L'augmentation du trafic aérien à proximité des zones à forte densité démographique impose aux constructeurs aéronautiques de développer des appareils de plus en plus silencieux. Les systèmes propulsifs figurent parmi les principaux contributeurs du rayonnement acoustique des aéronefs. Plus particulièrement, il est admis que la chambre de combustion est responsable d'une génération acoustique large-bande et basse fréquence. Deux principaux mécanismes générateurs de bruit ont été identifié dans les moteurs d'avions dans les années 70. Le premier correspond à l'émission d'ondes acoustiques par le dégagement de chaleur instationnaire induit par la combustion turbulente au sein de la chambre, bruit qualifié de direct. Le second mécanisme est la génération acoustique dans les étages de turbine par l'accélération des fluctuations de températures et de vorticité crées par la flamme et l'écoulement turbulent dans la chambre, bruit qualifié d'indirect. Ces deux mécanismes ont été largement mis en évidence au travers de travaux académiques analytiques, expérimentaux et numériques. Par contre, l'importance du bruit de combustion sur des moteurs réels a été peu étudiée. Dans ce travail, une méthodologie de calcul basée sur des simulations aux grandes échelles de chambres de combustion couplées à une méthode analytique pour calculer le bruit de combustion dans une configuration réelle est évaluée. Cette chaîne de calcul nommée CONOCHAIN est comparée aux résultats expérimentaux analysés dans cette thèse et issus du projet TEENI (projet européen FP7) où un moteur complet TURBOMECA a été instrumenté pour identifier les sources de bruits large-bandes. Dans un premier temps, un secteur de la chambre TEENI est calculée pour deux points de fonctionnements expérimentaux. Ensuite, la chambre annulaire complète est simulée au point de fonctionnement maximal pour évaluer l'apport du champ aérodynamique complet sur la prédiction du bruit. Enfin, les niveaux de bruits direct et indirect sont calculés, à partir des fluctuations extraites des précédentes simulations en sortie de brûleur, dans les étages de turbines et comparés aux données expérimentales. / The growth of air traffic at the vicinity of areas at high population density imposes to make quieter aircrafts on aeronautical manufacturers.The engine noise is one of the major contributors to the overall sound levels. Furthermore, the combustion is known to be responsible for a broadband noise generation at low-frequency. The combustion noise can be put into two main mechanisms. The first one is the emission of sound pulses by the unsteady heat release of the combustion process and is called the direct combustion noise. The second one is the generation of acoustic waves within the turbine stages by the acceleration of the temperature inhomogeneities and vorticity waves induced by the combustion and the turbulent flow within the combustor. This noise is the indirect combustion noise. These mechanisms were fully investigated in academic cases using experimental, analytical and numerical approaches contrary to the combustion noise within real engines. In this work, a hybrid approach called CONOCHAIN and based on LES of combustion chamber and an analytical disk theory to compute the combustion noise in a real turboshaft engine is evaluated. The predicted noise levels are compared with the experimental results obtained from a TURBOMECA engine in the framework of TEENI project (European project FP7) and analysed in this work where a turboshaft engine was instrumented to locate and identify the broadband noise sources. Two LES of a single sector of the TEENI combustion chamber representative of two experimental operating points are performed as well as a LES of the full-scale combustor at high power. The unsteady fields provided by the LES are used to compute direct and indirect combustion noise within the turbine stages in both cases and compared with the experimental results.

