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Injecting hydrogen into the natural gas network to reduce CO2 emissions in the EU residential sector is considered a critical element of the zero CO2 emissions target for 2050. Burning natural gas and hydrogen mixtures has potential risks, the main one being the flame flashback phenomenon that could occur in home appliances using premixed laminar burners. In the present study, two-dimensional transient computations of laminar CH4- + air and CH4 + H-2 + air flames are performed with the open-source CFD code OpenFOAM. A finite rate chemistry based solver is used to compute reaction rates and the laminar reacting flow. Starting from a flame stabilized at the rim of a cylindrical tube burner, the inlet bulk velocity of the premixture is gradually reduced to observe flashback. The results of the present work concern the effects of wall temperature and hydrogen addition on the flashback propensity of laminar premixed methane-hydrogen-air flames. Complete sequences of flame dynamics with gradual increases of premixture velocity are investigated. At the flame flashback velocities, strong oscillations at the flame leading edge emerge, causing broken flame symmetry and finally flame flashback. The numerical results reveal that flashback tendency increase with increasing wall temperature and hydrogen addition rate. (C) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Hidrojen teknolojileri araştırmaları; hidrojenin petrol rafinerilerinde, çelik üretimi ve kimyasal üretim gibi sanayi üretiminde ve çeşitli kaynaklarla yakıt elde edilmesinde kullanılması, güç kaynağı olarak sistemi güçlendirmesi, suyun elektrolizi yoluyla elde edilmesi, enerji taşıyıcısı olarak sisteme dahil olması, depolama, taşıma ve ulaşım, havacılık ve uzay, evsel kullanım, risk ve güvenlik sorunları konularının etkileştiği bir araştırma alanıdır. Toplumsal gerçekliğe ve doğanın kendisine içkin olan bu karmaşık yapı, hidrojen teknolojileri araştırmalarının disiplinlerarası bir yaklaşımla ele alınmasını gerekli kılmaktadır. Bu nedenle çalışmanın amacı, hidrojen araştırmalarının disiplinlerarası bu yapısında toplumsal bilimler alanlarının rolünü ve katkısını belirlemektir. Bu amaçla ilgili kavramlar yardımıyla hidrojen teknolojileri araştırmaları alanında 1975-2021 yılları arasında Web of Science (WoS) Sosyal Bilimler Atıf Endeksinde dizinlenmiş 379 makale analiz edilmiştir. Çalışmanın sonuçlarına göre son 10 yılda hidrojen teknolojileri araştırmaları alanında yayın sayısı ve atıf sayısı hızla artmaktadır. Yayınların yarısından fazlasının birden fazla yazarlı olması, bu alanda yazarlar arasındaki etkileşim ve işbirliğinin yoğun olduğunu göstermektedir. Hidrojen teknolojileri alanındaki bu araştırmalar, “Teknik ve Sistemsel Bağlam”, “Enerji taşıyıcı olarak Hidrojen”, “Politik Arkaplan”, “Ulaşım ve Davranışsal Faktörler” ve “Altyapı ve Tasarım” olarak isimlendirilen beş tematik kümede toplanmıştır ve bu kümelerin birleşimini bütüncül bir toplumsal-teknik sistem olarak ele almak mümkündür. Anahtar kelime analizi, zaman geçtikçe alandaki araştırmaların toplumsal boyutlarının ve etkilerinin daha detaylı incelendiğini göstermektedir. Tematik konu haritası ise, hidrojen teknolojileri sisteminin bir bütün olarak tüm boyutlarıyla ele alındığı göstermektedir., Hydrogen technologies research is an area where various research areas interact with each other such as the use of hydrogen in oil refineries, industrial production such as steel production and chemical production and obtaining fuel from various sources, strengthening the system as a power source, obtaining water through electrolysis, inclusion in the system as an energy carrier, storage, transportation, aviation and space, risk and security issues. This complex structure, which is immanent to social reality and nature itself, requires hydrogen technologies research to be handled with an interdisciplinary approach. Therefore, the study aims to determine the role and contribution of social science fields in this interdisciplinary structure of hydrogen technologies research. For this purpose, 379 articles indexed in the Web of Science (WoS) Social Sciences Citation Index between 1975-2021 in the field of hydrogen technologies research were analyzed with the help of related keywords. According to findings, the number of publications and citations in the field of hydrogen technologies research has been increasing rapidly in the last 10 years. The fact that more than half of the publications have more than one author shows that the interaction and cooperation between the co-authors in this field are intense. Studies in the field of hydrogen technologies are gathered in five thematic clusters named as “Technical and Systemic Context”, “Hydrogen as an Energy carrier”, “Political Background”, “Transportation and Behavioral Factors” and “Infrastructure and Design”, combining these clusters into a holistic socio-technical system. Keyword analysis shows that as time passes, the social dimensions and effects of research in the field are examined in more detail. The thematic topic map, on the other hand, shows that the hydrogen technologies system as a whole is handled with all its dimensions.

