Auswahl der wissenschaftlichen Literatur zum Thema „Expanding turbulent flames“

In this paper we study the essential role of Darrieus–Landau (DL), hydrodynamic, cellular flame-front instability in the propagation of expanding turbulent flames. First, we analyse and compare the characteristic time scales of flame wrinkling under the simultaneous actions of DL instability and turbulent eddies, based on which three turbulent flame propagation regimes are identified, namely, instability dominated, instability–turbulence interaction and turbulence dominated regimes. We then perform experiments over an extensive range of conditions, including high pressures, to promote and manipulate the DL instability. The results clearly demonstrate the increase in the acceleration exponent of the turbulent flame propagation as these three regimes are traversed from the weakest to the strongest, which are respectively similar to those of the laminar cellularly unstable flame and the turbulent flame without flame-front instability, and thus validating the scaling analysis. Finally, based on the scaling analysis and the experimental results, we propose a modification of the conventional turbulent flame regime diagram to account for the effects of DL instability.

In the present study, the effect of hydrogen addition on turbulent flame propagation characteristics is investigated in a fan-stirred combustion chamber. The turbulent burning velocities of methane/hydrogen mixture are determined over a wide range of hydrogen fractions, and four classical unified scaling models (the Zimont model, Gulder model, Schmidt model, and Peters model) are evaluated by the experimental data. The acceleration onset, cellular structure, and acceleration exponent of turbulent expanding flames are determined, and an empirical model of turbulent flame acceleration is proposed. The results indicate that turbulent burning velocity increases nonlinearly with the hydrogen addition, which is similar to that of laminar burning velocity. Turbulent flame acceleration weakens with the hydrogen addition, which is different from that of laminar flame acceleration. Turbulent flame acceleration is dominated by turbulent stretch, and flame intrinsic instability is negligible. Turbulent stretch reduces with hydrogen addition, because the interaction duration between turbulent vortexes and flamelets is shortened. The relative data and conclusions can provide useful reference for the model optimization and risk assessment of hydrogen-enriched gas explosion.

In this study, 3D premixed turbulent ammonia-hydrogen flames in air were studied using DNS. Mixtures with 75%, 50% and 25% ammonia (by mole fraction in the fuel mixture) and equivalence ratios of 0.8, 1.0 and 1.2 were studied. The studies were conducted in a decaying turbulence field with an initial Karlowitz number of 10. The flame structure and the influence of ammonia and the equivalence ratio were first studied. It was observed that the increase in equivalence ratio smoothened out the small scale wrinkles while leading to strongly curved leading edges. Increasing the amount of hydrogen in the fuel mixtures also led to increasingly distorted flames. These effects are attributed to local increases in the equivalence ratio due to the preferential diffusion effects of hydrogen. The effects of curvature on the flame chemistry were studied by looking at fuel consumption rates and key reactions. It was observed that the highly mobile H2 and H species were responsible for differential rates of fuel consumption in the positively curved and negatively curved regions of the flame. The indication of a critical amount of hydrogen in the fuel mixture was observed, after which the trends of reactions involving H radical reactions were flipped with respect to the sign of the curvature. This also has implications on NO formation. Finally, the spatial profiles of heat release and temperature for 50% hydrogen were studied, which showed that the flame brush of the lean case increases in width and that the flame propagation is slow for stoichiometric and rich cases attributed to suppression of flame chemistry due to preferential diffusion effects.

