The relationship between dilatation, displacement speed, flame tangential strain rate and flame normal velocity gradient for a premixed turbulent flame in a corrugated/wrinkled flame regime is analyzed. The decomposition of dilatation into the flame tangential and normal strains and their relationship with curvature is studied. Three-dimensional, fully compressible direct numerical simulations (DNS) of premixed flames in a cube have been performed using a uniform 256^3 grid. For the turbulent case, decaying isotropic homogeneous turbulent velocity field is considered with an initial turbulence spectrum imposed. Simple single-step chemistry with an Arrhenius reaction rate is used. This simplification is valid as the flame considered is in the corrugated/wrinkled regime where the flame thickness is smaller than the smallest scales of turbulence. A single laminar flame is initially inserted into the turbulent field. A strongly linear relationship between dilatation and curvature has been seen which is due to the high correlation of displacement speed with curvature. The correlation between tangential strain rate and curvature is shown to be negative with a breakdown due to the curvature reaching the scale of the flame thickness at the cusps. To isolate the effect of heat release and turbulence, cases of a laminar sinusoidal wrinkled flame and a turbulent 𝛕=0 flame have been carried out. For a laminar sinusoidal wrinkled flame, a negative correlation between a^𝛕 and curvature was seen. This contradicts previous hypotheses (Haworth and Poinsot, 1992) (Chakraborty and Cant, 2004) where the negative correlation between a^𝛕 and curvature was explained to be due to different turbulence levels in front and behind the flame. Turbulence and alignment of flame surface with expansive tangential strains is shown to be responsible for the scatter seen in a^n and a^𝛕 relationships with curvature. Changing the peak reaction location towards the front of the flame did not change the trend in the plots of dilatation, tangential and normal strain rates versus curvature, confirming that dilatation relationship with curvature in particular is not due to any curvature distortion of the flame interior. However, it did thicken the flame and reduce the dilatation (and consequently its components, an and at) plot versus curvature and the magnitude of their curvature dependence. / Thesis / Master of Applied Science (MASc)

Despite mounting concerns of looming global warming and fuel shortages, combustion will remain the predominant source of fulfilling the world’s ever-increasing demand for energy in the foreseeable future. In light of these issues, the combustion regime known as Moderate and Intense Low oxygen Dilution (MILD) combustion has the potential of offering increased efficiency whilst lowering pollutant emissions. Essentially, MILD combustion relies on the reuse of the exhaust gases from the combustion process to simultaneously dilute the oxygen concentration of the oxidant stream, and increase its temperature. The benefit of this technique is that it results in a vast reduction in emissions, especially oxides of nitrogen. In addition, the thermal efficiency of the combustion process is increased, reducing fuel demands, as well as producing a more uniform heating profile and subsequently better product quality for many applications. The recirculation of exhaust gas and heat has been utilised for applications in the past. MILD combustion aims to extend the advantages of heat recovery and exhaust gas recirculation beyond the boundaries that are otherwise possible using conventional techniques. The relatively new concept of MILD combustion is a major advancement to the previous technology, and many fundamental issues have not yet been resolved. In a furnace environment, the dilution and preheating of the reactants generate a unique “distributed” reaction zone. There is a need to better understand the structure of this combustion regime and the parameters which control it. To emulate MILD combustion conditions in a controlled experimental environment, a Jet in Hot Coflow (JHC) burner is used in this study. The MILD combustion regime is examined using laser diagnostic techniques. The two key flame intermediates hydroxyl radical (OH) and formaldehyde (H2CO), as well as temperature, are imaged simultaneously to reveal details relating to the reaction zone. Simultaneous imaging enables not only the spatial distribution of each scalar to be investigated, but also the combined effect of the interactions of the three measured scalars. The role of four key variables are investigated as part of this work, namely; the coflow oxygen (O2) level, the jet Reynolds number, fuel dilution and fuel type. Also considered is the effect of surrounding air entrainment into the hot and diluted coflow, which causes a deviation from MILD combustion conditions. The local oxygen (O2) concentration is a key parameter in the establishment of MILD combustion conditions. The effect of lowering the O2 level is to lead to reductions in the OH and temperature in the reaction zone, in effect leading to a less intense reaction. When comparatively high oxygen laden, cold surrounding air mixes with the hot and low O2 coflow, MILD combustion conditions no longer exist. In this case, the flame front can become locally extinguished and subsequent premixing with the high O2 concentrations can lead to increased reaction rates and hence higher temperatures. It is therefore essential that fresh air must be excluded from a MILD combustor to maintain the stable reaction which typifies MILD