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.

Hydrothermal carbonization (HTC) of Agave Salmiana bagasse was investigated to assess the potential of hydrochar as an energy resource. The effect of operating conditions on the mass yield and the energy quality of hydrochar was examined by varying the temperature and the holding time over the ranges of 180–250 ℃ (T) and 0–60 min (t), respectively. These parameters were screened using the response surface methodology through a Doehlert design. Performances of HTC were assessed on the basis of the hydrochar mass yield higher heating value ultimate and proximate analyses. In the studied domain, the modeling results indicated that the hydrochar mass yield varies between 45% and 86% and higher heating valuesfrom18 to 23 MJ.kg-1, which is similar to that of peat. In addition, the volatile matter and fixed carbon fractions of hydrochars ranged from 54 to 80% and between 12 and 36%, respectively. Energy yield modeling indicated that the mass yield is the most influent parameter. The maximal energy yield value was obtained at 180 ℃ with time equal to 0 min. Globally, the evolution of H/C ratio is amplified for temperatures greater than 215 ℃ by increasing the holding time from 30 to 60 min. For O/C ratio the maximal variation is below these conditions. It is concluded that increased hydrothermal carbonization conditions favor dehydration reactions, while decarboxylation reactions are favored in mild HTC conditions. The combustion characteristics such as ignition peak and burnout temperatures were significantly modified for hydrochars. The model results indicated that lowest ignition and peak temperature values are obtained at both low-temperature and low-holding time, and high-temperature and high-holding time zones. An increase in the burnout temperature was correlated with the increase of the fixed carbon content in hydrochars.

Computational Fluid Dynamics simulation was used to study hydrothermal carbonization of avocado stone (AS) in a batch stirred reactor, using an open-loop controller system. The corresponding simulations were carried out in COMSOL Multiphysics 5.2. The different biomass-to-water ratio was investigated. The hydrodynamic study shows that the ideal stirring speed to obtain a homogeneous mixture inside the reactor is 550 rpm, due to the high density of AS particles (1547.64 ± 27.33 kg/m3). However, a stagnant zone was observed just below the impeller. To validate the CFD simulations temperature profiles with experimental data, the heat transfer coefficient of the insulator was determined (11.66 W/m2.K), this value was used to set the heat loss in the CFD simulation. According to the model, the difference between the thermal properties of biomass and water under hydrothermal carbonization conditions is negligible. However, experimentally, an increase in temperature was observed with increasing biomass to-water-ratio; this is due to the global hydrothermal carbonization of AS is exothermic reaction. The heat released during 8 h, including heat-up time, was −7.25 ± 0.32 MJ/kg of feedstock. Finally, a kinetic model was proposed taking into account the influence of temperature, heat-up time, reactor volume, and biomass concentration.

International audience; The main aim of this study is to investigate the usability of the process water obtained from the hydrothermal carbonization (HTC) of two different types of biomass waste for microalgae production. In this context, the process waters released as a result of hydrothermal carbonization of olive and orange pomace were diluted in different proportions (50×, 100×, 200×, and 400×) and used in the cultivation of Chlorella minutissima and Botryococcus braunii microalgae. It was observed that the growth rates were higher and the doubling times were shorter at low dilution rates for both microalgae species. The highest growth rate and the shortest doubling time values were found as 0.130 g/L/day and 5.33 day, respectively, for Chlorella minutissima microalgae cultivated in the HTC process water of olive pomace. It was also observed that the process water dilution rate had no specific effect on microalgae biochemical characteristics. It can be concluded that the aqueous phases from HTC of orange and olive pomaces can be a good source of nutrients for different types of microalgae, opening a sustainable, cost effective and environmentally friendly approach for microalgae cultivation and, at the same time, for the cleaning of the otherwise biochemically toxic HTC process waters.

