Methane/air premixed and partially premixed turbulent flames at high Reynolds number are characterized using Direct Numerical Simulations (DNS) with detailed chemistry in a spatially evolving slot Bunsen configuration. Two sets of simulations are performed. A first set of simulations with fully premixed inlet conditions is considered in order to assess the effect of turbulence on the flame. Four simulations are performed at increasing Reynolds number and up to 22400, defined based on the bulk velocity, slot width, and the reactants' properties, and 22 billion grid points, making it one of the largest simulations in turbulent combustion. The simulations feature finite rate chemistry with a 16 species mechanism. To perform these simulations, few preliminary steps were required: (i) two skeletal mechanisms were developed reducing GRI-3.0; (ii) a convergence study is performed to select the proper spatial and temporal discretization and (iii) simulations of fully developed turbulent channel flows are preformed to generate the inlet conditions of the jet. The study covers different aspects of flame-turbulence interaction. It is found that the thickness of the reaction zone is similar to that of a laminar flame, while the preheat zone has a lower mean temperature gradient, indicating flame thickening. The characteristic length scales of turbulence are investigated and the effect of the Reynolds number on these quantities is assessed. The tangential rate of strain is responsible for the production of flame surface in the mean and surface destruction is due to the curvature term. A second set of simulations with inhomogeneous inlet conditions is performed to study how partial premixing and turbulence interact with the flame and with each other. The jet Reynolds number is 5600, and a 33 species mechanism is used. The effect of the inlet fluctuations is reflected on heat release rate fluctuations, however the conditional mean is not affected. The flames show thickening of the preheat zone, and for the lowest level of mixing a slight thickening of the reaction zone is observed. The effect of partially mixed mixture on the NOx formation is analyzed and no major impact was found.

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The thesis describes the characterisation and application of the laser induced incandescence technique for making soot measurements in practical devices. Laser induced incandescence is the phenomenon whereby particulates such a soot absorb laser radiation and are heated to a temperature much higher than the bath gas. The broadband incandescence signal from the hot particles can be detected and the signal is proportional to volume fraction. The technique was used to study soot in partially premixed counterflow ethylene air flames, iso-octane explosion flames, and to image soot in a D. I. Diesel engine. Mie scattering, OH-LIF and absorption were used a complementary diagnostics. Appropriate ratios of LII and Mie images allowed the relative particle size and number density to be imaged. The counter flow burner measurements were used to study the effects of strain on soot formation while the bomb work demonstrated soot production in hydro-dynamically unstable cellular flames. The Diesel engine measurements are a demonstration of optical diagnostics in a real device. In order to characterise the LII signal behaviour two types of carbon aerosol generators were built. The liquid dispersion device produces almost spherical sub-micron carbon black particles. The device was used to characterise the soot field response, laser fluence response, signal decay and spectrum of the LII signal.

The main topic of this thesis concerns the development and application of laser diagnostic techniques for accurate temperature measurements and for the determination of flamefront properties in premixed flames that can serve as input data for computational fluid dynamical (CFD) models in technical combustion. The work comprises of a number of related studies, to address problems of relevance in the field of combustion research. The first part of this work involves the development and testing of an improved method for the computation of flamefront curvature in lean premixed turbulent flames. Measurements of spatially resolved heat release rate along the flamefront were then compared with the curvature data and it could be shown that a significant correlation exists between local rate of heat release and flamefront curvature. The results here agree with predictions from CFD models and improve on previous experimental attempts to find a correlation between curvature and heat release. In the second part of this work, the focus was shifted towards the development and application of improved thermometry techniques. One study was on the improvement and application of a coherent anti-Stokes Raman spectroscopy (CARS) setup to an acoustically-forced turbulent lean premixed flame stabilised on a burner, whose design was modelled to mimic phenomena of relevance in industrial combustors. In a related previous study reported in the literature two-line OH planar laser induced fluorescence had been applied to this flame and it was suspected that the results were inaccurate. Using CARS, these inaccuracies could be verified, amounting to discrepancies in temperature of up to 47% compared to the true temperatures. A major effort towards the end of this project was focused on the improvement of traditional thermometry techniques, in order to make them more accurate, faster, and spatially resolved. A technique based on indium two-line atomic fluorescence (TLAF) thermometry was developed and applied, which employed a novel extended cavity diode laser design, and it was shown for the first time that temperature measurements with high accuracy and precision could be performed in low pressure sooting flames without recourse to calibration. Both the high precision and accuracy of the technique allowed for the deduction that the temperature in the flames studied here is relatively insensitive to changes in pressure in stark contrast to the soot volume fraction. Finally, it is shown for the first time that low power diode lasers can be used in combination with indium TLAF to measure spatially and temporally highly resolved temperatures in a quasi-continuous fashion. We demonstrated such measurements at effective rates of 3.5 kHz in a steady laminar test flame yielding an unprecedented precision of 1.5 % at ~2000 K at this measurement rate.

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.

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.

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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