combustion. It is found that the flame structure is relatively insensitive to both the type of hydrocarbon fuel and the Reynolds number. Each of these parameters can lead to changes in some intermediate species, namely formaldehyde, yet the OH and temperature measurements show comparatively minor variation. Nevertheless, fuel type and Reynolds number, in the form of increased flow convolution, can lead to striking differences in the flame structure. One of the most prominent effects is noted with the dilution of the fuel with various diluents. Some of the flames visually appear lifted, whereas the measurements reveal the occurrence of pre-ignition reactions in the “lifted” region. The unique characteristics of the stabilisation for these particular cases has lead to the term transitional flames. The fundamental aspects discovered by this study shed new light on the reaction zone structure under MILD combustion conditions. By advancing understanding of MILD combustion, future combustion systems will be able to better utilise the efficiency increases and lower pollutant benefits it offers. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1293788 / Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2007.

Despite mounting concerns of looming global warming and fuel shortages, combustion will remain the predominant source of fulfilling the world’s ever-increasing demand for energy in the foreseeable future. In light of these issues, the combustion regime known as Moderate and Intense Low oxygen Dilution (MILD) combustion has the potential of offering increased efficiency whilst lowering pollutant emissions. Essentially, MILD combustion relies on the reuse of the exhaust gases from the combustion process to simultaneously dilute the oxygen concentration of the oxidant stream, and increase its temperature. The benefit of this technique is that it results in a vast reduction in emissions, especially oxides of nitrogen. In addition, the thermal efficiency of the combustion process is increased, reducing fuel demands, as well as producing a more uniform heating profile and subsequently better product quality for many applications. The recirculation of exhaust gas and heat has been utilised for applications in the past. MILD combustion aims to extend the advantages of heat recovery and exhaust gas recirculation beyond the boundaries that are otherwise possible using conventional techniques. The relatively new concept of MILD combustion is a major advancement to the previous technology, and many fundamental issues have not yet been resolved. In a furnace environment, the dilution and preheating of the reactants generate a unique “distributed” reaction zone. There is a need to better understand the structure of this combustion regime and the parameters which control it. To emulate MILD combustion conditions in a controlled experimental environment, a Jet in Hot Coflow (JHC) burner is used in this study. The MILD combustion regime is examined using laser diagnostic techniques. The two key flame intermediates hydroxyl radical (OH) and formaldehyde (H2CO), as well as temperature, are imaged simultaneously to reveal details relating to the reaction zone. Simultaneous imaging enables not only the spatial distribution of each scalar to be investigated, but also the combined effect of the interactions of the three measured scalars. The role of four key variables are investigated as part of this work, namely; the coflow oxygen (O2) level, the jet Reynolds number, fuel dilution and fuel type. Also considered is the effect of surrounding air entrainment into the hot and diluted coflow, which causes a deviation from MILD combustion conditions. The local oxygen (O2) concentration is a key parameter in the establishment of MILD combustion conditions. The effect of lowering the O2 level is to lead to reductions in the OH and temperature in the reaction zone, in effect leading to a less intense reaction. When comparatively high oxygen laden, cold surrounding air mixes with the hot and low O2 coflow, MILD combustion conditions no longer exist. In this case, the flame front can become locally extinguished and subsequent premixing with the high O2 concentrations can lead to increased reaction rates and hence higher temperatures. It is therefore essential that fresh air must be excluded from a MILD combustor to maintain the stable reaction which typifies MILD combustion. It is found that the flame structure is relatively insensitive to both the type of hydrocarbon fuel and the Reynolds number. Each of these parameters can lead to changes in some intermediate species, namely formaldehyde, yet the OH and temperature measurements show comparatively minor variation. Nevertheless, fuel type and Reynolds number, in the form of increased flow convolution, can lead to striking differences in the flame structure. One of the most prominent effects is noted with the dilution of the fuel with various diluents. Some of the flames visually appear lifted, whereas the measurements reveal the occurrence of pre-ignition reactions in the “lifted” region. The unique characteristics of the stabilisation for these particular cases has lead to the term transitional flames. The fundamental aspects discovered by this study shed new light on the reaction zone structure under MILD combustion conditions. By advancing understanding of MILD combustion, future combustion systems will be able to better utilise the efficiency increases and lower pollutant benefits it offers. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1293788 / Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2007.