International audience; Five coal samples from different origins are characterized for their pyrolysis and combustion features using coupled thermogravimetry - mass spectrometry (TG-MS). The samples are heated at temperatures ranging from 25 to 900 degrees C in air atmosphere under heating rates of 10, 20, and 30 degrees C/min. The characteristic temperatures (initial, peak, burn out), reaction intervals, mass loss rate, ash residues, ignition and combustion performances, an ignition-combustion index and the reactivities of the coal samples are calculated and the associated heating rate effects are estimated. The less reactive coal exhibits higher peak temperature in conversion and low value of ignition index and reactivity. The reactivity of the coal samples varied in between 2.31 and 7.84 x10(2) (1/min). The main volatile and combustion products (H-2, CH4, H2O, CO, CO2, C6H6, COS, SO2) release for the corresponding TG and temperatures are estimated using online 3D measurements of MS based on their relative intensities and relevancy. Ozawa-Flynn-Wall (OFW) and Kissinger-Akahira-Sunose (KAS) type model free kinetic methods are applied to determine the activation energies of the coals which are exhibited in-between 44.6 - 75.1 kJ/mol and 50.5-96.6 kJ/mol using OFW and KAS methods respectively. The estimated kinetic constants are compared and found consistent with combustion performance parameters.

This study evaluates and compares the responses of two kinds of lignocellulosic biomass (pine and acacia) and sewage sludge to their hydrothermal carbonization treatment. All materials were subjected to HTC processes in an aqueous environment at 200 °C for 4 h with a 1:8 biomass to water ratio, using a Zipperclave Stirred Reactor. The physical, chemical analyses and, additionally, fibre, mercury porosimetry analyses and optical microscope images applied to both raw materials and to hydrochars, proved that HTC enhances the energy related properties of lignocellulosic biomass. Whereas, for sewage sludge, the HTC effect is negligible, because most of the carbon is transferred to the HTC liquid phase, confirmed by high COD and high ash content in the sewage sludge hydrochar. Subsequently, the thermo-chemical conversion kinetics of the raw and HTC materials were examined in the TGA analyser through nonisothermal runs. Finally, kinetics analyses were performed by two methods based on the TGA results. The estimated apparent activation energy values obtained by the Coats & Redfern method were higher than those given by the Arrhenius method, which also gave almost similar values for all samples, shedding doubt on the accuracy of this method for the current experiments. The Coats & Redfern analysis gave activation energy values which were lower by one half for sewage sludge hydrochars compared to lignocellulosic hydrochars. This indicates the potential of the HTC process for generating sewage sludge hydrochars suitable for co-combustion or co-gasification with other biomass solid fuels.

Hydrothermal carbonization (HTC) of dried olive pomace (DOP) was performed in subcritical water under HTC conditions (180–250 °C) to assess the potential of the generated hydrochar to produce energy. The effects of process parameters (holding time, temperature and DOP/water weight ratio) on the yield and the quality of the hydrochar were examined. The mass yield of hydrochar was between 56% and 71% and its HHV increased by more than 23% compared to the HHV of DOP giving a hydrochar similar to a lignite-type coal. The results reveal that a 30 min treatment at 215 °C with a DOP/water weight ratio of 1/6 maximizes the energy yield (83%) of the HTC process. Both the DOP and the hydrochar were characterized by ultimate, proximate and thermogravimetric analyses. The ultimate and thermogravimetric analyses showed that the hydrochar is more carbonaceous and more thermally stable than untreated DOP. The HTC conversion of raw DOP was carried out by dehydration reactions. The proximate analysis showed that hydrochars contained lower ash and volatile matter compared to the raw DOP.

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.

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 .

International audience; A techno-economic feasibility analysis for a 5 MWe scrap tire granulés gasification facility is presented. Electricity generation is based on gas engines with the option of waste heat recovery. Based on our previous work, it is demonstrated that the best thermochemical option for waste tire disposal and energetic valorization is gasification using a mixture of preheated air and steam as the gasification agent. Also based on our previous work, the best gasification technology is shown to be the circulating fluidized bed technology. The study shows that under the regulatory and financial conditions prevalent in Turkey, the project is both technically and economically feasible .

Char particles of high ash Turkish coal of two sizes were produced by pyrolysis in an atmospheric pressure thermo-gravimetric apparatus using 3 heating rates (100 K/min, 500 K/min and 800 K/min). It is observed that the particle size (for particles between 0.8 and 3 mm) did not change the pyrolysis process results. Char gasification rates in CO 2 , steam and mixture of CO 2 + steam are investigated in the same TGA system over the temperature range of 850–950 °C. The pyrolysis heating rate for char formation is observed to have a marked influence on the subsequent gasification reactivity of the char. The results indicate that the char produced from high heating rates show improved gasification rates. Smaller particles exhibit higher char–CO 2 and char–steam gasification rates. Increasing the temperature from 850 to 950 °C, leads to a significant reduction of the time required for 50% char conversion both for small and large particles. The maximum reaction rate is shifted to higher conversion degrees when the chars are produced from high heating rates. The gasification rate of char–H 2 O strongly depends on the H 2 O partial pressure. Gasification rates in CO 2 ambiance are much lower than those in steam. For CO 2 –steam blended ambiance, no obvious CO 2 inhibition effect is observed. But this topic requires more investigation. Kinetic parameters of the char particles are estimated using three different kinetic models. The obtained activation energies for char gasification essentially depend on the particle size and the three kinetic models give very close activation energy values for all the tested conditions.

