La simulation numérique est l’un des outils les plus puissants pour concevoir etoptimiser les systèmes industriels. Dans le domaine de la Dynamique des FluidesNumériques (CFD, "Computational Fluid Dynamics"), la simulation auxgrandes échelles (LES, "Large Eddy Simulation") est aujourd’hui largementutilisée pour calculer les écoulements turbulents réactifs, où les tourbillons degrande taille sont calculés explicitement, tandis que l’effet de ceux de petitetaille est modelisé. Des modèles de sous-mailles sont requis pour fermer leséquations de transport en LES, et dans le contexte de la simulation de la combustionturbulente, le plissement de la surface de flamme de sous-maille doitêtre modélisé.En général, augmenter le plissement de la surface de flamme de sous-maille favorisela combustion. L’amplitude de la promotion est donnée par une fonctiond’efficacité, qui est dérivée d’une hypothèse d’équilibre entre la production etla destruction de la surface de flamme. Dans les méthodes conventionnelles,le calcul de la fonction d’efficacité nécessite une constante qui dépend de lagéométrie de la chambre de combustion, de l’intensité de turbulence, de larichesse du mélange de air-carburant etc, et cette constante doit être fixée audébut de la simulation. Autrement dit, elle doit être déterminé empiriquement.Cette thèse développe un modèle de sous-maille pour la LES en combustionturbulente, qui est appelé le modèle dynamique de flammelette épaissie (DTF,"dynamic thickened flamelet model"), qui détermine la valeur de la constanteen fonction des conditions de l’écoulement sans utiliser des données empiriques.Ce modèle est tout d’abord testé sur une flamme laminaire unidimensionnellepour vérifier la convergence de la fonction d’efficacité vers l’unité (aucun plissementde la surface de flamme de sous-maille). Puis il est appliqué en combinaisonavec le modèle dynamique de Smagorinsky (Dynamic Smagorinskymodel) aux simulations multidimensionnelles d’une flamme en V, stabilisée enaval d’un dièdre. Les résultats de la simulation en trois dimensions sont alorscomparés avec les données expérimentales obtenues sur une expérience de mêmegéométrie. La comparaison montre la faisabilité de la formulation dynamique. / Numerical simulation is one of the most powerful tools to design and optimizeindustrial facilities. In the field of Computational Fluid Dynamics (CFD),Large Eddy Simulation (LES) is widely used to compute turbulent reactingflows, where larger turbulent motions are explicitly computed, while only theeffect of smaller ones is modeled. Subgrid models are required to close thetransport equations in LES, and in the context of the simulation of turbulentcombustion, the subgrid-scale wrinkling of the flame front must be modeled.In general, subgrid-scale flame wrinkling promotes the chemical reaction. Themagnitude of the promotion is given through an efficiency function derivedfrom an equilibrium assumption between production and destruction of flamesurface. In conventional methods, the calculation of the efficiency functionrequires a constant which depends on the geometry of the combustion chamber,turbulence intensity, the equivalence ratio of the fuel-air mixture, and so on;this constant must be prescribed at the beginning of the simulation. In otherwords, empirical knowledge is required.This thesis develops a subgrid-scale model for LES of turbulent combustion,called the dynamic thickened flamelet (DTF) model, which determines the valueof the constant from the flow conditions without any empirical input.The model is first tested in a one-dimensional laminar flame to verify the convergenceof the efficiency function to unity (no subgrid-scale flame front wrinkling).Then it is applied to multi-dimensional simulations of V-shape flamestabilized downstream of a triangular flame holder in combination with the dynamicSmagorinsky model. The results of the three-dimensional simulation arethen compared with the experimental data obtained through the experimentof the same geometry. The comparison proves the feasibility of the dynamicformulation.

