a specific segmentation post-treatment to obtain a quantitative access to the flame propagation and morphology. In a second time, CH* chemiluminescence is used concurrently with high-speed ILIDS (Interferometric Laser Imaging for Droplet Sizing) or laser tomography for a simultaneous characterization of the flame structure and aerosol properties. By a comparison with equivalent gaseous flames, the impact of the two-phase configuration is assessed. It is shown that it not only triggers the instability earlier, but that it also decreases the minimum size within the cell population. In addition to the minimum and maximum cell sizes, the distribution of cell size is provided, the extrema of which gives a reliable characteristic size of the cell population.

Butenes are intermediates ubiquitously formed by decomposition and oxidation of larger hydrocarbons (e.g. alkanes) or alcohols present in conventional or reformulated fuels. This study provides new complementary data for the oxidation of trans-2-butene. The oxidation of trans-2-butene was studied for measuring stable species concentration profiles during the oxidation of the fuel at atmospheric pressure, over a range of equivalence ratios (0.5 ⩽ φ ⩽ 2), and temperatures (900–1450 K). A combustion bomb apparatus was used to determine laminar flame velocities of trans-2-butene in air at 1 atm, 300 K, and for equivalence ratios of 0.8–1.4. The oxidation of trans-2-butene was simulated under these experimental conditions using an extended detail chemical kinetic reaction mechanism (201 species involved in 1788 reactions). This mechanism is based on a previously proposed scheme for the oxidation of hydrocarbons. Good agreement with experimental data presented in this article was obtained which significantly improves kinetic modeling ability. The structure of a trans-2-butene premixed low pressure flat flame recently published was also successfully modeled. Sensitivity and reaction pathways analyses were performed to get insights into the processes involved in the oxidation of trans-2-butene.

International audience; This study focuses on the response of premixed flames to a transient hydrodynamic perturbation in an intermediate situation between laminar stretched flames and turbulent flames: an axisymmetric vortex interacting with a flame. The reasons motivating this choice are discussed in the framework of turbulent combustion models and flame response to the stretch rate. We experimentally quantify the dependence of the flame kinematic properties (displacement and consumption speeds) to geometrical scalars (stretch rate and curvature) in flames characterized by different effective Lewis numbers. Whilst the displacement speed can be readily measured using particle image velocimetry and tomographic diagnostics, providing a reliable estimate of the consumption speed from experiments remains particularly challenging. In the present work, a method based on a budget of fuel on a well chosen domain is proposed and validated both experimentally and numerically using two-dimensional direct numerical simulations of flame/vortex interactions. It is demonstrated that the Lewis number impact neither the geometrical nor the kinematic features of the flames, these quantities being much more influenced by the vortex intensity. While interacting with the vortex, the flame displacement (at an isotherm close to the leading edge) and consumption speeds are found to increase almost independently of the type of fuel. We show that the total stretch rate is not the only scalar quantity impacting the flame displacement and consumption speeds and that curvature has a significant influence. Experimental data are interpreted in the light of asymptotic theories revealing the existence of two distinct Markstein numbers, one characterizing the dependence of flame speed to curvature, the other to the total stretch rate. This theory appears to be well suited for representing the evolution of the displacement speed with respect to either the total stretch rate, curvature or strain rate. It also explains the limited dependence of the flame displacement speed to Lewis number and the strong correlation with curvature observed in the experiments. An explicit relationship between displacement and consumption speeds is also given, indicating that the fuel consumption rate is likely to be altered by both the total stretch rate and curvature.

International audience; Spray 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.

International audience; The turbulent combustion of three different lean fuel–air mixtures has been investigated in a spherical vessel running at different turbulent intensities. Those mixtures were selected to present almost the same unstretched laminar flame speed but various Markstein lengths and Lewis numbers which are relevant parameters to describe flame-stretch interactions. Mie-scattering tomography has been used to measure a global flame stretch and an equivalent flame propagation speed. Results prove that mixtures have same ranking in terms of flame stretch sensitivity as in laminar regime and show the importance of taking into account this parameter in modeling and to evaluate fuel performance in terms of burning speed in Spark-Ignition engine. Flame wrinkling and curvature measurement were also carried out and the wrinkling effect on the flame propagation has been identified.