Mass losses of high ash coal and the mole fractions of evolved species were measured during pyrolysis using a thermo-gravimetric analyzer (TGA) and a mass spectrometric analyzer, respectively. Coal samples of three sizes from Bilaspur, India were pyrolyzed using the TGA in an argon atmosphere under dynamic conditions at atmospheric pressure under the heating rates of 40, 100, 500, and 1000 °C/min until the furnace temperature reached 1000 °C and maintain the temperature to complete the pyrolysis process. The structural properties of the produced char such as pore surface area (BET) obtained by N 2 adsorption method and SEM analysis were performed in order to explain the differences in structure based on char generation method. The BET surface area is increased by more than twofold when the heating rate is raised from 40 °C/min to 1000 °C/min. The kinetic parameters of activation energies ( E ) and Arrhenius constant (A) of thermal pyrolysis for different sized coal particles at various conversion levels were correlated from the thermogravimetric data under four heating rates. The dependencies of kinetic parameters on particle size and conversion levels during coal pyrolysis were estimated. The results revealed that higher pyrolysis rate of coal would greatly increase the yield of light gases such as CO, H 2 when compared to slower pyrolysis process.

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.

The topic of this study is the numerical simulation of a turbulent non-premixed hydrogen flame with different micromixing models in order to investigate their predictive capability. The two micromixing models are compared. The first model is the interaction by exchange with the mean (IEM) model (Dopazo and O'Brien Acta Astronaut 1(9-10):1239, 1974). The second one is the Euclidean minimum spanning tree (EMST) model (Subramaniam and Pope Combust Flame 115(4):487, 1998). The dynamic model for the mixing time-scale is handled, by computing the individual time-scales for the reactive scalars in each cell during the simulation course using the Fluent/MM-INTAS CFD code. The predictions are validated against experimental data provided by Raman and Laser Doppler anemometry measurements for a turbulent jet Hydrogen-air diffusion flame. The interaction of turbulence-chemistry is handled with TPDF method. The chemical model used here consists of 11 chemical species and 23 reactions. Comparisons with experimental data demonstrate that predictions based on the EMST model are slightly better. The EMST improves largely the precision of the results to the detriment of the RAM and the CPU performances. Overall, profile predictions of mixture fraction, flame temperature and major species are in reasonable agreement with experimental data.

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.

Apprehensions relating to global warming, climate change, pollution, rising energy demands as well as fluctuating crude oil prices and supply are leading to a shift in global interest to find suitable alternatives to fossil fuels. This review aims to highlight the many different facets of butanol as an advanced next-generation transportation biofuel. Butanol has fuel properties almost on a par with gasoline, such as high energy content, low vapor pressure, non-hygroscopic nature, less volatility, flexible fuel blends and high octane number. The paper reviews some recent advances in acetone-butanol-ethanol fermentation with special emphasis on the primary challenges encountered in butanol fermentation, including butanol toxicity, solvent intolerance and bacteriophage contamination. The mechanisms for butanol recovery techniques have been covered along with their benefits and limitations. A comprehensive discussion of genetic and metabolic engineering of butanol-producing microorganisms is made for the prospective development of industrially-relevant strains that can overcome the technical challenges involved in efficient butanol production.

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; 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.

The energetic conversion of biomass into syngas or biogas is a more and more important topic. In the framework of these studies, improved understanding of glucose and fructose thermal decomposition and oxidation appears crucial. For this task, thermodynamic data are needed to make possible, for instance, the building of a detailed chemical kinetic model of glucose and fructose reactivity at high temperature. A semitheoretical protocol, presented elsewhere, is used for the estimation of the thermodynamic data of glucose and fructose in the gas phase. Five isomers of glucose and five isomers of fructose are considered and the lowest-energy conformers are found to be β- d -glucopyranose for glucose and β- d -fructopyranose for fructose. The data for all 10 isomers are provided in the CHEMKIN-NASA format.

International audience; An experimental study has been conducted to determine flame propagation velocities in clouds of micro- (4.8 lm) and nano- (187 nm) aluminum particles in air at various concentrations. The experimental results show faster flame propagation in nanoparticle cloud with respect to the case of microparticles.Maximum flame temperature has been measured using a high-resolution spectrometer operating in the visible range. Analysis of combustion residual shows that nanoparticles combustion is realized via the gas-phase mechanism. A three-stage particle combustion model has been proposed based on these observations. Model parameters have been fitted to match the experimental results on the flame velocity and maximum temperature. Particle burning time is estimated from the flame simulations.