No presente trabalho é apresentado um estudo do efeito da diluição com inertes sobre as características da transferência de calor por radiação em chamas laminares não pré-misturadas de metano. O trabalho também apresenta um estudo sobre a modelagem do fluxo radiativo proveniente de chamas turbulentas visando à obtenção de fatores de ponderação para o modelo das múltiplas fontes pontuais. Em ambos os estudos, a distribuição do fluxo radiante é obtida através de medições ao longo do eixo da chama e os valores de fração radiante são calculados a partir da integração dessa distribuição. É mostrado qualitativamente que a adição de gás inerte ao combustível propicia a inibição da formação de fuligem. É mostrado quantitativamente que a adição de gás inerte pode reduzir ou ampliar a fração radiante da chama, dependendo do tipo de gás, dos níveis de diluição e do tempo de residência característico da chama. São reportados valores para os fatores de ponderação utilizados no modelo de múltiplas fontes pontuais obtidos experimentalmente a partir de medições nas chamas. O formato da curva formada pelos fatores de ponderação é semelhante para os diferentes níveis de diluição com gás carbônico. A utilização desses fatores de ponderação no modelo de múltiplas fontes pontuais apresenta resultados satisfatórios em comparação com a distribuição dos fluxos radiativos medidos ao longo do eixo da chama. / In this work it is presented a study of the effect of fuel dilution with inerts on the radiative heat transfer characteristics of laminar non premixed methane flames. A study on the radiative flux distribution from turbulent flames is conducted in order to obtain weighting factors for the model of multiple point sources. The distribution of radiative heat flux is obtained by measuring the fluxes along the axis of the flame and the radiant fraction is calculated by the integration of such distribution. It is qualitatively shown that the addition of inert gas in the fuel leads to the inhibition of soot formation. It is quantitatively shown that the addition of inert may decrease or increase the radiant fraction depending on gas type, dilution levels and characteristic residence times of the flame. It is also reported values for the weighting factors used in the model of multiple point sources experimentally obtained from measurements. The shapes of the curves formed by the weighting factors are similar for the different dilution levels of carbon dioxide. The use of these weighting factors in the multi-point source model shows satisfactory results in comparison to the distribution of radiative fluxes measured along the flame axis.

L'allumage d'ergols injectés dans une chambre de combustion, la propagation du noyau de flamme puis sa stabilisation sont autant de paramètres déterminants pour la conception d'un moteur fusée. Pour ce type d'application, il est nécessaire - du point de vue de la modélisation - de tenir compte du couplage existant entre les effets de compressibilité, les processus de mélange turbulent ainsi que de cinétique chimique, dans un environnement diphasique puisque les ergols sont injectés à l'état liquide. Un modèle Lagrangien a été implanté dans le code de calcul compressible N3S-Natur afin de disposer d'un outil numérique capable de simuler le transitoire d'allumage d'un moteur fusée. La physique représentative de chacun des processus physiques impliqués pendant la phase d'allumage a été incorporée puis validée sur des configurations académiques. Ce travail a permis de mettre en évidence l'importance de la description du mélange à petites échelles pour capturer correctement le développement de la flamme. Il a aussi mis en exergue la nécessité de prendre en compte le transitoire thermique des gouttes d'oxygène liquide afin de reproduire fidèlement sa stabilisation. Enfin, il a nécessité l'extension de la notion de fraction de mélange à des cas pratiques présentant plus de deux entrées afin d'être en mesure de simuler la propagation de l'allumage sur la plaque d'injection. Cette approche basée sur l'introduction d'un injecteur fictif est non seulement utile pour la simulation de l'allumage des moteurs-fusées mais peut aussi être employée dans tout autre système impliquant le mélange entre des courants de réactifs issus de deux entrées ou plus. / In the field of liquid rocket propulsion, ignition, propagation and stabilization of the flame are of first importance for the design of the engine. Computational fluid dynamics (CFD) solvers may provide a great deal of help to proceed with the primary design choice but need to be fed with suited physical models. Important modelling efforts are therefore required to provide reliable computational representations able to take into account compressibility effects, turbulent mixing and chemical kinetics in two-phase flows since ergols are injected at the liquid state. A Lagrangian model has been implemented in the compressible solver N3S-Natur so as to obtain a computational tool able to compute the transient ignition of rocket engines. The physical processes involved at each step of this ignition sequence have been integrated and validated on academically configurations. Three significant contributions rose from this work. First of all, it is highlighted that the description of the micro-mixing is of first importance to correctly capture the flame development. This study also emphasized the need to consider the transient heating of liquid oxygen droplets in order to accurately compute the flame stabilization. Finally, the notion of mixture fraction must be extended to practical devices implying more than two inlets. The proposed approach which is based on the introduction of a fictive injector is not only well suited to rocket engine ignition application but also to deal with other practical devices implying two inlets and more.