International audience; We propose a model for assessing the unresolved wrinkling factor in the large eddy simulation of turbulent premixed combustion. It relies essentially on a power-law dependence of the wrinkling factor on the filter size and an original expression for the ‘active’ corrugating strain rate. The latter is written as the turbulent strain multiplied by an efficiency function that accounts for viscous effects and the kinematic constraint of Peters. This yields functional expressions for the fractal dimension and the inner cut-off length scale, the latter being (i) filter-size independent and (ii) consistent with the Damköhler asymptotic behaviours at both large and small Karlovitz numbers. A new expression for the wrinkling factor that incorporates finite Reynolds number effects is further proposed. Finally, the model is successfully assessed on an experimental filtered database.

International audience; 1-Hexanol is among the promising renewable long-chain alcohols usable as an alternative to petrol-derived gasoline and diesel fuels. To better understand the combustion characteristics of 1-hexanol, new experimental data for the oxidation and combustion of 1-hexanol were obtained. Stable species concentration profiles were measured in a jet stirred reactor (JSR) at 10 atm over a range of equivalence ratios and temperatures. Burning velocities of 1-hexanol/air mixtures were measured at 1−10 bar and 423 K, over a range of equivalence ratios. The effect of total pressure on flame speed was determined. The oxidation of 1-hexanol in these experimental conditions was modeled using an extended detailed chemical kinetic reaction mechanism (2977 reactions involving 600 species). The proposed mechanism shows good agreement with the present experimental data. Reaction path analyses and sensitivity analyses were performed to interpret the results.

International audience; This work presents an experimental study of combustion characteristics of micron-sized aluminum particles in the transition regime under constant volume combustion experiments. Burning velocities were estimated from the measured pressure traces using both a simplified model for aerosol combustion on closed spherical bombs and a semi-empirical correlation, and compared to previous literature. Flame temperatures were measured by bi-color pyrometry and indicate that for particles smaller than 12 m, the flame moves closer to the particle's surface, since flame temperatures were close to aluminum boiling point. For 17:9 µm particles, flame temperatures were close to predicted adiabatic flame temperature and alumina vaporization-dissociation temperature, indicating a classical vapor phase flame under a diffusion-controlled mechanism. However, spectroscopy measurements did not detect significant reductions on molecular AlO emissions for ner particles. This indicates a still very significant presence of vapor phase reaction for powders with a Sauter mean diameter at least as large as 7 µm, which is further supported by the presence of nanometric spheres in the combustion residues, since alumina formed under a vapor phase reaction is expected to condensate into nanometric droplets.

International audience; Significant amounts of unsaturated hydrocarbons, such as butene isomers, are formed as intermediate products during the oxidation of higher hydrocarbons. In this study, new experimental data were obtained for the oxidation of 1-butene and cis-2-butene. The experiments were conducted in a jet-stirred reactor in the temperature range of 900−1440 K, at atmospheric pressure, for different equivalence ratios (0.25 ≤ φ ≤ 2), and in a combustion vessel at p = 1 atm and unburned gas temperatures in the range of 300−450 K. From gas sampled in the jet-stirred reactor, concentration profiles of stable species were measured by gas chromatography and infrared spectrometry. A combustion vessel was used to determine laminar burning velocities of butene−air mixtures at atmospheric pressure and over the equivalence ratio range of 0.8−1.4. Additional data were obtained over a range of pressure (1−5 atm). A detailed chemical kinetic mechanism based on a previously proposed scheme for the oxidation of hydrocarbons was used to reproduce the present experimental data (201 species involved in 1787 reactions). The present mechanism was also tested against literature data: the structure of 1-butene premixed low pressure flat flames and 1-butene/oxygen/argon mixtures ignition delays were simulated, showing satisfactory agreement. Sensitivity analyses and reaction paths analyses were used to rationalize the results. Finally, the oxidations of cis-2-butene and trans-2-butene were compared and discussed.