En vue d'atteindre la neutralité carbone d'ici 2050, l’Union européenne envisage l'hydrogène comme un vecteur énergétique prometteur afin de réduire la consommation des ressources fossiles. Alors que les piles à combustible et les véhicules électriques occupent déjà une place importante dans la décarbonation du secteur des transports, l'hydrogène est également considéré comme une alternative aux carburants conventionnels pour les véhicules lourds. Toutefois, de nombreux obstacles liés aux propriétés physico-chimiques ainsi qu’à la combustion pauvre en hydrogène sont encore à l’étude : apparition de phénomènes de combustion anormale, production d'oxydes d'azote, instabilités dues aux effets thermodiffusifs. Cette thèse contribue à la compréhension du processus d’auto-inflammation des mélanges pauvres hydrogène/air ainsi qu’au phénomène de propagation des flammes prémélangées laminaires et turbulentes. Dans une première partie, des mesures de délais d’auto-inflammation hydrogène/air et hydrogène/air/oxydes d’azote sont réalisées à l’aide d’une machine à compression rapide afin de revisiter et valider un mécanisme cinétique dans des conditions similaires à celles rencontrées dans un moteur à allumage commandé. Dans une deuxième partie, des flammes laminaires prémélangées en expansion sont étudiées au sein d’une chambre de combustion sphérique à volume constant, en faisant varier la température initiale ainsi que la dilution à la vapeur d’eau et en considérant les instabilités intrinsèques liées aux propriétés physico-chimiques de l’hydrogène : instabilités thermodiffusives, hydrodynamiques et liées à la pesanteur. Dans une dernière partie,des flammes turbulentes prémélangées en expansion sont caractérisées par génération d’une turbulence homogène et isotrope au sein d’une chambre sphérique. Une étude paramétrique est réalisée par rapport à un cas de référence en faisant varier l’intensité turbulente, la pression initiale et la richesse. Finalement, une corrélation turbulente est proposée afin de décrire la propagation de ces flammes et en vue d’être utilisée dans des modèles numériques
In order to reach carbon neutrality by 2050, the European Union is considering hydrogen as a promising energy carrier to reduce reliance on fossil fuels. While fuel cells and electric vehicles already play an important role in decarbonizing the transport sector, hydrogen is also seen as an alternative to conventional fuels for heavy-duty vehicles. Yet, a number of challenges linked to the physico-chemical properties of lean hydrogen combustion are still under investigation: abnormal combustion phenomena, production of nitrogen oxides,instabilities due to thermodiffusive effects, to state a few. This thesis contributes to the understanding of the auto-ignition process in lean hydrogen/air mixtures, as well as the propagation of laminar and turbulent premixed flames. First, measurements of hydrogen/air and hydrogen/air/nitrogen oxides ignition delay times are carried out using a rapid compression machine, to update and validate a kinetic mechanism under spark ignition engine-like conditions. Second, outwardly propagating spherical premixed laminar flames were studiedin a constant-volume combustion chamber, varying the initial temperature and steam dilution, and considering the intrinsic instabilities linked to the physico-chemical properties of hydrogen namely thermodiffusive,hydrodynamic and gravity-related instabilities. Then, expanding premixed turbulent flames are characterized by the generation of a homogeneous and isotropic turbulence zone within a spherical chamber. A parametric study is conducted by varying turbulent intensity, initial pressure and equivalence ratio. Finally, a turbulent correlation is proposed to describe the turbulent propagation of such flames, for use in numerical models

Le moteur downsizé à allumage commandé constitue l'une des voies principales explorées par les constructeurs automobiles pour améliorer le rendement et réduire les émissions de dioxyde de carbone des motorisations essence. Il s'agit de combiner une réduction de la cylindrée avec une forte suralimentation afin d'améliorer le rendement du moteur, en particulier à faibles et moyennes charges. Leur mise au point est limitée par l'augmentation des combustions anormales, dont le contrôle par forte dilution peut également entraîner l'apparition de variabilités cycliques importantes. Actuellement, la compréhension des nombreux paramètres intervenant dans l'apparition de ces phénomènes et de leurs interactions, reste encore imparfaite. Dans ce contexte, l'objectif de ce travail est de contribuer à la compréhension des mécanismes impliqués dans les processus de propagation des flammes turbulentes. Cette étude est réalisée dans une enceinte de combustion sphérique haute pression haute température, équipée de ventilateurs générant une turbulence homogène et isotrope. La première partie de ce travail est consacrée à l'étude de la combustion prémélangée laminaire isooctane/air. Dans un deuxième temps, l'aérodynamique de l'écoulement dans l'enceinte est finement caractérisée par Vélocimétrie Laser Doppler et Vélocimétrie par Images de Particules. Enfin, la propagation des flammes turbulentes est étudiée en termes de vitesse à partir de visualisations par ombroscopie. Une loi unifiée, permettant de décrire la propagation des flammes turbulentes indépendamment des conditions thermodynamiques initiales, de l'intensité de la turbulence et de la nature du mélange réactif est notamment proposée.

