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.

Large-eddy simulation (LES) is emerging as a promising computational tool for reacting flows. High-order schemes for LES are desirable to achieve improved solution accuracy with reduced computational cost. In this study, a parallel, block-based, three-dimensional high-order central essentially non-oscillatory (CENO) finite-volume scheme for LES of premixed turbulent combustion is developed for Cartesian mesh. This LES formulation makes use of the flame surface density (FSD) for subfilter-scale reaction rate modelling. An algebraic model is used to approximate the FSD. A detailed explanation of the governing equations for LES and the mathematical framework for CENO schemes are presented. The CENO reconstruction is validated and is also applied to three-dimensional Euler equations prior to its application to the equations governing LES of reacting flows.

Large-eddy simulation (LES) is emerging as a promising computational tool for reacting flows. High-order schemes for LES are desirable to achieve improved solution accuracy with reduced computational cost. In this study, a parallel, block-based, three-dimensional high-order central essentially non-oscillatory (CENO) finite-volume scheme for LES of premixed turbulent combustion is developed for Cartesian mesh. This LES formulation makes use of the flame surface density (FSD) for subfilter-scale reaction rate modelling. An algebraic model is used to approximate the FSD. A detailed explanation of the governing equations for LES and the mathematical framework for CENO schemes are presented. The CENO reconstruction is validated and is also applied to three-dimensional Euler equations prior to its application to the equations governing LES of reacting flows.

Heat release rate disturbances are the sources of additional thermal stresses, direct and indirect combustion noise and undesirable vibrations. In extreme cases, these perturbations may even cause destructive combustion instabilities. These quantities are difficult to measure in practical burners. The objective of this work is to develop two alternative diagnostics to measure heat release rate fluctuations in unsteady flames. These techniques are validated in generic configurations for perfectly premixed laminar flames. The first method is an acoustic technique, which is based on the measurement of the travel time of ultrasonic waves through the flames. Fluctuations of the sound propagation time transmission through unsteady flames are used to estimate perturbations in the burned gases width along the acoustic path. This information is then used to reconstruct heat release rate fluctuations. This technique is validated in the cases of unstable laminar premixed flames driven by buoyancy forces and for flames submitted to harmonic flow velocity modulations. Analytical expressions are derived linking fluctuations in heat release rate and disturbances of the sound travel time. Measurements made with this acoustic technique are compared with optical detections based on the flame chemiluminescence and with predictions from an analytical model. Good agreements are obtained between these different methods validating the proposed technique. The second method envisaged is an optical technique based on a Laser Interferometric Vibrometer used to measure integrated density perturbations along the optical path of a laser beam. It is shown that density disturbances along this path result mainly from heat release rate fluctuations when the flames are confined. A link is established to reconstruct heat release rate disturbances from the signal of the interferometer. The technique is validated in the case of pulsated laminar premixed flames. Measurements are compared to line-of-sight integrated chemiluminescence emission measurements. A good agreement is obtained for harmonic flow modulations at different forcing frequencies and perturbation levels for flames operating at different flow conditions. This work validates the principle of this alternative technique for detecting heat release rate perturbations.

