O interesse no desenvolvimento de sistemas eficientes de combustão para reduzir a poluição ambiental e aumentar a eficiência de queima tem chamado a atenção para a combustão em meios porosos inertes. A recirculação de calor, induzida pela matriz sólida a partir dos produtos quentes para os reagentes frios, aumenta a temperatura da chama melhorando a sua estabilidade e permitindo a utilização de combustíveis com baixo poder calorífico. Um estudo teórico recente mostra que uma chama estabilizada por um plano de estagnação imersa em um meio poroso pode, sob certas condições, estender os limites de inflamabilidade de uma mistura de ar e combustível. Por outro lado, o plano de estagnação é um problema que simula o efeito da taxa de deformação do escoamento sobre a estabilidade da chama, o que é relevante para várias configurações de queimador poroso. Portanto, o foco deste trabalho é o estudo da combustão em um queimador poroso com um plano de estagnação. Um experimento é conduzido com empacotamento de esferas, onde uma chama pode ser estabilizada por plano de estagnação devido a um anteparo. A razão de equivalência e a taxa de deformação são controladas pelos fluxos de ar e de combustível e da distância entre injetor e anteparo. A posição da chama é aproximadamente determinada pelo campo de temperaturas medidas por termopares. Complementarmente é realizada uma análise numérica simplificada do problema na qual se pode verificar o efeito da taxa de deformação sobre a estabilidade de chamas em queimadores porosos. Os resultados mostram que é possível estabilizar chamas no interior do meio poroso com plano de estagnação, porém, não foi possível atribuir um aumento de temperatura devido ao aumento da taxa de deformação. / The interest in developing efficient combustion systems to reduce environmental pollution and increase the burning efficiency has called attention to the combustion in inert porous media. The heat recirculation, induced by the solid matrix, from the hot products to the incoming cold reactants, increases the flame temperature and improves its stability, allowing for the use of fuels with low heat content. A recent study shows theoretically that a flame stabilized by a stagnation plane immersed in a porous medium may, under certain conditions, to extend the flammability limits of a mixture of fuel and air. On the other hand, the stagnation plane imposes a certain strain rate on the flow field, which is relevant to various porous burner configurations. Therefore, the focus of this work is the study of combustion in a porous burner with a stagnation plane. An experiment is conducted with packing bed of spheres where a flame can be stabilized against a stagnation plane. The equivalence ratio and the strain rate are controlled by the flows of air and fuel and the distance between the injector and the stagnation plane. The flame position is approximately determined by the temperature field measured by thermocouples. In addition, it is performed a simplified numerical analysis of the problem in which one can see the effect of the strain rate on the stability of flames in porous burners. The results show that it is possible to stabilize flames within the porous medium with stagnation plane, however, it has not been possible to assign a temperature increase due to the increased strain rate.

Low swirl burners (LSBs) have gained popularity in heating and gas power generation industries, in part due to their proven capacity for reducing the production of NOx, which in addition to reacting to form smog and acid rain, plays a central role in the formation of the tropospheric ozone layer. With lean operating conditions, LSBs are susceptible to combustion instability, which can result in flame extinction or equipment failure. Extensive work has been performed to understand the nature of LSB combustion, but scaling trends between laboratory- and industrial-sized burners have not been established. Using hydrogen addition as the primary method of flame stabilization, the current work presents results for a 2.54 cm LSB to investigate potential effects of burner outlet diameter on the nature of flame stability, with focus on flashback and lean blowout conditions. In the lean regime, the onset of instability and flame extinction have been shown to occur at similar equivalence ratios for both the 2.54 cm and a 3.81 cm LSB and depend on the resolution of equivalence ratios incremented. Investigations into flame structures are also performed. Discussion begins with a derivation for properties in a multicomponent gas mixture used to determine the Reynolds number (Re) to develop a condition for turbulent intensity similarity in differently-sized LSBs. Based on this requirement, operating conditions are chosen such that the global Reynolds number for the 2.54 cm LSB is within 2% of the Re for the 3.81 cm burner. With similarity obtained, flame structure investigations focus on flame front curvature and flame surface density (FSD). As flame structure results of the current 2.54 cm LSB work are compared to results for the 3.81 cm LSB, no apparent relationship is shown to exist between burner diameter and the distribution of flame surface density. However, burner diameter is shown to have a definite effect on the flame front curvature. In corresponding flow conditions, a decrease in burner diameter results a broader distribution of curvature and an increased average curvature, signifying that compared to the larger 3.81 cm LSB, the flame front of the smaller burner contains tighter, smaller scale wrinkling.

