Lean premixed combustors have been designed to lower NOx and other pollutant levels in land based gas turbines. These combustors are often susceptible to thermo-acoustic instabilities, which manifest as pressure and heat release oscillations in the combustor. To be able to predict and control these instabilities, it is required that both the acoustics of the system, and a frequency-resolved response of the combustion process to incoming perturbations be understood. Currently, a system-level approach is being used widely to predict the thermoacoustic instabilities. This approach requires simple, yet accurate models which would describe the behavior of each dynamic block within the loop. The present study is directed toward using computational fluid dynamics (CFD) as a tool in developing reduced order models for the dynamics of laminar flat flames and swirl stabilized turbulent flames. A finite-volume based approach is being used to simulate reacting flows in both laminar and turbulent combustors. The study has been divided into three parts -- the first part involves the modeling of a self-excited combustor (the acoustics of the combustor are coupled with the unsteady heat release); the second part of the research aims to study the effect of velocity perturbations on the unsteady heat release rate from a burner stabilized laminar flat flame; the third and final part of work involves an extension of the laminar flat flame study to turbulent reacting flows in a swirl stabilized combustor, and study the effects on the turbulent heat release due to the velocity perturbations. A Rijke tube combustor was selected to study self-excited combustion phenomenon. A laminar premixed methane-air flat flame was stabilized on a honeycomb flame-stabilizer. The flame stabilizer was placed at the center of the 5 feet vertical tube. The position of the flame at the center of the tube leads to a thermoacoustic instability of the 2nd acoustic mode. The fundamental thermoacoustic frequency was predicted accurately by the CFD model and the amplitude was reasonably matched (for a flow rate of Q = 120 cc/s and equivalence ratio phi = 1.0). Other characteristics of the pressure power spectrum were captured to a good degree of accuracy. This included the amplitude modulation of the fundamental and the harmonics due to a subsonic pulsating instability. The flat flame study has been being conducted for Q = 200 cc/s and equivalence ratio phi = 0.75. The objective has been to obtain a frequency response function (FRF) of the unsteady heat release rate (output) due to incoming velocity perturbations (input). A range of frequencies (15 Hz - 500 Hz) have been selected for generating the FRF. The aim of this part of the study has been to validate the computational model against the experimental results and propose a physics based interpretation of the flame response. Detailed heat transfer modeling (including radiation heat transfer) and two-step chemistry models have been implemented in the model. The FRF generated has been able to reproduce the experimentally observed phenomena, like the low frequency pulsating instability occurring at 30 Hz. A heat transfer study has been conducted to explain the pulsating instability and a fuel variability study has been performed. Both the heat transfer study and the fuel variability study proved the role of heat transfer in creating the pulsating instability. The final part of the study involves simulation of reacting flow in a turbulent swirl stabilized combustor. The effect of velocity perturbations on the unsteady heat release has been studied by creating an FRF between the unsteady velocity and the unsteady heat release rate. A Large Eddy Simulation (LES) approach has been selected. A swirl number of S = 1.19 corresponding to a flow rate of Q = 20 SCFM with an equivalence ratio of phi = 0.75 have been implemented. Reduced reaction chemistry modeling, turbulence-chemistry interaction and heat transfer modeling have been incorporated in the model. The LES of reacting flow has shown vortex-flame interaction occurring inside the combustor. This interaction has been shown to occur at 255 Hz. The FRF obtained between unsteady velocity and unsteady heat release rate shows good comparison with the experimentally obtained FRF. / Ph. D.

A joint experimental and theoretical project has been initiated at Virginia Tech to study the effects of dual-mode combustion at high pressures for a two-dimensional Wolfhard-Parker burner. This thesis is the first stage of the theoretical part of the project, and contains a numerical study of laminar coflow diffusion flames stabilized on a confined Wolfhard-Parker burner. A global finite difference method is used where the nonlinear equations written on a stream function-vorticity formulation are solved with a flame sheet approach. The pseudotransient, approximative factorization method is utilized to solve the coupled system of equations. Adaptive gridding, numerical evaluation of Jacobians and iterations within time step are implemented for computational efficiency. Numerical results have been obtained for different fuels under different conditions. Comparison with measured data by Smyth et al. (1985) for a buoyancy dominated methane-air flame is made. The location of the flame front is accurately predicted. The temperature is over predicted in the fuel rich zone since pyrolysis and radiation effects have not been accounted for in the numerical model. Good agreement is observed for major species and velocities. As expected, large velocity increase and horizontal inflow of nitrogen and combustion products associated with buoyancy occur in the lower region of the flame. / Master of Science