International audience; Due to its high lubricity, nitromethane is a fuel regularly used in model engine or more generally in race engine. The objective of this study is to improve our knowledge and understanding of the combustion of nitromethane for better evaluating its potential as fuel for automotive spark-ignition engines. To achieve this goal, unstretched laminar burning velocities of nitromethane-air mixtures were measured using spherical propagation methodology at 423 K over a pressure range from 0.5 to 3 bar and equivalence ratios from 0.5 to 1.3. The data indicated a typical adverse effect of pressure on laminar burning velocities. Based on the work done by Zhang et al., Proc. Combust. Inst., 33 (2011) 407-414, a modified detailed kinetic model including 88 species and 701 reactions was proposed. Comparisons between experimental and simulated un-stretched laminar flame speed were made and showed good agreement. The new kinetic mechanism was also used to successfully simulate published experiments and rationalize the unusual occurrence of maximum flame speed in the fuel-lean region.

International audience; In Spark Ignition engines, the heat release rate is not only piloted by the mixture reactivity but also by its sensitivity to stretch effects. Only few results can be found in the literature about flame stretch effect in SI engine configurations.For this study, three different fuels (Methane, Propane, Iso-octane) were studied, but at different air-fuel lean mixture conditions, to present almost equivalent laminar flame speeds and thermo-dynamical properties at ignition timing condition. Besides those mixtures present different Lewis numbers which are relevant parameters to describe flame-stretch interactions. Mie-scattering tomography was then performed in an optical Spark Ignition (S.I.) engine. Using a high speed camera, flame propagation images were acquired through the piston. Thermodynamic analyses based on in-cylinder pressure traces were performed to estimate in-cylinder temperature and burnt mass fraction during the engine cycle.From the determination of flame areas, the global flame stretch and an equivalent propagation speed have been then defined and estimated. Results prove that mixtures have the same ranking in terms of flame stretch sensitivities as in the laminar regime. Probability density functions of flame curvature are centered on 0.05 mm−1 for the three mixtures whereas differences are observed on the global wrinkling. Finally the impact of the Lewis number on the Burned Mass Fraction curved was studied thus showing a linear relationship between crank angle corresponding to 5 and 10 % of burned mass fraction and the Lewis number.

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International audience; A potential implication of Volatile Organic Compounds (VOCs) emitted by vegetal species has been introduced in the literature to explain accelerating forest fires. The main purpose of this paper is to determine the combustion characteristics of a VOC emitted by Rosmarinus officinalis shrubs, namely p-cymene. The emission of this compound is studied for the temperature range 353-475 K and an emission peak is found at 448 K. Laminar burning speeds, Markstein lengths and flame thicknesses are determined using outwardly propagating spherical flames in a combustion chamber at atmospheric pressure. The effects of equivalence ratio (0.8 to 1.4) and unburned gas temperature (358 to 453 K) are studied. A correlation is proposed to estimate laminar burning speeds as a function of equivalence ratio and temperature. Due to the lack of data concerning the combustion characteristics of p-cymene, our results are compared to experimental data of heavy molecules like ethylbenzene, iso-octane, -pinene and to computed ones of JP-10 and n-decane. The computed laminar burning speeds of these last two molecules are obtained using the PREMIX code of the CHEMKIN package

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Several researchers have reported that under certain conditions, forest fires with normal behavior suddenly start to propagate at unusual and very high rate of spread. Over the last decades, these accelerating forest fires were responsible for many fatalities in Europe. A thermochemical approach, based on the ignition of a volatile organic compounds (VOCs) cloud, has been proposed previously to explain this phenomenon. Indeed, some vegetal species emit volatile substances when they are heated. A typical Mediterranean plant, Rosmarinus officinalis, emits 14 components, mainly α-pinene and limonene. The acceleration of the rate of spread can be the consequence of the ignition of these emitted gases. The determination of α-pinene/air and limonene/air premixed flame speeds is essential to take into account this approach in forest fire modeling. It is the main purpose of this article. The spherical expanding flames methodology coupled with a nonlinear model was used to determine the unstretched premixed flame ...

