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.

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.

The utilization of ammonia as a fuel is a pragmatic approach to pave the way towards a low-carbon economy. Ammonia compromises almost 18 % of hydrogen by mass and accepted as one of the hydrogen combustion enablers with existing infrastructure for transportation and storage. From an environmental and sustainability standpoint, ammonia combustion is an attractive energy source with zero carbon dioxide emissions. However, from a practical point of view, the direct combustion of ammonia is not feasible due to the low reactive nature of ammonia. Due to the low combustion intensity, and the higher nitrogen oxide emission, ammonia was not fully investigated and there is still a lack of fundamental knowledge of ammonia combustion. In this thesis, the computational study of ammonia premixed flame characteristics under various hydrogen addition ratios and moderate or intense low oxygen dilution (MILD) conditions were investigated. Particularly, the heat release characteristics and dominant reaction pathways were analyzed. The analysis revealed that the peak of heat release for ammonia flame occurs near burned gas, which raises a question regarding the physics of this. Further analysis identified the dominant reaction pathways and the intermediate species (NH2 and OH), which are mainly produced in the downstream and back diffused to the leading edge and produce some heat in the low-temperature zone. To overcome low reactivity and poor combustion performance of pure ammonia mixture, the onboard ammonia decomposition to hydrogen and nitrogen followed by blending ammonia with hydrogen is a feasible approach to improve ammonia combustion intensity. With increasing hydrogen amount in the mixture, the enhancement of heat release occurs due to both transport and chemical effect of hydrogen. Another approach to mitigate the low reactive nature of ammonia may be eliminated by applying the promising combustion concept known as MILD combustion. The heat release characteristics and flame marker of ammonia turbulent premixed MILD combustion were investigated. The high fidelity numerical simulation was performed to answer fundamental questions of ammonia turbulent premixed combustion characteristics.

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

An experimental study of sooting characteristics of laminar underventillated ethylene non-premixed flames in hot vitiated environments was performed using a modified Wolfhard-Parker co-flowing slot burner. The burner could be operated in "single mode" with a cold air/oxygen mixture as the oxidizer for the non-premixed flame or in varying degrees of "dual mode" where the products of lean premixed hydrogen/air/oxygen flames were used as the oxidizer for the non-premixed flame. Premixed flame stoichiometries of 0.3 and 0.5 were considered for the dual mode cases. Dual mode operation of the burner was intended to simulate the conditions of fuel rich pockets of gas burning in the wake of previously burned fuel/air mixture as typically found in real nonpremixed combustion devices. Dual mode operation introduced competing thermal and chemical effects on soot chemistry. Experimental conditions were chosen to match peak nonpremixed flame temperatures among the cases by varying oxidizer inert (N2) concentration to minimize the dual mode thermal effect. In addition the molecular oxygen (post premixed flame for dual mode cases) and ethylene fuel flow rates were held constant to maintain the same overall equivalence ratio from case to case. Thermocouple thermometry utilizing a rapid insertion technique and radiation corrections yielded the gas temperature field. Soot volume fractions were measured simultaneously with temperature using Thermocouple Particle Densitometry (TPD). Soot volume fraction, particle size and particle number density fields were measured using laser light scattering and extinction. Gas velocities were measured using Particle Imaging Velocimetry (PIV) on the non-premixed flame centerline by seeding the ethylene flow and calculated in the oxidizer flow stream. Porous sinters in the oxidizer slots prevented oxidizer particle seeding required for PIV measurements. In general as the degree of dual mode operation was increased (i.e. increasing stoichiometry of the premixed flames) soot volume fractions decreased, particle sizes increased and soot particle number densities decreased. This trend is suspected to be result of water vapor elevating OH concentrations near the flame front in dual mode operation reducing soot particle nucleation early in the flame by oxidizing soot precursors. The larger particle sizes measured at later stages of dual mode flames are suspected to be the result of lower competition for surface growth species for the lower particle number densities in those flames. Integrated soot volume fraction and particle number fluxes at various heights in the flame decreased with increasing degree of dual mode operation. / Master of Science