Increasing concern for energy sustainability has created motivation for the combustion of renewable, CO2 neutral fuels. Biomass co-firing with coal provides a means of utilizing the scaled efficiencies of coal with the lower supply availability of biomass. One of the challenges of co-firing is the burnout of biomass particles which are typically larger than coal but must be oxidized in the same residence time. Larger biomass particles also can increase the length of the radiative region and alter heat flux profiles. As a result, oxygen injection is being investigated as a means of improving biomass combustion performance.An Air Liquide designed burner was used to investigate the impact of oxygen enrichment on biomass combustion using two size distributions of ground wood pellets (fine grind 220 µm and medium grind 500 µm mass mean diameter). Flame images were obtained with a calibrated RGB digital camera allowing a calculation of visible radiative heat flux. Ash samples and exhaust NO were collected for a matrix of operating conditions with varying injection strategies. The results showed that oxygen can be both beneficial and detrimental to the flame length depending on the momentum of the oxygen jet. Oxygen injection was found to improve carbon burnout, particularly in the larger wood particles. Low flow rates of oxygen enrichment (2 to 6 kg/hr) also produced a modest increase in NO formation up to 30%. The results showed medium grind ~500 µm mass mean diameter particle combustion could improve LOI from 30% to 15% with an oxygen flow rate of 8 kg/hr. Flame images showed low flow rates of O2 (2 kg/hr) in the center of the burner with the fine particles produced a dual flame, one flame surrounding the center oxygen jet and a second flame between the volatiles and secondary air. The flame surrounding the center oxygen jet produced a very high intensity and temperature (2100 K). This center flame can be used to help stabilize the flame, increase devolatilization rates, and potentially improve the trade-off between NO and burnout.

High-pressure experiments and chemical kinetics modeling were performed for laminar spherically expanding flames for methane/air, ethane/air, methane/ethane/air and propane/air mixtures at pressures between 1 and 10 atm and equivalence ratios ranging from 0.7 to 1.3. All experiments were performed in a new flame speed facility capable of withstanding initial pressures up to 15 atm. The facility consists of a cylindrical pressure vessel rated up to 2200 psi. Vacuums down to 30 mTorr were produced before each experiment, and mixtures were created using the partial pressure method. Ignition was obtained by an automotive coil and a constant current power supply capable of reducing the spark energy close to the minimum ignition energy. Optical cine-photography was provided via a Z-type schlieren set up and a high-speed camera (2000 fps). A full description of the facility is given including a pressure rating and a computational conjugate heat transfer analysis predicting temperature rises at the walls. Additionally, a detailed uncertainty analysis revealed total uncertainty in measured flame speed of approximately +-0.7 cm/s. This study includes first-ever measurements of methane/ethane flame speeds at elevated pressures as well as unique high pressure ethane flame speed measurements. Three chemical kinetic models were used and compared against measured flame velocities. GRI 3.0 performed remarkably well even for high-pressure ethane flames. The C5 mechanism performed acceptably at low pressure conditions and under-predicted the experimental data at elevated pressures. Measured Markstein lengths of atmospheric methane/air flames were compared against values found in the literature. In this study, Markstein lengths increased for methane/air flames from fuel lean to fuel rich. A reverse trend was observed for ethane/air mixtures with the Markstein length decreasing from fuel lean to fuel rich conditions. Flame cellularity was observed for mixtures at elevated pressures. For both methane and ethane, hydrodynamic instabilities dominated at stoichiometric conditions. Flame acceleration was clearly visible and used to determine the onset of cellular instabilities. The onset of flame acceleration for each high-pressure experiment was recorded.