International audience; Several researches have reported that under certain conditions forest fires with normal behavior suddenly start to propagate at unusual and very fast rate of spread. A thermochemical approach, based on the ignition of a Volatile Organic Compounds (VOCs) cloud, has been proposed previously to explain these accelerating forest fires. Indeed, some vegetal species when heated emit volatile substances. We have shown using a flash pyrolysis apparatus that a typical Mediterranean plant, Rosmarinus officinalis, emits eighteen components, mainly α-pinene. Laminar burning speeds and Markstein lengths as well as flame thicknesses of α-pinene/air premixed flames are determined using the spherical expanding flames method. Experiments are carried out in a spherical vessel at atmospheric pressure. The effects of equivalence ratio (0.7-1.4) and unburned gas temperature (353-453 K) are studied. Combustion characteristics are obtained using a nonlinear methodology. A correlation is developed to calculate the laminar burning speeds as a function of equivalence ratio and temperature. The experimental results are compared to the computed ones of JP-10 and n-decane as well as to those found in the literature for these compounds.

To better understand the combustion characteristics of ethyl esters that could become second-generation biofuels, new experimental data were obtained for the combustion of a series of small ethyl esters. Unstretched laminar burning velocities of C4–C7 ethyl ester–air premixed laminar flames were measured in a spherical combustion chamber over a range of pressures (1–10 bar), initial temperatures (323–473 K), and equivalence ratios (0.7–1.5). These experiments were simulated as well as low-pressure flame structures, jet-stirred reactor species profiles, and ignition delays in shock tubes and rapid compression machines. A new detailed chemical kinetic reaction mechanism (1845 reactions versus 232 species) was developed and used in this work. The present model shows good agreement with the present experimental data and those taken from the literature. To interpret the results, sensitivity analyses were performed.

New experimental results were obtained to better assess the effect of ozone on the burning velocity of premixed methane–air flames at atmospheric pressure and room temperature. Ozone was produced using a dielectric barrier discharge device, and its quantity was fixed equal to 5 g/Nm3 in air (2369 ppm of ozone). Measurements were performed using a Bunsen burner. Simultaneous to flame height measurements, a 1D Rayleigh scattering system was set up to investigate the impact of ozone on the thermal flame structure. The experimental results showed that the partial conversion of molecular oxygen into ozone has a moderate positive effect on the burning velocity of methane–air flames, confirming previous measurements in the literature. The injection of 5 g/Nm3 of ozone in air increased the burning velocities by ca. 0.8–1.3 cm/s (ca. 3–8%). The oxidation of methane in the presence or absence of ozone was modeled using a detailed chemical kinetic scheme taken from the literature, to which an ozone submechanism was ...

International audience; New experimental results were obtained to better characterize and understand the oxidation and combustion of 1-propanol, which is a renewable alcohol usable as an alternative to petrol-derived gasoline. A pressurized jet-stirred reactor (JSR) was used to measure concentration profiles of stable species (reactants, intermediates, and final products) at 10 atm, over a range of temperatures (T = 770-1190 K) and equivalence ratios (φ = 0.35-2). A combustion bomb was used to measure burning velocities of 1-propanol/air mixtures at pressures of P = 1-10 bar and T = 423 K, over a range of equivalence ratios (0.7 ≤ φ ≤ 1.4) and at 1 bar for temperatures in the range of 323-473 K. The effects of total pressure and temperature on burning velocity were determined under stoichiometric conditions. The oxidation of 1-propanol under these conditions was modeled using a detailed chemical kinetic scheme taken from the literature and a kinetic scheme of ours was extended to the oxidation of 1-propanol. The computational results agreed reasonably well with the present set of experimental data, but the prediction of some intermediates and the burning velocities of 1-propanol/air mixtures under fuel-rich conditions could only be represented using the mechanism proposed here. Reaction path and sensitivity analyses were used to rationalize the results.

Effects of dilution on premixed methane/air combustion are investigated through experiments and numerical simulations on laminar burning velocities. Experiments are carried out at atmospheric pressure and 393 K; computed results are obtained using the GRI mechanism and the Premix code of the Chemkin package. Laminar burning velocities are determined for several diluents (nitrogen, carbon dioxide, vapor water, a mixture of the three previous gases representative of exhaust gases, helium, and argon) and for different dilution percentages. The equivalence ratio is kept constant equal to stoichiometric. Excellent agreements between experimental data and computed results are obtained. Because heat capacity seems to possess the predominant effect on the laminar flame, an explicit correlation between the heat capacity of the diluent and the laminar burning velocity is proposed.