In this paper, a method based on the combination of transport equations and PDF (Probability Density Function) calculations is tested through the study of a free axisymmetric turbulent jet and non-premixed flames. This method solves the difficulties related to the closure of the chemical source term. PDF calculations provide the statistical properties of the flow by which the mean values of physical quantities are calculated. In addition, Favre averaged quantities, i.e. temperature, species concentration and velocity fields, can be obtained by using PDF. A comparison has been made with previous experimental and numerical studies in the same field. The model is then applied to the investigation of a diffusive turbulent flame for which experimental data exist. The results obtained show good accuracy for velocities, main species and temperature and are very encouraging for use in combustion simulations.

International audience; An analytical model describing the kinetics of carbon dissolution in burning aluminum droplets has been developed in order to simulate its effects under solid rocket motor conditions. A carbon dissolution rate (k) was introduced in different droplet regression laws and depending on the heterogeneous kinetics between the Al sur-face and the surrounding gases. The model was validated using previous experiments performed by the authors on millimeter-sized Al droplets burning in several CO2-containing atmospheres at atmospheric pressure (P=1atm). It has been shown that the carbon dissolution is affected by the presence of hydrogen due to competition betweenCO and H2 chemisorption. The model was then applied to aluminized propellants (AP/HTPB) at high pressures(P=60 atm) and high temperatures (T=3000 and 3500 K), as well as at various burning rates and adsorption conditions. Though the accuracy of the extrapolation results needs further improvement, it has been shown that the carbon dissolution process should not be neglected in order to achieve global understanding of the combustion of Al particles, particularly agglomerates.

International audience; This study focuses on the geometrical properties of turbulent flame fronts and other interfaces. Toward that end, we use an original tool based on proper orthogonal decomposition (POD), which is applied to the interface spatial coordinates. The focus is mainly on the degree of roughness of the flame front, which is quantified through the scale dependence of its coverage arclength. POD is first validated by comparing with the caliper technique. Fractal characteristics are extracted in an unambiguous fashion using a parametric expression which appears to be impressively well suited for representing Richardson plots. Then it is shown that, for the range of Reynolds numbers investigated here, the scale-by-scale contribution to the arclength does not comply with scale similarity, irrespectively of the type of similarity which is invoked. The finite ratios between large and small scales, referred to as finite Reynolds number effects, are likely to explain this observation. In this context, the Reynolds number that ought to be achieved for a proper inertial range to be discernible, and for scale similarity to be likely to apply, is calculated. Fractal characteristics of flame folding are compared to available predictions. It is confirmed that the inner cutoff satisfactorily correlates with the Kolmogorov scale while the outer cutoff appears to be proportional to the integral length scale. However, the scaling for the fractal dimension is much less obvious. It is argued that much higher Reynolds numbers have to be reached for drawing firm statements about the evolution (or constancy) of the fractal dimension with respect to flame and flow parameters. Finally, a heuristic phenomenology of corrugated interfaces is highlighted. The degree of generality of the latter phenomenology is confirmed by comparing the folding of different interfaces including a turbulent-nonturbulent interface, a liquid jet destabilized by a surrounding air jet, a cavitating flow, and an isoscalar evolving in a turbulent medium. The latter outcome is likely to have strong implications for modeling the corrugation of turbulent interfaces occurring in many physical situations.

The paper explores the feasibility of using ballistic hopper vehicles with CO2/metal rocket engines instead of rovers in Mars missions. The hopper mission scenario involves several ballistic flights and hence visiting several sites on Mars surface. The Martian CO2 is acquired before every takeoff; the liquid CO2 is used directly as oxidiser for powdered magnesium fuel transported from Earth. The mission analysis takes into account the power and time required for CO2 acquisition. The mathematical simulation shows possibilities to fulfil the exploration programme typically assigned for a rover. It is also shown that the CO2/metal propulsion promises much longer hops as compared with engines using conventional bipropellant transported from Earth.

In this paper, the influence of carbon dioxide on the combustion of aluminum droplets is investigated. Millimeter-sized droplets were heated and ignited by a laser in an aerodynamic levitation system in several CO2 containing atmospheres (H2O/CO2, H2O/CO2/N2) with a large range of compositions (wet – xH2O

Accurate initial conditions based on a 2D temporal self-similarity hypothesis are developed for the direct numerical simulation of compressible binary free shear layers with high density ratio. Sample results illustrate effects of density ratio, convective Mach number and free streams temperature on the similarity solution. Direct numerical simulation of a stiff test case (density ratio ∼ 32) shows how the amplitude of early acoustic waves is strongly reduced, with regards to those generated by classical analytical distributions of velocity, temperature and mass fractions. Hence, stable simulations using higher order centered schemes can be achieved for a reasonable number of grid points, even at moderate Reynolds number.