Jusqu’à aujourd’hui, les moteurs automobiles et aéronautiques étaient conçus pour fonctionner avec des combustibles fossiles. Pour relever les défis économiques et environnementaux du monde actuel et de la transition énergétique, des carburants de substitution sont développés et testés. Ils sont utilisés pour remplacer directement les carburants classiques ou bien sous forme de mélanges pour obtenir les propriétés thermochimiques souhaitées. Cependant, l’impact de ces nouveaux carburants sur les performances des chambres de combustion reste partiellement connu. Dans cette perspective, des simulations haute-fidélité de la combustion turbulente de carburants de substitution ne peuvent être réalisées qu’à la condition de prendre en compte une description détaillée multi-composants des mélanges de liquides et de gaz. L’objectif de cette thèse est de contribuer à la modélisation instationnaire des flammes de spray en présence de phénomènes complexes multi-composants : évaporation différentielle, mélange multi-espèces, nombreuses réactions chimiques en phase gazeuse. À cet effet, le carburant est traité comme un ensemble de mélanges à plusieurs composants, qui peuvent être différents dans les phases liquide et gazeuse en fonction de la précision requise. Différents modèles pour les phénomènes susmentionnés sont disponibles dans la littérature. Le principal défi est le couplage de ces différentes approches et leur validation dans des conditions réalistes et complexes. Premièrement, l’approche multi-composants choisie pour la phase gazeuse, basée sur le transport d’un grand nombre d’espèces et sur la chimie de type Arrhénius, est validée pour les flammes prémélangées. La configuration de flamme sphérique en expansion a été choisie pour étudier la vitesse de consommation de la flamme, paramètre important de la combustion. En collaboration avec l’équipe expérimentale du laboratoire CORIA, un formalisme pour l’évaluation précise de la vitesse de consommation de flamme est établi pour les flammes en expansion sphérique confinées et non confinées. Ce formalisme permet d’avoir une comparaison précise des résultats expérimentaux et numériques pour les flammes méthane / air et iso-octane / air et de valider ainsi le modèle de transport en phase gazeuse. Deuxièmement, nous nous sommes concentrés sur le processus physique d’évaporation. Le modèle d’évaporation multi-composants d’Abramzon-Sirignano est implémenté dans le solveur fluide YALES2 en se basant sur une approche de point-force pour les gouttelettes de carburant. Ce modèle a été adapté pour permettre de décrire l’évaporation d’un ou plusieurs composants avec ou sans évaporation différentielle. En tant que tel, le modèle est capable de traiter divers surrogates de carburant. Ce modèle d’évaporation est comparé au modèle de Spalding et validé sur les résultats expérimentaux de Chauveau et al. [33], Nomura et al. [158], Ghassemi et al. [82] et Daïf et al. [47] pour une gouttelette à un seul composant, puis une gouttelette isolée à deux composants avec et sans convection. Enfin, la simulation aux grandes échelles (LES) d’une flamme de spray complexe n-heptane/air est réalisée avec une chimie analytiquement réduite (ARC, [169, 205]). Cette flamme a été étudiée expérimentalement au laboratoire CORIA avec des diagnostics haute-fidélité pour caractériser la structure de la flamme et fournir des données quantitatives telles que la vitesse et la température en phase gazeuse ainsi que les distributions locales de taille et de vitesse des gouttelettes de carburant. La comparaison avec les données expérimentales [225] et avec les simulations réalisées dans le cadre du 6ème atelier sur la combustion turbulente de spray (TCS6), montre que les LES actuelles reproduisent fidèlement l’écoulement gazeux et les propriétés de la phase dispersée. Cette configuration ouvre la voie à la simulation de flammes de spray encore plus complexes avec des combustibles à plusieurs composants
Until recently, automotive and aeronautical engines were designed to operate with fossil fuels. To better meet the economic and environmental challenges of the modern world and of the energy transition, alternative fuels are developed and tested. They are used to replaceconventional fuels or as a blend to achieve the desired thermo-chemical properties. However, the impact of these new fuels on the performance of combustion chambers remains partially known. From this perspective, high-fidelity simulations of turbulent combustion of alternative fuels can be reached only if a detailed multi-component description of the liquid and gas mixtures is considered. The objective of this thesis is to contribute to the unsteady modeling of spray flames where complex multi-component phenomena occur : differential evaporation, multi-species mixing, gas phase chemical reactions. To this aim, the fuel is treated as a set of multi-component mixtures, which may be different in the liquid and gas phases depending on the required accuracy. Different models for the aforementioned phenomena are available in the literature, and the main challenge is the coupling of these different approaches and their validation in realistic and complex conditions. First, the chosen multi-component approach for the gas phase, based on the transport of a large number of species and on finite-rate chemistry, is validated for premixed flames. The expanding spherical flame configuration was chosen to study the flame consumption speed, which is an important parameter in combustion. In collaboration with the experimental team at the CORIA laboratory, a flame consumption speed formalism is established for non-confined and confined spherical expanding flames. This formalism enables to have a precise comparison of experimental and numerical results for methane/air and iso-octane/air flames and to validate the gas phase models. Second, we focused on the physical process of evaporation. The multi-component evaporation model of Abramzon-Sirignano is implemented in the YALES2 flow solver based on a point-particle approach for the fuel droplets. This model is adapted to enable the description of single- or multi-component evaporation with or without differential evaporation. As such, the model is capable of dealing with various fuel surrogates. The evaporation model is compared to the Spalding model and validated on experimental results of Chauveau et al. [33], Nomura et al. [158], Ghassemi et al. [82] and Daïf et al. [47] for a single component droplet and then two-component isolated droplet with and without convection. Finally, the 3D Large-Eddy Simulation (LES) of a complex n-heptane/air spray flame is conducted with analytical reduced chemistry (ARC, [169, 205]). This flame was experimentally studied at the CORIA laboratory with high fidelity diagnostics to characterize the flame structure and provide quantitative data such as gas-phase velocity and temperature as well as local droplet size and velocity distributions. Comparison with the experimental data [225] and with the simulations carried out within the framework of the 6th Workshop on Turbulent Combustion of Spray, shows that the current LES accurately reproduce the gas flow and properties of the dispersed phase. This configuration paves the way for the simulation of even more complex spray flames with multi-component fuels