The combustion of petroleum-based fuels leads to the formation of several pollutants. Among them, soot particles are particularly harmful due to their severe consequences on human health. Over the past decades, strict regulations have been placed on automotive and aircraft engines to limit these particulate matter emissions. This work is primarily focused on understanding the fundamental behaviour of soot particles and their formation. Though the focus of this work is on soot formation and growth pathways, the study of the gas-phase combustion process was also an integral part to validate the mechanism. A reduced mechanism is developed with retaining the larger PAH species till coronene from KAUST-ARAMCO mechanism. Counterflow diffusion flames had emphasized the simulation of canonical configuration where the reduced mechanism is validated and the soot growth pathways are evaluated. The importance of the significant contribution of larger PAH species on the soot growth pathways in both SF and SFO flames is evident in this analysis. The sensitivity of these flames with respect to strain rates, dilution, and at higher pressures are analysed. Direct Numerical Simulation (DNS) of two-dimensional counterflow diffusion flames is conducted to understand the impact of vortex interactions on soot characteristics. The results indicate that the larger PAH species contributes to the soot formation in the air-side perturbation regimes, whereas the soot formation is dominated by the soot transport in fuel-side perturbation. The study is extended to simulate and compare coflow laminar flame using different statistical moment methods MOMIC, HMOM and CQMOM.

Heat-release in CI engines occurs in the presence of concentration and temperature gradients. Recognizing the need for a validation of chemical kinetic models in transport-affected systems, this study employs non-premixed systems to better understand complex couplings between low/high temperature oxidation kinetics and diffusive transport. This dissertation is divided into two sections. In the first section, a two-stage Lagrangian model compares model prediction of ignition delay time and experimental data from the KAUST ignition quality tester, and ignition data for liquid sprays in constant volume combustion chambers. The TSL employed in this study utilizes detailed chemical kinetics while also simulating basic mixing processes. The TSL model was found to be efficient in simulating IQT in long ignition delay time fuels; it was also effective in CVCC experiments with high injection pressures, where physical processes contributed little to ignition delay time. In section two, an atmospheric pressure counterflow burner was developed and fully validated. The counterflow burner was employed to examine the effects of molecular structure on low/high temperature reactivity of various fuels in transport-affected systems. These effects were investigated through measurement of conditions of extinction and ignition of various fuel/oxidizer mixtures. Data generated were used to validate various chemical kinetic models in diffusion flames. Where necessary, suggestions were made for improving these models. For hot flames studies, tested fuels included C3-C4 alcohols and six FACE gasoline fuels. Results for alcohols indicated that the substituted alcohols were less reactive than the normal alcohols. The ignition temperature of FACE gasoline was found to be nearly identical, while there was a slight difference in their extinction limits. Predictions by Sarathy et al. (2014) alcohol combustion model, and by the gasoline surrogate model (Sarathy et al., 2015), agreed with the experimental data. For cool diffusion flames studies, tested fuels included butane isomers, naphtha, gasolines and their surrogates. Results revealed that the addition of ozone successfully established cool flames in the fuels at low and moderate strain rates. Numerical simulations were performed to replicate the extinction limits of the cool flames of butane isomers. The model captured experimental trends for both fuels; but over-predicted their extinction limits.

Considering the importance of combustion characteristics in combustion applications including spark ignition engines and gas turbines, both laminar and turbulent burning velocities were measured for gasoline related fuels. The first part of the present work focused on the measurements of laminar burning velocities of Fuels for Advanced Combustion Engines (FACE) gasolines and their surrogates using a spherical constant volume combustion chamber (CVCC) that can provide high-pressure high-temperature (HPHT) combustion mode up to 0.6 MPa, 395 K, and the equivalence ratios ranging 0.7-1.6. The data reduction was based on the linear and nonlinear extrapolation models considering flame stretch effect. The effect of flame instability was investigated based on critical Peclet and Karlovitz, and Markstein numbers. The sensitivity of the laminar burning velocity of the aforementioned fuels to various fuel additives being knows as octane boosters and gasoline extenders including alcohols, olfins, and SuperButol was investigated. This part of the study was further extended by examining exhaust gas re-circulation effect. Tertiary mixtures of toluene primary reference fuel (TPRF) were shown to successfully emulate the laminar burning characteristics of FACE gasolines associated with different RONs under various experimental conditions. A noticeable enhancement of laminar burning velocities was observed for blends with high ethanol content (vol ≥ 45 %). However, such enhancement effect diminished as the pressure increased. The reduction of laminar burning velocity cause by real EGR showed insensitivity to the variation of the equivalence ratio. The second part focused on turbulent burning velocities of FACE-C gasoline and its surrogates subjected to a wide range of turbulence intensities measured in a fan-stirred CVCC dedicated to turbulent combustion up to initial pressure of 1.0 MP. A Mie scattering imaging technique was applied revealing the mutual flame-turbulence interaction. Furthermore, considerable efforts were made towards designing and commissioning a new optically-accessible fan-stirred HPHT combustion vessel. A time-resolved stereoscopic particle image velocimetry (TR-PIV) technique was applied for the characterization of turbulent flow revealing homogeneous-isotropic turbulence in the central region to be utilized successfully for turbulent burning velocity measurement. Turbulent burning velocities were measured for FACE-C and TPRF surrogate fuels along with the effect of ethanol addition for a wide range of initial pressure and turbulent intensity. FACE-C gasoline was found to be more sensitive to both primarily the primary contribution of turbulence intensification and secondarily from pressure in enhancing its turbulent burning velocity. Several correlations were validated revealing a satisfactory scaling with turbulence and thermodynamic parameters. The final part focused on the turbulent burning characteristics of piloted lean methane-air jet flames subjected to a wide range of turbulence intensity by adopting TR-SPIV and OH-planar laser-induced florescence (OH-PLIF) techniques. Both of the flame front thickness and volume increased reasonably linearly as normalized turbulence intensity, u^'/ S_L^0, increased. As u^'/ S_L^0 increased, the flame front exhibited more fractalized structure and occasionally localized extinction (intermittency). Probability density functions of flame curvature exhibited a Gaussian like distribution at all u^'/ S_L^0. Two-dimensional flame surface density (2D-FSD) decreased for low and moderate u^'/ S_L^0, while it increased for high u^'/ S_L^0Turbulent burning velocity was estimated using flame area and fractal dimension methods showing a satisfactory agreement with the flamelet models by Peters and Zimont. Mean stretch factor was estimated and found to increase linearly as u^'/ S_L^0increased. Conditioned velocity statistics were obtained revealing the mutual flame-turbulence interaction.