Un des principaux enjeux de l'industrie aéronautique est la recherche du moteur au meilleur rendement possible, pour satisfaire des contraintes économiques, techniques et environnementales. Les turbomachines bénéficient d'un constant perfectionnement depuis plus de 60 ans, et cette technologie semble avoir atteint un plateau. Une rupture technologique est aujourd'hui nécessaire, comme la combustion à volume constant (CVC). Le gain attendu est suffisant pour tenter de remplacer les systèmes actuels où la combustion se fait à pression constante. La combustion à isovolume fait appel à des mécanismes encore rarement maitrisés dans le contexte aéronautique. Sa compréhension passe par des expérimentations et des modèles théoriques et numériques. L’objectif de cette thèse est de développer une théorie et un outil de simulation LES (Large Eddy Simulation) appliqué au cas du concept ‘thermoréacteur’. Ainsi, la première étape a consisté à mettre en place un outil de simulation 0D traduisant l’évolution d’un cycle moteur de type CVC (Combustion à Volume Constant). Certains modèles utilisés dans cet outil 0D sont basés sur des corrélations expérimentales. D'autres présentent des paramètres à déterminer à partir de simulations numériques. La simulation 3D d’un système de type CVC est envisageable aujourd’hui grâce aux progrès récents des méthodes LES. Ainsi, des simulations du thermoréacteur ont pu être réalisées, et confrontées aux résultats expérimentaux obtenus au laboratoire Pprime sur trois points de fonctionnement. Les variabilités cycle à cycle observées expérimentalement ont été analysées dans les calculs LES. Les vitesses importantes au niveau de l'allumage et le taux de résidus du cycle précédent semblent être les principaux facteurs à l'origine de ces variations cycle à cycle. / A major challenge for the aircraft industry is to improve engine efficiency and to reduce pollutant emissions for economic, technical and environmental reasons. Aeronautical gas turbines have enjoyed a constant improvement for more than 60 years. This technology seems to have reached such efficiency levels that a technological breakthrough is necessary. Constant Volume Combustion (CVC) offers significant gain in consumption and could replace classical constant pressure combustion technologies, currently used in aeronautical engines. Mechanisms involved in isovolume combustion are not accurately controlled in the context of aeronautical chambers. Experimental, theoretical and numerical studies should provide a better understanding of CVC devices. The objective of this thesis is to develop simulation tools to study the thermoreacteur concept. First, a zero-dimensional (0D) simulation tool is developed to describe the evolution of a CVC cycle. Models based on experimental correlations are used to build the 0D tool. Parameters have to be determined from numerical simulations. Today, the 3D simulation of a CVC system is possible thanks to the recent progress of the LES (Large Eddy Simulation) methods developed at CERFACS. Simulations of the thermoreacteur concept have been carried out, and compared to experimental results obtained at the Pprime laboratory. Three operating points have been calculated. The main conclusion is the existence of significant cyclic variations which are observed in the experiment and analyzed in the LES: the local flow velocity at spark timing and the level of residuals gases are the major factors leading to cyclic variations.