La dynamique des flammes de prémélange est étudiée par deux approches numériques différentes. La première résout les équations compressibles de Navier-Stokes avec une chimie simplifiée (DNS). Afin de réduire les coûts de calcul, nous analysons et développons un schéma numérique à grille décalée. Le traitement des ondes acoustiques aux sorties est connu pour rendre les flammes cylindriques légèrement carrées. Ces déformations non-physiques sont expliquées en mettant en évidence la modélisation insuffisamment précise de l'accélération du fluide lorsque l'écoulement est oblique à la sortie. Une étude paramétrique et statistique de flammes turbulentes est menée en 2D et une simulation parallèle 3D est réalisée dans un domaine de (3cm)3. En considérant la flamme infiniment mince, l'approche EEM diminue considérablement les coûts de calcul. Les mêmes simulations sont réalisées et comparées aux résultats de DNS pour tester la capacité du modèle EEM à fournir des résultats quantitatifs.

Considering the importance of combustion characteristics in combustion applications including spark ignition engines and gas turbines, both laminar and turbulent burning velocities were measured for gasoline related fuels. The first part of the present work focused on the measurements of laminar burning velocities of Fuels for Advanced Combustion Engines (FACE) gasolines and their surrogates using a spherical constant volume combustion chamber (CVCC) that can provide high-pressure high-temperature (HPHT) combustion mode up to 0.6 MPa, 395 K, and the equivalence ratios ranging 0.7-1.6. The data reduction was based on the linear and nonlinear extrapolation models considering flame stretch effect. The effect of flame instability was investigated based on critical Peclet and Karlovitz, and Markstein numbers. The sensitivity of the laminar burning velocity of the aforementioned fuels to various fuel additives being knows as octane boosters and gasoline extenders including alcohols, olfins, and SuperButol was investigated. This part of the study was further extended by examining exhaust gas re-circulation effect. Tertiary mixtures of toluene primary reference fuel (TPRF) were shown to successfully emulate the laminar burning characteristics of FACE gasolines associated with different RONs under various experimental conditions. A noticeable enhancement of laminar burning velocities was observed for blends with high ethanol content (vol ≥ 45 %). However, such enhancement effect diminished as the pressure increased. The reduction of laminar burning velocity cause by real EGR showed insensitivity to the variation of the equivalence ratio. The second part focused on turbulent burning velocities of FACE-C gasoline and its surrogates subjected to a wide range of turbulence intensities measured in a fan-stirred CVCC dedicated to turbulent combustion up to initial pressure of 1.0 MP. A Mie scattering imaging technique was applied revealing the mutual flame-turbulence interaction. Furthermore, considerable efforts were made towards designing and commissioning a new optically-accessible fan-stirred HPHT combustion vessel. A time-resolved stereoscopic particle image velocimetry (TR-PIV) technique was applied for the characterization of turbulent flow revealing homogeneous-isotropic turbulence in the central region to be utilized successfully for turbulent burning velocity measurement. Turbulent burning velocities were measured for FACE-C and TPRF surrogate fuels along with the effect of ethanol addition for a wide range of initial pressure and turbulent intensity. FACE-C gasoline was found to be more sensitive to both primarily the primary contribution of turbulence intensification and secondarily from pressure in enhancing its turbulent burning velocity. Several correlations were validated revealing a satisfactory scaling with turbulence and thermodynamic parameters. The final part focused on the turbulent burning characteristics of piloted lean methane-air jet flames subjected to a wide range of turbulence intensity by adopting TR-SPIV and OH-planar laser-induced florescence (OH-PLIF) techniques. Both of the flame front thickness and volume increased reasonably linearly as normalized turbulence intensity, u^'/ S_L^0, increased. As u^'/ S_L^0 increased, the flame front exhibited more fractalized structure and occasionally localized extinction (intermittency). Probability density functions of flame curvature exhibited a Gaussian like distribution at all u^'/ S_L^0. Two-dimensional flame surface density (2D-FSD) decreased for low and moderate u^'/ S_L^0, while it increased for high u^'/ S_L^0Turbulent burning velocity was estimated using flame area and fractal dimension methods showing a satisfactory agreement with the flamelet models by Peters and Zimont. Mean stretch factor was estimated and found to increase linearly as u^'/ S_L^0increased. Conditioned velocity statistics were obtained revealing the mutual flame-turbulence interaction.