Le contrôle de la production des particules de suies est aujourd'hui un enjeu industriel majeur en raison de leur impact néfaste tant sur le climat que sur la santé humaine et de leur forte contribution aux transferts radiatifs. Pour mieux comprendre et contrôler la production de ces polluants dans les foyers industriels, il est primordial d’améliorer nos connaissances à ce sujet dans un brûleur turbulent. L’objectif de cette thèse est donc de mettre en place des diagnostics optiques pour l’étude des flammes suitées turbulentes et pour caractérise la production de suies dans une nouvelle configuration de combustion prémélangée,confinée, swirlée turbulente académique tout en se rapprochant des configurations industrielles. Une première configuration expérimentale laminaire est donc considérée afin de valider la mise en place de la technique d’Incandescence Induite par Laser (LII) pour mesurer la fraction volumique de suies fv. Il s’agit d’un brûleur conçu à l’université de Yale qui permet la stabilisation d’une flamme laminaire de diffusion éthylène/air. Ce brûleur a été largement étudié dans la littérature nous permettant ainsi de comparer nos mesures aux résultats de différentes équipes internationales. La calibration du signal LII avec la technique MAE (Modulated Absorption Emission) a été effectuée via une collaboration avec l’UPMC, permettant de mesurer quantitativement fv et de comparer les techniques MAE et LII. Le brûleur a ensuite été équipé d’un haut-parleur afin de moduler l’écoulement et de pouvoir étudier les effets d’une perturbation contrôlée sur la production de suies, se rapprochant ainsi des phénomènes instationnaires caractéristiques des écoulements turbulents. Enfin, les effets d’élargissement de la nappe laser sur les résultats de la LII sont examinés afin de pouvoir appliquer ce diagnostic optique dans une configuration turbulente innovante caractérisée par de grandes dimensions. Ce brûleur (EM2Soot) a été développé pour mesurer la production de suies dans une flamme turbulente swirlée riche confinée prémélangée. Il permet de quantifier indépendamment les effets de la richesse, de la puissance et de l’environnement thermique sur la production de suies. Un point de fonctionnement représentatif a alors été étudié et, en parallèle avec la LII, les techniques de vélocimétrie par images de particules (PIV), et de mesure de température des parois par phosphorescence induite par laser (LIP) ont été employées afin de caractériser l’effet de la turbulence sur la production des suies et d’établir une base de données pour la validation de futures simulations numériques. Enfin, la géométrie du brûleur a été modifiée permettant une stabilisation différente de la flamme (en forme d’un V). Un nouveau point de fonctionnement a alors été étudié afin de mettre en évidence le rôle de la géométrie de l’injecteur sur la stabilisation de la flamme et, par conséquent, la production totale de suies. / The control of soot particles production represents today a major industrial issue because of their harmful impact on both the climate and the human health and their strong contribution to the radiative transfers. To better understand and control the production of these polluting emissions, it is essential to improve our knowledge on this subject in a turbulent burner. The objective of this Ph.D. is to set up optical diagnostics for the study of turbulent flames and to experimentally characterize soot production in a new academic turbulent premixed combustion configuration while approaching industrial configurations, generally confined and swirled flows. For this, a laminar experimental configuration is first considered to validate the implementation of the Laser Induced Incandescence (LII) technique to measure the soot volume fraction fv. This burner designed at Yale University allows the stabilization of a laminar ethylene/air diffusion flame. This burner has been widely studied in the literature, so that it is possible to compare the quality of our measurements with the results of different international teams. Through collaborations with the UPMC, we calibrated the LII signal with the MAE (Modulated Absorption Emission) technique, making it possible to quantitatively measure fv and to compare the MAE and LII techniques. Finally, the burner was equipped with a loudspeaker to modulate the flow and to study the effects of a controlled perturbation on the soot production, thus approaching the unsteady phenomena characteristics of turbulent flows. Finally, the effects of the enlargement of the laser sheet on LII results were also investigated in order to be able to apply this diagnostic technique in an innovative large turbulent configuration. This experimental configuration, called EM2Soot, was developed to measure the production of soot in a turbulent swirled rich confined premixed ethylene/air flame. This burner makes it possible to independently quantify the effects of the equivalence ratio, the total flame power and the thermal environment on the total soot production. A representative operating point was then characterized, in parallel with LII measurements, Particle Image Velocimetry (PIV) and Laser Induced Phosphorescence (LIP) techniques have been employed in order to characterize the effect of the turbulence on soot production and to establish a database for the validation of future numerical simulations. Finally, the geometry of the burner has been modified allowing a different stabilization of the flame (V flame shape). A new operating point is then studied in order to highlight the role of the injector geometry on the stabilization of the flame and, consequently, on the total soot production.

No description available.

No description available.

No description available.