The quenching of undesired flames by cold surfaces has been investigated for more than a century. The current quenching theory can predict simple configurations, this is not the case for real environments such as fuel management systems. Flames are sensitive to numerous parameters, such as fuel, mixture fraction, pressure, temperature, flow properties, acoustics, radiation, and surface interactions. The effects of some of these parameters are very well documented but there is a lack of information regarding the effects of acoustics and flow. This dissertation work will focus on improving the understanding of flow effect on the quenching of premixed gaseous flames. First, the effect of apparent velocity on flame quenching was investigated for different fuels and equivalence ratios. An experimental facility is designed such that the apparent flame velocity at which the flame enters and propagates through the channel can be varied without changing the initial mixture condition. High-speed (15,000 frames per second (FPS)) Schlieren and dynamic pressure measurement were used to measure the apparent flame velocity and to assess the flame quenching, respectively. This study showed that the high-speed laminar flames are harder to quench compared to self-propagating and turbulent flames. A similar trend was obtained for all the conditions investigated, lean and stoichiometric methane-air, lean propane-air, and lean ethylene-air mixtures. Further investigation was carried out to understand the quenching of high-speed laminar flames. The flame propagation through the channel was investigated using Hydroxyl (OH) planar laser induced fluorescence (PLIF). This study showed that the OH intensity fell below the detection threshold in the later part of the channel when quenching is observed. Then, the influence of heat transfer was investigated using spatial and temporal evolution of the temperature in the quenching channel. A high-speed (10 kHz) filtered Rayleigh scattering (FRS) technique was used to measure the one-dimensional time-resolved temperature profile. Three different channel heights (H = 1.3, 1.5, 2.0 mm) were investigated. Based on the evolution of the temperature profile in the quenching channel, a new parameter was identified and the importance of its evolution on the flame quenching was discussed.

This present research effort was directed towards developing reduced order models for the dynamics of laminar flat flames, swirl stabilized turbulent flames, and in evaluating the effects of the variation in fuel composition on flame dynamics. The laminar flat flame study was conducted on instrument grade methane, propane, and ethane flames for four total flow rates from 145 cc/sec to 200 cc/sec, and five equivalence ratios from 0.5 to 0.75. The analysis was done by measuring the frequency resolved velocity perturbations, u', and the OH* chemiluminescence, as a measure of unsteady heat release rate, q'. The experimental data showed the corresponding flame dynamics to be fourth order in nature with a pure time delay. One of the resonance was shown to represent the pulsation of the flame location caused by fluctuation in the flame speed and fluctuating heat losses to the flame stabilizer. The other resonance was correlated to the dynamics of the chemical kinetics involved in the combustion process. The time delay was correlated to the chemical time delay. Upon comparing the results of the experiments with the three fuels, it was concluded that for all equivalence ratios studied, propane flame had a higher dynamic gain than methane flames. Ethane flames exhibited a higher dynamic gain than methane flame in the frequency range of 20-100 Hz. Thus, burning of propane instead of methane increased the likelihood of the occurrence of thermo-acoustic instabilities. The experimental techniques developed during the dynamic studies conducted on laminar flat flames were applied to swirl stabilized turbulent flames. Experiments were performed for QAir = 15 scfm and 20 scfm, F = 0.55, 0.6, 0.65, and S = 0.79 and 1.19. The results of fully premixed experiments showed that the flame behaved as a 8th order low pass filter. The results of the partially premixed experiment exhibited a rich spectra, which maintained its bandwidth over the entire range of frequency studied. Comparison of fully and partially premixed flames in the frequency range of 200-400 Hz, indicated that at overall lean conditions the dynamic gain of the totally premixed flames was almost an order of magnitude lower than that of the partially premixed conditions. Thus, it was concluded that combustors with fully premixed flames have a higher probability of being thermo-acoustically stable than those with partially premixed flames. / Ph. D.

