Predictive modeling of pre-chamber combustion engines relies primarily on the correct prediction of laminar and turbulent flame speeds. While the latter has been rigorously derived, the former correlations are mostly semi-empirical and valid for a limited range of operating conditions. The current work aims at highlighting the fundamental significance of correct laminar flame speed prediction on numerical modeling of ultralean prechamber engine combustion. Gulder's empirical correlation for laminar flame speed was chosen for the current work. It was modified for ranges beyond what it was originally derived for. It was initially observed that the numerical results that utilize Gulder's correlation for the laminar flame speed underperform compared to the one computed from the skeletal GRI3.0 by Lu and Law. In all cases, Peters' turbulent flame speed correlation was used, which evidences that any potential difference comes from the laminar flame speed. Using Lu and Law's chemical mechanism as a reference for laminar flame speed calculations, the values of the empirical constants α, η, and ξ in Gulder's correlation were optimized to yield more accurate flame speeds at ultralean engine conditions. The updated Gulder's correlation for methane was implemented in CONVERGE, a three-dimensional computational fluid dynamics (CFD) solver, and validated against the experimental engine results from KAUST. The flame topology was also explored to correlate the observed behaviors in the pressure predictions among all tested cases. Finally, the Borghi-Peters diagram provides insightful information on combustion regimes encountered in pre-chamber combustion engines.

The thesis comprises effects of electric fields on a fundamental study of spheri­cal premixed flame propagation.Outwardly-propagating spherical laminar premixed flames have been investigated in a constant volume combustion vessel by applying au uni-directional electric potential.Direct photography and schlieren techniques have been adopted and captured images were analyzed through image processing. Unstretched laminar burning velocities under the influence of electric fields and their associated Markstein length scales have been determined from outwardly prop­agating spherical flame at a constant pressure. Methane and propane fuels have been tested to assess the effect of electric fields on the differential diffusion of the two fuels.The effects of varying equivalence ratios and applied voltages have been in­vestigated, while the frequency of AC was fixed at 1 KHz. Directional propagating characteristics were analyzed to identify the electric filed effect. The flame morphology varied appreciably under the influence of electric fields which in turn affected the burning rate of mixtures.The flame front was found to propagate much faster toward to the electrode at which the electric fields were supplied while the flame speeds in the other direction were minimally influenced. When the voltage was above 7 KV the combustion is markedly enhanced in the downward direction since intense turbulence is generated and as a result the mixing process or rather the heat and mass transfer within the flame front will be enhanced.The com­bustion pressure for the cases with electric fields increased rapidly during the initial stage of combustion and was relatively higher since the flame front was lengthened in the downward direction.

Tritium generation presents a significant hazard during operation of nuclear reactors and necessitates safety precautions in the case of a combustion incident. Accurate kinetic models should inform these safety precautions, however, reliable tritium data do not exist to generate such models. This work focuses on laminar flame speed, which is an important component of kinetic models. Current estimates based on established kinetic theories and experimental measurements of the other isotopes of hydrogen predict tritium to have a flame speed approximately 70% that of standard protium over a broad range of stoichiometries at one atmosphere in air. These estimates are based solely on isotopic mass differences and do not account for radioactive decay, which, in the case of tritium, is energetic enough to cause significant radiolysis reactions and potentially alter the radical pool for combustion. Simulations of a protium flame present compelling evidence that hydrogen flames are controlled by preferential radical diffusion from the rigorous flame region towards the unburned gases and not by heat conduction and dissociation of stable molecules. These flames rely on very low radical concentrations at the initiation region of the flame and the chemistry may be altered by a slight increase in radicals due to radioactive decay. This work also presents an experimental method suitable for measuring these radioactivity effects on tritium flame speed utilizing direct measurements of a flame propagating through a transparent tube. Measurements of protium and deuterium flame speeds using this method have proven highly repeatable and consistent with literature values while consuming much less reactant than other potential methods.