Les fluctuations du taux de dégagement de chaleur sont souvent responsables d’intensification des flux thermiques aux parois, de vibrations et d’émissions sonores qui peuvent éventuellement dégénérer en instabilités thermo-acoustiques auto-entretenues. Ces phénomènes instationnaires dégradent les performances des foyers, provoquent un vieillissement prématuré de certains éléments de la chambre de combustion, voire des dégâts plus importants sur l’installation. Ces perturbations sont cependant difficiles à mesurer dans les foyers car il n’existe pas de diagnostic qui permette d'accéder directement au taux de dégagement de chaleur. L’objectif de ce travail est d'explorer deux alternatives aux solutions existantes pour accéder aux fluctuations du taux de dégagement de chaleur avec une bonne résolution temporelle. Ces nouvelles méthodes sont testées dans des configurations génériques parfaitement prémélangées pour des écoulements laminaires. La première méthode est une technique acoustique, qui repose sur la détermination du temps de vol d’ondes ultrasonores qui traversent l’écoulement. Les fluctuations du temps de vol de ces ondes sont utilisées pour détecter des perturbations de la largeur des gaz brûlés le long du chemin acoustique. Cette information permet de reconstituer les fluctuations du taux de dégagement de chaleur dans des flammes prémélangées. Les premières validations de cette méthode sont présentées pour des flammes en l'absence de perturbation externe lorsqu'elles présentent une instabilité de type Kelvin-Helmholtz pilotée par les phénomènes de flottabilité du panache des gaz brûlés. Des mesures sont ensuite conduites pour des flammes soumises à des modulations harmoniques de l'écoulement. Les données obtenues dans ces configurations sont comparées à des mesures optiques ainsi qu'à des prévisions analytiques. La seconde méthode est une technique optique utilisant un système d’interférométrie laser Doppler permettant de déterminer les fluctuations de densité intégrées le long du chemin optique. On montre dans un premier temps que les perturbations de densité sont principalement causées par des fluctuations du taux de dégagement de chaleur lorsque les flammes sont confinées. Un lien est établi pour reconstituer les perturbations du taux de dégagement de chaleur exploitant le signal de l'interféromètre. La technique est validée pour des flammes pulsées pour différentes richesses et débits. Les données obtenues sont comparées à des mesures reposant sur la chimiluminescence de la flamme. Un bon accord est obtenu pour des modulations harmoniques de l'écoulement à différentes fréquences et niveaux de perturbation. Ce travail permet de valider le principe de ces deux techniques pour détecter les perturbations du taux de dégagement de chaleur lorsque l'accès optique à la zone de combustion est réduit et lorsque des informations quantitatives résolues temporellement sont nécessaires. / Heat release rate disturbances are the sources of additional thermal stresses, direct and indirect combustion noise and undesirable vibrations. In extreme cases, these perturbations may even cause destructive combustion instabilities. These quantities are difficult to measure in practical burners. The objective of this work is to develop two alternative diagnostics to measure heat release rate fluctuations in unsteady flames. These techniques are validated in generic configurations for perfectly premixed laminar flames. The first method is an acoustic technique, which is based on the measurement of the travel time of ultrasonic waves through the flames. Fluctuations of the sound propagation time transmission through unsteady flames are used to estimate perturbations in the burned gases width along the acoustic path. This information is then used to reconstruct heat release rate fluctuations. This technique is validated in the cases of unstable laminar premixed flames driven by buoyancy forces and for flames submitted to harmonic flow velocity modulations. Analytical expressions are derived linking fluctuations in heat release rate and disturbances of the sound travel time. Measurements made with this acoustic technique are compared with optical detections based on the flame chemiluminescence and with predictions from an analytical model. Good agreements are obtained between these different methods validating the proposed technique. The second method envisaged is an optical technique based on a Laser Interferometric Vibrometer used to measure integrated density perturbations along the optical path of a laser beam. It is shown that density disturbances along this path result mainly from heat release rate fluctuations when the flames are confined. A link is established to reconstruct heat release rate disturbances from the signal of the interferometer. The technique is validated in the case of pulsated laminar premixed flames. Measurements are compared to line-of-sight integrated chemiluminescence emission measurements. A good agreement is obtained for harmonic flow modulations at different forcing frequencies and perturbation levels for flames operating at different flow conditions. This work validates the principle of this alternative technique for detecting heat release rate perturbations.

