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International audience; This work experimentally addresses aluminum combustion in steam, pure or mixed with diluents, for aluminum particles in size range 40 ∼80 μm, using an electrodynamic levitator. High-speed videos unveil an unreported and complex mechanism in steam, not observed in other oxidizers. The detached flame is quite faint and very close to the surface. Alumina smoke around the droplet rapidly condenses and coalesces into a large, single orbiting alumina satellite. It eventually collides the main aluminum droplet while generating secondary alumina droplets. A unique feature is the presence of several distinct oxide lobes on the droplet, which merge only at the end of burning and encapsulate the remaining aluminum, possibly promoting an incomplete combustion. The measured burning times in pure water vapor are longer than expected and the efficiency of steam is found to be 30% that of oxygen, lower than the usually accepted value of 60%. A general correlation on burning time, including the major oxidizers, is proposed. Direct numerical simulations are conducted and are in line with experiments, in terms of burning rate or flame stand off ratio. Combustion in steam seems mostly supported by surface reactions, giving a faint flame with low gas temperatures and high hydrogen content. It is speculated that those two specific features could help explain the peculiarity of steam.

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; This work focuses on the diluent influence on stable methane flames in a fused silica micro flow reactor (MFR). Experimental and numerical investigations are performed. The first part presents the method of performing experiments and extracting data from an MFR in order to increase the reliability of the experimental results necessary for subsequent numerical simulations. Those methods are applied to premixed stoichiometric methane flames diluted by nitrogen, argon, helium and xenon. Methods to point out the influence of mixture thermodynamic and transport properties influence is presented. A numerical study of the influence of diluent on stable flames is also undertaken to support the experimental observations difficult to realize in such micro-devices.

International audience; Among all phenomena occurring in combustion chambers, one basic process is the interaction of a single droplet with a flamelet. Therefore, it is essential to understand the physics of individual droplet evaporation and its effects on the flame front to deal with more complex flames.The aim of this study is to investigate the ethanol droplet evaporation interacting with a premixed laminar CH4/air flame. In the experimental part of the study, the droplet was injected into the flame and visualization of the flame front and temporal monitoring of the droplet evaporation were performed using planar laser tomography (PIV, PTV, ILIDS). In the numerical part of the study, single droplet evaporation under constant temperature and stagnant environment was studied via Eulerian-Lagrangian approach with YALES2 solver. The variations of the droplet and gas properties were computed under N2 atmosphere and flame conditions. From the vaporization rate calculations, it is observed that there is a slight effect of the flame stretch on the evaporation due to the change in temperature profiles. It is also observed that the flame temperature has a leading effect on the evaporation rate rather than the burnt gas composition due to the changes in thermo-physical properties.

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International audience; Given the experimental difficulties, most of the available flame speed database is for relatively reduced thermodynamic conditions and for non-simultaneous variations of pressure and temperature. This limitation may be overpassed by using spherically expanding flames with the constant volume method. This methodology, introduced in the 30 s by Lewis and von Elbe, requires the knowledge of the pressure evolution in the combustion chamber. It has been penalized for a long time because of the underlying assumptions and problems in flame instability detection. This method has been greatly renewed recently by Egolfopoulos following a coupled experimental/numerical approach integrating the effects of radiation and dissociation while maintaining moderate computing costs. In parallel with this study, we have worked on an alternative method providing a maximum of information for each test minimizing uncertainties. The current study uses a new experimental device allowing simultaneous recording of pressure and flame radius inside the chamber during the full combustion process. The direct use of these data over the whole flame propagation allows testing kinetic schemes over large pressure and temperature domains with good accuracy. These new experimental targets allowed the identification of key reactions needing improvements.

International audience; The present paper addresses measurement techniques of the most important fundamental property of a combustible mixture, the laminar flame speed. The accurate determination of this parameter has been investigated carefully using two different approaches: the constant pressure method based on the flame front trajectory obtained by means of a Schlieren optical technique, and the constant volume method based only on the pressure-time history recorded during the combustion process. Methane/air flames are selected, because they are relevant for many technical applications and believed to be well understood. Measurements were performed using two high-pressure, high-temperature constant volume vessels at RWTH and ICARE. First, uncertainties stemming from experiments are quantified, and the different sources of errors are identified. Second, the assumption that the unburned gas is compressed isentropically due to the burned gas expansion is verified experimentally for the first time to the authors’ knowledge. More importantly, it was found that combining the two approaches can provide high-fidelity data w.r.t. stretch, radiative heat loss, and instabilities. The data will be very helpful for mechanism development and combustion modeling. Ideally, both approaches are applied simultaneously in the same vessel to obtain a high confidence level in the measured data. Finally, a methodology is proposed to provide flame speeds over a wide range of pressures and temperatures up to 14 bars and 620 K, close to engine-relevant conditions.