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The arising environmental concerns make emission reduction from combustion devices one of the greatest challenges of the century. Modern dry low-NOx emission combustion systems often operate under lean premixed turbulent conditions. In order to design and operate these systems efficiently, it is necessary to have a thorough understanding of combustion process in these devices. In premixed combustion, flame speed determines the conversion rate of fuel. The flame speed under highly turbulent conditions is defined as turbulent flame speed. Turbulent flame speed depends on laminar flame speed, which is a property of the combustible mixture. The goal of this thesis is to estimate laminar flame speed and NOx emissions under certain conditions for specific industrial gas turbines. For this purpose, an in-house one-dimensional code, GENE-AC, is used. At first, a data validation is performed in order to select an optimized chemical reaction mechanism which can be used safely with the fuels of interest in gas turbines. Results show that GRI-Mech 3.0 performs well in most cases. This mechanism is selected for further simulations. Secondly, laminar flame speed is calculated using GRI-Mech 3.0 at SGT-800 conditions. Results show that at gas turbine conditions, increasing ambient temperature and fuel to air ratio enhances flame speed, mainly due to faster reaction rates. Moreover, laminar flame speed is highly affected by fuel composition. In particular, adding hydrogen to a fuel changes chemical processes significantly, because hydrogen is relatively light and highly diffusive. Calculations are conducted over a range of equivalence ratios and hydrogen fractions in methane at atmospheric as well as gas turbine operating conditions. Results reveal some trends for changes in laminar flame speed, depending on hydrogen content in the mixture. The final part of the thesis involves the development of a reactor network model for the SGT-700 combustor in order to predict NOx emissions. The network model is built in GENE-AC based on results from available computational fluid dynamics (CFD) simulations of the combustor. The model is developed for full load conditions with variable pilot fuel ratios. The NOx emissions are predicted using GRI-Mech 3.0 mechanism. A parametric study shows the dependency of NOx emissions on equivalence ratio and residence time. For SGT-700 running on natural gas, NOx emissions are fitted to measurement data by tuning equivalence ratio and residence time. The model is then tested for a range of ambient temperatures and fuel compositions. It is found that, although the model can correctly predict the trends of ambient temperature and fuel effects on NOx emissions, these effects are to some extent over-estimated. Using future engine tests and amending calibration can improve the results.

Combustion has been used for a long time as a means of energy extraction. However, in the recent years there has been further increase in air pollution, through pollutants such as nitrogen oxides, acid rain etc. To solve this problem, there is a need to reduce carbon and nitrogen oxides through lean burning, fuel dilution and usage of bi-product fuel gases. A numerical analysis has been carried out to investigate the effectiveness of several reduced mechanisms, in terms of computational time and accuracy. The cases were tested for the combustion of hydrocarbons diluted with hydrogen, syngas, and bi-product fuel in a cylindrical combustor. The simulations were carried out using the ANSYS Fluent 19.1. By solving the conservations equations, several global reduced mechanisms (2-5-10 steps) were obtained. The reduced mechanisms were used in the simulations for a 2D cylindrical tube with dimensions of 40 cm in length and 2.0 cm diameter. The mesh of the model included a proper fine quad mesh, within the first 7 cm of the tube and around the walls. By developing a proper boundary layer, several simulations were performed on hydrocarbon/air and syngas blends to visualize the flame characteristics. To validate the results “PREMIX and CHEMKIN” codes were used to calculate 1D premixed flame based on the temperature, composition of burned and unburned gas mixtures. Numerical calculations were carried for several hydrocarbons by changing the equivalence ratios (lean to rich) and adding small amounts of hydrogen into the fuel blends. The changes in temperature, radical formation, burning velocities and the reduction in NOx and CO2 emissions were observed. The results compared to experimental data to study the changes. Once the results were within acceptable range, different fuels compositions were used for the premixed combustion through adding H2/CO/CO2 by volume and changing the equivalence ratios and preheat temperatures, in the fuel blends. The results on flame temperature, shape, burning velocity and concentrations of radicals and emissions were observed. The flame speed was calculated by finding the surface area of the flame, through the mass fractions of fuel components and products conversions that were simulated through the tube. The area method was applied to determine the flame speed. It was determined that the reduced mechanisms provided results within an acceptable range. The variation of the inlet velocity had neglectable effects on the burning velocity. The highest temperatures were obtained in lean conditions (0.5-0.9) equivalence ratio and highest flame speed was obtained for Blast Furnace Gas (BFG) at elevated preheat temperature and methane-hydrogen fuels blends in the combustor. The results included; reduction in CO2 and NOx emissions, expansion of the flammable limit, under the condition of having the same laminar flow. The usage of diluted natural gases, syngas and bi-product gases provides a step in solving environmental problems and providing efficient energy.