Diplomová práce se zabývá spalováním zemního plynu při využití vzduchu s vyšším obsahem kyslíku (21–46 % kyslíku ve spalovacím vzduchu), tzv. kyslíkem obohaceným spalováním (OEC). Technologie OEC nalezla uplatnění v průmyslu, kde se jsou nároky na zvýšenou produktivitu, dosažení vyšší tepelné účinnosti, zlepšení vlastností plamene, snížení náklady, či zlepšení kvality výsledného produktu. Ačkoliv OEC přináší řadu výhod, je nutné zmínit i nevýhody jako: poškození zařízení, nestejnoměrné zahřívání, narušení plamene, zvýšené emise anebo zpětný zášleh plamene. Zkoušky proběhly na zkušebně hořáků, která umožňuje testovat hořáky nejen na plynná a kapalná paliva, ale i hořáky navržené pro kombinované spalování při maximálním výkonu hořáku 1 800 kW. Při zkouškách byl použit plynový „low-NOx“ hořák se stupňovitým přívodem paliva. V diplomové práci je popsán vliv obsahu kyslíku ve spalovacím vzduchu na emise oxidů dusíku (NOx), teplotu plamene, přenos tepla ze spalin do stěn spalovací komory, a také vlastnosti plamene, zvláště pak jeho stabilitu, tvar a rozměry. Zkoušky proběhly při výkonech 300 kW, 500 kW a 750 kW, přičemž pro výkon 750 kW proběhly testy jak při jednostupňové, tak dvoustupňové konfiguraci.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21100号 / 工博第4464号 / 新制||工||1694(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 黒瀬 良一, 教授 中部 主敬, 教授 吉田 英生 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

A pulsed Nd:YAG laser is used to study laser spark ignition of methane counter-flow diffusion flames with the use of helium and argon as diluents to achieve a wide range of variations in transport properties. The global strain rate and Damkohler number on successful ignition were investigated for the effects of Lewis number and transport properties, which are dependent on the diluent type and dilution level. A high-speed camera is used to record the ignition events and a software is used for pre-ignition flow field and mixing calculations. It is found that the role of effective Lewis number on the critical global strain rate, beyond which ignition is not possible, is qualitatively similar that on the extinction strain rate. With the same level of dilution, the inert diluent with smaller Lewis number yields larger critical global strain rate. The critical Damkohler number below which no ignition is possible is found to be within approximately 20% for all the fuel-inert gas mixtures studied. When successful ignition takes place, the ignition time increases as the level of dilution of argon is increased. The ignition time decreases with increasing level of helium dilution due to decreases in thermal diffusion time, which causes rapid cooling of the flammable layer during the ignition process. However, the critical strain for ignition with helium dilution rapidly decreases as the dilution level is increased. The experimental results show that with the increase of strain rate the time to steady flame decreases, and that with the increase of dilution level time for the flame to become steady increases. For the same level of dilution, the time for steady flame is observed to be longer for He-diluted flames than for Ar-diluted flames due to its thermal diffusivity being larger than that of Ar.

The role of intrinsic hydrodynamic instability of the premixed flame (known as Landau-Darrieus instability) in various flame phenomena is studied by means of direct numerical simulations of the complete system of hydrodynamic equations. Rigorous study of flame dynamics and effect of Landau-Darrieus instability is essential for all premixed combustion problems where multidimensional effects cannot be disregarded. The present thesis consists of three parts. The first part deals with the fundamental problem of curved stationary flames propagation in tubes of different widths. It is shown that only simple "single-hump" slanted stationary flames are possible in wide tubes, and "multi-hump" flames in a laminar flow are possible in wide tubes only as a non-stationary mode of flame propagation. The stability limits of curved stationary flames in wider tubes are obtained, together with the dependence of the velocity of the stationary flame on the tube width. The flame dynamics in wider tubes is shown to be governed by a large-scale stability mechanism resulting in a highly slanted flame front. The second part of the thesis is dedicated to studies of acceleration and fractal structure of outward freely propagating flames. It is shown that in direct numerical simulation the development of Landau-Darrieus instability results in the formation of fractal-like flame front structure. The fractal excess for radially expanding flames in cylindrical geometry is evaluated. Two-dimensional simulation of radially expanding flames in cylindrical geometry displays a radial growth with 1.25 power law temporal behavior after some transient time. It is shown that the fractal excess for 2D geometry obtained in the numerical simulation is in good agreement with theoretical predictions. The difference in fractal dimension between 2D cylidrical and three-dimensional spherical radially expanding flames is outlined. Extrapolation of the obtained results for the case of spherical expanding flames gives a radial growth power law that is consistent with temporal behavior obtained in the survey of experimental data. The last part of the thesis concerns the role of Landau-Darrieus instability in the transition from deflagration to detonation. It is found that in sufficiently wide channels Landau-Darrieus instability may invoke nucleation of hot spots within the folds of the developing wrinkled flame, triggering an abrupt transition from deflagrative to detonative combustion. It is found that the mechanism of the transition is the temperature increase due to the influx of heat from the folded reaction zone, followed by autoignition. The transition occurs when the pressure elevation at the accelerating reaction front becomes high enough to produce a shock capable of supporting detonation. / QC 20101119