碩士
國立中央大學
能源工程研究所
102
This thesis measures quantitatively the turbulent flame speed of premixed flames over an initial pressure range of p = 1 ~ 5 atm. The main objective is to investigate the effect of the thermodiffusive instability on the self-similar propagation of expanding spherical premixed flames. Such a self-similar propagation phenomenon was first found by Chaudhri et al. (2012). In it they measured the turbulent flame speed (d/dt) of unity Lewis number (Le) methane-air mixtures at the equivalence ratio  = 0.9, such that all d/dt data measured at various values of the root-mean-square turbulent fluctuation velocity (u') and pressures (p) can be represented by a normalized relationship: (d/dt)/S_L^b ≈ 0.102ReT,flame0.54. is the average flame radius, t is time, S_L^b is the laminar burning velocity before density correlation, and flame turbulent Reynolds number ReT,flame= u'/DT where DT is the thermal diffusivity of unburned mixtures. All present experiments are carried out in a recently-built high-pressure, double-chamber, cruciform fan-stirred premixed turbulent explosion facility, capable of generating intense near-isotropic turbulence and making combustion experiments conducted at fixed p and u' conditions possible. Three different gas fuels/air mixtures with different values of Le are measured, respectively (i) syngas (35%H2/65%CO) at  = 0.5 having Le ≈ 0.76 < 1, (ii) methane CH4 at  = 0.9 with Le ≈ 1 (same as Chaudhri et al. for comparison), and (iii) propane C3H8 at  = 0.7 having Le ≈ 1.62 > 1. Each case covers a wide range of u' = 1.4 ~ 6 m/s and p = 1 ~ 5 atm. Results show that the effect of Le has an important impact on the turbulent flame speed. The corresponding normalized relationships for the aforesaid three different mixtures were: (d/dt)/S_L^b ≈ 0.190ReT,flame0.55 for Le < 1 syngas flames, d/dt)/S_L^b ≈ 0.116ReT,flame0.54 for Le ≈ 1 methane flames, and (d/dt)/S_L^b ≈ 0.102ReT,flame0.51 for Le > 1 propane flames. In comparison with methane flames, values of d/dt)/S_L^b of syngas and propane flames are 1.64 times higher and 0.88 times lower, respectively. This is because Le < 1 turbulent flames are not only influence by the inherent hydrodynamics instability, but also strongly affected by the thermaldiffusive instability, while Le ≈ 1 and Le > 1 turbulent flames are only influenced by the hydrodynamics instability, resulting in lower values of (d/dt)/S_L^b than that of Le < 1 turbulent flames at the same ReT,flame. Here we propose a correction function f(Le) = 2.15|Le - 1| based on the Lewis number for non-unity Lewis number turbulent flames and f(Le) = 1 if Le ≈ 1, such that the above-mentioned three different normalized relationship curves can be collapsed onto one single normalized relationship curve, f(Le)[(d/dt)/S_L^b] ≈ 0.113ReT,flame0.54. These results should be useful to our understanding of high-pressure premixed turbulent combustion and applicable to automobile and aviation internal combustion engines.