The focus of the present work is on the empirical characterization and modeling of ignition trends of ternary blends of three distinct hydrocarbon classes, namely a methyl ester (C9H18O2), a linear alkane (n-C9H20), and a cycloalkane (MCH). Numerous surrogate biofuel formulations have been proposed in the literature, yet specific blending of these species has not been studied. Moreover, the effects of blending biofuel compounds with conventional hydrocarbons are not widely studied and a further point is the lack of studies paying specific attention to the effects of fuel variation within a given blended biofuel. To this end, a statistical Design of Experiments L9 array, comprised of 4 parameters (%MO, %MCH, pressure, and equivalence ratio) with 3 levels of variation, constructed in order to systematically study the effects of relative fuel concentrations within the ternary blend enabled variations in fuel concentration for methyl octanoate and MCH of 10% - 30% and 20% - 40%, respectively. Variation in pressure of 1 atm, 5 atm, and 10 atm and in equivalence ratio of 0.5, 1.0, and 2.0 were used, respectively. The fuel-volume percentage of n-nonane varied from 30% - 70%. In total, 10 ternary blends were studied. Ignition delay times for the ternary blends and for the three constituents were obtained by monitoring excited-state OH or CH transitions, A2Epsilon+ -> X2Pi or A2Delta -> X2Pi, respectively, behind reflected shock waves using a heated shock tube facility. Dilute conditions of 99% Ar (vol.) were maintained in all shock tube experiments with the exception of a separate series of n-nonane and MCH experiments under stoichiometric conditions which used 4% oxygen (corresponding to ~ 95% Ar dilution). Temperatures behind reflected shock waves were varied over the range 1243 < T (K) < 1672. From over 450 shock tube experiments, empirical ignition delay time correlations were constructed for all three pure fuels and a master correlation equation for the blended fuels. Ignition experiments conducted on the pure fuels at 1.5 atm indicated the following ignition delay time order, from shortest to longest: methyl octanoate < n-nonane < MCH. With increased pressure to 10 atm (nominal) the order remained, in general, consistent. Under fuel-lean conditions, ignition trends between methyl octanoate and n-nonane exhibited overlap at temperatures below 1350 K, below which the trends diverged with methyl octanoate having shorter ignition delay times. Similar behavior was observed under fuel-rich conditions, yet with the overlap occurring above 1450 K. Stoichiometric ignition trends did not display overlapping behavior under either 1.5 atm or 10 atm pressure. Laminar flame speed measurements were performed at 1 atm and an initial temperature of 443 K on the pure fuel constituents. Additional flame speed measurements of MCH were conducted at 403 K to compare with literature values and were shown to agree strongly with experiments conducted in a constant-volume apparatus. The experiments conducted herein, for the first time, measure laminar flame speeds methyl octanoate. A detailed chemical kinetics mechanism was compiled from three independent, well-validated models for the constituent fuels, where the sub-mechanisms for methyl octanoate and MCH were extracted for integration into a base n-nonane model. The compiled mechanism in the present study (4785 reactions and 1082 species) enables modeling of oxidation processes of the ternary fuel blends of interest. Calculations were performed using the compiled model relative to the base models to assess the impact of utilizing different base chemistry sets. In general, results were reproduced well relative to base models for both n-nonane and MCH, however results for methyl octanoate from both the compiled model and the base model are in disagreement with the results measured herein. Ignition delay times of the fuel blends are well-predicted for several conditions, specifically for blends at lean/high-pressure and stoichiometric/high-pressure conditions, however are not accurately modeled at fuel-rich, high-pressure conditions.

Research with heterogeneous mixtures involving solid particulate in closed, constant-volume bombs is typically limited by the powder dispersion technique. This work details the development of an experimental apparatus that promotes ideal conditions, namely a quiescent atmosphere and uniform particle distribution, for measuring laminar, heterogeneous flame propagation. In this thesis, two methods of dispersing particles are investigated. In the first, heterogeneous mixtures are made in a secondary vessel that is connected to the main experiment. Particles are dispersed into the secondary vessel by adapting a piston-driven particle injector, which has been shown to produce uniform particle distributions. The heterogeneous mixture is then transferred to the main bomb facility and ignited after laminar conditions are achieved. In the second method of dispersion, particles are directly injected into the main experimental facility using a strong blast of compressed air. As with the first approach, enough time is given (~4 minutes) for the mixture to become quiescent before ignition occurs. An extinction diagnostic is also applied to the secondary mixing vessel as well as the primary experimental facility (for both dispersion methods) to provide a qualitative understanding of the dispersion technique. To perform this diagnostic a 632.8-nm, 5-mW Helium-Neon (HeNe) laser was employed. Aluminum nano-particles with an average diameter of 100 nm were used in this study. It was found that for typical dust loadings produced with both dispersion techniques, a pure dust-air system would not ignite due to the current spark ignition system. Thus, a hybrid mixture of Al/CH4/O2/N2 was employed to achieve the project goal of demonstrating a system for controlled laminar flame speed measurements in aerosol mixtures. With the hybrid mixture, the combustion characteristics were studied both with and without the presence of nano-Al particles. Based on the experimental results, the simplicity of the "direct-injection" methodology compared to that of the "side-vessel" is desirable and will be further investigated as a viable alternative, or improvement, to the side-vessel technology.