Numerical simulations have been done with the CHEMKINsoftware to study different aspects of wall effects in thecombustion of lean, laminar and premixed flames in anaxisymmetric boundary-layer flow. The importance of the chemical wall effects compared to thethermal wall effects caused by the development of the thermaland velocity boundary layer has been investigated in thereaction zone by using different wall boundary conditions, walltemperatures and fuel/air ratios. Surface mechanisms include acatalytic surface (Platinum), a surface that promotesrecombination of active intermediates and a completely inertwall with no species and reactions as the simplest possibleboundary condition. When hydrogen is the model fuel, the analysis of the resultsshow that for atmospheric pressure and a wall temperature of600 K, the surface chemistry gives significant wall effects atthe richer combustion case (f=0.5), while the thermal andvelocity boundary layer gives rather small effects. For theleaner combustion case (f=0.1) the thermal and velocityboundary layer gives more significant wall effects, whilesurface chemistry gives less significant wall effects comparedto the other case. For methane as model fuel, the thermal and velocity boundarylayer gives significant wall effects at the lower walltemperature (600 K), while surface chemistry gives rather smalleffects. The wall can then be modelled as chemically inert forthe lean mixtures used (f=0.2 and 0.4). For the higher walltemperature (1200 K) the surface chemistry gives significantwall effects. For both model fuels, the catalytic wall unexpectedlyretards homogeneous combustion of the fuel more than the wallthat acts like a sink for active intermediates. This is due toproduct inhibition by catalytic combustion. For hydrogen thisoccurs at atmospheric pressure, but for methane only at thehigher wall temperature (1200 K) and the higher pressure (10atm). As expected, the overall wall effects (i.e. a lowerconversion) were more pronounced for the leaner fuel-air ratiosand at the lower wall temperatures. To estimate a possible discrepancy in flame position as aresult of neglecting the axial diffusion in the boundary layerassumption, calculations have been performed with PREMIX, alsoa part of the CHEMKIN software. With PREMIX, where axialdiffusion is considered, steady, laminar, one-dimensionalpremixed flames can be modelled. Results obtained with the sameinitial conditions as in the boundary layer calculations showthat for the richer mixtures at atmospheric pressure the axialdiffusion generally has a strong impact on the flame position,but in the other cases the axial diffusion may beneglected. Keywords:wall effects, laminar premixed flames,platinum surfaces, boundary layer flow / QC 20100504

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Turbulent combustion systems are physically complex processes that involve many interdependent phenomena---including turbulent fluid dynamics, multi-component mass transfer, convective and radiative heat transfer, and multiphase flow---that occur over a wide range of length and time scales. Modeling and simulation studies complement experimental work by implementing and validating models and providing predictive capabilities, but current software tools are often limited by a lack of standardization and best practices, non-robust implementation, or over-specialization. Some topics in combustion CFD research, notably radiative heat transfer and soot modeling, are critically underrepresented in simulation studies as a result of software limitations. This project establishes and develops three computational tools designed for use in combustion CFD: the ODT code implements the one-dimensional turbulence (ODT) model in its most reliable form, increasing its potential for application to turbulent flow problems of interest to engineers; RadLib is a standalone library of validated radiative property models intended for application to combustion systems; and SootLib is a library of validated models for soot chemistry and particle size distribution treatments, including four moment methods and one sectional model. All three tools are open-source, cross-platform model implementations that incorporate aspects of modern software design intended to make them flexible, consistent, and easy to use and expand upon. The tools developed in this project provide researchers with convenient access to modeling tools for complex phenomena that might otherwise require significant investments of time and resources to implement individually. They also provide established frameworks on which new models can be developed and communicated, offering unparalleled potential for comparative and parametric studies of combustion processes.

The prevalence of gas turbines operating in primarily lean premixed modes is predicated on the need for lower emissions and increased efficiency. An enhancement in the mixing process through the introduction of swirl in the combustion reactants is also necessary for flame stabilization. The resulting lean swirling flames are often characterized by a susceptibility to feedback between velocity, pressure and heat release perturbations with a potential for unstable self-amplifying dynamics. The existing literature on combustion dynamics is predominantly dedicated to premixed flame configurations motivated by power generation and propulsive gas turbine applications. In the present research effort, an investigation into the response of atmospheric, non-premixed swirling flames to acoustic perturbations at various frequencies (fp = 0-315Hz) and swirl intensities (S=0.09 and S=0.34) is carried out. The primary objective of the research effort is to broaden the scope of fundamental understanding in flame dynamics in the literature to include non-premixed swirling flames. Applications of the research effort include control strategies to mitigate the occurrence of thermoacoustic instabilities in future power generation gas turbines. Flame heat release is quantitatively measured using a photomultiplier with a 430nm bandpass filter for observing CH* chemiluminescence which is simultaneously imaged with a phase-locked CCD camera. Acoustic perturbations are generated with a loudspeaker at the base of an atmospheric co-flow burner with resulting velocity oscillation amplitudes, u'/Uavg in the 0.03-0.30 range. The dependence of flame dynamics on the relative richness of the flame is investigated by studying various constant fuel flow rate flame configurations. The effect of varying fuel flow rates on the flame response is also examined using with dynamic time-dependent fuel supply rates over the data acquisition period. The Particle Image Velocimetry (PIV) method is used to study the isothermal flow field associated with acoustic pulsing. The acoustic impedance, wavelet analysis, Rayleigh criteria and phase conditioning methods are used to identify fundamental mechanisms common to highly responsive flame configurations.

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