In this study, we investigate some preliminary reaction model predictions analytically in comparison with experimental premixed turbulent combustion data from four different flame configurations, which include i) high-jet enveloped, ii) expanding spherical, iii) Bunsen-like, and iv) wide-angled diffuser flames. The special intent of the present work is to evaluate the workability range of the model to hydrogen and hydrogen-doped hydrocarbon mixtures, emphasizing on the significance of preferential diffusion, PD, and Le effects in premixed turbulent flames. This is carried out in two phases: first, involving pure hydrocarbon and pure hydrogen mixtures from two independent measured data, and second, with the blended mixtures from two other data sets. For this purpose, a novel reaction closure embedded with explicit high-pressure and exponential Lewis number terms developed in the context of hydrocarbon mixtures is used. These comparative studies based on the global quantity, turbulent flame speed, indicate that the model predictions are encouraging yielding proper quantification along with reasonable characterization of all the four different flames, over a broad range of turbulence, fuel-types and for varied equivalence ratios. However, with each flame involved the model demands tuning of the (empirical) constant to allow for either or both of these effects, or for the influence of the burner geometry. This provisional stand remains largely insufficient. Therefore, a submodel for chemical time scale from the leading point analysis based on the critically curved laminar flames employed in earlier studies for expanding spherical flames is introduced here. By combining the submodel and the reaction closure, the dependence of turbulent flame speed on physicochemical properties of the burning mixtures including the strong dependence of preferential diffusion and/or Le effects can be determined.

A typical stationary premixed turbulent flame is the developing flame, as indicated by the growth of mean flame thickness with distance from flame-stabilization point. The goal of this work is to assess the importance of modeling flame development for RANS simulations of confined stationary premixed turbulent flames. For this purpose, submodels for developing turbulent diffusivity and developing turbulent burning velocity, which were early suggested by our group (FSC model) and validated for expanding spherical flames [4], have been incorporated into the so-called Zimont model of premixed turbulent combustion and have been implemented into the CFD package Fluent 6.2. The code has been run to simulate a stationary premixed turbulent flame stabilized behind a triangular bluff body in a rectangular channel using both the original and extended models. Results of these simulations show that the mean temperature and velocity fields in the flame are markedly affected by the development of turbulent diffusivity and burning velocity.

The flame speed is a central element not only in theoretical combustion and basic research but also a key parameter for several combustion models. Various methods to define and measure laminar flame speed have been applied. One method that is being relevant to determine flame speed and flame stretch (quantified by Markstein numbers) under high pressure and high temperature conditions is the bomb method, where an unsteady spherical expanding flame is investigated in a closed vessel. Especially for higher pressures, instabilities occur in the flame front of spherically expanding flames, due to the decreased flame thickness and diffusion processes. These so called cellular structures increase the flame surface and therefore the laminar flame speed cannot be determined in the usual manner. As high pressures are common under engine conditions, there is a need to be still able to determine the flame speed within these pressure ranges. By exposing the flame to a turbulent flow field, the vortex interactions are mainly responsible for the deformation of the flame surface. This fact can be used to deduce the laminar flame speed from a turbulent flame. In the present work, a turbulent flame speed model to determine unstretched laminar flame speed and Markstein numbers is introduced and validated. For this purpose the flame is exposed to a well-known turbulent flow field that is nearly homogenous and isotropic. By estimating the flame surface, the laminar flame speed can be calculated, using a turbulent stretch model as well as the flamelet assumption in an implicit approach. A high speed 2D laser imaging technique is used to capture the flame propagation. The most significant issue of this method is to correctly identify the surface and the volume of the flame, because solely a cross section of the flame surface can be visualized in the laser sheet. In case of spherical flames, the radius of the cross section corresponds to the flame surface and volume whereas a single radius is not sufficient to quantify a turbulent flame. The idea is to use the circumference of the cross section in a power law approach to quantify the cellular structures as a mean. The implicit model has been validated for methane and hydrogen flame speed at various equivalence ratios at atmospheric and elevated pressure. The results are in a good agreement with laminar determined flame speed and also the Markstein number is obtained qualitatively correct. Although the flame surface and volume cannot be determined directly by 2D measurement techniques, a model has been developed to determine laminar unstretched flame speed and Markstein numbers by investigating unsteady propagating flames in a turbulent flow field.