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La réduction des émissions de polluants et l’augmentation du rendement des moteurs ont conduit à une large utilisation de régimes de combustion pauvres en carburant dans les foyers de type moteurs aéronautiques et turbines à gaz. Des phénomènes de bruit et d’instabilités de combustion peuvent alors apparaître. Des fluctuations cycliques auto-entretenues de la pression au sein d’un foyer peuvent conduire à une limitation des régimes de fonctionnement ou une usure rapide et indésirable des installations et dans certains cas une destruction du système. L’objectif de ce travail de thèse est d’étudier les mécanismes responsables du bruit de combustion et des instabilités dans un foyer turbulent prémélangé swirlé. L’étude repose sur une analyse du champ de pression au sein du foyer, de la dynamique de la combustion et une caractérisation détaillée des conditions limites en amont, aval et dans les lignes d’alimentation en combustible et en comburant. Le banc expérimental CESAM ("Combustion Étagée Swirlée Acoustiquement Maîtrisée") est utilisé au cours de ce travail. Basée sur des observations expérimentales, une étude théorique de l’acoustique du foyer est tout d’abord réalisée grâce à un modèle à deux cavités couplées qui modélisent le tube de prémélange et la chambre de combustion de ce banc. Les fréquences et les structures spatiales des modes propres du foyer sont examinées, et des comparaisons sont menées avec les résultats expérimentaux. La condition limite au fond du tube de prémélange est mesurée, et utilisée comme entrée dans le modèle. L’effet de cette condition sur la prévision des fréquences des modes propres est examiné. Par la suite, le code de calcul AVSP est utilisé pour valider les résultats obtenus avec le modèle couplé. L’interaction entre ces modes acoustiques et la flamme est mise en évidence en caractérisant la dynamique de l’écoulement réactif. La vélocimétrie par images de particules (PIV) à haute cadence est utilisée. Une première étude est menée sur les champs de vitesse moyens et fluctuants puis on s’intéresse à l’analyse spectrale des champs de vitesse instantanés, rendue possible par la haute cadence du diagnostic. Un post-traitement faisant intervenir une méthode de détection des tourbillons est ensuite mis en oeuvre en utilisant le critère _2. Des structures cohérentes sont convectées le long du front de flamme à la fréquence du second mode instable du foyer. Le chapitre précédent ayant permis de montrer que ce mode acoustique était essentiellement associé au tube de prémélange, le mécanisme de couplage est clairement identifié. Par la suite, un traitement en moyenne de phase est appliqué aux champs de vitesse axiale. Des mouvements de battements des bras de la flamme dans les directions longitudinale et transverse sont mis en évidence aux fréquences des modes instables. L’émission naturelle de la flamme est également mesurée avec une caméra rapide. Une analyse spectrale et un traitement en moyenne phase avec transformée d’Abel sont appliqués aux images pour caractériser les régions de la flamme présentant une forte réponse aux fréquences des modes acoustiques du foyer. Les mécanismes à l’origine du bruit sont analysés en corrélant les mesures optiques et acoustiques. Au cours de cette étude, des fonctions de transfert de flamme FTF sont également caractérisées aux fréquences des modes propres du foyer, liant perturbations amont et réponse de flamme. La vitesse acoustique est reconstruite dans le tube de prémélange à partir des mesures des microphones. La FTF est calculée grâce aux mesures de vitesse par PIV, à l’émission des radicaux OH* et CH* et à l’émission naturelle de la flamme obtenue par caméra rapide. La caractérisation et la modélisation du système composé du tube de prémélange et de la chambre de combustion montrent qu’il est nécessaire de s’intéresser à l’influence des conditions aux limites sur les propriétés de la flamme et la stabilité du brûleur. / Lean premixed combustion is widely used to limit pollutant emissions and improve efficiency. However in this situation combustion instabilities and associated noise may occur. The growth of self-sustained pressure fluctuations within the combustor may limit the operating conditions and eventually damage the installation. The objective of this work is to study the mechanisms induced in combustion noise and instabilities in a turbulent premixed swirled burner. The study is based on a detailed analysis of the pressure field of the combustor, the flame dynamics and a characterization of the upstream and downstream acoustic boundary conditions and in the air and fuel feeding lines. Based upon experimental investigations, a theoretical study of the burner acoustics is carried out using a low-order model with two coupled cavities. The eigenfrequencies and spatial distribution of the pressure field are obtained, allowing comparisons with experimental results. The impact of the inlet acoustic impedance on the prediction of the eigenmodes is examined through the use of the measured impedance in the model. Thereafter calculations with the AVSP Helmholtz code are carried out to confirm the results obtained with the loworder model. The interactions between the burner acoustic modes and the flame are investigated and the reacting flow dynamics is characterized, using High Speed Particle Image Velocimetry HSPIV at 15 kHz. A first analysis concerns the mean and fluctuating velocity fields and a spectral analysis of the collection of instantaneous velocity fields is carried out. Then a method based on the _2 criterion is used to detect vortices, showing that coherent structures are convected through the flame front at the frequency of the second unstable combustor mode. It is shown in the previous chapter that this mode is essentially associated with the premixer acoustics, allowing a clear coupling scenario between the acoustics and the flame. A phase locked averaging method is applied to the axial velocity fields. Flapping motions of the flame branches are highlighted in longitudinal and transverse directions at the unstable modes frequencies. The natural light emission from the flame is also measured using a fast camera. Spectral analysis and phase locked averaging with Abel transform are applied to images in order to determine the flame regions where a strong response is visible at the acoustic modes. Mechanisms underlying combustion noise are analyzed by correlating the optical and acoustic measurements. Flame transfer functions FTF are also characterized between upstream disturbances and the flame response at the combustor eigenfrequencies. Acoustic velocity is reconstructed in the premixer using microphones measurements. The FTF is calculated using PIV velocity fields, OH* or CH* intensities and flame natural light emissions measurements. Measurements and modeling show that boundary conditions play a crucial role in the burner stability. The acoustic impedance at the premixer inlet can be modified using an impedance control system (ICS). Thus, the pressure field and flame dynamics are characterized for different boundary conditions imposed by the ICS. The acoustic boundary conditions in the feeding lines are characterized using an Impedance Measurement Device (IMD) equipped with microphones and mounted within the supplies.