Les instabilités de combustion induites par le couplage combustion-acoustique se produisent dans de nombreux systèmes industriels et domestiques tels que les chaudières, les turbines à gaz et les moteurs de fusée. Ces instabilités se traduisent par des fluctuations de pression et un dégagement de chaleur qui peuvent provoquer une défaillance mécanique ou des dégâts désastreux dans certains cas extrêmes. Ces phénomènes ont été largement étudiés par le passé, et les mécanismes responsables du couplage ont déjà été identifiés. Cependant, il apparaît que la plupart des systèmes se comportent différemment lors du démarrage à froid ou en régime permanent. Le couplage entre la température des parois et les instabilités de combustion reste encore méconnu et n’a pas été étudié en détail jusqu’à présent. Dans le cadre de ces travaux de thèse, on s’intéresse à ce mécanisme. Ces travaux présentent une étude expérimentale des instabilités de combustion pour une flamme laminaire de pré-mélange stabilisée sur un brûleur à fente. Pour certaines conditions de fonctionnement, le système présente un mode instable autour du mode de Helmholtz du brûleur. Il est démontré que l’instabilité peut être contrôlée, et même supprimée, en changeant uniquement la température de la surface du brûleur. Une analyse de stabilité linéaire peut être mise en œuvre afin d’identifier les paramètres jouant un rôle dans les mécanismes d’instabilité, et il est possible de modéliser analytiquement les phénomènes observés expérimentalement. Des études expérimentales détaillées de différents processus élémentaires impliqués dans le couplage thermo-acoustique ont été menées pour évaluer la sensibilité de ces paramètres à la température de la paroi. Enfin un modèle théorique du couplage entre le transfert de chaleur instationnaire à la paroi et la fluctuation du pied de flamme a été proposé. Par ailleurs, d’autres mesures expérimentales ont permis de comprendre les mécanismes physiques responsables de la dépendance de la réponse de la flamme à la température de paroi. / Combustion instabilities, induced by the resonant coupling of acoustics and combustion occur in many practical systems such as domestic boilers, gas turbine and rocket engines. They produce pressure and heat release fluctuations that in some extreme cases can provoke mechanical failure or catastrophic damage. These phenomena have been extensively studied in the past, and the basic driving and coupling mechanisms have already been identified. However, it is well known that most systems behave differently at cold start and in the permanent regime and the coupling between the temperature of the solid material and combustion instabilities still remains unclear. The aim of this thesis is to study this mechanism. This work presents an experimental investigation of combustion instabilities for a laminar premixed flame stabilized on a slot burner with controlled wall temperature. For certain operating conditions, the system exhibits a combustion instability locked on the Helmholtz mode of the burner. It is shown that this instability can be controlled and even suppressed by changing solely the temperature of the burner rim. A linear stability analysis is used to identify the parameters playing a role in the resonant coupling and retrieves the features observed experimentally. Detailed experimental studies of the different elementary processes involved in the thermo-acoustic coupling are used to evaluate the sensitivity of these parameters to the wall temperature. Finally a theoretical model of unsteady heat transfer from the flame root to the burner-rim and detailed experimental measurements permit to establish the physical mechanism for the temperature dependance on the flame response.

Acoustic instabilities in a combustion chamber arise due to the coupling of acoustic pressure with in-phase heat-release, and are characterized by large amplitude oscillations of one or more natural modes of combustor. Even though an array of studies, both theoretical and experimental, has been conducted by a number of authors in this field to extract the flame response, most of these are based on kinematic flame models. In this dissertation, an experimental study of a subsonic flame's intrinsic response to acoustic pressure perturbations is performed for the case of a tube closed at one end and the other end opened to the atmospheric conditions. Pressure fluctuations inside the tube are measured for hot and cold side flows, and their varying trend is explained. The frequencies obtained from Fourier transform analysis exhibit a strong dependence with the distance between the stabilized flame position and open end of the tube. For different values of flame position (xf ), the values of growth constant 's' are calculated from the pressure versus time data readings procured from acoustic pressure transducer and dominant frequencies are analyzed from windowed FFT of the same. The expression for obtaining response function from the measured pressure fluctuations has been derived from the 1-D linearized conservation equations. The undamped response function plot is obtained by adding the decay rates at different frequencies inside the tube to the corresponding growth rates. Finally, the effect of blockage of pre-mixed flow on the growth rates inside the tube and consequently, the flame response values, is studied by repeating the experiment with different types of flame holders. A large number of theoretical flame-response models have been developed in modern literature, and some of these models are compared with the experimentally obtained response. Suggestions are also cited in this study so as to account for the observed deviations in trends. This includes a revisit of the intrinsic flame model by incorporating the effect of flame-area perturbations, with the aid of analyzed steady flame images.