Abstract This paper presents a RANS turbulent combustion model for CH4/H2/air mixtures which includes the effect of heat losses and flame stretch. This approach extends a previous model concept designed for methane/air mixtures and improves the prediction of flame stabilization when hydrogen is added to the fuel. Heat loss and stretch effects are modelled by tabulating the consumption speed of laminar counter flow flames in a fresh-to-burnt configuration with detailed chemistry at various heat loss and flame stretch values. These computed values are then introduced in the turbulent combustion model by means of a turbulent flame speed expression which is derived as a function of flame stretch, heat loss and H2 addition. The model proposed in this paper is compared to existing models on experimental data of spherical expanding turbulent flame speeds. The performance of the model is further validated by comparing CFD predictions to experimental data of an atmospheric turbulent premixed bluff-body stabilized flame fed with CH4/H2/air mixtures ranging from pure methane to pure hydrogen.

Abstract Natural gas is a major fuel source for many industrial and power-generation applications. The primary constituent of natural gas is methane (CH4), while smaller quantities of higher order hydrocarbons such as ethane (C2H6) and propane (C3H8) can also be present. Detailed understanding of natural gas combustion is important to obtain the highest possible combustion efficiency with minimal environmental impact in devices such as gas turbines and industrial furnaces. For a better understanding the combustion performance of natural gas, several important parameters to study are the flame temperature, heat release zone, flame front evolution, and laminar flame speed as a function of flame equivalence ratio. Spectrally and temporally resolved, high-speed chemiluminescence imaging can provide direct measurements of some of these parameters under controlled laboratory conditions. A series of experiments were performed on premixed methane/ethane-air flames at different equivalence ratios inside a closed flame speed vessel that allows the direct observation of the spherically expanding flame front. The vessel was filled with the mixtures of CH4 and C2H6 along with respective partial pressures of O2 and N2, to obtain the desired equivalence ratios at 1 atm initial pressure. A high-speed camera coupled with an image intensifier system was used to capture the chemiluminescence emitted by the excited hydroxyl (OH*) and methylidyne (CH*) radicals, which are two of the most important species present in the natural gas flames. The calculated laminar flame speeds for an 80/20 methane/ethane blend based on high-speed chemiluminescence images agreed well with the previously conducted Z-type schlieren imaging-based measurements. A high-pressure test, conducted at 5 atm initial pressure, produced wrinkles in the flame and decreased flame propagation rate. In comparison to the spherically expanding laminar flames, subsequent turbulent flame studies showed the sporadic nature of the flame resulting from multiple flame fronts that were evolved discontinuously and independently with the time. This paper documents some of the first results of quantitative spherical flame speed experiments using high-speed chemiluminescence imaging.

For 3D observation of high speed flames, non-scanning 3D-CT technique using a multi-directional quantitative schlieren system with flash light source, is proposed for instantaneous density distribution of unsteady premixed flames. This “Schlieren 3D-CT” is based on (i) simultaneous acquisition of flash-light schlieren images taken from numerous directions, and (ii) 3D-CT reconstruction of the images by an appropriate CT algorithm. In this technique, for simultaneous schlieren photography, the custom-made 20-directional schlieren camera has been constructed and used. This camera consists of 20 optical systems of single-directional quantitative schlieren system. Each system is composed of two convex achromatic lenses of 50 mm in diameter and 300 mm in focal length, a light source unit, a schlieren stop of a vertical knife edge and a digital camera. The light unit has a flash (9 micro-sec duration) light source of a uniform luminance rectangular area of 1 mm × 1 mm. Both of the uniformity of the luminosity and the definite shape are essential for a quantitative schlieren observation. Sensitivity of the digital cameras are calibrated with a stepped neutral density filter. Target flames are located at the center of the camera. The image set of 20 directional schlieren images are processed as follows. First the schlieren picture brightness is shifted by no-flame-schlieren picture brightness in order to obtain the real schlieren brightness images. Second, brightness of these images is scaled by Gladstone-Dale constant of air. Finally, the scaled brightness is horizontally integrated to form “density thickness images”, which can be used for CT reconstruction of density distribution. The density thickness images are used for CT reconstruction by MLEM (maximum likelihood-expectation maximization) CT-algorithm to obtain the 3D reconstruction of instantaneous density distribution. In this investigation, the “density thickness” projection images of 400(H) × 500(V) pixel (32.0 mm × 40.0 mm) are used for 3D-CT reconstruction to produce 3D data of 400(x) × 400(y) × 500(z) pixel (32.0 mm × 32.0 mm × 40.0 mm). The voxel size is 0.08 mm each direction. In this investigation, the target flame is spark-ignited flame kernels. The flame kernels are made by spark ignition for a fuel-rich propane-air premixed gas. First, laminar flow is selected as the premixed gas flow to establish the spherically expanding laminar flame. The CT reconstruction result show the spherical shape of flame kernel with a pair of deep wrinkles. The wrinkle is considered to be caused by spark electrodes. Next turbulent flows behind turbulence promoting grid is selected. The corrugated shape flame kernel is obtained. The schlieren 3D-CT measurements are made for the complicated kernels. CT results expresses the instantaneous 3D turbulent flame kernel shapes.