This paper presents an attempt to realize experimental isotropicturbulence at low Reynolds number. For this aim an experimentalapparatus, a turbulence chamber “Box”, was designed and built togenerate a turbulent flow field in the center of the chamber. Theturbulent airflow field was generated by eight electrical fans placedsymmetrically at the eight internal corners of the externally cubicchamber. The turbulence intensity was controlled by the fans speed.Laser Doppler velocimeter (LDV) in single and two-point velocitymeasurements was used to fully characterize the turbulent field insidethe chamber. The main results indicate that the turbulence ishomogeneous and isotropic with a quasi-zero mean velocity within aspherical region of 20 mm radius from the center of the chamber. Themeasured turbulent integral length scale was found to be constant andindependent of the turbulence intensity (or fans speed). Furthermore, anoticeable spectral inertial subrange as prescribed by the Kolmogorovtheory has not been observed at the range of Reynolds number exploredhere, where Reλ < 100. But rather a scaling region characterized by anexponent that is lower than the Kolmogorov value, −5/3, has beenidentified. Moreover, the value of this exponent showed no definedtrend, while the width of the inertial scaling region expands as themicroscale Reynolds number increases.

International audience; Aluminum, in its normal passive oxidized state, does not react with water. In this work, aluminum activation is carried out using liquid metal eutectics Ga-In-Sn-Zn (60:25:10:5). Subsequently, the reaction with water of activated aluminum to produce hydrogen has been examined. The effects of aluminum particle size, liquid eutectics content, and reaction temperature on hydrogen production rates are investigated. The liquid eutectics interaction with aluminum is discussed and the mechanisms of liquid eutectics penetration within the Al particles have been investigated. The specific surface area of the Al particles, the mass ratio of Al to eutectics content and the reaction temperature were found to determine the hydrogen production rate and yield. It is observed that micro-aluminum particles of ~30 μm size display lower reaction rates and hydrogen yields than ~350 μm size particles.

This paper presents a new correlation for mass transfer from single liquid droplets into a turbulent environment. Experiments were carried out under ambient room temperature and pressure. Homogeneous isotropic turbulence with zero-mean velocity was generated by eight identical electrical fans placed on the eight corners of a cubic chamber. The LDV technique was used to characterize the turbulence inside the chamber. The vaporization of fiber suspended droplets of five different n -alkanes and the bi-component droplet of n -heptane and n -decane mixtures subjected to varying turbulent kinetic energy is investigated by imaging techniques. For mono-component droplets the d 2 -law holds for all fuels and turbulent kinetic energies, and the vaporization rates increase with increasing the turbulent kinetic energy. Bi-component droplets exhibit a sequential vaporization behavior for all mixtures and turbulent kinetic energies. The instantaneous vaporization rates increase with increasing turbulence kinetic energy and increasing volume fraction of the highest volatility component. The proposed correlation predicts the vaporization rates of mono and bi-component n -alkane droplets subjected to isotropic turbulence with zero-mean velocity.

Resume Une etude numerique d'un jet rond, libre, turbulent, incompressible et a masse volumique variable est consideree. L'influence de la variation de la masse volumique est etudiee particulierement dans le cas d'un jet d'air turbulent non-isotherme et de melange heterogene. Un modele de turbulence k – e base sur la decomposition de Favre est utilise. L'approche numerique adoptee est celle de Patankar et Spalding [Heat and Mass Transfer in Boundary Layer, Intertext Books, London, 1970]. Les resultats de la simulation numerique sont en accord satisfaisant avec les resultats experimentaux de Chassaing [Melange turbulent de gaz inertes dans un jet de tube libre, These d'etat, INPT, 1979], ceux de Birch et al. [J. Fluid Mech. 88 (3) (1978) 431–449] et ceux de Panchapakesan et Lumley [J. Fluid Mech. 246 (1993) 197–223, 225–247]. Ils montrent que le taux de decroissance axiale de la vitesse, de la temperature et de la fraction massique moyennes augmentent par rapport au cas a densite constante, lorsque la densite du jet est faible par rapport a celle de l'environnement. Ceci confirme que l'efficacite du melange (parametre qui indique la quantite de masse ou de chaleur issue du jet qui se trouve loin de l'axe de symetrie) augmente lorsque le rapport de masse volumique entre le jet debouchant et le milieu exterieur diminue. On montre egalement que les effets de densite sont fortement influences par la gravite dans la zone de similitude.