<p> Lifted non-premixed turbulent jet flames in the Transverse Instability Combustor (TIC) have been analyzed using qualitative and quantitative methods. Lifted flames in the TIC have been observed to stabilize about zero to five injector exit diameters downstream of the dump plane into the chamber and exhibit pulsating, unsteady burning. Anchored flames immediately begin reacting in the injector recess and burn evenly in a uniform jet from the injector exit through the entire optically accessible region. Statistically significant, repeatable behavior lifted flames are observed. It is shown that the occurrence of lifted flames is most likely for an injector configuration with close wall-spacing, second greatest for a configuration with close middle-element spacing, and lowest for a configuration with even element-spacing. For all configurations, of those elements that have been observed to lift, the center element is most likely to lift while the second element from the wall was likely. Flames at the wall elements were never observed to lift. Evidence is shown to support that close injector element spacing and stronger transverse pressure waves aid lateral heat transfer which supports flame stability in the lifted position. It is hypothesized that the stability of lifted flames is influenced by neighboring ignition sources, often a neighboring anchored flame. It is also shown that instances of lifted flames increase with the root-mean-squared magnitude of pressure fluctuation about its mean (P’ RMS) up to a threshold, after which flames stabilize in the anchored recess position.</p> <p>Dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD) analyses of CH* chemiluminescence data is performed. It is found that lateral ignition of the most upstream portion of lifted flames is dominated by the 1W mode. Furthermore, it is shown that low-frequency high energy modes with spatial layers resemble intensity-pulses, possibly attributable to ignition. These modes are trademarks of CH* chemiluminescent intensity data of lifted flames. It was also shown that the residence time in the chamber may be closely associated with those low-frequency modes around 200 Hz. DMD and POD were repeated for a downstream region on the center element, as well as a near-wall element, highlighting differences between the lifted flame dynamics in all three regions. </p> <p>It is shown that lifted flames are best characterized by their burning behavior and in rare cases may stabilize in the recess, while still being “lifted”. Furthermore, it is shown that flame position differentiation can extend into an initial period of highly stable combustor operation. Dynamic mode decomposition is explored as potential method to understand physical building blocks of proper orthogonal spatial layers. Non-visual indicators of lifted flames within the high-frequency (HF) pressure signal are sought to seek a method that allows for observation of lifted flames in optically inaccessible combustors, such as those in industry. Some attributes of power-spectral diagrams and cross-correlations of pressure signals are provided as potential indicators. </p>

This thesis describes computational and theoretical studies of fundamental physical processes that influence the heat-release response of turbulent premixed flames to acoustic forcing. Attached turbulent flames, as found in many practical devices, have a non-zero mean velocity component tangential to the turbulent flame brush. Hence, flame surface wrinkles generated at a given location travel along the flame sheet while being continuously modified by local flow velocity disturbances, thereby, causing the flame sheet to respond in a non-local manner to upstream turbulence fluctuations. The correlation length and time scales of these flame sheet motions are significantly different from those of the upstream turbulence fluctuations. These correlation lengths and times increase with turbulence intensity, due to the influence of kinematic restoration. This non-local nature of flame sheet wrinkling (called 'non-locality') results in a spatially varying distribution of local consumption speed (i.e. local mass burning rate) even when the upstream flow statistics are isotropic and stationary. Non-locality and kinematic restoration result in coupling between the responses of the flame surface to coherent acoustic forcing and random turbulent fluctuations respectively, thereby, causing the coherent ensemble averaged component of the global heat-release fluctuation to be different in magnitude and phase from its nominal (laminar) value even in the limit of small coherent forcing amplitudes (i.e. linear forcing limit). An expression for this correction, derived from an asymptotic analysis to leading order in turbulence intensity, shows that its magnitude decreases with increasing forcing frequency because kinematic restoration limits flame surface wrinkling amplitudes. Predictions of ensemble averaged heat release response from a different, generalized modeling approach using local consumption and displacement speed distributions from unforced analysis shows good agreement with the exact asymptotic result at low frequencies.