This thesis presents research work which is concerned with the development of analytical and numerical methods for the dynamic analysis of the crankshaft-crankcase assembly. The effects of interaction of crankshaft and crankcase on the dynamic response of an IC engine block structure are studied. These methods are especially attractive for the simulation of the steady state response of rotating systems with many degrees of freedom which are forced by multiple periodic excitations. A major novelty of the methods is the ability to model the system non-linearities successfully as frequency dependent properties.

Two mathematical models have been developed to simulate two-dimensional, premixed, laminar, stationary, axisymmetric methane-air flames, and successfully validated with non-intrusive Coherent Anti-Stokes Raman Spectroscopy (CARS) temperature measurements. With the first model, the heat releaser ate model, volumetric heat release rate was generalised from one-dimensional computations. This approximation greatly simplified the set of governing equations that need to be solved. However, it cannot describe the effects of high stretch rates or of negative stretch rate. The second model made use of a number of reduced chemical kinetic schemes, with realistic elementary reactions. These were drawn from the literature and realistic transport properties have been included. With this model, based on the work of Peters (1985), the effects of stretch are automatically accounted for. Practical experimental validation was obtained with a multiple slot burner, supplied by the collaborating body, British Gas p1c. Temperature fields, obtained with the CARS technique, partially validated the reduced chemical kinetic scheme model. Some uncertainty arose in the prediction of heat loss to the burner tube. A numerical algorithm based upon the SUVIPLE method was employed,with a fully staggered grid. Various discretisation schemes were examined with the heat release model. Based on these tests, the hybrid scheme was selected for use in the reduced model. With this approach, a few reduced kinetic schemes have been selected and implemented. The most successfuol ne was the Peters( 1985) scheme. This consisted of 4 global reaction steps with 18 elementary reactions and 7 non-steady chemical species. The scheme has been employed in all the detailed computations in the present study. With this scheme, two-dimensional field solutions, for methane-air mixtures with equivalence ratios of 0.75,0.84 and 1.0, with slot widths of 2 mm, and 3 mm, and mean inlet velocities ranging from 0.3 m/s to 2.8 m/s have been obtained. Detailed flame structures have been obtained for all these conditions. Under these conditions, a number of parameters, essential in burner design and stability analysis, have been investigated. These includes flame height, flame thickness, and heat loss to the burner tube. The loss can range between 3% and 32% of the chemical energy in the premixture. The computations reveal the stretch rates acting on the flame and their effects on the burning velocity. At low flow rates the base of the flame has a negative stretch rate, while the flame tip is positively stretched. These effects are reversed at high flow rates. From the localised relationships between stretch rate and burning velocity, Markstein lengths have been evaluated,for different mixtures and the values compared with those obtained experimentally by other researchers. In general, there was good agreement despite the large scatter in the experimental values. The results further showed that the effects of the two components of flame stretch, namely flame curvature and aerodynamic straining, on burning velocities were very different. It seems appropriate to introduce two Markstein lengths to correlate burning velocity and the two components of stretch and these have been evaluated. Aerodynamic straining has a significantly larger effect on burning velocity than has flame curvature.

One of the most important industrial uses of pyrotechnic compositions is as delay fuses in electric detonators. Many factors influence the rate of burning of such fuses. These include (a) the primary choice of chemical components, followed by (b) the physical properties of these components, particularly the particle-size and distribution of the fuel, (c) the composition of the system chosen and (d) the presence of additives and/or impurities. A full experimental study of the influences of even a few of these factors, while attempting to hold other potential variables constant, would be extremely time consuming and hence attention has been focused on the possibilities of modelling pyrotechnic combustion. Various approaches to the modelling of pyrotechnic combustion are discussed. These include:- (i) one-dimensional finite-difference models; (ii) two-dimensional finite-element models; (iii) particle-packing considerations; (iv) Monte Carlo models. Predicted behaviour is compared with extensive experimental information for the widely-used antimony/potassium permanganate pyrotechnic system, and the tungsten /potassium dichromate pyrotechnic system. The one-dimensional finite-difference model was investigated to give a simple means of investigating the effects of some parameters on the combustion of a pyrotechnic. The two-dimensional finite-difference model used similar inputs, but at the expense of considerably more computer power, gave more extensive information such as the shape of the burning front and the temperature gradients throughout the column and within the casing material. Both these models gave improved results when allowance was made for autocatalytic kinetics in place of the usual assumption of an "order-of-reaction", n ≤ 1. The particle-packing model investigated the qualitative relationship between the maximum burning rate of a pyrotechnic system and the maximum number of contact points (per 1.00 g composition) calculated for that system. Qualitative agreement was found for those systems which are presumed to burn mainly via solid-solid reactions. The Monte Carlo model investigated the effect of the random packing of fuel and oxidant particles on the variability of the burning rate of a pyrotechnic composition.