La combustion diphasique implique de nombreux phénomènes physiques complexes, comprenant l'atomisation, la dispersion, l'évaporation et la combustion. Bien que la simulation numérique soit un outil performant pour aborder ces différentes interactions entre les phases liquides et gazeuses, la méthode doit être validée par des études expérimentales fiables. Par conséquent, des données expérimentales précises sur la structure de la flamme et sur les propriétés de la phase liquide et gazeuse le long des étapes d'évaporation et de combustion sont nécessaires. La complexité des configurations aéronautiques réelles implique d'étudier l'effet des propriétés locales sur la dynamique des flammes pour une configuration canonique. Ce travail, réalisé dans le cadre du projet ANR TIMBER, a pour objectif d'améliorer la compréhension de la combustion en flux diphasique, ainsi que de produire une base de données efficace et originale pour la validation des modèles utilisés dans les LES. / Liquid fuels are the primary energy source in a wide range of applications including industrial and residential furnaces, internal combustion engines and propulsion systems. Pollutant emission reduction is currently one of the major constraints for the design of the next generation combustion chamber. Spray combustion involves many complex physical phenomena including atomization, dispersion, evaporation and combustion, which generally take place simultaneously or within very small regions in the combustion chambers. Although numerical simulation is a valuable tool to tackle these different interactions between liquid and gas phases, the method needs to be validated through reliable experimental studies. Therefore, accurate experimental data on flame structure and on liquid and gas properties along the evaporation and combustion steps are needed and are still challenging. A joint effort between numerical and experimental teams is necessary to meet tomorrow's energy challenges and opportunities. The complexity of the real aeronautical configurations implies to study the effect of local properties in flame dynamics on a canonical configuration, which presents the essential feature of very well defined boundary conditions. This work, carried out within the framework of the ANR TIMBER project, aims to improve the understanding of two-phase flow combustion, as well as to produce an efficient and original database for the validation of the models used in LES.

<div>This research has been studied a laser-induced spark in flowing gases. The relationship between the minimum ignition energy (MIE), the turbulence intensity, and the flame kernel propagation speed is considered. Plasma emission, produced by the laser-induced spark, and flame kernel generation by the plasma are investigated. The energy balance equation between an ignition energy and energy losses by heat transfer is studied at laminar flows and turbulent flows. Hydrogen and air mixtures were used in a premixed jet burner for ignition experiments. Particle image velocimetry (PIV) examined the velocity and the turbulence intensity under the turbulent flows. The flame kernel development was visualized using Schlieren imaging and infrared images (IR camera). Flame kernel temperatures were measured through Rayleigh scattering and infrared images (IR camera). Plasma evaluations were captured through an intensified CCD camera (ICCD camera). Minimum ignition energies were measured at the laminar flows and the turbulent flows. The MIE decreases with an increase in the turbulence intensity which changed by ignition locations and perforated plates at the constant bulk velocity. Improved mixing rates due to the ignition locations or the geometry of the perforated plates decrease the MIE at the constant bulk velocity. The turbulence intensity increases wrinkles in the flame kernel surface, thus the contact between the flame kernel and reactants increases due to the wrinkles. Therefore, the flame kernel propagation speed increases as the turbulence intensity is higher since the increased reaction by the wrinkles and the contact. Thus, the MIE decreases as the turbulence intensity increases at the constant ignition condition, including bulk velocities and ignition heights, since the high turbulence intensity increases the flame kernel propagation speed. Laser energy differences affect the plasma expansions by the laser absorption. Laser-supported radiation (LSR) wave speeds were measured and calculated using energy balance equations. Velocity does not affect the flame kernel temperature distribution during the early reaction steps because the plasma generates a flame kernel and determines the flame kernel temperature distribution. The MIE increases with increasing the bulk velocity. The energy losses considering convection, conduction, and radiation were calculated using the flame kernel radius, the flame kernel temperature, mixture properties, and the flame speed. The energy balance equation in the ignition of flowing gases is newly written at the laminar flows and the turbulent flows.</div>

Thesis: M.S., Massachusetts Institute of Technology, Department of Mechanical Engineering, 1982 / Includes bibliographical references. / by Dinah Georgianna McNutt. / M.S. / M.S. Massachusetts Institute of Technology, Department of Mechanical Engineering

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