International audience; Application of oxygen-enhanced combustion to existing fossil fuel energy systems to facilitate CO2 capture presents several challenges. This work investigates the combustion characteristics of methane oxygen enriched air turbulent non-premixed swirling flames. It focuses on the stability of flames, NOx, CO2 and CO emissions and the flow field dynamics. The burner configuration consists of two concentric tubes with a swirler placed in the annular part for the oxydant. The experiments are conducted using a 25 kW water cooled combustion chamber. The exhaust gas compositions are measured using gas analyzers. OH chemiluminescence experiments are conducted to investigate the structure and the stability of the flames without and with oxygen enrichment. Flame liftoff heights, fluctuations of the flame base and flame lengths are determined. Particle Image Velocimetry is used to analyze the dynamics of swirling flows. The measurements are performed for oxygen concentrations ranging from 21% to 30% by volume, with swirl numbers from 0.8 to 1.4 and global equivalence ratios from 0.8 to 1. The results show that the addition of oxygen to air, while keeping the oxidant flow rate constant, enhances the combustion efficiency and flame stability. It is observed that increasing oxygen concentration leads to lower lift-off heights and reduces flame height fluctuations. Increasing the swirl number significantly improves the flame stability. The results demonstrate that the CO2 emissions in the exhaust gases linearly increase with increasing O2 content in the oxidant. The CO emissions are shown to decay exponentially, whereas the NOx emissions, mainly produced through the thermal pathway, increase strongly with oxygen enrichment. The PIV results illustrate that increasing the swirl intensity increases the reverse flow velocities close to the burner exit. The decay of axial velocity presents favorable flow patterns for the stabilization of the flame.

This study concerns the regime of distributed reaction zones of a well-stirred reactor. No mention has been made until now on the fields of the interaction between combustion and turbulence in this regime, where the Damkohler number is below unity. The aim of this paper is to investigate the fluctuating temperature structures inside the well stirred-reactor for various mean pressure situations. The experimental study is conducted by the technique of fine-wire thermoanemometry. In parallel, the fuel conversion rates of propane oxidation in all the experimental conditions are determined from a simulation work which takes into account a detailed kinetic model. The different relevant characteristics of the dynamic field are estimated by means of the k - e model. The variation of the unmixedness and the time scales of the temperature field is compared for various pressures in the reacting and non-reacting cases. The discussion is conducted in terms of the ratio between the time scales of the scalar and dynamic fields. The main conclusions are as follows. When the reacting and non-reacting temperature fields are compared at the same mean pressure, the effect of the chemical reactions appears to be the feeding of more energy into the small scale turbulence structures. It is important to notice that the systematic decrease of time and length scales with the mean pressure in the reactor, together with a very significant increase of the dissipation rate of the temperature fluctuations, gives evidence for a positive effect of the increase of the pressure on the micromixing.

International audience; A Franco-Japanese cooperative research program on microgravity combustion has been initiated. One of the studies is devoted to the experimental investigation of high pressure combustion of methanol droplets, under normal and reduced gravity conditions, which are obtained in a drop tower and during the parabolic flight of an airplane. A methanol droplet is suspended at the center of a high-pressure combustion chamber, and is ignited with an electrically heated kanthal wire. A high-speed video camera is used to obtain the time histories of the squared droplet diameter, which are used to determine the droplet burning rate. The results show that the d-square law is valid for all the conditions of the present experiments. The droplet burning rate increases with ambient pressure and does not show a maximum around the critical pressure of methanol. Experiments also show that the droplet burning rate decreases with the reduction of gravitational acceleration.

A method for measuring the burning rate integral BT in turbulent premixed flames and data obtained from several V-flames via the method are reported. BT is determined by integration over the flame brush of data for the mass fluxes normal and tangent to c¯ constant surfaces. The determination of both convective and turbulent mass fluxes is made by simultaneous two-component velocity (axial and lateral) and temperature measurements using a combination of laser Doppler velocimetry (LDV) and compensated, finewire thermocouples, where the instantaneous gas density is obtained from the instantaneous temperature through the equation of state and the assumption of constant pressure. BT data for turbulent premixed CH4-air flames, with equivalence ratios between 0.7 and 0.75, mean axial flow velocities between 2 and 6.37 m/s, and ratios of turbulence intensities to laminar burning velocities between 0.643 and 3.16, are reported. The data demonstrate the utility of simultaneous velocity and temperature measurements for determining the burning rate integral for premixed flames in laboratory burners. They further show (1) that conditional velocity data alone cannot be used with confidence to estimate mass fluxes in the flamelet regime, (2) that the use of velocity data from the reactant side of the turbulent flame brush to estimate BT (or the turbulent burning velocity) results in estimates that are quite high, and (3) that BT is a strong function of u′/SL.