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.

Thermodiffusive and hydrodynamic instabilities cause a departure from laminar combustion and, as such, are responsible for flame self-acceleration behavior. Due to the presence of hydrogen, syngas is a fuel prone to instabilities and is frequently diluted with CO2 or co-fired with CH4. This paper, thus presents an experimental study on how syngas mixed with CO2 and/or CH4 influence the onset of both kinds of instability. First, the results show that the laminar flame thickness is the controlling parameter in determining the critical radius at which cellularity appeared when CH4 is added to syngas. Moreover, higher levels of CO2 dilution translated into a constant critical radius, illustrating that the thermodiffusive mechanism is counterbalanced by the hydrodynamic one. To help differentiate between both kinds of instability, it is suggested herein to use the coefficient of self-acceleration or the ratio of the flame speed at the critical flame radius on the laminar flame speed. Finally, a correlation predicting the onset of cellularity is proposed based on the equation format proposed by Jomaas et al. (2007), derived from the stability analysis of a spherically expending flame. The correlation expresses the critical Peclet as a function of hydrodynamic and thermodiffusive instabilities and was successfully validated against experimental data from this study and the literature.

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.

International audience; The successful implementation of innovative fuels relies on many factors and requires a precise description of thecombustion characteristics. In practice, traditional indicators, such as octane or cetane numbers, are used. Obtaining thesenumbers however requires a specific setup. Moreover, they are not sufficient to predict the combustion timing for innovativeconcepts, such as homogeneous charge compression ignition (HCCI). Evaluating and reporting the characteristics of new fuels istherefore challenging. Using a regular diesel engine converted to HCCI operation, we developed a screening methodology toeasily characterize and present key features of new fuels. This paper describes the methodology and then applies it to new fuelsproduced from biomass: valeric biofuels. The methodology presented in this study intends to bridge the work on fundamentalchemical kinetics with conventional engine experiments.

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International audience; The current experimental study aims at analyzing the self-sustained combustion of 30-120 μm Al particles using an electrostatic levitation system. In this improved set-up, a single particle is isolated in air with different ambient pressure (1-31 bar) to be then ignited by a CO2 laser beam. The integrated optical signature of burning Al particle is recorded using filtered photomultipliers and a high-speed camera. The photomultiplier is used to estimate burning time while the camera direct visualization allows to measure the evolution of combustion parameters in real time such as droplet diameter (including initial particle diameter) and flame diameter with important resolution. Important information are also provided to accurately describe combustion phenomenology. Measured burning times according to the pressure are compared to experimental correlations issued from literature and similar trends are observed. The existence of different stages during combustion is clearly evidenced and early assumptions can be made to explain dependence between combustion time and identified phenomena.

This is the post-print version of the following article: "An analysis of the droplet support fiber effect on the evaporation process", which has been published in final form at https://www.sciencedirect.com/science/article/abs/pii/S001793101833196X?via%3Dihub; International audience; This paper presents an analysis of the effect of the droplet support fiber on the droplet evaporation process. This effect is evaluated for a droplet evaporating in a hot environment at atmospheric pressure using the experimental results of the present study and those in the literature. Selected published results are acquired using similar test conditions and experimental setups as the present data. The only main difference between these studies is the droplet support fiber diameter which varies between 14 µm and 225 µm. The ambient temperature explored in these studies ranges from room temperature up to 973K. n-heptane is selected because it is the most common fuel used in these studies. The main findings are that the cross-fiber technique, which uses 14 µm fiber diameters, induces no noticeable heat transfer into the droplet and consequently does not interfere with the evaporation process. In contrast, the classical fiber technique, which uses relatively larger fibers, greatly enhances the droplet evaporation rate as a consequence of increased conduction heat transfer through the fiber. A correlation is proposed to quantify the level of this increase as a function of ambient temperature and the fiber cross-sectional area, 2 .

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.