[ES] El objetivo de esta tesis es la investigación y análisis de la estructura interna de los chorros diésel reactivos y el efecto de las condiciones de contorno en los parámetros asociados a la combustión. Este objetivo se consigue por medio de la simulación numérica del chorro con modelos de turbulencia RANS y LES usando un modelo de combustión avanzado basado en el concepto flamelet. Para este estudio, se aplica una aproximación simplificada de las flamelets de difusión, conocidas en la literatura como Flamelets de Difusión Aproximadas (ADF en inglés), como fundamento del modelo de combustión. En una primera etapa, el modelo se valida con combustibles de diferente complejidad química en regímenes estacionarios y transitorios para el conjunto de posibles velocidades de deformación. Una vez se confirma su idoneidad para condiciones encontradas en chorros diésel, se aplica a la simulación del chorro A del Engine Combustion Network (ECN), representativo de chorros diésel. Para proporcionar un cuadro completo de los fenómenos subyacentes, la combustión se analiza inicialmente para condiciones homogéneas y llamas laminares para las distintas condiciones de contorno de este experimento. Después este análisis se complementa con la simulación de diferentes mecanismos químicos para determinar cómo las características del encendido predichas por el esquema de oxidación afectan a la propagación de llama. Los resultados obtenidos en esta etapa se enlazan con el análisis del chorro turbulento en el contexto de simulaciones RANS y LES para describir cómo el fenómeno de la combustión se modifica con los diferentes niveles de complejidad física. La estructura del chorro turbulento se describe profundamente para las distintas condiciones de contorno y mecanismos químicos en términos de mezcla y escalares reactivos para las fases temporales y las regiones espaciales de la llama. La satisfactoria concordancia con los resultados experimentales muestran que el concepto flamelet, y más particularmente el modelo ADF, es adecuado para las simulaciones de chorros diésel. / [CA] L'objectiu d'esta tesi és la investigació i anàlisi de l'estructura interna dels dolls dièsel reactius i l'efecte de les condicions de contorn en els paràmetres associats a la combustió. Este objectiu s'aconsegueix per mitjà de la simulació numèrica del doll amb models de turbulència RANS i LES usant un model de combustió avançat basat en el concepte flamelet. Per a este estudi, s'aplica una aproximació simplificada de les flamelets de difusió, conegudes a la literatura com Flamelets de Difusió Aproximades (ADF en anglés), com a fonament del model de combustió. En una primera etapa, el model es valida amb combustibles de diferent complexitat química en règims estacionaris i transitoris per al conjunt de possibles velocitats de deformació. Una vegada es confirma la seua idoneïtat per a condicions trobades en dolls dièsel, s'aplica a la simulació del doll A del Engine Combustion Network (ECN), representatiu de dolls dièsel. Per a proporcionar un cuadre complet dels fenòmens subjacents, la combustió s'analitza inicialment per a condicions homogènies i flames laminars per a les distintes condicions de contorn d'aquest experiment. Després esta anàlisi es complementa amb la simulació de diferents mecanismes químics per a determinar com les característiques de l'encesa predites per l'esquema d'oxidació afecten la propagació de flama. Els resultats obtinguts en esta etapa s'enllacen amb l'anàlisi del doll turbulent en el context de simulacions RANS i LES per a descriure com el fenomen de la combustió es modifica amb els diferents nivells de complexitat física. L'estructura del doll turbulent es descriu profundament per a les distintes condicions de contorn i mecanismes químics en termes de mescla i escalars reactius per a les fases temporals i les regions espacials de la flama. La satisfactòria concordança amb els resultats experimentals mostren que el concepte flamelet, i més particularment el model ADF, és adequat per a les simulacions de dolls dièsel. / [EN] The objective of this thesis is the investigation and analysis of the internal structure of diesel-like reacting sprays and the effect of boundary conditions on combustion related parameters. This objective is achieved by means of the numerical simulation of the spray with RANS and LES turbulence models using an advanced combustion model based on the flamelet concept. For this study, a simplified approach for diffusion flamelets, known in the literature as Approximated Diffusion Flamelet (ADF), is applied as the basis of the combustion model. In a first step, this model is validated for fuels with different chemical complexity in steady and transient regimes for the whole set of possible strain rates. Once its suitability is confirmed for conditions found in diesel sprays, it is applied