Increasingly stringent emission requirements and dwindling petroleum reserves have generated interest in expanding the role of synthesis gas (syngas) fuels in power generation applications. Syngas fuels are the product of gasifying organic-based feedstock such as coal and biomass and are composed of mainly H₂ and CO. However, the use of syngas fuels in lean premixed gas turbine systems has been limited in part because the behavior of turbulent flames in these mixtures at practical gas turbine operating conditions are not well understood. This thesis presents an investigation of the influence of fuel composition and pressure on the turbulent consumption speed, ST,GC, and the turbulent flame brush thickness, FBT, for these mixtures. ST,GC and FBT are global parameters which represent the average rate of conversion of reactants to products and the average heat release distribution of the turbulent flame respectively. A comprehensive database of turbulent consumption speed measurements obtained at pressures up to 20 atm and H₂/CO ratios of 30/70 to 90/10 by volume is presented. There are two key findings from this database. First, mixtures of different H₂/CO ratios but with the same un-stretched laminar flame speeds, SL,0, exposed to the same turbulence intensities, u'rms , have different turbulent consumption speeds. Second, higher pressures augment the turbulent consumption speed when SL,0 is held constant across pressures and H₂/CO ratios. These observations are attributed to the mixture stretch sensitivities, which are incorporated into a physics-based model for the turbulent consumption speed using quasi-steady leading points concepts. The derived scaling law closely resembles Damkhler's classical turbulent flame speed scaling, except that the maximum stretched laminar flame speed, SL,max, arises as the normalizing parameter. Scaling the ST,GC data by SL,max shows good collapse of the data at fixed pressures, but systematic differences between data taken at different pressures are observed. These differences are attributed to non-quasi-steady chemistry effects, which are quantified with a Damkhler number defined as the ratio of the chemical time scale associated with SL,max and a fluid mechanic time scale. The observed scatter in the normalized turbulent consumption speed data correlates very well with this Damkhler number, suggesting that ST,GC can be parameterized by u'rms/SL,max and the leading point Damkhler number. Finally, a systematic investigation of the influence of pressure and fuel composition on the flame brush thickness is presented. The flame brush thickness is shown to be independent of the H₂/CO ratio if SL,0 is held constant across the mixtures. However, increasing the equivalence ratio for lean mixtures at a constant H₂/CO ratio, results in a thicker flame brush. Increasing the pressure is shown to augment the flame brush thickness, a result which has not been previously reported in the literature. Classical correlations based on turbulent diffusion concepts collapse the flame brush thickness data obtained at fixed u'rms/U₀ and pressure reasonably well, but systematic differences exist between the data at different u'rms/U₀ and pressures.

Lean premixed combustors are highly susceptible to combustion instabilities, caused by the coupling between heat release fluctuations and combustor acoustics. In order to predict the conditions under which these instabilities occur and their limit cycle amplitudes, understanding of the amplitude dependent response of the flame to acoustic excitation is required. Extensive maps of the flame response were obtained as a function of perturbation amplitude, frequency, and flow velocity. These maps illustrated substantial nonlinearity in the perturbation velocity - heat release relationship, with complex topological dependencies that illustrate folds and kinks when plotted in frequency-amplitude-heat release space. A detailed analysis of phase locked OH PLIF images of acoustically excited swirl flames was used to identify the key controlling physical processes and qualitatively discuss their characteristics. The results illustrate that the flame response is not controlled by any single physical process but rather by several simultaneously occurring processes which are potentially competing, and whose relative significance depends upon forcing frequency, amplitude of excitation, and flame stabilization dynamics. An in-depth study on the effect of acoustic forcing on the turbulent flame properties was conducted in a turbulent Bunsen flame using PIV measurements. The results showed that the flame brush thickness and the local consumption speed were modulated in the presence of acoustic forcing. These results will not only be a useful input to help improve combustion dynamics predictions but will also help serve as validation data for models.