Periodicity in evolution of premixed methane–air V-shaped flames in the space domain is investigated experimentally. The experiments were performed using the Mie scattering and Particle Image Velocimetry techniques. Three Reynolds numbers of 510, 790, and 1057 along with two fuel–air equivalence ratios of 0.6 and 0.7 were tested in the experiments. The analyses were performed using the Proper Orthogonal Decomposition (POD) technique for the flame front position as well as the velocity data pertaining to non-reacting flow condition. The POD analysis shows that the spectral characteristics of the mode shapes associated with the velocity and the flame front position data feature similarities; however, the corresponding temporal coefficients are significantly different. Specifically, the POD mode shapes pertaining to both velocity and flame front position data feature dominant instabilities. It was shown that the normalized wave number pertaining to these instabilities are similar and equal to the Strouhal number corresponding to non-reacting flow over a circular cylinder. Comparison of the normalized temporal coefficients show that, for the flame front position data, the normalized first and second coefficients are mainly centered close to the origin; however, those associated with the velocity data are positioned around a unity radius circle. This was argued to be linked to the ratio of the corresponding first and second eigenvalues. Specifically, it was shown that, as this ratio approached to unity, the signal energy becomes distributed between the first and the second POD modes. As a result, the normalized temporal coefficients follow a circular pattern.

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; In a context of decreasing pollutant emissions, the transport sector has to tackle improvements to the engine concept as well as fuel diversification. The use of these different fuels often has an impact on the combustion performance itself. In the case of Spark Ignition (SI) engines, efficiency is a function of the combustion speed, i.e. the speed at which the fresh air–fuel mixture is consumed by the flame front. Every expanding flame is subject to flame curvature and strain rate, which both contribute to flame stretch. As each air–fuel mixture responds differently to flame stretch, this paper focuses on understanding the impact of flame stretch on fuel performances in SI engines. Different air–fuel mixtures (different fuels or equivalence ratios) with similar unstretched laminar burning speeds and thermodynamic properties but different responses to stretch were selected. The mixtures were studied in a turbulent spherical vessel and in an optical engine using Mie-Scattering tomography. The combustion phasing was also investigated in both optical and all-metal single cylinder engines. Results show that flame stretch sensitivity properties such as Markstein length and Lewis number, determined in laminar combustion conditions, are relevant parameters that need to be taken into consideration to predict the global performance of fuels, either experimentally or for modeling simulation.

International audience; This 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 thewrinkled 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 kinetic energy dissipation rate that was farther from the flame than the region of maximum vorticity. This region was upstream of positively curved flame segments. The advective time over which the vortex transports flame-generated products to the ignition site corresponded well to the time between the beginning of the flame/vortex interaction and ignition. It therefore is hypothesized that the vortex transports HO 2 and H 2 O 2 to the low-temperature, low-dissipation region wherein ignition is promoted by preferential diffusion of H due to the positively curved flame. Evidence of additional ignition pockets was found upstream of otherflame wrinkles, preferentially near the highest magnitude flame curvatures. These results provide a novel test case for validating diffusion and low-temperature kinetic models, and also have potential implications for reaction rate closure models.

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; High pressure turbulent premixed flames have been significantly studied in the past decade. The eloquent flame images obtained under high pressure conditions attest the large changes in flame front wrinkling enhanced by the pressure increase. Among the explanation attempts, pressure was assumed to promote Darrieus–Landau (DL) instabilities. More recently the role of the laminar flame thickness was also identified as a key parameter for the flame/turbulence interactions. The objective of the current study is to contribute to this debate by isolating the effects of the suspected parameters. To do so, turbulent methane/air flames are investigated in a high pressure combustion test facility. A multi-grid turbulence promoter system has been developed and implemented to obtain a more intense, isotropic and homogeneous turbulence at the burner exit. The pressure range is 0.1–0.4 MPa, and the mixture composition is varied between ER = 0.7 and ER = 1.0. Instantaneous flame images have been collected using Mie scattering tomography and exploited to analyze flame–turbulence interactions under controlled conditions. Flame and turbulence parameters have been independently varied under DL instability free conditions to isolate the effect of the laminar flame thickness and that of the small scale turbulence eddies. The stretching of the turbulence energy spectrum towards smaller turbulent length scales is identified as the main reason for the enhanced flame front wrinkling under high pressure flame conditions, together with the reduction of the laminar flame thickness. The Taylor micro-scale appears to be the regulating turbulence scale for flame–turbulence interactions.

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.