to the simulation of spray A from the Engine Combustion Network (ECN), representative of diesel-like sprays. In order to provide a complete picture of the underlying phenomena, combustion is initially analysed in homogeneous conditions and laminar flames for the different boundary conditions of this experiment. Later, this analysis is complemented with the simulation of different chemical mechanisms in order to determine how the ignition characteristics predicted by the oxidation scheme affect to the flame propagation. The results obtained at this stage are connected with the analysis of the turbulent spray in the context of RANS and LES simulations as a way to track how combustion phenomenon is modified at the different levels of physical complexity. The turbulent spray structure is thoroughly described for the different boundary conditions and chemical schemes in terms of mixing and reactive variables for both temporal phases and spatial flame regions. The satisfactory agreement with experimental results shows that the flamelet concept, and more particularly the ADF model, is suitable for diesel-like sprays simulations. / Pérez Sánchez, EJ. (2019). Application of a flamelet-based combustion model to diesel-like reacting sprays [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/117316

Les suies et les hydrocarbures polyaromatiques, qui sont émis à l’échappement des moteurs Diesel, constituent une part importante de la pollution urbaine. Depuis plusieurs années, de nombreuses recherches ont permis de progresser dans le domaine de la cinétique de la combustion des hydrocarbures qui est maintenant relativement bien connue. Il reste néanmoins des zones d’ombre concernant la formation des composés aromatiques (benzène, toluène,…) et des hydrocarbures aromatiques polycycliques (HAP) dont la formation provient d’espèces composées de deux à six atomes de carbone et qui mènent directement ou indirectement à la formation des suies.L’allène, le propyne, le butadiène et le cyclopentène sont des produits intermédiaires importants de la combustion dans les moteurs. Dans ce contexte, ce travail de thèse a permis de mieux comprendre les voies réactionnelles importantes dans la formation du benzène et du toluène. Le chapitre I de ce mémoire présente une revue bibliographique des travaux antérieurs sur l’oxydation du méthane, de l’allène, de propyne, de 1,3-butadiène et du cyclopentène. Le chapitre II présente une description détaillée du montage expérimental utilisé durant cette étude afin d’étudier la structure de la flamme laminaire de prémélange. Les chapitres III, IV et V présentent les résultats expérimentaux obtenus en flamme laminaire de prémélange, ainsi qu’une comparaison avec des simulations effectuées à l’aide de mécanismes réactionnels élaborés durant ce travail de thèse / Soots and polyaromatic hydrocarbons (PAH), which are present in the exhaust gas of diesel engines, represent a large part of the urban pollution. Many efforts have then been focused on reducing the emissions of these compounds. The formation of soot precursors and PAH in combustion involves small unsaturadted hydrocarbons the chemistry of which is still very uncertain. Allene, propyne, 1,3-butadiene and cyclopentene are intermediate products in the combustion in cars engines. This work has led to a better understanding of several important paths in the formation of benzene and toluene. The chapter I of this report presents a bibliographic review of former work on the oxidation of methane, allene, propyne, 1,3-butadiene and cyclopentene. Chapter II gives a detailed description of the experimental set up used during this work to study the structure of the premixed flat laminar flames. Chapters III, IV and V present our experimental results obtained in laminar premixed flat flames and also the comparison with simulations

An experimental study of the effects of turbulent flow regime on the flame structure is conducted by using perforated-plate-stabilized hydrogen-piloted lean premixed methane/air turbulent flames. The underlying non-reacting turbulent flow field was investigated using two-dimensional three-components particle imaging velocimetry (2D3C-PIV) with and without three perforated plates. The non-reacting flow data allowed a separation of the turbulent flow regime into axial velocity dominated and vortex dominated flows. A plate with 62\% blockage ratio was used to represent the stream-dominant flow regime and another with 86\% blockage ratio was used to represent the vortex-dominant flow regime. OH laser-induced fluorescence was used to study the effects of the turbulent flow regime on the mean progress variable, flame brush thickness, flame surface density, and global consumption speed. In comparison with the stream-dominant flow, the vortex-dominant flow makes a wider and shorter flame. Also, the vortex-dominant flow has a thicker horizontal flame brush thickness and a thinner longitudinal flame brush thickness. Especially, the horizontal flame brush thickness for the vortex-dominant flow does not follow the turbulence diffusion theory. Then, the vortex-dominant flow shows a relatively constant flame surface density along the stream-wise direction, while the stream-dominant flow shows a decreasing flame surface density. Lastly, the vortex-dominant turbulent flow improves the consumption speed in comparison to the stream-dominant turbulent flow regime with the same velocity fluctuation level.