Thermal spraying is a technology, which is used for coating of components and structures in order to achieve certain tribological characteristics, or for protection against corrosion, excessive temperature and wear. Within thermal spray, there are processes, which utilise combustion of liquid fuel to obtain high velocities flows providing, therefore, good adhesion of coating materials to substrates. These include High Velocity Oxygen Flame (HVOF) and High Velocity Air Flame (HVAF) process, of which the former one is widely used as it has been developed for at least two decades, while HVAF is less common. However, some studies indicate that HVAF has a number of advantages over HVOF, including the economic benefits. The thermal spray gun, based on the HVAF process, has been developed before, but the system was controlled manually. Therefore, there is a need to develop a fully automated controller of an HVAF thermal spray system. Process control of thermal spraying is highly complex as it involves simultaneous control of a number of processes, including; ignition process, combustion process, spraying material melting, as well as control and monitoring of auxiliary equipment. This paper presents the development of a control system for an HVAF thermal spray system, based on a Microchip PIC microcontroller. The designed control system was applied for controlling of thermal spraying of carbides powders, and provided a reliable ignition and stable combustion process, powder feeding and all other functions of control.

In the analysis of thermoacoustic systems, a flame is usually characterised by the way its heat release responds to acoustic forcing. This response depends on the hydrodynamic stability of the flame. Some flames, such as a premixed bunsen flame, are hydrodynamically globally stable. They respond only at the forcing frequency. Other flames, such as a jet diffusion flame, are hydrodynamically globally unstable. They oscillate at their own natural frequencies and are often assumed to be insensitive to low-amplitude forcing at other frequencies. If a hydrodynamically globally unstable flame really is insensitive to forcing at other frequencies, then it should be possible to weaken thermoacoustic oscillations by detuning the frequency of the natural hydrodynamic mode from that of the natural acoustic modes. This would be very beneficial for industrial combustors. In this thesis, that assumption of insensitivity to forcing is tested experimentally. This is done by acoustically forcing two different self-excited flows: a non-reacting jet and a reacting jet. Both jets have regions of absolute instability at their base and this causes them to exhibit varicose oscillations at discrete natural frequencies. The forcing is applied around these frequencies, at varying amplitudes, and the response examined over a range of frequencies (not just at the forcing frequency). The overall system is then modelled as a forced van der Pol oscillator. The results show that, contrary to some expectations, a hydrodynamically self-excited jet oscillating at one frequency is sensitive to forcing at other frequencies. When forced at low amplitudes, the jet responds at both frequencies as well as at several nearby frequencies, and there is beating, indicating quasi-periodicity. When forced at high amplitudes, however, it locks into the forcing. The critical forcing amplitude required for lock-in increases with the deviation of the forcing frequency from the natural frequency. This increase is linear, indicating a Hopf bifurcation to a global mode. The lock-in curve has a characteristic ∨ shape, but with two subtle asymmetries about the natural frequency. The first asymmetry concerns the forcing amplitude required for lock-in. In the non-reacting jet, higher amplitudes are required when the forcing frequency is above the natural frequency. In the reacting jet, lower amplitudes are required when the forcing frequency is above the natural frequency. The second asymmetry concerns the broadband response at lock-in. In the non-reacting jet, this response is always weaker than the unforced response, regardless of whether the forcing frequency is above or below the natural frequency. In the reacting jet, that response is weaker than the unforced response when the forcing frequency is above the natural frequency, but is stronger than it when the forcing frequency is below the natural frequency. In the reacting jet, weakening the global instability – by adding coflow or by diluting the fuel mixture – causes the flame to lock in at lower forcing amplitudes. This finding, however, cannot be detected in the flame describing function. That is because the flame describing function captures the response at only the forcing frequency and ignores all other frequencies, most notably those arising from the natural mode and from its interactions with the forcing. Nevertheless, the flame describing function does show a rise in gain below the natural frequency and a drop above it, consistent with the broadband response. Many of these features can be predicted by the forced van der Pol oscillator. They include (i) the coexistence of the natural and forcing frequencies before lock-in; (ii) the presence of multiple spectral peaks around these competing frequencies, indicating quasi-periodicity; (iii)the occurrence of lock-in above a critical forcing amplitude; (iv) the ∨-shaped lock-in curve; and (v) the reduced broadband response at lock-in. There are, however, some features that cannot be predicted. They include (i) the asymmetry of the forcing amplitude required for lock-in, found in both jets; (ii) the asymmetry of the response at lock-in, found in the reacting jet; and (iii) the interactions between the fundamental and harmonics of both the natural and forcing frequencies, found in both jets.