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International audience; In this study, a combined experimental and numerical investigation of a toroidal vortex interacting with a stagnation premixed flame is carried out with the aim of quantifying the ability of such a vortex to stretch the flame. By scrutinizing the literature, it was found that, although inferred from exactly similar numerical simulations, existing parametric expressions for the efficiency function (the ratio of the flame stretch to vortex strain) do not agree in the way the latter should behave when the ratio of the vortex rotational velocity U θ to the laminar flame speed S L is increased. These expressions also appear to be unequally accurate when compared to experimental data and do not feature the non monotonic evolution of the efficiency function with U θ / S L which is observed in both experimental data and numerical simulations of a ‘isothermal’ propagating interface. In addition, whilst previous studies have focused only on the impact of U θ / S L and R v / δL ( R v being the vortex typical size and δL the laminar flame thickness) our study reveals the importance of other parameters, the most important of which being the residence time of the vortex associated with its convection velocity. These results yield a new formulation for the efficiency function which compares favorably well with experimental data.

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.

The comparison of butanol/isooctane blends with an ethanol/isooctane blend was characterized with respect to laminar combustion, by using the spherical expanding flame methodology, in a constant volume vessel. This paper presents the results obtained for blends with alcohol concentrations of 0%, 25%, 50%, 75% and 100% (in volume) and an equivalence ratio ranging from 0.7 to 1.4, at different initial pressures (0.1, 0.3, 0.5 and 1.0 MPa) and an initial temperature of 423 K. The addition of alcohol to isooctane increases the laminar burning velocity, but increasing the initial pressure limits this increase. From burned gas Markstein length estimates, the flame of ethanol/isooctane and air mixture seems to be less sensitive to the total stretch rate and thermo-diffusive instabilities. From experimental data, a correlation with laminar burning velocity is suggested as a function of initial pressure, equivalence ratio and alcohol concentration.

International audience; The main objective of the present study is to explore the degree of interactions between turbulence and the flame front in the context of lean premixed combustion. Premixed methane Bunsen flames are investigated using Mie scattering at three different pressure magnitudes (0.1, 0.2, and 0.3 MPa) and equivalence ratios (0.6, 0.7, 0.8) through six operating conditions at the same mean bulk velocity (3.6 m/s). Nearly homogeneous and isotropic turbulence is generated by a multi-scale grid consisting of three distinct perforated plates. The multi-scale nature of the flame front is characterized using an original method based on Proper Orthogonal Decomposition (POD). Particular emphasis is shed to the scales responsible for the flame brush and the wrinkling of the flame front. The flame front curvilinear length is analyzed as a function of the mode (or equivalently the scale) and yields the well known Richardson plot that is usually observed using fractal analysis. The inner and outer cut-off length-scales, together with the fractal dimension can thus be evaluated using the present method. An analytical expression for unambiguously assessing the latter three parameters is introduced and compares favorably well with experimental data. Second, the reconstruction of the flame using only the first few modes suggests that, the flame brush is characteristic of a rather large-scale phenomena whilst the smallest scales act in decreasing the total flame brush.

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.

Spherically expanding flames are employed to measure flame velocities, from which are derived the corresponding laminar burning velocities at zero stretch rate. Iso-octane/air mixtures at initial temperatures between 323 and 473 K, and pressures between 1 and 10 bar, are studied over an extensive range of equivalence ratios, using a high-speed shadowgraph system. Effects of dilution are investigated with nitrogen and for several dilution percentages (from 5 to 25 vol% N2). Over 270 experimental values have been obtained, providing an exhaustive data base for iso-octane/air combustion. Experimental results are in excellent agreement with recently published experimental data. An explicit correlation giving the laminar burning velocity from the initial pressure, the initial temperature, the dilution rate, and the equivalence ratio is finally proposed. Computed results using the two kinetic schemes and the Cantera code are compared to the present measurements. It is found that the mechanisms yield substantially higher values of laminar flame velocities than the present experimental results. Effects of oxygen enrichment are also investigated. A linear trend relating the percentage of oxygen in air and the unstretched laminar burning velocity is observed. Effects of high pressure, high temperature, and high dilution rate on Markstein lengths are also studied. As already done for the laminar burning velocity, an empirical correlation is proposed to describe the Markstein length for burned gases as a function of initial temperature and pressure, for equivalence ratios between 0.9 and 1.1, which has never been done before in the literature.