Flame retardant chemicals were used to make flammable fibres or fabrics flame retardant. Flame retardants protect the flammable material from fire by delaying or preventing the ignition process. The problem with flame retardants is unreliable durability when applied physically or bonded chemically on the surface of the fibre or fabric. This thesis project investigated the implementation of inherently flame retardant fibres as a shield form flame for flammable fibres. The most widely used flammable textiles fibres (cotton and polyester) were mixed with inherently flame retardant fibres (modacrylic and Lenzing FR) pairwise at fibre level for non-woven fabric and both fibre & yarn level for knitted fabric. The vertical flame test, where the fabric hung vertically and burned from the bottom, was used to characterise their burning behaviour. With the vertical flame test, it was found that flame shielding ability of inherently flame retardant fibres towards flammable fibres improves with an increasing proportion of inherently flame retardant fibres in the fabric. Also, fabric structure influences the shielding properties of the flame retardant fibres. A comparison between fibre and yarn level mixing for knitted fabric yarn level mixing was found to have better flame shielding properties. Thesis work points out the issue with flame retardant chemical and presents an alternative approach for conventional flame retardant.

To contribute to the design of next-generation high performance and low emission combustion devices, this study provides a series of high fidelity numerical simulations of turbulent premixed combustion and autoignition with different clean fuels. The first part of the thesis consists of the direct numerical simulations (DNS) of the lean hydrogen-air turbulent premixed flames at a wide range of Karlovitz number (Ka) conditions up to Ka = 1,126. Turbulence-chemistry interaction is discussed in terms of statistical analysis of the turbulent flame speed and flame structure. Global and local flame speed are separately studied through the fuel consumption speed and displacement speed of the flame front, respectively, and the results are compared with the reference laminar flames as well as similar studies in the literature. The global flame structure is assessed via cross-sectional and conditional averages, and modeling implication is further discussed. Detailed analysis of the local flame structure along the positive and negative curvature is also conducted, providing an understanding of the different behavior of local heat release response. Finally, as the modeling perspectives for Reynolds-averaged Navier-Stokes (RANS) and large eddy simulations (LES), the mean quantities of major species, intermediate species, density, the reaction rate of the progress variable, and heat release rate are assessed in the context of the probability density function (PDF). The second part of the thesis consists of applications of the advanced mathematical tool called the computational singular perturbation (CSP). A skeletal chemical mechanism is developed using the CSP algorithm for the autoignition of methanol and dimethyl ether blends, and the ignition delay time and laminar flame speed are validated for a wide range of mixture conditions. A series of autoignition simulations are carried out in the canonical counter flow mixing layer using the developed skeletal mechanism, and detailed analyses of the autoignition for the methanol and dimethyl ether blends at a wide range of strain rate conditions are provided using the CSP diagnostics tools for a wide range of chemical and fluid combinations.

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Experimental measurements of the lift-off velocity and lift-off height, and numerical simulations were conducted on the liftoff and stabilization phenomena of laminar jet diffusion flames of inert-diluted C3H8 and CH4 fuels. Both non-reacting and reacting jets were investigated, including effects of multi-component diffusivities and heat release (buoyancy and gas expansion). The role of Schmidt number for non-reacting jets was investigated, with no conclusive Schmidt number criterion for liftoff previously known in similarity solutions. The cold-flow simulation for He-diluted CH4 fuel does not predict flame liftoff; however, adding heat release reaction leads to the prediction of liftoff, which is consistent with experimental observations. Including reaction was also found to improve liftoff height prediction for C3H8 flames, with the flame base location differing from that in the similarity solution - the intersection of the stoichiometric and iso-velocity contours is not necessary for flame stabilization (and thus lift-off). Possible mechanisms other than that proposed for similarity solution may better help to explain the stabilization and liftoff phenomena. The stretch rate at a wide range of isotherms near the base of the lifted tribrachial flame were also quantitatively plotted and analyzed.