International audience; 2-Propanol is a renewable alcohol usable as an alternative to petrol-derived gasoline, for which new experimental data for its oxidation and combustion were obtained to better characterize and understand its combustion. Concentration profiles of stable species (reactants, intermediates, and final products) were measured in a pressurized jet-stirred reactor (JSR) over the temperature range of 770−1190 K and for equivalence ratios in the range of 0.35−4. Flame speeds of 2-propanol/air mixtures were measured at 1−10 bar and 423 K, over a range of equivalence ratios (0.7−1.5). The effect of the total pressure on the flame speed was determined for stoichiometric 2-propanol/air flames. The oxidation of 2-propanol in these experimental conditions was modeled using a detailed chemical kinetic reaction mechanism recently proposed for 2-propanol ignition. The kinetic mechanism showed reasonably good agreement with the present experimental data. Reaction path analyses and sensitivity analyses were performed to interpret the results.

International audience; 1-Pentanol is among the possible alcohols usable as an alternative to conventional gasoline and diesel fuels. In order to better understand the combustion characteristics of 1-pentanol, this study presents new experimental data for 1-pentanol in two complementary experimental configurations. Stable species concentration profiles were measured in a jet-stirred reactor (JSR) at 10 atm over a range of equivalence ratios and temperatures. Laminar flame speeds of 1-pentanol-air premixed laminar flame were measured at 1 atm and 423 K for equivalence ratios of 0.7–1.4. The oxidation of 1-pentanol in these experimental configurations was modeled using an extended detailed chemical kinetic reaction mechanism (2099 reactions involving 261 species) derived from a previously proposed scheme for the oxidation of methanol-, ethanol-, and 1-butanol-based fuels. The proposed mechanism shows good agreement with the present experimental data. Reaction path analyses were conducted to interpret the JSR results.

International audience; This paper assesses different butanol–gasoline blends used in a port fuel-injection, spark-ignition engine to quantify the influence of butanol addition on the emission of unburned hydrocarbons, carbon monoxide, and nitrogen oxide. Furthermore, in-cylinder pressure was measured to quantify combustion stability and to compare the ignition delay and fully developed turbulent combustion phases as given by 0%–10% and 10%–90% Mass Fraction Burned (MFB). The main findings are: 1) a 40% butanol/60% gasoline blend by volume (B40) minimizes HC emissions; 2) no significant change in NOx emissions were observed, with the exception of the 80% butanol/20% gasoline blend; 3) the addition of butanolimproves combustion stability as measured by the COV of IMEP; 4) butanol added to gasoline reduces ignition delay (0%–10% MFB); and 5) the specific fuel consumption of B40 blend is within 10% of that of pure gasoline for stoichiometric mixture.

International audience; In all experimental configurations, the flames are affected by stretch (curvature and/or strain rate). To obtain the unstretched flame speed, independent of the experimental configuration, the measured flame speed needs to be corrected. Usually, a linear relationship linking the flame speed to stretch is used. However, this linear relation is the result of several assumptions, which may be incorrected. The present study aims at evaluating the error in the laminar burning speed evaluation induced by using the traditional linear methodology. Experiments were performed in a closed vessel at atmospheric pressure for two different mixtures: methane/air and iso-octane/air. The initial temperatures were respectively 300 K and 400 K for methane and iso-octane. Both methodologies (linear and nonlinear) are applied and results in terms of laminar speed and burned gas Markstein length are compared. Methane and iso-octane were chosen because they present opposite evolutions in their Markstein length when the equivalence ratio is increased. The error induced by the linear methodology is evaluated, taking the nonlinear methodology as the reference. It is observed that the use of the linear methodology starts to induce substantial errors after an equivalence ratio of 1.1 for methane/air mixtures and before an equivalence ratio of 1 for iso-octane/air mixtures. One solution to increase the accuracy of the linear methodology for these critical cases consists in reducing the number of points used in the linear methodology by increasing the initial flame radius

International audience; The impact of dilution on laminar burning speed of two different fuels (methane and isooctane) is studied. In the present study, three different diluents are used--nitrogen, carbon dioxide, and a mixture representative of exhaust gases issued from a stoichiometric combustion of methane. Experimental results and PREMIX computations of the CHEMKIN package, using two different kinetic schemes, are presented and compared with literature results, when available. Initial pressure and temperature conditions are respectively 0.1 MPa and 300 K. For both fuels, a larger decrease of the laminar burning speed is obtained for carbon dioxide dilution than for nitrogen dilution. This observation is directly linked to the increase in heat capacity of the dilution gas but also to the carbon dioxide dissociation, even if the heat capacity effect seems to be predominant.