Abstract Turbulent combustion is a very active and challenging research topic of direct interest to the design and operation of gas turbine engines. A spherically expanding flame immersed in a turbulent field is one way to gain fundamental insight on the effect of turbulence on combustion. This kind of experiment is often conducted inside a fan-stirred flame bomb, preferably at conditions of high pressure, high temperature, and intense turbulence. A new fan-stirred flame bomb was designed and built to provide a device for conducting fundamental turbulent flame measurements at conditions of interest to gas turbine engines. A literature review on existing systems was used as guidance in the design of the turbulence-generation elements in the present rig. A few options of impellers were explored. The flow field produced by the chosen impeller was measured with Laser Doppler Velocimetry (LDV). A detailed exposition of the vessel engineering and construction are presented, including current activities that will extend the use of the facility for heated experiments up to at least 400 K. Before turbulent experiments were attempted, a validation of the rig accuracy and pressure worthiness was made. Finally, a demonstration of the new apparatus was made by testing a lean mixture of syngas. The experiment matrix using hydrogen and H2/CO mixtures included three levels of pressure (1, 5, and, 10 bar) and three levels of turbulence fluctuation rms (1.4, 2.8, and 5.5 m/s). Data based on the high-speed schlieren diagnostic are presented.

Natural gas is the primary fuel for stationary, powergeneration gas turbines, and it is necessary to understand its combustion characteristics under engine-relevant (turbulent) conditions. Since its composition varies depending on the fuel source, a natural gas surrogate (NG 18% C2+) and admixtures with H2 have been utilized recently by the authors to aid chemical kinetics modeling using ignition delay times and laminar flame speed experiments. The present study focused on measuring turbulent flame speeds (displacement speeds) of natural gas (NG2) and methane with H2 using a fan-stirred flame bomb. The apparatus is a closed, cylindrical chamber fitted with four radial impellers that generate a central spherical volume of homogeneous and isotropic turbulence with negligible mean flow. Schlieren imaging was used to visually track the growth of the spherically expanding turbulent kernels during the constant-pressure period. The turbulence levels were fixed at an average RMS intensity level of 1.5 m/s and at an integral length scale of 27 mm. Turbulent flame speeds (ST,0.1) of NG2 blends were measured over a wide range of equivalence ratios between 0.7 and 1.3. ST,0.1 for the natural gas surrogate closely matched with those of methane for near-stoichiometric mixtures. However, preferential-diffusion effects (fuel effects) were observed under turbulent conditions for off-stoichiometric cases. The effects of hydrogen addition on the turbulent flame speeds of NG2 (25/75 and 50/50 (by volume) blends of H2/NG2) were also investigated and were compared with the flame speeds reported in a recent paper by the authors (ASME GT2014-26742) on the effects of hydrogen addition to turbulent flame speeds of methane. The effect of the hydrogen addition was to increase the turbulent flame speed (by about a factor of two for 50% H2 addition), although this effect was much more pronounced for the lean and stoichiometric mixtures. Interestingly, the flame speeds (both laminar and turbulent) of the CH4 blends with H2 were slightly larger than those for the NG2 blend at equivalent conditions, or about 10–20% larger at 50% H2 addition. This behavior can be explained kinetically by the increased importance of the inhibiting reaction CH3 + H (+M) ↔ CH4 (+M), where ethane oxidation produces more CH3 radicals than methane at similar conditions.