Experimental tests were carried out on a single-cylinder port-fuel injection SI engine to quantify the potential of butanol/isooctane blends (with an alcohol concentration of 0%vol, 25%vol, 50%vol, 75%vol and 100%vol) to reduce regulated pollutants (CO, CO2, Nox and THC), and non-regulated pollutants (methane, acetylene, ethylene, benzene, acetaldehyde and formaldehyde), without deteriorating the engine performance. A comparison with results obtained for ethanol/isooctane blends is given. It was observed that the addition of alcohol results in an increase of the fuel consumption of about 30% with butanol and 60% with ethanol, but only a slight increase (about 2%) in CO2 emissions. A strong decrease in HC and Nox emissions was obtained for both alcohols. For the non-regulated pollutants, the addition of ethanol induces a reduction of up to 100% in ethylene emissions, whereas with butanol, a strong increase is observed. The addition of butanol induces a greater decrease, of up to 100%, in methane emissions than ethanol. For benzene emissions, a strong reduction is observed for both alcohols. Moreover, the addition of alcohol engenders a reduction in acetylene emissions and an increase in formaldehyde emissions, but at different levels for both alcohols. Finally, no effects or trends were distinguishable for acetaldehyde and CO emissions.

The potential of butanol as an additive in iso-octane used as gasoline fuel was characterized with respect to laminar combustion, and compared with ethanol. New sets of data of laminar burning velocity are provided by using the spherical expanding flame methodology, in a constant volume vessel. This paper presents the first results obtained for pure fuels (iso-octane, ethanol and butanol) at an initial pressure of 0.1 MPa and a temperature of 400 K, and for an equivalence range from 0.8 to 1.4. New data of laminar burning velocity for three fuel blends containing up to 75% alcohol by liquid volume are also provided. From these new experimental data, a correlation to estimate the laminar burning velocity of any butanol or ethanol blend iso-octane-air mixture is proposed.

International audience; Syngas laminar flame speed measurements have been carried out at atmospheric pressure and ambient temperature using spherically expanding flames. Mixture compositions ranging from 5/95% to 50/50% H2/CO and equivalence ratios from 0.4 to 5.0 have been investigated. Both state-of-the-art linear and non-linear extrapolation methodologies have been tested and extracted laminar flame speeds have been compared to datasets available in the literature as well as computations using two leading kinetic mechanisms for syngas combustion. Syngas flame sensitivities to stretch have been characterized by extracting the corresponding Markstein lengths. In order to explain important disparities found for rich syngas flames among datasets of the literature, computations have been performed to quantify the possible extent of velocity reduction corresponding to small contents of iron pentacarbonyl.

The combined effects of hydrogen addition and nitrogen dilution on laminar flame characteristics for iso-octane–air stoichiometric mixtures were investigated. The spherical expanding flames technique, in a constant volume bomb, was employed. The mole fraction of hydrogen in the iso-octane–hydrogen mixture was varied from 0 to 1 and the mole fraction of nitrogen in the total mixture (iso-octane–hydrogen–air-diluent) from 0 to 0.4. Measurements were performed at an initial pressure of 0.1 MPa and an initial temperature of 300 K. The laminar burning velocity strongly decreases with nitrogen dilution and for all dilution rates increases linearly with hydrogen mass percentage. The burned gas Markstein length is reduced with the increase in hydrogen amount in the mixture until it reaches negative values, the opposite effect of nitrogen dilution. From these new experimental data, an experimental correlation to estimate the laminar burning velocity of iso-octane–hydrogen–air–nitrogen mixtures is proposed. The experimental values of laminar burning velocities are found to be in very good agreement with those predicted by the high temperature mechanism developed by Jerzembeck et al. [1] .

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.

Three different methodologies used for the extraction of laminar information are compared and discussed. Starting from an asymptotic analysis assuming a linear relation between the propagation speed and the stretch acting on the flame front, temporal radius evolutions of spherically expanding laminar flames are postprocessed to obtain laminar burning velocities and Markstein lengths. The first methodology fits the temporal radius evolution with a polynomial function, while the new methodology proposed uses the exact solution of the linear relation linking the flame speed and the stretch as a fit. The last methodology consists in an analytical resolution of the problem. To test the different methodologies, experiments were carried out in a stainless steel combustion chamber with methane/air mixtures at atmospheric pressure and ambient temperature. The equivalence ratio was varied from 0.55 to 1.3. The classical shadowgraph technique was used to detect the reaction zone. The new methodology has proven to be the most robust and provides the most accurate results, while the polynomial methodology induces some errors due to the differentiation process. As original radii are used in the analytical methodology, it is more affected by the experimental radius determination. Finally, laminar burning velocity and Markstein length values determined with the new methodology are compared with results reported in the literature.