International audience; In premixed turbulent combustion, reaction rates can be estimated from the flame surface density. This parameter, which measures the mean flame surface area available per unit volume, may be obtained from algebraic expressions or by solving a transport equation. In this study, detailed measurements were performed on a Bunsen-type burner fed with methane/air mixtures in order to determine the local flame surface density experimentally. This burner, located in a high-pressure combustion chamber, allows investigation of turbulent premixed flames under various flow, mixture, and pressure conditions. In the present work, equivalence ratio was varied from 0.6 to 0.8 and pressure from 0.1 to 0.9 MPa. Flame front visualizations by Mie scattering laser tomography are used to obtain experimental data on the instantaneous flame front dynamics. The exact equation given by Pope is used to obtain flame surface density maps for different flame conditions. Some assumptions are made in order to access three-dimensional information from our two-dimensional experiments. Two different methodologies are proposed and tested in term of global mass balance (what enters compared to what is burned). The detailed experimental flame surface data provided for the first time in this work should progressively allow improvement of turbulent premixed flame modeling approaches.

International audience; Planar Rayleigh scattering measurements were used to investigate the inner structure of flamelets in premixed turbulent Bunsen flames. Measurements were performed in a high pressure chamber, where pressure is varied from 0.3 to 0.9 MPa. Two lean equivalence ratios (0.6 and 0.7) of methane=air mixtures were studied. Local information regarding the thickness and the local curvature of the flamelets were obtained. It is observed that an increase of the equivalence ratio induces a decrease of the mean local flamelet thickness. When pressure is increased from 0.3 to 0.9 MPa, the mean flamelet thickness does not decrease as for laminar case. The different external phenomena (turbulence and stretch (curvature and strain rate)) which could explain this trend are investigated.

International audience; The use of hydrogenated fuels shows considerable promise for applications in gas turbines and internal combustion engines. In the present work, the effects of hydrogen addition in methane/air flames are investigated using both a laminar flame propagation facility and a high-pressure turbulent flame facility. The aim of this research is to contribute to the characterization of lean methane/hydrogen/air premixed turbulent flames at high pressures, by studying the flame front geometry, the flame surface density and the instantaneous flame front thermal thickness distributions. The experiments and analyses show that a small amount of hydrogen addition in turbulent premixed methane–air flames introduces changes in both instantaneous and average flame characteristics.

International audience

International audience

As an alternative to second generation ethanol, valeric esters can be produced from lignocellulose through levulinic acid. While some data on these fuels are available, only few experiments have been performed to analyze their combustion characteristics under engine conditions. Using a traditional spark ignition engine converted to mono-cylinder operation, we have investigated the engine performances and emissions of methyl and ethyl valerates. This paper compares the experimental results for pure valeric esters and for blends of 20% of esters in PRF95, with PRF95 as the reference fuel. The esters propagate faster than PRF95 which requires a slight change of ignition timing to optimise the work output. However, both the performances and the emissions are not significantly changed compared to the reference. Accordingly, methyl and ethyl valerate represent very good alternatives as biofuels for SI engines. Future studies will focus on testing these esters in real application engines and performing endurance tests.

International audience; This paper reports experimental studies of two-phase combustion carried out with an aerosol of monosized ethanol droplets generated by rapid expansion under microgravity conditions. Optical diagnostics consisting of coupling laser tomography with the high-speed ILIDS technique are successfully applied for the aerosol combustion study. Droplet diameter reduction together with the radial droplet displacement ahead of the flame front is accurately measured. It is found that fuel droplets evaporate by following the d 2 law which confirms the validity of this model in an aerosol configuration. The laser tomography technique allows the identification of droplets which are present in the burnt gases. A regime diagram has been built using relevant non-dimensional parameters of two-phase combustion to exhibit the observations of droplets crossing. The results demonstrate that the droplet size and the droplet interdistance are the most important parameters which control the possibility for the droplet to penetrate the burnt gases. Moreover, it is observed that typical topologies of flames in terms of size and number of cells are related to the droplet number density.