Ce travail de recherche est consacré à l'étude des processus d'allumage et de stabilisation des flammes turbulentes en configuration aéronautique. Cette thèse, entièrement expérimentale, se base dans une première partie sur l'étude et la compréhension de l'effet des paramètres locaux (vitesse et concentration en combustible) et de l'historique du noyau de flamme au cours de sa propagation sur les statistiques d'allumage. Pour expliquer ces statistiques, des scénarios sont proposés et validés pour différentes configurations opératoires. La deuxième partie de la thèse est dédiée à l'amélioration de la stabilité des flammes kérosène/air par dopage en hydrogène. Nous avons montré qu'à même structure de flamme, l'amplitude des fluctuations de pression était abaissée par la présence d'hydrogène. / This research is dedicated to the study of turbulent flames ignition and stabilization processes in representatives' aircraft combustion chambers. This PHD thesis, fully experimental, is based on studying and understanding local parameters (velocity and fuel concentration) and historical effects of the flame kernel's environment during its propagation on ignition statistics. To explain this statistics, various scenarios arre proposed and discussed. In this way, we justify the existence of high and low efficiency areas thanks to a time development criterion of the flame kernel. The second part of this thesis is dedicated to flame stabilization improvment by using H2 enrichment. Turbulent kerosene/air flames have been doped in gaseous hydrogen. We demonstrate a reduction of pressure oscillations' amplitude for the same flame structure.

L'optimisation des systèmes d'allumage est un paramètre critique pour la définition des foyers de combustion industriels. Des simulations aux grandes échelles (ou LES pour Large-Eddy Simulation) d'un brûleur de type bluff-body non pré-mélangé ont été menées afin de comprendre l'influence de la position de la bougie sur la probabilité d'allumage. La prise en compte de la combustion est basée sur une méthode de tabulation de la chimie détaillée (PCM-FPI pour Presumed Conditional Moments - Flame Prolongation of ILDM). Les résultats de ces simulations ont été confrontés des résultats expérimentaux disponibles dans la littérature. Dans un premier temps, les mesures de vitesse et du champ de richesse à froid sont comparées aux résultats de la simulation pour évaluer les capacités de prédiction en terme de structure de l'écoulement et de mélange turbulent. Un suivi temporel des vitesses et de la fraction de mélange est réalisé à différents points pour déterminer les fonctions de densité de probabilité (ou PDF)des variables caractéristiques de l'écoulement, à partir des champs résolus en LES. Les PDFs ainsi obtenues servent l'analyse des phénomènes d'allumages réussis ou déficients rencontrés expérimentalement. Des simulations d'allumage forcé ont été effectuées pour analyser les différents scénarios de développement de la flamme. Les corrélations entre les valeurs locales (fraction de mélange, vitesse) autour de la position d'allumage et les chances de succès de développement du noyau de gaz brûlés sont alors discutées. Enfin, une extension de la méthode PCM-FPI avec prise en compte des effets d'étirement est développée à l'aide d'une analyse asymptotique, puis confrontée aux résultats de mesures expérimentales. / The optimization of the ignition process is a crucial issue in the design of many combustion systems. Large eddy simulation (LES) of a conical shaped bluff-body turbulent non-premixed burner has been performed to study the impact of spark location on ignition success. The chemistry part of the simulation is done using tabulated detailed chemistry approach. This burner was experimentally investigated by Ahmed et al at Cambridge (UK). The present work focuses on the case without swirl for which detailed measurements are available. First, cold fkow measurements of velocities and mixture fraction are compared with their LES counterparts, to assess the prediction capabilities of simulations in terms of flow and turbulent mixing. Time history of velocities and mixture fraction are recorded at selected spots, to probe the resolved probability density function (pdf) of flow variables, in an attempt to reproduce, from the knowledge of LES resolved instantaneous flow conditions, the experimentally observed reasons of success or failure of spark ignition. A flammability map is also constructed from the resolved mixture fraction pdf and compared with its experimental counterpart. LES of forced ignition is then performed using flamelet fully detailed tabulated chemistry combined with presumed pdfs (PCM-FPI). Various scenarios of flame kernel development are analyzed and correlated with typical flow conditions observed in this burner. The correlations between velocities and mixture fraction values at the sparking time and the success or failure of ignition are then further discussed and analysed. The rate of flame development during successful or unsuccessful ignition events are analysed and compared against experimental observations. Finally, from asymptotic flame analysis, a novel approach has been proposed to include flame straining effects in the PCM-FPI method developped at CORIA-CNRS. The new model overcomes the problem associated with classical PCM-FPI closure to model kernel quenching due to intense local turbulence. Computations are done including the flame straining effects and the effect brought by the new model on kernel development is analysed in detail.