The potential of using the laminar flamelet model for predictions of NOx emissions from turbulent hydrogen jet diffusion flames with various amounts of helium dilution is explored in this study. The flamelet approach treats turbulent flames as an ensemble of laminar flames subject to local fluid dynamic stretch which causes chemical kinetics to deviate from equilibrium. This flame stretch rate can be described as the scalar dissipation rate or as the strain rate. Numerical modeling is performed to assess the merits of these two choices by comparing predicted scaling behavior of NOx emission index with the experimental data of Driscoll and co-workers [1, 2]. The present study reveals that only with the scalar dissipation rate as the nonequilibrium parameter will the predicted NOx emission indices exhibit a clear scaling relation vs the Damkohler number. The flamelet predictions are found to improve when the Damkohler number increases. However, the absolute NOx levels are overpredicted, which is attributed to differential diffusion effects.

The standard κ-e model and a second-order Reynolds stress model (RSM) are used to investigate variable density effects in axisymmetric turbulent jets. Without buoyancy, both models predict no effect of the varying density on the far field turbulence parameters and the classical effective diameter concept works for the decay rates. No conclusive disagreement with experimental data is observed. Effects of turbulence production due to buoyancy are found to be small compared to the effect of the mean buoyancy term in the momentum equation. However, this turbulence production has a large influence on the axial scalar flux, for which the experimental trend is predicted with the second-order model but not at all with the k-e model.

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Gökalp Iskender. Contribution à l’étude de la structure et de la dynamique du réseau ferré ottoman, 1850-1960. In: Anatolia moderna - Yeni anadolu, Tome 5, 1994. pp. 141-154.

International audience; The d2-law validity during n-decane droplet vaporization in microgravity environment is examined experimentally. Two sets of experiments are performed, under normal and microgravity, in stagnant hot atmospheric environment. The environment temperature is varied in the range up to 967 K. The droplet is suspended onto the cross point of two micro-fibers of 14 lm in diameter. This technique enables to greatly minimize the effect of fiber on droplet heat and mass transfer. The results show that, for ambient temperatures below approximately 950 K, departure from the d2-law is observed during droplet vaporization in microgravity environment. In addition, the droplet lifetime is longer in microgravity than in normal gravity under the same ambient test conditions. However, for temperatures exceeding approximately 950 K, the experimental results demonstrate that the d2-law holds throughout the entire droplet lifetime, and the mass transfer rate is identical in both microgravity and normal gravity environments.

International audience; Nowadays, numerous projects are focussed on finding non-polluting energy sources. Interest in the use of biomass for energy purposes has been also revived in this context. Among others, agricultural biomass waste is one of the most promising resources. Its conversion into biofuel for further application via combustion increases its economic interest despite the initial low calorific value. In the present study, the conversion process is carried out in supercritical water (SCW), thus improving significantly the energy balance by eliminating the need for drying the biomass. The SCW dissolution, hydrolysis and oxidation of pure substances in a hydrothermal diamond anvil cell (HDAC) have been studied previously and unique results concerning the homogeneous dissolution of naphthalene, cellulose and glucose were acquired. The current investigation was performed for samples representing real biomass (winery residues) rather than pure substances. Visualizations of the SCW biomass gasification process were obtained and the products of the decomposition were also analyzed. It is expected that this work will contribute in valorizing winery waste by decreasing the cost required for its elimination and will also contribute significantly to green house gases mitigation.

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.

An experimental study on CH 4 –CO 2 –air flames at various pressures is conducted by using both laminar and turbulent Bunsen flame configurations. The aim of this research is to contribute to the characterization of fuel lean methane/carbon dioxide/air premixed laminar and turbulent flames at different pressures, by studying laminar and turbulent flame propagation velocities, the flame surface density and the instantaneous flame front wrinkling parameters. PREMIX computations and experimental results indicate a decrease of the laminar flame propagation velocities with increasing CO 2 dilution rate. Instantaneous flame images are obtained by Mie scattering tomography. The image analysis shows that although the height of the turbulent flame increases with the CO 2 addition rate, the flame structure is quite similar. This implies that the flame wrinkling parameters and flame surface density are indifferent to the CO 2 addition. However, the pressure increase has a drastic effect on both parameters. This is also confirmed by a fractal analysis of instantaneous images. It is also observed that the combustion intensity S T / S L increases both with pressure and the CO 2 rate. Finally, the mean fuel consumption rate decreases with the CO 2 addition rate but increases with the pressure.